Cryptography glossary

Cryptanalysis

Codebreaking · Code breaking

The discipline of attacking a ciphertext without knowing the key. The "offensive half" of cryptology — where cryptography protects, cryptanalysis attacks.

Cryptography

Encryption

The science and practice of protecting information through transformation. The "defensive half" of cryptology — designing ciphers that resist cryptanalysis.

Cryptology

Science of secrecy

The umbrella discipline grouping cryptography (designing ciphers) and cryptanalysis (breaking them). The term also covers steganography (hiding the message's existence).

Frequency analysis

Letter counting

A cryptanalysis technique that exploits the uneven frequency of letters in any natural language. Instantly breaks any monoalphabetic cipher and remains useful against short polyalphabetic ones.

Index of coincidence

IC · IoC · Indice de coïncidence

A statistical measure introduced by William Friedman in 1922. Compares the probability that two randomly drawn letters from a text are identical — used to distinguish a monoalphabetic cipher from a polyalphabetic one.

Monoalphabetic

Simple substitution

Family of ciphers in which each plaintext letter is always replaced by the same ciphertext letter, regardless of its position. Crackable in minutes by frequency analysis.

Polyalphabetic

Multiple substitution

Family of ciphers that switches substitution tables along the message — typically in a cycle dictated by a key. Far more resistant to frequency analysis than monoalphabetic substitution.

Substitution

Substitution cipher

Encryption principle in which each letter (or group of letters) of the plaintext is replaced by another symbol — letter, glyph, number. Stands in contrast with transposition, which only reorders.

Transposition

Transposition cipher · Permutation

Encryption principle that reorders the plaintext letters without replacing them. The same characters appear in the ciphertext, just in different positions.

Key

Secret key · Cryptographic key

The private parameter shared between the sender and recipient of an enciphered message. The cipher's security rests entirely on the secrecy of the key, never on that of the procedure.

Plaintext

Clear text · Source text · Cleartext

The original message before encryption, or the message recovered after decryption. The text anyone reads in the source language, with no cryptographic transformation.

Ciphertext

Cryptogram · Cipher text · Encrypted text

The result of applying a cipher to a plaintext message. Text rendered unintelligible to anyone without the key — without it, it looks like random noise.

Hash

Cryptographic fingerprint · Digest · Hashing

A fixed-length fingerprint computed from a piece of data through a one-way function. It is (in practice) impossible to recover the data from its hash.

Brute-force

Brute-force attack · Exhaustive search

An attack method that systematically tries every possible key until it finds the right one. Theoretically unstoppable, practically unmanageable as soon as the key space is large enough.

Kasiski test

Kasiski examination · Kasiski method

A cryptanalysis technique that determines the key length of a polyalphabetic cipher by looking for identical sequences repeated in the ciphertext.

Kerckhoffs's principle

Kerckhoffs's law · Shannon's maxim

A founding principle of modern cryptography (1883) — the security of a cipher must rest only on the secrecy of the key, never on the secrecy of the procedure.

Steganography

Hidden writing · Concealment

The art of hiding the very existence of a message — invisible ink, the last bit of an image, a message inside a song. Where cryptography makes things unreadable, steganography makes them invisible.