Add self-contained wNAF scalar multiplication in primeorder#1690
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tob-scott-a wants to merge 1 commit intoRustCrypto:wnaffrom
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Add self-contained wNAF scalar multiplication in primeorder#1690tob-scott-a wants to merge 1 commit intoRustCrypto:wnaffrom
tob-scott-a wants to merge 1 commit intoRustCrypto:wnaffrom
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The upstream `group::Wnaf` has two bugs that break SEC1/NIST curves: 1. `wnaf_form()` assumes `Scalar::to_repr()` returns little-endian bytes, but `ff::PrimeField` documents repr endianness as implementation-specific. All SEC1/NIST curves use big-endian. 2. `wnaf_form()` drops the final carry when the scalar fills all `bit_len` bits. This is masked on BLS12-381 (255-bit modulus in 256-bit repr) but causes wrong results for p256/k256/p384/p521. Rather than depending on an upstream group crate fix, this adds `primeorder::wnaf::WnafScalarMul` — a self-contained wNAF that handles big-endian repr and the carry correctly. The `WnafGroup` impl on `ProjectivePoint` is filled in (was `todo!()`) but warns that `group::Wnaf` itself remains broken for these curves. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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This sort of approach seems like a reasonable stopgap, but I'd probably prefer to vendor |
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The upstream
group::Wnafhas two bugs that break SEC1/NIST curves:wnaf_form()assumesScalar::to_repr()returns little-endian bytes, butff::PrimeFielddocuments repr endianness as implementation-specific. All SEC1/NIST curves use big-endian.wnaf_form()drops the final carry when the scalar fills allbit_lenbits. This is masked on BLS12-381 (255-bit modulus in 256-bit repr) but causes wrong results for p256/k256/p384/p521.Rather than depending on an upstream group crate fix, this adds
primeorder::wnaf::WnafScalarMul— a self-contained wNAF that handles big-endian repr and the carry correctly.The
WnafGroupimpl onProjectivePointis filled in (wastodo!()) but warns thatgroup::Wnafitself remains broken for these curves.This is an alternative to #1689 that doesn't rely on an external change to the group crate.