channels/sip/sdp_crypto: Handle SRTP keys negotiated with key lifetime/MKI

Prior to this patch, SDP offers negotiating SDES-SRTP crypto attributes would
be rejected if those crypto attributes contained either a key lifetime or a
MKI parameter. While from a theoretical point of view this was defensible -
Asterisk does not support key lifetimes or multiple crypto keys - from a
practical point of view, this is quite a problem. A large number of endpoints
offer lifetimes/MKI, which Asterisk can tolerate so long as it doesn't actually
have to support anything more than a single key or refresh the key.
In reality, this is (so far as we've seen) always the case.

This patch is a forward port of Olle's work in the lingon-srtp-key-lifetime-1.8
branch. To quote Olle from ASTERISK-17721, it handles lifetime/MKI parameters
in the following fashion:

> The Lingon branch now handle lifetime and MKI parameters.
>
> We only accept lifetimes up to max for the crypto and higher than 10 hours
> for packetization of 20 ms (50 pps).
>
> We only handle MKI with index 1.
>
> We do not really bother with counting packets and reinviting at end of
> lifetime, so the min of 10 hours kind of takes care of most calls. If there
> are longer ones, we rely on the other side for re-invites.
>
> It's still not perfect, but I personally think this is an improvement. A
> configuration option for minimum lifetime accepted could be added.

When the patch was ported forward, I decided against adding a configuration
option as Olle's handling was more than sufficient for every case I've seen
come through the issue tracker or through interoperability testing. We can
revisit that decision if it proves to be false.

A few small other tweaks were made to the surrounding code to reduce
indentation and provide better type safety for the 'tag' parameter.

Review: https://reviewboard.asterisk.org/r/4419/
Review: https://reviewboard.asterisk.org/r/4418/

ASTERISK-17721 #close
Reported by: Terry Wilson

ASTERISK-17899 #close
Reported by: Dwayne Hubbard
patches:
  lingon-srtp-key-lifetime-1.8.diff uploaded by oej (License 5267)

ASTERISK-20233
Reported by: tootai

ASTERISK-22748
Reported by: Alejandro Mejia
........

Merged revisions 432239 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@432258 65c4cc65-6c06-0410-ace0-fbb531ad65f3

1 files changed, 81 insertions, 28 deletions

diff --git a/main/sdp_srtp.c b/main/sdp_srtp.c
index 31d633e38..fee3fea56 100644
--- a/main/sdp_srtp.c
+++ b/main/sdp_srtp.c
@@ -34,6 +34,7 @@
 
 ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 
+#include <math.h>
 #include "asterisk/options.h"
 #include "asterisk/utils.h"
 #include "asterisk/sdp_srtp.h"
@@ -68,7 +69,7 @@ void ast_sdp_srtp_destroy(struct ast_sdp_srtp *srtp)
 struct ast_sdp_crypto {
 	char *a_crypto;
 	unsigned char local_key[SRTP_MASTER_LEN];
-	char *tag;
+	int tag;
 	char local_key64[SRTP_MASTER_LEN64];
 	unsigned char remote_key[SRTP_MASTER_LEN];
 };
@@ -79,8 +80,6 @@ void ast_sdp_crypto_destroy(struct ast_sdp_crypto *crypto)
 {
 	ast_free(crypto->a_crypto);
 	crypto->a_crypto = NULL;
-	ast_free(crypto->tag);
-	crypto->tag = NULL;
 	ast_free(crypto);
 }
 
@@ -97,6 +96,7 @@ struct ast_sdp_crypto *ast_sdp_crypto_alloc(void)
 	if (!(p = ast_calloc(1, sizeof(*p)))) {
 		return NULL;
 	}
+	p->tag = 1;
 
 	if (res_srtp->get_random(p->local_key, sizeof(p->local_key)) < 0) {
 		ast_sdp_crypto_destroy(p);
@@ -109,13 +109,13 @@ struct ast_sdp_crypto *ast_sdp_crypto_alloc(void)
 
 	if (key_len != SRTP_MASTER_LEN) {
 		ast_log(LOG_ERROR, "base64 encode/decode bad len %d != %d\n", key_len, SRTP_MASTER_LEN);
-		ast_free(p);
+		ast_sdp_crypto_destroy(p);
 		return NULL;
 	}
 
 	if (memcmp(remote_key, p->local_key, SRTP_MASTER_LEN)) {
 		ast_log(LOG_ERROR, "base64 encode/decode bad key\n");
-		ast_free(p);
+		ast_sdp_crypto_destroy(p);
 		return NULL;
 	}
 
@@ -211,13 +211,15 @@ int ast_sdp_crypto_process(struct ast_rtp_instance *rtp, struct ast_sdp_srtp *sr
 	char *key_params = NULL;
 	char *key_param = NULL;
 	char *session_params = NULL;
-	char *key_salt = NULL;
-	char *lifetime = NULL;
+	char *key_salt = NULL;       /* The actual master key and key salt */
+	char *lifetime = NULL;       /* Key lifetime (# of RTP packets) */
+	char *mki = NULL;            /* Master Key Index */
 	int found = 0;
 	int key_len = 0;
 	int suite_val = 0;
 	unsigned char remote_key[SRTP_MASTER_LEN];
 	int taglen = 0;
+	double sdes_lifetime;
 	struct ast_sdp_crypto *crypto = srtp->crypto;
 
 	if (!ast_rtp_engine_srtp_is_registered()) {
@@ -236,6 +238,11 @@ int ast_sdp_crypto_process(struct ast_rtp_instance *rtp, struct ast_sdp_srtp *sr
 		return -1;
 	}
 
