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      Technically, a code is defined as substitution at the level of words or phrases, whereas a cipher is defined as substitution at the level of letters. Hence the term encipher means to scramble a message using a cipher, while encode means to scramble a message using a code. Similarly, the term decipher applies to unscrambling an enciphered message, and decode to unscrambling an encoded message. The terms encrypt and decrypt are more general, and cover scrambling and unscrambling with respect to both codes and ciphers. Figure 6 presents a brief summary of these definitions. In general, I shall keep to these definitions, but when the sense is clear, I might use a term such as codebreaking to describe a process that is really cipher breaking – the latter phrase might be technically accurate, but the former phrase is widely accepted.

      At first sight, codes seem to offer more security than ciphers, because words are much less vulnerable to frequency analysis than letters. To decipher a monoalphabetic cipher you need only identify the true value of each of the twenty-six characters, whereas to decipher a code you need to identify the true value of hundreds or even thousands of codewords. However, if we examine codes in more detail, we see that they suffer from two major practical failings when compared with ciphers. First, once the sender and receiver have agreed upon the twenty-six letters in the cipher alphabet (the key), they can encipher any message, but to achieve the same level of flexibility using a code they would need to go through the painstaking task of defining a codeword for every one of the thousands of possible plaintext words. The codebook would consist of hundreds of pages, and would look something like a dictionary. In other words, compiling a codebook is a major task, and carrying it around is a major inconvenience.

      Second, the consequences of having a codebook captured by the enemy are devastating. Immediately, all the encoded communications would become transparent to the enemy. The senders and receivers would have to go through the process of having to compile an entirely new codebook, and then this hefty new book would have to be distributed to everyone in the communications network, which might mean securely transporting it to every ambassador in every state. In comparison, if the enemy succeeds in capturing a cipher key, then it is relatively easy to compile a new cipher alphabet of twenty-six letters, which can be memorized and easily distributed.

      Even in the sixteenth century, cryptographers appreciated the inherent weaknesses of codes and instead relied largely on ciphers, or sometimes nomenclators. A nomenclator is a system of encryption that relies on a cipher alphabet, which is used to encrypt the majority of a message, and a limited list of codewords. For example, a nomenclator book might consist of a front page containing the cipher alphabet, and then a second page containing a list of codewords. Despite the addition of codewords, a nomenclator is not much more secure than a straightforward cipher, because the bulk of a message can be deciphered using frequency analysis, and the remaining encoded words can be guessed from the context.