This package provides an implementation of the Secure Real-time Transport Protocol (SRTP), the Universal Security Transform (UST), and a supporting cryptographic kernel. The SRTP API is documented in include/srtp.h, and the library is in libsrtp2.a (after compilation).
This document describes libSRTP, the Open Source Secure RTP library from Cisco Systems, Inc. RTP is the Real-time Transport Protocol, an IETF standard for the transport of real-time data such as telephony, audio, and video, defined by RFC 3550. Secure RTP (SRTP) is an RTP profile for providing confidentiality to RTP data and authentication to the RTP header and payload. SRTP is an IETF Standard, defined in RFC 3711, and was developed in the IETF Audio/Video Transport (AVT) Working Group. This library supports all of the mandatory features of SRTP, but not all of the optional features. See the Supported Features section for more detailed information.
This document is also used to generate the documentation files in the /doc/ folder where a more detailed reference to the libSRTP API and related functions can be created (requires installing doxygen.). The reference material is created automatically from comments embedded in some of the C header files. The documentation is organized into modules in order to improve its clarity. These modules do not directly correspond to files. An underlying cryptographic kernel provides much of the basic functionality of libSRTP but is mostly undocumented because it does its work behind the scenes.
libsrtp@lists.packetizer.com general mailing list for news / announcements / discussions. This is an open list, see https://lists.packetizer.com/mailman/listinfo/libsrtp for singing up.
libsrtp-security@lists.packetizer.com for disclosing security issues to the libsrtp maintenance team. This is a closed list but anyone can send to it.
libSRTP is distributed under the following license, which is included in the source code distribution. It is reproduced in the manual in case you got the library from another source.
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.
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libSRTP provides functions for protecting RTP and RTCP. RTP packets
can be encrypted and authenticated (using the srtp_protect()
function), turning them into SRTP packets. Similarly, SRTP packets
can be decrypted and have their authentication verified (using the
srtp_unprotect()
function), turning them into RTP packets. Similar
functions apply security to RTCP packets.
The typedef srtp_stream_t
points to a structure holding all of the
state associated with an SRTP stream, including the keys and
parameters for cipher and message authentication functions and the
anti-replay data. A particular srtp_stream_t
holds the information
needed to protect a particular RTP and RTCP stream. This datatype
is intentionally opaque in order to better seperate the libSRTP
API from its implementation.
Within an SRTP session, there can be multiple streams, each
originating from a particular sender. Each source uses a distinct
stream context to protect the RTP and RTCP stream that it is
originating. The typedef srtp_t
points to a structure holding all of
the state associated with an SRTP session. There can be multiple
stream contexts associated with a single srtp_t
. A stream context
cannot exist indepent from an srtp_t
, though of course an srtp_t
can
be created that contains only a single stream context. A device
participating in an SRTP session must have a stream context for each
source in that session, so that it can process the data that it
receives from each sender.
In libSRTP, a session is created using the function srtp_create()
.
The policy to be implemented in the session is passed into this
function as an srtp_policy_t
structure. A single one of these
structures describes the policy of a single stream. These structures
can also be linked together to form an entire session policy. A linked
list of srtp_policy_t
structures is equivalent to a session policy.
In such a policy, we refer to a single srtp_policy_t
as an element.
An srtp_policy_t
structure contains two srtp_crypto_policy_t
structures
that describe the cryptograhic policies for RTP and RTCP, as well as
the SRTP master key and the SSRC value. The SSRC describes what to
protect (e.g. which stream), and the srtp_crypto_policy_t
structures
describe how to protect it. The key is contained in a policy element
because it simplifies the interface to the library. In many cases, it
is desirable to use the same cryptographic policies across all of the
streams in a session, but to use a distinct key for each stream. A
srtp_crypto_policy_t
structure can be initialized by using either the
srtp_crypto_policy_set_rtp_default()
or srtp_crypto_policy_set_rtcp_default()
functions, which set a crypto policy structure to the default policies
for RTP and RTCP protection, respectively.