+	if (sscanf(tag, "%30d", &crypto->tag) != 1 || crypto->tag <= 0 || crypto->tag > 9) {
+		ast_log(LOG_WARNING, "Unacceptable a=crypto tag: %s\n", tag);
+		return -1;
+	}
+
 	if (!ast_strlen_zero(session_params)) {
 		ast_log(LOG_WARNING, "Unsupported crypto parameters: %s\n", session_params);
 		return -1;
@@ -255,33 +262,88 @@ int ast_sdp_crypto_process(struct ast_rtp_instance *rtp, struct ast_sdp_srtp *sr
 	}
 
 	while ((key_param = strsep(&key_params, ";"))) {
+		unsigned int n_lifetime;
 		char *method = NULL;
 		char *info = NULL;
 
 		method = strsep(&key_param, ":");
 		info = strsep(&key_param, ";");
+		sdes_lifetime = 0;
 
-		if (!strcmp(method, "inline")) {
-			key_salt = strsep(&info, "|");
-			lifetime = strsep(&info, "|");
+		if (strcmp(method, "inline")) {
+			continue;
+		}
 
-			if (lifetime) {
-				ast_log(LOG_NOTICE, "Crypto life time unsupported: %s\n", attr);
-				continue;
-			}
+		key_salt = strsep(&info, "|");
 
+		/* The next parameter can be either lifetime or MKI */
+		lifetime = strsep(&info, "|");
+		if (!lifetime) {
 			found = 1;
 			break;
 		}
+
+		mki = strchr(lifetime, ':');
+		if (mki) {
+			mki = lifetime;
+			lifetime = NULL;
+		} else {
+			mki = strsep(&info, "|");
+		}
+
+		if (mki && *mki != '1') {
+			ast_log(LOG_NOTICE, "Crypto MKI handling is not supported: ignoring attribute %s\n", attr);
+			continue;
+		}
+
+		if (lifetime) {
+			if (!strncmp(lifetime, "2^", 2)) {
+				char *lifetime_val = lifetime + 2;
+
+				/* Exponential lifetime */
+				if (sscanf(lifetime_val, "%30u", &n_lifetime) != 1) {
+					ast_log(LOG_NOTICE, "Failed to parse lifetime value in crypto attribute: %s\n", attr);
+					continue;
+				}
+
+				if (n_lifetime > 48) {
+					/* Yeah... that's a bit big. */
+					ast_log(LOG_NOTICE, "Crypto lifetime exponent of '%u' is a bit large; using 48\n", n_lifetime);
+					n_lifetime = 48;
+				}
+				sdes_lifetime = pow(2, n_lifetime);
+			} else {
+				/* Decimal lifetime */
+				if (sscanf(lifetime, "%30u", &n_lifetime) != 1) {
+					ast_log(LOG_NOTICE, "Failed to parse lifetime value in crypto attribute: %s\n", attr);
+					continue;
+				}
+				sdes_lifetime = n_lifetime;
+			}
+
+			/* Accept anything above 10 hours. Less than 10; reject. */
+			if (sdes_lifetime < 1800000) {
+				ast_log(LOG_NOTICE, "Rejecting crypto attribute '%s': lifetime '%f' too short\n", attr, sdes_lifetime);
+				continue;
+			}
+		}
+
+		ast_debug(2, "Crypto attribute '%s' accepted with lifetime '%f', MKI '%s'\n",
+			attr, sdes_lifetime, mki ? mki : "-");
+
+		found = 1;
+		break;
 	}
 
 	if (!found) {
-		ast_log(LOG_NOTICE, "SRTP crypto offer not acceptable\n");
+		ast_log(LOG_NOTICE, "SRTP crypto offer not acceptable: '%s'\n", attr);
 		return -1;
 	}
 
-	if ((key_len = ast_base64decode(remote_key, key_salt, sizeof(remote_key))) != SRTP_MASTER_LEN) {
-		ast_log(LOG_WARNING, "SRTP descriptions key %d != %d\n", key_len, SRTP_MASTER_LEN);
+	key_len = ast_base64decode(remote_key, key_salt, sizeof(remote_key));
+	if (key_len != SRTP_MASTER_LEN) {
+		ast_log(LOG_WARNING, "SRTP descriptions key length '%d' != master length '%d'\n",
+			key_len, SRTP_MASTER_LEN);
 		return -1;
 	}
 
@@ -296,15 +358,6 @@ int ast_sdp_crypto_process(struct ast_rtp_instance *rtp, struct ast_sdp_srtp *sr
 		return -1;
 	}
 
-	if (!crypto->tag) {
-		ast_debug(1, "Accepting crypto tag %s\n", tag);
-		crypto->tag = ast_strdup(tag);
-		if (!crypto->tag) {
-			ast_log(LOG_ERROR, "Could not allocate memory for tag\n");
-			return -1;
-		}
-	}
-
 	/* Finally, rebuild the crypto line */
 	if (ast_sdp_crypto_build_offer(crypto, taglen)) {
 		return -1;
@@ -321,8 +374,8 @@ int ast_sdp_crypto_build_offer(struct ast_sdp_crypto *p, int taglen)
 		ast_free(p->a_crypto);
 	}
 
-	if (ast_asprintf(&p->a_crypto, "%s AES_CM_128_HMAC_SHA1_%i inline:%s",
-			 p->tag ? p->tag : "1", taglen, p->local_key64) == -1) {
+	if (ast_asprintf(&p->a_crypto, "%d AES_CM_128_HMAC_SHA1_%i inline:%s",
+			 p->tag, taglen, p->local_key64) == -1) {
 			ast_log(LOG_ERROR, "Could not allocate memory for crypto line\n");
 		return -1;
 	}