Network Working Group M. Baugher
Request for Comments: 3711 D. McGrew
Category: Standards Track Cisco Systems, Inc.
M. Naslund
E. Carrara
K. Norrman
Ericsson Research
March 2004
The Secure Real-time Transport Protocol (SRTP)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes the Secure Real-time Transport Protocol
(SRTP), a profile of the Real-time Transport Protocol (RTP), which
can provide confidentiality, message authentication, and replay
protection to the RTP traffic and to the control traffic for RTP, the
Real-time Transport Control Protocol (RTCP).
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Goals and Features . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Features . . . . . . . . . . . . . . . . . . . . . . . . 5
3. SRTP Framework . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Secure RTP . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. SRTP Cryptographic Contexts. . . . . . . . . . . . . . . 7
3.2.1. Transform-independent parameters . . . . . . . . 8
3.2.2. Transform-dependent parameters . . . . . . . . . 10
3.2.3. Mapping SRTP Packets to Cryptographic Contexts . 10
3.3. SRTP Packet Processing . . . . . . . . . . . . . . . . . 11
3.3.1. Packet Index Determination, and ROC, s_l Update. 13
3.3.2. Replay Protection. . . . . . . . . . . . . . . . 15
3.4. Secure RTCP . . . . . . . . . . . . . . . . . . . . . . . 15
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4. Pre-Defined Cryptographic Transforms . . . . . . . . . . . . . 19
4.1. Encryption . . . . . . . . . . . . . . . . . . . . . . . 19
4.1.1. AES in Counter Mode. . . . . . . . . . . . . . . 21
4.1.2. AES in f8-mode . . . . . . . . . . . . . . . . . 22
4.1.3. NULL Cipher. . . . . . . . . . . . . . . . . . . 25
4.2. Message Authentication and Integrity . . . . . . . . . . 25
4.2.1. HMAC-SHA1. . . . . . . . . . . . . . . . . . . . 25
4.3. Key Derivation . . . . . . . . . . . . . . . . . . . . . 26
4.3.1. Key Derivation Algorithm . . . . . . . . . . . . 26
4.3.2. SRTCP Key Derivation . . . . . . . . . . . . . . 28
4.3.3. AES-CM PRF . . . . . . . . . . . . . . . . . . . 28
5. Default and mandatory-to-implement Transforms. . . . . . . . . 28
5.1. Encryption: AES-CM and NULL. . . . . . . . . . . . . . . 29
5.2. Message Authentication/Integrity: HMAC-SHA1. . . . . . . 29
5.3. Key Derivation: AES-CM PRF . . . . . . . . . . . . . . . 29
6. Adding SRTP Transforms . . . . . . . . . . . . . . . . . . . . 29
7. Rationale. . . . . . . . . . . . . . . . . . . . . . . . . . . 30
7.1. Key derivation . . . . . . . . . . . . . . . . . . . . . 30
7.2. Salting key. . . . . . . . . . . . . . . . . . . . . . . 30
7.3. Message Integrity from Universal Hashing . . . . . . . . 31
7.4. Data Origin Authentication Considerations. . . . . . . . 31
7.5. Short and Zero-length Message Authentication . . . . . . 32
8. Key Management Considerations. . . . . . . . . . . . . . . . . 33
8.1. Re-keying . . . . . . . . . . . . . . . . . . . . . . . 34
8.1.1. Use of the for re-keying. . . . . . . 34
8.2. Key Management parameters. . . . . . . . . . . . . . . . 35
9. Security Considerations. . . . . . . . . . . . . . . . . . . . 37
9.1. SSRC collision and two-time pad. . . . . . . . . . . . . 37
9.2. Key Usage. . . . . . . . . . . . . . . . . . . . . . . . 38
9.3. Confidentiality of the RTP Payload . . . . . . . . . . . 39
9.4. Confidentiality of the RTP Header. . . . . . . . . . . . 40
9.5. Integrity of the RTP payload and header. . . . . . . . . 40
9.5.1. Risks of Weak or Null Message Authentication. . . 42
9.5.2. Implicit Header Authentication . . . . . . . . . 43
10. Interaction with Forward Error Correction mechanisms. . . . . 43
11. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 43
11.1. Unicast. . . . . . . . . . . . . . . . . . . . . . . . . 43
11.2. Multicast (one sender) . . . . . . . . . . . . . . . . . 44
11.3. Re-keying and access control . . . . . . . . . . . . . . 45
11.4. Summary of basic scenarios . . . . . . . . . . . . . . . 46
12. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 46
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 47
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
14.1. Normative References . . . . . . . . . . . . . . . . . . 47
14.2. Informative References . . . . . . . . . . . . . . . . . 48
Appendix A: Pseudocode for Index Determination . . . . . . . . . . 51
Appendix B: Test Vectors . . . . . . . . . . . . . . . . . . . . . 51
B.1. AES-f8 Test Vectors. . . . . . . . . . . . . . . . . . . 51
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B.2. AES-CM Test Vectors. . . . . . . . . . . . . . . . . . . 52
B.3. Key Derivation Test Vectors. . . . . . . . . . . . . . . 53
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 55
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 56
1. Introduction
This document describes the Secure Real-time Transport Protocol
(SRTP), a profile of the Real-time Transport Protocol (RTP), which
can provide confidentiality, message authentication, and replay
protection to the RTP traffic and to the control traffic for RTP,
RTCP (the Real-time Transport Control Protocol) [RFC3350].
SRTP provides a framework for encryption and message authentication
of RTP and RTCP streams (Section 3). SRTP defines a set of default
cryptographic transforms (Sections 4 and 5), and it allows new
transforms to be introduced in the future (Section 6). With
appropriate key management (Sections 7 and 8), SRTP is secure
(Sections 9) for unicast and multicast RTP applications (Section 11).
SRTP can achieve high throughput and low packet expansion. SRTP
proves to be a suitable protection for heterogeneous environments
(mix of wired and wireless networks). To get such features, default
transforms are described, based on an additive stream cipher for
encryption, a keyed-hash based function for message authentication,
and an "implicit" index for sequencing/synchronization based on the
RTP sequence number for SRTP and an index number for Secure RTCP
(SRTCP).
