res_srtp.c

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/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 2005, Mikael Magnusson
 *
 * Mikael Magnusson <[email protected]>
 *
 * See http://www.asterisk.org for more information about
 * the Asterisk project. Please do not directly contact
 * any of the maintainers of this project for assistance;
 * the project provides a web site, mailing lists and IRC
 * channels for your use.
 *
 * This program is free software, distributed under the terms of
 * the GNU General Public License Version 2. See the LICENSE file
 * at the top of the source tree.
 *
 * Builds on libSRTP http://srtp.sourceforge.net
 */

/*! \file res_srtp.c
 *
 * \brief Secure RTP (SRTP)
 *
 * Secure RTP (SRTP)
 * Specified in RFC 3711.
 *
 * \author Mikael Magnusson <[email protected]>
 */

/*** MODULEINFO
   <depend>srtp</depend>
   <use type="external">openssl</use>
   <support_level>core</support_level>
***/

/* See https://wiki.asterisk.org/wiki/display/AST/Secure+Calling */

#include "asterisk.h"                   /* for NULL, size_t, memcpy, etc */

#include <math.h>                       /* for pow */

#if HAVE_SRTP_VERSION > 1
# include <srtp2/srtp.h>
# include "srtp/srtp_compat.h"
# include <openssl/rand.h>
#else
# include <srtp/srtp.h>
# ifdef HAVE_OPENSSL
#  include <openssl/rand.h>
# else
#  include <srtp/crypto_kernel.h>
# endif
#endif

#include "asterisk/astobj2.h"           /* for ao2_t_ref, etc */
#include "asterisk/frame.h"             /* for AST_FRIENDLY_OFFSET */
#include "asterisk/logger.h"            /* for ast_log, ast_debug, etc */
#include "asterisk/module.h"            /* for ast_module_info, etc */
#include "asterisk/sdp_srtp.h"
#include "asterisk/res_srtp.h"          /* for ast_srtp_cb, ast_srtp_suite, etc */
#include "asterisk/rtp_engine.h"        /* for ast_rtp_engine_register_srtp, etc */
#include "asterisk/utils.h"             /* for ast_free, ast_calloc */

struct ast_srtp {
   struct ast_rtp_instance *rtp;
   struct ao2_container *policies;
   srtp_t session;
   const struct ast_srtp_cb *cb;
   void *data;
   int warned;
   unsigned char buf[8192 + AST_FRIENDLY_OFFSET];
   unsigned char rtcpbuf[8192 + AST_FRIENDLY_OFFSET];
};

struct ast_srtp_policy {
   srtp_policy_t sp;
};

/*! Tracks whether or not we've initialized the libsrtp library */
static int g_initialized = 0;

/* SRTP functions */
static int ast_srtp_create(struct ast_srtp **srtp, struct ast_rtp_instance *rtp, struct ast_srtp_policy *policy);
static int ast_srtp_replace(struct ast_srtp **srtp, struct ast_rtp_instance *rtp, struct ast_srtp_policy *policy);
static void ast_srtp_destroy(struct ast_srtp *srtp);
static int ast_srtp_add_stream(struct ast_srtp *srtp, struct ast_srtp_policy *policy);
static int ast_srtp_change_source(struct ast_srtp *srtp, unsigned int from_ssrc, unsigned int to_ssrc);

static int ast_srtp_unprotect(struct ast_srtp *srtp, void *buf, int *len, int rtcp);
static int ast_srtp_protect(struct ast_srtp *srtp, void **buf, int *len, int rtcp);
static void ast_srtp_set_cb(struct ast_srtp *srtp, const struct ast_srtp_cb *cb, void *data);
static int ast_srtp_get_random(unsigned char *key, size_t len);

/* Policy functions */
static struct ast_srtp_policy *ast_srtp_policy_alloc(void);
static void ast_srtp_policy_destroy(struct ast_srtp_policy *policy);
static int ast_srtp_policy_set_suite(struct ast_srtp_policy *policy, enum ast_srtp_suite suite);
static int ast_srtp_policy_set_master_key(struct ast_srtp_policy *policy, const unsigned char *key, size_t key_len, const unsigned char *salt, size_t salt_len);
static void ast_srtp_policy_set_ssrc(struct ast_srtp_policy *policy, unsigned long ssrc, int inbound);

static struct ast_srtp_res srtp_res = {
   .create = ast_srtp_create,
   .replace = ast_srtp_replace,
   .destroy = ast_srtp_destroy,
   .add_stream = ast_srtp_add_stream,
   .change_source = ast_srtp_change_source,
   .set_cb = ast_srtp_set_cb,
   .unprotect = ast_srtp_unprotect,
   .protect = ast_srtp_protect,
   .get_random = ast_srtp_get_random
};

static struct ast_srtp_policy_res policy_res = {
   .alloc = ast_srtp_policy_alloc,
   .destroy = ast_srtp_policy_destroy,
   .set_suite = ast_srtp_policy_set_suite,
   .set_master_key = ast_srtp_policy_set_master_key,
   .set_ssrc = ast_srtp_policy_set_ssrc
};

static const char *srtp_errstr(int err)
{
   switch(err) {
   case err_status_ok:
      return "nothing to report";
   case err_status_fail:
      return "unspecified failure";
   case err_status_bad_param:
      return "unsupported parameter";
   case err_status_alloc_fail:
      return "couldn't allocate memory";
   case err_status_dealloc_fail:
      return "couldn't deallocate properly";
   case err_status_init_fail:
      return "couldn't initialize";
   case err_status_terminus:
      return "can't process as much data as requested";
   case err_status_auth_fail:
      return "authentication failure";
   case err_status_cipher_fail:
      return "cipher failure";
   case err_status_replay_fail:
      return "replay check failed (bad index)";
   case err_status_replay_old:
      return "replay check failed (index too old)";
   case err_status_algo_fail:
      return "algorithm failed test routine";
   case err_status_no_such_op:
      return "unsupported operation";
   case err_status_no_ctx:
      return "no appropriate context found";
   case err_status_cant_check:
      return "unable to perform desired validation";
   case err_status_key_expired:
      return "can't use key any more";
   default:
      return "unknown";
   }
}

