Edited by author 03-03-2003 02:11 PM
One other thing. The reason you cannot reuse a one-time pad is easy to show. Given a plaintext P and a key K you obtain a ciphertext C by using XOR:

C = P XOR K

You transmit C and your partner knows K. He recovers P by XORing again.

C XOR K = P XOR K XOR K = P

XORing twice gives you the original.

Let's say you were stupid enough to send another transmission, Q, using the same K.

D = Q XOR K

Your attacker presumably heard C and D, and wants to know what P and Q are. He can get you easily with the following by XORing the two ciphertexts, as follows:

C XOR D = P XOR K XOR Q XOR K
        = (P XOR Q XOR Q) XOR K XOR Q XOR K
        = P XOR Q XOR Q XOR K XOR Q XOR K
        = P XOR Q

It is trivial to attack the XOR of plaintexts P and Q using frequency analysis. Note that this is NOT the same thing as the Prizm boys imply. I need NOT know the plaintext in advance, I only need to intercept TWO ciphertext transmissions. From that I get BOTH plaintexts. I can get the key after that if I want, but who gives a fuck about the key? I already own you and the missiles are on their way.