ecmult_gen: add variable-time generator point multiplication (no API behaviour changes yet) - #1883
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ecmult_gen: add variable-time generator point multiplication (no API behaviour changes yet)#1883theStack wants to merge 2 commits into
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Having this available as a global constant allows to introduce an alternative `ecmult_gen_gej` function that doesn't need access to a context, see next commit. Note that the precomputed constant takes the name of the function that previously generated it at run-time (`secp256k1_ecmult_gen_scalar_diff`), while the function is now renamed to include the "compute" verb (`secp256k1_ecmult_gen_compute_scalar_diff`), to match the naming of the table generation function. Can be reviewed with `--color-moved=dimmed-zebra` for easier checking of the move-only parts.
Add faster variable-time variants for generator point multiplication. This is essentially `ecmult_gen` without side-channel mitigations and without requiring a context object. Intended for use cases where the scalar is not representing sensitive data. On my arm64 machine, this is ~86% faster than the constant-time variant (with the default build table size, i.e. ECMULT_GEN_KB=86): ``` $ ./build/bin/bench_ecmult Benchmark , Min(us) , Avg(us) , Max(us) ecmult_gen , 9.32 , 9.35 , 9.60 ecmult_gen_var , 5.02 , 5.02 , 5.02 ..... ```
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The PR is a rebased subset of #1843: it adds a variable-time variant for generator point multiplication, with the idea of enabling potential performance gains for use cases where the scalar isn't treated as secret. The implementation is essentially a copy of the existing
secp256k1_ecmult_gen_gejfunction, but with all side-channel mitigations (i.e., constant-time code, random scalar blinding, and memory clearing) removed. The EC point operation calls (addition, doubling) are replaced with their faster variable-time equivalents.The first commit moves the calculation of the
ecmult_gen_scalar_diffconstant (scalar(2^COMB_BITS - 1) / 2) from run-time to compile-time, in order to avoid needing a context object and thus enable computing the result statelessly.On my arm64 machine, the variable-time variant is ~86% faster than the constant-time variant (with the default build table size, i.e. ECMULT_GEN_KB=86):
Note that in contrast to #1843, this PR doesn't take use of the new functions in any user-facing code (API functions) yet, to keep discussions about potential behavior changes w.r.t. constant-timing guarantees separated. The only visible change for users after merging this PR would be that the library is a tiny bit larger, as the generated tables now also contain the
ecmult_gen_scalar_diffconstant. On my arm64 machine, this leads to an increase of 56 bytes for the .so file (1392672 bytes master vs. 1392728 bytes PR on a default release build), which I think we can treat as negligible.One relatively obvious scenario where using the variable-time variant would make sense is for verifying P2TR script-paths spends in Bitcoin Core via
secp256k1_xonly_pubkey_tweak_add_check(see #1843 (comment)). However, whether to change the behavior of the existing API function or adding a new one (e.g. with_varsuffix) still has to be decided and might be a topic for a different PR (see #1843 (comment)). Another scenario could be scanning of Silent Payments, though in this case it has to be carefully evaluated whether it's safe to treat the tweak t_k as non-secret.