static int policy_hash_fn(const void *obj, const int flags)
{
   const struct ast_srtp_policy *policy = obj;

   return policy->sp.ssrc.type == ssrc_specific ? policy->sp.ssrc.value : policy->sp.ssrc.type;
}

static int policy_cmp_fn(void *obj, void *arg, int flags)
{
   const struct ast_srtp_policy *one = obj, *two = arg;

   return one->sp.ssrc.type == two->sp.ssrc.type && one->sp.ssrc.value == two->sp.ssrc.value;
}

static struct ast_srtp_policy *find_policy(struct ast_srtp *srtp, const srtp_policy_t *policy, int flags)
{
   struct ast_srtp_policy tmp = {
      .sp = {
         .ssrc.type = policy->ssrc.type,
         .ssrc.value = policy->ssrc.value,
      },
   };

   return ao2_t_find(srtp->policies, &tmp, flags, "Looking for policy");
}

static struct ast_srtp *res_srtp_new(void)
{
   struct ast_srtp *srtp;

   if (!(srtp = ast_calloc(1, sizeof(*srtp)))) {
      ast_log(LOG_ERROR, "Unable to allocate memory for srtp\n");
      return NULL;
   }

   srtp->policies = ao2_t_container_alloc_hash(AO2_ALLOC_OPT_LOCK_MUTEX, 0, 5,
      policy_hash_fn, NULL, policy_cmp_fn, "SRTP policy container");
   if (!srtp->policies) {
      ast_free(srtp);
      return NULL;
   }

   srtp->warned = 1;

   return srtp;
}

/*
  struct ast_srtp_policy
*/
static void srtp_event_cb(srtp_event_data_t *data)
{
   switch (data->event) {
   case event_ssrc_collision:
      ast_debug(1, "SSRC collision\n");
      break;
   case event_key_soft_limit:
      ast_debug(1, "event_key_soft_limit\n");
      break;
   case event_key_hard_limit:
      ast_debug(1, "event_key_hard_limit\n");
      break;
   case event_packet_index_limit:
      ast_debug(1, "event_packet_index_limit\n");
      break;
   }
}

static void ast_srtp_policy_set_ssrc(struct ast_srtp_policy *policy,
      unsigned long ssrc, int inbound)
{
   if (ssrc) {
      policy->sp.ssrc.type = ssrc_specific;
      policy->sp.ssrc.value = ssrc;
   } else {
      policy->sp.ssrc.type = inbound ? ssrc_any_inbound : ssrc_any_outbound;
   }
}

static void policy_destructor(void *obj)
{
   struct ast_srtp_policy *policy = obj;

   if (policy->sp.key) {
      ast_free(policy->sp.key);
      policy->sp.key = NULL;
   }
}

static struct ast_srtp_policy *ast_srtp_policy_alloc()
{
   struct ast_srtp_policy *tmp;

   if (!(tmp = ao2_t_alloc(sizeof(*tmp), policy_destructor, "Allocating policy"))) {
      ast_log(LOG_ERROR, "Unable to allocate memory for srtp_policy\n");
   }

   return tmp;
}

static void ast_srtp_policy_destroy(struct ast_srtp_policy *policy)
{
   ao2_t_ref(policy, -1, "Destroying policy");
}

static int policy_set_suite(crypto_policy_t *p, enum ast_srtp_suite suite)
{
   switch (suite) {
   case AST_AES_CM_128_HMAC_SHA1_80:
      crypto_policy_set_aes_cm_128_hmac_sha1_80(p);
      return 0;

   case AST_AES_CM_128_HMAC_SHA1_32:
      crypto_policy_set_aes_cm_128_hmac_sha1_32(p);
      return 0;

#ifdef HAVE_SRTP_192
   case AST_AES_CM_192_HMAC_SHA1_80:
      crypto_policy_set_aes_cm_192_hmac_sha1_80(p);
      return 0;

   case AST_AES_CM_192_HMAC_SHA1_32:
      crypto_policy_set_aes_cm_192_hmac_sha1_32(p);
      return 0;
#endif
#ifdef HAVE_SRTP_256
   case AST_AES_CM_256_HMAC_SHA1_80:
      crypto_policy_set_aes_cm_256_hmac_sha1_80(p);
      return 0;

   case AST_AES_CM_256_HMAC_SHA1_32:
      crypto_policy_set_aes_cm_256_hmac_sha1_32(p);
      return 0;
#endif
#ifdef HAVE_SRTP_GCM
   case AST_AES_GCM_128:
      crypto_policy_set_aes_gcm_128_16_auth(p);
      return 0;

   case AST_AES_GCM_256:
      crypto_policy_set_aes_gcm_256_16_auth(p);
      return 0;

   case AST_AES_GCM_128_8:
      crypto_policy_set_aes_gcm_128_8_auth(p);
      return 0;

   case AST_AES_GCM_256_8:
      crypto_policy_set_aes_gcm_256_8_auth(p);
      return 0;
#endif

   default:
      ast_log(LOG_ERROR, "Invalid crypto suite: %u\n", suite);
      return -1;
   }
}

static int ast_srtp_policy_set_suite(struct ast_srtp_policy *policy, enum ast_srtp_suite suite)
{
   return policy_set_suite(&policy->sp.rtp, suite) | policy_set_suite(&policy->sp.rtcp, suite);
}

static int ast_srtp_policy_set_master_key(struct ast_srtp_policy *policy, const unsigned char *key, size_t key_len, const unsigned char *salt, size_t salt_len)
{
   size_t size = key_len + salt_len;
   unsigned char *master_key;

   if (policy->sp.key) {
      ast_free(policy->sp.key);
      policy->sp.key = NULL;
   }

