Fingerprint Analysis

CLASSICAL ENCRYPTION TECHNIQUES

 There are two basic building blocks of all encryption techniques: substitution and transposition. 

I.SUBSTITUTION TECHNIQUES

 A substitution technique is one in which the letters of plaintext are replaced by other letters or by numbers or symbols. If the plaintext is viewed as a sequence of bits, then substitution involves replacing plaintext bit patterns with ciphertext bit patterns.

   

 (i)Caesar cipher (or) shift cipher

The earliest known use of a substitution cipher and the simplest was by Julius Caesar. The Caesar cipher involves replacing each letter of the alphabet with the letter standing 3 places further down the alphabet.  

e.g., plain text: pay more money   

Ciphertext: SDB PRUH PRQHB Note that the alphabet is wrapped around, so that letter following „z‟ is „a‟.

 For each plaintext letter p, substitute the ciphertext letter c such that 

          C = E(p) = (p+3) mod 26

 A shift may be any amount, so that general Caesar algorithm is 

          C = E (p) = (p+k) mod 26

Where k takes on a value in the range 1 to 25.

The decryption algorithm is simply   

           P = D(C) = (C-k) mod 26  

(ii)Playfair cipher

The best known multiple letter encryption cipher is the Playfair, which treats diagrams in the plaintext as single units and translates these units into ciphertext diagrams. 

The Playfair algorithm is based on the use of 5x5 matrix of letters constructed using a keyword. Let the keyword be „monarchy‟. The matrix is constructed by filling in the letters of the keyword (minus duplicates) from left to right and from top to bottom, and then filling in the remainder of the matrix with the remaining letters in alphabetical order. 

The letter „i‟ and „j‟ count as one letter. Plaintext is encrypted two letters at a time according to the following rules:

  Repeating plaintext letters that would fall in the same pair are separated with a filler letter such as „x‟.

 Plaintext letters that fall in the same row of the matrix are each replaced by the letter to the right, with the first element of the row following the last.

 Plaintext letters that fall in the same column are replaced by the letter beneath, with the top element of the column following the last.

 Otherwise, each plaintext letter is replaced by the letter that lies in its own row and the column occupied by the other plaintext letter. 

M	O	N	A	R
C	H	Y	B	D
E	F	G	I/J	K
L	P	Q	S	T
U	V	W	X	Z

 

Plaintext = meet me at the school house 

Splitting two letters as a unit => me  et   me  at   th  es   ch   ox  ol   ho  us  ex Corresponding cipher text     => CL KL CL RS PD IL HY AV MP HF XL IU 

Strength of playfair cipher

  Playfair cipher is a great advance over simple mono alphabetic ciphers.

 Since there are 26 letters, 26x26 = 676 diagrams are possible, so identification of individual digram is more difficult.

 Frequency analysis is much more difficult.

(iii)Polyalphabetic ciphers

 Another way to improve on the simple monoalphabetic technique is to use different monoalphabetic substitutions as one proceeds through the plaintext message.

The general name for this approach is the polyalphabetic cipher.

All the techniques have the following features in common.

 A set of related monoalphabetic substitution rules are used

 A key determines which particular rule is chosen for a given transformation.

II .TRANSPOSITION TECHNIQUES

All the techniques examined so far involve the substitution of a ciphertext symbol for a plaintext symbol. A very different kind of mapping is achieved by performing some sort of permutation on the plaintext letters. This technique is referred to as a transposition cipher.

 Rail fence is simplest of such cipher, in which the plaintext is written down as a sequence of diagonals and then read off as a sequence of rows.

Plaintext = meet at the school house

To encipher this message with a rail fence of depth 2, we write the message as follows:

                 m   e   a   t   e   c   o   l   o   s

                       e   t    t    h   s   h   o   h   u   e

The encrypted message is   MEATECOLOSETTHSHOHUE

Row Transposition Ciphers-A more complex scheme is to write the message in a rectangle, row by row, and read the message off, column by column, but permute the order of the columns.

 The order of columns then becomes the key of the algorithm. e.g., 

 plaintext = meet at the school house

Key = 4       3        1      2     5       6      7

PT   = m      e        e       t       a      t       t

          h       e        s       c      h      o      o

          l       h        o       u      s      e

CT   = ESOTCUEEHMHLAHSTOETO

A pure transposition cipher is easily recognized because it has the same letter frequencies as the original plaintext. The transposition cipher can be made significantly more secure by performing more than one stage of transposition. The result is more complex permutation that is not easily reconstructed.