1. Purpose

2. Introduction

3. Definitions and Abbreviations

4. Authenticator Assurance Levels

5. Authenticator and Verifier Requirements

6. Authenticator Lifecycle Requirements

7. Session Management

8. Threats and Security Considerations

9. Privacy Considerations

10. Usability Considerations

11. References

Appendix A — Strength of Memorized Secrets

This table contains changes that have been incorporated into Special Publication 800-63B. Errata updates can include corrections, clarifications, or other minor changes in the publication that are either editorial or substantive in nature.

Date	Type	Change	Location
2017-12-01	Editorial	Updated AAL descriptions for consistency with other text in document	Introduction
	Editorial	Deleted “cryptographic” to consistently reflect authenticator options at AAL3	§4.3
	Substantive	Refined the requirements about processing of attributes	§4.4
	Editorial	Make language regarding activation factors for multifactor authenticators consistent	§5.1.5.1, 5.1.8.1, and 5.1.9.1
	Substantive	Recognize use of hardware TPM as hardware crypto authenticator	§5.1.7.1, 5.1.9.1
	Editorial	Improve normative language on authenticated protected channels for biometrics	§5.2.3
	Editorial	Changed “transaction” to “binding transaction” to emphasize that requirement doesn’t apply to authentication transactions	§6.1.1
	Editorial	Replaced out-of-context note at end of section 7.2	§7.2
	Editorial	Changed IdP to CSP to match terminology used elsewhere in this document	Table 8-1
	Editorial	Corrected capitalization of Side Channel Attack	Table 8-2
	Substantive	Changed the title to processing limitation; clarified the language, incorporated privacy objectives language, and specified that consent is explicit	§9.3
	Editorial	Added NISTIR 8062 as a reference	§11.1
	Editorial	Corrected title of SP 800-63C	§11.3
2020-03-02	Substantive	Clarified wording of verifier impersonation resistance requirement	§4.3.2
	Editorial	Emphasized use of key unlocked by additional factor to sign nonce	§5.1.9.1
	Editorial	Provided examples of risk-based behavior observations	§5.2.2
	Editorial	Removed redundant phrase	§5.2.3
	Editorial	Updated URL for reference [Blacklists]	§11.1

1 Purpose

This section is informative.

This document and its companion documents, Special Publication (SP) 800-63, SP 800-63A, and SP 800-63C, provide technical guidelines to agencies for the implementation of digital authentication.

2 Introduction

This section is informative.

Digital identity is the unique representation of a subject engaged in an online transaction. A digital identity is always unique in the context of a digital service, but does not necessarily need to be traceable back to a specific real-life subject. In other words, accessing a digital service may not mean that the underlying subject’s real-life representation is known. Identity proofing establishes that a subject is actually who they claim to be. Digital authentication is the process of determining the validity of one or more authenticators used to claim a digital identity. Authentication establishes that a subject attempting to access a digital service is in control of the technologies used to authenticate. For services in which return visits are applicable, successfully authenticating provides reasonable risk-based assurances that the subject accessing the service today is the same as the one who accessed the service previously. Digital identity presents a technical challenge because it often involves the proofing of individuals over an open network and always involves the authentication of individuals over an open network. This presents multiple opportunities for impersonation and other attacks which can lead to fraudulent claims of a subject’s digital identity.

The ongoing authentication of subscribers is central to the process of associating a subscriber with their online activity. Subscriber authentication is performed by verifying that the claimant controls one or more authenticators (called tokens in earlier versions of SP 800-63) associated with a given subscriber. A successful authentication results in the assertion of an identifier, either pseudonymous or non-pseudonymous, and optionally other identity information, to the relying party (RP).

This document provides recommendations on types of authentication processes, including choices of authenticators, that may be used at various Authenticator Assurance Levels (AALs). It also provides recommendations on the lifecycle of authenticators, including revocation in the event of loss or theft.

This technical guideline applies to digital authentication of subjects to systems over a network. It does not address the authentication of a person for physical access (e.g., to a building), though some credentials used for digital access may also be used for physical access authentication. This technical guideline also requires that federal systems and service providers participating in authentication protocols be authenticated to subscribers.

The strength of an authentication transaction is characterized by an ordinal measurement known as the AAL. Stronger authentication (a higher AAL) requires malicious actors to have better capabilities and expend greater resources in order to successfully subvert the authentication process. Authentication at higher AALs can effectively reduce the risk of attacks. A high-level summary of the technical requirements for each of the AALs is provided below; see Sections 4 and 5 of this document for specific normative requirements.

Authenticator Assurance Level 1: AAL1 provides some assurance that the claimant controls an authenticator bound to the subscriber’s account. AAL1 requires either single-factor or multi-factor authentication using a wide range of available authentication technologies. Successful authentication requires that the claimant prove possession and control of the authenticator through a secure authentication protocol.

Authenticator Assurance Level 2: AAL2 provides high confidence that the claimant controls an authenticator(s) bound to the subscriber’s account. Proof of possession and control of two different authentication factors is required through secure authentication protocol(s). Approved cryptographic techniques are required at AAL2 and above.

Authenticator Assurance Level 3: AAL3 provides very high confidence that the claimant controls authenticator(s) bound to the subscriber’s account. Authentication at AAL3 is based on proof of possession of a key through a cryptographic protocol. AAL3 authentication requires a hardware-based authenticator and an authenticator that provides verifier impersonation resistance; the same device may fulfill both these requirements. In order to authenticate at AAL3, claimants are required to prove possession and control of two distinct authentication factors through secure authentication protocol(s). Approved cryptographic techniques are required.

The following table states which sections of the document are normative and which are informative:

Section Name	Normative/Informative
1. Purpose	Informative
2. Introduction	Informative
3. Definitions and Abbreviations	Informative
4. Authenticator Assurance Levels	Normative
5. Authenticator and Verifier Requirements	Normative
6. Authenticator Lifecycle Management	Normative
7. Session Management	Normative
8. Threat and Security Considerations	Informative
9. Privacy Considerations	Informative
10. Usability Considerations	Informative
11. References	Informative
Appendix A — Strength of Memorized Secrets	Informative

3 Definitions and Abbreviations

See SP 800-63, Appendix A for a complete set of definitions and abbreviations.

4 Authenticator Assurance Levels

This section contains both normative and informative material.

To satisfy the requirements of a given AAL, a claimant SHALL be authenticated with at least a given level of strength to be recognized as a subscriber. The result of an authentication process is an identifier that SHALL be used each time that subscriber authenticates to that RP. The identifier MAY be pseudonymous. Subscriber identifiers SHOULD NOT be reused for a different subject but SHOULD be reused when a previously-enrolled subject is re-enrolled by the CSP. Other attributes that identify the subscriber as a unique subject MAY also be provided.

Detailed normative requirements for authenticators and verifiers at each AAL are provided in Section 5.

See SP 800-63 Section 6.2 for details on how to choose the most appropriate AAL.

FIPS 140 requirements are satisfied by FIPS 140-2 or newer revisions.

At IAL1, it is possible that attributes are collected and made available by the digital identity service. Any PII or other personal information — whether self-asserted or validated — requires multi-factor authentication. Therefore, agencies SHALL select a minimum of AAL2 when self-asserted PII or other personal information is made available online.

4.1 Authenticator Assurance Level 1

This section is normative.

AAL1 provides some assurance that the claimant controls an authenticator bound to the subscriber’s account. AAL1 requires either single-factor or multi-factor authentication using a wide range of available authentication technologies. Successful authentication requires that the claimant prove possession and control of the authenticator through a secure authentication protocol.

4.1.1 Permitted Authenticator Types

AAL1 authentication SHALL occur by the use of any of the following authenticator types, which are defined in Section 5:

• Memorized Secret (Section 5.1.1)
• Look-Up Secret (Section 5.1.2)
• Out-of-Band Devices (Section 5.1.3)
• Single-Factor One-Time Password (OTP) Device (Section 5.1.4)
• Multi-Factor OTP Device (Section 5.1.5)
• Single-Factor Cryptographic Software (Section 5.1.6)
• Single-Factor Cryptographic Device (Section 5.1.7)
• Multi-Factor Cryptographic Software (Section 5.1.8)
• Multi-Factor Cryptographic Device (Section 5.1.9)

4.1.2 Authenticator and Verifier Requirements

Cryptographic authenticators used at AAL1 SHALL use approved cryptography. Software-based authenticators that operate within the context of an operating system MAY, where applicable, attempt to detect compromise (e.g., by malware) of the user endpoint in which they are running and SHOULD NOT complete the operation when such a compromise is detected.

Communication between the claimant and verifier (using the primary channel in the case of an out-of-band authenticator) SHALL be via an authenticated protected channel to provide confidentiality of the authenticator output and resistance to man-in-the-middle (MitM) attacks.

Verifiers operated by government agencies at AAL1 SHALL be validated to meet the requirements of FIPS 140 Level 1.

4.1.3 Reauthentication

Periodic reauthentication of subscriber sessions SHALL be performed as described in Section 7.2. At AAL1, reauthentication of the subscriber SHOULD be repeated at least once per 30 days during an extended usage session, regardless of user activity. The session SHOULD be terminated (i.e., logged out) when this time limit is reached.

4.1.4 Security Controls

The CSP SHALL employ appropriately-tailored security controls from the low baseline of security controls defined in SP 800-53 or equivalent federal (e.g. FEDRAMP) or industry standard. The CSP SHALL ensure that the minimum assurance-related controls for low-impact systems, or equivalent, are satisfied.

4.1.5 Records Retention Policy

The CSP shall comply with its respective records retention policies in accordance with applicable laws, regulations, and policies, including any National Archives and Records Administration (NARA) records retention schedules that may apply. If the CSP opts to retain records in the absence of any mandatory requirements, the CSP SHALL conduct a risk management process, including assessments of privacy and security risks, to determine how long records should be retained and SHALL inform the subscriber of that retention policy.

4.2 Authenticator Assurance Level 2

This section is normative.

AAL2 provides high confidence that the claimant controls authenticator(s) bound to the subscriber’s account. Proof of possession and control of two distinct authentication factors is required through secure authentication protocol(s). Approved cryptographic techniques are required at AAL2 and above.

4.2.1 Permitted Authenticator Types

At AAL2, authentication SHALL occur by the use of either a multi-factor authenticator or a combination of two single-factor authenticators. A multi-factor authenticator requires two factors to execute a single authentication event, such as a cryptographically-secure device with an integrated biometric sensor that is required to activate the device. Authenticator requirements are specified in Section 5.

When a multi-factor authenticator is used, any of the following MAY be used:

• Multi-Factor OTP Device (Section 5.1.5)
• Multi-Factor Cryptographic Software (Section 5.1.8)
• Multi-Factor Cryptographic Device (Section 5.1.9)

When a combination of two single-factor authenticators is used, it SHALL include a Memorized Secret authenticator (Section 5.1.1) and one possession-based (i.e., “something you have”) authenticator from the following list:

• Look-Up Secret (Section 5.1.2)
• Out-of-Band Device (Section 5.1.3)
• Single-Factor OTP Device (Section 5.1.4)
• Single-Factor Cryptographic Software (Section 5.1.6)
• Single-Factor Cryptographic Device (Section 5.1.7)

Note: When biometric authentication meets the requirements in Section 5.2.3, the device has to be authenticated in addition to the biometric — a biometric is recognized as a factor, but not recognized as an authenticator by itself. Therefore, when conducting authentication with a biometric, it is unnecessary to use two authenticators because the associated device serves as “something you have,” while the biometric serves as “something you are.”

4.2.2 Authenticator and Verifier Requirements

Cryptographic authenticators used at AAL2 SHALL use approved cryptography. Authenticators procured by government agencies SHALL be validated to meet the requirements of FIPS 140 Level 1. Software-based authenticators that operate within the context of an operating system MAY, where applicable, attempt to detect compromise of the platform in which they are running (e.g., by malware) and SHOULD NOT complete the operation when such a compromise is detected. At least one authenticator used at AAL2 SHALL be replay resistant as described in Section 5.2.8. Authentication at AAL2 SHOULD demonstrate authentication intent from at least one authenticator as discussed in Section 5.2.9.

Communication between the claimant and verifier (the primary channel in the case of an out-of-band authenticator) SHALL be via an authenticated protected channel to provide confidentiality of the authenticator output and resistance to MitM attacks.

Verifiers operated by government agencies at AAL2 SHALL be validated to meet the requirements of FIPS 140 Level 1.

When a device such as a smartphone is used in the authentication process, the unlocking of that device (typically done using a PIN or biometric) SHALL NOT be considered one of the authentication factors. Generally, it is not possible for a verifier to know that the device had been locked or if the unlock process met the requirements for the relevant authenticator type.

When a biometric factor is used in authentication at AAL2, the performance requirements stated in Section 5.2.3 SHALL be met, and the verifier SHOULD make a determination that the biometric sensor and subsequent processing meet these requirements.

4.2.3 Reauthentication

Periodic reauthentication of subscriber sessions SHALL be performed as described in Section 7.2. At AAL2, authentication of the subscriber SHALL be repeated at least once per 12 hours during an extended usage session, regardless of user activity. Reauthentication of the subscriber SHALL be repeated following any period of inactivity lasting 30 minutes or longer. The session SHALL be terminated (i.e., logged out) when either of these time limits is reached.

Reauthentication of a session that has not yet reached its time limit MAY require only a memorized secret or a biometric in conjunction with the still-valid session secret. The verifier MAY prompt the user to cause activity just before the inactivity timeout.

4.2.4 Security Controls

The CSP SHALL employ appropriately-tailored security controls from the moderate baseline of security controls defined in SP 800-53 or equivalent federal (e.g., FEDRAMP) or industry standard. The CSP SHALL ensure that the minimum assurance-related controls for moderate-impact systems or equivalent are satisfied.

4.2.5 Records Retention Policy

The CSP shall comply with its respective records retention policies in accordance with applicable laws, regulations, and policies, including any NARA records retention schedules that may apply. If the CSP opts to retain records in the absence of any mandatory requirements, the CSP SHALL conduct a risk management process, including assessments of privacy and security risks to determine how long records should be retained and SHALL inform the subscriber of that retention policy.

4.3 Authenticator Assurance Level 3

This section is normative.

AAL3 provides very high confidence that the claimant controls authenticator(s) bound to the subscriber’s account. Authentication at AAL3 is based on proof of possession of a key through a cryptographic protocol. AAL3 authentication SHALL use a hardware-based authenticator and an authenticator that provides verifier impersonation resistance — the same device MAY fulfill both these requirements. In order to authenticate at AAL3, claimants SHALL prove possession and control of two distinct authentication factors through secure authentication protocol(s). Approved cryptographic techniques are required.

4.3.1 Permitted Authenticator Types

AAL3 authentication SHALL occur by the use of one of a combination of authenticators satisfying the requirements in Section 4.3. Possible combinations are:

• Multi-Factor Cryptographic Device (Section 5.1.9)
• Single-Factor Cryptographic Device (Section 5.1.7) used in conjunction with Memorized Secret (Section 5.1.1)
• Multi-Factor OTP device (software or hardware) (Section 5.1.5) used in conjunction with a Single-Factor Cryptographic Device (Section 5.1.7)
• Multi-Factor OTP device (hardware only) (Section 5.1.5) used in conjunction with a Single-Factor Cryptographic Software (Section 5.1.6)
• Single-Factor OTP device (hardware only) (Section 5.1.4) used in conjunction with a Multi-Factor Cryptographic Software Authenticator (Section 5.1.8)
• Single-Factor OTP device (hardware only) (Section 5.1.4) used in conjunction with a Single-Factor Cryptographic Software Authenticator (Section 5.1.6) and a Memorized Secret (Section 5.1.1)

4.3.2 Authenticator and Verifier Requirements

Communication between the claimant and verifier SHALL be via an authenticated protected channel to provide confidentiality of the authenticator output and resistance to MitM attacks. At least one cryptographic authenticator used at AAL3 SHALL be verifier impersonation resistant as described in Section 5.2.5 and SHALL be replay resistant as described in Section 5.2.8. All authentication and reauthentication processes at AAL3 SHALL demonstrate authentication intent from at least one authenticator as described in Section 5.2.9.

