frost/secp256k1-zkp
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Eliminate harmless non-constant time operations on secret data.

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There were several places where the code was non-constant time
 for invalid secret inputs.  These are harmless under sane use
 but get in the way of automatic const-time validation.

(Nonce overflow in signing is not addressed, nor is s==0 in
 signing)
This commit is contained in:
Gregory Maxwell
2020-01-11 01:01:05 +00:00
parent 856a01d6ad
commit 34a67c773b
16 changed files with 154 additions and 101 deletions
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17
src/ecdsa_impl.h
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@@ -280,6 +280,7 @@ static int secp256k1_ecdsa_sig_sign(const secp256k1_ecmult_gen_context *ctx, sec
secp256k1_ge r;
secp256k1_scalar n;
int overflow = 0;
int high;
secp256k1_ecmult_gen(ctx, &rp, nonce);
secp256k1_ge_set_gej(&r, &rp);
@@ -295,7 +296,7 @@ static int secp256k1_ecdsa_sig_sign(const secp256k1_ecmult_gen_context *ctx, sec
/* The overflow condition is cryptographically unreachable as hitting it requires finding the discrete log
* of some P where P.x >= order, and only 1 in about 2^127 points meet this criteria.
*/
*recid = (overflow ? 2 : 0) | (secp256k1_fe_is_odd(&r.y) ? 1 : 0);
*recid = (overflow << 1) | secp256k1_fe_is_odd(&r.y);
}
secp256k1_scalar_mul(&n, sigr, seckey);
secp256k1_scalar_add(&n, &n, message);
@@ -304,16 +305,12 @@ static int secp256k1_ecdsa_sig_sign(const secp256k1_ecmult_gen_context *ctx, sec
secp256k1_scalar_clear(&n);
secp256k1_gej_clear(&rp);
secp256k1_ge_clear(&r);
if (secp256k1_scalar_is_zero(sigs)) {
return 0;
high = secp256k1_scalar_is_high(sigs);
secp256k1_scalar_cond_negate(sigs, high);
if (recid) {
*recid ^= high;
}
if (secp256k1_scalar_is_high(sigs)) {
secp256k1_scalar_negate(sigs, sigs);
if (recid) {
*recid ^= 1;
}
}
return 1;
return !secp256k1_scalar_is_zero(sigs);
}
#endif /* SECP256K1_ECDSA_IMPL_H */