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Add invariant checking for scalars #1373

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Aug 18, 2023
Merged

Add invariant checking for scalars #1373

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ad15215
update max scalar in scalar_cmov_test and fix schnorrsig_verify exhau…
stratospher Jul 11, 2023
c7d0454
add verification for scalars
stratospher Jul 3, 2023
d23da6d
use secp256k1_scalar_verify checks
stratospher Jul 12, 2023
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10 changes: 5 additions & 5 deletions src/modules/schnorrsig/tests_exhaustive_impl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -110,15 +110,15 @@ static void test_exhaustive_schnorrsig_verify(const secp256k1_context *ctx, cons
if (!e_done[e]) {
/* Iterate over the possible valid last 32 bytes in the signature.
0..order=that s value; order+1=random bytes */
int count_valid = 0, s;
int count_valid = 0;
unsigned int s;
for (s = 0; s <= EXHAUSTIVE_TEST_ORDER + 1; ++s) {
int expect_valid, valid;
if (s <= EXHAUSTIVE_TEST_ORDER) {
secp256k1_scalar s_s;
secp256k1_scalar_set_int(&s_s, s);
secp256k1_scalar_get_b32(sig64 + 32, &s_s);
memset(sig64 + 32, 0, 32);
secp256k1_write_be32(sig64 + 60, s);
expect_valid = actual_k != -1 && s != EXHAUSTIVE_TEST_ORDER &&
(s_s == (actual_k + actual_d * e) % EXHAUSTIVE_TEST_ORDER);
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Unrelated note: looking at that replaced line, I was confused for a moment that comparing secp256k1_scalar directly with an integer is possible, until I realized that it's simply a uint32_t typedef in the scalar_low implementation (would have expected that it's still wrapped in a struct), and that's of course the only one used in exhaustive tests.

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Yep, indeed. I guess that's okay for test code, introducing another function here will be overkill.

(s == (actual_k + actual_d * e) % EXHAUSTIVE_TEST_ORDER);
} else {
secp256k1_testrand256(sig64 + 32);
expect_valid = 0;
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5 changes: 4 additions & 1 deletion src/scalar_low_impl.h
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Expand Up @@ -18,7 +18,10 @@ SECP256K1_INLINE static int secp256k1_scalar_is_even(const secp256k1_scalar *a)
}

SECP256K1_INLINE static void secp256k1_scalar_clear(secp256k1_scalar *r) { *r = 0; }
SECP256K1_INLINE static void secp256k1_scalar_set_int(secp256k1_scalar *r, unsigned int v) { *r = v; }

SECP256K1_INLINE static void secp256k1_scalar_set_int(secp256k1_scalar *r, unsigned int v) {
*r = v % EXHAUSTIVE_TEST_ORDER;
}

SECP256K1_INLINE static unsigned int secp256k1_scalar_get_bits(const secp256k1_scalar *a, unsigned int offset, unsigned int count) {
if (offset < 32)
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4 changes: 2 additions & 2 deletions src/tests.c
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Expand Up @@ -7652,8 +7652,8 @@ static void scalar_cmov_test(void) {
static const secp256k1_scalar zero = SECP256K1_SCALAR_CONST(0, 0, 0, 0, 0, 0, 0, 0);
static const secp256k1_scalar one = SECP256K1_SCALAR_CONST(0, 0, 0, 0, 0, 0, 0, 1);
static const secp256k1_scalar max = SECP256K1_SCALAR_CONST(
0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFFUL,
0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFFUL
0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFFUL, 0xFFFFFFFEUL,
0xBAAEDCE6UL, 0xAF48A03BUL, 0xBFD25E8CUL, 0xD0364140UL
);
secp256k1_scalar r = max;
secp256k1_scalar a = zero;
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