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.

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

substitution cipher

basic cryptanalisis

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