## Cryptography basics

Written on 3 December 2014, 11:07pm

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## 1. Steganography

The science of hiding the existence of a message, as opposed to cryptography. A type of security through obscurity.
Ex. message written on the head of a messenger and sent only when it’s covered by the messenger growing hair; microdots; physical templates applied to a long text to highlight only some of the words.
http://en.wikipedia.org/wiki/Steganography

## 2. Cryptography

The practice of secured communication. The science of encrypting a message, or concealing the meaning of a message.

• Transposition ciphers – letters do not change, but move position
• Substitution ciphers – letters change, but keep position
1. Caesar shift: all the letters of the alphabet shift a number of positions (from 1 to 26)
2. Simple monoalphabetic substitution: substituting a different letter for every letter. The cipher alphabet is fixed throughout the encryption. Both methods fail to basic frequency analysis
3. Monoalphabetic with Homophones: a plaintext letter can be enciphered in many ways (typically numbers or symbols) – making the encryption resistant to a basic frequency analysis
4. Polyalphabetic substitution – alphabet matrix + password repeated until it has the same length as the plain text message (Vigenère cypher). The cipher alphabet changes during the encryption; the change is defined by a key. The longer the key, the more secure; but less practical for everyday use.
• A mix between transposition and substitution: ADFGVX (used to send Morse code messages)
• One time pad – the only form of encryption that is unbreakable, relying on a random key that is the same length as the message. Each key can be used only once. Impractical for extended use.

## 3. Cryptanalysis

The science of deducting the plain text from a cyphertext, without knowledge of the key.
One of the most used methods at the beginning: frequency analysis
http://en.wikipedia.org/wiki/Cryptanalysis