In this section we review SRTP and introduce some terms that are used in libSRTP. An RTP session is defined by a pair of destination transport addresses, that is, a network address plus a pair of UDP ports for RTP and RTCP. RTCP, the RTP control protocol, is used to coordinate between the participants in an RTP session, e.g. to provide feedback from receivers to senders. An SRTP session is similarly defined; it is just an RTP session for which the SRTP profile is being used. An SRTP session consists of the traffic sent to the SRTP or SRTCP destination transport addresses. Each participant in a session is identified by a synchronization source (SSRC) identifier. Some participants may not send any SRTP traffic; they are called receivers, even though they send out SRTCP traffic, such as receiver reports.
RTP allows multiple sources to send RTP and RTCP traffic during the same session. The synchronization source identifier (SSRC) is used to distinguish these sources. In libSRTP, we call the SRTP and SRTCP traffic from a particular source a stream. Each stream has its own SSRC, sequence number, rollover counter, and other data. A particular choice of options, cryptographic mechanisms, and keys is called a policy. Each stream within a session can have a distinct policy applied to it. A session policy is a collection of stream policies.
A single policy can be used for all of the streams in a given session, though the case in which a single key is shared across multiple streams requires care. When key sharing is used, the SSRC values that identify the streams must be distinct. This requirement can be enforced by using the convention that each SRTP and SRTCP key is used for encryption by only a single sender. In other words, the key is shared only across streams that originate from a particular device (of course, other SRTP participants will need to use the key for decryption). libSRTP supports this enforcement by detecting the case in which a key is used for both inbound and outbound data.
This library supports all of the mandatory-to-implement features of
SRTP (as defined in RFC 3711). Some of these
features can be selected (or de-selected) at run time by setting an
appropriate policy; this is done using the structure srtp_policy_t
.
Some other behaviors of the protocol can be adapted by defining an
approriate event handler for the exceptional events; see the SRTPevents
section in the generated documentation.
Some options that are described in the SRTP specification are not supported. This includes
The user should be aware that it is possible to misuse this libary, and that the result may be that the security level it provides is inadequate. If you are implementing a feature using this library, you will want to read the Security Considerations section of RFC 3711. In addition, it is important that you read and understand the terms outlined in the License and Disclaimer section.
The srtp_protect()
function assumes that the buffer holding the
rtp packet has enough storage allocated that the authentication
tag can be written to the end of that packet. If this assumption
is not valid, memory corruption will ensue.
Automated tests for the crypto functions are provided through
the cipher_type_self_test()
and auth_type_self_test()
functions.
These functions should be used to test each port of this code
to a new platform.
Replay protection is contained in the crypto engine, and tests for it are provided.
This implementation provides calls to initialize, protect, and unprotect RTP packets, and makes as few as possible assumptions about how these functions will be called. For example, the caller is not expected to provide packets in order (though if they're called more than 65k out of sequence, synchronization will be lost).
The sequence number in the rtp packet is used as the low 16 bits
of the sender's local packet index. Note that RTP will start its
sequence number in a random place, and the SRTP layer just jumps
forward to that number at its first invocation. An earlier
version of this library used initial sequence numbers that are
less than 32,768; this trick is no longer required as the
rdbx_estimate_index(...)
function has been made smarter.
The replay window for (S)RTCP is hardcoded to 128 bits in length.
To install libSRTP, download the latest release of the distribution
from https://github.com/cisco/libsrtp/releases.
