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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19
src/field_10x26_impl.h
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@@ -320,6 +320,7 @@ static int secp256k1_fe_cmp_var(const secp256k1_fe *a, const secp256k1_fe *b) {
}
static int secp256k1_fe_set_b32(secp256k1_fe *r, const unsigned char *a) {
int ret;
r->n[0] = (uint32_t)a[31] | ((uint32_t)a[30] << 8) | ((uint32_t)a[29] << 16) | ((uint32_t)(a[28] & 0x3) << 24);
r->n[1] = (uint32_t)((a[28] >> 2) & 0x3f) | ((uint32_t)a[27] << 6) | ((uint32_t)a[26] << 14) | ((uint32_t)(a[25] & 0xf) << 22);
r->n[2] = (uint32_t)((a[25] >> 4) & 0xf) | ((uint32_t)a[24] << 4) | ((uint32_t)a[23] << 12) | ((uint32_t)(a[22] & 0x3f) << 20);
@@ -331,15 +332,17 @@ static int secp256k1_fe_set_b32(secp256k1_fe *r, const unsigned char *a) {
r->n[8] = (uint32_t)a[5] | ((uint32_t)a[4] << 8) | ((uint32_t)a[3] << 16) | ((uint32_t)(a[2] & 0x3) << 24);
r->n[9] = (uint32_t)((a[2] >> 2) & 0x3f) | ((uint32_t)a[1] << 6) | ((uint32_t)a[0] << 14);
if (r->n[9] == 0x3FFFFFUL && (r->n[8] & r->n[7] & r->n[6] & r->n[5] & r->n[4] & r->n[3] & r->n[2]) == 0x3FFFFFFUL && (r->n[1] + 0x40UL + ((r->n[0] + 0x3D1UL) >> 26)) > 0x3FFFFFFUL) {
return 0;
}
ret = !((r->n[9] == 0x3FFFFFUL) & ((r->n[8] & r->n[7] & r->n[6] & r->n[5] & r->n[4] & r->n[3] & r->n[2]) == 0x3FFFFFFUL) & ((r->n[1] + 0x40UL + ((r->n[0] + 0x3D1UL) >> 26)) > 0x3FFFFFFUL));
#ifdef VERIFY
r->magnitude = 1;
r->normalized = 1;
secp256k1_fe_verify(r);
if (ret) {
r->normalized = 1;
secp256k1_fe_verify(r);
} else {
r->normalized = 0;
}
#endif
return 1;
return ret;
}
/** Convert a field element to a 32-byte big endian value. Requires the input to be normalized */
@@ -1107,10 +1110,10 @@ static SECP256K1_INLINE void secp256k1_fe_cmov(secp256k1_fe *r, const secp256k1_
r->n[8] = (r->n[8] & mask0) | (a->n[8] & mask1);
r->n[9] = (r->n[9] & mask0) | (a->n[9] & mask1);
#ifdef VERIFY
if (a->magnitude > r->magnitude) {
if (flag) {
r->magnitude = a->magnitude;
r->normalized = a->normalized;
}
r->normalized &= a->normalized;
#endif
}