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© Copyright 1998, Jim Loy
A friend of mine says that any code (he means cipher) can be broken. Let's prove him wrong. Here's an encrypted message:
XMNB
You may guess the message correctly. But, I guarantee that nobody can logically deduce the message. I can even give you a clue, it is a simple substitution cipher (as used in Cryptograms).
Do you see what the problem is? The message isn't long enough. You don't have enough information to break the cipher.
There are much more complicated ciphers, with complicated keys. With one of these, you could encrypt an entire book and have it be unbreakable, because the message isn't long enough.
Now, let's prove that a message of any length can be unbreakable, if the cipher is complicated enough.
Let's say that if a message is more than 200 characters long, we can normally break the simple substitution cipher. And, let's say that if it is shorter than 150 characters, then we cannot break it. In between 150 and 200, we sometimes can break it and sometimes not. These numbers, 150 and 200, are just a wild guess. But, we can safely assume that there are two numbers for which the above statements are true. After all, we have found a 4-character message which cannot be broken, and we have all seen longer messages which can be broken.
Further, let's assume that I have a very long message (1,000,000,000 characters) that I want to encrypt. Here is how I will do that. I will encrypt the first 100 characters with one cipher, the second 100 characters with a second cipher, the third 100 characters with a third cipher, etc. We have encrypted our 1-billion character message with 10-million different ciphers. And, the whole thing is unbreakable. It's probably impractical, but it would work. The encrypted message is unbreakable.
What I am saying is that if the method of encryption is complicated enough, almost any encrypted message can be unbreakable.
A cipher is a method of substituting letters, to encrypt a message. A code requires a code book, which is a dictionary of codes and their corresponding words. See my story Sherlock Holmes and the Secret Code.