CRYPTO TERMINOLOGY

Useful Terminology

Much of the terminology of cryptography can be linked back to the time when only written messages were being encrypted and the same terminology is still used regardless of whether it is being applied to a written message or a stream of binary code between two computers.

CIPHERTEXT	The encrypted form of the PLAINTEXT.
CODE	An unvarying rule for replacing a piece of information with another object, not necessarily of the same sort e.g. ASCII.
CRYPTANALYSIS	The science (and art) of recovering information from ciphers without knowledge of the key.
CRYPTOGRAPHY	The science of the enciphering and deciphering of messages in secret code or cipher.
CRYPTOSYSTEM	A system for encrypting information.
DECRYPTION	The process of converting the CIPHER back into PLAINTEXT.
ENCRYPTION	The process of converting the PLAINTEXT into a CIPHER.
KEY	The secret information known only to the transmitter and the receiver which is used to secure the PLAINTEXT.
MONOALPHABETIC SUBSTITUTION	A method of encryption where a letter in the plaintext is always replaced by the same letter in the ciphertext.
PLAINTEXT	The source information to be secured.
POLYALPHABETIC SUBSTITUTION	A method of encryption where a letter in the plaintext is not always replaced by the same letter in the ciphertext.

N.B. Ciphers, as in the case of codes, also replace a piece of information (an element of the plaintext that may consist of a letter or word or string of symbols) with another object. The difference is that the replacement is made according to a rule defined by a secret key known only to the transmitter and legitimate receiver(s) in the expectation that an outsider, ignorant of the key, will not be able to undo the replacement and retrieve the original plaintext.