reset SRTP ROC to zero when SSRC changes

workaround as per issue #102 for broken audio after SSRC change by
hold/unhold. workaround before full multi-SSRC support is available.

daemon/crypto.h

@@ -73,6 +73,7 @@ struct crypto_context {
 	char session_salt[SRTP_MAX_SESSION_SALT_LEN]; /* k_s */
 	char session_auth_key[SRTP_MAX_SESSION_AUTH_LEN];
 
+	u_int32_t ssrc;
 	u_int64_t last_index;
 	/* XXX replay list */
 	/* <from, to>? */
@@ -138,6 +139,7 @@ INLINE void crypto_cleanup(struct crypto_context *c) {
 INLINE void crypto_reset(struct crypto_context *c) {
 	crypto_cleanup(c);
 	c->last_index = 0;
+	c->ssrc = 0;
 }
 INLINE void crypto_params_copy(struct crypto_params *o, const struct crypto_params *i) {
 	crypto_params_cleanup(o);

daemon/rtp.c

@@ -189,6 +189,14 @@ int rtp_avp2savp(str *s, struct crypto_context *c) {
 	if (check_session_keys(c))
 		return -1;
 
+	/* SSRC is part of the crypto context and ROC must be reset when it changes */
+	if (G_UNLIKELY(!c->ssrc))
+		c->ssrc = rtp->ssrc;
+	else if (G_UNLIKELY(c->ssrc != rtp->ssrc)) {
+		c->last_index = 0;
+		c->ssrc = rtp->ssrc;
+	}
+
 	index = packet_index(c, rtp);
 
 	/* rfc 3711 section 3.1 */