Cryptography &Network Security: Module 3

Module 3 Key management and Distribution

Symmetric Key Distribution Using Symmetric Encryption

For symmetric encryption to work, the two parties to an exchange must share the same
key, and that key must be protected from access by others. Therefore, the term that refers to the
means of delivering a key to two parties who wish to exchange data, without allowing others to
see the key. For two parties A and B, key distribution can be achieved in anumber of ways, as
follows:

1. A can select a key and physically deliver it to B.

2. A third party can select the key and physically deliver it to A and B.
3. If A and B have previously and recently used a key, one party can transmit the newkey
to the other, encrypted using the old key.
4. If A and B each has an encrypted connection to a third party C, C can deliver a key onthe
encrypted links to A and B.

Physical delivery (1 & 2) is simplest - but only applicable when there is personal contact
between recipient and key issuer. This is fine for link encryption where devices & keys occur in
pairs, but does not scale as number of parties who wish to communicate grows.3 is mostly based
on 1 or 2 occurring first.

A third party, whom all parties trust, can be used as a trusted intermediary to mediate
the establishment of secure communications between them (4). Must trust intermediary not to
abuse the knowledge of all session keys. As number of parties grow, some variant of 4 is only
practical solution to the huge growth in number of keys potentially needed.
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Key distribution centre:

The use of a key distribution center is based on the use of a hierarchy of keys. At a
minimum, two levels of keys are used.
Communication between end systems is encrypted using a temporary key, often referredto
as a Session key.
Typically, the session key is used for the duration of a logical connection and then
discarded
Master key is shared by the key distribution center and an end system or user and used to
encrypt the session key.

Key Distribution Scenario:

Let us assume that user A wishes to establish a logical connection with B and requiresa
one-time session key to protect the data transmitted over the connection. A has a master key, Ka,
known only to itself and the KDC; similarly, B shares the master key K b with the KDC. The
following steps occur:
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1 A issues a request to the KDC for a session key to protect a logical connection to B. The
message includes the identity of A and B and a unique identifier, N 1, for this transaction,
which we refer to as a nonce. The nonce may be a timestamp, a counter, or a random
number; the minimum requirement is that it differs with each request. Also, to prevent
masquerade, it should be difficult for an opponent to guess the nonce. Thus, a random
number is a good choice for a nonce.

2. The KDC responds with a message encrypted using Ka Thus, A is the only one who can
successfully read the message, and A knows that it originated at the KDC. The message
includes two items intended for A:

 The one-time session key, Ks, to be used for the session

 The original request message, including the nonce, to enable A to match this
response with the appropriate request

Thus, A can verify that its original request was not altered before reception by the KDC and,
because of the nonce, that this is not a replay of some previous request.In addition,
the message includes two items intended for B:

 The one-time session key, Ks to be used for the session

 An identifier of A (e.g., its network address), IDA

These last two items are encrypted with Kb (the master key that the KDC shares with B).
They are to be sent to B to establish the connection and prove A's identity.

3. A stores the session key for use in the upcoming session and forwards to B the information
that originated at the KDC for B, namely, E(K b, [Ks || IDA]). Because this information is
encrypted with Kb, it is protected from eavesdropping. B now knows the session key (K s),
knows that the other party is A (from IDA), and knows that the information originated at
the KDC (because it is encrypted using Kb).At this point, a session key has
been securely delivered to A and B, and they may begin
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their protected exchange. However, two additional steps are desirable:

4. Using the newly minted session key for encryption, B sends a nonce, N2, to A.

5. Also using Ks, A responds with f(N2), where f is a function that performs some
transformation on N2 (e.g., adding one).

These steps assure B that the original message it received (step 3) was not a replay.

Note that the actual key distribution involves only steps 1 through 3 but that steps 4 and 5, aswell as
3, perform an authentication function.

