Encryption and Shannon's Perfect Secrecy

I was reading Claude Shannon's 1949 paper titled Communication Theory of Secrecy Systems" and much like Kerchoff, many of his high level concepts are very relevant today. The section on "perfect secrecy" is very relevant today and many of his ideas linger in encryption ciphers like RC4 and in the wireless world. Symmetric encryption have been around for thousands of years and many seek the perfect encryption algorithmn. He identifies 3 high level types of secrecy systems including concealment systems, privacy systems, and true secrecy systems. Concealment systems might include invisible ink and can be summarized as an attempt to physically hide some sensitive information (Security by Obscurity). Privacy systems assume there some shared equipment that average eavesdroppers might not have while true secrecy systems assume anyone can intercept and record the cipher text or any encrypted transmission. Shannon's world of "true secrecy" is very similar to the security challenges we face on the Internet today.
Perfect Secrecy approaches the idea of "perfect encryption". It basically states that if the cipher text, encryption key, and the plain text are changing at a consistent rate -- you are approaching perfect secrecy. The theory states that if someone intercepts a cryptogram or some cipher text, that person could not deduce anything about the original plain text by studying the cipher text or its patterns. There is no pattern one could extrapolate from the cryptogram over time and the likelihood of someone guessing the plain text message is the same whether they observed and recorded the cryptogram or not. This means that any encrypted message that an attacker sees is completely meaningless and cannot be used to reconstruct the plain text or "the a priori probability of a plain text message M is the same as the a posteriori probability of a plain text message M given the corresponding cipher text." Brute force attacks simply do not reveal anything because the key is constantly changing. One Time Pad is a theoretetically unbreakable symmetric encryption cipher that share Shandy's "perfect secrecy" property.

Thanksgiving :
I went over to my folks for Thanksgiving this afternoon and then later this evening to the in-laws. It was nice spending time with the family and taking a day to be thankful for all the things we take for granted. Happy Thanksgiving 2005 everyone !