Polyalphabetic Ciphers

Anyone have any experience with cryptography? I don't have the time to go too deep into number theory to work with RSA or anything of that nature, but I have read a few books on simple ciphers, and have written a simple program to handle polyalphabetic ciphers. I choose them because they are the one code system I have found that is perfectly invulnerable to passive attacks. I have even incorporated them into some of my programs to handle registration of software. Anyone have experience / interest in cryptography.

Chris

I do have an interest in cryptography and have been examining the RSA model, which seems pretty tough to break. I have been wondering if an extrapolation of it or a similar concept could be used to create a truly unbreakable code, one that would be mathematically impossible for the government to intercept, even if they had the "key"? It would apparently have to involve a passive third party that held a crucial part of the key, unavailable even to the individual wishing to decypher the messege except under certain and non-duplicatable circumstances.

I am definitely no expert, but here is my understanding. First you have to seperate physical attacks (trying to obtain the key from one of the individuals or intercept it) and passive attacks (looking at the encoded messages only and trying to break it). From a passive attack, the polyalphabetic cipher is the only one I have heard of that is perfectly unbreakable.

With most encoding methods, there are methods of breaking them, even if it entails a computer taking a million years. I have a book on simple ciphers, and the ways you break them are fascinating, including doing statistical analysis on the frequency of occurance of letters, and matching the coded letters to the frequency of occurance of letters of the uncoded message in a particular language. For instance, lets say "t" occurs with frequency 7% in the English language, and you have a "q" with 7% in the message. With the polyalphabetic cipher, each letter in the message is encoded under a different key. This of course makes it less than practical, but still unbreakable.

Of course, there can never be a system that is physically unbreakable. That relies on human security.