[RC5] Re: An idea to prevent cheating

Ryan Schmidt berrytech at bigfoot.com
Mon Aug 31 20:05:09 EDT 1998

>I just recalled all the trouble with Bitbucket because some people
>thought they were cheating in the DES-II-1 contest, and I thought about
>what could be done to prevent cheating. It is rather obvious, actually.
>This is the idea I got: we could simply compute a "checksum" over all
>the trial encryption results the client gets while he tries to find the
>right key. I.e. if the client is working on block 123456:00000000
>des(kp,123456:00000000) xor des(kp,123456:00000001) xor ...
>(kp is the known plaintext) and store the result it in the output block.
>If there is any doubt that a person or team processes the blocks
>properly, the blocks could be redone on a trusted machine and the
>checksums could be compared.
>I don´t see any way how this check could be worked around by a cheater.
>You only get the correct checksum by computing the encrypted string for
>all keys. It should help soften the paranoia in some people, and maybe
>then we could finally release the block data structure if we are sure
>that it won´t tempt people into cheating?

OK, I don't know if I understood your idea right, so correct me if I'm
wrong, but wouldn't that require that the client check each and every block?
Because if the client loses any block, then the computed checksum would be
incorrect. So if I download 100 blocks, lose 2 to a crash, but complete and
submit the other 98, 98 blocks would be thrown out because my checksum
wasn't the same as the checksum for all 100 blocks.

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