Multi-factor authenticators used at AAL3 SHALL be hardware cryptographic modules validated at FIPS 140 Level 2 or higher overall with at least FIPS 140 Level 3 physical security. Single-factor cryptographic devices used at AAL3 SHALL be validated at FIPS 140 Level 1 or higher overall with at least FIPS 140 Level 3 physical security.

Verifiers at AAL3 SHALL be validated at FIPS 140 Level 1 or higher.

Verifiers at AAL3 SHALL be verifier compromise resistant as described in Section 5.2.7 with respect to at least one authentication factor.

Hardware-based authenticators and verifiers at AAL3 SHOULD resist relevant side-channel (e.g., timing and power-consumption analysis) attacks. Relevant side-channel attacks SHALL be determined by a risk assessment performed by the CSP.

When a device such a smartphone is used in the authentication process — presuming that the device is able to meet the requirements above — the unlocking of that device SHALL NOT be considered to satisfy one of the authentication factors. This is because it is generally not possible for verifier to know that the device had been locked nor whether the unlock process met the requirements for the relevant authenticator type.

When a biometric factor is used in authentication at AAL3, the verifier SHALL make a determination that the biometric sensor and subsequent processing meet the performance requirements stated in Section 5.2.3.

4.3.3 Reauthentication

Periodic reauthentication of subscriber sessions SHALL be performed as described in Section 7.2. At AAL3, authentication of the subscriber SHALL be repeated at least once per 12 hours during an extended usage session, regardless of user activity, as described in Section 7.2. Reauthentication of the subscriber SHALL be repeated following any period of inactivity lasting 15 minutes or longer. Reauthentication SHALL use both authentication factors. The session SHALL be terminated (i.e., logged out) when either of these time limits is reached. The verifier MAY prompt the user to cause activity just before the inactivity timeout.

4.3.4 Security Controls

The CSP SHALL employ appropriately-tailored security controls from the high baseline of security controls defined in SP 800-53 or an equivalent federal (e.g., FEDRAMP) or industry standard. The CSP SHALL ensure that the minimum assurance-related controls for high-impact systems or equivalent are satisfied.

4.3.5 Records Retention Policy

The CSP shall comply with its respective records retention policies in accordance with applicable laws, regulations, and policies, including any NARA records retention schedules that may apply. If the CSP opts to retain records in the absence of any mandatory requirements, the CSP SHALL conduct a risk management process, including assessments of privacy and security risks, to determine how long records should be retained and SHALL inform the subscriber of that retention policy.

4.4 Privacy Requirements

The CSP SHALL employ appropriately-tailored privacy controls defined in SP 800-53 or equivalent industry standard.

If CSPs process attributes for purposes other than identity proofing, authentication, or attribute assertions (collectively “identity service”), related fraud mitigation, or to comply with law or legal process, CSPs SHALL implement measures to maintain predictability and manageability commensurate with the privacy risk arising from the additional processing. Measures MAY include providing clear notice, obtaining subscriber consent, or enabling selective use or disclosure of attributes. When CSPs use consent measures, CSPs SHALL NOT make consent for the additional processing a condition of the identity service.

Regardless of whether the CSP is an agency or private sector provider, the following requirements apply to an agency offering or using the authentication service:

1. The agency SHALL consult with their Senior Agency Official for Privacy (SAOP) and conduct an analysis to determine whether the collection of PII to issue or maintain authenticators triggers the requirements of the Privacy Act of 1974 [Privacy Act] (see Section 9.4).
   • The agency SHALL publish a System of Records Notice (SORN) to cover such collections, as applicable.
   • The agency SHALL consult with their SAOP and conduct an analysis to determine whether the collection of PII to issue or maintain authenticators triggers the requirements of the E-Government Act of 2002 [E-Gov].
   • The agency SHALL publish a Privacy Impact Assessment (PIA) to cover such collection, as applicable.

4.5 Summary of Requirements

This section is informative.

Table 4-1 summarizes the requirements for each of the AALs:

Requirement	AAL1	AAL2	AAL3
Permitted authenticator types	Memorized Secret; Look-up Secret; Out-of-Band; SF OTP Device; MF OTP Device; SF Crypto Software; SF Crypto Device; MF Crypto Software; MF Crypto Device	MF OTP Device; MF Crypto Software; MF Crypto Device; or Memorized Secret plus:  • Look-up Secret  • Out-of-Band  • SF OTP Device  • SF Crypto Software  • SF Crypto Device	MF Crypto Device; SF Crypto Device plus   Memorized Secret; SF OTP Device plus MF Crypto Device or Software; SF OTP Device plus SF Crypto Software plus Memorized Secret
FIPS 140 validation	Level 1 (Government agency verifiers)	Level 1 (Government agency authenticators and verifiers)	Level 2 overall (MF authenticators) Level 1 overall (verifiers and SF Crypto Devices) Level 3 physical security (all authenticators)
Reauthentication	30 days	12 hours or 30 minutes inactivity; MAY use one authentication factor	12 hours or 15 minutes inactivity; SHALL use both authentication factors
Security controls	SP 800-53 Low Baseline (or equivalent)	SP 800-53 Moderate Baseline (or equivalent)	SP 800-53 High Baseline (or equivalent)
MitM resistance	Required	Required	Required
Verifier-impersonation resistance	Not required	Not required	Required
Verifier-compromise resistance	Not required	Not required	Required
Replay resistance	Not required	Required	Required
Authentication intent	Not required	Recommended	Required
Records Retention Policy	Required	Required	Required
Privacy Controls	Required	Required	Required

5 Authenticator and Verifier Requirements

This section is normative.

This section provides the detailed requirements specific to each type of authenticator. With the exception of reauthentication requirements specified in Section 4 and the requirement for verifier impersonation resistance at AAL3 described in Section 5.2.5, the technical requirements for each of the authenticator types are the same regardless of the AAL at which the authenticator is used.

5.1 Requirements by Authenticator Type

5.1.1 Memorized Secrets

5.1.1.1 Memorized Secret Authenticators

Memorized secrets SHALL be at least 8 characters in length if chosen by the subscriber. Memorized secrets chosen randomly by the CSP or verifier SHALL be at least 6 characters in length and MAY be entirely numeric. If the CSP or verifier disallows a chosen memorized secret based on its appearance on a blacklist of compromised values, the subscriber SHALL be required to choose a different memorized secret. No other complexity requirements for memorized secrets SHOULD be imposed. A rationale for this is presented in Appendix A Strength of Memorized Secrets.

5.1.1.2 Memorized Secret Verifiers

Verifiers SHALL require subscriber-chosen memorized secrets to be at least 8 characters in length. Verifiers SHOULD permit subscriber-chosen memorized secrets at least 64 characters in length. All printing ASCII [RFC 20] characters as well as the space character SHOULD be acceptable in memorized secrets. Unicode [ISO/ISC 10646] characters SHOULD be accepted as well. To make allowances for likely mistyping, verifiers MAY replace multiple consecutive space characters with a single space character prior to verification, provided that the result is at least 8 characters in length. Truncation of the secret SHALL NOT be performed. For purposes of the above length requirements, each Unicode code point SHALL be counted as a single character.

If Unicode characters are accepted in memorized secrets, the verifier SHOULD apply the Normalization Process for Stabilized Strings using either the NFKC or NFKD normalization defined in Section 12.1 of Unicode Standard Annex 15 [UAX 15]. This process is applied before hashing the byte string representing the memorized secret. Subscribers choosing memorized secrets containing Unicode characters SHOULD be advised that some characters may be represented differently by some endpoints, which can affect their ability to authenticate successfully.

Memorized secrets that are randomly chosen by the CSP (e.g., at enrollment) or by the verifier (e.g., when a user requests a new PIN) SHALL be at least 6 characters in length and SHALL be generated using an approved random bit generator [SP 800-90Ar1].

Memorized secret verifiers SHALL NOT permit the subscriber to store a “hint” that is accessible to an unauthenticated claimant. Verifiers SHALL NOT prompt subscribers to use specific types of information (e.g., “What was the name of your first pet?”) when choosing memorized secrets.

When processing requests to establish and change memorized secrets, verifiers SHALL compare the prospective secrets against a list that contains values known to be commonly-used, expected, or compromised. For example, the list MAY include, but is not limited to:

• Passwords obtained from previous breach corpuses.
• Dictionary words.
• Repetitive or sequential characters (e.g. ‘aaaaaa’, ‘1234abcd’).
• Context-specific words, such as the name of the service, the username, and derivatives thereof.

If the chosen secret is found in the list, the CSP or verifier SHALL advise the subscriber that they need to select a different secret, SHALL provide the reason for rejection, and SHALL require the subscriber to choose a different value.

Verifiers SHOULD offer guidance to the subscriber, such as a password-strength meter [Meters], to assist the user in choosing a strong memorized secret. This is particularly important following the rejection of a memorized secret on the above list as it discourages trivial modification of listed (and likely very weak) memorized secrets [Blacklists].

Verifiers SHALL implement a rate-limiting mechanism that effectively limits the number of failed authentication attempts that can be made on the subscriber’s account as described in Section 5.2.2.

Verifiers SHOULD NOT impose other composition rules (e.g., requiring mixtures of different character types or prohibiting consecutively repeated characters) for memorized secrets. Verifiers SHOULD NOT require memorized secrets to be changed arbitrarily (e.g., periodically). However, verifiers SHALL force a change if there is evidence of compromise of the authenticator.

Verifiers SHOULD permit claimants to use “paste” functionality when entering a memorized secret. This facilitates the use of password managers, which are widely used and in many cases increase the likelihood that users will choose stronger memorized secrets.

In order to assist the claimant in successfully entering a memorized secret, the verifier SHOULD offer an option to display the secret — rather than a series of dots or asterisks — until it is entered. This allows the claimant to verify their entry if they are in a location where their screen is unlikely to be observed. The verifier MAY also permit the user’s device to display individual entered characters for a short time after each character is typed to verify correct entry. This is particularly applicable on mobile devices.

The verifier SHALL use approved encryption and an authenticated protected channel when requesting memorized secrets in order to provide resistance to eavesdropping and MitM attacks.

Verifiers SHALL store memorized secrets in a form that is resistant to offline attacks. Memorized secrets SHALL be salted and hashed using a suitable one-way key derivation function. Key derivation functions take a password, a salt, and a cost factor as inputs then generate a password hash. Their purpose is to make each password guessing trial by an attacker who has obtained a password hash file expensive and therefore the cost of a guessing attack high or prohibitive. Examples of suitable key derivation functions include Password-based Key Derivation Function 2 (PBKDF2) [SP 800-132] and Balloon [BALLOON]. A memory-hard function SHOULD be used because it increases the cost of an attack. The key derivation function SHALL use an approved one-way function such as Keyed Hash Message Authentication Code (HMAC) [FIPS 198-1], any approved hash function in SP 800-107, Secure Hash Algorithm 3 (SHA-3) [FIPS 202], CMAC [SP 800-38B] or Keccak Message Authentication Code (KMAC), Customizable SHAKE (cSHAKE), or ParallelHash [SP 800-185]. The chosen output length of the key derivation function SHOULD be the same as the length of the underlying one-way function output.

The salt SHALL be at least 32 bits in length and be chosen arbitrarily so as to minimize salt value collisions among stored hashes. Both the salt value and the resulting hash SHALL be stored for each subscriber using a memorized secret authenticator.

For PBKDF2, the cost factor is an iteration count: the more times the PBKDF2 function is iterated, the longer it takes to compute the password hash. Therefore, the iteration count SHOULD be as large as verification server performance will allow, typically at least 10,000 iterations.

In addition, verifiers SHOULD perform an additional iteration of a key derivation function using a salt value that is secret and known only to the verifier. This salt value, if used, SHALL be generated by an approved random bit generator [SP 800-90Ar1] and provide at least the minimum security strength specified in the latest revision of SP 800-131A (112 bits as of the date of this publication). The secret salt value SHALL be stored separately from the hashed memorized secrets (e.g., in a specialized device like a hardware security module). With this additional iteration, brute-force attacks on the hashed memorized secrets are impractical as long as the secret salt value remains secret.

5.1.2 Look-Up Secrets

5.1.2.1 Look-Up Secret Authenticators

CSPs creating look-up secret authenticators SHALL use an approved random bit generator [SP 800-90Ar1] to generate the list of secrets and SHALL deliver the authenticator securely to the subscriber. Look-up secrets SHALL have at least 20 bits of entropy.

Look-up secrets MAY be distributed by the CSP in person, by postal mail to the subscriber’s address of record, or by online distribution. If distributed online, look-up secrets SHALL be distributed over a secure channel in accordance with the post-enrollment binding requirements in Section 6.1.2.

If the authenticator uses look-up secrets sequentially from a list, the subscriber MAY dispose of used secrets, but only after a successful authentication.

5.1.2.2 Look-Up Secret Verifiers

Verifiers of look-up secrets SHALL prompt the claimant for the next secret from their authenticator or for a specific (e.g., numbered) secret. A given secret from an authenticator SHALL be used successfully only once. If the look-up secret is derived from a grid card, each cell of the grid SHALL be used only once.

Verifiers SHALL store look-up secrets in a form that is resistant to offline attacks. Look-up secrets having at least 112 bits of entropy SHALL be hashed with an approved one-way function as described in Section 5.1.1.2. Look-up secrets with fewer than 112 bits of entropy SHALL be salted and hashed using a suitable one-way key derivation function, also described in Section 5.1.1.2. The salt value SHALL be at least 32 in bits in length and arbitrarily chosen so as to minimize salt value collisions among stored hashes. Both the salt value and the resulting hash SHALL be stored for each look-up secret.

For look-up secrets that have less than 64 bits of entropy, the verifier SHALL implement a rate-limiting mechanism that effectively limits the number of failed authentication attempts that can be made on the subscriber’s account as described in Section 5.2.2.

The verifier SHALL use approved encryption and an authenticated protected channel when requesting look-up secrets in order to provide resistance to eavesdropping and MitM attacks.

5.1.3 Out-of-Band Devices

5.1.3.1 Out-of-Band Authenticators

The out-of-band authenticator SHALL establish a separate channel with the verifier in order to retrieve the out-of-band secret or authentication request. This channel is considered to be out-of-band with respect to the primary communication channel (even if it terminates on the same device) provided the device does not leak information from one channel to the other without the authorization of the claimant.

The out-of-band device SHOULD be uniquely addressable and communication over the secondary channel SHALL be encrypted unless sent via the public switched telephone network (PSTN). For additional authenticator requirements specific to the PSTN, see Section 5.1.3.3. Methods that do not prove possession of a specific device, such as voice-over-IP (VOIP) or email, SHALL NOT be used for out-of-band authentication.

The out-of-band authenticator SHALL uniquely authenticate itself in one of the following ways when communicating with the verifier:

• Establish an authenticated protected channel to the verifier using approved cryptography. The key used SHALL be stored in suitably secure storage available to the authenticator application (e.g., keychain storage, TPM, TEE, secure element).

• Authenticate to a public mobile telephone network using a SIM card or equivalent that uniquely identifies the device. This method SHALL only be used if a secret is being sent from the verifier to the out-of-band device via the PSTN (SMS or voice).

If a secret is sent by the verifier to the out-of-band device, the device SHOULD NOT display the authentication secret while it is locked by the owner (i.e., requires an entry of a PIN, passcode, or biometric to view). However, authenticators SHOULD indicate the receipt of an authentication secret on a locked device.

