Internet Engineering Task Force Baugher, McGrew,
AVT Working Group Oran (Cisco)
INTERNET-DRAFT Blom, Carrara, Naslund,
EXPIRES: December 2002 Norrman (Ericsson)
June 2002
The Secure Real-time Transport Protocol
<draft-ietf-avt-srtp-05.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference
material or cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/lid-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
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/RTCP traffic.
INTERNET-DRAFT SRTP June, 2002
TABLE OF CONTENTS
1. Introduction......................................................3
1.1. Notational Conventions..........................................3
2. Goals and Features................................................4
3. SRTP Framework....................................................5
3.1 Secure RTP......................................................6
3.2 SRTP Cryptographic Contexts.....................................7
3.2.1 Transform-independent parameters............................7
3.2.2 Transform-dependent parameters..............................9
3.2.3 Mapping SRTP Packets to Cryptographic Contexts.............10
3.3 SRTP Packet Processing.........................................10
3.3.1 Packet Index Determination, and ROC, s_l Update............12
3.3.2 Replay Protection..........................................14
3.4 Secure RTCP....................................................15
4. Pre-Defined Cryptographic Transforms.............................18
4.1 Encryption.....................................................18
4.1.1 AES in Counter Mode........................................20
4.1.2 AES in f8-mode.............................................21
4.1.3 NULL Cipher................................................23
4.2 Message Authentication and Integrity...........................23
4.2.1. HMAC-SHA1.................................................24
4.3 Key Derivation.................................................24
4.3.1 Key Derivation Algorithm...................................24
4.3.2 SRTCP Key Derivation.......................................26
4.3.3 AES-CM PRF.................................................26
5. Default and mandatory-to-implement Transforms....................27
5.1 Encryption: AES-CM and NULL....................................27
5.2 Message Authentication/Integrity: HMAC-SHA1....................27
5.3 Key Derivation: AES-CM PRF.....................................27
6. Adding SRTP Transforms...........................................27
7. Rationale........................................................28
7.1 Key derivation.................................................28
7.2 Salting key....................................................29
7.3 Message Integrity from Universal Hashing.......................29
7.4 Data Origin Authentication Considerations......................29
8. Key Management Considerations....................................30
8.1. Re-keying.....................................................31
8.2. Key Management parameters.....................................32
9. Security Considerations..........................................33
9.1 SSRC collision and two-time pad................................33
9.2 Key Usage......................................................34
9.3 Confidentiality of the RTP Payload.............................35
9.4 Confidentiality of the RTP Header..............................36
9.5 Integrity of the RTP payload and header........................36
10. Interaction with Forward Error Correction mechanisms............37
11. Scenarios.......................................................37
11.1 Unicast.......................................................37
Baugher, et al. [Page 2]
INTERNET-DRAFT SRTP June, 2002
11.2 Multicast.....................................................38
11.2.1 Small multicast with one sender...........................38
11.2.2 Large multicast with one sender...........................39
11.3 Re-keying and access control..................................40
11.4 Summary of basic scenarios....................................40
12. IANA Considerations.............................................41
13. Acknowledgements................................................41
14. Author's Addresses..............................................42
15. References......................................................42
Appendix A: Pseudocode for Index Determination......................45
Appendix B: Test Vectors............................................45
B.1 AES-f8 Test Vectors............................................45
B.2 AES-CM Test Vectors............................................46
B.3 Key Derivation Test Vectors....................................47
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/RTCP traffic.
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 and 10) 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.
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].
By convention, the adopted representation is the network byte order,
i.e. the left most bit (octet) is the most significant one. By XOR
Baugher, et al. [Page 3]
INTERNET-DRAFT SRTP June, 2002
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.
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 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,
Baugher, et al. [Page 4]
INTERNET-DRAFT SRTP June, 2002
* independence from the underlying transport, network, and physical
layers used by RTP, in particular high tolerance to packet loss
and re-ordering, and robustness to transmission bit-errors in the
encrypted payload.
These properties ensure that SRTP is a suitable protection scheme
for RTP/RTCP in both wired and wireless scenarios.
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" provides keying material for
confidentiality and integrity protection, both for the SRTP stream
and the corresponding SRTCP stream. This is achieved due to a key
derivation function (see Section 4.3), providing "session keys"
for the respective security primitive, securely derived from
the master key. Under additional SSRC uniqueness requirements, a
single master key can even protect several SRTP streams, see
Section 9.1.
* 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 attacks
[MF00].
Detailed rationale for these features can be found in Section 7.
3. SRTP Framework
RTP is the Real-time Transport Protocol [RFC1889]. We define SRTP as
a profile of RTP, in a way analogous to RFC1890 which defines the
audio/video profile for RTP. Conceptually, we consider it 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
which 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 to
protect the RTCP messages and thereby protect the RTP session that
Baugher, et al. [Page 5]
INTERNET-DRAFT SRTP June, 2002
uses RTP fields to keep track of membership, provide feedback to RTP
senders, or maintain packet sequence counters. SRTCP is described
in Section 3.4.
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 (OPTIONAL) ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+- 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. (Note: the "Encrypted Portion" MAY be the
exact size of the plaintext or MAY be larger. It is exact for the
pre-defined transforms and for NULL-encryption, which doesn't change
the payload in any way.)
The optional MKI and optional authentication tag are the only fields
defined by SRTP that are not in RTP. Only 8-bit alignment is
assumed.
MKI (Master Key Identifier): variable length, OPTIONAL
Baugher, et al. [Page 6]
INTERNET-DRAFT SRTP June, 2002
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: variable length, OPTIONAL
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, note that 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
cryptographically secure way.
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.
Baugher, et al. [Page 7]
INTERNET-DRAFT SRTP June, 2002
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 is the
highest received RTP sequence number (possibly authenticated, if
message authentication is provided),
* an identifier for the encryption algorithm, i.e., the cipher and
its mode of operation,
* an identifier for the message authentication algorithm (when
authentication is provided),
* 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,
* for each master key, there is a counter of the number of SRTP
packets that has 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 implementation, see Section 4.3.
Baugher, et al. [Page 8]
INTERNET-DRAFT SRTP June, 2002
* <"From", "To"> 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.
These values are absolute quantities, not relative. Whenever this
field is unspecified, the related master key is valid "from the
first observed packet" to "until further notice" (with maximum
lifetime as specified in Section 3.3.1).
SRTCP SHALL by default share the crypto context with SRTP and uses
the same cryptographic context parameters, 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 mean 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 SRTCP, if the pre-defined transforms (including the key
derivation) are used but the session key(s) MUST NOT be so shared.
In addition, there can be cases (see Sections 8 and 9.1) where
several SRTP streams, identified by their SSRCs, share most of the
crypto context parameters (including master 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, but the session keys MAY then be shared between SRTP
streams.
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 IV formation, etc. Future SRTP transform
specifications MUST include a section to list the additional
cryptographic context's parameters for that transform, if any.
Baugher, et al. [Page 9]
INTERNET-DRAFT SRTP June, 2002
3.2.3 Mapping SRTP Packets to Cryptographic Contexts
Recall that an RTP session for each participant is defined [RFC1889]
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 = <SSRC, destination network address, destination
transport port number>
where the destination network address and the destination transport
port are the ones in the current RTP packet (for the sender) or SRTP
packet (for the receiver). 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 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 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 from
further SRTP processing.