   if (!(master_key = ast_calloc(1, size))) {
      return -1;
   }

   memcpy(master_key, key, key_len);
   memcpy(master_key + key_len, salt, salt_len);

   policy->sp.key = master_key;

   return 0;
}

static int ast_srtp_get_random(unsigned char *key, size_t len)
{
#ifdef HAVE_OPENSSL
   return RAND_bytes(key, len) > 0 ? 0: -1;
#else
   return crypto_get_random(key, len) != err_status_ok ? -1: 0;
#endif
}

static void ast_srtp_set_cb(struct ast_srtp *srtp, const struct ast_srtp_cb *cb, void *data)
{
   if (!srtp) {
      return;
   }

   srtp->cb = cb;
   srtp->data = data;
}

/* Vtable functions */
static int ast_srtp_unprotect(struct ast_srtp *srtp, void *buf, int *len, int flags)
{
   int res = 0;
   int i;
   int rtcp  = (flags & 0x01) >> 0;
   int retry = (flags & 0x02) >> 1;
   struct ast_rtp_instance_stats stats = {0,};

tryagain:

   if (!srtp->session) {
      ast_log(LOG_ERROR, "SRTP unprotect %s - missing session\n", rtcp ? "rtcp" : "rtp");
      errno = EINVAL;
      return -1;
   }

   for (i = 0; i < 2; i++) {
      res = rtcp ? srtp_unprotect_rtcp(srtp->session, buf, len) : srtp_unprotect(srtp->session, buf, len);
      if (res != err_status_no_ctx) {
         break;
      }

      if (srtp->cb && srtp->cb->no_ctx) {
         if (ast_rtp_instance_get_stats(srtp->rtp, &stats, AST_RTP_INSTANCE_STAT_REMOTE_SSRC)) {
            break;
         }
         if (srtp->cb->no_ctx(srtp->rtp, stats.remote_ssrc, srtp->data) < 0) {
            break;
         }
      } else {
         break;
      }
   }

   if (retry == 0  && res == err_status_replay_old) {
      ast_log(AST_LOG_NOTICE, "SRTP unprotect failed with %s, retrying\n", srtp_errstr(res));

      if (srtp->session) {
         struct ast_srtp_policy *policy;
         struct ao2_iterator it;
         int policies_count;

         /* dealloc first */
         ast_debug(5, "SRTP destroy before re-create\n");
         srtp_dealloc(srtp->session);

         /* get the count */
         policies_count = ao2_container_count(srtp->policies);

         /* get the first to build up */
         it = ao2_iterator_init(srtp->policies, 0);
         policy = ao2_iterator_next(&it);

         ast_debug(5, "SRTP try to re-create\n");
         if (policy) {
            int res_srtp_create = srtp_create(&srtp->session, &policy->sp);
            if (res_srtp_create == err_status_ok) {
               ast_debug(5, "SRTP re-created with first policy\n");
               ao2_t_ref(policy, -1, "Unreffing first policy for re-creating srtp session");

               /* if we have more than one policy, add them */
               if (policies_count > 1) {
                  ast_debug(5, "Add all the other %d policies\n",
                     policies_count - 1);
                  while ((policy = ao2_iterator_next(&it))) {
                     srtp_add_stream(srtp->session, &policy->sp);
                     ao2_t_ref(policy, -1, "Unreffing n-th policy for re-creating srtp session");
                  }
               }

               retry++;
               ao2_iterator_destroy(&it);
               goto tryagain;
            }
            ast_log(LOG_ERROR, "SRTP session could not be re-created after unprotect failure: %s\n", srtp_errstr(res_srtp_create));

            /* If srtp_create() fails with a previously alloced session, it will have been dealloced before returning. */
            srtp->session = NULL;

            ao2_t_ref(policy, -1, "Unreffing first policy after srtp_create failed");
         }
         ao2_iterator_destroy(&it);
      }
   }

   if (!srtp->session) {
      errno = EINVAL;
      return -1;
   }

   if (res != err_status_ok && res != err_status_replay_fail ) {
      /*
       * Authentication failures happen when an active attacker tries to
       * insert malicious RTP packets. Furthermore, authentication failures
       * happen, when the other party encrypts the sRTP data in an unexpected
       * way. This happens quite often with RTCP. Therefore, when you see
       * authentication failures, try to identify the implementation
       * (author and product name) used by your other party. Try to investigate
       * whether they use a custom library or an outdated version of libSRTP.
       */
      if (rtcp) {
         ast_verb(2, "SRTCP unprotect failed on SSRC %u because of %s\n",
            ast_rtp_instance_get_ssrc(srtp->rtp), srtp_errstr(res));
      } else {
         if ((srtp->warned >= 10) && !((srtp->warned - 10) % 150)) {
            ast_verb(2, "SRTP unprotect failed on SSRC %u because of %s %d\n",
               ast_rtp_instance_get_ssrc(srtp->rtp), srtp_errstr(res), srtp->warned);
            srtp->warned = 11;
         } else {
            srtp->warned++;
         }
      }
      errno = EAGAIN;
      return -1;
   }

   return *len;
}

static int ast_srtp_protect(struct ast_srtp *srtp, void **buf, int *len, int rtcp)
{
   int res;
   unsigned char *localbuf;

   if (!srtp->session) {
      ast_log(LOG_ERROR, "SRTP protect %s - missing session\n", rtcp ? "rtcp" : "rtp");
      errno = EINVAL;
      return -1;
   }

   if ((*len + SRTP_MAX_TRAILER_LEN) > sizeof(srtp->buf)) {
      return -1;
   }

   localbuf = rtcp ? srtp->rtcpbuf : srtp->buf;

   memcpy(localbuf, *buf, *len);

   if ((res = rtcp ? srtp_protect_rtcp(srtp->session, localbuf, len) : srtp_protect(srtp->session, localbuf, len)) != err_status_ok && res != err_status_replay_fail) {
      ast_log(LOG_WARNING, "SRTP protect: %s\n", srtp_errstr(res));
      return -1;
   }

