"Karl A. Siil" writes:
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.
DES (and other block ciphers) has a couple of modes that let you do this -- CFB mode and OFB mode come to mind. Their security hasn't been that well studied to my knowledge. However...
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.
??? You ought to explain your application more clearly; it isn't necessarily the case that a stream cipher is appropriate for you. .pm