On Sunday,2009-07-19, at 13:24 , Paul Hoffman wrote:

At 7:54 AM -0600 7/18/09, Zooko Wilcox-O'Hearn wrote:
This involves deciding whether a 192-bit elliptic curve public key is strong enough...

Why not just go with 256-bit EC (128-bit symmetric strength)? Is the 8 bytes per signature the issue, or the extra compute time?

Those are two good guesses, but no. The main concern is the size of the public key. This is why (if I understand correctly), hyperelliptic curves might eventually offer public key signatures which are twice as good for the purposes of TahoeLAFS as elliptic curves. (By which I mean, the keys are half as big.) I discussed this topic a bit in a subsequent message to the cryptography mailing list entitled "Why hyperelliptic curves?".

Actually, the computation time matters, too. Our measurements on an ARM 266 MHz embedded system showed a significant penalty for 256-bit ECDSA vs. 192-bit:




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