Get Worksheet C.1 Introduction To Cryptology: Substitution Ciphers ...

A substitution cipher merely substitutes different letters, numbers, or other characters for each character in the original text. The most straightforward example is a simplistic substitution in which each letter of the alphabet is represented by a numerical digit, starting with 1 for A.

A substitution cipher simply means that each letter in the plaintext is substituted with another letter to form the ciphertext. If the same letter occurs more than once in the plaintext then it appears the same at each occurrence in the ciphertext. For example the phrase "HELLO THERE" has multiple H's, E's, and L's.

substitution cipher, data encryption scheme in which units of the plaintext (generally single letters or pairs of letters of ordinary text) are replaced with other symbols or groups of symbols.

Examples of substitution ciphers include monoalphabetic ciphers, homophonic ciphers, polyalphabetic ciphers, and polygraphic substitution ciphers. Monoalphabetic ciphers, or simple ciphers, are the most basic form of ciphers and substitute one letter with another character or letter.

The simple substitution cipher The basic idea of a substitution cipher is a simple one: take one letter in your message, let's say 'A', and replace it with a different letter, such as 'E'. Sounds familiar? Both the Atbash and Caesar ciphers used this basic principle, however they both have one weakness: predictability.

The simplest of all substitution ciphers are those in which the cipher alphabet is merely a cyclical shift of the plaintext alphabet. Of these, the best-known is the Caesar cipher, used by Julius Caesar, in which A is encrypted as D, B as E, and so forth.

Two of the most common substitution ciphers are the Atbash Cipher and the Caesar Cipher. Other examples include the Affine Cipher and Baconian Cipher.

A substitution cipher replaces any plain text character from a given fixed set of characters with another character from the same set based on a key. With a shift of one, for example, A would be replaced by B, B by C, and so on.

A Caesar cipher is a simple method of encoding messages. Caesar ciphers use a substitution method where letters in the alphabet are shifted by some fixed number of spaces to yield an encoding alphabet.