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[John Ioannidis: Alleged RC4 speed and key setup times]



John Ioannidis sent me the following interesting message. He's been
playing a bit with the alleged RC4 code posted anonymously to
cypherpunks a while back. He gave me permission to forward this to
cypherpunks.

Perry

------- Forwarded Message

From: John Ioannidis <[email protected]>
Subject: Alleged RC4 speed and key setup times


The key setup time for the alleged rc4 code is the same as the time to
encrypt about 1200 bytes of data. Key setup time is independent of key
length (if you discount possible cache interactions). I haven't tried to
optimize (other than what the compiler (gcc -O4) can do) the key setup
time, the way I optimized the cipher, but still...

Anyway, you can do slightly over 4000 key setups per second on a
S10/51 (the time to en/decrypt a 64-byte block is trivial), so you can
test 2^12 keys per second on a S10, or 2^12*16 = 2^16 keys/sec on a
16-processor SparcCenter-1000. That's 2^16 * 2^17 keys per day, so you
need 2^9 days to exhaustively search a 40-bit keyspace on a fairly
common machine such as the S1000. Move that back down to 2^13 days for
a S10; The CS dept easilly has 2^8 machines with the average speed of
a S10 (some are slower, some are a LOT faster, and tehy have more than
256 machines anyway). That means that, by dedicating the CS dept's
resources, we can crack a 40-bit key in 2^7 days (2^6 on the average,
of course). As another example, Lehman has 2^11 S10 equivalents, so
that figure would go down to 2^2 days or a 4-day weekend.

Well, the above goes to prove that 40bits isn't all that much (which
we already knew), but I wanted to get a feel of what it means it terms
of resources that mere mortals like us have access to.

/ji

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