Add API to require cryptex

allowing applications to reject packets not using cryptex using
  srtp_set_stream_require_cryptex(...)

as described in
  https://datatracker.ietf.org/doc/html/rfc9335#section-5.2

Fixes #804 (for 2_x_dev)

Author: fippo

include/srtp.h

@@ -1765,6 +1765,24 @@ srtp_err_status_t srtp_set_stream_use_cryptex(srtp_t session,
                                               const srtp_ssrc_t *ssrc,
                                               int enable);
 
+/**
+ * @brief srtp_set_stream_require_cryptex(session, ssrc, enable)
+ *
+ * Require cryptex, RFC 9335, processing for the stream identified by the given
+ * SSRC. For wildcard SSRC types the require cryptex setting is applied to the
+ * session template and any streams created from it.
+ *
+ * @param session is the SRTP session containing the stream to update.
+ * @param ssrc describes the SSRC to require cryptex for.
+ * @param enable whether to require sending and receiving cryptex.
+ *
+ * @returns srtp_err_status_ok on success, or srtp_err_status_bad_param if the
+ * stream or template cannot be found for the given SSRC.
+ */
+srtp_err_status_t srtp_set_stream_require_cryptex(srtp_t session,
+                                                 const srtp_ssrc_t *ssrc,
+                                                 int enable);
+
 /**
  * @}
  */

include/srtp_priv.h

@@ -146,6 +146,7 @@ typedef struct srtp_stream_ctx_t_ {
     int *enc_xtn_hdr;
     int enc_xtn_hdr_count;
     int use_cryptex;
+    int require_cryptex;
     uint32_t pending_roc;
     /*
     The next and prev pointers are here to allow for a stream list to be

srtp/srtp.c

@@ -235,6 +235,10 @@ static srtp_err_status_t srtp_cryptex_unprotect_init(
         *inuse = 0;
     }
 
+    if (stream->require_cryptex && !*inuse && hdr->x == 1) {
+        return srtp_err_status_cryptex_err;
+    }
+
     if (*inuse) {
         srtp_hdr_xtnd_t *xtn_hdr = srtp_get_rtp_xtn_hdr(hdr);
         *enc_start -=
@@ -5128,6 +5132,7 @@ static int set_cryptex_from_template_cb(srtp_stream_t stream, void *raw_data)
     if (stream->session_keys[0].rtp_auth ==
         data->template->session_keys[0].rtp_auth) {
         stream->use_cryptex = data->template->use_cryptex;
+        stream->require_cryptex = data->template->require_cryptex;
     }
 
     return 0;
@@ -5171,6 +5176,44 @@ srtp_err_status_t srtp_set_stream_use_cryptex(srtp_t session,
     return srtp_err_status_ok;
 }
 
+srtp_err_status_t srtp_set_stream_require_cryptex(srtp_t session,
+                                                 const srtp_ssrc_t *ssrc,
+                                                 int enable)
+{
+    srtp_stream_t stream;
+
+    if (session == NULL || ssrc == NULL) {
+        return srtp_err_status_bad_param;
+    }
+
+    switch (ssrc->type) {
+    case ssrc_specific:
+        stream = srtp_get_stream(session, htonl(ssrc->value));
+        if (stream == NULL) {
+            return srtp_err_status_bad_param;
+        }
+        stream->require_cryptex = enable != 0;
+        break;
+    case ssrc_any_inbound:
+    case ssrc_any_outbound: {
+        struct set_cryptex_from_template_data data;
+
+        if (session->stream_template == NULL) {
+            return srtp_err_status_bad_param;
+        }
+        session->stream_template->require_cryptex = enable != 0;
+        data.template = session->stream_template;
+        srtp_stream_list_for_each(session->stream_list,
+                                  set_cryptex_from_template_cb, &data);
+        break;
+    }
+    default:
+        return srtp_err_status_bad_param;
+    }
+
+    return srtp_err_status_ok;
+}
+
 #ifndef SRTP_NO_STREAM_LIST
 
 /* in the default implementation, we have an intrusive doubly-linked list */

test/srtp_driver.c

@@ -115,6 +115,8 @@ srtp_err_status_t srtp_test_cryptex_csrc_but_no_extension_header(void);
 
 srtp_err_status_t srtp_test_cryptex_disable(void);
 
+srtp_err_status_t srtp_test_require_cryptex(void);
+
 double srtp_bits_per_second(int msg_len_octets, const srtp_policy_t *policy);
 
 double srtp_rejections_per_second(int msg_len_octets,
@@ -684,6 +686,14 @@ int main(int argc, char *argv[])
             printf("failed\n");
             exit(1);
         }
+
+        printf("testing require_cryptex()...");
+        if (srtp_test_require_cryptex() == srtp_err_status_ok) {
+            printf("passed\n");
+        } else {
+            printf("failed\n");
+            exit(1);
+        }
     }
 
     if (do_stream_list) {
@@ -2707,6 +2717,41 @@ srtp_err_status_t srtp_test_cryptex_disable(void)
     return srtp_err_status_ok;
 }
 
+srtp_err_status_t srtp_test_require_cryptex(void)
+{
+    srtp_policy_t policy;
+    memset(&policy, 0, sizeof(policy));
+    srtp_crypto_policy_set_rtp_default(&policy.rtp);
+    srtp_crypto_policy_set_rtcp_default(&policy.rtcp);
+    policy.ssrc.type = ssrc_specific;
+    policy.ssrc.value = 0xcafebabe;
+    policy.key = test_key;
+    policy.window_size = 128;
+    policy.allow_repeat_tx = 0;
+    policy.next = NULL;
+
+    srtp_t srtp_snd, srtp_recv;
+    CHECK_OK(srtp_create(&srtp_snd, &policy));
+    CHECK_OK(srtp_set_stream_use_cryptex(srtp_snd, &policy.ssrc, 0));
+    CHECK_OK(srtp_create(&srtp_recv, &policy));
+    CHECK_OK(srtp_set_stream_require_cryptex(srtp_recv, &policy.ssrc, 1));
+
+    int packet_len;
+    srtp_hdr_t *packet =
+        srtp_create_test_packet_ext_hdr(100, policy.ssrc.value, &packet_len);
+
+    CHECK_OK(srtp_protect(srtp_snd, packet, &packet_len));
+
+    CHECK_RETURN(srtp_unprotect(srtp_recv, packet, &packet_len),
+                 srtp_err_status_cryptex_err);
+
+    CHECK_OK(srtp_dealloc(srtp_snd));
+    CHECK_OK(srtp_dealloc(srtp_recv));
+    free(packet);
+
+    return srtp_err_status_ok;
+}
+
 #ifdef GCM
 /*
  * srtp_validate_gcm() verifies the correctness of libsrtp by comparing