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.
Baugher, et al. [Page 10]
INTERNET-DRAFT SRTP June, 2002
2. Determine the index of the SRTP packet using the rollover counter
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.
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. If message authentication is provided, 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 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. Estimate the index of the SRTP packet using 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.
4. Determine the session keys, and session salt (if used by the
transform) as described in Section 4.3, using master key, master
Baugher, et al. [Page 11]
INTERNET-DRAFT SRTP June, 2002
salt, key_derivation_rate and session key-lengths in the
cryptographic context with the index, determined in Steps 2 and 3.
5. If message authentication and replay protection are provided,
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.
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). The index MAY also be used to determine the correct master key
when <"From", "To"> values are used to represent key lifetime
(Section 3.2.1).
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.
Baugher, et al. [Page 12]
INTERNET-DRAFT SRTP June, 2002
Receiver-side implementations use the RTP sequence number to estimate
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, ROC MUST be set to zero.
Receivers joining an on-going session MUST be given the current ROC
value using out of band 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 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.
After the packet has been processed using the estimated index, the
receiver MUST decide if s_l and ROC should be updated. For instance,
a simple (but not error robust) method is to simply set s_l to SEQ
(if SEQ > s_l) and, if the value v = ROC+1 was used, to update ROC to
v.
After a re-keying occurs (changing to a new master key), the
rollover counter maintains its sequence of values, i.e., it MUST NOT
be reset to zero, to avoid inconsistencies in key lifetimes.
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.
Baugher, et al. [Page 13]
INTERNET-DRAFT SRTP June, 2002
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 synchronisation 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.
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
[RFC1889].
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.
Baugher, et al. [Page 14]
INTERNET-DRAFT SRTP June, 2002
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
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 Report and
SDES packet.
Baugher, et al. [Page 15]
INTERNET-DRAFT SRTP June, 2002
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.
According to [RFC1889] there is a "recommended" packet format for
compound packets. SRTCP MUST be given packets according to that
recommendation in the sense that the first part MUST be a sender
report or a receiver report. However, the encryption prefix (Section
6.1 of [RFC1889]), a random 32-bit quantity intended to deter known
plaintext attacks, MUST NOT be used (see below).
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 [RFC1889] 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 (Section 3.3.1).
Authentication Tag: variable length, REQUIRED
The authentication tag is used to carry message
authentication data.
MKI: variable length, OPTIONAL
The MKI is the Master Key Indicator, and functions according
to the MKI definition in Section 3.
Baugher, et al. [Page 16]
INTERNET-DRAFT SRTP June, 2002
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.
* If the MKI indicator in the cryptographic context is zero, the
master key is determined by the current SRTCP index when that key is
shared between SRTP and SRTCP, even though SRTCP has its own index.
Since the SRTCP source as with any SSRC in an SRTP session has its
own sequence number space, the master key <"From", "To"> lifetime
MUST be based on the SRTP master key lifetime when the master key is
shared by both SRTP and SRTCP. The concomitant re-keying issues are
discussed in sections 8 and 9.
* 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 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) (except when SRTP is not authenticated).
* 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
Baugher, et al. [Page 17]
INTERNET-DRAFT SRTP June, 2002
and for security reasons the sender MUST also check the number of
RTCP packets processed, see Section 9.2.
As noted, the encryption prefix (Section 6.1 of RFC1889]) SHALL NOT
be used, as it is not needed by the cryptographic mechanisms used in
SRTP.
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 [RFC1889], 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 [RFC1889]), the value of "packet_size" MUST
include the size of the additional fields added by SRTCP.
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
larger packet intervals. The increase in the intervals will be
directly proportional to size of the added fields.
4. Pre-Defined Cryptographic Transforms
While there are numerous encryption and message authentication
algorithms that can be used in SRTP, we define below 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
Baugher, et al. [Page 18]
INTERNET-DRAFT SRTP June, 2002
* 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, an
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 pseudorandom 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. Decryption is done the same way, but swapping
the roles of the plaintext and ciphertext.
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.
+----+ +------------------+---------------------------------+
| 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 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 [RFC1889, Section 6.1]),
Baugher, et al. [Page 19]
INTERNET-DRAFT SRTP June, 2002
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), and we
define two modes of running AES, Segmented Integer Counter Mode AES
and AES in f8-mode. In the remainder of this section, let E(k,x) be
AES applied to key k and input block x.
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 (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 session key and salt.
Baugher, et al. [Page 20]
INTERNET-DRAFT SRTP June, 2002
Note that the initial value, IV, is fixed for each packet. The
number of blocks of keystream generated for any fixed value of IV
MUST NOT exceed 2^16. 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].
4.1.2 AES in f8-mode
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.
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
Baugher, et al. [Page 21]
INTERNET-DRAFT SRTP June, 2002
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 f8-mode for 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 what we
shall call "f8-mode of operation".
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) 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.
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:
Baugher, et al. [Page 22]
INTERNET-DRAFT SRTP June, 2002
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, 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.
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.
Baugher, et al. [Page 23]
INTERNET-DRAFT SRTP June, 2002
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. 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 specifed above, i.e. HMAC(k_a, M). The
HMAC output SHALL then be truncated to the n_tag left-most bits.
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 MAY 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.
Baugher, et al. [Page 24]
INTERNET-DRAFT SRTP June, 2002
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 be initially invoked before the first
packet and then, if derivation rate is r > 0, further invoked on
every r-th packet, and produce session keys according to the non-
zero key derivation rate. 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) is needed, and a default PRF transform is defined
in Section 4.3.3.
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 <label>, an
8-bit constant (see below), master_salt and key_derivation_rate, as
determined in the cryptographic context, and index, the packet index
(i.e., the 48-bit ROC || SEQ for SRTP):
* Let r = index DIV key_derivation_rate (with DIV as defined above).
* Let key_id = <label> || r.
* Let x = key_id XOR master_salt, where key_id and master_salt are
aligned so that their least significant bits agree (right-
alignment).
The n-bit SRTP key (or salt) for this packet SHALL then be
PRF_n(k_master, x).
(The PRF may internally specify additional formatting and padding
of x, see e.g. Section 4.3.3 for the default PRF.)
Baugher, et al. [Page 25]
INTERNET-DRAFT SRTP June, 2002
The session keys and salt SHALL now be derived using:
- k_e (SRTP encryption): <label> = 0x00, n = n_e.
- k_a (SRTP message authentication): <label> = 0x01, n = n_a.
- k_s (SRTP salting key) <label> = 0x02, n = n_s.
where n_e, n_s, and n_a are from the cryptographic context.
The master key and master salting key MUST be random, but the master
salt MAY be public.
Note that for a key_derivation_rate of 0, the initial application of
the key derivation SHALL take place exactly once.
The definition of DIV above is purely for notational convenience.
For a non-zero t among the set of allowed key derivation rates, "a
DIV t" can be implemented as a right-shift by the base-2 logarithm
of t. The derivation operation is further facilitated if the rates
are chosen to be powers of 256, but that granularity was considered
too coarse to be a requirement of this specification.
The upper limit on the number of packets that can be secured using
the same master key (see Section 9.2) is independent of the key
derivation.