1.1. Notational Conventions
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. The
terminology conforms to [RFC2828] with the following exception. For
simplicity we use the term "random" throughout the document to denote
randomly or pseudo-randomly generated values. Large amounts of
random bits may be difficult to obtain, and for the security of SRTP,
pseudo-randomness is sufficient [RFC1750].
By convention, the adopted representation is the network byte order,
i.e., the left most bit (octet) is the most significant one. By XOR
we mean bitwise addition modulo 2 of binary strings, and || denotes
concatenation. In other words, if C = A || B, then the most
significant bits of C are the bits of A, and the least significant
bits of C equal the bits of B. Hexadecimal numbers are prefixed by
0x.
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The word "encryption" includes also use of the NULL algorithm (which
in practice does leave the data in the clear).
With slight abuse of notation, we use the terms "message
authentication" and "authentication tag" as is common practice, even
though in some circumstances, e.g., group communication, the service
provided is actually only integrity protection and not data origin
authentication.
2. Goals and Features
The security goals for SRTP are to ensure:
* the confidentiality of the RTP and RTCP payloads, and
* the integrity of the entire RTP and RTCP packets, together with
protection against replayed packets.
These security services are optional and independent from each other,
except that SRTCP integrity protection is mandatory (malicious or
erroneous alteration of RTCP messages could otherwise disrupt the
processing of the RTP stream).
Other, functional, goals for the protocol are:
* a framework that permits upgrading with new cryptographic
transforms,
* low bandwidth cost, i.e., a framework preserving RTP header
compression efficiency,
and, asserted by the pre-defined transforms:
* a low computational cost,
* a small footprint (i.e., small code size and data memory for
keying information and replay lists),
* limited packet expansion to support the bandwidth economy goal,
* independence from the underlying transport, network, and physical
layers used by RTP, in particular high tolerance to packet loss
and re-ordering.
These properties ensure that SRTP is a suitable protection scheme for
RTP/RTCP in both wired and wireless scenarios.
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2.1. Features
Besides the above mentioned direct goals, SRTP provides for some
additional features. They have been introduced to lighten the burden
on key management and to further increase security. They include:
* A single "master key" can provide keying material for
confidentiality and integrity protection, both for the SRTP stream
and the corresponding SRTCP stream. This is achieved with a key
derivation function (see Section 4.3), providing "session keys"
for the respective security primitive, securely derived from the
master key.
* In addition, the key derivation can be configured to periodically
refresh the session keys, which limits the amount of ciphertext
produced by a fixed key, available for an adversary to
cryptanalyze.
* "Salting keys" are used to protect against pre-computation and
time-memory tradeoff attacks [MF00] [BS00].
Detailed rationale for these features can be found in Section 7.
3. SRTP Framework
RTP is the Real-time Transport Protocol [RFC3550]. We define SRTP as
a profile of RTP. This profile is an extension to the RTP
Audio/Video Profile [RFC3551]. Except where explicitly noted, all
aspects of that profile apply, with the addition of the SRTP security
features. Conceptually, we consider SRTP to be a "bump in the stack"
implementation which resides between the RTP application and the
transport layer. SRTP intercepts RTP packets and then forwards an
equivalent SRTP packet on the sending side, and intercepts SRTP
packets and passes an equivalent RTP packet up the stack on the
receiving side.
Secure RTCP (SRTCP) provides the same security services to RTCP as
SRTP does to RTP. SRTCP message authentication is MANDATORY and
thereby protects the RTCP fields to keep track of membership, provide
feedback to RTP senders, or maintain packet sequence counters. SRTCP
is described in Section 3.4.
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3.1. Secure RTP
The format of an SRTP packet is illustrated in Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
|V=2|P|X| CC |M| PT | sequence number | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| timestamp | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| synchronization source (SSRC) identifier | |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
| contributing source (CSRC) identifiers | |
| .... | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| RTP extension (OPTIONAL) | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | payload ... | |
| | +-------------------------------+ |
| | | RTP padding | RTP pad count | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
| ~ SRTP MKI (OPTIONAL) ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| : authentication tag (RECOMMENDED) : |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+- Encrypted Portion* Authenticated Portion ---+
Figure 1. The format of an SRTP packet. *Encrypted Portion is the
same size as the plaintext for the Section 4 pre-defined transforms.
The "Encrypted Portion" of an SRTP packet consists of the encryption
of the RTP payload (including RTP padding when present) of the
equivalent RTP packet. The Encrypted Portion MAY be the exact size
of the plaintext or MAY be larger. Figure 1 shows the RTP payload
including any possible padding for RTP [RFC3550].
None of the pre-defined encryption transforms uses any padding; for
these, the RTP and SRTP payload sizes match exactly. New transforms
added to SRTP (following Section 6) may require padding, and may
hence produce larger payloads. RTP provides its own padding format
(as seen in Fig. 1), which due to the padding indicator in the RTP
header has merits in terms of compactness relative to paddings using
prefix-free codes. This RTP padding SHALL be the default method for
transforms requiring padding. Transforms MAY specify other padding
methods, and MUST then specify the amount, format, and processing of
their padding. It is important to note that encryption transforms
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that use padding are vulnerable to subtle attacks, especially when
message authentication is not used [V02]. Each specification for a
new encryption transform needs to carefully consider and describe the
security implications of the padding that it uses. Message
authentication codes define their own padding, so this default does
not apply to authentication transforms.
The OPTIONAL MKI and the RECOMMENDED authentication tag are the only
fields defined by SRTP that are not in RTP. Only 8-bit alignment is
assumed.
MKI (Master Key Identifier): configurable length, OPTIONAL. The
MKI is defined, signaled, and used by key management. The
MKI identifies the master key from which the session
key(s) were derived that authenticate and/or encrypt the
particular packet. Note that the MKI SHALL NOT identify
the SRTP cryptographic context, which is identified
according to Section 3.2.3. The MKI MAY be used by key
management for the purposes of re-keying, identifying a
particular master key within the cryptographic context
(Section 3.2.1).