   *buf = localbuf;
   return *len;
}

static int ast_srtp_create(struct ast_srtp **srtp, struct ast_rtp_instance *rtp, struct ast_srtp_policy *policy)
{
   struct ast_srtp *temp;
   int status;

   if (!(temp = res_srtp_new())) {
      return -1;
   }
   ast_module_ref(ast_module_info->self);

   /* Any failures after this point can use ast_srtp_destroy to destroy the instance */
   status = srtp_create(&temp->session, &policy->sp);
   if (status != err_status_ok) {
      /* Session either wasn't created or was created and dealloced. */
      temp->session = NULL;
      ast_srtp_destroy(temp);
      ast_log(LOG_ERROR, "Failed to create srtp session on rtp instance (%p) - %s\n",
            rtp, srtp_errstr(status));
      return -1;
   }

   temp->rtp = rtp;
   *srtp = temp;

   ao2_t_link((*srtp)->policies, policy, "Created initial policy");

   return 0;
}

static int ast_srtp_replace(struct ast_srtp **srtp, struct ast_rtp_instance *rtp, struct ast_srtp_policy *policy)
{
   struct ast_srtp *old = *srtp;
   int res = ast_srtp_create(srtp, rtp, policy);

   if (!res && old) {
      ast_srtp_destroy(old);
   }

   if (res) {
      ast_log(LOG_ERROR, "Failed to replace srtp (%p) on rtp instance (%p) "
            "- keeping old\n", *srtp, rtp);
   }

   return res;
}

static void ast_srtp_destroy(struct ast_srtp *srtp)
{
   if (srtp->session) {
      srtp_dealloc(srtp->session);
   }

   ao2_t_callback(srtp->policies, OBJ_UNLINK | OBJ_NODATA | OBJ_MULTIPLE, NULL, NULL, "Unallocate policy");
   ao2_t_ref(srtp->policies, -1, "Destroying container");

   ast_free(srtp);
   ast_module_unref(ast_module_info->self);
}

static int ast_srtp_add_stream(struct ast_srtp *srtp, struct ast_srtp_policy *policy)
{
   struct ast_srtp_policy *match;

   /* For existing streams, replace if its an SSRC stream, or bail if its a wildcard */
   if ((match = find_policy(srtp, &policy->sp, OBJ_POINTER))) {
      if (policy->sp.ssrc.type != ssrc_specific) {
         ast_log(AST_LOG_WARNING, "Cannot replace an existing wildcard policy\n");
         ao2_t_ref(match, -1, "Unreffing already existing policy");
         return -1;
      } else {
         if (srtp_remove_stream(srtp->session, match->sp.ssrc.value) != err_status_ok) {
            ast_log(AST_LOG_WARNING, "Failed to remove SRTP stream for SSRC %u\n", match->sp.ssrc.value);
         }
         ao2_t_unlink(srtp->policies, match, "Remove existing match policy");
         ao2_t_ref(match, -1, "Unreffing already existing policy");
      }
   }

   ast_debug(3, "Adding new policy for %s %u\n",
      policy->sp.ssrc.type == ssrc_specific ? "SSRC" : "type",
      policy->sp.ssrc.type == ssrc_specific ? policy->sp.ssrc.value : policy->sp.ssrc.type);
   if (srtp_add_stream(srtp->session, &policy->sp) != err_status_ok) {
      ast_log(AST_LOG_WARNING, "Failed to add SRTP stream for %s %u\n",
         policy->sp.ssrc.type == ssrc_specific ? "SSRC" : "type",
         policy->sp.ssrc.type == ssrc_specific ? policy->sp.ssrc.value : policy->sp.ssrc.type);
      return -1;
   }

   ao2_t_link(srtp->policies, policy, "Added additional stream");

   return 0;
}

static int ast_srtp_change_source(struct ast_srtp *srtp, unsigned int from_ssrc, unsigned int to_ssrc)
{
   struct ast_srtp_policy *match;
   struct srtp_policy_t sp = {
      .ssrc.type = ssrc_specific,
      .ssrc.value = from_ssrc,
   };
   err_status_t status;

   /* If we find a match, return and unlink it from the container so we
    * can change the SSRC (which is part of the hash) and then have
    * ast_srtp_add_stream link it back in if all is well */
   if ((match = find_policy(srtp, &sp, OBJ_POINTER | OBJ_UNLINK))) {
      match->sp.ssrc.value = to_ssrc;
      if (ast_srtp_add_stream(srtp, match)) {
         ast_log(LOG_WARNING, "Couldn't add stream\n");
      } else if ((status = srtp_remove_stream(srtp->session, from_ssrc))) {
         ast_debug(3, "Couldn't remove stream (%u)\n", status);
      }
      ao2_t_ref(match, -1, "Unreffing found policy in change_source");
   }

   return 0;
}

struct ast_sdp_crypto {
   char *a_crypto;
   unsigned char local_key[SRTP_MAX_KEY_LEN];
   int tag;
   char local_key64[((SRTP_MAX_KEY_LEN) * 8 + 5) / 6 + 1];
   unsigned char remote_key[SRTP_MAX_KEY_LEN];
   int key_len;
};

static void res_sdp_crypto_dtor(struct ast_sdp_crypto *crypto)
{
   if (crypto) {
      ast_free(crypto->a_crypto);
      crypto->a_crypto = NULL;
      ast_free(crypto);

      ast_module_unref(ast_module_info->self);
   }
}

static struct ast_sdp_crypto *crypto_init_keys(struct ast_sdp_crypto *p, const int key_len)
{
   unsigned char remote_key[key_len];

   if (srtp_res.get_random(p->local_key, key_len) < 0) {
      return NULL;
   }

   ast_base64encode(p->local_key64, p->local_key, key_len, sizeof(p->local_key64));

