[HARDWARE] ongoing work

Matthew Smart mcsmart at engin.umich.edu
Wed Apr 28 12:10:47 EDT 1999

First, thanks to everyone for their comments.  I believe some notions of
what RC5 is and isn't and how the project is working were cleared up.

Second, in my original message I misspoke/mistyped and said you'd only have
to encrypt or decrypt "The ", but of course that's only the first 4 bytes.
As has been pointed out a few times, for RC5-32/12/8 the smallest block of
data to encrypt is 8 bytes.  It should be "The unkn" instead.

Third, the point about the IV is important, but doesn't change much.  It
simply adds an XOR at the beginning of the encryption algorithm or an XOR
and the end of the decryption algorithm.

Fourth, the code I wrote to encrypt and decrypt works on one of RSA Labs
sample (40-bit) tests, and numerous other published examples (RFC 2040,
Rivest's RC5 paper).


-----Original Message-----
From: owner-hardware at lists.distributed.net
[mailto:owner-hardware at lists.distributed.net]On Behalf Of Greg Hewgill
Sent: Wednesday, April 28, 1999 10:20 AM
To: hardware at lists.distributed.net
Subject: Re: [HARDWARE] ongoing work

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
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
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

The object of the contest is to recover the remainder of the plaintext, and
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
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
try all the keys.

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

The RSA page outlining the challenges is:


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

Greg Hewgill
distributed.net Coding Team
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