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field: correct fe_equal's b magnitude bound

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`secp256k1_fe_equal` negates `a` before adding `b`.
That gives the temporary value magnitude 2, and the following field addition requires the input magnitudes to sum to at most 32.
So the largest `b` magnitude the implementation can accept is 30, not 31.

Lower the documented and checked bound for `b` to 30.
Adjust the focused test to use random field elements with randomized magnitudes within the accepted `a <= 1` and `b <= 30` bounds.

Co-authored-by: Sebastian Falbesoner <sebastian.falbesoner@gmail.com>
Co-authored-by: Tim Ruffing <me@real-or-random.org>
This commit is contained in:
Lőrinc
2026-06-21 13:37:40 +02:00
parent 2ce4f71dc5
commit 994b35010d
3 changed files with 15 additions and 2 deletions
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13
src/tests.c
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@@ -3064,6 +3064,18 @@ static int fe_equal(const secp256k1_fe *a, const secp256k1_fe *b) {
return secp256k1_fe_equal(&an, &bn);
}
static void run_fe_equal_magnitude_boundaries(void) {
int i;
secp256k1_fe a, b;
for (i = 0; i < 100 * COUNT; ++i) {
testutil_random_fe(&a);
b = a;
testutil_random_fe_magnitude(&a, 1);
testutil_random_fe_magnitude(&b, 30);
CHECK(secp256k1_fe_equal(&a, &b));
}
}
static void run_field_convert(void) {
static const unsigned char b32[32] = {
0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
@@ -7970,6 +7982,7 @@ static const struct tf_test_entry tests_scalar[] = {
static const struct tf_test_entry tests_field[] = {
CASE(field_half),
CASE(field_misc),
CASE(fe_equal_magnitude_boundaries),
CASE(field_convert),
CASE(field_be32_overflow),
CASE(fe_mul),