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Handle even moduli in sp_exptmod_nct for RSA public-only builds #11849

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Handle even moduli in sp_exptmod_nct for RSA public-only builds #11849

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ab4a0a6
Handle even moduli in sp_exptmod_nct for RSA public-only builds
Oct 5, 2026
ee291fa
Reject even moduli in sp_exptmod_nct when the generic path is not built
Nov 6, 2026
dcdf6d2
Align even modulus test guard and MP_VAL docs with sp_exptmod_nct
Nov 6, 2026
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8 changes: 8 additions & 0 deletions .github/configs/os-check-linux.json
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Original file line number Diff line number Diff line change
Expand Up @@ -542,6 +542,14 @@
"comment": "RSA verify-only with ECC cofactor ECDH (HAVE_ECC_CDH, set by --enable-ecccustcurves=all), whose wc_ecc_shared_secret_gen_sync() calls mp_mul_d. SAKKE stays off, so the HAVE_ECC_CDH term in settings.h is the only reason sp_mul_d is built. This gives link-time coverage of that term only: the mp_mul_d call runs only when cofactor != 1, and no curve at or above ECC_MIN_KEY_SZ has another cofactor.",
"configure": ["--enable-cryptonly", "--enable-rsavfy",
"--enable-ecccustcurves=all", "--disable-examples"]},
{"name": "cryptonly-rsapub-no-dh-public-mp", "minutes": 1.0,
"comment": "RSA public only with DH off, so sp_exptmod_ex hands every modulus to sp_exptmod_nct and the constant-time generic exponentiation is not compiled. WOLFSSL_PUBLIC_MP lets wolfcrypt test run mp_test, whose even modulus checks confirm sp_exptmod_nct returns MP_VAL in this shape instead of a wrong Montgomery result.",
"configure": ["--enable-cryptonly", "--enable-rsapub", "--disable-dh",
"--disable-examples", "CPPFLAGS=-DWOLFSSL_PUBLIC_MP"]},
{"name": "cryptonly-rsapub-public-mp", "minutes": 1.0,
"comment": "RSA public only with DH at its default (on), so the generic exponentiation is compiled and sp_exptmod_nct must use it for an even modulus rather than Montgomery. WOLFSSL_PUBLIC_MP lets wolfcrypt test run mp_test's even modulus checks in this shape.",
"configure": ["--enable-cryptonly", "--enable-rsapub",
"--disable-examples", "CPPFLAGS=-DWOLFSSL_PUBLIC_MP"]},
{"name": "pkcs12-pbkdf-mp-api", "minutes": 1.0,
"comment": "WC_PKCS12_PBKDF_USING_MP_API selects a second wc_PKCS12_PBKDF_ex implementation that no configure option reaches.",
"configure": ["--enable-pkcs12", "--enable-des3", "--enable-opensslextra",
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22 changes: 17 additions & 5 deletions wolfcrypt/src/sp_int.c
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Expand Up @@ -14199,7 +14199,8 @@ static int _sp_exptmod_base_2(const sp_int* e, int digits, const sp_int* m,
*
* @return MP_OKAY on success.
* @return MP_VAL when b, e, m or r is NULL, digits is negative, or m <= 0 or
* e is negative.
* e is negative; or m is even in an RSA verify/public only build
* without DH or OPENSSL_ALL.
* @return MP_MEM when dynamic memory allocation fails.
*/
int sp_exptmod_ex(const sp_int* b, const sp_int* e, int digits, const sp_int* m,
Expand Down Expand Up @@ -14379,7 +14380,9 @@ int sp_exptmod_ex(const sp_int* b, const sp_int* e, int digits, const sp_int* m,
* @param [out] r SP integer to hold result.
*
* @return MP_OKAY on success.
* @return MP_VAL when b, e, m or r is NULL; or m <= 0 or e is negative.
* @return MP_VAL when b, e, m or r is NULL; or m <= 0 or e is negative; or
* m is even in an RSA verify/public only build without DH or
* OPENSSL_ALL.
* @return MP_MEM when dynamic memory allocation fails.
*/
int sp_exptmod(const sp_int* b, const sp_int* e, const sp_int* m, sp_int* r)
Expand Down Expand Up @@ -14839,7 +14842,9 @@ static int _sp_exptmod_nct(const sp_int* b, const sp_int* e, const sp_int* m,
* @param [out] r SP integer to hold result.
*
* @return MP_OKAY on success.
* @return MP_VAL when b, e, m or r is NULL; or m <= 0 or e is negative.
* @return MP_VAL when b, e, m or r is NULL; or m <= 0 or e is negative; or
* m is even in an RSA verify/public only build without DH or
* OPENSSL_ALL.
* @return MP_MEM when dynamic memory allocation fails.
*/
int sp_exptmod_nct(const sp_int* b, const sp_int* e, const sp_int* m, sp_int* r)
Expand Down Expand Up @@ -14886,11 +14891,18 @@ int sp_exptmod_nct(const sp_int* b, const sp_int* e, const sp_int* m, sp_int* r)
else if (m->used * 2 >= r->size) {
err = MP_VAL;
}
#if !defined(WOLFSSL_RSA_VERIFY_ONLY) && !defined(WOLFSSL_RSA_PUBLIC_ONLY)
else if (mp_iseven(m)) {
#if (defined(WOLFSSL_SP_MATH_ALL) && !defined(WOLFSSL_RSA_VERIFY_ONLY) && \
!defined(WOLFSSL_RSA_PUBLIC_ONLY)) || !defined(NO_DH) || \
defined(OPENSSL_ALL)
err = _sp_exptmod_ex(b, e, (int)(e->used * SP_WORD_SIZE), m, r);
}
#else
/* Montgomery form requires an odd modulus and the generic
* implementation is not compiled in (RSA verify/public only without
* DH). */
err = MP_VAL;
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#endif
}
else {
err = _sp_exptmod_nct(b, e, m, r);
}
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23 changes: 20 additions & 3 deletions wolfcrypt/test/test.c
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Expand Up @@ -95254,12 +95254,28 @@ static wc_test_ret_t mp_test_exptmod(mp_int* b, mp_int* e, mp_int* m, mp_int* r,