Authentication tag: configurable length, RECOMMENDED. The
authentication tag is used to carry message authentication
data. The Authenticated Portion of an SRTP packet
consists of the RTP header followed by the Encrypted
Portion of the SRTP packet. Thus, if both encryption and
authentication are applied, encryption SHALL be applied
before authentication on the sender side and conversely on
the receiver side. The authentication tag provides
authentication of the RTP header and payload, and it
indirectly provides replay protection by authenticating
the sequence number. Note that the MKI is not integrity
protected as this does not provide any extra protection.
3.2. SRTP Cryptographic Contexts
Each SRTP stream requires the sender and receiver to maintain
cryptographic state information. This information is called the
"cryptographic context".
SRTP uses two types of keys: session keys and master keys. By a
"session key", we mean a key which is used directly in a
cryptographic transform (e.g., encryption or message authentication),
and by a "master key", we mean a random bit string (given by the key
management protocol) from which session keys are derived in a
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cryptographically secure way. The master key(s) and other parameters
in the cryptographic context are provided by key management
mechanisms external to SRTP, see Section 8.
3.2.1. Transform-independent parameters
Transform-independent parameters are present in the cryptographic
context independently of the particular encryption or authentication
transforms that are used. The transform-independent parameters of
the cryptographic context for SRTP consist of:
* a 32-bit unsigned rollover counter (ROC), which records how many
times the 16-bit RTP sequence number has been reset to zero after
passing through 65,535. Unlike the sequence number (SEQ), which
SRTP extracts from the RTP packet header, the ROC is maintained by
SRTP as described in Section 3.3.1.
We define the index of the SRTP packet corresponding to a given
ROC and RTP sequence number to be the 48-bit quantity
i = 2^16 * ROC + SEQ.
* for the receiver only, a 16-bit sequence number s_l, which can be
thought of as the highest received RTP sequence number (see
Section 3.3.1 for its handling), which SHOULD be authenticated
since message authentication is RECOMMENDED,
* an identifier for the encryption algorithm, i.e., the cipher and
its mode of operation,
* an identifier for the message authentication algorithm,
* a replay list, maintained by the receiver only (when
authentication and replay protection are provided), containing
indices of recently received and authenticated SRTP packets,
* an MKI indicator (0/1) as to whether an MKI is present in SRTP and
SRTCP packets,
* if the MKI indicator is set to one, the length (in octets) of the
MKI field, and (for the sender) the actual value of the currently
active MKI (the value of the MKI indicator and length MUST be kept
fixed for the lifetime of the context),
* the master key(s), which MUST be random and kept secret,
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* for each master key, there is a counter of the number of SRTP
packets that have been processed (sent) with that master key
(essential for security, see Sections 3.3.1 and 9),
* non-negative integers n_e, and n_a, determining the length of the
session keys for encryption, and message authentication.
In addition, for each master key, an SRTP stream MAY use the
following associated values:
* a master salt, to be used in the key derivation of session keys.
This value, when used, MUST be random, but MAY be public. Use of
master salt is strongly RECOMMENDED, see Section 9.2. A "NULL"
salt is treated as 00...0.
* an integer in the set {1,2,4,...,2^24}, the "key_derivation_rate",
where an unspecified value is treated as zero. The constraint to
be a power of 2 simplifies the session-key derivation
implementation, see Section 4.3.
* an MKI value,
* values, specifying the lifetime for a master key,
expressed in terms of the two 48-bit index values inside whose
range (including the range end-points) the master key is valid.
For the use of , see Section 8.1.1. is an
alternative to the MKI and assumes that a master key is in one-
to-one correspondence with the SRTP session key on which the
range is defined.
SRTCP SHALL by default share the crypto context with SRTP, except:
* no rollover counter and s_l-value need to be maintained as the
RTCP index is explicitly carried in each SRTCP packet,
* a separate replay list is maintained (when replay protection is
provided),
* SRTCP maintains a separate counter for its master key (even if the
master key is the same as that for SRTP, see below), as a means to
maintain a count of the number of SRTCP packets that have been
processed with that key.
Note in particular that the master key(s) MAY be shared between SRTP
and the corresponding SRTCP, if the pre-defined transforms (including
the key derivation) are used but the session key(s) MUST NOT be so
shared.
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In addition, there can be cases (see Sections 8 and 9.1) where
several SRTP streams within a given RTP session, identified by their
synchronization source (SSRCs, which is part of the RTP header),
share most of the crypto context parameters (including possibly
master and session keys). In such cases, just as in the normal
SRTP/SRTCP parameter sharing above, separate replay lists and packet
counters for each stream (SSRC) MUST still be maintained. Also,
separate SRTP indices MUST then be maintained.
A summary of parameters, pre-defined transforms, and default values
for the above parameters (and other SRTP parameters) can be found in
Sections 5 and 8.2.
3.2.2. Transform-dependent parameters
All encryption, authentication/integrity, and key derivation
parameters are defined in the transforms section (Section 4).
Typical examples of such parameters are block size of ciphers,
session keys, data for the Initialization Vector (IV) formation, etc.
Future SRTP transform specifications MUST include a section to list
the additional cryptographic context's parameters for that transform,
if any.
3.2.3. Mapping SRTP Packets to Cryptographic Contexts
Recall that an RTP session for each participant is defined [RFC3550]
by a pair of destination transport addresses (one network address
plus a port pair for RTP and RTCP), and that a multimedia session is
defined as a collection of RTP sessions. For example, a particular
multimedia session could include an audio RTP session, a video RTP
session, and a text RTP session.
A cryptographic context SHALL be uniquely identified by the triplet
context identifier:
context id =
where the destination network address and the destination transport
port are the ones in the SRTP packet. It is assumed that, when
presented with this information, the key management returns a context
with the information as described in Section 3.2.