If the out-of-band authenticator sends an approval message over the secondary communication channel — rather than by the claimant transferring a received secret to the primary communication channel — it SHALL do one of the following:

• The authenticator SHALL accept transfer of the secret from the primary channel which it SHALL send to the verifier over the secondary channel to associate the approval with the authentication transaction. The claimant MAY perform the transfer manually or use a technology such as a barcode or QR code to effect the transfer.

• The authenticator SHALL present a secret received via the secondary channel from the verifier and prompt the claimant to verify the consistency of that secret with the primary channel, prior to accepting a yes/no response from the claimant. It SHALL then send that response to the verifier.

5.1.3.2 Out-of-Band Verifiers

For additional verification requirements specific to the PSTN, see Section 5.1.3.3.

If out-of-band verification is to be made using a secure application, such as on a smart phone, the verifier MAY send a push notification to that device. The verifier then waits for the establishment of an authenticated protected channel and verifies the authenticator’s identifying key. The verifier SHALL NOT store the identifying key itself, but SHALL use a verification method (e.g., an approved hash function or proof of possession of the identifying key) to uniquely identify the authenticator. Once authenticated, the verifier transmits the authentication secret to the authenticator.

Depending on the type of out-of-band authenticator, one of the following SHALL take place:

• Transfer of secret to primary channel: The verifier MAY signal the device containing the subscriber’s authenticator to indicate readiness to authenticate. It SHALL then transmit a random secret to the out-of-band authenticator. The verifier SHALL then wait for the secret to be returned on the primary communication channel.

• Transfer of secret to secondary channel: The verifier SHALL display a random authentication secret to the claimant via the primary channel. It SHALL then wait for the secret to be returned on the secondary channel from the claimant’s out-of-band authenticator.

• Verification of secrets by claimant: The verifier SHALL display a random authentication secret to the claimant via the primary channel, and SHALL send the same secret to the out-of-band authenticator via the secondary channel for presentation to the claimant. It SHALL then wait for an approval (or disapproval) message via the secondary channel.

In all cases, the authentication SHALL be considered invalid if not completed within 10 minutes. In order to provide replay resistance as described in Section 5.2.8, verifiers SHALL accept a given authentication secret only once during the validity period.

The verifier SHALL generate random authentication secrets with at least 20 bits of entropy using an approved random bit generator [SP 800-90Ar1]. If the authentication secret has less than 64 bits of entropy, the verifier SHALL implement a rate-limiting mechanism that effectively limits the number of failed authentication attempts that can be made on the subscriber’s account as described in Section 5.2.2.

5.1.3.3 Authentication using the Public Switched Telephone Network

Use of the PSTN for out-of-band verification is RESTRICTED as described in this section and in Section 5.2.10. If out-of-band verification is to be made using the PSTN, the verifier SHALL verify that the pre-registered telephone number being used is associated with a specific physical device. Changing the pre-registered telephone number is considered to be the binding of a new authenticator and SHALL only occur as described in Section 6.1.2.

Verifiers SHOULD consider risk indicators such as device swap, SIM change, number porting, or other abnormal behavior before using the PSTN to deliver an out-of-band authentication secret.

NOTE: Consistent with the restriction of authenticators in Section 5.2.10, NIST may adjust the RESTRICTED status of the PSTN over time based on the evolution of the threat landscape and the technical operation of the PSTN.

5.1.4 Single-Factor OTP Device

5.1.4.1 Single-Factor OTP Authenticators

Single-factor OTP authenticators contain two persistent values. The first is a symmetric key that persists for the device’s lifetime. The second is a nonce that is either changed each time the authenticator is used or is based on a real-time clock.

The secret key and its algorithm SHALL provide at least the minimum security strength specified in the latest revision of SP 800-131A (112 bits as of the date of this publication). The nonce SHALL be of sufficient length to ensure that it is unique for each operation of the device over its lifetime. OTP authenticators — particularly software-based OTP generators — SHOULD discourage and SHALL NOT facilitate the cloning of the secret key onto multiple devices.

The authenticator output is obtained by using an approved block cipher or hash function to combine the key and nonce in a secure manner. The authenticator output MAY be truncated to as few as 6 decimal digits (approximately 20 bits of entropy).

If the nonce used to generate the authenticator output is based on a real-time clock, the nonce SHALL be changed at least once every 2 minutes. The OTP value associated with a given nonce SHALL be accepted only once.

5.1.4.2 Single-Factor OTP Verifiers

Single-factor OTP verifiers effectively duplicate the process of generating the OTP used by the authenticator. As such, the symmetric keys used by authenticators are also present in the verifier, and SHALL be strongly protected against compromise.

When a single-factor OTP authenticator is being associated with a subscriber account, the verifier or associated CSP SHALL use approved cryptography to either generate and exchange or to obtain the secrets required to duplicate the authenticator output.

The verifier SHALL use approved encryption and an authenticated protected channel when collecting the OTP in order to provide resistance to eavesdropping and MitM attacks. Time-based OTPs [RFC 6238] SHALL have a defined lifetime that is determined by the expected clock drift — in either direction — of the authenticator over its lifetime, plus allowance for network delay and user entry of the OTP. In order to provide replay resistance as described in Section 5.2.8, verifiers SHALL accept a given time-based OTP only once during the validity period.

If the authenticator output has less than 64 bits of entropy, the verifier SHALL implement a rate-limiting mechanism that effectively limits the number of failed authentication attempts that can be made on the subscriber’s account as described in Section 5.2.2.

5.1.5 Multi-Factor OTP Devices

5.1.5.1 Multi-Factor OTP Authenticators

Multi-factor OTP authenticators operate in a similar manner to single-factor OTP authenticators (see Section 5.1.4.1), except that they require the entry of either a memorized secret or the use of a biometric to obtain the OTP from the authenticator. Each use of the authenticator SHALL require the input of the additional factor.

In addition to activation information, multi-factor OTP authenticators contain two persistent values. The first is a symmetric key that persists for the device’s lifetime. The second is a nonce that is either changed each time the authenticator is used or is based on a real-time clock.

The secret key and its algorithm SHALL provide at least the minimum security strength specified in the latest revision of [SP 800-131A] (112 bits as of the date of this publication). The nonce SHALL be of sufficient length to ensure that it is unique for each operation of the device over its lifetime. OTP authenticators — particularly software-based OTP generators — SHOULD discourage and SHALL NOT facilitate the cloning of the secret key onto multiple devices.

The authenticator output is obtained by using an approved block cipher or hash function to combine the key and nonce in a secure manner. The authenticator output MAY be truncated to as few as 6 decimal digits (approximately 20 bits of entropy).

If the nonce used to generate the authenticator output is based on a real-time clock, the nonce SHALL be changed at least once every 2 minutes. The OTP value associated with a given nonce SHALL be accepted only once.

Any memorized secret used by the authenticator for activation SHALL be a randomly-chosen numeric secret at least 6 decimal digits in length or other memorized secret meeting the requirements of Section 5.1.1.2 and SHALL be rate limited as specified in Section 5.2.2. A biometric activation factor SHALL meet the requirements of Section 5.2.3, including limits on the number of consecutive authentication failures.

The unencrypted key and activation secret or biometric sample — and any biometric data derived from the biometric sample such as a probe produced through signal processing — SHALL be zeroized immediately after an OTP has been generated.

5.1.5.2 Multi-Factor OTP Verifiers

Multi-factor OTP verifiers effectively duplicate the process of generating the OTP used by the authenticator, but without the requirement that a second factor be provided. As such, the symmetric keys used by authenticators SHALL be strongly protected against compromise.

When a multi-factor OTP authenticator is being associated with a subscriber account, the verifier or associated CSP SHALL use approved cryptography to either generate and exchange or to obtain the secrets required to duplicate the authenticator output. The verifier or CSP SHALL also establish, via the authenticator source, that the authenticator is a multi-factor device. In the absence of a trusted statement that it is a multi-factor device, the verifier SHALL treat the authenticator as single-factor, in accordance with Section 5.1.4.

The verifier SHALL use approved encryption and an authenticated protected channel when collecting the OTP in order to provide resistance to eavesdropping and MitM attacks. Time-based OTPs [RFC 6238] SHALL have a defined lifetime that is determined by the expected clock drift — in either direction — of the authenticator over its lifetime, plus allowance for network delay and user entry of the OTP. In order to provide replay resistance as described in Section 5.2.8, verifiers SHALL accept a given time-based OTP only once during the validity period. In the event a claimant’s authentication is denied due to duplicate use of an OTP, verifiers MAY warn the claimant in case an attacker has been able to authenticate in advance. Verifiers MAY also warn a subscriber in an existing session of the attempted duplicate use of an OTP.

If the authenticator output or activation secret has less than 64 bits of entropy, the verifier SHALL implement a rate-limiting mechanism that effectively limits the number of failed authentication attempts that can be made on the subscriber’s account as described in Section 5.2.2. A biometric activation factor SHALL meet the requirements of Section 5.2.3, including limits on the number of consecutive authentication failures.

5.1.6 Single-Factor Cryptographic Software

5.1.6.1 Single-Factor Cryptographic Software Authenticators

Single-factor software cryptographic authenticators encapsulate one or more secret keys unique to the authenticator. The key SHALL be stored in suitably secure storage available to the authenticator application (e.g., keychain storage, TPM, or TEE if available). The key SHALL be strongly protected against unauthorized disclosure by the use of access controls that limit access to the key to only those software components on the device requiring access. Single-factor cryptographic software authenticators SHOULD discourage and SHALL NOT facilitate the cloning of the secret key onto multiple devices.

5.1.6.2 Single-Factor Cryptographic Software Verifiers

The requirements for a single-factor cryptographic software verifier are identical to those for a single-factor cryptographic device verifier, described in Section 5.1.7.2.

5.1.7 Single-Factor Cryptographic Devices

5.1.7.1 Single-Factor Cryptographic Device Authenticators

Single-factor cryptographic device authenticators encapsulate one or more secret keys unique to the device that SHALL NOT be exportable (i.e., cannot be removed from the device). The authenticator operates by signing a challenge nonce presented through a direct computer interface (e.g., a USB port). Alternatively, the authenticator could be a suitably secure processor integrated with the user endpoint itself (e.g., a hardware TPM). Although cryptographic devices contain software, they differ from cryptographic software authenticators in that all embedded software is under control of the CSP or issuer and that the entire authenticator is subject to all applicable FIPS 140 requirements at the AAL being authenticated.

The secret key and its algorithm SHALL provide at least the minimum security length specified in the latest revision of SP 800-131A (112 bits as of the date of this publication). The challenge nonce SHALL be at least 64 bits in length. Approved cryptography SHALL be used.

Single-factor cryptographic device authenticators SHOULD require a physical input (e.g., the pressing of a button) in order to operate. This provides defense against unintended operation of the device, which might occur if the endpoint to which it is connected is compromised.

5.1.7.2 Single-Factor Cryptographic Device Verifiers

Single-factor cryptographic device verifiers generate a challenge nonce, send it to the corresponding authenticator, and use the authenticator output to verify possession of the device. The authenticator output is highly dependent on the specific cryptographic device and protocol, but it is generally some type of signed message.

The verifier has either symmetric or asymmetric cryptographic keys corresponding to each authenticator. While both types of keys SHALL be protected against modification, symmetric keys SHALL additionally be protected against unauthorized disclosure.

The challenge nonce SHALL be at least 64 bits in length, and SHALL either be unique over the authenticator’s lifetime or statistically unique (i.e., generated using an approved random bit generator [SP 800-90Ar1]). The verification operation SHALL use approved cryptography.

5.1.8 Multi-Factor Cryptographic Software

5.1.8.1 Multi-Factor Cryptographic Software Authenticators

Multi-factor software cryptographic authenticators encapsulate one or more secret keys unique to the authenticator and accessible only through the input of an additional factor, either a memorized secret or a biometric. The key SHOULD be stored in suitably secure storage available to the authenticator application (e.g., keychain storage, TPM, TEE). The key SHALL be strongly protected against unauthorized disclosure by the use of access controls that limit access to the key to only those software components on the device requiring access. Multi-factor cryptographic software authenticators SHOULD discourage and SHALL NOT facilitate the cloning of the secret key onto multiple devices.

Each authentication operation using the authenticator SHALL require the input of both factors.

Any memorized secret used by the authenticator for activation SHALL be a randomly-chosen numeric value at least 6 decimal digits in length or other memorized secret meeting the requirements of Section 5.1.1.2 and SHALL be rate limited as specified in Section 5.2.2. A biometric activation factor SHALL meet the requirements of Section 5.2.3, including limits on the number of consecutive authentication failures.

The unencrypted key and activation secret or biometric sample — and any biometric data derived from the biometric sample such as a probe produced through signal processing — SHALL be zeroized immediately after an authentication transaction has taken place.

5.1.8.2 Multi-Factor Cryptographic Software Verifiers

The requirements for a multi-factor cryptographic software verifier are identical to those for a single-factor cryptographic device verifier, described in Section 5.1.7.2. Verification of the output from a multi-factor cryptographic software authenticator proves use of the activation factor.

5.1.9 Multi-Factor Cryptographic Devices

5.1.9.1 Multi-Factor Cryptographic Device Authenticators

Multi-factor cryptographic device authenticators use tamper-resistant hardware to encapsulate one or more secret keys unique to the authenticator and accessible only through the input of an additional factor, either a memorized secret or a biometric. The authenticator operates by using a private key that was unlocked by the additional factor to sign a challenge nonce presented through a direct computer interface (e.g., a USB port). Alternatively, the authenticator could be a suitably secure processor integrated with the user endpoint itself (e.g., a hardware TPM). Although cryptographic devices contain software, they differ from cryptographic software authenticators in that all embedded software is under control of the CSP or issuer, and that the entire authenticator is subject to any applicable FIPS 140 requirements at the selected AAL.

The secret key and its algorithm SHALL provide at least the minimum security length specified in the latest revision of SP 800-131A (112 bits as of the date of this publication). The challenge nonce SHALL be at least 64 bits in length. Approved cryptography SHALL be used.

Each authentication operation using the authenticator SHOULD require the input of the additional factor. Input of the additional factor MAY be accomplished via either direct input on the device or via a hardware connection (e.g., USB, smartcard).

Any memorized secret used by the authenticator for activation SHALL be a randomly-chosen numeric value at least 6 decimal digits in length or other memorized secret meeting the requirements of Section 5.1.1.2 and SHALL be rate limited as specified in Section 5.2.2. A biometric activation factor SHALL meet the requirements of Section 5.2.3, including limits on the number of consecutive authentication failures.

The unencrypted key and activation secret or biometric sample — and any biometric data derived from the biometric sample such as a probe produced through signal processing — SHALL be zeroized immediately after an authentication transaction has taken place.

5.1.9.2 Multi-Factor Cryptographic Device Verifiers

The requirements for a multi-factor cryptographic device verifier are identical to those for a single-factor cryptographic device verifier, described in Section 5.1.7.2. Verification of the authenticator output from a multi-factor cryptographic device proves use of the activation factor.

5.2 General Authenticator Requirements

5.2.1 Physical Authenticators

CSPs SHALL provide subscriber instructions on how to appropriately protect the authenticator against theft or loss. The CSP SHALL provide a mechanism to revoke or suspend the authenticator immediately upon notification from subscriber that loss or theft of the authenticator is suspected.

5.2.2 Rate Limiting (Throttling)

When required by the authenticator type descriptions in Section 5.1, the verifier SHALL implement controls to protect against online guessing attacks. Unless otherwise specified in the description of a given authenticator, the verifier SHALL limit consecutive failed authentication attempts on a single account to no more than 100.

