1.1 Terminology

First, a few fundamental cryptography terms:

Cipher: a secret way of writing (way of encrypting)

Cryptography: Theory of secret writing

Cryptanalysis: Theory of "breaking" ciphers

Cryptology:Cryptography and Cryptanalysis

Plaintext: un-encrypted text

Ciphertext: encrypted text

Key: controls the En- and Decryption

In principal, there are two kinds of encryption:

Transposition: reorders the symbols of a text

Substitution: replaces (blocks of) symbols by other symbols

Usually it is assumed that the cryptanalyst knows the cipher algorithm (but not the key). For him, there are three typical situations:

Chiffretext only: The Cryptanalyst can determine the plain text only from the caught chiffre text.

known plain text: Knowledge of plaintext-ciphertext-pairs helps in decrypting the entire text, e.g. greeting phrases in letters, keywords in programming languages.

chosen plaintext: The Cryptanalyst reveives the cipher text for a plaintext chosen by him (e.g. on database systems).

Public-Key Cryptosystems allow further attacks of the Cryptanalyst.

2 Basics