# Classical encryption

## Introduction

### Plaintext and ciphertext

### ROT13

Rotate 13. It is its own inverse.

### Atbash

Revese the alphabet. It is its own inverse.

## Verifying decryptions

### Corpus

verifying solutions when spaces are omitted. can rate fitness using corpus information on popularity

## Caesar

### Caesar ciphers

Shift along in alphabet by \(c\).

### Affine cipher

page on affine cipher too. like caesar but rather than +c, mx+c

### Breaking

For Caesar, only 26 possible keys, can just brute force.

For Affine, can also brute force.

## Monoalphabetic substitution

### Monoalphabetic substitution ciphers and keys

(key plus algorithm encrpyts and decrypts)

### Breaking monoalphabetic substitution ciphers with frequency analysis

(need to identify algorithm and needs to identify key)

finding substitution cyphers

Search space is larger, \(26!=4*10^26\). need alternative to brute force.

Letter popularity. Compare against popularity for corpus. Monogram (ie letters); ngrams(ie n letter in a row frequency); common words.

Single letter words are I or A. More generally. corpus smaller for fewer letters

Can test substitution cypher by matching each word against a corpus

## Polyalphabetic substitution

### Polyalphabetic ciphers

#### Multiple substitution

#### Vigenere

#### Rotor machines

#### The Enigma machine

### Breaking polyalphabetic ciphers with the Kasiski examination

## Other

### Codebooks

( (eg sdrgdr is code for “meet at x on y”)

### Transposition ciphers

### Book cipher

Eg use Bible.

### One-time pads