CHAPTER 3: STUDY OF SOME CLASSICAL AND MODERN CRYPTOSYSTEMS
Introduction Most encryption algorithms apply a technique to perform. Traditional (classical) symmetric ciphers use substitution and/or transposition techniques while modern ones use exponentiation and discrete logarithm. Substitution techniques map plaintext elements (characters, bits) into ciphertext elements. Transposition techniques systematically transpose the positions of plaintext elements. Therefore a cipher is composed of its encryption and decryption algorithms applying a specific technique and its key(s).
1. Classical encryption techniques 1.1.
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. There are four types of substitution ciphers: -
Simple
substitution ciphers: they replace
each
character
of
plaintext
with
a
corresponding character of ciphertext; a single one-to-one mapping from plaintext to ciphertext characters is used to encipher an entire message. E.g Caesar and Substitution Ciphers -
Homophonic substitution ciphers: they are similar, except that the mapping is one-tomany, and each plaintext character is enciphered with a variety of ciphertext characters. E.g Beale Cipher
-
Polyalphabetic substitution ciphers: they use multiple mappings from plaintext to ciphertext characters; the mappings are usually one-to-one as in simple substitution. E.g Vigenère and Beaufort Ciphers
-
Polygram
substitution ciphers: they are
the most
general,
substitutions for groups of characters. E.g Playfair and Hill Ciphers
1.2.
Transposition techniques
permitting
arbitrary