libsrtp rtpw 例子代码分析
libsrtp安装
下载：https://github.com/cisco/libsrtp.git
cd your path/libsrtp-master
./configure
make
make runtest
make install
测试的方法是：
cd test
./rtpw_test.sh
其实分析里面的实现主要是创建了两个进程：
./rtpw -b Ky7cUDT2GnI0XKWYbXv9AYmqbcLsqzL9mvdN9t/G -a -e 128 -r 0.0.0.0 9999 &
./rtpw -b Ky7cUDT2GnI0XKWYbXv9AYmqbcLsqzL9mvdN9t/G -a -e 128 -s 127.0.0.1 9999 &
所以其实是执行了rtpw两次，一次指定为receiver，一次为sender。
/*
* rtpw.c
*
* rtp word sender/receiver
*
* David A. McGrew
* Cisco Systems, Inc.
*
* This app is a simple RTP application intended only for testing
* libsrtp. It reads one word at a time from words.txt (or
* whatever file is specified as DICT_FILE or with -w), and sends one word out
* each USEC_RATE microseconds. Secure RTP protections can be
* applied. See the usage() function for more details.
*
*/
/*
*
* Copyright (c) 2001-2017, Cisco Systems, Inc.
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
* Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
*
* Redistributions in binary form must reproduce the above
* copyright notice, this list of conditions and the following
* disclaimer in the documentation and/or other materials provided
* with the distribution.
*
* Neither the name of the Cisco Systems, Inc. nor the names of its
* contributors may be used to endorse or promote products derived
* from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
* COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
* INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
* (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
* STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
* OF THE POSSIBILITY OF SUCH DAMAGE.
*
*/
#ifdef HAVE_CONFIG_H
#include /* for printf, fprintf */
#include /* for atoi() */
#include /* for signal() */
#include /* for strncpy() */
#include /* for usleep() */
#ifdef HAVE_UNISTD_H
#include /* for close() */
#elif defined(_MSC_VER)
#include /* for _close() */
#define close _close
#endif
#ifdef HAVE_SYS_SOCKET_H
#include ]* [-k [-a][-e]] "
"[-s | -r] dest_ip dest_port\n"
"or %s -l\n"
"where -a use message authentication\n"
" -e use encryption (use 128 or 256 for key size)\n"
" -g Use AES-GCM mode (must be used with -e)\n"
" -t Tag size to use in GCM mode (use 8 or 16)\n"
" -k sets the srtp master key given in hexadecimal\n"
" -b sets the srtp master key given in base64\n"
" -s act as rtp sender\n"
" -r act as rtp receiver\n"
" -l list debug modules\n"
" -d turn on debugging for module \n"
" -w use for input, rather than %s\n" ,
string, string, DICT_FILE);
exit(1);
}
void leave_group(int sock, struct ip_mreq mreq, char *name)
{
int ret;
ret = setsockopt(sock, IPPROTO_IP, IP_DROP_MEMBERSHIP, (void *)&mreq,
sizeof(mreq));
if (ret < 0) {
fprintf(stderr, "%s: Failed to leave multicast group", name);
perror("");
}
}
void handle_signal(int signum)
{
interrupted = 1;
/* Reset handler explicitly, in case we don't have sigaction() (and signal()
has BSD semantics), or we don't have SA_RESETHAND */
signal(signum, SIG_DFL);
}
int setup_signal_handler(char *name)
{
#if HAVE_SIGACTION
struct sigaction act;
memset(&act, 0, sizeof(act));
act.sa_handler = handle_signal;
sigemptyset(&act.sa_mask);
#if defined(SA_RESETHAND)
act.sa_flags = SA_RESETHAND;
#else
act.sa_flags = 0;
#endif
/* Note that we're not setting SA_RESTART; we want recvfrom to return
* EINTR when we signal the receiver. */
if (sigaction(SIGTERM, &act, NULL) != 0) {
fprintf(stderr, "%s: error setting up signal handler", name);
perror("");
return -1;
}
#else
if (signal(SIGTERM, handle_signal) == SIG_ERR) {
fprintf(stderr, "%s: error setting up signal handler", name);
perror("");
return -1;
}
#endif
return 0;
}
接收端
（1）创建srtp_policy_t对象
（2）用之前创建的srtp_policy_t调用srtp_create（）初始化srtp_t这个对象，得到在这个policy下的一个srtp的session。
srtp_err_status_t srtp_create(srtp_t *session, /* handle for session */
const srtp_policy_t *policy)srtp_t结构体就是srtp_ctx_t。
（3）对于接收到的数据（本例中是从socket中接收过来的），调用srtp_unprotect（）解码出真实的数据。
srtp_err_status_t srtp_unprotect(srtp_ctx_t *ctx,
void *srtp_hdr,
int *pkt_octet_len)ctx就是步骤（2）创建和初始化的session。
发送端
（1）创建srtp_policy_t对象
（2）用之前创建的srtp_policy_t调用srtp_create（）初始化srtp_t这个对象，得到在这个policy下的一个srtp的session。
srtp_err_status_t srtp_create(srtp_t *session, /* handle for session */
const srtp_policy_t *policy)srtp_t结构体就是srtp_ctx_t。
（3）对于要发送的数据调用srtp_protect对数据进行加密，然后发送出去（本例中是通过socket发送出去的）。
srtp_err_status_t srtp_protect(srtp_ctx_t *ctx,
void *rtp_hdr,
int *pkt_octet_len)ctx就是步骤（2）创建和初始化的session。