   p->key_len = ast_base64decode(remote_key, p->local_key64, sizeof(remote_key));

   if (p->key_len != key_len) {
      ast_log(LOG_ERROR, "base64 encode/decode bad len %d != %d\n", p->key_len, key_len);
      return NULL;
   }

   if (memcmp(remote_key, p->local_key, p->key_len)) {
      ast_log(LOG_ERROR, "base64 encode/decode bad key\n");
      return NULL;
   }

   ast_debug(1 , "local_key64 %s len %zu\n", p->local_key64, strlen(p->local_key64));

   return p;
}

static struct ast_sdp_crypto *sdp_crypto_alloc(const int key_len)
{
   struct ast_sdp_crypto *p, *result;

   if (!(p = ast_calloc(1, sizeof(*p)))) {
      return NULL;
   }
   p->tag = 1;
   ast_module_ref(ast_module_info->self);

   /* default is a key which uses AST_AES_CM_128_HMAC_SHA1_xx */
   result = crypto_init_keys(p, key_len);
   if (!result) {
      res_sdp_crypto_dtor(p);
   }

   return result;
}

static struct ast_sdp_crypto *res_sdp_crypto_alloc(void)
{
   return sdp_crypto_alloc(SRTP_MASTER_KEY_LEN);
}

static int res_sdp_crypto_build_offer(struct ast_sdp_crypto *p, int taglen)
{
   int res;

   /* Rebuild the crypto line */
   ast_free(p->a_crypto);
   p->a_crypto = NULL;

   if ((taglen & 0x007f) == 8) {
      res = ast_asprintf(&p->a_crypto, "%d AEAD_AES_%d_GCM_%d inline:%s",
         p->tag, 128 + ((taglen & 0x0300) >> 2), taglen & 0x007f, p->local_key64);
   } else if ((taglen & 0x007f) == 16) {
      res = ast_asprintf(&p->a_crypto, "%d AEAD_AES_%d_GCM inline:%s",
         p->tag, 128 + ((taglen & 0x0300) >> 2), p->local_key64);
   } else if ((taglen & 0x0300) && !(taglen & 0x0080)) {
      res = ast_asprintf(&p->a_crypto, "%d AES_%d_CM_HMAC_SHA1_%d inline:%s",
         p->tag, 128 + ((taglen & 0x0300) >> 2), taglen & 0x007f, p->local_key64);
   } else {
      res = ast_asprintf(&p->a_crypto, "%d AES_CM_%d_HMAC_SHA1_%d inline:%s",
         p->tag, 128 + ((taglen & 0x0300) >> 2), taglen & 0x007f, p->local_key64);
   }
   if (res == -1 || !p->a_crypto) {
      ast_log(LOG_ERROR, "Could not allocate memory for crypto line\n");
      return -1;
   }

   ast_debug(1, "Crypto line: a=crypto:%s\n", p->a_crypto);

   return 0;
}

static int set_crypto_policy(struct ast_srtp_policy *policy, int suite_val, const unsigned char *master_key, int key_len, unsigned long ssrc, int inbound)
{
   if (policy_res.set_master_key(policy, master_key, key_len, NULL, 0) < 0) {
      return -1;
   }

   if (policy_res.set_suite(policy, suite_val)) {
      ast_log(LOG_WARNING, "Could not set remote SRTP suite\n");
      return -1;
   }

   policy_res.set_ssrc(policy, ssrc, inbound);

   return 0;
}

static int crypto_activate(struct ast_sdp_crypto *p, int suite_val, unsigned char *remote_key, int key_len, struct ast_rtp_instance *rtp)
{
   struct ast_srtp_policy *local_policy = NULL;
   struct ast_srtp_policy *remote_policy = NULL;
   struct ast_rtp_instance_stats stats = {0,};
   int res = -1;

   if (!p) {
      return -1;
   }

   if (!(local_policy = policy_res.alloc())) {
      return -1;
   }

   if (!(remote_policy = policy_res.alloc())) {
      goto err;
   }

   if (ast_rtp_instance_get_stats(rtp, &stats, AST_RTP_INSTANCE_STAT_LOCAL_SSRC)) {
      goto err;
   }

   if (set_crypto_policy(local_policy, suite_val, p->local_key, key_len, stats.local_ssrc, 0) < 0) {
      goto err;
   }

   if (set_crypto_policy(remote_policy, suite_val, remote_key, key_len, 0, 1) < 0) {
      goto err;
   }

   /* Add the SRTP policies */
   if (ast_rtp_instance_add_srtp_policy(rtp, remote_policy, local_policy, 0)) {
      ast_log(LOG_WARNING, "Could not set SRTP policies\n");
      goto err;
   }

   ast_debug(1 , "SRTP policy activated\n");
   res = 0;

err:
   if (local_policy) {
      policy_res.destroy(local_policy);
   }

   if (remote_policy) {
      policy_res.destroy(remote_policy);
   }

   return res;
}

static int res_sdp_crypto_parse_offer(struct ast_rtp_instance *rtp, struct ast_sdp_srtp *srtp, const char *attr)
{
   char *str = NULL;
   char *tag = NULL;
   char *suite = NULL;
   char *key_params = NULL;
   char *key_param = NULL;
   char *session_params = 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_from_sdp;
   int key_len_expected;
   int tag_from_sdp;
   int suite_val = 0;
   unsigned char remote_key[SRTP_MAX_KEY_LEN];
   int taglen;
   double sdes_lifetime;
   struct ast_sdp_crypto *crypto;
   struct ast_sdp_srtp *tmp;

   str = ast_strdupa(attr);

   tag = strsep(&str, " ");
   suite = strsep(&str, " ");
   key_params = strsep(&str, " ");
   session_params = strsep(&str, " ");

   if (!tag || !suite) {
      ast_log(LOG_WARNING, "Unrecognized crypto attribute a=%s\n", attr);
      return -1;
   }

