[RC5] RSA Secret-Key Challenge Termination

[Robert A. Rosenberg]

At 18:21 -0600 on 05/21/2007, Open Phugu wrote about Re: [RC5] RSA 
Secret-Key Challenge Termination:

>Once the key is found, anyone can decrypt the ciphertext with the key
>and if the plaintext starts with ``The unknown message is:'', we have found
>the key. RSA's role is only to verify that we have found the key and give us
>the prize money.

At 20:23 -0400 on 05/21/2007, Scott Dodson wrote about Re: [RC5] RSA 
Secret-Key Challenge Termination:

>Only they'd be able to confirm it for sure, however, assuming that
>there's only one key that produces coherent verbage it could be assumed
>we've found it.


Given the way the RC5 cypher hashes, I think the string "The unknown 
message is:" is longer than the chunk size so that the first part of 
the string is hashed/encoded separately from the the next section 
(which would include the start of the actual payload). Thus if you 
recover a message that begins "The unknown message is:", I think that 
this would be a unique result that could not be the result of using 
any other key. I MAY be wrong but that is my impression. A backup 
check would be the actually read with the Mark-1 Eyeball (as opposed 
to a computer program) the complete text to see that it is coherent.