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Instead of having the starting point of the ecmult_gen computation be offset, do it with the final point. This enables reasoning over the set of points reachable in intermediary computations, which can be leveraged by potential future optimization. Because the final point is in affine coordinates, its projective blinding is no longer possible. It will be reintroduced again in a different way, in a later commit. Also introduce some more comments and more descriptive names.
The old code overwrote the input at the start of the function, making a call like secp256k1_scalar_inverse(&x,&x) always fail.
This introduces the signed-digit multi-comb multiplication algorithm for constant-time G multiplications (ecmult_gen). It is based on section 3.3 of "Fast and compact elliptic-curve cryptography" by Mike Hamburg (see https://eprint.iacr.org/2012/309). Original implementation by Peter Dettman, with changes by Pieter Wuille to use scalars for recoding, and additional comments.
It is unnecessary to recompute this term needed by the SDMC algorithm for every multiplication; move it into the context scalar_offset value instead.
The old code would trigger UB when count=32.
The existing code needs to deal with the edge case that bit_pos >= 256, which would lead to an out-of-bounds read from secp256k1_scalar. Instead, recode the scalar into an array of uint32_t with enough zero padding at the end to alleviate the issue. This also simplifies the code, and is necessary for a security improvement in a follow-up commit. Original code by Peter Dettman, with modifications by Pieter Wuille.
Co-authored-by: Tim Ruffing <crypto@timruffing.de>
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jonasnick
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| SECP256K1_SCALAR_VERIFY(r); |
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I believe this is unrelated to #1058. Can you do that in a separate PR?
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I believe this is unrelated to #1058. Can you do that in a separate PR?
| *r = v % EXHAUSTIVE_TEST_ORDER; | ||
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| secp256k1_scalar_verify(r); | ||
| SECP256K1_SCALAR_VERIFY(r); |
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I believe this is unrelated to #1058. Can you do that in a separate PR?
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| echo " ecmult window size = $set_ecmult_window" | ||
| echo " ecmult gen prec. bits = $set_ecmult_gen_precision" | ||
| echo " ecmult gen table size = $set_ecmult_gen_kb KiB" | ||
| # Hide test-only options unless they're used. | ||
| if test x"$set_widemul" != xauto; then | ||
| echo " wide multiplication = $set_widemul" |
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Merge bitcoin-core/secp256k1#1058: Signed-digit multi-comb ecmult_gen algorithm
This PR can be recreated with
./contrib/sync-upstream.sh -b master range da51507.Tip: Use
git show --remerge-diffto show the changes manually added to the merge commit.