res_rtp_asterisk: Enable Forward Secrecy (PFS) for DTLS.

Since July 2014, TLS based protocols (SIP over TLS, Secure WebSockets, HTTPS)
support PFS thanks to ASTERISK-23905. In July 2015, the same feature was added
for DTLS. The source code from main/tcptls.c should have been re-used to ease
security audits. Therefore, this change rolls back the change from July 2015 and
re-uses the code from July 2014. This has the additional benefits to work under
CentOS 7 and enabling not just ECDHE but DHE based cipher suites as well.

ASTERISK-25659 #close
Reported by: StefanEng86, urbaniak, pay123
Tested by: sarumjanuch, traud
patches:
res_rtp_asterisk.patch submitted by sarumjanuch
dtls_centos_step_1.patch submitted by traud
dtls_centos_step_2.patch submitted by traud

Change-Id: I537cadf4421f092a613146b230f2c0ee1be28d5c

1 files changed, 34 insertions, 7 deletions

diff --git a/res/res_rtp_asterisk.c b/res/res_rtp_asterisk.c
index feab1ca80..8055e877f 100644
--- a/res/res_rtp_asterisk.c
+++ b/res/res_rtp_asterisk.c
@@ -1340,7 +1340,7 @@ static int ast_rtp_dtls_set_configuration(struct ast_rtp_instance *instance, con
 {
 	struct ast_rtp *rtp = ast_rtp_instance_get_data(instance);
 	int res;
-#ifndef HAVE_OPENSSL_ECDH_AUTO
+#ifdef HAVE_OPENSSL_EC
 	EC_KEY *ecdh;
 #endif
 
@@ -1368,15 +1368,42 @@ static int ast_rtp_dtls_set_configuration(struct ast_rtp_instance *instance, con
 
 	SSL_CTX_set_read_ahead(rtp->ssl_ctx, 1);
 
-#ifdef HAVE_OPENSSL_ECDH_AUTO
-	SSL_CTX_set_ecdh_auto(rtp->ssl_ctx, 1);
-#else
+#ifdef HAVE_OPENSSL_EC
+
+	if (!ast_strlen_zero(dtls_cfg->pvtfile)) {
+		BIO *bio = BIO_new_file(dtls_cfg->pvtfile, "r");
+		if (bio != NULL) {
+			DH *dh = PEM_read_bio_DHparams(bio, NULL, NULL, NULL);
+			if (dh != NULL) {
+				if (SSL_CTX_set_tmp_dh(rtp->ssl_ctx, dh)) {
+					long options = SSL_OP_CIPHER_SERVER_PREFERENCE |
+						SSL_OP_SINGLE_DH_USE | SSL_OP_SINGLE_ECDH_USE;
+					options = SSL_CTX_set_options(rtp->ssl_ctx, options);
+					ast_verb(2, "DTLS DH initialized, PFS enabled\n");
+				}
+				DH_free(dh);
+			}
+			BIO_free(bio);
+		}
+	}
+	/* enables AES-128 ciphers, to get AES-256 use NID_secp384r1 */
 	ecdh = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);
-	if (ecdh) {
-		SSL_CTX_set_tmp_ecdh(rtp->ssl_ctx, ecdh);
+	if (ecdh != NULL) {
+		if (SSL_CTX_set_tmp_ecdh(rtp->ssl_ctx, ecdh)) {
+			#ifndef SSL_CTRL_SET_ECDH_AUTO
+				#define SSL_CTRL_SET_ECDH_AUTO 94
+			#endif
+			/* SSL_CTX_set_ecdh_auto(rtp->ssl_ctx, on); requires OpenSSL 1.0.2 which wraps: */
+			if (SSL_CTX_ctrl(rtp->ssl_ctx, SSL_CTRL_SET_ECDH_AUTO, 1, NULL)) {
+				ast_verb(2, "DTLS ECDH initialized (automatic), faster PFS enabled\n");
+			} else {
+				ast_verb(2, "DTLS ECDH initialized (secp256r1), faster PFS enabled\n");
+			}
+		}
 		EC_KEY_free(ecdh);
 	}
-#endif
+
+#endif /* #ifdef HAVE_OPENSSL_EC */
 
 	rtp->dtls_verify = dtls_cfg->verify;