Signed-digit multi-comb for ecmult_gen #546
+414
−24
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -225,13 +225,14 @@ static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp25 | ||
| bit_pos = comb_off; | ||
| for (block = 0; block < COMB_BLOCKS; ++block) { | ||
| #if COMB_GROUPED | ||
| bits = recoded[bit_pos >> 5] >> (bit_pos & 0x1F); | ||
| bit_pos += COMB_TEETH; | ||
| #else | ||
| bits = 0; | ||
| for (tooth = 0; tooth < COMB_TEETH; ++tooth) { | ||
| bit = recoded[bit_pos >> 5] >> (bit_pos & 0x1F); | ||
| bits &= ~(1 << tooth); | ||
|
There was a problem hiding this comment. I think this bit-clearing line isn't actually needed, because There was a problem hiding this comment. Note that ‘bit’ may now contain junk (or noise) bits beyond bit 0. Iterations after the first therefore need to clear the target bit (and we try to leave noise in the high bits). The change is in response to [1], which I think @gmaxwell mentioned on IRC a while back. [1] https://www.usenix.org/system/files/conference/usenixsecurity18/sec18-alam.pdf There was a problem hiding this comment. Ah! I understand, I was misreading There was a problem hiding this comment. Because the variable is named |
||
| bits ^= bit << tooth; | ||
| bit_pos += COMB_SPACING; | ||
| } | ||
| #endif | ||
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Should this be done as part of a cmov ladder, like the old algorithm, to avoid cache-timing attacks?
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This line is reading a window from the scalar. IIUC, the cmov ladder you mean is the _ge_storage_cmov loop at lines 255-257.
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Oh, yes! I missed that - you do indeed have a cmov ladder at the point where the secret data is actually used. Thanks.