As noted above, SRTP and SRTCP by default share the bulk of the
parameters in the cryptographic context. Thus, retrieving the crypto
context parameters for an SRTCP stream in practice may imply a
binding to the correspondent SRTP crypto context. It is up to the
implementation to assure such binding, since the RTCP port may not be
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directly deducible from the RTP port only. Alternatively, the key
management may choose to provide separate SRTP- and SRTCP- contexts,
duplicating the common parameters (such as master key(s)). The
latter approach then also enables SRTP and SRTCP to use, e.g.,
distinct transforms, if so desired. Similar considerations arise
when multiple SRTP streams, forming part of one single RTP session,
share keys and other parameters.
If no valid context can be found for a packet corresponding to a
certain context identifier, that packet MUST be discarded.
3.3. SRTP Packet Processing
The following applies to SRTP. SRTCP is described in Section 3.4.
Assuming initialization of the cryptographic context(s) has taken
place via key management, the sender SHALL do the following to
construct an SRTP packet:
1. Determine which cryptographic context to use as described in
Section 3.2.3.
2. Determine the index of the SRTP packet using the rollover counter,
the highest sequence number in the cryptographic context, and the
sequence number in the RTP packet, as described in Section 3.3.1.
3. Determine the master key and master salt. This is done using the
index determined in the previous step or the current MKI in the
cryptographic context, according to Section 8.1.
4. Determine the session keys and session salt (if they are used by
the transform) as described in Section 4.3, using master key,
master salt, key_derivation_rate, and session key-lengths in the
cryptographic context with the index, determined in Steps 2 and 3.
5. Encrypt the RTP payload to produce the Encrypted Portion of the
packet (see Section 4.1, for the defined ciphers). This step uses
the encryption algorithm indicated in the cryptographic context,
the session encryption key and the session salt (if used) found in
Step 4 together with the index found in Step 2.
6. If the MKI indicator is set to one, append the MKI to the packet.
7. For message authentication, compute the authentication tag for the
Authenticated Portion of the packet, as described in Section 4.2.
This step uses the current rollover counter, the authentication
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algorithm indicated in the cryptographic context, and the session
authentication key found in Step 4. Append the authentication tag
to the packet.
8. If necessary, update the ROC as in Section 3.3.1, using the packet
index determined in Step 2.
To authenticate and decrypt an SRTP packet, the receiver SHALL do the
following:
1. Determine which cryptographic context to use as described in
Section 3.2.3.
2. Run the algorithm in Section 3.3.1 to get the index of the SRTP
packet. The algorithm uses the rollover counter and highest
sequence number in the cryptographic context with the sequence
number in the SRTP packet, as described in Section 3.3.1.
3. Determine the master key and master salt. If the MKI indicator in
the context is set to one, use the MKI in the SRTP packet,
otherwise use the index from the previous step, according to
Section 8.1.
4. Determine the session keys, and session salt (if used by the
transform) as described in Section 4.3, using master key, master
salt, key_derivation_rate and session key-lengths in the
cryptographic context with the index, determined in Steps 2 and 3.
5. For message authentication and replay protection, first check if
the packet has been replayed (Section 3.3.2), using the Replay
List and the index as determined in Step 2. If the packet is
judged to be replayed, then the packet MUST be discarded, and the
event SHOULD be logged.
Next, perform verification of the authentication tag, using the
rollover counter from Step 2, the authentication algorithm
indicated in the cryptographic context, and the session
authentication key from Step 4. If the result is "AUTHENTICATION
FAILURE" (see Section 4.2), the packet MUST be discarded from
further processing and the event SHOULD be logged.
6. Decrypt the Encrypted Portion of the packet (see Section 4.1, for
the defined ciphers), using the decryption algorithm indicated in
the cryptographic context, the session encryption key and salt (if
used) found in Step 4 with the index from Step 2.
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7. Update the rollover counter and highest sequence number, s_l, in
the cryptographic context as in Section 3.3.1, using the packet
index estimated in Step 2. If replay protection is provided, also
update the Replay List as described in Section 3.3.2.
8. When present, remove the MKI and authentication tag fields from
the packet.
3.3.1. Packet Index Determination, and ROC, s_l Update
SRTP implementations use an "implicit" packet index for sequencing,
i.e., not all of the index is explicitly carried in the SRTP packet.
For the pre-defined transforms, the index i is used in replay
protection (Section 3.3.2), encryption (Section 4.1), message
authentication (Section 4.2), and for the key derivation (Section
4.3).
When the session starts, the sender side MUST set the rollover
counter, ROC, to zero. Each time the RTP sequence number, SEQ, wraps
modulo 2^16, the sender side MUST increment ROC by one, modulo 2^32
(see security aspects below). The sender's packet index is then
defined as
i = 2^16 * ROC + SEQ.
Receiver-side implementations use the RTP sequence number to
determine the correct index of a packet, which is the location of the
packet in the sequence of all SRTP packets. A robust approach for
the proper use of a rollover counter requires its handling and use to
be well defined. In particular, out-of-order RTP packets with
sequence numbers close to 2^16 or zero must be properly handled.
The index estimate is based on the receiver's locally maintained ROC
and s_l values. At the setup of the session, the ROC MUST be set to
zero. Receivers joining an on-going session MUST be given the
current ROC value using out-of-band signaling such as key-management
signaling. Furthermore, the receiver SHALL initialize s_l to the RTP
sequence number (SEQ) of the first observed SRTP packet (unless the
initial value is provided by out of band signaling such as key
management).
On consecutive SRTP packets, the receiver SHOULD estimate the index
as
i = 2^16 * v + SEQ,
where v is chosen from the set { ROC-1, ROC, ROC+1 } (modulo 2^32)
such that i is closest (in modulo 2^48 sense) to the value 2^16 * ROC
+ s_l (see Appendix A for pseudocode).
Baugher, et al. Standards Track [Page 13]
RFC 3711 SRTP March 2004
After the packet has been processed and authenticated (when enabled
for SRTP packets for the session), the receiver MUST use v to
conditionally update its s_l and ROC variables as follows. If
v=(ROC-1) mod 2^32, then there is no update to s_l or ROC. If v=ROC,
then s_l is set to SEQ if and only if SEQ is larger than the current
s_l; there is no change to ROC. If v=(ROC+1) mod 2^32, then s_l is
set to SEQ and ROC is set to v.