You probably want to get the most recent release. Unpack the distribution and
extract the source files; the directory into which the source files
will go is named libsrtp-A-B-C
where A
is the version number, B
is the
major release number and C
is the minor release number.
libSRTP uses the GNU autoconf
and make
utilities (BSD make will not work; if
both versions of make are on your platform, you can invoke GNU make as
gmake
.). In the libsrtp
directory, run the configure script and then
make:
./configure [ options ]
make
The configure script accepts the following options:
Option | Description |
---|---|
--help -h | Display help |
--enable-debug-logging | Enable debug logging in all modules |
--enable-log-stdout | Enable logging to stdout |
--enable-openssl | Enable OpenSSL crypto engine |
--enable-openssl-kdf | Enable OpenSSL KDF algorithm |
--with-log-file | Use file for logging |
--with-openssl-dir | Location of OpenSSL installation |
By default there is no log output, logging can be enabled to be output to stdout or a given file using the configure options.
This package has been tested on the following platforms: Mac OS X (powerpc-apple-darwin1.4), Cygwin (i686-pc-cygwin), Solaris (sparc-sun-solaris2.6), RedHat Linux 7.1 and 9 (i686-pc-linux), and OpenBSD (sparc-unknown-openbsd2.7).
To build the ./configure
script mentioned above, libSRTP relies on the
automake toolchain. Since
./configure
is built from configure.in
by automake, if you make changes in
how ./configure
works (e.g., to add a new library dependency), you will need
to rebuild ./configure
and commit the updated version. In addition to
automake itself, you will need to have the pkgconfig
tools installed as well.
For example, on macOS:
brew install automake pkgconfig
# Edit configure.in
autoremake -ivf
On Windows one can use Visual Studio via CMake. CMake can be downloaded here: https://cmake.org/ . To create Visual Studio build files, for example run the following commands:
# Create build subdirectory
mkdir build
cd build
# Make project files
cmake .. -G "Visual Studio 15 2017"
# Or for 64 bit project files
cmake .. -G "Visual Studio 15 2017 Win64"
On all platforms including Windows, one can build using Meson. Steps to download Meson are here: https://mesonbuild.com/Getting-meson.html
To build with Meson, you can do something like:
# Setup the build subdirectory
meson setup --prefix=/path/to/prefix builddir
# Build the project
meson compile -C builddir
# Run tests
meson test -C builddir
# Optionally, install
meson install -C builddir
To build with Visual Studio, run the above commands from inside a Visual Studio
command prompt, or run vcvarsall.bat
with the appropriate arguments inside
a Command Prompt.
Note that you can also replace the above commands with the appropriate ninja
targets: ninja -C build
, ninja -C build test
, ninja -C build install
.
Several test drivers and a simple and portable srtp application are
included in the test/
subdirectory.
Test driver | Function tested |
---|---|
kernel_driver | crypto kernel (ciphers, auth funcs, rng) |
srtp_driver | srtp in-memory tests (does not use the network) |
rdbx_driver | rdbx (extended replay database) |
roc_driver | extended sequence number functions |
replay_driver | replay database |
cipher_driver | ciphers |
auth_driver | hash functions |
The app rtpw
is a simple rtp application which reads words from
/usr/dict/words
and then sends them out one at a time using [s]rtp.
Manual srtp keying uses the -k option; automated key management
using gdoi will be added later.
usage:
rtpw [[-d <debug>]* [-k|b <key> [-a][-e <key size>][-g]] [-s | -r] dest_ip dest_port] | [-l]
Either the -s (sender) or -r (receiver) option must be chosen. The
values dest_ip
, dest_port
are the IP address and UDP port to which
the dictionary will be sent, respectively.
The options are:
Option | Description |
---|---|
-s | (S)RTP sender - causes app to send words |
-r | (S)RTP receive - causes app to receive words |
-k | use SRTP master key , where the key is a hexadecimal (without the leading "0x") |
-b | same as -k but with base64 encoded key |
-e | encrypt/decrypt (for data confidentiality) (requires use of -k option as well) (use 128, 192, or 256 for keysize) |
-g | use AES-GCM mode (must be used with -e) |
-a | message authentication (requires use of -k option as well) |
-l | list the available debug modules |
-d | turn on debugging for module |