4.3.2 SRTCP Key Derivation
SRTCP SHALL by default use the same master key (and master salt) as
SRTP. To do this securely, the following changes SHALL be done to
the definitions in Section 4.3.1 when applying session key
derivation for SRTCP.
Replace the SRTP index by the 32-bit quantity: 0 || SRTCP index
(i.e. excluding the E-bit, replacing it with a fixed 0-bit), and use
<label> = 0x03 for the SRTCP encryption key, <label> = 0x04 for the
SRTCP authentication key, and, <label> = 0x05 for the SRTCP salting
key.
4.3.3 AES-CM PRF
The currently defined PRF, keyed by 128 to 256 bit master key, has
input block size m = 128 and can produce n-bit outputs for n up to
2^23. PRF_n(k_master,x) SHALL be AES in Counter Mode as described in
Section 4.1.1, applied to key k_master, and IV equal to (x*2^16),
and with the output keystream truncated to the n first (left-most)
bits. (Requiring n/128, rounded up, applications of AES.)
Baugher, et al. [Page 26]
INTERNET-DRAFT SRTP June, 2002
5. Default and mandatory-to-implement Transforms
The default transforms also are mandatory-to-implement transforms in
SRTP. Of course, "mandatory-to-implement" does not imply "mandatory-
to-use". Table 1 summarizes the pre-defined transforms.
mandatory-to-impl. optional default
encryption AES-CM, NULL AES-f8 AES-CM
message integrity HMAC-SHA1 - HMAC-SHA1
key derivation (PRF) AES-CM - AES-CM
Table 1: Mandatory-to-implement, optional and default transforms in
SRTP.
5.1 Encryption: AES-CM and NULL
AES running in Segmented Integer Counter Mode, as defined in Section
4.1.1, SHALL be the default encryption algorithm. The default key
lengths SHALL be 128-bit for the session encryption key (n_e). The
default session salt key-length (n_s) SHALL be 112 bits.
The NULL cipher SHALL also be mandatory-to-implement.
5.2 Message Authentication/Integrity: HMAC-SHA1
HMAC-SHA1, as defined in Section 4.2.1, SHALL be the default message
authentication code. The default session authentication key-length
(n_a) SHALL be 128 bits, the default authentication tag length
(n_tag) SHALL be 32 bits, and the SRTP_PREFIX_LENGTH SHALL be zero
for HMAC-SHA1.
5.3 Key Derivation: AES-CM PRF
The AES Counter Mode based key derivation and PRF defined in
Sections 4.3.1 to 4.3.3, using a 128-bit master key, SHALL be the
default method for generating session keys. The default master salt
length SHALL be 112 bits and the default key-derivation rate SHALL
be zero.
6. Adding SRTP Transforms
Baugher, et al. [Page 27]
INTERNET-DRAFT SRTP June, 2002
Section 4 provides examples of the level of detail needed for
defining transforms. Whenever a new transform is to be added to
SRTP, a companion standard track RFC MUST be written to exactly
define how the new transform can be used with SRTP (and SRTCP). Such
a companion RFC SHOULD avoid to overlap with the SRTP protocol
document. Note however, that it MAY be necessary to extend the SRTP
or SRTCP cryptographic context definition with new parameters
(including fixed or default values), or add steps to the packet
processing. The companion RFC SHALL explain any known issues
regarding interactions between the transform and other aspects of
SRTP.
Each new transform document SHOULD specify its key attributes, e.g.,
size of keys (minimum, maximum, recommended), format of keys,
recommended/required processing of input keying material,
requirements/recommendations on re-keying and key derivation, etc.
7. Rationale
7.1 Key derivation
Key derivation reduces the burden on the key establishment. As many
as six different keys are needed to protect the RTP/RTCP session
(SRTP and SRTCP encryption keys and salts, SRTP and SRTCP
authentication keys), but these are derived from a single master key
in a cryptographically secure way. Thus, the key management protocol
needs to exchange only one master key (plus master salt when
required), and then SRTP itself derives all the necessary session
keys (via the first, mandatory application of the key derivation
function). Note however that the key management protocol may provide
SRTP with more than one master key in advance, e.g., multiple
distinct master keys with their respective lifetime. Each of these
lifetimes MUST NOT be overlapping with the lifetime of the other
master keys, so that one and only one master key is active at each
point in time. Providing arrays of master keys in advance is for
example used when a certain rate of re-keying is wanted.
Multiple applications of the key derivation function are optional,
but will give security benefits when enabled. They prevent an
attacker from obtaining large amounts of ciphertext produced by a
single fixed session key. If the attacker was able to collect a
large amount of ciphertext for a certain session key, he might be
helped in mounting certain attacks.
Multiple applications of the key derivation function provide
backwards and forward security in the sense that a compromised
session key does not compromise other session keys derived from the
Baugher, et al. [Page 28]
INTERNET-DRAFT SRTP June, 2002
same master key. This means that the attacker who is able to recover
a certain session key, is anyway not able to have access to messages
secured under previous and later session keys (derived from the same
master key). (Note that, of course, a leaked master key reveals all
the session keys derived from it.)
Considerations arise with high-rate key-refresh, especially in large
multicast settings, see Section 11.
7.2 Salting key
The master salt guarantees security against off-line key-collision
attacks on the key derivation that might otherwise reduce the
effective key size.
The derived session salting key used in the encryption, has been
introduced to protect against some attacks on additive stream
ciphers, see Section 9.2. The explicit inclusion method of the salt
in the IV has been selected for ease of hardware implementation.
7.3 Message Integrity from Universal Hashing
The particular definition of the keystream given in Section 4.1 (the
keystream prefix) is to give provision for particular universal hash
functions, suitable for message authentication in the Wegman-Carter
paradigm [WC81]. Such functions are provably secure, simple, quick,
and especially appropriate for Digital Signal Processors and other
processors with a fast multiply operation.
No authentication transforms are currently provided in SRTP other
than HMAC-SHA1. Future transforms, like the above mentioned
universal hash functions, MAY be added following the guidelines in
Section 6.
7.4 Data Origin Authentication Considerations
Note that in unicast, integrity and data origin authentication are
provided together. However, in group scenarios where the keys are
shared between members, the MAC tag only proves that a member of the
group sent the packet, but does not prevent against a member
impersonating another. Data origin authentication (DOA) for
multicast and group RTP sessions is a hard problem that needs a
solution; while some promising proposals are being investigated
[PCST1, PCST2], more work is needed to rigorously specify these
technologies. Thus SRTP data origin authentication in groups is for
further study.
Baugher, et al. [Page 29]
INTERNET-DRAFT SRTP June, 2002
DOA can be done otherwise using signatures. However, this has high
impact in terms of bandwidth and processing time, therefore we do
not offer this form of authentication in the pre-defined packet-
integrity transform.
The presence of mixers and translators does not allow data origin
authentication in case the RTP payload and/or the RTP header are
manipulated. Note that these types of middle entities also disrupt
end-to-end confidentiality (as the IV formation depends e.g. on the
RTP header preservation). A certain trust model may choose to trust
the mixers/translators to decrypt/re-encrypt the media (this would
imply breaking the end-to-end security, with related security
implications).
8. Key Management Considerations
For initialization, an interoperable SRTP implementation SHOULD be
given the SSRC and MAY be given the initial RTP sequence number for
the RTP stream by key management (thus, key management has a
dependency on RTP operational parameters). Sending the RTP sequence
number in the key management may be useful e.g. when the initial
sequence number is close to wrapping (to avoid synchronization
problems), and to communicate the current sequence number to a
joining endpoint (to properly initialise its replay list).