After a re-keying occurs (changing to a new master key), the rollover
counter always maintains its sequence of values, i.e., it MUST NOT be
reset to zero.
As the rollover counter is 32 bits long and the sequence number is 16
bits long, the maximum number of packets belonging to a given SRTP
stream that can be secured with the same key is 2^48 using the pre-
defined transforms. After that number of SRTP packets have been sent
with a given (master or session) key, the sender MUST NOT send any
more packets with that key. (There exists a similar limit for SRTCP,
which in practice may be more restrictive, see Section 9.2.) This
limitation enforces a security benefit by providing an upper bound on
the amount of traffic that can pass before cryptographic keys are
changed. Re-keying (see Section 8.1) MUST be triggered, before this
amount of traffic, and MAY be triggered earlier, e.g., for increased
security and access control to media. Recurring key derivation by
means of a non-zero key_derivation_rate (see Section 4.3), also gives
stronger security but does not change the above absolute maximum
value.
On the receiver side, there is a caveat to updating s_l and ROC: if
message authentication is not present, neither the initialization of
s_l, nor the ROC update can be made completely robust. The
receiver's "implicit index" approach works for the pre-defined
transforms as long as the reorder and loss of the packets are not too
great and bit-errors do not occur in unfortunate ways. In
particular, 2^15 packets would need to be lost, or a packet would
need to be 2^15 packets out of sequence before synchronization is
lost. Such drastic loss or reorder is likely to disrupt the RTP
application itself.
The algorithm for the index estimate and ROC update is a matter of
implementation, and should take into consideration the environment
(e.g., packet loss rate) and the cases when synchronization is likely
to be lost, e.g., when the initial sequence number (randomly chosen
by RTP) is not known in advance (not sent in the key management
protocol) but may be near to wrap modulo 2^16.
Baugher, et al. Standards Track [Page 14]
RFC 3711 SRTP March 2004
A more elaborate and more robust scheme than the one given above is
the handling of RTP's own "rollover counter", see Appendix A.1 of
[RFC3550].
3.3.2. Replay Protection
Secure replay protection is only possible when integrity protection
is present. It is RECOMMENDED to use replay protection, both for RTP
and RTCP, as integrity protection alone cannot assure security
against replay attacks.
A packet is "replayed" when it is stored by an adversary, and then
re-injected into the network. When message authentication is
provided, SRTP protects against such attacks through a Replay List.
Each SRTP receiver maintains a Replay List, which conceptually
contains the indices of all of the packets which have been received
and authenticated. In practice, the list can use a "sliding window"
approach, so that a fixed amount of storage suffices for replay
protection. Packet indices which lag behind the packet index in the
context by more than SRTP-WINDOW-SIZE can be assumed to have been
received, where SRTP-WINDOW-SIZE is a receiver-side, implementation-
dependent parameter and MUST be at least 64, but which MAY be set to
a higher value.
The receiver checks the index of an incoming packet against the
replay list and the window. Only packets with index ahead of the
window, or, inside the window but not already received, SHALL be
accepted.
After the packet has been authenticated (if necessary the window is
first moved ahead), the replay list SHALL be updated with the new
index.
The Replay List can be efficiently implemented by using a bitmap to
represent which packets have been received, as described in the
Security Architecture for IP [RFC2401].
3.4. Secure RTCP
Secure RTCP follows the definition of Secure RTP. SRTCP adds three
mandatory new fields (the SRTCP index, an "encrypt-flag", and the
authentication tag) and one optional field (the MKI) to the RTCP
packet definition. The three mandatory fields MUST be appended to an
RTCP packet in order to form an equivalent SRTCP packet. The added
fields follow any other profile-specific extensions.
Baugher, et al. Standards Track [Page 15]
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According to Section 6.1 of [RFC3550], there is a REQUIRED packet
format for compound packets. SRTCP MUST be given packets according
to that requirement in the sense that the first part MUST be a sender
report or a receiver report. However, the RTCP encryption prefix (a
random 32-bit quantity) specified in that Section MUST NOT be used
since, as is stated there, it is only applicable to the encryption
method specified in [RFC3550] and is not needed by the cryptographic
mechanisms used in SRTP.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
|V=2|P| RC | PT=SR or RR | length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| SSRC of sender | |
+>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
| ~ sender info ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ~ report block 1 ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ~ report block 2 ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ~ ... ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |V=2|P| SC | PT=SDES=202 | length | |
| +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
| | SSRC/CSRC_1 | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ~ SDES items ~ |
| +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
| ~ ... ~ |
+>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
| |E| SRTCP index | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
| ~ SRTCP MKI (OPTIONAL) ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| : authentication tag : |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+-- Encrypted Portion Authenticated Portion -----+
Figure 2. An example of the format of a Secure RTCP packet,
consisting of an underlying RTCP compound packet with a Sender Report
and SDES packet.
Baugher, et al. Standards Track [Page 16]
RFC 3711 SRTP March 2004
The Encrypted Portion of an SRTCP packet consists of the encryption
(Section 4.1) of the RTCP payload of the equivalent compound RTCP
packet, from the first RTCP packet, i.e., from the ninth (9) octet to
the end of the compound packet. The Authenticated Portion of an
SRTCP packet consists of the entire equivalent (eventually compound)
RTCP packet, the E flag, and the SRTCP index (after any encryption
has been applied to the payload).
The added fields are:
E-flag: 1 bit, REQUIRED
The E-flag indicates if the current SRTCP packet is
encrypted or unencrypted. Section 9.1 of [RFC3550] allows
the split of a compound RTCP packet into two lower-layer
packets, one to be encrypted and one to be sent in the
clear. The E bit set to "1" indicates encrypted packet, and
"0" indicates non-encrypted packet.
SRTCP index: 31 bits, REQUIRED
The SRTCP index is a 31-bit counter for the SRTCP packet.