/* Even, multi-word modulus uses the generic exponentiation path for
* both mp_exptmod_ex and mp_exptmod_nct. Verify b^6 mod m against
* repeated modular multiplication. */
* repeated modular multiplication. RSA public only builds without DH
* do not compile the generic path, and mp_exptmod_ex hands every modulus
* to mp_exptmod_nct, so both must reject the even modulus instead of
* returning a wrong Montgomery result. */
if ((ret = mp_set(m, 1)) != MP_OKAY ||
(ret = mp_mul_2d(m, 521, m)) != MP_OKAY || /* m = 2^521 (even) */
(ret = mp_set(b, 7)) != MP_OKAY ||
(ret = mp_set(e, 6)) != MP_OKAY ||
(ret = mp_set(t, 1)) != MP_OKAY) {
(ret = mp_set(e, 6)) != MP_OKAY) {
return WC_TEST_RET_ENC_EC(ret);
}
#if (defined(WOLFSSL_RSA_VERIFY_ONLY) || defined(WOLFSSL_RSA_PUBLIC_ONLY)) && \
defined(NO_DH) && !defined(OPENSSL_ALL)
ret = mp_exptmod_ex(b, e, (int)e->used, m, r);
if (ret != WC_NO_ERR_TRACE(MP_VAL)) {
return WC_TEST_RET_ENC_EC(ret);
}
ret = mp_exptmod_nct(b, e, m, r);
if (ret != WC_NO_ERR_TRACE(MP_VAL)) {
return WC_TEST_RET_ENC_EC(ret);
}
#else
if ((ret = mp_set(t, 1)) != MP_OKAY) {
return WC_TEST_RET_ENC_EC(ret);
}
for (k = 0; k < 6; k++) {
Expand All @@ -95279,6 +95295,7 @@ static wc_test_ret_t mp_test_exptmod(mp_int* b, mp_int* e, mp_int* m, mp_int* r,
if (mp_cmp(r, t) != MP_EQ) {
return WC_TEST_RET_ENC_NC;
}
#endif

#if !defined(HAVE_FIPS) || FIPS_VERSION3_GE(7,0,0)

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