At 2:45 PM 4/22/96, Karl A. Siil wrote:

Folks,

As this sounds like a previously solved problem, I wanted to find out about using DES (or any block cipher) as a stream cipher, i.e., in a manner that keeps input and output data length equal. I don't want to use a true stream cipher, as I want to use the same key for multiple messages and stream ciphers tend to place the bulk of their overhead in the re-key. Since stream ciphers have "memory," I would have to "re-key" to the same key for each of my messages. I would rather key something like DES once and run it in CBC mode or use some other form of IV.

Help or pointers to help are greatly appreciated.

Just generate a stream of octets by running DES (or some other block cipher) in the following mode: C_-1 = IV C_n = E_K(C_n-1) i.e., start by encrypting the IV with your key, and thereafter generate a block by encrypting the previous block with your key. Here the IV is essentially part of the key. XOR the resulting bits (or octets) with your plaintext to generate an encrypted stream. Remember within your cipher's state the current output block and how much of it you've used. Note that if you reuse a key, you'll enable attacks that rely on the fact that similar messages will produce similar ciphertext. - Tim Dierks Tim Dierks -- timd@consensus.com -- www.consensus.com Head of Thing-u-ma-jig Engineering, Consensus Development