algorithm: A set of mathematical instructions forming a step-by-step procedure to encrypt or decrypt information.
ASCII: American Standard Code for Information Interchange, used to represent text in computers.
asymmetric key system: A cryptological system where a different ‘one-way’ key is needed for encrypting and decrypting.
authentication: The process of confirming identity.
bigram: Pair of letters, syllables or words, commonly used as the basis for statistical analysis of text.
binary code: Code system using only two characters or numbers, 0 and 1.
black chamber: General term for the cryptological offices set up by various governments since the 16th century where intercepted messages were studied for decryption.
block cipher: A cipher in which blocks of text are enciphered in groups, usually each of 64 bits.
Caesar shift cipher: Cipher in which a letter is replaced by another, a set number of places along in the alphabet.
cipher: A process in which individual letters are reordered or replaced to conceal the meaning of a text.
ciphertext: Enciphered text.
clear text: Another term for plaintext or en clair.
code: A system in which words or phrases are reordered or replaced for concealment. The word ‘code’ comes from the Latin for ‘book’, ‘codex’.
code book: The crucial ‘dictionary’ giving words and phrases and the character(s) to be used to represent them. For larger-scale codes, a reverse dictionary is required for decryption.
crib: A section of known plaintext, which can be used to break a code or cipher.
cryptanalysis: The art of breaking codes and ciphers.
cryptography: The art of devising codes and ciphers.
cryptology: The general term for cryptanalysis and cryptography.
cryptosystem: A system for encrypting and decrypting data.
decipher: To turn enciphered text into the original message, or plaintext.
decode: To turn a coded text into the original message, or plaintext.
decryption: The process of turning encoded or enciphered text into plaintext.
DES: Data Encryption Standard, the algorithm widely used for data encryption, adopted in 1976.
Diffie–Hellman–Merkle key exchange: Process for establishing a secret key through public discussion.
digital signature: Electronic identification of a person, using a public key algorithm.
digraph: Two letters representing one sound, such as ‘ph’ or ‘th’, forming common pairings that are useful in decryption.
encode: To turn plaintext into a coded message.
encipher: To turn plaintext into a cipher message.
Enigma: Most famous enciphering system in recent history, used by the Germans in World War II and, crucially, broken by the British.
en clair: Plaintext, un-encoded message.
fractionation: Process in which plaintext symbols are converted into new symbols prior to transposition, creating a more complex cipher.
frequency analysis: Decryption strategy in which ciphertext letters are counted to identify patterns, which relate to how often letters occur in natural text.
Greek square: Device for changing characters into numbers, using the grid references of a square. Also known as the Polybius square or the Greek checkerboard.
homophonic substitution: A cipher where a letter can be represented by several different characters, thus combating frequency analysis.
key: The set of characters that determines how a text is to be encrypted.
key length: The number of characters or bits in the key. The longer it is, the harder it is to decrypt.
monoalphabetic substitution cipher: Cipher in which the plaintext is encrypted using one alphabet.
nomenclator: Encryption using a mixture of homophonic substitution and codes for certain words and phrases. It was the main cryptological method for several centuries until World War I.
null: Part of the ciphertext, which indicates ends of sentences, or which can be ignored in decryption because it is there to confuse enemy code breakers.
one-time pad: The only known totally secure encryption method – a running key of totally random characters, used only once.
plaintext: A message before it is encrypted and after it is decrypted.
Polybius checkerboard: see Greek square.
polyalphabetic cipher: Cipher in which the plaintext is encrypted using more than one alphabet.
Pretty Good Privacy (PGP): A method for secure, encrypted email communication, developed by Phil Zimmerman.
private key: The ‘secret’ part of an asymmetric key system, also known as the decryption key.
public key: The ‘open’ part of an asymmetric key system, also known as the encryption key.
quantum cryptography: The use of quantum physics to create random bits on a computer, which can be used to create a one-time pad cipher.
running key: A key as long as the plaintext, as in a book cipher.
RSA: Rivest, Shamir and Adleman’s system enabling public key cryptography, invented in 1977.
steganography: Greek for ‘hidden writing’, this is the art of hiding the message itself, rather than concealing its meaning.
substitution cipher: Encryption system in which letters are replaced but remain in the correct position.
superencipherment: Encrypting a message twice, either with the same or with a different method for the second process. Also known as superencryption.
Transposition cipher: Encryption system in which letters change position, creating a giant anagram (in which the letters are also changed).
Vigenère square: The first polyalphabetic cipher made using a tabula recta combined with a keyword.