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Re: [openpgp] Followup on fingerprints

2015-08-03 11:59:18
On Mon, Aug 3, 2015 at 3:08 PM, Derek Atkins <derek(_at_)ihtfp(_dot_)com> wrote:
Remember, the fingerprint is over the public key, so you still have to
actually perform the ECC g^x operation for each trial.

Take care to not confuse what you would do with what an attacker _must_ do.

For each new key to generate the attacker can perform only a single
addition of G or a doubling (whichever is faster for the curve in
question), then a conversion to affine (which is nearly free--
marginally, ~one field multiply-- if done in a batch).

E.g. You compute,
P_0 = xG
P_1 = P_0 + G  (x_1 = x_0 + 1)
P_2 = P_1 + G  (x_2 = x_1 + 1)
...

There are even faster techniques available for some curves.

If software for this doesn't run in the rough ballpark of a million
per second on a current gen laptop/desktop or 10 million/sec on a GPU
even on a fairly generic curve, it's probably completely naieve.

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