Lecture 10 Encryption techniques, Lecture notes of Information Security and Markup Languages

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Classical Encryption

Techniques

Transposition Ciphers

▪ A transposition ciphers changes the position of the plaintext using various

techniques to get the cipher text instead of substitution

▪ Rail Fence Technique

▪ Alice wants to send Bob a message as follows: attack is at 3pm today

▪ Key known to both Bob and Alice is 3

▪ Grid is created as follows:

▪ Number of rows = key

▪ Number of columns =characters in the message

2 a c a m a t a k s t p t d y t i 3 o

Transposition cipher

▪ Key used is 4312567 ( or a keyword can be used eg:PASWORD )

▪ Plaintext is: attack postponed until two am

▪ Transpose the columns based on the key to get the cipher text(1234567)

▪ TTNAAPTMTSUOAODWCOIXKNLYPETZ

▪ Repeat the procedure with the ciphertext to achieve double transposition

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a t t a c k p o s t p o n e d u n t i l t w o a m x y z

Polyalphabetic ciphers

▪ Polyalphabetic substitution cipher – one that uses multiple monoalphabetic

substitutions for the plaintext message

▪ All such ciphers have the following features

▪ Set of related monoalphabetic substitution rules is used

▪ A key determines which particular rule is used for a given

transformation

▪ Polyalphabetic cipher examples

▪ Vignére cipher

▪ Vernam Cipher

▪ One-time pad

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Vigenére Cipher - Encryption

▪ Enter the shift for the plaintext message

▪ Cipher text will be as follows

7 m a k e i t h a p p e n n o w 15 0 18 22 3 15 0 18 22 3 15 0 18 22 3 m a k e i t h a p p e n n o w B A C A L I H S L S T N F K Z

Vigenére Cipher - Decryption

▪ Decrypt the following cipher text using the keyword PASWD

▪ Steps

▪ Each letter is shifted in the opposite direction to get the plaintext

▪ Once the keyword length is determined easy to break the cipher

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B A C A L I H S L S T N F K Z

B A C A L I H S L S T N F K Z

m a k e i t h a p p e n n o w

Vernam Cipher

▪ Uses a keyword that is as long as the plaintext and has no statistical

relationship to it

▪ Introduced by an AT&T engineer named Gilbert Vernam in 1918

▪ System worked with binary numbers instead of letters

▪ Cipher text is generated by performing the bitwise XOR of the plaintext and

key

▪ Vernam proposed the use of a running loop of tape that eventually

repeated the key

▪ Hence a long key was created but it was repeated

▪ Although cryptanalysis is difficult it can still be broken

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One-Time Pad - Features

▪ Improvement to Vernam cipher proposed by an Army Signal Corp officer,

Joseph Mauborgne

▪ Use a random key that is as long as the message so that the key need not

be repeated

▪ Key is used to encrypt and decrypt a single message and then is discarded

▪ Each new message requires a new key of the same length as the new

message

▪ Scheme is unbreakable

▪ Produces random output that bears no statistical relationship to the

plaintext

▪ Because the ciphertext contains no information whatsoever about the

plaintext, there is simply no way to break the code

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Other Methods

▪ Rotor Machines (Refer e-textbook)

▪ Provides multiple stages of encryption as it consists of independently

rotating cylinders through which electric pulses flow

▪ Has 26 input pins and 26 output pins with internal wiring that connects

each input pin to a unique output pin

▪ For every complete rotation of the inner cylinder rotates one pin

position.

▪ This is repeated for the outer cylinder when the middle cylinder rotates

▪ Hence there is a possibility of 26 x 26 x 26 = 17,576 substitution

alphabets before the system repeats

▪ Home Work : What is Steganography?

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