Additional techniques MAY be used to reduce the likelihood that an attacker will lock the legitimate claimant out as a result of rate limiting. These include:

• Requiring the claimant to complete a CAPTCHA before attempting authentication.

• Requiring the claimant to wait following a failed attempt for a period of time that increases as the account approaches its maximum allowance for consecutive failed attempts (e.g., 30 seconds up to an hour).

• Accepting only authentication requests that come from a white list of IP addresses from which the subscriber has been successfully authenticated before.

• Leveraging other risk-based or adaptive authentication techniques to identify user behavior that falls within, or out of, typical norms. These might, for example, include use of IP address, geolocation, timing of request patterns, or browser metadata.

When the subscriber successfully authenticates, the verifier SHOULD disregard any previous failed attempts for that user from the same IP address.

5.2.3 Use of Biometrics

The use of biometrics (something you are) in authentication includes both measurement of physical characteristics (e.g., fingerprint, iris, facial characteristics) and behavioral characteristics (e.g., typing cadence). Both classes are considered biometric modalities, although different modalities may differ in the extent to which they establish authentication intent as described in Section 5.2.9.

For a variety of reasons, this document supports only limited use of biometrics for authentication. These reasons include:

• The biometric False Match Rate (FMR) does not provide confidence in the authentication of the subscriber by itself. In addition, FMR does not account for spoofing attacks.
• Biometric comparison is probabilistic, whereas the other authentication factors are deterministic.
• Biometric template protection schemes provide a method for revoking biometric credentials that is comparable to other authentication factors (e.g., PKI certificates and passwords). However, the availability of such solutions is limited, and standards for testing these methods are under development.
• Biometric characteristics do not constitute secrets. They can be obtained online or by taking a picture of someone with a camera phone (e.g., facial images) with or without their knowledge, lifted from objects someone touches (e.g., latent fingerprints), or captured with high resolution images (e.g., iris patterns). While presentation attack detection (PAD) technologies (e.g., liveness detection) can mitigate the risk of these types of attacks, additional trust in the sensor or biometric processing is required to ensure that PAD is operating in accordance with the needs of the CSP and the subscriber.

Therefore, the limited use of biometrics for authentication is supported with the following requirements and guidelines:

Biometrics SHALL be used only as part of multi-factor authentication with a physical authenticator (something you have).

An authenticated protected channel between sensor (or an endpoint containing a sensor that resists sensor replacement) and verifier SHALL be established and the sensor or endpoint SHALL be authenticated prior to capturing the biometric sample from the claimant.

The biometric system SHALL operate with an FMR [ISO/IEC 2382-37] of 1 in 1000 or better. This FMR SHALL be achieved under conditions of a conformant attack (i.e., zero-effort impostor attempt) as defined in [ISO/IEC 30107-1].

The biometric system SHOULD implement PAD. Testing of the biometric system to be deployed SHOULD demonstrate at least 90% resistance to presentation attacks for each relevant attack type (i.e., species), where resistance is defined as the number of thwarted presentation attacks divided by the number of trial presentation attacks. Testing of presentation attack resistance SHALL be in accordance with Clause 12 of [ISO/IEC 30107-3]. The PAD decision MAY be made either locally on the claimant’s device or by a central verifier.

Note: PAD is being considered as a mandatory requirement in future editions of this guideline.

The biometric system SHALL allow no more than 5 consecutive failed authentication attempts or 10 consecutive failed attempts if PAD meeting the above requirements is implemented. Once that limit has been reached, the biometric authenticator SHALL either:

• Impose a delay of at least 30 seconds before the next attempt, increasing exponentially with each successive attempt (e.g., 1 minute before the following failed attempt, 2 minutes before the second following attempt), or
• Disable the biometric user authentication and offer another factor (e.g., a different biometric modality or a PIN/Passcode if it is not already a required factor) if such an alternative method is already available.

The verifier SHALL make a determination of sensor and endpoint performance, integrity, and authenticity. Acceptable methods for making this determination include, but are not limited to:

• Authentication of the sensor or endpoint.
• Certification by an approved accreditation authority.
• Runtime interrogation of signed metadata (e.g., attestation) as described in Section 5.2.4.

Biometric comparison can be performed locally on claimant’s device or at a central verifier. Since the potential for attacks on a larger scale is greater at central verifiers, local comparison is preferred.

If comparison is performed centrally:

• Use of the biometric as an authentication factor SHALL be limited to one or more specific devices that are identified using approved cryptography. Since the biometric has not yet unlocked the main authentication key, a separate key SHALL be used for identifying the device.
• Biometric revocation, referred to as biometric template protection in ISO/IEC 24745, SHALL be implemented.
• All transmission of biometrics SHALL be over the authenticated protected channel.

Biometric samples collected in the authentication process MAY be used to train comparison algorithms or — with user consent — for other research purposes. Biometric samples and any biometric data derived from the biometric sample such as a probe produced through signal processing SHALL be zeroized immediately after any training or research data has been derived.

Biometrics are also used in some cases to prevent repudiation of enrollment and to verify that the same individual participates in all phases of the enrollment process as described in SP 800-63A.

5.2.4 Attestation

An attestation is information conveyed to the verifier regarding a directly-connected authenticator or the endpoint involved in an authentication operation. Information conveyed by attestation MAY include, but is not limited to:

• The provenance (e.g., manufacturer or supplier certification), health, and integrity of the authenticator and endpoint.
• Security features of the authenticator.
• Security and performance characteristics of biometric sensor(s).
• Sensor modality.

If this attestation is signed, it SHALL be signed using a digital signature that provides at least the minimum security strength specified in the latest revision of SP 800-131A (112 bits as of the date of this publication).

Attestation information MAY be used as part of a verifier’s risk-based authentication decision.

5.2.5 Verifier Impersonation Resistance

Verifier impersonation attacks, sometimes referred to as “phishing attacks,” are attempts by fraudulent verifiers and RPs to fool an unwary claimant into authenticating to an impostor website. In prior versions of SP 800-63, protocols resistant to verifier-impersonation attacks were also referred to as “strongly MitM resistant.”

A verifier impersonation-resistant authentication protocol SHALL establish an authenticated protected channel with the verifier. It SHALL then strongly and irreversibly bind a channel identifier that was negotiated in establishing the authenticated protected channel to the authenticator output (e.g., by signing the two values together using a private key controlled by the claimant for which the public key is known to the verifier). The verifier SHALL validate the signature or other information used to prove verifier impersonation resistance. This prevents an impostor verifier, even one that has obtained a certificate representing the actual verifier, from replaying that authentication on a different authenticated protected channel.

Approved cryptographic algorithms SHALL be used to establish verifier impersonation resistance where it is required. Keys used for this purpose SHALL provide at least the minimum security strength specified in the latest revision of SP 800-131A (112 bits as of the date of this publication).

One example of a verifier impersonation-resistant authentication protocol is client-authenticated TLS, because the client signs the authenticator output along with earlier messages from the protocol that are unique to the particular TLS connection being negotiated.

Authenticators that involve the manual entry of an authenticator output, such as out-of-band and OTP authenticators, SHALL NOT be considered verifier impersonation-resistant because the manual entry does not bind the authenticator output to the specific session being authenticated. In a MitM attack, an impostor verifier could replay the OTP authenticator output to the verifier and successfully authenticate.

5.2.6 Verifier-CSP Communications

In situations where the verifier and CSP are separate entities (as shown by the dotted line in SP 800-63-3 Figure 4-1), communications between the verifier and CSP SHALL occur through a mutually-authenticated secure channel (such as a client-authenticated TLS connection) using approved cryptography.

5.2.7 Verifier-Compromise Resistance

Use of some types of authenticators requires that the verifier store a copy of the authenticator secret. For example, an OTP authenticator (described in Section 5.1.4) requires that the verifier independently generate the authenticator output for comparison against the value sent by the claimant. Because of the potential for the verifier to be compromised and stored secrets stolen, authentication protocols that do not require the verifier to persistently store secrets that could be used for authentication are considered stronger, and are described herein as being verifier compromise resistant. Note that such verifiers are not resistant to all attacks. A verifier could be compromised in a different way, such as being manipulated into always accepting a particular authenticator output.

Verifier compromise resistance can be achieved in different ways, for example:

• Use a cryptographic authenticator that requires the verifier store a public key corresponding to a private key held by the authenticator.

• Store the expected authenticator output in hashed form. This method can be used with some look-up secret authenticators (described in Section 5.1.2), for example.

To be considered verifier compromise resistant, public keys stored by the verifier SHALL be associated with the use of approved cryptographic algorithms and SHALL provide at least the minimum security strength specified in the latest revision of SP 800-131A (112 bits as of the date of this publication).

Other verifier compromise resistant secrets SHALL use approved hash algorithms and the underlying secrets SHALL have at least the minimum security strength specified in the latest revision of SP 800-131A (112 bits as of the date of this publication). Secrets (e.g., memorized secrets) having lower complexity SHALL NOT be considered verifier compromise resistant when hashed because of the potential to defeat the hashing process through dictionary lookup or exhaustive search.

5.2.8 Replay Resistance

An authentication process resists replay attacks if it is impractical to achieve a successful authentication by recording and replaying a previous authentication message. Replay resistance is in addition to the replay-resistant nature of authenticated protected channel protocols, since the output could be stolen prior to entry into the protected channel. Protocols that use nonces or challenges to prove the “freshness” of the transaction are resistant to replay attacks since the verifier will easily detect when old protocol messages are replayed since they will not contain the appropriate nonces or timeliness data.

Examples of replay-resistant authenticators are OTP devices, cryptographic authenticators, and look-up secrets.

In contrast, memorized secrets are not considered replay resistant because the authenticator output — the secret itself — is provided for each authentication.

5.2.9 Authentication Intent

An authentication process demonstrates intent if it requires the subject to explicitly respond to each authentication or reauthentication request. The goal of authentication intent is to make it more difficult for directly-connected physical authenticators (e.g., multi-factor cryptographic devices) to be used without the subject’s knowledge, such as by malware on the endpoint. Authentication intent SHALL be established by the authenticator itself, although multi-factor cryptographic devices MAY establish intent by reentry of the other authentication factor on the endpoint with which the authenticator is used.

Authentication intent MAY be established in a number of ways. Authentication processes that require the subject’s intervention (e.g., a claimant entering an authenticator output from an OTP device) establish intent. Cryptographic devices that require user action (e.g., pushing a button or reinsertion) for each authentication or reauthentication operation are also establish intent.

Depending on the modality, presentation of a biometric may or may not establish authentication intent. Presentation of a fingerprint would normally establish intent, while observation of the claimant’s face using a camera normally would not by itself. Behavioral biometrics similarly are less likely to establish authentication intent because they do not always require a specific action on the claimant’s part.

5.2.10 Restricted Authenticators

As threats evolve, authenticators’ capability to resist attacks typically degrades. Conversely, some authenticators’ performance may improve — for example, when changes to their underlying standards increases their ability to resist particular attacks.

To account for these changes in authenticator performance, NIST places additional restrictions on authenticator types or specific classes or instantiations of an authenticator type.

The use of a RESTRICTED authenticator requires that the implementing organization assess, understand, and accept the risks associated with that RESTRICTED authenticator and acknowledge that risk will likely increase over time. It is the responsibility of the organization to determine the level of acceptable risk for their system(s) and associated data and to define any methods for mitigating excessive risks. If at any time the organization determines that the risk to any party is unacceptable, then that authenticator SHALL NOT be used.

Further, the risk of an authentication error is typically borne by multiple parties, including the implementing organization, organizations that rely on the authentication decision, and the subscriber. Because the subscriber may be exposed to additional risk when an organization accepts a RESTRICTED authenticator and that the subscriber may have a limited understanding of and ability to control that risk, the CSP SHALL:

1. Offer subscribers at least one alternate authenticator that is not RESTRICTED and can be used to authenticate at the required AAL.

2. Provide meaningful notice to subscribers regarding the security risks of the RESTRICTED authenticator and availability of alternative(s) that are not RESTRICTED.

3. Address any additional risk to subscribers in its risk assessment.

4. Develop a migration plan for the possibility that the RESTRICTED authenticator is no longer acceptable at some point in the future and include this migration plan in its digital identity acceptance statement.

6 Authenticator Lifecycle Management

This section is normative.

A number of events can occur over the lifecycle of a subscriber’s authenticator that affect that authenticator’s use. These events include binding, loss, theft, unauthorized duplication, expiration, and revocation. This section describes the actions to be taken in response to those events.

6.1 Authenticator Binding

Authenticator binding refers to the establishment of an association between a specific authenticator and a subscriber’s account, enabling the authenticator to be used — possibly in conjunction with other authenticators — to authenticate for that account.

Authenticators SHALL be bound to subscriber accounts by either:

• Issuance by the CSP as part of enrollment; or
• Associating a subscriber-provided authenticator that is acceptable to the CSP.

These guidelines refer to the binding rather than the issuance of an authenticator as to accommodate both options.

Throughout the digital identity lifecycle, CSPs SHALL maintain a record of all authenticators that are or have been associated with each identity. The CSP or verifier SHALL maintain the information required for throttling authentication attempts when required, as described in Section 5.2.2. The CSP SHALL also verify the type of user-provided authenticator (e.g., single-factor cryptographic device vs. multi-factor cryptographic device) so verifiers can determine compliance with requirements at each AAL.

The record created by the CSP SHALL contain the date and time the authenticator was bound to the account. The record SHOULD include information about the source of the binding (e.g., IP address, device identifier) of any device associated with the enrollment. If available, the record SHOULD also contain information about the source of unsuccessful authentications attempted with the authenticator.

When any new authenticator is bound to a subscriber account, the CSP SHALL ensure that the binding protocol and the protocol for provisioning the associated key(s) are done at a level of security commensurate with the AAL at which the authenticator will be used. For example, protocols for key provisioning SHALL use authenticated protected channels or be performed in person to protect against man-in-the-middle attacks. Binding of multi-factor authenticators SHALL require multi-factor authentication or equivalent (e.g., association with the session in which identity proofing has been just completed) be used in order to bind the authenticator. The same conditions apply when a key pair is generated by the authenticator and the public key is sent to the CSP.

6.1.1 Binding at Enrollment

The following requirements apply when an authenticator is bound to an identity as a result of a successful identity proofing transaction, as described in SP 800-63A. Since Executive Order 13681 [EO 13681] requires the use of multi-factor authentication for the release of any personal data, it is important that authenticators be bound to subscriber accounts at enrollment, enabling access to personal data, including that established by identity proofing.

The CSP SHALL bind at least one, and SHOULD bind at least two, physical (something you have) authenticators to the subscriber’s online identity, in addition to a memorized secret or one or more biometrics. Binding of multiple authenticators is preferred in order to recover from the loss or theft of the subscriber’s primary authenticator.

While all identifying information is self-asserted at IAL1, preservation of online material or an online reputation makes it undesirable to lose control of an account due to the loss of an authenticator. The second authenticator makes it possible to securely recover from an authenticator loss. For this reason, a CSP SHOULD bind at least two physical authenticators to the subscriber’s credential at IAL1 as well.

At IAL2 and above, identifying information is associated with the digital identity and the subscriber has undergone an identity proofing process as described in SP 800-63A. As a result, authenticators at the same AAL as the desired IAL SHALL be bound to the account. For example, if the subscriber has successfully completed proofing at IAL2, then AAL2 or AAL3 authenticators are appropriate to bind to the IAL2 identity. While a CSP MAY bind an AAL1 authenticator to an IAL2 identity, if the subscriber is authenticated at AAL1, the CSP SHALL NOT expose personal information, even if self-asserted, to the subscriber. As stated in the previous paragraph, the availability of additional authenticators provides backup methods for authentication if an authenticator is damaged, lost, or stolen.

