Components of Cryptography

Codes

The word ‘code’ is not one we’ll use within the context of cryptography, although it’s a term often associated informally with cryptography. There are many different interpretations of the concept of a code.

Most generally, the term ‘code’ is often used for any scheme in which data is replaced by alternative data before being sent over a communication channel. This replacement is usually dictated by the contents of a codebook, which states precisely which replacement data to use. A good example is the Morse Code, which replaces the letters of the alphabet with short sequences of dots and dashes.

Note: Morse Code has nothing to do with secrecy, since the codebook, in this case, is well known.

Morse Code was designed to efficiently transmit messages over telegraph wires. Another example of a code is ASCII, which provides a means of converting keyboard symbols into data suitable for processing on a computer (see appendix).

If a codebook is kept secret and is only known by the sender and the receiver of some data, then the resulting code can be regarded as a type of cryptosystem. In this case, the encryption algorithm is simply to replace the plaintext with its matching ciphertext entry in the codebook. The decryption algorithm is the reverse process. The encryption (and decryption) key is the codebook specification itself.

For example, Morse Code is not a cryptosystem because there is only one way of replacing letters with dots and dashes. However, if the rule for replacing letters with dots and dashes was kept secret from everyone except a chosen sender and receiver, then we could regard this as a cryptosystem.