If the pre-defined transforms are used, a particular key management
system might allow different RTP sessions to share the same
cryptographic master keys. The SRTP sender and receiver typically
share a master key to derive session keys for encryption/decryption
and authentication; SRTCP sources will typically derive keys from
the same master key used by the correspondent SRTP. Sharing also
between SRTP streams is secure if the design of the synchronization
mechanism, i.e., the IV, avoids keystream re-use (the two-time pad,
Section 9.1). If this feature is used, the SSRCs MUST be unique
between all the RTP streams sharing the same master key. In other
words, when a master key is shared among RTP sessions, SRTP/SRTCP
cryptographic transforms are vulnerable to unfortunate SSRC
collisions owing to normal operation of a compliant RTP
implementation. SRTP implementations that share master keys
introduce a non-standard constraint on RTP operation: SSRC values
must be unique among RTP sessions that share an SRTP master key (see
Section 9.1).
The same considerations apply to message authentication: SRTP
streams authenticated under the same key MUST have a distinct SSRC.
Baugher, et al. [Page 30]
INTERNET-DRAFT SRTP June, 2002
Whenever uniqueness between the SSRCs can not be guaranteed, the
same master key MUST NOT be shared among the streams.
To share master keys between two SRTP streams, they MUST use
distinct SSRCs. Note that this is not guaranteed by standard RTP
operation, unless they belong to the same RTP session. However,
the fact that an SRTP stream and its associated SRTCP stream both
carry the same SSRC does not constitute a problem for the two time
pad due to the key derivation. Thus, SRTP and SRTCP corresponding to
one RTP session MAY share master keys.
8.1. Re-keying
A particular key management system might choose to provide re-key:
- by associating a master key for a crypto context with an MKI,
or
- by associating a master key for a crypto context directly with a
pair of index (sequence number and ROC) values, <"From", "To">. In
this case, the MKI is not included. Note that the range <"From",
"To"> gives also the lifetime of the master key itself. <"From",
"To"> are specified in the crypto context for a given master key, or
the default values, "from the first observed packet" and "until
further notice", respectively, are used. Also, in case the default
values are used, the SRTP implementation MUST never exceed the
maximum limit of SRTP/SRTCP packets sent for each given
master/session key.
The first method (using the MKI) has the advantage of easier master
key retrieval (see Scenarios in Section 11), but has the
disadvantage of adding extra bits to each packet. Using the MKI does
not exclude using <"From", "To"> key lifetime simultaneously. This
can for instance be useful to signal at which point in time an MKI
is to be made active.
The key management specification may therefore require the SRTP
implementation to check the index of an incoming SRTP packet against
the interval for the master key in the context before using the key.
SRTP senders SHALL count the amount of SRTP and SRTCP traffic being
used for a master key and invoke key management to re-key if needed.
These interactions are defined by the key management interface to
SRTP and are not defined by this protocol specification.
Baugher, et al. [Page 31]
INTERNET-DRAFT SRTP June, 2002
8.2. Key Management parameters
The table below lists all SRTP parameters that key management may
need to supply. For reference, it also provides a summary of the
default and mandatory-to-support values for an SRTP implementation
as described in Section 5.
Parameter Mandatory-to-support Default
--------- -------------------- -------
SRTP and SRTCP encr transf. AES_CM, NULL AES_CM
(Other possible values: AES_f8)
SRTP and SRTCP auth transf. HMAC-SHA1 HMAC-SHA1
SRTP and SRTCP auth params:
n_tag (tag length) 32 32
SRTP prefix_length 0 0
Key derivation PRF AES_CM AES_CM
Key material params
(for each master key):
master key
master key length 128 128
n_e (encr session key length) 128 128
n_a (auth session key length) 128 128
master salt key
length of the master salt 112 112
n_s (session salt key length) 112 112
key derivation rate 0 0
<"From", "To">
MKI indicator 0 0
length of the MKI 0 0
value of the MKI
Crypto context index params:
SSRC value
ROC
SEQ
SRTCP Index
Transport address
Port number
Relation to other RTP profiles:
sender's order between FEC and SRTP FEC-SRTP
Baugher, et al. [Page 32]
INTERNET-DRAFT SRTP June, 2002
9. Security Considerations
9.1 SSRC collision and two-time pad
Any fixed keystream output, generated from the same key and index
should only be used to encrypt once. Re-using such keystream
(jokingly called a "two-time pad" system by cryptographers), can
seriously compromise security. The NSA's VENONA project [C99]
provides a historical example of such a compromise. In SRTP, a "two-
time pad" is avoided by requiring the key, or some other parameter
of cryptographic significance, to be unique per RTP stream and
packet. The pre-defined SRTP transforms accomplish packet-uniqueness
by including the packet index and stream-uniqueness by inclusion of
the SSRC.
The pre-defined transforms (AES-CM and AES-f8) allow master keys to
be shared across streams by the inclusion of the SSRC in the IV.
Sharing a key among RTP sessions, however, requires the added
constraint that SSRC values be unique across RTP sessions (see
Section 8).
Thus, the SSRC MUST be unique between all the RTP streams and
sessions sharing the same master key. It is incumbent upon SRTP
implementations to ensure SSRC uniqueness across RTP streams that
share a master key, to avoid unfortunate IV combinations and end up
in a two-time pad. Even with distinct SSRCs, extensive use of the
same key might improve chances of probabilistic collision and time-
memory-tradeoff attacks succeeding.
It is RECOMMENDED that RTP senders on different hosts not use the
same master key to send. When a local host shares a master key
among its RTP/RTCP streams to an RTP session, it MUST check for
collisions among the SSRCs it is using at the time of SSRC
generation and generate a unique SSRC before sending the value in an
SRTP or SRTCP message. When a local host shares a master key among
RTP/RTCP streams in multiple RTP sessions (e.g. in a multimedia
session), it MUST check for collisions among the SSRCs it is using
for those sessions and enforce SSRC uniqueness even though SSRC
uniqueness among RTP sessions is not an RTP requirement. When
master keys are shared between RTP hosts, the effect of an eventual
RTP SSRC collision detection MUST be taken into account, as a
collision could duplicate the SSRC leading temporarily to a two-time
pad before the collision is detected. SRTP implementations SHOULD
obtain unique SSRCs from key management when they share a master
key. Failing this, an SRTP implementation MUST obtain a new master
key from key management for any session that experiences an SSRC
collision.
Baugher, et al. [Page 33]
INTERNET-DRAFT SRTP June, 2002
Manual keying SHOULD NOT be used in SRTP.
9.2 Key Usage
The effective key size is determined (upper bounded) by the size of
the master key and, for encryption, the size of the salting key. Any
additive stream cipher is vulnerable to attacks that use statistical
knowledge about the plaintext source to enable key collision and
time-memory tradeoff attacks [MF00,H80]. These attacks take
advantage of commonalities among plaintexts, and provide a way for a
cryptanalyst to amortize the computational effort of decryption over
many keys, thus reducing the effective key size of the cipher. A
detailed analysis of these attacks and their applicability to the
encryption of Internet traffic is provided in [MF00]. In summary,
the effective key size of SRTP when used in a security system in
which m distinct keys are used, is equal to the key size of the
cipher less the logarithm (base two) of m. Protection against such
attacks can be provided simply by increasing the size of the keys
used, which here can be accomplished by the use of the salting key.
