# Substitution Cipher: Description

A substitution cipher is a mono-alphabetic cipher, i.e. same letters in the plain-text map to the same but possibly different letter in the cipher-text. A substitution cipher is essentially a permutation of the letters of the alphabet which is done for each character of the plain-text. Hence substitution ciphers form a group and super-encipherment does not make sense.

Important subclasses are the Caesar Cipher and Linear Ciphers.

## Example

### Substitution Table

``` abcdefghijklmnopqrstuvwxyz
qwertyuiopasdfghjklzxcvbnm
```

### Plaintext

```i am so cool
```

### Ciphertext

```pqwfqzhqrhhd
```

## Breaking Substitution Cipher

Substitution ciphers are weak ciphers. They can be broken by just looking at a cipher-text and analyzing parameters like:
• Single Letter Occurrence (Monogram)
• Bigrams
• Trigrams
• Letters at the Beginning of Words
• Short Words
In our approach we will concentrate on trigrams.

If the cipher-text contains word separators the task is much easier than without them. We will assume that there are word separators.

Last Update: 13.04.96 (Format: DD.MM.YY)