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Chapter 2
Cryptologic Terms
Cryptography is the art or science of designing methods to send secret messages.
(In Greek crypt or kryptos means “secret” or “hidden” and graphy
means “writing”). Cryptologia, meaning “secrecy in speech”, and Cryptographia
meaning “secrecy in writing” were used first by John Wilkens in
his 1641 book “Mercury, of the Secret and the Swift Messenger,” the first English
book about cryptography. Wilkins went on to be the first secretary of
the Royal Society, which he co-founded with John Wallis. The words took their
modern forms, cryptology and cryptography, in 1645 and 1658, respectively.
Cipher or cypher comes from the same Arabic word that also provides the
root for zero, and has been used to mean a secret manner of writing in English
since the 1500’s. Once we have a cipher algorithm, telling us which cipher
is going to be used and how to use that cipher, and a key, we have a cipher
system or cryptosystem. For example, in the Caesar cryptosystem, the
algorithm is “shift the alphabet by a given amount” and the key is the amount.
The sender composes the plaintext, the original message which understandable
to all, uses the key to encipher it into the ciphertext, the “secret” version of
the message that is, hopefully, understandable only to those who have the key
and are able to decipher the message back into plaintext.
The word cryptanalysis was coined by William Friedman in 1923 and is the
name for the art or science of reading another person’s message without the key.
One decrypts a message and breaks a cipher system. While cryptography
held its meaning, cryptology no longer refers to speech but is the name for the
subject that combines cryptography and cryptanalysis. 1
It has become traditional to attach the names Alice, Bob and sometimes Eve
1 It will be clear that each of our secret messages is indeed a secret message. That is, we will
not worry about keeping the existence of the secret message secret, but rather about keeping
the meaning of the message secret. Stenanography (Greek steganos, meaning covered, apparently
first used by Trithemius in 1499) is the study of methods to keep the very existence
of the secret message secret, and is clearly a very interesting subject to spies.
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