   /* RFC4568 9.1 - tag is 1-9 digits */
   if (sscanf(tag, "%30d", &tag_from_sdp) != 1 || tag_from_sdp < 0 || tag_from_sdp > 999999999) {
      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;
   }

   /* On egress, Asterisk sent several crypto lines in the SIP/SDP offer
      The remote party might have choosen another line than the first */
   for (tmp = srtp; tmp && tmp->crypto && tmp->crypto->tag != tag_from_sdp;) {
      tmp = AST_LIST_NEXT(tmp, sdp_srtp_list);
   }
   if (tmp) { /* tag matched an already created crypto line */
      unsigned int flags = tmp->flags;

      /* Make that crypto line the head of the list, not by changing the
         list structure but by exchanging the content of the list members */
      crypto = tmp->crypto;
      tmp->crypto = srtp->crypto;
      tmp->flags = srtp->flags;
      srtp->crypto = crypto;
      srtp->flags = flags;
   } else {
      crypto = srtp->crypto;
      crypto->tag = tag_from_sdp;
   }

   ast_clear_flag(srtp, AST_SRTP_CRYPTO_TAG_8);
   ast_clear_flag(srtp, AST_SRTP_CRYPTO_TAG_16);
   ast_clear_flag(srtp, AST_SRTP_CRYPTO_TAG_32);
   ast_clear_flag(srtp, AST_SRTP_CRYPTO_TAG_80);
   ast_clear_flag(srtp, AST_SRTP_CRYPTO_AES_192);
   ast_clear_flag(srtp, AST_SRTP_CRYPTO_AES_256);
   ast_clear_flag(srtp, AST_SRTP_CRYPTO_OLD_NAME);

   if (!strcmp(suite, "AES_CM_128_HMAC_SHA1_80")) {
      suite_val = AST_AES_CM_128_HMAC_SHA1_80;
      ast_set_flag(srtp, AST_SRTP_CRYPTO_TAG_80);
      key_len_expected = 30;
   } else if (!strcmp(suite, "AES_CM_128_HMAC_SHA1_32")) {
      suite_val = AST_AES_CM_128_HMAC_SHA1_32;
      ast_set_flag(srtp, AST_SRTP_CRYPTO_TAG_32);
      key_len_expected = 30;
#ifdef HAVE_SRTP_192
   } else if (!strcmp(suite, "AES_192_CM_HMAC_SHA1_80")) {
      suite_val = AST_AES_CM_192_HMAC_SHA1_80;
      ast_set_flag(srtp, AST_SRTP_CRYPTO_TAG_80);
      ast_set_flag(srtp, AST_SRTP_CRYPTO_AES_192);
      key_len_expected = 38;
   } else if (!strcmp(suite, "AES_192_CM_HMAC_SHA1_32")) {
      suite_val = AST_AES_CM_192_HMAC_SHA1_32;
      ast_set_flag(srtp, AST_SRTP_CRYPTO_TAG_32);
      ast_set_flag(srtp, AST_SRTP_CRYPTO_AES_192);
      key_len_expected = 38;
   /* RFC used a different name while in draft, some still use that */
   } else if (!strcmp(suite, "AES_CM_192_HMAC_SHA1_80")) {
      suite_val = AST_AES_CM_192_HMAC_SHA1_80;
      ast_set_flag(srtp, AST_SRTP_CRYPTO_TAG_80);
      ast_set_flag(srtp, AST_SRTP_CRYPTO_AES_192);
      ast_set_flag(srtp, AST_SRTP_CRYPTO_OLD_NAME);
      key_len_expected = 38;
   } else if (!strcmp(suite, "AES_CM_192_HMAC_SHA1_32")) {
      suite_val = AST_AES_CM_192_HMAC_SHA1_32;
      ast_set_flag(srtp, AST_SRTP_CRYPTO_TAG_32);
      ast_set_flag(srtp, AST_SRTP_CRYPTO_AES_192);
      ast_set_flag(srtp, AST_SRTP_CRYPTO_OLD_NAME);
      key_len_expected = 38;
#endif
#ifdef HAVE_SRTP_256
   } else if (!strcmp(suite, "AES_256_CM_HMAC_SHA1_80")) {
      suite_val = AST_AES_CM_256_HMAC_SHA1_80;
      ast_set_flag(srtp, AST_SRTP_CRYPTO_TAG_80);
      ast_set_flag(srtp, AST_SRTP_CRYPTO_AES_256);
      key_len_expected = 46;
   } else if (!strcmp(suite, "AES_256_CM_HMAC_SHA1_32")) {
      suite_val = AST_AES_CM_256_HMAC_SHA1_32;
      ast_set_flag(srtp, AST_SRTP_CRYPTO_TAG_32);
      ast_set_flag(srtp, AST_SRTP_CRYPTO_AES_256);
      key_len_expected = 46;
   /* RFC used a different name while in draft, some still use that */
   } else if (!strcmp(suite, "AES_CM_256_HMAC_SHA1_80")) {
      suite_val = AST_AES_CM_256_HMAC_SHA1_80;
      ast_set_flag(srtp, AST_SRTP_CRYPTO_TAG_80);
      ast_set_flag(srtp, AST_SRTP_CRYPTO_AES_256);
      ast_set_flag(srtp, AST_SRTP_CRYPTO_OLD_NAME);
      key_len_expected = 46;
   } else if (!strcmp(suite, "AES_CM_256_HMAC_SHA1_32")) {
      suite_val = AST_AES_CM_256_HMAC_SHA1_32;
      ast_set_flag(srtp, AST_SRTP_CRYPTO_TAG_32);
      ast_set_flag(srtp, AST_SRTP_CRYPTO_AES_256);
      ast_set_flag(srtp, AST_SRTP_CRYPTO_OLD_NAME);
      key_len_expected = 46;
#endif
#ifdef HAVE_SRTP_GCM
   } else if (!strcmp(suite, "AEAD_AES_128_GCM")) {
      suite_val = AST_AES_GCM_128;
      ast_set_flag(srtp, AST_SRTP_CRYPTO_TAG_16);
      key_len_expected = AES_128_GCM_KEYSIZE_WSALT;
   } else if (!strcmp(suite, "AEAD_AES_256_GCM")) {
      suite_val = AST_AES_GCM_256;
      ast_set_flag(srtp, AST_SRTP_CRYPTO_TAG_16);
      ast_set_flag(srtp, AST_SRTP_CRYPTO_AES_256);
      key_len_expected = AES_256_GCM_KEYSIZE_WSALT;
   /* RFC contained a (too) short auth tag for RTP media, some still use that */
   } else if (!strcmp(suite, "AEAD_AES_128_GCM_8")) {
      suite_val = AST_AES_GCM_128_8;
      ast_set_flag(srtp, AST_SRTP_CRYPTO_TAG_8);
      key_len_expected = AES_128_GCM_KEYSIZE_WSALT;
   } else if (!strcmp(suite, "AEAD_AES_256_GCM_8")) {
      suite_val = AST_AES_GCM_256_8;
      ast_set_flag(srtp, AST_SRTP_CRYPTO_TAG_8);
      ast_set_flag(srtp, AST_SRTP_CRYPTO_AES_256);
      key_len_expected = AES_256_GCM_KEYSIZE_WSALT;
#endif
   } else {
      ast_verb(1, "Unsupported crypto suite: %s\n", suite);
      return -1;
   }

