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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34
src/modules/ecdh/main_impl.h
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@@ -32,36 +32,40 @@ int secp256k1_ecdh(const secp256k1_context* ctx, unsigned char *output, const se
secp256k1_gej res;
secp256k1_ge pt;
secp256k1_scalar s;
unsigned char x[32];
unsigned char y[32];
VERIFY_CHECK(ctx != NULL);
ARG_CHECK(output != NULL);
ARG_CHECK(point != NULL);
ARG_CHECK(scalar != NULL);
if (hashfp == NULL) {
hashfp = secp256k1_ecdh_hash_function_default;
}
secp256k1_pubkey_load(ctx, &pt, point);
secp256k1_scalar_set_b32(&s, scalar, &overflow);
if (overflow || secp256k1_scalar_is_zero(&s)) {
ret = 0;
} else {
unsigned char x[32];
unsigned char y[32];
secp256k1_ecmult_const(&res, &pt, &s, 256);
secp256k1_ge_set_gej(&pt, &res);
overflow |= secp256k1_scalar_is_zero(&s);
secp256k1_scalar_cmov(&s, &secp256k1_scalar_one, overflow);
/* Compute a hash of the point */
secp256k1_fe_normalize(&pt.x);
secp256k1_fe_normalize(&pt.y);
secp256k1_fe_get_b32(x, &pt.x);
secp256k1_fe_get_b32(y, &pt.y);
secp256k1_ecmult_const(&res, &pt, &s, 256);
secp256k1_ge_set_gej(&pt, &res);
ret = hashfp(output, x, y, data);
}
/* Compute a hash of the point */
secp256k1_fe_normalize(&pt.x);
secp256k1_fe_normalize(&pt.y);
secp256k1_fe_get_b32(x, &pt.x);
secp256k1_fe_get_b32(y, &pt.y);
ret = hashfp(output, x, y, data);
memset(x, 0, 32);
memset(y, 0, 32);
secp256k1_scalar_clear(&s);
return ret;
return !!ret & !overflow;
}
#endif /* SECP256K1_MODULE_ECDH_MAIN_H */