Substitution ciphers are an encryption scheme in which the identity
unit is substituted for a cipher text unit. A clear text unit is generally a letter
of the alphabet but may be a number or form of punctuation [which when
encrypted makes cryptanalysis much harder]. The ciphertext unit may be
anything - a letter, number, punctuation mark, symbol, plot on a graph, etc...
of meaningless symbols. This is the purpose of substitution: hiding the
meaning of the message by hiding the identity of the symbolsthat
comprise the message. As you will see throughout this manual, each added
layer of complexity in the enciphering method hides a particular dimension
or structure of the original message.
symbol [uniliteral vs multiliteral]
2. the number of clear text symbols encrypted at a time [monographic
3. the number of enciphering alphabets used [monoalphabetic vs poly
was added as the techniques of cryptanalysts became more sophisticated.
Each level of complexity is similar to adding a new dimension to the crypto-
graphic process -this manual will recursively deal with each dimension in
traversing from the simplest to the most complex cipher system.