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refactor: introduce _ecmult_gen_ge helper (preventing accidental gej leaks)

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Scalar multiplication with the generator point frequently involves a
conversion to affine coordinates and clearing out the temporary Jacobian
group element object after to avoid leaking secret key material, i.e.
executing the following three steps:
    - secp256k1_ecmult_gen(ctx, &rj, ...)
    - secp256k1_ge_set_gej(&r, &rj)
    - secp256k1_gej_clear(&rj)

This commit introduces a corresponding helper to deduplicate code
and mitigate the risk that last step is forgotten (which can easily
happen and is not detected by tests).

The idea came up during a conversation with furszy, see
https://github.com/bitcoin-core/secp256k1/pull/1765#issuecomment-4482838033
This commit is contained in:
Sebastian Falbesoner
2026-05-19 16:29:05 +02:00
parent c63062380f
commit a3296d5e23
5 changed files with 14 additions and 16 deletions
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5
src/ecdsa_impl.h
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@@ -273,14 +273,12 @@ static int secp256k1_ecdsa_sig_verify(const secp256k1_scalar *sigr, const secp25
static int secp256k1_ecdsa_sig_sign(const secp256k1_ecmult_gen_context *ctx, secp256k1_scalar *sigr, secp256k1_scalar *sigs, const secp256k1_scalar *seckey, const secp256k1_scalar *message, const secp256k1_scalar *nonce, int *recid) {
unsigned char b[32];
secp256k1_gej rp;
secp256k1_ge r;
secp256k1_scalar n;
int overflow = 0;
int high;
secp256k1_ecmult_gen(ctx, &rp, nonce);
secp256k1_ge_set_gej(&r, &rp);
secp256k1_ecmult_gen_ge(ctx, &r, nonce);
secp256k1_fe_normalize(&r.x);
secp256k1_fe_normalize(&r.y);
secp256k1_fe_get_b32(b, &r.x);
@@ -296,7 +294,6 @@ static int secp256k1_ecdsa_sig_sign(const secp256k1_ecmult_gen_context *ctx, sec
secp256k1_scalar_inverse(sigs, nonce);
secp256k1_scalar_mul(sigs, sigs, &n);
secp256k1_scalar_clear(&n);
secp256k1_gej_clear(&rp);
secp256k1_ge_clear(&r);
high = secp256k1_scalar_is_high(sigs);
secp256k1_scalar_cond_negate(sigs, high);