Note that the salting key MUST be random but MAY be public. A salt
size of (the suggested) size 112 bits protects against attacks in
scenarios where at most 2^112 keys are in use. This is sufficient
for all practical purposes.
Implementations SHOULD use keys that are as large as possible.
Please note that in many cases increasing the key size of a cipher
does not affect the throughput of that cipher.
The use of the SRTP and SRTCP indexes in the pre-defined transforms
fixes the maximum number of packets that can be secured with the
same key. This limit is fixed to 2^48 SRTP packets for an SRTP
stream, and 2^31 SRTCP packets, when SRTP and SRTCP are considered
independently. Due to for example re-keying, reaching this limit may
or may not coincide with wrapping of the indices, and thus the
sender MUST keep packet counts. However, when the session keys for
related SRTP and SRTCP streams are derived from the same master key
(the default behavior, Section 4.3), the upper bound that has to be
considered is in practice the minimum of the two quantities. That
is, when 2^48 SRTP packets or 2^31 SRTCP packets have been secured
with the same key (whichever occurs before), the key management MUST
be called to provide new master key(s) (previously stored and used
keys MUST NOT be used again), or the session MUST be terminated. If
a sender of RTCP discovers that the sender of SRTP (or SRTCP) has
not updated the master or session key prior to sending 2^48 SRTP (or
2^31 SRTCP) packets belonging to the same SRTP (SRTCP) stream, it is
up to the security policy of the RTCP sender how to behave, e.g.
Baugher, et al. [Page 34]
INTERNET-DRAFT SRTP June, 2002
whether an RTCP BYE-packet should be sent and/or if the event should
be logged.
Note: in most typical applications (assuming at least one RTCP
packet for every 128,000 RTP packets), it will be the SRTCP index
that first reaches the upper limit, although the time until this
occurs is very long: even at 200 SRTCP packets/sec, the 2^31 index
space of SRTCP is enough to secure approximately 4 months of
communication.
Note that if the master key is to be shared between SRTP streams
having distinct SSRCs (Section 9.1), although the above bounds are
on a per stream (i.e. per SSRC) basis, the sender MUST base re-key
decision on the stream whose sequence number space is the first to
be exhausted.
Key derivation limits the amount of plaintext that is encrypted with
a fixed session key, and made available to an attacker for analysis,
but key derivation does not extend the master key's lifetime. To see
this, simply consider our requirements to avoid two-time pad: two
distinct packets MUST either be processed with distinct IVs, or with
distinct session keys, and both the distinctness of IV and of the
session keys are (for the pre-defined transforms) dependent on the
distinctness of the packet indicies.
9.3 Confidentiality of the RTP Payload
SRTP's pre-defined ciphers are "seekable" stream ciphers, i.e.
ciphers able to efficiently seek to arbitrary locations in their
keystream (so that the encryption or decryption of one packet does
not depend on preceding packets). By using "seekable" stream
ciphers, SRTP avoids the denial of service attacks that are possible
on stream ciphers that lack this property. It is important to be
aware that, as with any stream cipher, the exact length of the
payload is revealed by the encryption. This means that it may be
possible to deduce certain "formatting bits" of the payload, as the
length of the codec output might vary due to certain parameter
settings etc. This, in turn, implies that the corresponding bit of
the keystream can be deduced. However, if the stream cipher is
secure (counter mode and f8 are provably secure under certain
assumptions [BDJR,KSYH]), knowledge of a few bits of the keystream
will not aid an attacker in predicting subsequent keystream bits.
Thus, the payload length (and information deducible from this) will
leak, but nothing else.
As some RTP packet could contain highly predictable data, e.g. SID,
it is important to use a cipher designed to resist known plaintext
attacks (which is the current practice).
Baugher, et al. [Page 35]
INTERNET-DRAFT SRTP June, 2002
9.4 Confidentiality of the RTP Header
In SRTP, RTP headers are sent in the clear to allow for header
compression. This means that data such as payload type,
synchronization source identifier, and timestamp are available to an
eavesdropper. Moreover, since RTP allows for future extensions of
headers, we cannot foresee what kind of possibly sensitive
information might also be "leaked".
SRTP is a low-cost method, which allows header compression to reduce
bandwidth. It is up to the endpoints' policies to decide about the
security protocol to employ. If one really needs to protect headers,
and is allowed to do so by the surrounding environment, then one
should also look at alternatives, e.g., IPsec.
9.5 Integrity of the RTP payload and header
Additive stream ciphers do not provide any security service other
than confidentiality. In particular, they do not provide message
authentication (see [RK99] or [HAC] for a discussion of this
security service). SRTP uses a message authentication code to
provide a message authentication service.
HMAC is a well-studied message authentication code that is based on
a provably secure construction. The security against MAC forgery
depends on the key-size and the size of the output tags (or for some
attacks, half the size of the tag due to the "birthday-paradox").
The default tag size for SRTP HMAC is 32 bits. Other size values MAY
be chosen (via the key management protocol). The use of a truncated
size is motivated by the fact that it may be desirable, e.g., in
wireless environments, to save bandwidth. The choice of such a
truncation MUST be evaluated to the reduction in security it
implies. The default 32-bit size is a compromise, offering a
reasonable level of security, taking into account the real-time
aspects of the protected protocol. High security applications SHOULD
however use larger tags.
The fact that message authentication is optional (for SRTP) is
motivated by the fact that, while the function is typically highly
desired, there are certain cases (notably in cellular environments)
where it has an impact in terms of cost, e.g. for bandwidth
consumption. Also, independently of the tag length, a single
transmission bit error in the protected part of the packet or in the
tag itself forces the entire packet to be dropped. Given a fixed
quality of service, it implies the necessity of higher protection of
Baugher, et al. [Page 36]
INTERNET-DRAFT SRTP June, 2002
the transmitted unit, hence higher cost. In those cases, it is up to
the user's security profile to request authentication.
The use of error detection mechanism (e.g., Unequal Error Detection,
UED and UEP) is compatible with SRTP and the pre-defined encryption
transforms, since stream ciphers operate on each bit individually.
However, the use of UED/UEP may be difficult to combine with
authentication because any bit error will cause authentication to
fail.
The IV formation of the f8-mode gives implicit authentication (IHA)
of the RTP header, even if no cryptographic integrity protection is
present. This means that modifying bits of the RTP header will cause
the decryption process at the receiver to produce essentially random
garbage.
10. Interaction with Forward Error Correction mechanisms
The default processing when using Forward Error Correction (e.g. RFC
2733) processing with SRTP SHALL be to perform FEC processing prior
to SRTP processing on the sender side and to perform SRTP processing
prior to FEC processing on the receiver side. Any change to this
ordering (reversing it, or, placing FEC between SRTP encryption and
SRTP authentication) SHALL be signaled out of band.
11. Scenarios
SRTP can be used as security protocol for the RTP/RTCP traffic in
many different scenarios. SRTP has a number of configuration
options, and can have impact on the total performance of the
application according to the way it is used. Hence, the use of SRTP
is dependent on the kind of scenario and application it is used
with. In the following, we briefly illustrate some use cases for
SRTP, and give some guidelines for recommended setting of its
options.
