Monoalphabetic Ciphers

Polyalphabetic Ciphers

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Substitution ciphers are an encryption scheme in which the identity

of each clear text unit is changed but its position is retained - the cleartext

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...

What is important is that one symbol is replaced by another symbol

in a consistent manner to render a string of meaningful symbols as a string

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.

There are three primary properties of substutition ciphers:

1. the number of cipher text symbols that represent a single cleartext

symbol [uniliteral vs multiliteral]

2. the number of clear text symbols encrypted at a time [monographic

vs polygraphic]

3. the number of enciphering alphabets used [monoalphabetic vs poly


How each of these properties is implemented determines the relative

complexity of the cipher.Historically, complexity to ciphers, by cryptographers,

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.

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