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[SECP256K1] Add tests to exercise lambda split near bounds
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Summary:
Partial backport 5/11 of secp256k1 [[bitcoin-core/secp256k1#830 | PR830]]:
bitcoin-core/secp256k1@9d2f2b4

Depends on D8040.

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8041
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@sipa @Fabcien
sipa authored and Fabcien committed Oct 22, 2020
1 parent 51e4185 commit e2d35a9
Showing 1 changed file with 94 additions and 5 deletions.
99 changes: 94 additions & 5 deletions src/secp256k1/src/tests.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2633,6 +2633,87 @@ void test_point_times_order(const secp256k1_gej *point) {
ge_equals_ge(&res3, &secp256k1_ge_const_g);
}

/* These scalars reach large (in absolute value) outputs when fed to secp256k1_scalar_split_lambda.
*
* They are computed as:
* - For a in [-2, -1, 0, 1, 2]:
* - For b in [-3, -1, 1, 3]:
* - Output (a*LAMBDA + (ORDER+b)/2) % ORDER
*/
static const secp256k1_scalar scalars_near_split_bounds[20] = {
SECP256K1_SCALAR_CONST(0xd938a566, 0x7f479e3e, 0xb5b3c7fa, 0xefdb3749, 0x3aa0585c, 0xc5ea2367, 0xe1b660db, 0x0209e6fc),
SECP256K1_SCALAR_CONST(0xd938a566, 0x7f479e3e, 0xb5b3c7fa, 0xefdb3749, 0x3aa0585c, 0xc5ea2367, 0xe1b660db, 0x0209e6fd),
SECP256K1_SCALAR_CONST(0xd938a566, 0x7f479e3e, 0xb5b3c7fa, 0xefdb3749, 0x3aa0585c, 0xc5ea2367, 0xe1b660db, 0x0209e6fe),
SECP256K1_SCALAR_CONST(0xd938a566, 0x7f479e3e, 0xb5b3c7fa, 0xefdb3749, 0x3aa0585c, 0xc5ea2367, 0xe1b660db, 0x0209e6ff),
SECP256K1_SCALAR_CONST(0x2c9c52b3, 0x3fa3cf1f, 0x5ad9e3fd, 0x77ed9ba5, 0xb294b893, 0x3722e9a5, 0x00e698ca, 0x4cf7632d),
SECP256K1_SCALAR_CONST(0x2c9c52b3, 0x3fa3cf1f, 0x5ad9e3fd, 0x77ed9ba5, 0xb294b893, 0x3722e9a5, 0x00e698ca, 0x4cf7632e),
SECP256K1_SCALAR_CONST(0x2c9c52b3, 0x3fa3cf1f, 0x5ad9e3fd, 0x77ed9ba5, 0xb294b893, 0x3722e9a5, 0x00e698ca, 0x4cf7632f),
SECP256K1_SCALAR_CONST(0x2c9c52b3, 0x3fa3cf1f, 0x5ad9e3fd, 0x77ed9ba5, 0xb294b893, 0x3722e9a5, 0x00e698ca, 0x4cf76330),
SECP256K1_SCALAR_CONST(0x7fffffff, 0xffffffff, 0xffffffff, 0xffffffff, 0xd576e735, 0x57a4501d, 0xdfe92f46, 0x681b209f),
SECP256K1_SCALAR_CONST(0x7fffffff, 0xffffffff, 0xffffffff, 0xffffffff, 0xd576e735, 0x57a4501d, 0xdfe92f46, 0x681b20a0),
SECP256K1_SCALAR_CONST(0x7fffffff, 0xffffffff, 0xffffffff, 0xffffffff, 0xd576e735, 0x57a4501d, 0xdfe92f46, 0x681b20a1),
SECP256K1_SCALAR_CONST(0x7fffffff, 0xffffffff, 0xffffffff, 0xffffffff, 0xd576e735, 0x57a4501d, 0xdfe92f46, 0x681b20a2),
SECP256K1_SCALAR_CONST(0xd363ad4c, 0xc05c30e0, 0xa5261c02, 0x88126459, 0xf85915d7, 0x7825b696, 0xbeebc5c2, 0x833ede11),
SECP256K1_SCALAR_CONST(0xd363ad4c, 0xc05c30e0, 0xa5261c02, 0x88126459, 0xf85915d7, 0x7825b696, 0xbeebc5c2, 0x833ede12),
SECP256K1_SCALAR_CONST(0xd363ad4c, 0xc05c30e0, 0xa5261c02, 0x88126459, 0xf85915d7, 0x7825b696, 0xbeebc5c2, 0x833ede13),
SECP256K1_SCALAR_CONST(0xd363ad4c, 0xc05c30e0, 0xa5261c02, 0x88126459, 0xf85915d7, 0x7825b696, 0xbeebc5c2, 0x833ede14),
SECP256K1_SCALAR_CONST(0x26c75a99, 0x80b861c1, 0x4a4c3805, 0x1024c8b4, 0x704d760e, 0xe95e7cd3, 0xde1bfdb1, 0xce2c5a42),
SECP256K1_SCALAR_CONST(0x26c75a99, 0x80b861c1, 0x4a4c3805, 0x1024c8b4, 0x704d760e, 0xe95e7cd3, 0xde1bfdb1, 0xce2c5a43),
SECP256K1_SCALAR_CONST(0x26c75a99, 0x80b861c1, 0x4a4c3805, 0x1024c8b4, 0x704d760e, 0xe95e7cd3, 0xde1bfdb1, 0xce2c5a44),
SECP256K1_SCALAR_CONST(0x26c75a99, 0x80b861c1, 0x4a4c3805, 0x1024c8b4, 0x704d760e, 0xe95e7cd3, 0xde1bfdb1, 0xce2c5a45)
};

