--- a/src/plugins/janus_nosip.c
+++ b/src/plugins/janus_nosip.c
@@ -392,6 +392,31 @@ static void janus_nosip_message_free(janus_nosip_message *msg) {
 
 
 /* SRTP stuff (in case we need SDES) */
+/* SRTP is decided per media line: a line uses SDES-SRTP (in both directions)
+ * only when an SRTP context exists for it in both directions. The peer may
+ * use SDES on a line and plain RTP on another one (e.g., SRTP audio and RTP
+ * video): checking a single session-wide flag would make us use contexts
+ * that were never created for the plain RTP line (and crash, for the
+ * inbound one), or send SRTP to a peer that expects RTP. */
+static gboolean janus_nosip_srtp_active(janus_nosip_session *session, gboolean video) {
+	if(video)
+		return session->media.video_srtp_in != NULL && session->media.video_srtp_out != NULL;
+	return session->media.audio_srtp_in != NULL && session->media.audio_srtp_out != NULL;
+}
+/* Whether the peer uses SDES-SRTP on some media lines but not on this one:
+ * then we must not add a crypto attribute to this line in our SDP. Only a
+ * line the peer has already negotiated (it has a remote port) can be a plain
+ * line: a line that is new in a re-offer (e.g., video added to an audio call)
+ * has no remote SRTP context yet only because the peer has not answered it,
+ * and must be offered with crypto like the other lines. */
+static gboolean janus_nosip_srtp_plain_line(janus_nosip_session *session, gboolean video) {
+	if(!session->media.has_srtp_remote)
+		return FALSE;
+	if((video ? session->media.remote_video_rtp_port : session->media.remote_audio_rtp_port) == 0)
+		return FALSE;
+	return (video ? session->media.video_srtp_in : session->media.audio_srtp_in) == NULL;
+}
+
 static int janus_nosip_srtp_set_local(janus_nosip_session *session, gboolean video, char **profile, char **crypto) {
 	if(session == NULL)
 		return -1;
@@ -1089,7 +1114,7 @@ void janus_nosip_incoming_rtp(janus_plugin_session *handle, janus_plugin_rtp *pa
 			/* Save the frame if we're recording */
 			janus_recorder_save_frame(video ? session->vrc : session->arc, buf, len);
 			/* Is SRTP involved? */
-			if(session->media.has_srtp_local) {
+			if(janus_nosip_srtp_active(session, video)) {
 				char sbuf[2048];
 				memcpy(&sbuf, buf, len);
 				int protected = len;
@@ -1150,7 +1175,7 @@ void janus_nosip_incoming_rtcp(janus_plugin_session *handle, janus_plugin_rtcp *
 				(video ? session->media.video_ssrc : session->media.audio_ssrc),
 				(video ? session->media.video_ssrc_peer : session->media.audio_ssrc_peer));
 			/* Is SRTP involved? */
-			if(session->media.has_srtp_local) {
+			if(janus_nosip_srtp_active(session, video)) {
 				char sbuf[2048];
 				memcpy(&sbuf, buf, len);
 				int protected = len;
@@ -1930,7 +1955,7 @@ char *janus_nosip_sdp_manipulate(janus_nosip_session *session, janus_sdp *sdp, g
 		m->proto = g_strdup(session->media.require_srtp ? "RTP/SAVP" : "RTP/AVP");
 		if(m->type == JANUS_SDP_AUDIO) {
 			m->port = session->media.local_audio_rtp_port;
-			if(session->media.has_srtp_local) {
+			if(session->media.has_srtp_local && !janus_nosip_srtp_plain_line(session, FALSE)) {
 				if(!session->media.audio_srtp_local_profile || !session->media.audio_srtp_local_crypto) {
 					janus_nosip_srtp_set_local(session, FALSE, &session->media.audio_srtp_local_profile, &session->media.audio_srtp_local_crypto);
 				}
@@ -1942,7 +1967,7 @@ char *janus_nosip_sdp_manipulate(janus_nosip_session *session, janus_sdp *sdp, g
 			}
 		} else if(m->type == JANUS_SDP_VIDEO) {
 			m->port = session->media.local_video_rtp_port;
-			if(session->media.has_srtp_local) {
+			if(session->media.has_srtp_local && !janus_nosip_srtp_plain_line(session, TRUE)) {
 				if(!session->media.video_srtp_local_profile || !session->media.video_srtp_local_crypto) {
 					janus_nosip_srtp_set_local(session, TRUE, &session->media.video_srtp_local_profile, &session->media.video_srtp_local_crypto);
 				}
@@ -2474,7 +2499,7 @@ static void *janus_nosip_relay_thread(void *data) {
 							video ? session->media.video_ssrc_peer : session->media.audio_ssrc_peer);
 					}
 					/* Is this SRTP? */
-					if(session->media.has_srtp_remote) {
+					if(janus_nosip_srtp_active(session, video)) {
 						int buflen = bytes;
 						srtp_err_status_t res = srtp_unprotect(
 							(video ? session->media.video_srtp_in : session->media.audio_srtp_in),
@@ -2528,7 +2553,7 @@ static void *janus_nosip_relay_thread(void *data) {
 						/* Not an RTCP packet? */
 						continue;
 					}
-					if(session->media.has_srtp_remote) {
+					if(janus_nosip_srtp_active(session, video)) {
 						int buflen = bytes;
 						srtp_err_status_t res = srtp_unprotect_rtcp(
 							(video ? session->media.video_srtp_in : session->media.audio_srtp_in),