If enrollment and binding cannot be completed in a single physical encounter or electronic transaction (i.e., within a single protected session), the following methods SHALL be used to ensure that the same party acts as the applicant throughout the processes:

For remote transactions:

1. The applicant SHALL identify themselves in each new binding transaction by presenting a temporary secret which was either established during a prior transaction, or sent to the applicant’s phone number, email address, or postal address of record.

2. Long-term authenticator secrets SHALL only be issued to the applicant within a protected session.

For in-person transactions:

1. The applicant SHALL identify themselves in person by either using a secret as described in remote transaction (1) above, or through use of a biometric that was recorded during a prior encounter.

2. Temporary secrets SHALL NOT be reused.

3. If the CSP issues long-term authenticator secrets during a physical transaction, then they SHALL be loaded locally onto a physical device that is issued in person to the applicant or delivered in a manner that confirms the address of record.

6.1.2 Post-Enrollment Binding

6.1.2.1 Binding of an Additional Authenticator at Existing AAL

With the exception of memorized secrets, CSPs and verifiers SHOULD encourage subscribers to maintain at least two valid authenticators of each factor that they will be using. For example, a subscriber who usually uses an OTP device as a physical authenticator MAY also be issued a number of look-up secret authenticators, or register a device for out-of-band authentication, in case the physical authenticator is lost, stolen, or damaged. See Section 6.1.2.3 for more information on replacement of memorized secret authenticators.

Accordingly, CSPs SHOULD permit the binding of additional authenticators to a subscriber’s account. Before adding the new authenticator, the CSP SHALL first require the subscriber to authenticate at the AAL (or a higher AAL) at which the new authenticator will be used. When an authenticator is added, the CSP SHOULD send a notification to the subscriber via a mechanism that is independent of the transaction binding the new authenticator (e.g., email to an address previously associated with the subscriber). The CSP MAY limit the number of authenticators that may be bound in this manner.

6.1.2.2 Adding an Additional Factor to a Single-Factor Account

If the subscriber’s account has only one authentication factor bound to it (i.e., at IAL1/AAL1) and an additional authenticator of a different authentication factor is to be added, the subscriber MAY request that the account be upgraded to AAL2. The IAL would remain at IAL1.

Before binding the new authenticator, the CSP SHALL require the subscriber to authenticate at AAL1. The CSP SHOULD send a notification of the event to the subscriber via a mechanism independent of the transaction binding the new authenticator (e.g., email to an address previously associated with the subscriber).

6.1.2.3 Replacement of a Lost Authentication Factor

If a subscriber loses all authenticators of a factor necessary to complete multi-factor authentication and has been identity proofed at IAL2 or IAL3, that subscriber SHALL repeat the identity proofing process described in SP 800-63A. An abbreviated proofing process, confirming the binding of the claimant to previously-supplied evidence, MAY be used if the CSP has retained the evidence from the original proofing process pursuant to a privacy risk assessment as described in SP 800-63A Section 4.2. The CSP SHALL require the claimant to authenticate using an authenticator of the remaining factor, if any, to confirm binding to the existing identity. Reestablishment of authentication factors at IAL3 SHALL be done in person, or through a supervised remote process as described in SP 800-63A Section 5.3.3.2, and SHALL verify the biometric collected during the original proofing process.

The CSP SHOULD send a notification of the event to the subscriber. This MAY be the same notice as is required as part of the proofing process.

Replacement of a lost (i.e., forgotten) memorized secret is problematic because it is very common. Additional “backup” memorized secrets do not mitigate this because they are just as likely to also have been forgotten. If a biometric is bound to the account, the biometric and associated physical authenticator SHOULD be used to establish a new memorized secret.

As an alternative to the above re-proofing process when there is no biometric bound to the account, the CSP MAY bind a new memorized secret with authentication using two physical authenticators, along with a confirmation code that has been sent to one of the subscriber’s addresses of record. The confirmation code SHALL consist of at least 6 random alphanumeric characters generated by an approved random bit generator [SP 800-90Ar1]. Those sent to a postal address of record SHALL be valid for a maximum of 7 days but MAY be made valid up to 21 days via an exception process to accommodate addresses outside the direct reach of the U.S. Postal Service. Confirmation codes sent by means other than physical mail SHALL be valid for a maximum of 10 minutes.

6.1.3 Binding to a Subscriber-provided Authenticator

A subscriber may already possess authenticators suitable for authentication at a particular AAL. For example, they may have a two-factor authenticator from a social network provider, considered AAL2 and IAL1, and would like to use those credentials at an RP that requires IAL2.

CSPs SHOULD, where practical, accommodate the use of subscriber-provided authenticators in order to relieve the burden to the subscriber of managing a large number of authenticators. Binding of these authenticators SHALL be done as described in Section 6.1.2.1. In situations where the authenticator strength is not self-evident (e.g., between single-factor and multi-factor authenticators of a given type), the CSP SHOULD assume the use of the weaker authenticator unless it is able to establish that the stronger authenticator is in fact being used (e.g., by verification with the issuer or manufacturer of the authenticator).

6.1.4 Renewal

The CSP SHOULD bind an updated authenticator an appropriate amount of time before an existing authenticator’s expiration. The process for this SHOULD conform closely to the initial authenticator binding process (e.g., confirming address of record). Following successful use of the new authenticator, the CSP MAY revoke the authenticator that it is replacing.

6.2 Loss, Theft, Damage, and Unauthorized Duplication

Compromised authenticators include those that have been lost, stolen, or subject to unauthorized duplication. Generally, one must assume that a lost authenticator has been stolen or compromised by someone that is not the legitimate subscriber of the authenticator. Damaged or malfunctioning authenticators are also considered compromised to guard against any possibility of extraction of the authenticator secret. One notable exception is a memorized secret that has been forgotten without other indications of having been compromised, such as having been obtained by an attacker.

Suspension, revocation, or destruction of compromised authenticators SHOULD occur as promptly as practical following detection. Agencies SHOULD establish time limits for this process.

To facilitate secure reporting of the loss, theft, or damage to an authenticator, the CSP SHOULD provide the subscriber with a method of authenticating to the CSP using a backup or alternate authenticator. This backup authenticator SHALL be either a memorized secret or a physical authenticator. Either MAY be used, but only one authentication factor is required to make this report. Alternatively, the subscriber MAY establish an authenticated protected channel to the CSP and verify information collected during the proofing process. The CSP MAY choose to verify an address of record (i.e., email, telephone, postal) and suspend authenticator(s) reported to have been compromised. The suspension SHALL be reversible if the subscriber successfully authenticates to the CSP using a valid (i.e., not suspended) authenticator and requests reactivation of an authenticator suspended in this manner. The CSP MAY set a time limit after which a suspended authenticator can no longer be reactivated.

6.3 Expiration

CSPs MAY issue authenticators that expire. If and when an authenticator expires, it SHALL NOT be usable for authentication. When an authentication is attempted using an expired authenticator, the CSP SHOULD give an indication to the subscriber that the authentication failure is due to expiration rather than some other cause.

The CSP SHALL require subscribers to surrender or prove destruction of any physical authenticator containing attribute certificates signed by the CSP as soon as practical after expiration or receipt of a renewed authenticator.

6.4 Revocation and Termination

Revocation of an authenticator — sometimes referred to as termination, especially in the context of PIV authenticators — refers to removal of the binding between an authenticator and a credential the CSP maintains.

CSPs SHALL revoke the binding of authenticators promptly when an online identity ceases to exist (e.g., subscriber’s death, discovery of a fraudulent subscriber), when requested by the subscriber, or when the CSP determines that the subscriber no longer meets its eligibility requirements.

The CSP SHALL require subscribers to surrender or certify destruction of any physical authenticator containing certified attributes signed by the CSP as soon as practical after revocation or termination takes place. This is necessary to block the use of the authenticator’s certified attributes in offline situations between revocation/termination and expiration of the certification.

Further requirements on the termination of PIV authenticators are found in FIPS 201.

7 Session Management

This section is normative.

Once an authentication event has taken place, it is often desirable to allow the subscriber to continue using the application across multiple subsequent interactions without requiring them to repeat the authentication event. This requirement is particularly true for federation scenarios — described in SP 800-63C — where the authentication event necessarily involves several components and parties coordinating across a network.

To facilitate this behavior, a session MAY be started in response to an authentication event, and continue the session until such time that it is terminated. The session MAY be terminated for any number of reasons, including but not limited to an inactivity timeout, an explicit logout event, or other means. The session MAY be continued through a reauthentication event — described in Section 7.2 — wherein the user repeats some or all of the initial authentication event, thereby re-establishing the session.

Session management is preferable over continual presentation of credentials as the poor usability of continual presentation often creates incentives for workarounds such as cached unlocking credentials, negating the freshness of the authentication event.

7.1 Session Bindings

A session occurs between the software that a subscriber is running — such as a browser, application, or operating system (i.e., the session subject) — and the RP or CSP that the subscriber is accessing (i.e., the session host). A session secret SHALL be shared between the subscriber’s software and the service being accessed. This secret binds the two ends of the session, allowing the subscriber to continue using the service over time. The secret SHALL be presented directly by the subscriber’s software or possession of the secret SHALL be proven using a cryptographic mechanism.

The secret used for session binding SHALL be generated by the session host in direct response to an authentication event. A session SHOULD inherit the AAL properties of the authentication event which triggered its creation. A session MAY be considered at a lower AAL than the authentication event but SHALL NOT be considered at a higher AAL than the authentication event.

Secrets used for session binding:

1. SHALL be generated by the session host during an interaction, typically immediately following authentication.
2. SHALL be generated by an approved random bit generator [SP 800-90Ar1] and contain at least 64 bits of entropy.
3. SHALL be erased or invalidated by the session subject when the subscriber logs out.
4. SHOULD be erased on the subscriber endpoint when the user logs out or when the secret is deemed to have expired.
5. SHOULD NOT be placed in insecure locations such as HTML5 Local Storage due to the potential exposure of local storage to cross-site scripting (XSS) attacks.
6. SHALL be sent to and received from the device using an authenticated protected channel.
7. SHALL time out and not be accepted after the times specified in Sections 4.1.4, 4.2.4, and 4.3.4, as appropriate for the AAL.
8. SHALL NOT be available to insecure communications between the host and subscriber’s endpoint. Authenticated sessions SHALL NOT fall back to an insecure transport, such as from https to http, following authentication.

URLs or POST content SHALL contain a session identifier that SHALL be verified by the RP to ensure that actions taken outside the session do not affect the protected session.

There are several mechanisms for managing a session over time. The following sections give different examples along with additional requirements and considerations particular to each example technology. Additional informative guidance is available in the OWASP Session Management Cheat Sheet [OWASP-session].

7.1.1 Browser Cookies

Browser cookies are the predominant mechanism by which a session will be created and tracked for a subscriber accessing a service.

Cookies:

1. SHALL be tagged to be accessible only on secure (HTTPS) sessions.
2. SHALL be accessible to the minimum practical set of hostnames and paths.
3. SHOULD be tagged to be inaccessible via JavaScript (HttpOnly).
4. SHOULD be tagged to expire at, or soon after, the session’s validity period. This requirement is intended to limit the accumulation of cookies, but SHALL NOT be depended upon to enforce session timeouts.

7.1.2 Access Tokens

An access token — such as found in OAuth — is used to allow an application to access a set of services on a subscriber’s behalf following an authentication event. The presence of an OAuth access token SHALL NOT be interpreted by the RP as presence of the subscriber, in the absence of other signals. The OAuth access token, and any associated refresh tokens, MAY be valid long after the authentication session has ended and the subscriber has left the application.

7.1.3 Device Identification

Other methods of secure device identification — including but not limited to mutual TLS, token binding, or other mechanisms — MAY be used to enact a session between a subscriber and a service.

7.2 Reauthentication

Continuity of authenticated sessions SHALL be based upon the possession of a session secret issued by the verifier at the time of authentication and optionally refreshed during the session. The nature of a session depends on the application, including:

1. A web browser session with a “session” cookie, or
2. An instance of a mobile application that retains a session secret.

Session secrets SHALL be non-persistent. That is, they SHALL NOT be retained across a restart of the associated application or a reboot of the host device.

Periodic reauthentication of sessions SHALL be performed to confirm the continued presence of the subscriber at an authenticated session (i.e., that the subscriber has not walked away without logging out).

A session SHALL NOT be extended past the guidelines in Sections 4.1.3, 4.2.3, and 4.3.3 (depending on AAL) based on presentation of the session secret alone. Prior to session expiration, the reauthentication time limit SHALL be extended by prompting the subscriber for the authentication factor(s) specified in Table 7-1.

When a session has been terminated, due to a time-out or other action, the user SHALL be required to establish a new session by authenticating again.

AAL	Requirement
1	Presentation of any one factor
2	Presentation of a memorized secret or biometric
3	Presentation of all factors

Note: At AAL2, a memorized secret or biometric, and not a physical authenticator, is required because the session secret is something you have, and an additional authentication factor is required to continue the session.

7.2.1 Reauthentication from a Federation or Assertion

When using a federation protocol as described in SP 800-63C, Section 5 to connect the CSP and RP, special considerations apply to session management and reauthentication. The federation protocol communicates an authentication event between the CSP and the RP but establishes no session between them. Since the CSP and RP often employ separate session management technologies, there SHALL NOT be any assumption of correlation between these sessions. Consequently, when an RP session expires and the RP requires reauthentication, it is entirely possible that the session at the CSP has not expired and that a new assertion could be generated from this session at the CSP without reauthenticating the user.

An RP requiring reauthentication through a federation protocol SHALL — if possible within the protocol — specify the maximum acceptable authentication age to the CSP, and the CSP SHALL reauthenticate the subscriber if they have not been authenticated within that time period. The CSP SHALL communicate the authentication event time to the RP to allow the RP to decide if the assertion is sufficient for reauthentication and to determine the time for the next reauthentication event.

8 Threats and Security Considerations

This section is informative.

8.1 Authenticator Threats

An attacker who can gain control of an authenticator will often be able to masquerade as the authenticator’s owner. Threats to authenticators can be categorized based on attacks on the types of authentication factors that comprise the authenticator:

• Something you know may be disclosed to an attacker. The attacker might guess a memorized secret. Where the authenticator is a shared secret, the attacker could gain access to the CSP or verifier and obtain the secret value or perform a dictionary attack on a hash of that value. An attacker may observe the entry of a PIN or passcode, find a written record or journal entry of a PIN or passcode, or may install malicious software (e.g., a keyboard logger) to capture the secret. Additionally, an attacker may determine the secret through offline attacks on a password database maintained by the verifier.

• Something you have may be lost, damaged, stolen from the owner, or cloned by an attacker. For example, an attacker who gains access to the owner’s computer might copy a software authenticator. A hardware authenticator might be stolen, tampered with, or duplicated. Out-of-band secrets may be intercepted by an attacker and used to authenticate their own session.

• Something you are may be replicated. For example, an attacker may obtain a copy of the subscriber’s fingerprint and construct a replica.

This document assumes that the subscriber is not colluding with an attacker who is attempting to falsely authenticate to the verifier. With this assumption in mind, the threats to the authenticator(s) used for digital authentication are listed in Table 8-1, along with some examples.

