Introduction To Codes, Ciphers, & Codebreaking
by Greg Goebel

Codes are unsurprisingly defined by "codebooks", which are dictionaries of codegroups listed with their corresponding their plaintext. Codes originally had the codegroups in the same order as their plaintext. For example, in a code based on codenumbers, a word starting with "a" would have a low-value codenumber, while one starting with "z" would have a high-value codenumber. This meant that the same codebook could be used to "encode" a plaintext message into a coded message or "codetext", and "decode" a codetext back into plaintext message.

However, such "one-part" codes had a certain predictability that made it easier for outsiders to figure out the pattern and "crack" or "break" the message, revealing its secrets. In order to make life more difficult for codebreakers, codemakers then designed codes where there was no predictable relationship between the order of the codegroups and the order of the matching plaintext. This meant that two codebooks were required, one to look up plaintext to find codegroups for encoding, the other to look up codegroups to find plaintext for decoding. This was in much the same way that a student of a foreign language, say French, needs an English-French and a French-English dictionary to translate back and forth between the two languages. Such "two-part" codes required more effort to implement and use, but they were harder to break.

* In contrast to a code, a "cipher" conceals a plaintext message by replacing or scrambling its letters. This process is known as "enciphering" and results in a "ciphertext" message. Converting a ciphertext message back to a plaintext message is known as "deciphering". Coded messages are often enciphered to improve their security, a process known as "superencipherment".

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There are two classes of ciphers. A "substitution cipher" changes the letters in a message to another set of letters, or "cipher alphabet", while a "transposition cipher" shuffles the letters around. In some usages, the term "cipher" always means "substitution cipher", while "transpositions" are not referred to as ciphers at all. In this document, the term "cipher" will mean both substitution ciphers and transposition ciphers. It is useful to refer to them together, since the two approaches are often combined in the same cipher scheme. However, transposition ciphers will be referred to as "transpositions" for simplicity.

"Encryption" covers both encoding and enciphering, while "decryption" covers both decoding and deciphering. This should also imply the term "cryptotext" to cover both codetext and ciphertext, but this term doesn't seem to be in use, although the term "encicode" is sometimes seen.

The science of creating codes and ciphers is known as "cryptography", while the science of breaking them is known as "cryptanalysis". Together, the two fields make up the science of "cryptology".

Despite the fact that the term "code" is misleading, for the sake of readability this document will retain the use of general terms like "codebreaker", "code bureau", "code expert", and "army codes", rather than continually belaboring the distinction between codes and ciphers. As long as the distinction between "code" and "cipher" is clearly understood, this usage should cause no difficulty.

By the way, in cryptographic examples, cryptographers like to use the fictional characters "Alice" and "Bob", with Alice writing encrypted messages and Bob decrypting them. This convention will be followed in this document, along with the unconventional use of "Holmes" as a fictional codebreaker.

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