[RC5] What do we cracking?

Ian Clelland iclelland at centralindustrial.com
Thu Sep 30 12:44:34 EDT 1999


At 10:22 AM 9/27/99 +0200, you wrote:
>Hi!
>Does someone know what look information that we cracking? Where is that
>endoded information?
>I've search d.net and RSA without success ...
>Greetings
>Artur

 From the RSALabs site ( 
http://www.rsasecurity.com/rsalabs/challenges/secretkey/secret-key.html ),
this is the contest d.net is currently working on:


Contest identifier: RC5-32/12/8
Cipher: RC5-32/12/8 (RC5 with 32-bit wordsize, 12 rounds, and 8*8=64-bit key)
Start of contest: 28 January 1997, 9 am PST
State of contest: ongoing
IV: 79 ce d5 d5 50 75 ea fc
Hexadecimal ciphertext:

  bf 55 01 55 dc 26 f2 4b 26 e4 85 4d f9 0a d6 79
66 93 ab 92 3c 72 f1 37 c8 b7 0d 1f 60 11 0c 92
ae 2e cd fd 70 d3 fd 17 df b0 42 12 b9 7d cf 22
18 6b a7 15 ce 2c 84 bf ce 0d d0 4d 00 6b e1 46


That long hex string is the encoded message; the only other information we 
have is that the message begins with "The unknown message is: ", and the 
Initialisation Vector shown above, which (AFAIK; someone correct me if I'm 
mistaken here) is XOR-ed with the first block of the plaintext before it 
was encrypted.

Our brute force attack means that we can ignore everything but the first 
block (64 bits) and our problem is reduced to this:

1. First 8 characters of plaintext: "The unkn" = 54 68 65 20 75 6e 6b 6e
2. The initialisation vector: 79 ce d5 d5 50 75 ea fc
3. XOR of (1) and (2): 2d a6 b0 f5 25 1b 81 92
4. First block of ciphertext: bf 55 01 55 dc 26 f2 4b

All we have to do is find the one key, which, given the input in (3) will 
produce the output (4). The first person or group to do that gets $10K from 
RSALabs



Ian Clelland
Central Industrial Wire

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