Major Issues with KDC:

1. Hierarchical Key Control

It is not necessary to limit the key distribution function to a single KDC.Indeed,for verylarge
networks,it may not be practical to do so.As an alternative,a hierarchy of KDCs canbe
established.
For example, there can be local KDCs, each responsible for a small domain of the overall
internetwork, such as a single LAN or a single building.
If two entities in different domains desire a shared key, then the corresponding local
KDCs can communicate through a global KDC.
The hierarchical concept can be extended to three or even more layers, depending on thesize of
the user population and the geographic scope of the internetwork.
A hierarchical scheme minimizes the effort involved in master key distribution,
because most master keys are those shared by a local KDC with its local entities.

2. Session Key Lifetime

The distribution of session keys delays the start of any exchange and places a burden on
network capacity. A security manager must try to balance these competing considerationsin
determining the lifetime of a particular session key.
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For connection-oriented protocols, one obvious choice is to use the same session key for
the length of time that the connection is open, using a new session key for each
newsession.
If a logical connection has a very long lifetime, then it would be prudent to change the
session key periodically, perhaps every time the PDU (protocol data unit) sequence
number cycles.
For a connectionless protocol, such as a transaction-oriented protocol, there is no
explicit connection initiation or termination.
Thus, it is not obvious how often one needs to change the session key. The most secure
approach is to use a new session key for each exchange.
A better strategy is to use a given session key for a certain fixed period only or for a
certain number of transactions.

3. A Transparent Key Control Scheme

The approach suggested in Figure 14.3is useful for providing end-to-end encryption at a
network or transport level in a way that is transparent to the end users.
The approach assumes that communication makes use of a connection-oriented end-to- end
protocol, such as TCP.
The noteworthy element of this approach is a session security module (SSM), which may
consist of functionality at one protocol layer,that performs end-to-end encryption and obtains
session keys on behalf of its host or terminal.

The steps involved in establishing a connection are shown in Figure 14.4.

1. When one host wishes to set up a connection to another host, it transmits a
connection-request packet.
2. The SSM saves that packet and applies to the KDC for permission to
establishthe connection.
3. The communication between the SSM and the KDC is encrypted using a
master key shared only by this SSM and the KDC.If the KDC approves the
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connection request,it generates the session key and delivers it to the two
appropriate SSMs,using a unique permanent key for each SSM.
4. The requesting SSM can now release the connection request packet, and a
connection is set up between the two end systems.
5. All user data exchanged between the two end systems are encrypted by their
respective SSMs using the onetime session key.

 The automated key distribution approach provides the flexibility and dynamic
characteristics needed to allow a number of terminal users to access a number of
hostsand for the hosts to exchange data with each other.

4. Decentralized Key Control

 The use of a key distribution center imposes the requirement that the KDC be trusted
and be protected from subversion. This requirement can be avoided if key distributionis
fully decentralized.
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 Although full decentralization is not practical for larger networks using symmetric
encryption only, it may be useful within a local context.
 A decentralized approach requires that each end system be able to communicate in a
secure manner with all potential partner end systems for purposes of session key
distribution.
 Thus, there may need to be as many as 𝑛 (𝑛 − 1)/2master keys for a configuration with
𝑛 end systems.
 A session key may be established with the following sequence of steps (Figure 14.5).
1. A issues a request to B for a session key and includes a nonce, .
2. B responds with a message that is encrypted using the shared master key. The response
includes the session key selected by B,an identifier of B,the value f(N 1), andanother
nonce N2.
3. Using the new session key,A returns f(N2) to B.

5. Controlling Key Usage

The concept of a key hierarchy and the use of automated key distribution techniques greatly
reduce the number of keys that must be manually managed and distributed. It also may be
desirable to impose some control on the way in which automatically distributed keys are used.
For example, in addition to separating master keys from session keys, we may wishto define
different types of session keys on the basis of use, such as
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 Data-encrypting key, for general communication across a network

 PIN-encrypting key, for personal identification numbers (PINs) used in
electronic funds transfer and point-of-sale applications
 File-encrypting key, for encrypting files stored in publicly accessible locations

To illustrate the value of separating keys by type, consider the risk that a master key is
imported as a data-encrypting key into a device. Normally, the master key is physically secured
within the cryptographic hardware of the key distribution center and of the end systems. Session
keys encrypted with this master key are available to application programs, as are the data
encrypted with such session keys.
However, if a master key is treated as a session key, it may be possible for an
unauthorized application to obtain plaintext of session keys encrypted with that master key.
The proposed technique is for use with DES and makes use of the extra 8 bits in
each64- bit DES key. That is, the eight non-key bits ordinarily reserved for parity checking
form the key tag. The bits have the following interpretation:
• One bit indicates whether the key is a session key or a master key.
• One bit indicates whether the key can be used for encryption.
• One bit indicates whether the key can be used for decryption.
• The remaining bits are spares for future use.