The index is explicitly included in each packet, in contrast
to the "implicit" index approach used for SRTP. The SRTCP
index MUST be set to zero before the first SRTCP packet is
sent, and MUST be incremented by one, modulo 2^31, after
each SRTCP packet is sent. In particular, after a re-key,
the SRTCP index MUST NOT be reset to zero again.
Authentication Tag: configurable length, REQUIRED
The authentication tag is used to carry message
authentication data.
MKI: configurable length, OPTIONAL
The MKI is the Master Key Indicator, and functions according
to the MKI definition in Section 3.
SRTCP uses the cryptographic context parameters and packet processing
of SRTP by default, with the following changes:
* The receiver does not need to "estimate" the index, as it is
explicitly signaled in the packet.
* Pre-defined SRTCP encryption is as specified in Section 4.1, but
using the definition of the SRTCP Encrypted Portion given in this
section, and using the SRTCP index as the index i. The encryption
transform and related parameters SHALL by default be the same
selected for the protection of the associated SRTP stream(s),
while the NULL algorithm SHALL be applied to the RTCP packets not
to be encrypted. SRTCP may have a different encryption transform
Baugher, et al. Standards Track [Page 17]
RFC 3711 SRTP March 2004
than the one used by the corresponding SRTP. The expected use for
this feature is when the former has NULL-encryption and the latter
has a non NULL-encryption.
The E-flag is assigned a value by the sender depending on whether the
packet was encrypted or not.
* SRTCP decryption is performed as in Section 4, but only if the E
flag is equal to 1. If so, the Encrypted Portion is decrypted,
using the SRTCP index as the index i. In case the E-flag is 0,
the payload is simply left unmodified.
* SRTCP replay protection is as defined in Section 3.3.2, but using
the SRTCP index as the index i and a separate Replay List that is
specific to SRTCP.
* The pre-defined SRTCP authentication tag is specified as in
Section 4.2, but with the Authenticated Portion of the SRTCP
packet given in this section (which includes the index). The
authentication transform and related parameters (e.g., key size)
SHALL by default be the same as selected for the protection of the
associated SRTP stream(s).
* In the last step of the processing, only the sender needs to
update the value of the SRTCP index by incrementing it modulo 2^31
and for security reasons the sender MUST also check the number of
SRTCP packets processed, see Section 9.2.
Message authentication for RTCP is REQUIRED, as it is the control
protocol (e.g., it has a BYE packet) for RTP.
Precautions must be taken so that the packet expansion in SRTCP (due
to the added fields) does not cause SRTCP messages to use more than
their share of RTCP bandwidth. To avoid this, the following two
measures MUST be taken:
1. When initializing the RTCP variable "avg_rtcp_size" defined in
chapter 6.3 of [RFC3550], it MUST include the size of the fields
that will be added by SRTCP (index, E-bit, authentication tag, and
when present, the MKI).
2. When updating the "avg_rtcp_size" using the variable "packet_size"
(section 6.3.3 of [RFC3550]), the value of "packet_size" MUST
include the size of the additional fields added by SRTCP.
Baugher, et al. Standards Track [Page 18]
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With these measures in place the SRTCP messages will not use more
than the allotted bandwidth. The effect of the size of the added
fields on the SRTCP traffic will be that messages will be sent with
longer packet intervals. The increase in the intervals will be
directly proportional to size of the added fields. For the pre-
defined transforms, the size of the added fields will be at least 14
octets, and upper bounded depending on MKI and the authentication tag
sizes.
4. Pre-Defined Cryptographic Transforms
While there are numerous encryption and message authentication
algorithms that can be used in SRTP, below we define default
algorithms in order to avoid the complexity of specifying the
encodings for the signaling of algorithm and parameter identifiers.
The defined algorithms have been chosen as they fulfill the goals
listed in Section 2. Recommendations on how to extend SRTP with new
transforms are given in Section 6.
4.1. Encryption
The following parameters are common to both pre-defined, non-NULL,
encryption transforms specified in this section.
* BLOCK_CIPHER-MODE indicates the block cipher used and its mode of
operation
* n_b is the bit-size of the block for the block cipher
* k_e is the session encryption key
* n_e is the bit-length of k_e
* k_s is the session salting key
* n_s is the bit-length of k_s
* SRTP_PREFIX_LENGTH is the octet length of the keystream prefix, a
non-negative integer, specified by the message authentication code
in use.
The distinct session keys and salts for SRTP/SRTCP are by default
derived as specified in Section 4.3.
The encryption transforms defined in SRTP map the SRTP packet index
and secret key into a pseudo-random keystream segment. Each
keystream segment encrypts a single RTP packet. The process of
encrypting a packet consists of generating the keystream segment
corresponding to the packet, and then bitwise exclusive-oring that
keystream segment onto the payload of the RTP packet to produce the
Encrypted Portion of the SRTP packet. In case the payload size is
not an integer multiple of n_b bits, the excess (least significant)
bits of the keystream are simply discarded. Decryption is done the
same way, but swapping the roles of the plaintext and ciphertext.
Baugher, et al. Standards Track [Page 19]
RFC 3711 SRTP March 2004
+----+ +------------------+---------------------------------+
| KG |-->| Keystream Prefix | Keystream Suffix |---+
+----+ +------------------+---------------------------------+ |
|
+---------------------------------+ v
| Payload of RTP Packet |->(*)
+---------------------------------+ |
|
+---------------------------------+ |
| Encrypted Portion of SRTP Packet|<--+
+---------------------------------+
Figure 3: Default SRTP Encryption Processing. Here KG denotes the
keystream generator, and (*) denotes bitwise exclusive-or.
The definition of how the keystream is generated, given the index,
depends on the cipher and its mode of operation. Below, two such
keystream generators are defined. The NULL cipher is also defined,
to be used when encryption of RTP is not required.
The SRTP definition of the keystream is illustrated in Figure 3. The
initial octets of each keystream segment MAY be reserved for use in a
message authentication code, in which case the keystream used for
encryption starts immediately after the last reserved octet. The
initial reserved octets are called the "keystream prefix" (not to be
confused with the "encryption prefix" of [RFC3550, Section 6.1]), and
the remaining octets are called the "keystream suffix". The
keystream prefix MUST NOT be used for encryption. The process is
illustrated in Figure 3.