Authenticator Threat/Attack	Description	Examples
Assertion Manufacture or Modification	The attacker generates a false assertion	Compromised CSP asserts identity of a claimant who has not properly authenticated
	The attacker modifies an existing assertion	Compromised proxy that changes AAL of an authentication assertion
Theft	A physical authenticator is stolen by an Attacker.	A hardware cryptographic device is stolen.
		An OTP device is stolen.
		A look-up secret authenticator is stolen.
		A cell phone is stolen.
Duplication	The subscriber’s authenticator has been copied with or without their knowledge.	Passwords written on paper are disclosed.
		Passwords stored in an electronic file are copied.
		Software PKI authenticator (private key) copied.
		Look-up secret authenticator copied.
		Counterfeit biometric authenticator manufactured.
Eavesdropping	The authenticator secret or authenticator output is revealed to the attacker as the subscriber is authenticating.	Memorized secrets are obtained by watching keyboard entry.
		Memorized secrets or authenticator outputs are intercepted by keystroke logging software.
		A PIN is captured from a PIN pad device.
		A hashed password is obtained and used by an attacker for another authentication (pass-the-hash attack).
	An out-of-band secret is intercepted by the attacker by compromising the communication channel.	An out-of-band secret is transmitted via unencrypted Wi-Fi and received by the attacker.
Offline Cracking	The authenticator is exposed using analytical methods outside the authentication mechanism.	A software PKI authenticator is subjected to dictionary attack to identify the correct password to use to decrypt the private key.
Side Channel Attack	The authenticator secret is exposed using physical characteristics of the authenticator.	A key is extracted by differential power analysis on a hardware cryptographic authenticator.
		A cryptographic authenticator secret is extracted by analysis of the response time of the authenticator over a number of attempts.
Phishing or Pharming	The authenticator output is captured by fooling the subscriber into thinking the attacker is a verifier or RP.	A password is revealed by subscriber to a website impersonating the verifier.
		A memorized secret is revealed by a bank subscriber in response to an email inquiry from a phisher pretending to represent the bank.
		A memorized secret is revealed by the subscriber at a bogus verifier website reached through DNS spoofing.
Social Engineering	The attacker establishes a level of trust with a subscriber in order to convince the subscriber to reveal their authenticator secret or authenticator output.	A memorized secret is revealed by the subscriber to an officemate asking for the password on behalf of the subscriber’s boss.
		A memorized secret is revealed by a subscriber in a telephone inquiry from an attacker masquerading as a system administrator.
		An out of band secret sent via SMS is received by an attacker who has convinced the mobile operator to redirect the victim’s mobile phone to the attacker.
Online Guessing	The attacker connects to the verifier online and attempts to guess a valid authenticator output in the context of that verifier.	Online dictionary attacks are used to guess memorized secrets.
		Online guessing is used to guess authenticator outputs for an OTP device registered to a legitimate claimant.
Endpoint Compromise	Malicious code on the endpoint proxies remote access to a connected authenticator without the subscriber’s consent.	A cryptographic authenticator connected to the endpoint is used to authenticate remote attackers.
	Malicious code on the endpoint causes authentication to other than the intended verifier.	Authentication is performed on behalf of an attacker rather than the subscriber.
		A malicious app on the endpoint reads an out-of-band secret sent via SMS and the attacker uses the secret to authenticate.
	Malicious code on the endpoint compromises a multi-factor software cryptographic authenticator.	Malicious code proxies authentication or exports authenticator keys from the endpoint.
Unauthorized Binding	An attacker is able to cause an authenticator under their control to be bound to a subscriber’s account.	An attacker intercepts an authenticator or provisioning key en route to the subscriber.

8.2 Threat Mitigation Strategies

Related mechanisms that assist in mitigating the threats identified above are summarized in Table 8-2.

Authenticator Threat/Attack	Threat Mitigation Mechanisms	Normative Reference(s)
Theft	Use multi-factor authenticators that need to be activated through a memorized secret or biometric.	4.2.1, 4.3.1
	Use a combination of authenticators that includes a memorized secret or biometric.	4.2.1, 4.3.1
Duplication	Use authenticators from which it is difficult to extract and duplicate long-term authentication secrets.	4.2.2, 4.3.2, 5.1.7.1
Eavesdropping	Ensure the security of the endpoint, especially with respect to freedom from malware such as key loggers, prior to use.	4.2.2
	Avoid use of non-trusted wireless networks as unencrypted secondary out-of-band authentication channels.	5.1.3.1
	Authenticate over authenticated protected channels (e.g., observe lock icon in browser window).	4.1.2, 4.2.2, 4.3.2
	Use authentication protocols that are resistant to replay attacks such as pass-the-hash.	5.2.8
	Use authentication endpoints that employ trusted input and trusted display capabilities.	5.1.6.1, 5.1.8.1
Offline Cracking	Use an authenticator with a high entropy authenticator secret.	5.1.2.1, 5.1.4.1, 5.1.5.1, 5.1.7.1, 5.1.9.1
	Store memorized secrets in a salted, hashed form, including a keyed hash.	5.1.1.2, 5.2.7
Side Channel Attack	Use authenticator algorithms that are designed to maintain constant power consumption and timing regardless of secret values.	4.3.2
Phishing or Pharming	Use authenticators that provide verifier impersonation resistance.	5.2.5
Social Engineering	Avoid use of authenticators that present a risk of social engineering of third parties such as customer service agents.	6.1.2.1, 6.1.2.3
Online Guessing	Use authenticators that generate high entropy output.	5.1.2.1, 5.1.7.1, 5.1.9.1
	Use an authenticator that locks up after a number of repeated failed activation attempts.	5.2.2
Endpoint Compromise	Use hardware authenticators that require physical action by the subscriber.	5.2.9
	Maintain software-based keys in restricted-access storage.	5.1.3.1, 5.1.6.1, 5.1.8.1
Unauthorized Binding	Use MitM-resistant protocols for provisioning of authenticators and associated keys.	6.1

Several other strategies may be applied to mitigate the threats described in Table 8-1:

• Multiple factors make successful attacks more difficult to accomplish. If an attacker needs to both steal a cryptographic authenticator and guess a memorized secret, then the work to discover both factors may be too high.

• Physical security mechanisms may be employed to protect a stolen authenticator from duplication. Physical security mechanisms can provide tamper evidence, detection, and response.

• Requiring the use of long memorized secrets that don’t appear in common dictionaries may force attackers to try every possible value.

• System and network security controls may be employed to prevent an attacker from gaining access to a system or installing malicious software.

• Periodic training may be performed to ensure subscribers understand when and how to report compromise — or suspicion of compromise — or otherwise recognize patterns of behavior that may signify an attacker attempting to compromise the authentication process.

• Out of band techniques may be employed to verify proof of possession of registered devices (e.g., cell phones).

8.3 Authenticator Recovery

The weak point in many authentication mechanisms is the process followed when a subscriber loses control of one or more authenticators and needs to replace them. In many cases, the options remaining available to authenticate the subscriber are limited, and economic concerns (e.g., cost of maintaining call centers) motivate the use of inexpensive, and often less secure, backup authentication methods. To the extent that authenticator recovery is human-assisted, there is also the risk of social engineering attacks.

To maintain the integrity of the authentication factors, it is essential that it not be possible to leverage an authentication involving one factor to obtain an authenticator of a different factor. For example, a memorized secret must not be usable to obtain a new list of look-up secrets.

8.4 Session Attacks

The above discussion focuses on threats to the authentication event itself, but hijacking attacks on the session following an authentication event can have similar security impacts. The session management guidelines in Section 7 are essential to maintain session integrity against attacks, such as XSS. In addition, it is important to sanitize all information to be displayed [OWASP-XSS-prevention] to ensure that it does not contain executable content. These guidelines also recommend that session secrets be made inaccessible to mobile code in order to provide extra protection against exfiltration of session secrets.

Another post-authentication threat, cross-site request forgery (CSRF), takes advantage of users’ tendency to have multiple sessions active at the same time. It is important to embed and verify a session identifier into web requests to prevent the ability for a valid URL or request to be unintentionally or maliciously activated.

9 Privacy Considerations

These privacy considerations supplement the guidance in Section 4. This section is informative.

9.1 Privacy Risk Assessment

Sections 4.1.5, 4.2.5, and 4.3.5 require the CSP to conduct a privacy risk assessment for records retention. Such a privacy risk assessment would include:

1. The likelihood that the records retention could create a problem for the subscriber, such as invasiveness or unauthorized access to the information.
2. The impact if such a problem did occur.

CSPs should be able to reasonably justify any response they take to identified privacy risks, including accepting the risk, mitigating the risk, and sharing the risk. The use of subscriber consent is a form of sharing the risk, and therefore appropriate for use only when a subscriber could reasonably be expected to have the capacity to assess and accept the shared risk.

9.2 Privacy Controls

Section 4.4 requires CSPs to employ appropriately-tailored privacy controls. SP 800-53 provides a set of privacy controls for CSPs to consider when deploying authentication mechanisms. These controls cover notices, redress, and other important considerations for successful and trustworthy deployments.

9.3 Use Limitation

Section 4.4 requires CSPs to use measures to maintain the objectives of predictability (enabling reliable assumptions by individuals, owners, and operators about PII and its processing by an information system) and manageability (providing the capability for granular administration of PII, including alteration, deletion, and selective disclosure) commensurate with privacy risks that can arise from the processing of attributes for purposes other than identity proofing, authentication, authorization, or attribute assertion, related fraud mitigation, or to comply with law or legal process NISTIR8062.

CSPs may have various business purposes for processing attributes, including providing non-identity services to subscribers. However, processing attributes for other purposes than those specified at collection can create privacy risks when individuals are not expecting or comfortable with the additional processing. CSPs can determine appropriate measures commensurate with the privacy risk arising from the additional processing. For example, absent applicable law, regulation or policy, it may not be necessary to get consent when processing attributes to provide non-identity services requested by subscribers, although notices may help subscribers maintain reliable assumptions about the processing (predictability). Other processing of attributes may carry different privacy risks that call for obtaining consent or allowing subscribers more control over the use or disclosure of specific attributes (manageability). Subscriber consent needs to be meaningful; therefore, as stated in Section 4.4, when CSPs use consent measures, acceptance by the subscriber of additional uses SHALL NOT be a condition of providing authentication services.

Consult your SAOP if there are questions about whether the proposed processing falls outside the scope of the permitted processing or the appropriate privacy risk mitigation measures.

9.4 Agency-Specific Privacy Compliance

Section 4.4 covers specific compliance obligations for federal CSPs. It is critical to involve your agency’s SAOP in the earliest stages of digital authentication system development in order to assess and mitigate privacy risks and advise the agency on compliance requirements, such as whether or not the collection of PII to issue or maintain authenticators triggers the Privacy Act of 1974 Privacy Act or the E-Government Act of 2002 E-Gov requirement to conduct a PIA. For example, with respect to centralized maintenance of biometrics, it is likely that the Privacy Act requirements will be triggered and require coverage by either a new or existing Privacy Act system of records due to the collection and maintenance of PII and any other attributes necessary for authentication. The SAOP can similarly assist the agency in determining whether a PIA is required.

These considerations should not be read as a requirement to develop a Privacy Act SORN or PIA for authentication alone. In many cases it will make the most sense to draft a PIA and SORN that encompasses the entire digital authentication process or include the digital authentication process as part of a larger programmatic PIA that discusses the service or benefit to which the agency is establishing online.

Due to the many components of digital authentication, it is important for the SAOP to have an awareness and understanding of each individual component. For example, other privacy artifacts may be applicable to an agency offering or using federated CSP or RP services (e.g., Data Use Agreements, Computer Matching Agreements). The SAOP can assist the agency in determining what additional requirements apply. Moreover, a thorough understanding of the individual components of digital authentication will enable the SAOP to thoroughly assess and mitigate privacy risks either through compliance processes or by other means.

10 Usability Considerations

This section is informative.

ISO/IEC 9241-11 defines usability as the “extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use.” This definition focuses on users, their goals, and the context of use as key elements necessary for achieving effectiveness, efficiency, and satisfaction. A holistic approach that accounts for these key elements is necessary to achieve usability.

A user’s goal for accessing an information system is to perform an intended task. Authentication is the function that enables this goal. However, from the user’s perspective, authentication stands between them and their intended task. Effective design and implementation of authentication makes it easy to do the right thing, hard to do the wrong thing, and easy to recover when the wrong thing happens.

Organizations need to be cognizant of the overall implications of their stakeholders’ entire digital authentication ecosystem. Users often employ one or more authenticator, each for a different RP. They then struggle to remember passwords, to recall which authenticator goes with which RP, and to carry multiple physical authentication devices. Evaluating the usability of authentication is critical, as poor usability often results in coping mechanisms and unintended work-arounds that can ultimately degrade the effectiveness of security controls.

Integrating usability into the development process can lead to authentication solutions that are secure and usable while still addressing users’ authentication needs and organizations’ business goals.

The impact of usability across digital systems needs to be considered as part of the risk assessment when deciding on the appropriate AAL. Authenticators with a higher AAL sometimes offer better usability and should be allowed for use for lower AAL applications.

Leveraging federation for authentication can alleviate many of the usability issues, though such an approach has its own tradeoffs, as discussed in SP 800-63C.

This section provides general usability considerations and possible implementations, but does not recommend specific solutions. The implementations mentioned are examples to encourage innovative technological approaches to address specific usability needs. Further, usability considerations and their implementations are sensitive to many factors that prevent a one-size-fits-all solution. For example, a font size that works in the desktop computing environment may force text to scroll off of a small OTP device screen. Performing a usability evaluation on the selected authenticator is a critical component of implementation. It is important to conduct evaluations with representative users, realistic goals and tasks, and appropriate contexts of use.

ASSUMPTIONS

In this section, the term “users” means “claimants” or “subscribers.”

Guidelines and considerations are described from the users’ perspective.

Accessibility differs from usability and is out of scope for this document. Section 508 was enacted to eliminate barriers in information technology and require federal agencies to make their online public content accessible to people with disabilities. Refer to Section 508 law and standards for accessibility guidance.

10.1 Usability Considerations Common to Authenticators

When selecting and implementing an authentication system, consider usability across the entire lifecycle of the selected authenticators (e.g., typical use and intermittent events), while being mindful of the combination of users, their goals, and context of use.

A single authenticator type usually does not suffice for the entire user population. Therefore, whenever possible — based on AAL requirements — CSPs should support alternative authenticator types and allow users to choose based on their needs. Task immediacy, perceived cost benefit tradeoffs, and unfamiliarity with certain authenticators often impact choice. Users tend to choose options that incur the least burden or cost at that moment. For example, if a task requires immediate access to an information system, a user may prefer to create a new account and password rather than select an authenticator requiring more steps. Alternatively, users may choose a federated identity option — approved at the appropriate AAL — if they already have an account with an identity provider. Users may understand some authenticators better than others, and have different levels of trust based on their understanding and experience.

Positive user authentication experiences are integral to the success of an organization achieving desired business outcomes. Therefore, they should strive to consider authenticators from the users’ perspective. The overarching authentication usability goal is to minimize user burden and authentication friction (e.g., the number of times a user has to authenticate, the steps involved, and the amount of information he or she has to track). Single sign-on exemplifies one such minimization strategy.

Usability considerations applicable to most authenticators are described below. Subsequent sections describe usability considerations specific to a particular authenticator.

Usability considerations for typical usage of all authenticators include:

• Provide information on the use and maintenance of the authenticator, e.g., what to do if the authenticator is lost or stolen, and instructions for use — especially if there are different requirements for first-time use or initialization.

• Authenticator availability should also be considered as users will need to remember to have their authenticator readily available. Consider the need for alternate authentication options to protect against loss, damage, or other negative impacts to the original authenticator.

• Whenever possible, based on AAL requirements, users should be provided with alternate authentication options. This allows users to choose an authenticator based on their context, goals, and tasks (e.g., the frequency and immediacy of the task). Alternate authentication options also help address availability issues that may occur with a particular authenticator.

• Characteristics of user-facing text:
  • Write user-facing text (e.g., instructions, prompts, notifications, error messages) in plain language for the intended audience. Avoid technical jargon and, typically, write for a 6th to 8th grade literacy level.
  • Consider the legibility of user-facing and user-entered text, including font style, size, color, and contrast with surrounding background. Illegible text contributes to user entry errors. To enhance legibility, consider the use of:
    • High contrast. The highest contrast is black on white.
    • Sans serif fonts for electronic displays. Serif fonts for printed materials.
    • Fonts that clearly distinguish between easily confusable characters (e.g., the capital letter “O” and the number “0”).
    • A minimum font size of 12 points as long as the text fits for display on the device.
• User experience during authenticator entry:
  • Offer the option to display text during entry, as masked text entry is error-prone. Once a given character is displayed long enough for the user to see, it can be hidden. Consider the device when determining masking delay time, as it takes longer to enter memorized secrets on mobile devices (e.g., tablets and smartphones) than on traditional desktop computers. Ensure masking delay durations are consistent with user needs.
  • Ensure the time allowed for text entry is adequate (i.e., the entry screen does not time out prematurely). Ensure allowed text entry times are consistent with user needs.
  • Provide clear, meaningful and actionable feedback on entry errors to reduce user confusion and frustration. Significant usability implications arise when users do not know they have entered text incorrectly.
  • Allow at least 10 entry attempts for authenticators requiring the entry of the authenticator output by the user. The longer and more complex the entry text, the greater the likelihood of user entry errors.
  • Provide clear, meaningful feedback on the number of remaining allowed attempts. For rate limiting (i.e., throttling), inform users how long they have to wait until the next attempt to reduce confusion and frustration.
• Minimize the impact of form-factor constraints, such as limited touch and display areas on mobile devices:
  • Larger touch areas improve usability for text entry since typing on small devices is significantly more error prone and time consuming than typing on a full-size keyboard. The smaller the onscreen keyboard, the more difficult it is to type, due to the size of the input mechanism (e.g., a finger) relative to the size of the on-screen target.
  • Follow good user interface and information design for small displays.

Intermittent events include events such as reauthentication, account lock-out, expiration, revocation, damage, loss, theft, and non-functional software.

Usability considerations for intermittent events across authenticator types include:

• To prevent users from needing to reauthenticate due to user inactivity, prompt users in order to trigger activity just before (e.g., 2 minutes) an inactivity timeout would otherwise occur.

• Prompt users with adequate time (e.g., 1 hour) to save their work before the fixed periodic reauthentication event required regardless of user activity.

• Clearly communicate how and where to acquire technical assistance. For example, provide users with information such as a link to an online self-service feature, chat sessions or a phone number for help desk support. Ideally, sufficient information can be provided to enable users to recover from intermittent events on their own without outside intervention.

10.2 Usability Considerations by Authenticator Type

In addition to the previously described general usability considerations applicable to most authenticators (Section 10.1), the following sections describe other usability considerations specific to particular authenticator types.

10.2.1 Memorized Secrets

Typical Usage

Users manually input the memorized secret (commonly referred to as a password or PIN).

Usability considerations for typical usage include:

• Memorability of the memorized secret.
  • The likelihood of recall failure increases as there are more items for users to remember. With fewer memorized secrets, users can more easily recall the specific memorized secret needed for a particular RP.
  • The memory burden is greater for a less frequently used password.
• User experience during entry of the memorized secret.
  • Support copy and paste functionality in fields for entering memorized secrets, including passphrases.

Intermittent Events

Usability considerations for intermittent events include:

• When users create and change memorized secrets:
  • Clearly communicate information on how to create and change memorized secrets.
  • Clearly communicate memorized secret requirements, as specified in Section 5.1.1.
  • Allow at least 64 characters in length to support the use of passphrases. Encourage users to make memorized secrets as lengthy as they want, using any characters they like (including spaces), thus aiding memorization.
  • Do not impose other composition rules (e.g. mixtures of different character types) on memorized secrets.
  • Do not require that memorized secrets be changed arbitrarily (e.g., periodically) unless there is a user request or evidence of authenticator compromise. (See Section 5.1.1 for additional information).
• Provide clear, meaningful and actionable feedback when chosen passwords are rejected (e.g., when it appears on a “black list” of unacceptable passwords or has been used previously).

10.2.2 Look-Up Secrets

Typical Usage

Users use the authenticator — printed or electronic — to look up the appropriate secret(s) needed to respond to a verifier’s prompt. For example, a user may be asked to provide a specific subset of the numeric or character strings printed on a card in table format.

Usability considerations for typical usage include:

• User experience during entry of look-up secrets.
  • Consider the prompts’ complexity and size. The larger the subset of secrets a user is prompted to look up, the greater the usability implications. Both the cognitive workload and physical difficulty for entry should be taken into account when selecting the quantity and complexity of look-up secrets for authentication.

10.2.3 Out-of-Band

Typical Usage

Out-of-band authentication requires users have access to a primary and secondary communication channel.

Usability considerations for typical usage:

• Notify users of the receipt of a secret on a locked device. However, if the out of band device is locked, authentication to the device should be required to access the secret.

• Depending on the implementation, consider form-factor constraints as they are particularly problematic when users must enter text on mobile devices. Providing larger touch areas will improve usability for entering secrets on mobile devices.

• A better usability option is to offer features that do not require text entry on mobile devices (e.g., a single tap on the screen, or a copy feature so users can copy and paste out-of-band secrets). Providing users such features is particularly helpful when the primary and secondary channels are on the same device. For example, it is difficult for users to transfer the authentication secret on a smartphone because they must switch back and forth—potentially multiple times—between the out of band application and the primary channel.

10.2.4 Single-Factor OTP Device

Typical Usage

Users access the OTP generated by the single-factor OTP device. The authenticator output is typically displayed on the device and the user enters it for the verifier.

Usability considerations for typical usage include:

• Authenticator output allows at least one minute between changes, but ideally allows users the full two minutes as specified in Section 5.1.4.1. Users need adequate time to enter the authenticator output (including looking back and forth between the single-factor OTP device and the entry screen).

• Depending on the implementation, the following are additional usability considerations for implementers:

  • If the single-factor OTP device supplies its output via an electronic interface (e.g, USB) this is preferable since users do not have to manually enter the authenticator output. However, if a physical input (e.g., pressing a button) is required to operate, the location of the USB ports could pose usability difficulties. For example, the USB ports of some computers are located on the back of the computer and will be difficult for users to reach.
  • Limited availability of a direct computer interface such as a USB port could pose usability difficulties. For example, the number of USB ports on laptop computers is often very limited. This may force users to unplug other USB peripherals in order to use the single-factor OTP device.

10.2.5 Multi-Factor OTP Device

Typical Usage

Users access the OTP generated by the multi-factor OTP device through a second authentication factor. The OTP is typically displayed on the device and the user manually enters it for the verifier. The second authentication factor may be achieved through some kind of integral entry pad to enter a memorized secret, an integral biometric (e.g., fingerprint) reader, or a direct computer interface (e.g., USB port). Usability considerations for the additional factor apply as well — see Section 10.2.1 for memorized secrets and Section 10.4 for biometrics used in multi-factor authenticators.

Usability considerations for typical usage include:

• User experience during manual entry of the authenticator output.
  • For time-based OTP, provide a grace period in addition to the time during which the OTP is displayed. Users need adequate time to enter the authenticator output, including looking back and forth between the multi-factor OTP device and the entry screen.
  • Consider form-factor constraints if users must unlock the multi-factor OTP device via an integral entry pad or enter the authenticator output on mobile devices. Typing on small devices is significantly more error prone and time-consuming than typing on a traditional keyboard. The smaller the integral entry pad and onscreen keyboard, the more difficult it is to type. Providing larger touch areas improves usability for unlocking the multi-factor OTP device or entering the authenticator output on mobile devices.
  • Limited availability of a direct computer interface like a USB port could pose usability difficulties. For example, laptop computers often have a limited number of USB ports, which may force users to unplug other USB peripherals to use the multi-factor OTP device.

10.2.6 Single-Factor Cryptographic Software

Typical Usage

Users authenticate by proving possession and control of the cryptographic software key.

Usability considerations for typical usage include:

• Give cryptographic keys appropriately descriptive names that are meaningful to users since users have to recognize and recall which cryptographic key to use for which authentication task. This prevents users from having to deal with multiple similarly- and ambiguously-named cryptographic keys. Selecting from multiple cryptographic keys on smaller mobile devices may be particularly problematic if the names of the cryptographic keys are shortened due to reduced screen size.

10.2.7 Single-Factor Cryptographic Device

Typical Usage

Users authenticate by proving possession of the single-factor cryptographic device.

Usability considerations for typical usage include:

• Requiring a physical input (e.g., pressing a button) to operate the single-factor cryptographic device could pose usability difficulties. For example, some USB ports are located on the back of computers, making it difficult for users to reach.

• Limited availability of a direct computer interface like a USB port could pose usability difficulties. For example, laptop computers often have a limited number of USB ports, which may force users to unplug other USB peripherals to use the single-factor cryptographic device.

10.2.8 Multi-Factor Cryptographic Software

Typical Usage

In order to authenticate, users prove possession and control of the cryptographic key stored on disk or some other “soft” media that requires activation. The activation is through the input of a second authentication factor, either a memorized secret or a biometric. Usability considerations for the additional factor apply as well — see Section 10.2.1 for memorized secrets and Section 10.4 for biometrics used in multi-factor authenticators.

Usability considerations for typical usage include:

• Give cryptographic keys appropriately descriptive names that are meaningful to users since users have to recognize and recall which cryptographic key to use for which authentication task. This prevents users from having to deal with multiple similarly- and ambiguously-named cryptographic keys. Selecting from multiple cryptographic keys on smaller mobile devices may be particularly problematic if the names of the cryptographic keys are shortened due to reduced screen size.

10.2.9 Multi-Factor Cryptographic Device

Typical Usage

Users authenticate by proving possession of the multi-factor cryptographic device and control of the protected cryptographic key. The device is activated by a second authentication factor, either a memorized secret or a biometric. Usability considerations for the additional factor apply as well — see Section 10.2.1 for memorized secrets and Section 10.4 for biometrics used in multi-factor authenticators.

Usability considerations for typical usage include:

• Do not require users to keep multi-factor cryptographic devices connected following authentication. Users may forget to disconnect the multi-factor cryptographic device when they are done with it (e.g., forgetting a smartcard in the smartcard reader and walking away from the computer).
  • Users need to be informed regarding whether the multi-factor cryptographic device is required to stay connected or not.

• Give cryptographic keys appropriately descriptive names that are meaningful to users since users have to recognize and recall which cryptographic key to use for which authentication task. This prevents users being faced with multiple similarly and ambiguously named cryptographic keys. Selecting from multiple cryptographic keys on smaller mobile devices (such as smartphones) may be particularly problematic if the names of the cryptographic keys are shortened due to reduced screen size.

• Limited availability of a direct computer interface like a USB port could pose usability difficulties. For example, laptop computers often have a limited number of USB ports, which may force users to unplug other USB peripherals to use the multi-factor cryptographic device.

10.3 Summary of Usability Considerations

Table 10-1 summarizes the usability considerations for typical usage and intermittent events for each authenticator type. Many of the usability considerations for typical usage apply to most of the authenticator types, as demonstrated in the rows. The table highlights common and divergent usability characteristics across the authenticator types. Each column allows readers to easily identify the usability attributes to address for each authenticator. Depending on users’ goals and context of use, certain attributes may be valued over others. Whenever possible, provide alternative authenticator types and allow users to choose between them.

Multi-factor authenticators (e.g., multi-factor OTP devices, multi-factor cryptographic software, and multi-factor cryptographic devices) also inherit their secondary factor’s usability considerations. As biometrics are only allowed as an activation factor in multi-factor authentication solutions, usability considerations for biometrics are not included in Table 10-1 and are discussed in Section 10.4.

10.4 Biometrics Usability Considerations

This section provides a high-level overview of general usability considerations for biometrics. A more detailed discussion of biometric usability can be found in Usability & Biometrics, Ensuring Successful Biometric Systems NIST Usability.

Although there are other biometric modalities, the following three biometric modalities are more commonly used for authentication: fingerprint, face and iris.

Typical Usage

• For all modalities, user familiarity and practice with the device improves performance.

• Device affordances (i.e., properties of a device that allow a user to perform an action), feedback, and clear instructions are critical to a user’s success with the biometric device. For example, provide clear instructions on the required actions for liveness detection.

• Ideally, users can select the modality they are most comfortable with for their second authentication factor. The user population may be more comfortable and familiar with — and accepting of — some biometric modalities than others.

• User experience with biometrics as an activation factor.
  • Provide clear, meaningful feedback on the number of remaining allowed attempts. For example, for rate limiting (i.e., throttling), inform users of the time period they have to wait until next attempt to reduce user confusion and frustration.
• Fingerprint Usability Considerations:
  • Users have to remember which finger(s) they used for initial enrollment.
  • The amount of moisture on the finger(s) affects the sensor’s ability for successful capture.
  • Additional factors influencing fingerprint capture quality include age, gender, and occupation (e.g., users handling chemicals or working extensively with their hands may have degraded friction ridges).
• Face Usability Considerations:
  • Users have to remember whether they wore any artifacts (e.g., glasses) during enrollment because it affects facial recognition accuracy.
  • Differences in environmental lighting conditions can affect facial recognition accuracy.
  • Facial expressions affect facial recognition accuracy (e.g., smiling versus neutral expression).
  • Facial poses affect facial recognition accuracy (e.g., looking down or away from the camera).
• Iris Usability Considerations:
  • Wearing colored contacts may affect the iris recognition accuracy.
  • Users who have had eye surgery may need to re-enroll post-surgery.
  • Differences in environmental lighting conditions can affect iris recognition accuracy, especially for certain iris colors.

Intermittent Events

As biometrics are only permitted as a second factor for multi-factor authentication, usability considerations for intermittent events with the primary factor still apply. Intermittent events with biometrics use include, but are not limited to, the following, which may affect recognition accuracy:

• If users injure their enrolled finger(s), fingerprint recognition may not work. Fingerprint authentication will be difficult for users with degraded fingerprints.
• The time elapsed between the time of facial recognition for authentication and the time of the initial enrollment can affect recognition accuracy as a user’s face changes naturally over time. A user’s weight change may also be a factor.
• Iris recognition may not work for people who had eye surgery, unless they re-enroll.

Across all biometric modalities, usability considerations for intermittent events include:

• An alternative authentication method must be available and functioning. In cases where biometrics do not work, allow users to use a memorized secret as an alternative second factor.
• Provisions for technical assistance:
  • Clearly communicate information on how and where to acquire technical assistance. For example, provide users information such as a link to an online self-service feature and a phone number for help desk support. Ideally, provide sufficient information to enable users to recover from intermittent events on their own without outside intervention.
  • Inform users of factors that may affect the sensitivity of the biometric sensor (e.g., cleanliness of the sensor).

11 References

This section is informative.

11.1 General References

[BALLOON] Boneh, Dan, Corrigan-Gibbs, Henry, and Stuart Schechter. “Balloon Hashing: A Memory-Hard Function Providing Provable Protection Against Sequential Attacks,” Asiacrypt 2016, October, 2016. Available at: https://eprint.iacr.org/2016/027.

[Blacklists] Habib, Hana, Jessica Colnago, William Melicher, Blase Ur, Sean Segreti, Lujo Bauer, Nicolas Christin, and Lorrie Cranor. “Password Creation in the Presence of Blacklists,” 2017. Available at: https://www.ndss-symposium.org/wp-content/uploads/2017/09/usec2017_01_3_Habib_paper.pdf

[Composition] Komanduri, Saranga, Richard Shay, Patrick Gage Kelley, Michelle L Mazurek, Lujo Bauer, Nicolas Christin, Lorrie Faith Cranor, and Serge Egelman. “Of Passwords and People: Measuring the Effect of Password-Composition Policies.” In Proceedings of the SIGCHI Conference on Human Factors in Computing Systems, 2595–2604. ACM, 2011. Available at: https://www.ece.cmu.edu/~lbauer/papers/2011/chi2011-passwords.pdf.