Because the tag is embedded in the key, it is encrypted along with the key when thatkey
is distributed, thus providing protection. The drawbacks of this scheme are
1. The tag length is limited to 8 bits, limiting its flexibility and functionality.
2. Because the tag is not transmitted in clear form, it can be used only at the
point of decryption, limiting the ways in which key use can be controlled.

A more flexible scheme, referred to as the control vector, is described here. In this
scheme, each session key has an associated control vector consisting of a number of fields
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that specify the uses and restrictions for that session key. The length of the control vectormay
vary.

The control vector is cryptographically coupled with the key at the time of key
generation at the KDC.

As a first step, the control vector is passed through a hash function that produces a
value whose length is equal to the encryption key length. In essence, a hash function maps
values from a larger range into a smaller range with a reasonably uniform spread. Thus, for
example, if numbers in the range 1 to 100 are hashed into numbers in the range 1 to 10,
approximately 10% of the source values should map into each of the target values. The
hashvalue is then XORed with the master key to produce an output that
is used as the key input for encrypting the session key. Thus,

Hash value = H = h(CV)Key input = Km ⊕H

Ciphertext = E([Km ⊕H], Ks)

where is the master key and is the session key. The session key is recovered inplaintext
by the reverse operation:
D([Km⊕H], E([Km ⊕H], Ks))
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When a session key is delivered to a user from the KDC, it is accompanied by the control
vector in clear form. The session key can be recovered only by usingboth the master key that the user
shares with the KDC and the control vector. Thus,the linkage between the session key and its control
vector is maintained.

Use of the control vector has two advantages over use of an 8-bit tag. First, there is no
restriction on length of the control vector, which enables arbitrarily complex controls to be imposed
on key use. Second, the control vector is available inclear form at all stages of operation. Thus,
control of key use can be exercised in multiple locations.
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SYMMETRIC KEY DISTRIBUTION USING ASYMMETRIC ENCRYPTION

Once public keys have been distributed or have become accessible, secure communication that
thwarts eavesdropping, tampering, or both, is possible.
Public-key encryption provides for the distribution of secret keys to be used forconventional
encryption.

Simple Secret Key Distribution

 A generates a public/private key pair {PUa, PRa} and transmits a message to Bconsisting of
PUa and an identifier of A, IDA
 B generates a secret key, Ks, and transmits it to A, encrypted with A's public key.
 A computes D(PRa, E(PUa, Ks)) to recover the secret key. Because only A candecrypt the
message, only A and B will know the identity of Ks.
 A discards PUa and PRa and B discards PUa.

Here third party can intercept messages and then either relay the intercepted message or
substitute another message Such an attack is known as a man-in-the-middle attack.
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Secret Key Distribution with Confidentiality and Authentication:

A uses B's public key to encrypt a message to B containing an identifier of A (ID A) and anonce
(N1), which is used to identify this transaction uniquely
B sends a message to A encrypted with PUa and containing A's nonce (N1) as well as anew nonce
generated by B (N2) Because only B could have decrypted message (1), thepresence of N 1 in
message (2) assures A that the correspondent is B
A returns N2 encrypted using B's public key, to assure B that its correspondent is A.
A selects a secret key Ks and sends M = E(PUb, E(PRa, Ks)) to B. Encryption of this message
with B's public key ensures that only B can read it; encryption with A's privatekey ensures that
only A could have sent it.
B computes D(PUa, D(PRb, M)) to recover the secret key.

A Hybrid Scheme:

Yet another way to use public-key encryption to distribute secret keys is a hybrid approach.