11.1 Unicast
A typical example would be a voice call or video-on-demand
application.
Consider one bi-directional RTP stream. It is possible for the two
parties to share the same master key in the two directions. The
first round of the key derivation splits the master key into any or
all of the following session keys (according to the provided
security functions):
Baugher, et al. [Page 37]
INTERNET-DRAFT SRTP June, 2002
SRTP_encr_key, SRTP_auth_key, SRTCP_encr_key, and SRTCP_auth key.
(For simplicity, we omit discussion of the salts, which are also
derived.) In this scenario, it will in most cases suffice to have a
single master key with unspecified lifetime (i.e. unrestricted key
lifetime, not using explicit <"From", "To"> values). This guarantees
sufficiently long lifetime of the keys and a minimum set of keys in
place for most practical purposes. Also, in this case RTCP
protection can be applied smoothly. Under these assumptions, use of
the MKI can be omitted. As the key-derivation in combination with
large difference in the packet rate in the respective directions may
require simultaneous storage of several session keys, if storage is
an issue, we recommended to use low-rate key derivation.
The same considerations can be extended to the unicast scenario with
multiple RTP sessions sharing the master key if particular care is
taken to guarantee unique SSRCs for the streams.
11.2 Multicast
Just as with (unprotected) RTP, a scalability issue arises in big
groups due to the possibly very large amount of SRTCP Receiver
Reports that the sender might need to process. In SRTP, the sender
may have to keep state (the cryptographic context) for each
receiver, or more precisely, for the SRTCP used to protect Receiver
Reports. The overhead increases proportionally to the size of the
group. In particular, re-keying requires special concern, see below.
We describe in the following multicast for small groups, and give
guidelines for use of SRTP/SRTCP with large group multicast.
11.2.1 Small multicast with one sender
The sender secures his RTP stream using one cryptographic context.
The sender's RTP and RTCP are secured with the same master key. Key
derivation gives the necessary session keys, i.e.
SRTP_encr_key, SRTP_auth_key, SRTCP_encr_key, and SRTCP_auth key.
If there are multiple RTP streams, their SSRCs MUST (as noted) be
unique to avoid two-time pad (see Section 9.1), or else distinct
per-stream master keys MUST be used.
There are a few possible setups with the distribution of master keys
among the receivers. One possibility is that the receivers share the
same master key to secure all their respective RTCP traffic. This
shared master key could then be the same one used by the sender to
Baugher, et al. [Page 38]
INTERNET-DRAFT SRTP June, 2002
protect its outbound traffic. Alternatively, it could be a master key
shared only among the receivers and used solely for their SRTCP
traffic. Both alternatives requires the receivers to trust each
other.
Considering SRTCP and key storage, it is recommended to use low-rate
(or zero) key_derivation (except the mandatory initial one), so that
the sender does not need to store too many session keys (each SRTCP
stream might otherwise have a different session key at a given point
in time, as the SRTCP sources send at different times). Thus, in
case key derivation is wanted for SRTP, the cryptographic context
for SRTP can be kept separate from the SRTCP crypto context, so that
it is possible to have a key_derivation_rate of 0 for SRTCP and a
non-zero value for SRTP.
Re-keying gives two problems: the number of master keys stored at
the sender side, and re-keying triggering. Forcing re-keying using
the <"From", "To"> fields creates the problem that the sender needs
to maintain multiple master keys, as the re-keying will typically
happen at different times on each SRTCP stream from the receivers
(because each SSRC defines a sequence number space). Also, problems
may occur in retrieving the current master key for the SRTCP packets
in some cases, since that is done based on SRTP index, not on the
SRTCP index. Use of the MKI for re-keying is recommended for most
applications (see Section 8.1).
If there are more than one outgoing SRTP stream sharing master key,
the upper limit of 2^48 SRTP packets / 2^31 SRTCP packets means
that, before one of the streams reaches such maximum number of
packets, re-keying MUST be triggered on ALL streams sharing the
master key. (From strict security point of view, only the stream
reaching the maximum would need to be re-keyed, but then the streams
would no longer be sharing master key, which is the intention.) A
local policy at the sender side should force rekeying in a way that
the maximum packet limit is not reached on any of the streams. The
MKI or <"From", "To"> fields may be employed for key synchronization
during changeover to a new key, see Section 11.3.
11.2.2 Large multicast with one sender
The same considerations as for the small group multicast hold. The
biggest issue in this scenario is the additional load placed at the
sender side, due to the state (cryptographic contexts) that has to
be maintained for each receiver, sending back RTCP Receiver Reports.
At minimum, a replay window might need to be maintained for each
RTCP source. Therefore, with big groups and where the load at the
sender is considered not acceptable, an RTP sender may choose not to
authenticate or protect against replay for incoming SRTCP messages
Baugher, et al. [Page 39]
INTERNET-DRAFT SRTP June, 2002
(nor to negotiate encryption for them). Of course, security impacts
of neglecting to authenticate certain packets MUST be carefully
considered. This is therefore strongly NOT RECOMMENDED from a
security point of view, but may appear a reasonable compromise in
order to have at least security guaranteed on the outgoing RTP
traffic.
11.3 Re-keying and access control
Re-keying may occur due to access control (e.g., when a member is
removed during a multicast RTP session), or, for pure cryptographic
reasons (e.g. the key is at the end of its lifetime). When using
SRTP default transforms, the master key MUST be replaced before any
of the index spaces are exhausted for any of the streams protected
by one and the same master key.
How key management rekeys SRTP implementations is out of our scope,
but it is clear that there are straightforward ways to manage keys
for a multicast group. In one-sender multicast, for example, it is
typically the responsibility of the sender to determine when a new
key is needed. The sender is the one entity that can keep track of
when the maximum number of packets has been sent, as receivers may
join and leave the session at any time, there may be packet loss and
delay etc. In scenarios other than one-sender multicast, other
methods can be used. Here, one must take into consideration that key
exchange can be a costly operation, taking several seconds for a
single exchange. Hence, some time before the master key is
exhausted/expires, out-of-band key management is initiated,
resulting in a new master key shared with the receiver(s). In any
event, to maintain synchronization when switching to the new key,
group policy might choose between using the MKI or the <"From",
"To">, as described in Section 8.1.
For access control purposes, the <"From", "To"> periods are set at
the desired granularity, dependent on the packet rate. High rate re-
keying can be problematic for some large-group SRTP scenarios, with
SRTCP. There are potential problems in using the SRTP index, rather
than the SRTCP index, for determining the master key. In particular,
for short periods during switching of master keys, it may be the
case that SRTCP packets are not under the current master key of the
correspondent SRTP. Therefore, using the MKI for re-keying in such
scenarios is likely to produce better results.
11.4 Summary of basic scenarios
Baugher, et al. [Page 40]
INTERNET-DRAFT SRTP June, 2002
The description of these scenarios highlights some recommendations
on the use of SRTP, mainly related to re-keying and large scale
multicast:
- Do not use SRTP for fast re-keying using the <"From", "To">
feature. It may, in particular, give problems in retrieving the
correct SRTCP key, if an SRTCP packet arrives close to the re-
keying time. The MKI SHOULD be used in this case.
- If multiple SRTP streams share the same master key, also moderate
rate re-keying MAY have the same problems, and the MKI SHOULD be
used.
