Backport DH_check from OpenSSL 1.1.0.

OpenSSL 1.0.2's DH_check considers the q parameter, allowing it
validate more generators and primes; however, OpenSSL 1.1.0's DH_check
includes code to handle errors in BN functions, so it's preferred.

src/_cffi_src/openssl/dh.py

@@ -16,7 +16,6 @@
 DH *DH_new(void);
 void DH_free(DH *);
 int DH_size(const DH *);
-int DH_check(const DH *, int *);
 int DH_check_pub_key(const DH *, const BIGNUM *, int *);
 int DH_generate_key(DH *);
 int DH_compute_key(unsigned char *, const BIGNUM *, DH *);
@@ -28,6 +27,9 @@
 int DHparams_print(BIO *, const DH *);
 DH *DHparams_dup(DH *);
 
+int Cryptography_DH_check(const DH *, int *);
+
+
 /* added in 1.1.0 when the DH struct was opaqued */
 void DH_get0_pqg(const DH *, const BIGNUM **, const BIGNUM **,
                  const BIGNUM **);
@@ -114,4 +116,100 @@
     return 1;
 }
 #endif
+
+#ifdef CRYPTOGRAPHY_OPENSSL_LESS_THAN_110 || defined(LIBRESSL_VERSION_NUMBER)
+/*-
+ * Check that p is a safe prime and
+ * if g is 2, 3 or 5, check that it is a suitable generator
+ * where
+ * for 2, p mod 24 == 11
+ * for 3, p mod 12 == 5
+ * for 5, p mod 10 == 3 or 7
+ * should hold.
+ */
+
+int Cryptography_DH_check(const DH *dh, int *ret)
+{
+    int ok = 0, r;
+    BN_CTX *ctx = NULL;
+    BN_ULONG l;
+    BIGNUM *t1 = NULL, *t2 = NULL;
+
+    *ret = 0;
+    ctx = BN_CTX_new();
+    if (ctx == NULL)
+        goto err;
+    BN_CTX_start(ctx);
+    t1 = BN_CTX_get(ctx);
+    if (t1 == NULL)
+        goto err;
+    t2 = BN_CTX_get(ctx);
+    if (t2 == NULL)
+        goto err;
+
+    if (dh->q) {
+        if (BN_cmp(dh->g, BN_value_one()) <= 0)
+            *ret |= DH_NOT_SUITABLE_GENERATOR;
+        else if (BN_cmp(dh->g, dh->p) >= 0)
+            *ret |= DH_NOT_SUITABLE_GENERATOR;
+        else {
+            /* Check g^q == 1 mod p */
+            if (!BN_mod_exp(t1, dh->g, dh->q, dh->p, ctx))
+                goto err;
+            if (!BN_is_one(t1))
+                *ret |= DH_NOT_SUITABLE_GENERATOR;
+        }
+        r = BN_is_prime_ex(dh->q, BN_prime_checks, ctx, NULL);
+        if (r < 0)
+            goto err;
+        if (!r)
+            *ret |= DH_CHECK_Q_NOT_PRIME;
+        /* Check p == 1 mod q  i.e. q divides p - 1 */
+        if (!BN_div(t1, t2, dh->p, dh->q, ctx))
+            goto err;
+        if (!BN_is_one(t2))
+            *ret |= DH_CHECK_INVALID_Q_VALUE;
+        if (dh->j && BN_cmp(dh->j, t1))
+            *ret |= DH_CHECK_INVALID_J_VALUE;
+
+    } else if (BN_is_word(dh->g, DH_GENERATOR_2)) {
+        l = BN_mod_word(dh->p, 24);
+        if (l == (BN_ULONG)-1)
+            goto err;
+        if (l != 11)
+            *ret |= DH_NOT_SUITABLE_GENERATOR;
+    } else if (BN_is_word(dh->g, DH_GENERATOR_5)) {
+        l = BN_mod_word(dh->p, 10);
+        if (l == (BN_ULONG)-1)
+            goto err;
+        if ((l != 3) && (l != 7))
+            *ret |= DH_NOT_SUITABLE_GENERATOR;
+    } else
+        *ret |= DH_UNABLE_TO_CHECK_GENERATOR;
+
+    r = BN_is_prime_ex(dh->p, BN_prime_checks, ctx, NULL);
+    if (r < 0)
+        goto err;
+    if (!r)
+        *ret |= DH_CHECK_P_NOT_PRIME;
+    else if (!dh->q) {
+        if (!BN_rshift1(t1, dh->p))
+            goto err;
+        r = BN_is_prime_ex(t1, BN_prime_checks, ctx, NULL);
+        if (r < 0)
+            goto err;
+        if (!r)
+            *ret |= DH_CHECK_P_NOT_SAFE_PRIME;
+    }
+    ok = 1;
+ err:
+    if (ctx != NULL) {
+        BN_CTX_end(ctx);
+        BN_CTX_free(ctx);
+    }
+    return (ok);
+}
+#else
+int (*Cryptography_DH_check)(const DH *dh, int *ret) = DH_check;
+#endif
 """

src/cryptography/hazmat/backends/openssl/backend.py

@@ -1763,7 +1763,7 @@ def load_dh_private_numbers(self, numbers):
         self.openssl_assert(res == 1)
 
         codes = self._ffi.new("int[]", 1)
-        res = self._lib.DH_check(dh_cdata, codes)
+        res = self._lib.Cryptography_DH_check(dh_cdata, codes)
         self.openssl_assert(res == 1)
 
         if codes[0] != 0:
@@ -1815,7 +1815,7 @@ def dh_parameters_supported(self, p, g):
         self.openssl_assert(res == 1)
 
         codes = self._ffi.new("int[]", 1)
-        res = self._lib.DH_check(dh_cdata, codes)
+        res = self._lib.Cryptography_DH_check(dh_cdata, codes)
         self.openssl_assert(res == 1)
 
         return codes[0] == 0