[E-Gov] E-Government Act [includes FISMA] (P.L. 107-347), December 2002, available at: http://www.gpo.gov/fdsys/pkg/PLAW-107publ347/pdf/PLAW-107publ347.pdf.

[EO 13681] Executive Order 13681, Improving the Security of Consumer Financial Transactions, October 17, 2014, available at: https://www.federalregister.gov/d/2014-25439.

[FEDRAMP] General Services Administration, Federal Risk and Authorization Management Program, available at: https://www.fedramp.gov/.

[ICAM] National Security Systems and Identity, Credential and Access Management Sub-Committee Focus Group, Federal CIO Council, ICAM Lexicon, Version 0.5, March 2011.

[M-03-22] OMB Memorandum M-03-22, OMB Guidance for Implementing the Privacy Provisions of the E-Government Act of 2002, September 26, 2003, available at: https://georgewbush-whitehouse.archives.gov/omb/memoranda/m03-22.html.

[M-04-04] OMB Memorandum M-04-04, E-Authentication Guidance for Federal Agencies, December 16, 2003, available at: https://georgewbush-whitehouse.archives.gov/omb/memoranda/fy04/m04-04.pdf.

[Meters] de Carné de Carnavalet, Xavier and Mohammad Mannan. “From Very Weak to Very Strong: Analyzing Password-Strength Meters.” In Proceedings of the Network and Distributed System Security Symposium (NDSS), 2014. Available at: http://www.internetsociety.org/sites/default/files/06_3_1.pdf

[NISTIR8062] NIST Internal Report 8062, An Introduction to Privacy Engineering and Risk Management in Federal Systems, January 2017, available at: http://nvlpubs.nist.gov/nistpubs/ir/2017/NIST.IR.8062.pdf.

[NIST Usability] National Institute and Standards and Technology, Usability & Biometrics, Ensuring Successful Biometric Systems, June 11, 2008, available at: http://www.nist.gov/customcf/get_pdf.cfm?pub_id=152184.

[OWASP-session] Open Web Application Security Project, Session Management Cheat Sheet, available at: https://www.owasp.org/index.php/Session_Management_Cheat_Sheet.

[OWASP-XSS-prevention] Open Web Application Security Project, XSS (Cross Site Scripting) Prevention Cheat Sheet, available at: https://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet.

[Persistence] herley, cormac, and Paul van Oorschot. “A Research Agenda Acknowledging the Persistence of Passwords,” IEEE Security&Privacy Magazine, 2012. Available at: http://research.microsoft.com/apps/pubs/default.aspx?id=154077.

[Privacy Act] Privacy Act of 1974 (P.L. 93-579), December 1974, available at: https://www.justice.gov/opcl/privacy-act-1974.

[Policies] Weir, Matt, Sudhir Aggarwal, Michael Collins, and Henry Stern. “Testing Metrics for Password Creation Policies by Attacking Large Sets of Revealed Passwords.” In Proceedings of the 17th ACM Conference on Computer and Communications Security, 162–175. CCS ‘10. New York, NY, USA: ACM, 2010. doi:10.1145/1866307.1866327.

[Section 508] Section 508 Law and Related Laws and Policies (January 30, 2017), available at: https://www.section508.gov/content/learn/laws-and-policies.

[Shannon] Shannon, Claude E. “A Mathematical Theory of Communication,” Bell System Technical Journal, v. 27, pp. 379-423, 623-656, July, October, 1948.

[Strength] Kelley, Patrick Gage, Saranga Komanduri, Michelle L Mazurek, Richard Shay, Timothy Vidas, Lujo Bauer, Nicolas Christin, Lorrie Faith Cranor, and Julio Lopez. “Guess Again (and Again and Again): Measuring Password Strength by Simulating Password-Cracking Algorithms.” In Security and Privacy (SP), 2012 IEEE Symposium On, 523–537. IEEE, 2012. Available at: http://ieeexplore.ieee.org/iel5/6233637/6234400/06234434.pdf.

11.2 Standards

[BCP 195] Sheffer, Y., Holz, R., and P. Saint-Andre, Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS), BCP 195, RFC 7525,DOI 10.17487/RFC7525, May 2015, https://doi.org/10.17487/RFC7525.

[ISO 9241-11] International Standards Organization, ISO/IEC 9241-11 Ergonomic requirements for office work with visual display terminals (VDTs) — Part 11: Guidance on usability, March 1998, available at: https://www.iso.org/standard/16883.html.

[ISO/IEC 2382-37] International Standards Organization, Information technology — Vocabulary — Part 37: Biometrics, 2017, available at: http://standards.iso.org/ittf/PubliclyAvailableStandards/c066693_ISO_IEC_2382-37_2017.zip.

[ISO/IEC 10646] International Standards Organization, Universal Coded Character Set, 2014, available at: http://standards.iso.org/ittf/PubliclyAvailableStandards/c063182_ISO_IEC_10646_2014.zip.

[ISO/IEC 24745] International Standards Organization, Information technology — Security techniques — Biometric information protection, 2011, available at: http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=52946.

[ISO/IEC 30107-1] International Standards Organization, Information technology — Biometric presentation attack detection — Part 1: Framework, 2016, available at: http://standards.iso.org/ittf/PubliclyAvailableStandards/c053227_ISO_IEC_30107-1_2016.zip.

[ISO/IEC 30107-3] International Standards Organization, Information technology — Biometric presentation attack detection — Part 3: Testing and reporting, 2017.

[RFC 20] Cerf, V., ASCII format for network interchange, STD 80, RFC 20, DOI 10.17487/RFC0020, October 1969, https://doi.org/10.17487/RFC0020.

[RFC 5246] IETF, The Transport Layer Security (TLS) Protocol Version 1.2, RFC 5246, DOI 10.17487/RFC5246, August 2008, https://doi.org/10.17487/RFC5246.

[RFC 5280] IETF, Internet X.509 Public Key Infrastructure Certificate and CRL Profile, RFC 5280, DOI 10.17487/RFC5280, May 2008, https://doi.org/10.17487/RFC5280.

[RFC 6238] IETF, TOTP: Time-Based One-Time Password Algorithm,RFC 6238, DOI 10.17487/RFC6238, https://doi.org/10.17487/RFC6238.

[RFC 6960] IETF, X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP, RFC 6960, DOI 10.17487/RFC6960, https://doi.org/10.17487/RFC6960.

[UAX 15] Unicode Consortium, Unicode Normalization Forms, Unicode Standard Annex 15, Version 9.0.0, February, 2016, available at: http://www.unicode.org/reports/tr15/.

11.3 NIST Special Publications

NIST 800 Series Special Publications are available at: http://csrc.nist.gov/publications/nistpubs/index.html. The following publications may be of particular interest to those implementing systems of applications requiring digital authentication.

[SP 800-38B] NIST Special Publication 800-38B, Recommendation for Block Cipher Modes of Operation: the CMAC Mode for Authentication, October, 2016, http://dx.doi.org/10.6028/NIST.SP.800-38B.

[SP 800-52] NIST Special Publication 800-52 Revision 1, Guidelines for the Selection, Configuration, and Use of Transport Layer Security (TLS) Implementations, April, 2014, http://dx.doi.org/10.6028/NIST.SP.800-52r1

[SP 800-53] NIST Special Publication 800-53 Revision 4, Recommended Security and Privacy Controls for Federal Information Systems and Organizations, April 2013 (updated January 22, 2015), http://dx.doi.org/10.6028/NIST.SP.800-53r4.

[SP 800-57 Part 1] NIST Special Publication 800-57 Part 1, Revision 4, Recommendation for Key Management, Part 1: General, January 2016, http://dx.doi.org/10.6028/NIST.SP.800-57pt1r4.

[SP 800-63-3] NIST Special Publication 800-63-3, Digital Identity Guidelines, June 2017, https://doi.org/10.6028/NIST.SP.800-63-3.

[SP 800-63A] NIST Special Publication 800-63A, Digital Identity Guidelines: Enrollment and Identity Proofing Requirements, June 2017, https://doi.org/10.6028/NIST.SP.800-63a.

[SP 800-63C] NIST Special Publication 800-63C, Digital Identity Guidelines: Federation and Assertions, June 2017, https://doi.org/10.6028/NIST.SP.800-63c.

[SP 800-90Ar1] NIST Special Publication 800-90A Revision 1, Recommendation for Random Number Generation Using Deterministic Random Bit Generators, June 2015, http://dx.doi.org/10.6028/NIST.SP.800-90Ar1.

[SP 800-107] NIST Special Publication 800-107 Revision 1, Recommendation for Applications Using Approved Hash Algorithms, August 2012, http://dx.doi.org/10.6028/NIST.SP.800-107r1.

[SP 800-131A] NIST Special Publication 800-131A Revision 1, Transitions: Recommendation for Transitioning the Use of Cryptographic Algorithms and Key Lengths, November 2015, http://dx.doi.org/10.6028/NIST.SP.800-131Ar1

[SP 800-132] NIST Special Publication 800-132, Recommendation for Password-Based Key Derivation, December 2010, http://dx.doi.org/10.6028/NIST.SP.800-132.

[SP 800-185] NIST Special Publication 800-185, SHA-3 Derived Functions: cSHAKE, KMAC, TupleHash, and ParallelHash, December, 2016, https://doi.org/10.6028/NIST.SP.800-185.

11.4 Federal Information Processing Standards

[FIPS 140-2] Federal Information Processing Standard Publication 140-2, Security Requirements for Cryptographic Modules, May 25, 2001 (with Change Notices through December 3, 2002), https://doi.org/10.6028/NIST.FIPS.140-2.

[FIPS 198-1] Federal Information Processing Standard Publication 198-1, The Keyed-Hash Message Authentication Code (HMAC), July 2008, https://doi.org/10.6028/NIST.FIPS.198-1.

[FIPS 201] Federal Information Processing Standard Publication 201-2, Personal Identity Verification (PIV) of Federal Employees and Contractors, August 2013, http://dx.doi.org/10.6028/NIST.FIPS.201-2.

[FIPS 202] Federal Information Processing Standard Publication 202, SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions, August 2015, http://dx.doi.org/10.6028/NIST.FIPS.202.

Appendix A—Strength of Memorized Secrets

This appendix is informative.

Throughout this appendix, the word “password” is used for ease of discussion. Where used, it should be interpreted to include passphrases and PINs as well as passwords.

A.1 Introduction

Despite widespread frustration with the use of passwords from both a usability and security standpoint, they remain a very widely used form of authentication [Persistence]. Humans, however, have only a limited ability to memorize complex, arbitrary secrets, so they often choose passwords that can be easily guessed. To address the resultant security concerns, online services have introduced rules in an effort to increase the complexity of these memorized secrets. The most notable form of these is composition rules, which require the user to choose passwords constructed using a mix of character types, such as at least one digit, uppercase letter, and symbol. However, analyses of breached password databases reveal that the benefit of such rules is not nearly as significant as initially thought [Policies], although the impact on usability and memorability is severe.

Complexity of user-chosen passwords has often been characterized using the information theory concept of entropy [Shannon]. While entropy can be readily calculated for data having deterministic distribution functions, estimating the entropy for user-chosen passwords is difficult and past efforts to do so have not been particularly accurate. For this reason, a different and somewhat simpler approach, based primarily on password length, is presented herein.

Many attacks associated with the use of passwords are not affected by password complexity and length. Keystroke logging, phishing, and social engineering attacks are equally effective on lengthy, complex passwords as simple ones. These attacks are outside the scope of this Appendix.

A.2 Length

Password length has been found to be a primary factor in characterizing password strength [Strength] [Composition]. Passwords that are too short yield to brute force attacks as well as to dictionary attacks using words and commonly chosen passwords.

The minimum password length that should be required depends to a large extent on the threat model being addressed. Online attacks where the attacker attempts to log in by guessing the password can be mitigated by limiting the rate of login attempts permitted. In order to prevent an attacker (or a persistent claimant with poor typing skills) from easily inflicting a denial-of-service attack on the subscriber by making many incorrect guesses, passwords need to be complex enough that rate limiting does not occur after a modest number of erroneous attempts, but does occur before there is a significant chance of a successful guess.

Offline attacks are sometimes possible when one or more hashed passwords is obtained by the attacker through a database breach. The ability of the attacker to determine one or more users’ passwords depends on the way in which the password is stored. Commonly, passwords are salted with a random value and hashed, preferably using a computationally expensive algorithm. Even with such measures, the current ability of attackers to compute many billions of hashes per second with no rate limiting requires passwords intended to resist such attacks to be orders of magnitude more complex than those that are expected to resist only online attacks.

Users should be encouraged to make their passwords as lengthy as they want, within reason. Since the size of a hashed password is independent of its length, there is no reason not to permit the use of lengthy passwords (or pass phrases) if the user wishes. Extremely long passwords (perhaps megabytes in length) could conceivably require excessive processing time to hash, so it is reasonable to have some limit.

A.3 Complexity

As noted above, composition rules are commonly used in an attempt to increase the difficulty of guessing user-chosen passwords. Research has shown, however, that users respond in very predictable ways to the requirements imposed by composition rules [Policies]. For example, a user that might have chosen “password” as their password would be relatively likely to choose “Password1” if required to include an uppercase letter and a number, or “Password1!” if a symbol is also required.

Users also express frustration when attempts to create complex passwords are rejected by online services. Many services reject passwords with spaces and various special characters. In some cases, the special characters that are not accepted might be an effort to avoid attacks like SQL injection that depend on those characters. But a properly hashed password would not be sent intact to a database in any case, so such precautions are unnecessary. Users should also be able to include space characters to allow the use of phrases. Spaces themselves, however, add little to the complexity of passwords and may introduce usability issues (e.g., the undetected use of two spaces rather than one), so it may be beneficial to remove repeated spaces in typed passwords prior to verification.

Users’ password choices are very predictable, so attackers are likely to guess passwords that have been successful in the past. These include dictionary words and passwords from previous breaches, such as the “Password1!” example above. For this reason, it is recommended that passwords chosen by users be compared against a “black list” of unacceptable passwords. This list should include passwords from previous breach corpuses, dictionary words, and specific words (such as the name of the service itself) that users are likely to choose. Since user choice of passwords will also be governed by a minimum length requirement, this dictionary need only include entries meeting that requirement.

Highly complex memorized secrets introduce a new potential vulnerability: they are less likely to be memorable, and it is more likely that they will be written down or stored electronically in an unsafe manner. While these practices are not necessarily vulnerable, statistically some methods of recording such secrets will be. This is an additional motivation not to require excessively long or complex memorized secrets.

A.4 Randomly-Chosen Secrets

Another factor that determines the strength of memorized secrets is the process by which they are generated. Secrets that are randomly chosen (in most cases by the verifier or CSP) and are uniformly distributed will be more difficult to guess or brute-force attack than user-chosen secrets meeting the same length and complexity requirements. Accordingly, at LOA2, SP 800-63-2 permitted the use of randomly generated PINs with 6 or more digits while requiring user-chosen memorized secrets to be a minimum of 8 characters long.

As discussed above, the threat model being addressed with memorized secret length requirements includes rate-limited online attacks, but not offline attacks. With this limitation, 6 digit randomly-generated PINs are still considered adequate for memorized secrets.

A.5 Summary

Length and complexity requirements beyond those recommended here significantly increase the difficulty of memorized secrets and increase user frustration. As a result, users often work around these restrictions in a way that is counterproductive. Furthermore, other mitigations such as blacklists, secure hashed storage, and rate limiting are more effective at preventing modern brute-force attacks. Therefore, no additional complexity requirements are imposed.