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ellswift: fix overflow flag handling in secp256k1_ellswift_xdh #1821

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Merged
real-or-random merged 2 commits into from
Feb 17, 2026
+29 −1
Merged

ellswift: fix overflow flag handling in secp256k1_ellswift_xdh #1821

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307b49f
ellswift: fix overflow flag handling in secp256k1_ellswift_xdh
ZhiqiangXiongmao Feb 11, 2026
b99a94c
Add tests for bad scalar inputs in ellswift XDH
ZhiqiangXiongmao Feb 12, 2026
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2 changes: 1 addition & 1 deletion src/modules/ellswift/main_impl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -564,7 +564,7 @@ int secp256k1_ellswift_xdh(const secp256k1_context *ctx, unsigned char *output,

/* Load private key (using one if invalid). */
secp256k1_scalar_set_b32(&s, seckey32, &overflow);
overflow = secp256k1_scalar_is_zero(&s);
overflow |= secp256k1_scalar_is_zero(&s);
secp256k1_scalar_cmov(&s, &secp256k1_scalar_one, overflow);

/* Compute shared X coordinate. */
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28 changes: 28 additions & 0 deletions src/modules/ellswift/tests_impl.h
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Expand Up @@ -460,6 +460,33 @@ void ellswift_hash_init_tests(void) {
test_sha256_tag_midstate(&sha_optimized, bip324_tag, sizeof(bip324_tag));
}

void ellswift_xdh_bad_scalar_tests(void) {
unsigned char s_zero[32] = { 0 };
unsigned char s_overflow_minus1[32] = { 0 };
unsigned char s_overflow_plus1[32] = { 0 };
unsigned char s_good[32] = { 0 };
unsigned char ell_a64[64], ell_b64[64];
unsigned char output[32];
secp256k1_scalar rand_scalar;

testutil_random_scalar_order(&rand_scalar);
secp256k1_scalar_get_b32(s_good, &rand_scalar);

CHECK(secp256k1_ellswift_create(CTX, ell_a64, s_good, NULL) == 1);

testrand256_test(ell_b64);
testrand256_test(ell_b64 + 32);

memcpy(s_overflow_minus1, secp256k1_group_order_bytes, 32);
s_overflow_minus1[31] -= 1;
memcpy(s_overflow_plus1, secp256k1_group_order_bytes, 32);
s_overflow_plus1[31] += 1;
CHECK(secp256k1_ellswift_xdh(CTX, output, ell_a64, ell_b64, s_zero, 0, &ellswift_xdh_hash_x32, NULL) == 0);
CHECK(secp256k1_ellswift_xdh(CTX, output, ell_a64, ell_b64, secp256k1_group_order_bytes, 0, &ellswift_xdh_hash_x32, NULL) == 0);
CHECK(secp256k1_ellswift_xdh(CTX, output, ell_a64, ell_b64, s_overflow_plus1, 0, &ellswift_xdh_hash_x32, NULL) == 0);
CHECK(secp256k1_ellswift_xdh(CTX, output, ell_a64, ell_b64, s_overflow_minus1, 0, &ellswift_xdh_hash_x32, NULL) == 1);
}

/* --- Test registry --- */
static const struct tf_test_entry tests_ellswift[] = {
CASE1(ellswift_encoding_test_vectors_tests),
Expand All @@ -470,6 +497,7 @@ static const struct tf_test_entry tests_ellswift[] = {
CASE1(ellswift_compute_shared_secret_tests),
CASE1(ellswift_xdh_correctness_tests),
CASE1(ellswift_hash_init_tests),
CASE1(ellswift_xdh_bad_scalar_tests),
};

#endif