One more post on encryption before moving on to other things.

In Part I, I submitted an encryption program that "should prevent the casual snoop from decoding your files." The program's security can be increased tremendously by using the random number generator to generate the key. As it stands the key must be typed in on the command line, a feature that encourages the use of short keys. When the program gets to the end of the key it goes back to the key's beginning and reuses it. In our example we used "buttermilk" as the key. In a file of 500 characters a codebreaker would then get 50 clues to the key.

The random number generator will produce sequences of tens of thousands of characters before it begins to repeat itself. The first problem we have to solve is how to get the random number generator to generate the same sequence during decoding. We solve this by giving the program a "pin" of four characters. Since each number is 8 bits we have a 32 bit seed or 4,294,967,296 possible keys. Of course the user is most likely to use pins made up of alphanumeric characters so there are more like 15,000,000 most probable keys. A user who limits himself to keys made up of easily remembered words limits himself even further. Nevertheless, the number of most probable keys is still in the 100s of thousands...