   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")) {
         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 ~5.8 hours. Less than ~5.8; reject. */
         if (sdes_lifetime < 1048576) {
            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: '%s'\n", attr);
      return -1;
   }

   key_len_from_sdp = ast_base64decode(remote_key, key_salt, sizeof(remote_key));
   if (key_len_from_sdp != key_len_expected) {
      ast_log(LOG_WARNING, "SRTP descriptions key length is '%d', not '%d'\n",
         key_len_from_sdp, key_len_expected);
      return -1;
   }

   /* on default, the key is 30 (AES-128); throw that away (only) when the suite changed actually */
   /* ingress: optional, but saves one expensive call to get_random(.) */
   /*  egress: required, because the local key was communicated before the remote key is processed */
   if (crypto->key_len != key_len_from_sdp) {
      if (!crypto_init_keys(crypto, key_len_from_sdp)) {
         return -1;
      }
   } else if (!memcmp(crypto->remote_key, remote_key, key_len_from_sdp)) {
      ast_debug(1, "SRTP remote key unchanged; maintaining current policy\n");
      return 0;
   }

   if (key_len_from_sdp > sizeof(crypto->remote_key)) {
      ast_log(LOG_ERROR,
         "SRTP key buffer is %zu although it must be at least %d bytes\n",
         sizeof(crypto->remote_key), key_len_from_sdp);
      return -1;
   }
   memcpy(crypto->remote_key, remote_key, key_len_from_sdp);

   if (crypto_activate(crypto, suite_val, remote_key, key_len_from_sdp, rtp) < 0) {
      return -1;
   }

   if (ast_test_flag(srtp, AST_SRTP_CRYPTO_TAG_32)) {
      taglen = 32;
   } else if (ast_test_flag(srtp, AST_SRTP_CRYPTO_TAG_16)) {
      taglen = 16;
   } else if (ast_test_flag(srtp, AST_SRTP_CRYPTO_TAG_8)) {
      taglen = 8;
   } else {
      taglen = 80;
   }
   if (ast_test_flag(srtp, AST_SRTP_CRYPTO_AES_256)) {
      taglen |= 0x0200;
   } else if (ast_test_flag(srtp, AST_SRTP_CRYPTO_AES_192)) {
      taglen |= 0x0100;
   }
   if (ast_test_flag(srtp, AST_SRTP_CRYPTO_OLD_NAME)) {
      taglen |= 0x0080;
   }

   /* Finally, rebuild the crypto line */
   if (res_sdp_crypto_build_offer(crypto, taglen)) {
      return -1;
   }

   ast_set_flag(srtp, AST_SRTP_CRYPTO_OFFER_OK);
   return 0;
}

static const char *res_sdp_srtp_get_attr(struct ast_sdp_srtp *srtp, int dtls_enabled, int default_taglen_32)
{
   int taglen;

   if (!srtp) {
      return NULL;
   }

   /* Set encryption properties */
   if (!srtp->crypto) {
      if (AST_LIST_NEXT(srtp, sdp_srtp_list)) {
         srtp->crypto = res_sdp_crypto_alloc();
         ast_log(LOG_ERROR, "SRTP SDP list was not empty\n");
      } else {
         const int len = default_taglen_32 ? AST_SRTP_CRYPTO_TAG_32 : AST_SRTP_CRYPTO_TAG_80;
         const int attr[][3] = {
         /* This array creates the following list:
          * a=crypto:1 AES_CM_128_HMAC_SHA1_ ...
          * a=crypto:2 AEAD_AES_128_GCM ...
          * a=crypto:3 AES_256_CM_HMAC_SHA1_ ...
          * a=crypto:4 AEAD_AES_256_GCM ...
          * a=crypto:5 AES_192_CM_HMAC_SHA1_ ...
          * something like 'AEAD_AES_192_GCM' is not specified by the RFCs
          *
          * If you want to prefer another crypto suite or you want to
          * exclude a suite, change this array and recompile Asterisk.
          * This list cannot be changed from rtp.conf because you should
          * know what you are doing. Especially AES-192 and AES-GCM are
          * broken in many VoIP clients, see
          * https://github.com/cisco/libsrtp/pull/170
          * https://github.com/cisco/libsrtp/pull/184
          * Furthermore, AES-GCM uses a shorter crypto-suite string which
          * causes Nokia phones based on Symbian/S60 to reject the whole
          * INVITE with status 500, even if a matching suite was offered.
          * AES-256 might just waste your processor cycles, especially if
          * your TLS transport is not secured with equivalent grade, see
          * https://security.stackexchange.com/q/61361
          * Therefore, AES-128 was preferred here.
          *
          * If you want to enable one of those defines, please, go for
          * CFLAGS='-DENABLE_SRTP_AES_GCM' ./configure && sudo make install
          */
            { len, 0, 30 },
#if defined(HAVE_SRTP_GCM) && defined(ENABLE_SRTP_AES_GCM)
            { AST_SRTP_CRYPTO_TAG_16, 0, AES_128_GCM_KEYSIZE_WSALT },
#endif
#if defined(HAVE_SRTP_256) && defined(ENABLE_SRTP_AES_256)
            { len, AST_SRTP_CRYPTO_AES_256, 46 },
#endif
#if defined(HAVE_SRTP_GCM) && defined(ENABLE_SRTP_AES_GCM) && defined(ENABLE_SRTP_AES_256)
            { AST_SRTP_CRYPTO_TAG_16, AST_SRTP_CRYPTO_AES_256, AES_256_GCM_KEYSIZE_WSALT },
#endif
#if defined(HAVE_SRTP_192) && defined(ENABLE_SRTP_AES_192)
            { len, AST_SRTP_CRYPTO_AES_192, 38 },
#endif
         };
         struct ast_sdp_srtp *tmp = srtp;
         int i;

