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