- Carefully consider the additional load at the sender side in
multicast scenarios. Optionally, but NOT RECOMMENDED, SRTCP
Receiver Reports' authentication could be left unverified by the
sender (and SRTCP Receiver Reports' encryption not selected).
- Though offering increased security, a non-zero key_derivation_rate
is NOT RECOMMENDED when trying to minimize the number of keys in
use with multiple streams.
12. IANA Considerations
The RTP specification establishes a registry of profile names for
use by higher-level control protocols, such as the Session
Description Protocol (SDP), to refer to transport methods. This
profile registers the name "RTP/SAVP".
SRTP uses cryptographic transforms, which a key management protocol
signals. It is the task of each particular key management protocol
to register the cryptographic transforms or suites of transforms
with IANA. The key management protocol conveys these protocol
numbers, not SRTP, and each key management protocol chooses the
numbering scheme and syntax that it requires.
Specification of a key management protocol for SRTP is out of scope
here. Section 8.2, however, provides guidance on the parameters that
need to be defined for the default and mandatory transforms.
13. Acknowledgements
The authors would like to thank Magnus Westerlund, Brian Weis,
Robert Fairlie-Cuninghame, Adrian Perrig, and the AVT WG for their
reviews and comments.
Baugher, et al. [Page 41]
INTERNET-DRAFT SRTP June, 2002
14. Author's Addresses
Questions and comments should be directed to the authors and
avt@ietf.org:
Mark Baugher
Cisco Systems, Inc.
5510 SW Orchid Street Phone: +1 408-853-4418
Portland, OR 97219 USA Email: mbaugher@cisco.com
Rolf Blom
Ericsson Research
SE-16480 Stockholm Phone: +46 8 58531707
Sweden EMail: rolf.blom@era.ericsson.se
Elisabetta Carrara
Ericsson Research
SE-16480 Stockholm Phone: +46 8 50877040
Sweden EMail: elisabetta.carrara@era.ericsson.se
David A. McGrew
Cisco Systems, Inc.
San Jose, CA 95134-1706 Phone: +1 301-349-5815
USA EMail: mcgrew@cisco.com
Mats Naslund
Ericsson Research
SE-16480 Stockholm Phone: +46 8 58533739
Sweden EMail: mats.naslund@era.ericsson.se
Karl Norrman
Ericsson Research
SE-16480 Stockholm Phone: +46 8 4044502
Sweden EMail: karl.norrman@era.ericsson.se
David Oran
Cisco Systems, Inc.
San Jose, CA 95134-1706
USA EMail: oran@cisco.com
15. References
Normative
Baugher, et al. [Page 42]
INTERNET-DRAFT SRTP June, 2002
[AES] NIST, "Advanced Encryption Standard (AES)", FIPS PUB 197,
http://www.nist.gov/aes/
[HMAC] Krawczyk, H., Bellare, M., and Canetti, R.: "HMAC: Keyed-
hashing for message authentication". IETF RFC 2104, February
1997.
[RFC1889] Schulzrinne, H., Casner, S., Frederick, R., Jacobson,V.,
"RTP: A Transport Protocol for Real-time Applications", IETF
RFC 1889.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", IETF RFC 2119, March 1997.
[RFC2401] Kent, S., and R. Atkinson, "Security Architecture for IP",
IETF RFC 2401, November 1998.
[RFC2675] Borman, D., Deering, S., Hinden, R., "IPv6 Jumbograms",
IETF RFC 2675, August 1999.
[RFC2828] Shirey, R., "Internet Security Glossary", IETF RFC 2828,
May 2000.
Informative
[AES-CTR] Lipmaa, H., Rogaway, P., Wagner, D., "CTR-Mode
Encryption", NIST,
http://csrc.nist.gov/encryption/modes/workshop1/papers/
lipmaa-ctr.pdf
[BDJR] Bellare, M., Desai, A., Jokipii, E., and Rogaway, P.,
"A Concrete Treatment of Symmetric Encryption: Analysis of
DES Modes of Operation", Proceedings 38th IEEE FOCS,
pp. 394-403, 1997.
[C99] Crowell, W. P., "Introduction to the VENONA Project",
http://www.nsa.gov:8080/docs/venona/index.html.
[CTR] Morris Dworkin, NIST Special Publication 800-38A,
"Recommendation for Block Cipher Modes of Operation: Methods
and Techniques", 2001. Online at
http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-
38a.pdf.
[f8-a] 3GPP TS 35.201 V4.1.0 (2001-12)
Technical Specification 3rd Generation Partnership Project;
Technical Specification Group Services and System Aspects;
Baugher, et al. [Page 43]
INTERNET-DRAFT SRTP June, 2002
3G Security; Specification of the 3GPP Confidentiality and
Integrity Algorithms; Document 1: f8 and f9 Specification
(Release 4).
[f8-b] 3GPP TR 33.908 V4.0.0 (2001-09) Technical Report 3rd
Generation Partnership Project; Technical Specification Group
Services and System Aspects; 3G Security; General Report on
the Design, Specification and Evaluation of 3GPP Standard
Confidentiality and Integrity Algorithms (Release 4).
[HAC] Menezes, A., Van Oorschot, P., and Vanstone, S., "Handbook of
Applied Cryptography", CRC Press, 1997, ISBN 0-8493-8523-7.
[H80] Hellman, M. E., "A cryptanalytic time-memory trade-off",
IEEE Transactions on Information Theory, July 1980,
pp. 401-406.
[KSYH] Kang, J-S., Shin, S-U., Hong, D., and Yi, O., "Provable
Security of KASUMI and 3GPP Encryption Mode f8",
Proceedings Asiacrypt 2001, Springer Verlag LNCS 2248,
pp. 255-271, 2001.
[MF00] McGrew, D., and Fluhrer, S., "Attacks on Encryption of
Redundant Plaintext and Implications on Internet Security",
the Proceedings of the Seventh Annual Workshop on Selected
Areas in Cryptography (SAC 2000), Springer-Verlag.
[RK99] Rescorla, E., and Korver, B., "Guidelines for Writing RFC
Text on Security Considerations," draft-rescorla-sec-cons-
00.txt
[PCST1] Perrig, A., Canetti, R., Tygar, D., Song, D., "Efficient and
Secure Source Authentication for Multicast", in Proc. of
Network and Distributed System Security Symposium NDSS 2001,
pp. 35-46, 2001.
[PCST2] Perrig, A., Canetti, R., Tygar, D., Song, D., "Efficient
Authentication and Signing of Multicast Streams over Lossy
Channels", in Proc. of IEEE Security and Privacy Symposium
S&P2000, pp. 56-73, 2000.
[WC81] M. N. Wegman and J. L. Carter, "New Hash Functions and Their
Use in Authentication and Set Equality", JCSS 22, 265-279,
1981.
Baugher, et al. [Page 44]
INTERNET-DRAFT SRTP June, 2002
Appendix A: Pseudocode for Index Determination
The following is an example of pseudocode for the algorithm to
determine the index i of an SRTP packet with sequence number SEQ. In
the following, signed arithmetic is assumed.
if (s_l < 32,768)
if (SEQ - s_l > 32,768)
set v to (ROC-1) mod 2^32
else
set v to ROC
endif
else
if (s_l - 32,768 > SEQ)
set v to (ROC+1) mod 2^32
else
set v to ROC
endif
endif
return SEQ + v*65,536
Appendix B: Test Vectors
All values are in hexadecimal.