void test_ecmult_target(const secp256k1_scalar* target, int mode) {
/* Mode: 0=ecmult_gen, 1=ecmult, 2=ecmult_const */
secp256k1_scalar n1, n2;
secp256k1_ge p;
secp256k1_gej pj, p1j, p2j, ptj;
static const secp256k1_scalar zero = SECP256K1_SCALAR_CONST(0, 0, 0, 0, 0, 0, 0, 0);

/* Generate random n1,n2 such that n1+n2 = -target. */
random_scalar_order_test(&n1);
secp256k1_scalar_add(&n2, &n1, target);
secp256k1_scalar_negate(&n2, &n2);

/* Generate a random input point. */
if (mode != 0) {
random_group_element_test(&p);
secp256k1_gej_set_ge(&pj, &p);
}

/* EC multiplications */
if (mode == 0) {
secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &p1j, &n1);
secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &p2j, &n2);
secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &ptj, target);
} else if (mode == 1) {
secp256k1_ecmult(&ctx->ecmult_ctx, &p1j, &pj, &n1, &zero);
secp256k1_ecmult(&ctx->ecmult_ctx, &p2j, &pj, &n2, &zero);
secp256k1_ecmult(&ctx->ecmult_ctx, &ptj, &pj, target, &zero);
} else {
secp256k1_ecmult_const(&p1j, &p, &n1, 256);
secp256k1_ecmult_const(&p2j, &p, &n2, 256);
secp256k1_ecmult_const(&ptj, &p, target, 256);
}

/* Add them all up: n1*P + n2*P + target*P = (n1+n2+target)*P = (n1+n1-n1-n2)*P = 0. */
secp256k1_gej_add_var(&ptj, &ptj, &p1j, NULL);
secp256k1_gej_add_var(&ptj, &ptj, &p2j, NULL);
CHECK(secp256k1_gej_is_infinity(&ptj));
}

void run_ecmult_near_split_bound(void) {
int i;
unsigned j;
for (i = 0; i < 4*count; ++i) {
for (j = 0; j < sizeof(scalars_near_split_bounds) / sizeof(scalars_near_split_bounds[0]); ++j) {
test_ecmult_target(&scalars_near_split_bounds[j], 0);
test_ecmult_target(&scalars_near_split_bounds[j], 1);
test_ecmult_target(&scalars_near_split_bounds[j], 2);
}
}
}

void run_point_times_order(void) {
int i;
secp256k1_fe x = SECP256K1_FE_CONST(0, 0, 0, 0, 0, 0, 0, 2);
Expand Down Expand Up @@ -3552,14 +3633,12 @@ void run_ecmult_gen_blind(void) {

#ifdef USE_ENDOMORPHISM
/***** ENDOMORPHISH TESTS *****/
void test_scalar_split(void) {
secp256k1_scalar full;
void test_scalar_split(const secp256k1_scalar* full) {
secp256k1_scalar s1, slam;
const unsigned char zero[32] = {0};
unsigned char tmp[32];

random_scalar_order_test(&full);
secp256k1_scalar_split_lambda(&s1, &slam, &full);
secp256k1_scalar_split_lambda(&s1, &slam, full);

/* check that both are <= 128 bits in size */
if (secp256k1_scalar_is_high(&s1)) {
Expand All @@ -3575,8 +3654,17 @@ void test_scalar_split(void) {
CHECK(secp256k1_memcmp_var(zero, tmp, 16) == 0);
}


void run_endomorphism_tests(void) {
test_scalar_split();
unsigned i;
for (i = 0; i < 100U * count; ++i) {
secp256k1_scalar full;
random_scalar_order_test(&full);
test_scalar_split(&full);
}
for (i = 0; i < sizeof(scalars_near_split_bounds) / sizeof(scalars_near_split_bounds[0]); ++i) {
test_scalar_split(&scalars_near_split_bounds[i]);
}
}
#endif

Expand Down Expand Up @@ -5596,6 +5684,7 @@ int main(int argc, char **argv) {
/* ecmult tests */
run_wnaf();
run_point_times_order();
run_ecmult_near_split_bound();
run_ecmult_chain();
run_ecmult_constants();
run_ecmult_gen_blind();
Expand Down

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