Introduction To Codes, Ciphers, & Codebreaking
by Greg Goebel

1.5] CRACKING CODES / CODES VERSUS CIPHERS
* Solving a monoalphabetic substitution cipher is easy. Solving even a simple code is difficult. Decrypting a coded message is a little like trying to translate a document written in an alien language, with the task basically amounting to building up a "dictionary" of the codegroups and the plaintext words they represent.

One fingerhold on a simple code is the fact, mentioned in the previous section, that some words are more common than others, such as "the" or "a" in English. In telegraphic messages, the codegroup for "STOP" (end of sentence) is usually very common. This helps define the structure of the message in terms of sentences, if not their meaning.

Further progress can be made against a code by collecting many messages encrypted with the same code and then obtaining intelligence background on the messages, such as the location from where a message was sent, and where it was being sent to; the time the message was sent; events occurring before and after the message was sent; and the normal habits of the people sending the coded messages. For example, a particular codegroup found almost exclusively in messages from a particular army and nowhere else might very well indicate the commander of that army. A codegroup that appears in messages preceding an attack on a particular location may very well stand for that location.

Of course, cribs are an immediate giveaway to the definitions of codegroups. As codegroups are determined, they gradually build up a critical mass, with more and more codegroups revealed from context and educated guesswork. One-part codes are more vulnerable to such educated guesswork than two-part codes, since if the codenumber "26839" of a one-part code is determined to stand for "bulldozer", then the lower codenumber "17598" must stand for a plaintext word that starts with "a" or "b".

top>

 

Various tricks can be used to "plant" or "sow" information into a code, for example by executing a raid at a particular time and location against an enemy, and then examining code messages in response to the raid. Coding errors are a particularly useful fingerhold into a code, and naturally people are bound to make errors, sometimes disastrous ones, sooner or later. Of course, planting data and exploiting errors works against ciphers as well.

* The most obvious and, in principle at least, simplest way of cracking a code is to steal the codebook through bribery, burglary, or raiding parties -- procedures sometimes glorified by the phrase "practical cryptology" -- and this is the weakness of a code. While a good code may be harder to break than a cipher, the need to write and distribute codebooks is troublesome.

Constructing a new code is like building a new language and writing a dictionary for it, which is a big job. If a code is compromised, the whole task has to be done all over again, and that means a lot of work for both cryptographers and the code users. In practice, when codes were in widespread use, they were changed on a periodic basis to frustrate codebreakers.

Once codes have been created, their distribution is logistically clumsy, and makes it likely that the code will be compromised. There is a saying that two people can keep a secret if one of them is dead, and though that may be something of an exaggeration, a secret becomes harder to keep if it is shared among more people. Codes can be reasonably secure if they are only used between a few people, but if whole armies use the same code keeping them secure becomes that much more difficult.

In contrast, the security of ciphers is, as mentioned earlier, generally dependent on protecting the cipher keys. Cipher keys can be stolen and people can betray them, but they are much easier to change and communicate.

1 2 3 4 5 6