 This scheme retains the use of a key distribution center (KDC) that shares a secretmaster key
with each user and distributes secret session keys encrypted with the master key.
 A public key scheme is used to distribute the master keys.
 The addition of a public-key layer provides a secure, efficient means of distributingmaster keys.
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Distribution of Public Keys:

Several techniques have been proposed for the distribution of public keys, which can mostlybe grouped
into the categories shown.

Public announcement
Publicly available directory
Public-key authority
Public-key certificates

Public Announcement of Public Keys

The point of public-key encryption is that the public key is public, hence any participant can
send his or her public key to any other participant, or broadcast the key to the community at large.
eg. append PGP keys to email messages or post to news groups or email list

Its major weakness is forgery, anyone could pretend to be user A and send a public key to

another participant or broadcast such a public key. Until the forgery is discovered they can masquerade
as the claimed user.
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Publicly Available Directory

 can obtain greater security by registering keys with a public directory

 directory must be trusted with properties:

The authority maintains a directory with a {name, public key} entry for each
participant.
Each participant registers a public key with the directory authority.
A participant may replace the existing key with a new one at any time because the
corresponding private key has been compromised in some way.
Participants could also access the directory electronically. For this purpose, secure,
authenticated communication from the authority to the participant is mandatory.

This scheme is clearly more secure than individual public announcements but still has
vulnerabilities.
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If an adversary succeeds in obtaining or computing the private key of the directory

authority, the adversary could authoritatively pass out counterfeit public keys and subsequently
impersonate any participant and eavesdrop on messages sent to any participant.Another way to
achieve the same end is for the adversary to tamper with the records kept bythe authority.

Public-Key Authority:

Stronger security for public-key distribution can be achieved by providing tighter

controlover the distribution of public keys from the directory.
It requires users to know the public key for the directory, and that they interact with
directory in real-time to obtain any desired public key securely.
Totally seven messages are required.
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1.
A sends a timestamped message to the public-key authority containing a request for the
current public key of B.

2.
The authority responds with a message that is encrypted using the authority's private key,
PRauth Thus, A is able to decrypt the message using the authority's public key. Therefore, A
is assured that the message originated with the authority. The message includes the
following:

 B's public key, PUb which A can use to encrypt messages destined for B


The original request, to enable A to match this response with the corresponding
earlier request and to verify that the original request was not altered beforereception
by the authority.

The original timestamp, so A can determine that this is not an old message from the
authority containing a key other than B's current public key.

3. A stores B's public key and also uses it to encrypt a message to B containing an
identifier of A (IDA) and a nonce (N1), which is used to identify this transaction
uniquely.

4. B retrieves A's public key from the authority in the same manner as A retrieved B's
public key.
5. At this point, public keys have been securely delivered to A and B, and they may
begin their protected exchange. However, two additional steps are desirable:

6. B sends a message to A encrypted with PUa and containing A's nonce (N1) as well as
a new nonce generated by B (N2) Because only B could have decrypted message (3),
the presence of N1 in message (6) assures A that the correspondent is B.

7. A returns N2, encrypted using B's public key, to assure B that its correspondent is A.
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Public-Key Certificates

A user must appeal to the authority for a public key for every other user that it wishes to
contact and it is vulnerable to tampering too.
Public key certificates can be used to exchange keys without contacting a public-key
authority.
A certificate binds an identity to public key, with all contents signed by a trusted Public-Key
or Certificate Authority (CA).
This can be verified by anyone who knows the public-key authorities public-key.

A participant can also convey its key information to another by transmitting its
certificate.

Other participants can verify that the certificate was created by the authority. We can
place the following requirements on this scheme:

1. Any participant can read a certificate to determine the name and public key of the
certificate's owner.
2. Any participant can verify that the certificate originated from the certificate
authorityand is not counterfeit.
3. Only the certificate authority can create and update certificates.

4. Any participant can verify the currency of the certificate.

One scheme has become universally accepted for formatting public-key certificates:the
X.509 standard.

X.509 certificates are used in most network security applications, including IP security,
secure sockets layer (SSL), secure electronic transactions (SET), and S/MIME.
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