## 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.

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