The number of octets in the keystream prefix is denoted as
SRTP_PREFIX_LENGTH. The keystream prefix is indicated by a positive,
non-zero value of SRTP_PREFIX_LENGTH. This means that, even if
confidentiality is not to be provided, the keystream generator output
may still need to be computed for packet authentication, in which
case the default keystream generator (mode) SHALL be used.
The default cipher is the Advanced Encryption Standard (AES) [AES],
and we define two modes of running AES, (1) Segmented Integer Counter
Mode AES and (2) AES in f8-mode. In the remainder of this section,
let E(k,x) be AES applied to key k and input block x.
Baugher, et al. Standards Track [Page 20]
RFC 3711 SRTP March 2004
4.1.1. AES in Counter Mode
Conceptually, counter mode [AES-CTR] consists of encrypting
successive integers. The actual definition is somewhat more
complicated, in order to randomize the starting point of the integer
sequence. Each packet is encrypted with a distinct keystream
segment, which SHALL be computed as follows.
A keystream segment SHALL be the concatenation of the 128-bit output
blocks of the AES cipher in the encrypt direction, using key k = k_e,
in which the block indices are in increasing order. Symbolically,
each keystream segment looks like
E(k, IV) || E(k, IV + 1 mod 2^128) || E(k, IV + 2 mod 2^128) ...
where the 128-bit integer value IV SHALL be defined by the SSRC, the
SRTP packet index i, and the SRTP session salting key k_s, as below.
IV = (k_s * 2^16) XOR (SSRC * 2^64) XOR (i * 2^16)
Each of the three terms in the XOR-sum above is padded with as many
leading zeros as needed to make the operation well-defined,
considered as a 128-bit value.
The inclusion of the SSRC allows the use of the same key to protect
distinct SRTP streams within the same RTP session, see the security
caveats in Section 9.1.
In the case of SRTCP, the SSRC of the first header of the compound
packet MUST be used, i SHALL be the 31-bit SRTCP index and k_e, k_s
SHALL be replaced by the SRTCP encryption session key and salt.
Note that the initial value, IV, is fixed for each packet and is
formed by "reserving" 16 zeros in the least significant bits for the
purpose of the counter. The number of blocks of keystream generated
for any fixed value of IV MUST NOT exceed 2^16 to avoid keystream
re-use, see below. The AES has a block size of 128 bits, so 2^16
output blocks are sufficient to generate the 2^23 bits of keystream
needed to encrypt the largest possible RTP packet (except for IPv6
"jumbograms" [RFC2675], which are not likely to be used for RTP-based
multimedia traffic). This restriction on the maximum bit-size of the
packet that can be encrypted ensures the security of the encryption
method by limiting the effectiveness of probabilistic attacks [BDJR].
For a particular Counter Mode key, each IV value used as an input
MUST be distinct, in order to avoid the security exposure of a two-
time pad situation (Section 9.1). To satisfy this constraint, an
implementation MUST ensure that the combination of the SRTP packet
Baugher, et al. Standards Track [Page 21]
RFC 3711 SRTP March 2004
index of ROC || SEQ, and the SSRC used in the construction of the IV
are distinct for any particular key. The failure to ensure this
uniqueness could be catastrophic for Secure RTP. This is in contrast
to the situation for RTP itself, which may be able to tolerate such
failures. It is RECOMMENDED that, if a dedicated security module is
present, the RTP sequence numbers and SSRC either be generated or
checked by that module (i.e., sequence-number and SSRC processing in
an SRTP system needs to be protected as well as the key).
4.1.2. AES in f8-mode
To encrypt UMTS (Universal Mobile Telecommunications System, as 3G
networks) data, a solution (see [f8-a] [f8-b]) known as the f8-
algorithm has been developed. On a high level, the proposed scheme
is a variant of Output Feedback Mode (OFB) [HAC], with a more
elaborate initialization and feedback function. As in normal OFB,
the core consists of a block cipher. We also define here the use of
AES as a block cipher to be used in what we shall call "f8-mode of
operation" RTP encryption. The AES f8-mode SHALL use the same
default sizes for session key and salt as AES counter mode.
Figure 4 shows the structure of block cipher, E, running in f8-mode.
Baugher, et al. Standards Track [Page 22]
RFC 3711 SRTP March 2004
IV
|
v
+------+
| |
+--->| E |
| +------+
| |
m -> (*) +-----------+-------------+-- ... ------+
| IV' | | | |
| | j=1 -> (*) j=2 -> (*) ... j=L-1 ->(*)
| | | | |
| | +-> (*) +-> (*) ... +-> (*)
| | | | | | | |
| v | v | v | v
| +------+ | +------+ | +------+ | +------+
k_e ---+--->| E | | | E | | | E | | | E |
| | | | | | | | | | |
+------+ | +------+ | +------+ | +------+
| | | | | | |
+------+ +--------+ +-- ... ----+ |
| | | |
v v v v
S(0) S(1) S(2) . . . S(L-1)
Figure 4. f8-mode of operation (asterisk, (*), denotes bitwise XOR).
The figure represents the KG in Figure 3, when AES-f8 is used.
4.1.2.1. f8 Keystream Generation
The Initialization Vector (IV) SHALL be determined as described in
Section 4.1.2.2 (and in Section 4.1.2.3 for SRTCP).
Let IV', S(j), and m denote n_b-bit blocks. The keystream,
S(0) ||... || S(L-1), for an N-bit message SHALL be defined by
setting IV' = E(k_e XOR m, IV), and S(-1) = 00..0. For
j = 0,1,..,L-1 where L = N/n_b (rounded up to nearest integer if it
is not already an integer) compute
S(j) = E(k_e, IV' XOR j XOR S(j-1))
Notice that the IV is not used directly. Instead it is fed through E
under another key to produce an internal, "masked" value (denoted
IV') to prevent an attacker from gaining known input/output pairs.
