[RC5] RC5 core questions

John Girvin john.girvin at unibol.com
Fri Jan 22 13:14:02 EST 1999


if you look at the source code to any of the RC5 cores for 32
bit processors ( http://www.distributed.net/cores/ ) you will
see that they work by encrypting the given plaintext with the
key being tested, then comparing the cyphertext produced with
the expected result 32 bits at a time (high 32 bits are only
checked if the low 32 bits match).

I was wondering:
1. is it possible and would there be any performance benefit
   in implementing a core that *decrypts* the given contest
   cyphertext with the key being tested and compares it to the
   plaintext? I mean, is RC5 decryption any easier than RC5
   encryption? Would this be compatible with the rest of the
   client/network? Would it even work!?

2. are there any statistics on how often you could expect the
   low 32 bits of the produced cyphertext to match the low 32
   bits of the expected result? eg: 50% of keys, 25%, 10% ... ?

Just looking to squeeze more keys/s out of the cores :)

/John (m68k cores)
John Girvin
girv at girvnet.freeserve.co.uk

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