ACA and YOU : Chapter 6

How to use a computer

You may have ideas of using a computer to solve the ciphers shown in The Cryptogram. In many instances, this is very practicable - at least to assist in some of the leg-work, if not to obtain complete solution. There are even programs available for those who have computers, but no ability or desire to do their own programming. In these cases, the computer is used as a tool just like a pencil.

As the computer must be programmed for a particular set of circumstances, there will always be a possibility that a manual encipherment will have a twist that is outside the range of the particular program. You should not therefore expect a 100% success rate; a study of why a cipher cannot be solved may lead to a better program, and this is one of the challenges of using a computer.

Some types of cipher can use a blend of human thought and machine trial of the solution very successfully. The Aristocrat is an example. Programs are available which will replace a chosen letter in all its occurrences in the text, saving a great deal of paper and pencil work. Other types of cipher will yield to "brute force" trials, in which every possible combination is tried by the computer until a valid answer is obtained. Examples of this type are Caesar ciphers and some Cryptarithmetic puzzles. A human would rarely wish to use such a method, and part of the computer challenge is to see whether a short-cut can be programmed.

The computer can be very helpful in placing tips at the correct places in ciphertexts, providing an entry for cryptanalysis. If a tip is not available, there are programs that will run letter frequency counts, distributions, di- and trigram frequencies, indices of coincidence, etc., which not only assist with the cryptanalysis, but may enable the type of cipher to be determined (if unknown) and possibly the original language as well.

The computer naturally lends itself to the solution of ciphers produced by mechanical or computer means. Some of the famous machines of W.W.II, such as the Enigma, can be duplicated quite simply on small home computers. However, as these ciphers are usually outside the range of paper and pencil solution, they are not normally considered in The Cryptogram.

The computer will, of course, do a very fast job of encipherment, and in some cases a suitable enciphering program will lead to the design of a better deciphering algorithm.

It is not usually vital that the computer program run at maximum possible speed. An answer in half an hour, or a trial running all night, is not unreasonable, so many of the published programs deliberately use a very unsophisticated BASIC in order to be compatible with the largest number of machines (and the largest number of computerists!). For the same reason, it is not necessary to have the latest, biggest, or fastest computer; much good work is still being done with old first-generation machines. The size of the programs is often quite small. Of course, if you wish to hold and 80,000 word dictionary in memory to help with patternword searches, you will need a large machine; and if you wish to try a brute force technique, you may be glad of a faster language than BASIC.

A Computer Supplement is published by the ACA for the benefit of computerists, particularly beginners in both ciphers and computers. A regular Computer Column also appears in The Cryptogram for more advanced computerists. A comprehensive reading list is available from the editor of the Computer Supplement.