README
1Secure RTP (SRTP) Reference Implementation
2David A. McGrew
3Cisco Systems, Inc.
4mcgrew@cisco.com
5
6
7This package provides an implementation of the Secure Real-time
8Transport Protocol (SRTP), the Universal Security Transform (UST), and
9a supporting cryptographic kernel. These mechanisms are documented in
10the Internet Drafts in the doc/ subdirectory. The SRTP API is
11documented in include/srtp.h, and the library is in libsrtp.a (after
12compilation). An overview and reference manual is available in
13doc/libsrtp.pdf. The PDF documentation is more up to date than this
14file.
15
16
17Installation:
18
19./configure [ options ] # GNU autoconf script
20make # or gmake if needed; use GNU make
21
22The configure script accepts the following options:
23
24 --help provides a usage summary
25 --disable-debug compile without the runtime debugging system
26 --enable-syslog use syslog for error reporting
27 --disable-stdout use stdout for error reporting
28 --enable-console use /dev/console for error reporting
29 --gdoi use GDOI key management (disabled at present)
30
31By default, debbuging is enabled and stdout is used for debugging.
32You can use the above configure options to have the debugging output
33sent to syslog or the system console. Alternatively, you can define
34ERR_REPORTING_FILE in include/conf.h to be any other file that can be
35opened by libSRTP, and debug messages will be sent to it.
36
37This package has been tested on Mac OS X (powerpc-apple-darwin1.4),
38Cygwin (i686-pc-cygwin), and Sparc (sparc-sun-solaris2.6). Previous
39versions have been tested on Linux and OpenBSD on both x86 and sparc
40platforms.
41
42A quick tour of this package:
43
44Makefile targets: all, clean, ...
45README this file
46CHANGES change log
47VERSION version number of this package
48LICENSE legal details (it's a BSD-like license)
49crypto/ciphers/ ciphers (null, aes_icm, ...)
50crypto/math/ crypto math routines
51crypto/hash/ crypto hashing (hmac, tmmhv2, ...)
52crypto/replay/ replay protection
53doc/ documentation: rfcs, apis, and suchlike
54include/ include files for all code in distribution
55srtp/ secure real-time transport protocol implementation
56tables/ apps for generating tables (useful in porting)
57test/ test drivers
58
59
60Applications
61
62 Several test drivers and a simple and portable srtp application
63 are included in the test/ subdirectory.
64
65 test driver function tested
66 -------------------------------------------------------------
67 kernel_driver crypto kernel (ciphers, auth funcs, rng)
68 srtp_driver srtp in-memory tests (does not use the network)
69 rdbx_driver rdbx (extended replay database)
70 roc_driver extended sequence number functions
71 replay_driver replay database (n.b. not used in libsrtp)
72 cipher_driver ciphers
73 auth_driver hash functions
74
75 The app rtpw is a simple rtp application which reads words from
76 /usr/dict/words and then sends them out one at a time using [s]rtp.
77 Manual srtp keying uses the -k option; automated key management
78 using gdoi will be added later.
79
80usage: rtpw [-d <debug>]* [-k <key> [-a][-e]] [-s | -r] dest_ip dest_port
81or rtpw -l
82
83 Either the -s (sender) or -r (receiver) option must be chosen.
84
85 The values dest_ip, dest_port are the ip address and udp port to
86 which the dictionary will be sent, respectively.
87
88 options:
89
90 -s (s)rtp sender - causes app to send words
91
92 -r (s)rtp receive - causes app to receve words
93
94 -k <key> use srtp master key <key>, where the
95 key is a hexadecimal value (without the
96 leading "0x")
97
98 -e encrypt/decrypt (for data confidentiality)
99 (requires use of -k option as well)
100
101 -a message authentication
102 (requires use of -k option as well)
103
104 -l list debug modules
105
106 -d <debug> turn on debugging for module <debug>
107
108
109In order to get random 30-byte values for use as key/salt pairs , you
110can use the following bash function to format the output of
111/dev/random (where that device is available).
112
113function randhex() {
114 cat /dev/random | od --read-bytes=32 --width=32 -x | awk '{ print $2 $3 $4 $5 $6 $7 $8 $9 $10 $11 $12 $13 $14 $15 $16 }'
115}
116
117
118An example of an SRTP session using two rtpw programs follows:
119
120set k=c1eec3717da76195bb878578790af71c4ee9f859e197a414a78d5abc7451
121
122[sh1]$ test/rtpw -s -k $k -ea 0.0.0.0 9999
123Security services: confidentiality message authentication
124set master key/salt to C1EEC3717DA76195BB878578790AF71C/4EE9F859E197A414A78D5ABC7451
125setting SSRC to 2078917053
126sending word: A
127sending word: a
128sending word: aa
129sending word: aal
130...
131
132[sh2]$ test/rtpw -r -k $k -ea 0.0.0.0 9999
133security services: confidentiality message authentication
134set master key/salt to C1EEC3717DA76195BB878578790AF71C/4EE9F859E197A414A78D5ABC7451
13519 octets received from SSRC 2078917053 word: A
13619 octets received from SSRC 2078917053 word: a
13720 octets received from SSRC 2078917053 word: aa
13821 octets received from SSRC 2078917053 word: aal
139...
140
141Implementation Notes
142
143 * The srtp_protect() function assumes that the buffer holding the
144 rtp packet has enough storage allocated that the authentication
145 tag can be written to the end of that packet. If this assumption
146 is not valid, memory corruption will ensue.
147
148 * Automated tests for the crypto functions are provided through
149 the cipher_type_self_test() and auth_type_self_test() functions.
150 These functions should be used to test each port of this code
151 to a new platform.
152
153 * Replay protection is contained in the crypto engine, and
154 tests for it are provided.
155
156 * This implementation provides calls to initialize, protect, and
157 unprotect RTP packets, and makes as few as possible assumptions
158 about how these functions will be called. For example, the
159 caller is not expected to provide packets in order (though if
160 they're called more than 65k out of sequence, synchronization
161 will be lost).
162
163 * The sequence number in the rtp packet is used as the low 16 bits
164 of the sender's local packet index. Note that RTP will start its
165 sequence number in a random place, and the SRTP layer just jumps
166 forward to that number at its first invocation. An earlier
167 version of this library used initial sequence numbers that are
168 less than 32,768; this trick is no longer required as the
169 rdbx_estimate_index(...) function has been made smarter.
170
171 * The replay window is 128 bits in length, and is hard-coded to this
172 value for now.
173
174
175