Human Computing
CRYPTOGRAPHY – DATA ENCRYPTION PART 2
Classical Ciphers
Classical ciphers used one of the following methods for the protection of
information
Substitution
To encrypt a message with the Caesar cipher, each letter of message is replaced by the
letter three positions later in the alphabet.
"WIKIPEDIA" encrypts as "ZLNLSHGLD".
Caesar rotated the alphabet by three letters, but any number works
Permutation/Transposition - Substitution ciphers replace each letter with a
different letter or symbol to produce the ciphertext, in a Transposition cipher, the
letters are just moved around.
Poly alphabetic ciphers
Weddingday encypts to YADGNIDDEW
Practice time:
Using Shift cipher encrypt
Weddingday
K = 13
Formula: (n+x) mod
26
https://www.khanacademy.org/computing/computer-science/cryptography/ciphers/a/
shift-cipher
Classical Ciphers
Book / Running Key
A large body of text is used as the key
Vulnerable to redundancy attacks
Codes
Construction of words/phrase mappings to other phrases, number or
symbols
Stenography
From the Greek for covered writing
Hiding the existence of a message
Microdots, watermarks
Symmetric Cryptography
An encryption system in which the sender and receiver of a message share a single,
common key that is used to encrypt and decrypt the message.
The most popular symmetric–key system is the Data Encryption Standard (DES)
Symmetric Cryptography
The process makes use of public and private components
Public
Algorithm to be used
The cipher text
Private
They key to be used
The exact transformation used out of a number of possibilities
Asymmetric Cryptography
The encryption process where different keys are used for encrypting and decrypting the
information.
Keys are different but are mathematically related, such that retrieving the plain text by decrypting
ciphertext is feasible.
Fairly new (45 years?)
Asymmetric Cryptography
Makes use of multiple keys for greater security, and solving the problem of key
distribution
Each party has their own keys, obviating the need for a shared secret key
Bases on the work by Diffie & Hellman Rivest, Shamir & Adleman
named after its authors (Rivest, Shamir and Adleman) one of the most popular: patented 1977,
published academically 1978
Finding large prime factors of numbers is a problem
Computationally intense
Rivest, Ronald L., Adi Shamir, and Leonard Adleman. “A method for obtaining digital signatures and public-key cryptosystems.” Communications of the
ACM 21.2 (1978): 120-126.
Asymmetric Cryptography
Some points to note:
Public key cannot decrypt a message it encrypted
Ideally a private key cannot be derived from a public key
A message encrypted with one key can only be decrypted with the
corresponding half
The private key MUST be kept private
Asymmetric Cryptography
RSA is the most widely used form of public key encryption,
RSA Algorithm
RSA stands for Rivest, Shamir, and Adelman
Both public and private key are interchangeable
Variable Key Size (512, 1024, or 2048 bits)
2 ^n
Pros & Cons : Asymmetric
Strengths
Better Key distribution
Scalability
Provides Confidentiality, authentication, non- repudiation
Weaknesses
Slower and more resource intensive than symmetric systems
Pros & Cons : Symmetric
Strengths
Faster than asymmetric systems
Hard to break if a sufficiently large key size is used
Weaknesses
Key distribution
Scalability
Limited security
Confidentiality only
No Authentication or Non-repudiation
The Key distribution Problem
Symmetric Asymmetric
Separate key required for each pair Each user has their own keys
of people You only need the public portion of a
Key distribution is a problem key
Only provides C Provides CIA
Complex to manage large numbers Improved manageability
Public key infrastructure (PKI)
Public key infrastructure (PKI) refers to the deployment and management of a public
key encryption system, where asymmetric encryption keys are used to enable secure
authentication and encryption-key exchange.
PKI’s authentication and key-exchange mechanisms enable security functionality such as
access control, secure messaging, data encryption, data/message signing, secure boot,
and more.
Provides Authentication, Confidentiality, Integrity, Non-Repudiation, Access Control
PKI is considered highly secure but also hard to manage, as it requires a set of roles,
policies, and procedures to create, manage, distribute, use, store, and revoke
encryption keys and related digital certificate
A PKI consists of the software, data formats, programs, procedures, algorithms,
communications and security policies which enable a dispersed group of people to
communicate in a secure manner
Certificate Authority (CA)
Certificate Authority is a trusted third party entity that issues
digital certificates and manages the public keys and credentials
for data encryption for the end user.
The responsibility of the CA in this process is to ensure that the
company or user receives a unique certificate for an efficient
identity authentication.
CA checks with a Qualified Information Source (QIS) to verify the
data supplied by the applicant, before issuing the digital
certificate.
A registration authority (RA) is an authority in a network that
verifies user requests for a digital certificate and tells the
certificate authority (CA) to issue it
CA is trusted by people to perform some kind of verification on clients
I trust you because the RA trusts you
Do you know whose web site it is?
Is the URL correct?
Are you likely to send any
confidential information over this
link?
When you should really worry
Are you doing anything
involving:
Financial transactions?
Passwords?
Personal data?
Downloading anything?
Summary
Cryptography is present in our everyday lives
Encryption and decryption form some of the basic building blocks of many security systems
Just because you are encrypting data does not mean it is safe
As with most security, the human factor is the weakest.
Encryption algorithms are basically step-by-step directions for performing cryptographic
functions (such as encryption, decryption, hashing, etc.)
Algorithms come in asymmetric and symmetric varieties.
In the first, one key encrypts data and another decrypts data.
In the latter, one key performs both the encryption and decryption functions.
Symmetric algorithms are efficient processes that are designed for encrypting data in
secure, non-public-facing channels.