B.1 AES-f8 Test Vectors
SRTP PREFIX LENGTH : 0
RTP packet header : 806e5cba50681de55c621599
RTP packet payload : 70736575646f72616e646f6d6e657373
20697320746865206e65787420626573
74207468696e67
ROC : d462564a
key : 234829008467be186c3de14aae72d62c
salt key : 32f2870d
key-mask (m) : 32f2870d555555555555555555555555
key XOR key-mask : 11baae0dd132eb4d3968b41ffb278379
IV : 006e5cba50681de55c621599d462564a
IV' : 595b699bbd3bc0df26062093c1ad8f73
Baugher, et al. [Page 45]
INTERNET-DRAFT SRTP June, 2002
j : 0
IV' XOR j : 595b699bbd3bc0df26062093c1ad8f73
S(-1) : 00000000000000000000000000000000
S(-1) XOR IV' XOR j : 595b699bbd3bc0df26062093c1ad8f73
S(0) : 71ef82d70a172660240709c7fbb19d8e
plaintext : 70736575646f72616e646f6d6e657373
ciphertext : 019ce7a26e7854014a6366aa95d4eefd
j : 1
IV' XOR j : 595b699bbd3bc0df26062093c1ad8f72
S(0) : 71ef82d70a172660240709c7fbb19d8e
S(0) XOR IV' XOR j : 28b4eb4cb72ce6bf020129543a1c12fc
S(1) : 3abd640a60919fd43bd289a09649b5fc
plaintext : 20697320746865206e65787420626573
ciphertext : 1ad4172a14f9faf455b7f1d4b62bd08f
j : 2
IV' XOR j : 595b699bbd3bc0df26062093c1ad8f70
S(1) : 3abd640a60919fd43bd289a09649b5fc
S(1) XOR IV' XOR j : 63e60d91ddaa5f0b1dd4a93357e43a8c
S(2) : 584d14a591acfca846b3aa3a0ab50fec
plaintext : 74207468696e67
ciphertext : 2c6d60cdf8c29b
B.2 AES-CM Test Vectors
Keystream segment length: 1044512 octets (65282 AES blocks)
Session Key: 2B7E151628AED2A6ABF7158809CF4F3C
Rollover Counter: 00000000
Sequence Number: 0000
SSRC: 00000000
Session Salt: F0F1F2F3F4F5F6F7F8F9FAFBFCFD0000 (already shifted)
Offset: F0F1F2F3F4F5F6F7F8F9FAFBFCFD0000
Counter Keystream
F0F1F2F3F4F5F6F7F8F9FAFBFCFD0000 E03EAD0935C95E80E166B16DD92B4EB4
F0F1F2F3F4F5F6F7F8F9FAFBFCFD0001 D23513162B02D0F72A43A2FE4A5F97AB
F0F1F2F3F4F5F6F7F8F9FAFBFCFD0002 41E95B3BB0A2E8DD477901E4FCA894C0
... ...
F0F1F2F3F4F5F6F7F8F9FAFBFCFDFEFF EC8CDF7398607CB0F2D21675EA9EA1E4
F0F1F2F3F4F5F6F7F8F9FAFBFCFDFF00 362B7C3C6773516318A077D7FC5073AE
F0F1F2F3F4F5F6F7F8F9FAFBFCFDFF01 6A2CC3787889374FBEB4C81B17BA6C44
Baugher, et al. [Page 46]
INTERNET-DRAFT SRTP June, 2002
Nota Bene: this test case is contrived so that the latter part of the
keystream segment coincides with the test case in Section F.5.1 of
[CTR].
B.3 Key Derivation Test Vectors
This section provides test data for the default key derivation
function, which uses AES-128 in Counter Mode. In the following, we
walk through the initial key derivation for the AES-128 Counter Mode
cipher, which requires a 16 octet session encryption key and a 14
octet session salt, and an authentication function which requires a
94-octet session authentication key. These values are called the
cipher key, the cipher salt, and the auth key in the following.
Since this is the initial key derivation, the value of (index DIV
key_derivation_rate) is zero (actually, a six-octet string of
zeros). In the following, we shorten key_derivation_rate to kdr.
The inputs to the key derivation function are the 16 octet master
key and the 14 octet master salt:
master key: E1F97A0D3E018BE0D64FA32C06DE4139
master salt: 0EC675AD498AFEEBB6960B3AABE6
We first show how the cipher key is generated. The input block for
AES-CM is generated by exclusive-oring the master salt with the
concatenation of the encryption key label 0x00 with (index DIV kdr),
then padding on the right with two null octets (which implements the
multiply-by-2^16 operation, see Section 4.3.3). The resulting value
is then AES-CM- encrypted using the master key to get the cipher
key.
index DIV kdr: 000000000000
label: 00
master salt: 0EC675AD498AFEEBB6960B3AABE6
-----------------------------------------------
xor: 0EC675AD498AFEEBB6960B3AABE6 (x, PRF input)
x*2^16: 0EC675AD498AFEEBB6960B3AABE60000 (AES-CM input)
cipher key: C61E7A93744F39EE10734AFE3FF7A087 (AES-CM output)
Next, we show how the cipher salt is generated. The input block for
AES-CM is generated by exclusive-oring the master salt with the
concatenation of the encryption salt label. That value is padded
and encrypted as above.
index DIV kdr: 000000000000
Baugher, et al. [Page 47]
INTERNET-DRAFT SRTP June, 2002
label: 02
master salt: 0EC675AD498AFEEBB6960B3AABE6
----------------------------------------------
xor: 0EC675AD498AFEE9B6960B3AABE6 (x, PRF input)
x*2^16: 0EC675AD498AFEE9B6960B3AABE60000 (AES-CM input)
30CBBC08863D8C85D49DB34A9AE17AC6 (AES-CM ouptut)
cipher salt: 30CBBC08863D8C85D49DB34A9AE1
We now show how the auth key is generated. The input block for
AES-CM is generated as above, but using the authentication key
label.
index DIV kdr: 000000000000
label: 01
master salt: 0EC675AD498AFEEBB6960B3AABE6
-----------------------------------------------
xor: 0EC675AD498AFEEAB6960B3AABE6 (x, PRF input)
x*2^16: 0EC675AD498AFEEAB6960B3AABE60000 (AES-CM input)
Below, the auth key is shown on the left, while the corresponding
AES input blocks are shown on the right.
auth key AES input blocks
CEBE321F6FF7716B6FD4AB49AF256A15 0EC675AD498AFEEAB6960B3AABE60000
6D38BAA48F0A0ACF3C34E2359E6CDBCE 0EC675AD498AFEEAB6960B3AABE60001
E049646C43D9327AD175578EF7227098 0EC675AD498AFEEAB6960B3AABE60002
6371C10C9A369AC2F94A8C5FBCDDDC25 0EC675AD498AFEEAB6960B3AABE60003
6D6E919A48B610EF17C2041E47403576 0EC675AD498AFEEAB6960B3AABE60004
6B68642C59BBFC2F34DB60DBDFB2 0EC675AD498AFEEAB6960B3AABE60005
This Internet-Draft expires in December 2002.
Baugher, et al. [Page 48]
