[HARDWARE] ongoing work

Greg Hewgill greg at hewgill.com
Wed Apr 28 08:19:43 EDT 1999


On Wed, Apr 29, 1998 at 02:22:20AM -0500, soul wrote:
> Okay so your Saying that the Clients and the keyservers have no idea what
> the right key says?
> 
> Then Could You Please explain Exactly how the D.net rc5 project works?
> There was a web page about it but i can't find it any more.....

In a symmetric cipher, there are three important pieces of data: (a) The
plaintext, (b) the ciphertext, and (c) the key. The encryption algorithm takes
the plaintext and key as input, and produces the ciphertext. The decryption
algorithm takes the ciphertext and key as input, and produces the plaintext.

We do know part of the plaintext and the full ciphertext. The part of the
plaintext we do know begins with "The unknown message is: ". The start of the
ciphertext is the hex bytes bf 55 01 55 dc 26 f2 4b (for the full data, see
http://www.rsa.com/rsalabs/97challenge/html/secret-key.html, contest identifier
RC5-32/12/8).

The object of the contest is to recover the remainder of the plaintext, and the
key originally used to encrypt the data. This is called a "known-plaintext"
attack. We know what part of the plaintext says, and we want to find out what
the rest of it says. To do this we need to discover what key was used to
encrypt the plaintext. The most direct method of finding this key is to simply
try all the keys.

There is actually a fourth piece of data involved in the encryption called the
Initial Value vector, or IV, but that has more to do with encrypting a block of
data than it does with the fundamental algorithm.

The RSA page outlining the challenges is:

  http://www.rsa.com/rsalabs/97challenge/

An excellent reference book for all crypto topics is Bruce Schneier's "Applied
Cryptography". Highly recommended reading.

Greg Hewgill
distributed.net Coding Team
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