Make the DH parameter we use for TLS match the one from Apache's mod_ssl

Our regular DH parameters that we use for circuit and rendezvous
crypto are unchanged.  This is yet another small step on the path of
protocol fingerprinting resistance.

changes/dhparam

@@ -0,0 +1,3 @@
+  o Minor features
+    - Adjust our TLS Diffie-Hellman parameters to match those used by
+      Apache's mod_ssl.

src/common/crypto.c

@@ -1685,8 +1685,10 @@ crypto_hmac_sha1(char *hmac_out,
 
 /* DH */
 
-/** Shared P parameter for our DH key exchanged. */
+/** Shared P parameter for our circuit-crypto DH key exchanges. */
 static BIGNUM *dh_param_p = NULL;
+/** Shared P parameter for our TLS DH key exchanges. */
+static BIGNUM *dh_param_p_tls = NULL;
 /** Shared G parameter for our DH key exchanges. */
 static BIGNUM *dh_param_g = NULL;
 
@@ -1695,14 +1697,16 @@ static BIGNUM *dh_param_g = NULL;
 static void
 init_dh_param(void)
 {
-  BIGNUM *p, *g;
+  BIGNUM *p, *p2, *g;
   int r;
-  if (dh_param_p && dh_param_g)
+  if (dh_param_p && dh_param_g && dh_param_p_tls)
     return;
 
   p = BN_new();
+  p2 = BN_new();
   g = BN_new();
   tor_assert(p);
+  tor_assert(p2);
   tor_assert(g);
 
   /* This is from rfc2409, section 6.2.  It's a safe prime, and
@@ -1716,10 +1720,20 @@ init_dh_param(void)
                 "A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6"
                 "49286651ECE65381FFFFFFFFFFFFFFFF");
   tor_assert(r);
+  /* This is the 1024-bit safe prime that Apache uses for its DH stuff; see
+   * modules/ssl/ssl_engine_dh.c */
+  r = BN_hex2bn(&p2,
+                  "D67DE440CBBBDC1936D693D34AFD0AD50C84D239A45F520BB88174CB98"
+                "BCE951849F912E639C72FB13B4B4D7177E16D55AC179BA420B2A29FE324A"
+                "467A635E81FF5901377BEDDCFD33168A461AAD3B72DAE8860078045B07A7"
+                "DBCA7874087D1510EA9FCC9DDD330507DD62DB88AEAA747DE0F4D6E2BD68"
+                "B0E7393E0F24218EB3");
+  tor_assert(r);
 
   r = BN_set_word(g, 2);
   tor_assert(r);
   dh_param_p = p;
+  dh_param_p_tls = p2;
   dh_param_g = g;
 }
 
@@ -1728,18 +1742,26 @@ init_dh_param(void)
 /** Allocate and return a new DH object for a key exchange.
  */
 crypto_dh_env_t *
-crypto_dh_new(void)
+crypto_dh_new(int dh_type)
 {
   crypto_dh_env_t *res = tor_malloc_zero(sizeof(crypto_dh_env_t));
 
+  tor_assert(dh_type == DH_TYPE_CIRCUIT || dh_type == DH_TYPE_TLS ||
+             dh_type == DH_TYPE_REND);
+
   if (!dh_param_p)
     init_dh_param();
 
   if (!(res->dh = DH_new()))
     goto err;
 
-  if (!(res->dh->p = BN_dup(dh_param_p)))
-    goto err;
+  if (dh_type == DH_TYPE_TLS) {
+    if (!(res->dh->p = BN_dup(dh_param_p_tls)))
+      goto err;
+  } else {
+    if (!(res->dh->p = BN_dup(dh_param_p)))
+      goto err;
+  }
 
   if (!(res->dh->g = BN_dup(dh_param_g)))
     goto err;

src/common/crypto.h

@@ -195,7 +195,10 @@ void crypto_hmac_sha1(char *hmac_out,
                       const char *msg, size_t msg_len);
 
 /* Key negotiation */
-crypto_dh_env_t *crypto_dh_new(void);
+#define DH_TYPE_CIRCUIT 1
+#define DH_TYPE_REND 2
+#define DH_TYPE_TLS 3
+crypto_dh_env_t *crypto_dh_new(int dh_type);
 int crypto_dh_get_bytes(crypto_dh_env_t *dh);
 int crypto_dh_generate_public(crypto_dh_env_t *dh);
 int crypto_dh_get_public(crypto_dh_env_t *dh, char *pubkey_out,

src/common/tortls.c

@@ -807,7 +807,7 @@ tor_tls_context_new(crypto_pk_env_t *identity, unsigned int key_lifetime)
   if (!SSL_CTX_check_private_key(result->ctx))
     goto error;
   {
-    crypto_dh_env_t *dh = crypto_dh_new();
+    crypto_dh_env_t *dh = crypto_dh_new(DH_TYPE_TLS);
     SSL_CTX_set_tmp_dh(result->ctx, _crypto_dh_env_get_dh(dh));
     crypto_dh_free(dh);
   }

src/or/onion.c

@@ -184,7 +184,7 @@ onion_skin_create(crypto_pk_env_t *dest_router_key,
   *handshake_state_out = NULL;
   memset(onion_skin_out, 0, ONIONSKIN_CHALLENGE_LEN);
 
-  if (!(dh = crypto_dh_new()))
+  if (!(dh = crypto_dh_new(DH_TYPE_CIRCUIT)))
     goto err;
 
   dhbytes = crypto_dh_get_bytes(dh);
@@ -258,7 +258,7 @@ onion_skin_server_handshake(const char *onion_skin, /*ONIONSKIN_CHALLENGE_LEN*/
     goto err;
   }
 
-  dh = crypto_dh_new();
+  dh = crypto_dh_new(DH_TYPE_CIRCUIT);
   if (crypto_dh_get_public(dh, handshake_reply_out, DH_KEY_LEN)) {
     log_info(LD_GENERAL, "crypto_dh_get_public failed.");
     goto err;

src/or/rendclient.c

@@ -121,7 +121,7 @@ rend_client_send_introduction(origin_circuit_t *introcirc,
     cpath = rendcirc->build_state->pending_final_cpath =
       tor_malloc_zero(sizeof(crypt_path_t));
     cpath->magic = CRYPT_PATH_MAGIC;
-    if (!(cpath->dh_handshake_state = crypto_dh_new())) {
+    if (!(cpath->dh_handshake_state = crypto_dh_new(DH_TYPE_REND))) {
       log_warn(LD_BUG, "Internal error: couldn't allocate DH.");
       goto err;
     }

src/or/rendservice.c

@@ -1100,7 +1100,7 @@ rend_service_introduce(origin_circuit_t *circuit, const uint8_t *request,
   }
 
   /* Try DH handshake... */
-  dh = crypto_dh_new();
+  dh = crypto_dh_new(DH_TYPE_REND);
   if (!dh || crypto_dh_generate_public(dh)<0) {
     log_warn(LD_BUG,"Internal error: couldn't build DH state "
              "or generate public key.");

src/test/test_crypto.c

@@ -12,8 +12,8 @@
 static void
 test_crypto_dh(void)
 {
-  crypto_dh_env_t *dh1 = crypto_dh_new();
-  crypto_dh_env_t *dh2 = crypto_dh_new();
+  crypto_dh_env_t *dh1 = crypto_dh_new(DH_TYPE_CIRCUIT);
+  crypto_dh_env_t *dh2 = crypto_dh_new(DH_TYPE_CIRCUIT);
   char p1[DH_BYTES];
   char p2[DH_BYTES];
   char s1[DH_BYTES];