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.
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
- Caesar shift: all the letters of the alphabet shift a number of positions (from 1 to 26)
- 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
- 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
- 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.
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