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fix: curve25519 clamp check missing rule 3 (bit6 of byte31)

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This commit is contained in:
Mark Atwood
2026-04-30 10:41:10 -07:00
parent c098e53948
commit 52fb6031ad
3 changed files with 64 additions and 3 deletions
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tests/api/test_curve25519.c
+55
View File
@@ -547,3 +547,58 @@ int test_wc_curve25519_import_private(void)
return EXPECT_RESULT();
} /* END test_wc_curve25519_import */
/*
* Test curve25519_priv_clamp_check via wc_curve25519_make_pub.
*
* RFC 7748 section 5 requires three clamping invariants on a Curve25519
* private scalar before use:
* Rule 1: bits 0-2 of byte 0 must be clear (scalar &= 0xF8)
* Rule 2: bit 7 of byte 31 must be clear (scalar &= 0x7F)
* Rule 3: bit 6 of byte 31 must be SET (scalar |= 0x40)
*
* Test vectors are derived from RFC 7748 s5; they are the independent oracle.
* Before the fix, rule 3 was not checked, so a scalar with byte[31]==0x00
* (bit 6 clear) was silently accepted -- regression covered below.
*/
int test_wc_curve25519_priv_clamp_check(void)
{
EXPECT_DECLS;
#ifdef HAVE_CURVE25519
/* Valid clamped scalar: all bytes 0x00 except byte[31] = 0x40
* (bit 7 clear, bit 6 set, byte[0] bits 0-2 clear). */
static const byte kValidPriv[CURVE25519_KEYSIZE] = {
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x40
};
byte pub[CURVE25519_KEYSIZE];
byte priv[CURVE25519_KEYSIZE];
/* Valid key succeeds. */
ExpectIntEQ(wc_curve25519_make_pub(CURVE25519_KEYSIZE, pub,
CURVE25519_KEYSIZE, kValidPriv), 0);
/* Rule 1 violation: bit 0 of byte[0] set (byte[0] = 0x01). */
XMEMCPY(priv, kValidPriv, sizeof(priv));
priv[0] |= 0x01;
ExpectIntEQ(wc_curve25519_make_pub(CURVE25519_KEYSIZE, pub,
CURVE25519_KEYSIZE, priv), WC_NO_ERR_TRACE(ECC_BAD_ARG_E));
/* Rule 2 violation: bit 7 of byte[31] set (byte[31] = 0xC0, keeping
* bit 6 set so only rule 2 is violated). */
XMEMCPY(priv, kValidPriv, sizeof(priv));
priv[CURVE25519_KEYSIZE - 1] = 0xC0;
ExpectIntEQ(wc_curve25519_make_pub(CURVE25519_KEYSIZE, pub,
CURVE25519_KEYSIZE, priv), WC_NO_ERR_TRACE(ECC_BAD_ARG_E));
/* Rule 3 violation: bit 6 of byte[31] clear (byte[31] = 0x00).
* Regression: this was silently accepted before the fix. */
XMEMCPY(priv, kValidPriv, sizeof(priv));
priv[CURVE25519_KEYSIZE - 1] = 0x00;
ExpectIntEQ(wc_curve25519_make_pub(CURVE25519_KEYSIZE, pub,
CURVE25519_KEYSIZE, priv), WC_NO_ERR_TRACE(ECC_BAD_ARG_E));
#endif
return EXPECT_RESULT();
} /* END test_wc_curve25519_priv_clamp_check */
tests/api/test_curve25519.h
+3 -1
View File
@@ -35,6 +35,7 @@ int test_wc_curve25519_export_public_ex(void);
int test_wc_curve25519_export_private_raw_ex(void);
int test_wc_curve25519_import_private_raw_ex(void);
int test_wc_curve25519_import_private(void);
int test_wc_curve25519_priv_clamp_check(void);
#define TEST_CURVE25519_DECLS \
TEST_DECL_GROUP("curve25519", test_wc_curve25519_init), \
@@ -47,6 +48,7 @@ int test_wc_curve25519_import_private(void);
TEST_DECL_GROUP("curve25519", test_wc_curve25519_export_public_ex), \
TEST_DECL_GROUP("curve25519", test_wc_curve25519_export_private_raw_ex), \
TEST_DECL_GROUP("curve25519", test_wc_curve25519_import_private_raw_ex), \
TEST_DECL_GROUP("curve25519", test_wc_curve25519_import_private)
TEST_DECL_GROUP("curve25519", test_wc_curve25519_import_private), \
TEST_DECL_GROUP("curve25519", test_wc_curve25519_priv_clamp_check)
#endif /* WOLFCRYPT_TEST_CURVE25519_H */
wolfcrypt/src/curve25519.c
+6 -2
View File
@@ -108,10 +108,14 @@ static WC_INLINE int curve25519_priv_clamp(byte* priv)
}
static WC_INLINE int curve25519_priv_clamp_check(const byte* priv)
{
/* check that private part of key has been clamped */
/* check that private part of key has been clamped per RFC 7748 section 5:
* bits 0-2 of byte 0 must be clear (priv[0] &= 248)
* bit 7 of byte 31 must be clear (priv[31] &= 127)
* bit 6 of byte 31 must be set (priv[31] |= 64) */
int ret = 0;
if ((priv[0] & ~248) ||
(priv[CURVE25519_KEYSIZE-1] & 128)) {
(priv[CURVE25519_KEYSIZE-1] & 128) ||
!(priv[CURVE25519_KEYSIZE-1] & 64)) {
ret = ECC_BAD_ARG_E;
}
return ret;