         for (i = 0; i < ARRAY_LEN(attr); i++) {
            if (attr[i][0]) {
               ast_set_flag(tmp, attr[i][0]);
            }
            if (attr[i][1]) {
               ast_set_flag(tmp, attr[i][1]);
            }
            tmp->crypto = sdp_crypto_alloc(attr[i][2]); /* key_len */
            tmp->crypto->tag = (i + 1); /* tag starts at 1 */

            if (i < ARRAY_LEN(attr) - 1) {
               AST_LIST_NEXT(tmp, sdp_srtp_list) = ast_sdp_srtp_alloc();
               tmp = AST_LIST_NEXT(tmp, sdp_srtp_list);
            }
         }
      }
   }

   if (dtls_enabled) {
      /* If DTLS-SRTP is enabled the key details will be pulled from TLS */
      return NULL;
   }

   /* set the key length based on INVITE or settings */
   if (ast_test_flag(srtp, AST_SRTP_CRYPTO_TAG_80)) {
      taglen = 80;
   } else if (ast_test_flag(srtp, AST_SRTP_CRYPTO_TAG_32)) {
      taglen = 32;
   } else if (ast_test_flag(srtp, AST_SRTP_CRYPTO_TAG_16)) {
      taglen = 16;
   } else if (ast_test_flag(srtp, AST_SRTP_CRYPTO_TAG_8)) {
      taglen = 8;
   } else {
      taglen = default_taglen_32 ? 32 : 80;
   }
   if (ast_test_flag(srtp, AST_SRTP_CRYPTO_AES_256)) {
      taglen |= 0x0200;
   } else if (ast_test_flag(srtp, AST_SRTP_CRYPTO_AES_192)) {
      taglen |= 0x0100;
   }
   if (ast_test_flag(srtp, AST_SRTP_CRYPTO_OLD_NAME)) {
      taglen |= 0x0080;
   }

   if (srtp->crypto && (res_sdp_crypto_build_offer(srtp->crypto, taglen) >= 0)) {
      return srtp->crypto->a_crypto;
   }

   ast_log(LOG_WARNING, "No SRTP key management enabled\n");
   return NULL;
}

static struct ast_sdp_crypto_api res_sdp_crypto_api = {
   .dtor = res_sdp_crypto_dtor,
   .alloc = res_sdp_crypto_alloc,
   .build_offer = res_sdp_crypto_build_offer,
   .parse_offer = res_sdp_crypto_parse_offer,
   .get_attr = res_sdp_srtp_get_attr,
};

static void res_srtp_shutdown(void)
{
   ast_sdp_crypto_unregister(&res_sdp_crypto_api);
   ast_rtp_engine_unregister_srtp();
   srtp_install_event_handler(NULL);
#ifdef HAVE_SRTP_SHUTDOWN
   srtp_shutdown();
#endif
   g_initialized = 0;
}

static int res_srtp_init(void)
{
   if (g_initialized) {
      return 0;
   }

   if (srtp_init() != err_status_ok) {
      ast_log(AST_LOG_WARNING, "Failed to initialize libsrtp\n");
      return -1;
   }

   srtp_install_event_handler(srtp_event_cb);

   if (ast_rtp_engine_register_srtp(&srtp_res, &policy_res)) {
      ast_log(AST_LOG_WARNING, "Failed to register SRTP with rtp engine\n");
      res_srtp_shutdown();
      return -1;
   }

   if (ast_sdp_crypto_register(&res_sdp_crypto_api)) {
      ast_log(AST_LOG_WARNING, "Failed to register SDP SRTP crypto API\n");
      res_srtp_shutdown();
      return -1;
   }

#ifdef HAVE_SRTP_GET_VERSION
   ast_verb(2, "%s initialized\n", srtp_get_version_string());
#else
   ast_verb(2, "libsrtp initialized\n");
#endif

   g_initialized = 1;
   return 0;
}

/*
 * Exported functions
 */

static int load_module(void)
{
   return res_srtp_init();
}

static int unload_module(void)
{
   res_srtp_shutdown();
   return 0;
}

AST_MODULE_INFO(ASTERISK_GPL_KEY, AST_MODFLAG_GLOBAL_SYMBOLS | AST_MODFLAG_LOAD_ORDER, "Secure RTP (SRTP)",
   .support_level = AST_MODULE_SUPPORT_CORE,
   .load = load_module,
   .unload = unload_module,
   .load_pri = AST_MODPRI_CHANNEL_DEPEND,
);