what are data encryption methods or traditional ciphers? substitution ciphers, Transposition ciphers , Monoalphabetic cipher, shift cipher and polyalbhabetic cipher

Data Encryption Methods or Traditional Ciphers 

The traditional ciphers are character-oriented and are organized into two categories (see fig.):

1.      Substitution ciphers

2.      Transposition ciphers

        Substitution ciphers .

·        In substitution ciphers each letter or group of letters is replaced by another letter or group.

·        For example, we can replace character A with E and character S with W. If the symbols are digits (0 to 9), we can replace 3 with 5, 4 with 7.

·        Substitution ciphers are of two types : Monoalphabetic and Polyalphabetic ciphers.

       Monoalphabetic cipher

1.      In monoalphabetic cipher a character of symbol in the plaintext is always changed to the same character.

2.      For example if character A is changed to character, D, every occurrence of character A should always be changed with character D.

3.      There exists one-to-one relationship between the characters in the plaintext and the ciphertext.

4.      For example, each letter in plaintext is mapped to some other letter as shown below:

Plaintext :abcdefghijklmnopqrstuvwxyz

Ciphertext :QWERTYUIOPASDFGHJKLZxCVBNM

5.      In such a system a plaintext "Hello" would be changed into a ciphertext "ITSSG"

Plaintext : Hello

Ciphertext : ITSSG

       Polyalphabetic cipher

1.       In polyalphabetic cipher, each occurrence of a character can have different substitute.

2.      The relationship between a character in the plaintext to a character in the ciphertext is a one-to-many relationship

3.      Character A could be changed to D in the beginning of the text, but it could be changed to N at the middle. Thus the relationship between the character A in plaintext to characters D & N is ciphertext is one-to-many.

4.      For example, a plaintext "Hello" changed to a ciphertext "ABNZF" represent the polyalbhatetic cipher because each occurrence of L in plaintext is encrypted by different character. The first L is encrypted as N and the second L as Z.

       Shift Cipher  

·        Shift cipher is monoalphabetic cipher .

·        In this cipher, the encryption algorithm is "shift key character down", with key equal to some number.

·        The decryption algorithm is "shift key character up".

·        Example of shift cipher is the Caesar cipher attribute to Julius Caesar. In this method a becomes D, b becomes E and c becomes F,.... and z becomes C. In Caesar cipher the "attack" becomes "DWWDFN"

Plaintext: attack

Ciphertext: DWWDFN

Transposition Cipher

·        Transposition cipher does not substitute a character with another character, rather it reorders the letters to change their locations.

·        A character in first position of plaintext may appear in the tenth position of the ciphertext. A character in the eighth position may appear in first position.

·        Thus a transpositions cipher reorders the symbols in a block of symbols.

·        Transposition cipher makes use of keys. The key provides the mapping between the position of the symbols in the plaintext and the ciphertext.

·        The key is usually a phrase or word that does not contain any repeated letters.

·        In our example, we take "BOENTIKA" as a key (see fig.). . .

·        The purpose of key is to number the columns. Column 1 is usually made under that key letter which is closest to the start of alphabet and so, on (see fig.).

·        The plaintext is written horizontally, in rows. The rows can also be padded to fill the matrix if required.

·        The ciphertext is read out by columns, starting with column whose key letter is lowest.

·        In our example, the plaintext is "please transfer ten thousand rupees to my swiss bank account, seven zero zero". The generation of cipher text is shown in fig.14.5 .

Plaintext: Please transfer ten thousand rupees to my swiss bank account seven zero zero. Ciphertext:RENTSONOPANDOBUZESHUYNTRERSEICEVETTASSCERAFOPSKSOLNTRMANESEUEWAEZ

·        The transposition encryption can be broken down. If the snooper guesses the right number of columns, the permutation and combinations can produce the right output.