Baugher, et al. Standards Track [Page 23]
RFC 3711 SRTP March 2004
The role of the internal counter, j, is to prevent short keystream
cycles. The value of the key mask m SHALL be
m = k_s || 0x555..5,
i.e., the session salting key, appended by the binary pattern 0101..
to fill out the entire desired key size, n_e.
The sender SHOULD NOT generate more than 2^32 blocks, which is
sufficient to generate 2^39 bits of keystream. Unlike counter mode,
there is no absolute threshold above (below) which f8 is guaranteed
to be insecure (secure). The above bound has been chosen to limit,
with sufficient security margin, the probability of degenerative
behavior in the f8 keystream generation.
4.1.2.2. f8 SRTP IV Formation
The purpose of the following IV formation is to provide a feature
which we call implicit header authentication (IHA), see Section 9.5.
The SRTP IV for 128-bit block AES-f8 SHALL be formed in the following
way:
IV = 0x00 || M || PT || SEQ || TS || SSRC || ROC
M, PT, SEQ, TS, SSRC SHALL be taken from the RTP header; ROC is from
the cryptographic context.
The presence of the SSRC as part of the IV allows AES-f8 to be used
when a master key is shared between multiple streams within the same
RTP session, see Section 9.1.
4.1.2.3. f8 SRTCP IV Formation
The SRTCP IV for 128-bit block AES-f8 SHALL be formed in the
following way:
IV= 0..0 || E || SRTCP index || V || P || RC || PT || length || SSRC
where V, P, RC, PT, length, SSRC SHALL be taken from the first header
in the RTCP compound packet. E and SRTCP index are the 1-bit and
31-bit fields added to the packet.
Baugher, et al. Standards Track [Page 24]
RFC 3711 SRTP March 2004
4.1.3. NULL Cipher
The NULL cipher is used when no confidentiality for RTP/RTCP is
requested. The keystream can be thought of as "000..0", i.e., the
encryption SHALL simply copy the plaintext input into the ciphertext
output.
4.2. Message Authentication and Integrity
Throughout this section, M will denote data to be integrity
protected. In the case of SRTP, M SHALL consist of the Authenticated
Portion of the packet (as specified in Figure 1) concatenated with
the ROC, M = Authenticated Portion || ROC; in the case of SRTCP, M
SHALL consist of the Authenticated Portion (as specified in Figure 2)
only.
Common parameters:
* AUTH_ALG is the authentication algorithm
* k_a is the session message authentication key
* n_a is the bit-length of the authentication key
* n_tag is the bit-length of the output authentication tag
* SRTP_PREFIX_LENGTH is the octet length of the keystream prefix as
defined above, a parameter of AUTH_ALG
The distinct session authentication keys for SRTP/SRTCP are by
default derived as specified in Section 4.3.
The values of n_a, n_tag, and SRTP_PREFIX_LENGTH MUST be fixed for
any particular fixed value of the key.
We describe the process of computing authentication tags as follows.
The sender computes the tag of M and appends it to the packet. The
SRTP receiver verifies a message/authentication tag pair by computing
a new authentication tag over M using the selected algorithm and key,
and then compares it to the tag associated with the received message.
If the two tags are equal, then the message/tag pair is valid;
otherwise, it is invalid and the error audit message "AUTHENTICATION
FAILURE" MUST be returned.
4.2.1. HMAC-SHA1
The pre-defined authentication transform for SRTP is HMAC-SHA1
[RFC2104]. With HMAC-SHA1, the SRTP_PREFIX_LENGTH (Figure 3) SHALL
be 0. For SRTP (respectively SRTCP), the HMAC SHALL be applied to
the session authentication key and M as specified above, i.e.,
HMAC(k_a, M). The HMAC output SHALL then be truncated to the n_tag
left-most bits.
Baugher, et al. Standards Track [Page 25]
RFC 3711 SRTP March 2004
4.3. Key Derivation
4.3.1. Key Derivation Algorithm
Regardless of the encryption or message authentication transform that
is employed (it may be an SRTP pre-defined transform or newly
introduced according to Section 6), interoperable SRTP
implementations MUST use the SRTP key derivation to generate session
keys. Once the key derivation rate is properly signaled at the start
of the session, there is no need for extra communication between the
parties that use SRTP key derivation.
packet index ---+
|
v
+-----------+ master +--------+ session encr_key
| ext | key | |---------->
| key mgmt |-------->| key | session auth_key
| (optional | | deriv |---------->
| rekey) |-------->| | session salt_key
| | master | |---------->
+-----------+ salt +--------+
Figure 5: SRTP key derivation.
At least one initial key derivation SHALL be performed by SRTP, i.e.,
the first key derivation is REQUIRED. Further applications of the
key derivation MAY be performed, according to the
"key_derivation_rate" value in the cryptographic context. The key
derivation function SHALL initially be invoked before the first
packet and then, when r > 0, a key derivation is performed whenever
index mod r equals zero. This can be thought of as "refreshing" the
session keys. The value of "key_derivation_rate" MUST be kept fixed
for the lifetime of the associated master key.
Interoperable SRTP implementations MAY also derive session salting
keys for encryption transforms, as is done in both of the pre-
defined transforms.
Let m and n be positive integers. A pseudo-random function family is
a set of keyed functions {PRF_n(k,x)} such that for the (secret)
random key k, given m-bit x, PRF_n(k,x) is an n-bit string,
computationally indistinguishable from random n-bit strings, see
[HAC]. For the purpose of key derivation in SRTP, a secure PRF with
m = 128 (or more) MUST be used, and a default PRF transform is
defined in Section 4.3.3.
Baugher, et al. Standards Track [Page 26]
RFC 3711 SRTP March 2004
Let "a DIV t" denote integer division of a by t, rounded down, and
with the convention that "a DIV 0 = 0" for all a. We also make the
convention of treating "a DIV t" as a bit string of the same length
as a, and thus "a DIV t" will in general have leading zeros.
Key derivation SHALL be defined as follows in terms of
RFC, FYI, BCP