vimwiki/tech/substitution.wiki

= Substitution =

A Cipher where characters are replaced based on a mapping

== Monoalphabetic Subsititutions ==

Cipher where each occurrence of a plaintext symbol is replaced by a
corresponding ciphertext symbol

=== Afline ===

An affine cipher E(x) = (ax + b) MOD 26 is a monoalphabetic sub cipher, where a
and b are given constants

=== Columnar Transposition ===

Use letters to create columns, then fill all remaining columns with remaining
letters in alphabetic order




== Cryptoanalysis ==

=== Monoalphabetic Substitution ===

We make some assumptions

* Its english and has same stats as standard english
* word divisions are not preserved
* use frequency analysis to guess high frequency letters
  - E E N O R I A S are 70% of letters in english
* identify
 - vowels
 - diagraphs (common combos of letters ie TH)
* Use cribs
* guess and rely on luck

== Also see ==

* [[gcd]]

[[index]]