DH: honor explicit prime in wc_DhCheckPrivKey_ex (Fenrir F5317)

Author: dgarske

wolfcrypt/src/dh.c

@@ -1808,12 +1808,9 @@ int wc_DhCheckPrivKey_ex(DhKey* key, const byte* priv, word32 privSz,
     if (ret == 0) {
         if (mp_iszero(q) == MP_NO) {
             /* priv (x) shouldn't be greater than q - 1 */
-            if (mp_copy(&key->q, q) != MP_OKAY)
-                ret = MP_INIT_E;
-            if (ret == 0) {
-                if (mp_sub_d(q, 1, q) != MP_OKAY)
-                    ret = MP_SUB_E;
-            }
+            /* q already holds the supplied prime or key->q; do not clobber it */
+            if (mp_sub_d(q, 1, q) != MP_OKAY)
+                ret = MP_SUB_E;
             if (ret == 0) {
                 if (mp_cmp(x, q) == MP_GT)
                     ret = DH_CHECK_PRIV_E;

wolfcrypt/test/test.c

@@ -30504,6 +30504,9 @@ static wc_test_ret_t dh_check_priv_key_test(DhKey* key, const byte* priv,
 {
     wc_test_ret_t ret;
     word32 pSz, qSz, gSz;
+    byte pBuf[DH_TEST_BUF_SIZE];
+    byte qBuf[DH_TEST_BUF_SIZE];
+    byte gBuf[DH_TEST_BUF_SIZE];
 
     ret = wc_DhCheckPrivKey(NULL, NULL, 0);
     if (ret != WC_NO_ERR_TRACE(BAD_FUNC_ARG))
@@ -30521,6 +30524,22 @@ static wc_test_ret_t dh_check_priv_key_test(DhKey* key, const byte* priv,
     if (ret != WC_NO_ERR_TRACE(LENGTH_ONLY_E))
         ERROR_OUT(WC_TEST_RET_ENC_EC(ret), exit_dh_check_priv);
 
+    /* Regression: the explicit prime argument to wc_DhCheckPrivKey_ex must be
+     * honored and not overwritten by key->q. This key is set with p and g
+     * only, so key->q is empty; supplying the modulus p as a generous bound
+     * must accept any valid priv (priv < p). The prior defect clobbered the
+     * loaded bound with the empty key->q and rejected a valid key. */
+    if (pSz <= (word32)sizeof(pBuf) && qSz <= (word32)sizeof(qBuf) &&
+            gSz <= (word32)sizeof(gBuf)) {
+        ret = wc_DhExportParamsRaw(key, pBuf, &pSz, qBuf, &qSz, gBuf, &gSz);
+        if (ret != 0)
+            ERROR_OUT(WC_TEST_RET_ENC_EC(ret), exit_dh_check_priv);
+
+        ret = wc_DhCheckPrivKey_ex(key, priv, privSz, pBuf, pSz);
+        if (ret != 0)
+            ERROR_OUT(WC_TEST_RET_ENC_EC(ret), exit_dh_check_priv);
+    }
+
     ret = 0;
 
 exit_dh_check_priv: