Internet Engineering Task Force Flemming Andreasen
MMUSIC Working Group Mark Baugher
INTERNET-DRAFT Dan Wing
EXPIRES: August 2004 Cisco Systems
February, 2004
Session Description Protocol Security Descriptions
for Media Streams
<draft-ietf-mmusic-sdescriptions-03.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
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document defines a Session Description Protocol (SDP)
cryptographic attribute for unicast media streams. The attribute
describes a cryptographic key and other parameters, which serve to
configure security for a unicast media stream in either a single
message or a roundtrip exchange. The attribute can be used with a
variety of SDP media transports and this document defines how to use
it for the Secure Real-time Transport Protocol (SRTP) unicast media
streams. The SDP crypto attribute requires the services of a data
security protocol to secure the SDP message.
INTERNET-DRAFT SDP Security Descriptions February, 2004
Table of Contents
1. Notational Conventions............................................3
2. Introduction......................................................3
3. SDP "Crypto" Attribute and Parameters.............................5
3.1 Tag.............................................................5
3.2 Crypto-suite....................................................5
3.3 Key Parameters..................................................6
3.4 Session Parameters..............................................6
3.5 Example.........................................................7
4. General Use of the crypto Attribute...............................7
4.1 Use With Offer/Answer...........................................8
4.1.1 Generating the Initial Offer - Unicast Streams............8
4.1.2 Generating the Initial Answer - Unicast Streams...........9
4.1.3 Offerer Processing of the Initial Answer - Unicast Streams10
4.1.4 Modifying the Session....................................10
4.2 Use Outside Offer/Answer.......................................10
4.3 General Backwards Compatibility Considerations.................10
5. SRTP Security Descriptions.......................................11
5.1 SRTP Key Parameter.............................................12
5.2 Crypto-suites..................................................14
5.2.1 AES_CM_128_HMAC_SHA1_80..................................15
5.2.2 AES_CM_128_HMAC_SHA1_32..................................15
5.2.3 F8_128_HMAC_SHA1_80......................................16
5.2.4 Adding new Crypto-suite Definitions......................16
5.3 Session Parameters.............................................16
5.3.1 KDR=n....................................................16
5.3.2 UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP...................16
5.3.3 UNAUTHENTICATED_SRTP.....................................17
5.3.4 FEC_ORDER=order..........................................17
5.3.5 Window Size Hint (WSH)...................................17
5.3.6 Defining New SRTP Session Parameters.....................18
5.4 SRTP Crypto Context Initialization.............................18
5.5 Removal of Crypto Contexts.....................................20
6. SRTP-Specific Use of the crypto Attribute........................21
6.1 Use with Offer/Answer..........................................21
6.1.1 Generating the Initial Offer - Unicast Streams...........21
6.1.2 Generating the Initial Answer - Unicast Streams..........21
6.1.3 Offerer Processing of the Initial Answer - Unicast Streams22
6.1.4 Modifying the Session....................................22
6.1.5 Offer/Answer Example.....................................23
6.2 SRTP-Specific Use Outside Offer/Answer.........................24
6.3 Support for SIP Forking........................................24
6.4 SRTP-Specific Backwards Compatibility Considerations...........25
6.5 Operation with KEYMGT= and k= lines............................25
7. Security Considerations..........................................26
7.1 Authentication of packets......................................26
7.2 Keystream Reuse................................................26
7.3 Signaling Authentication and Signaling Encryption..............26
8. Grammar..........................................................28
8.1 Generic "Crypto" Attribute Grammar.............................28
Andreasen, Baugher & Wing [Page 2]
INTERNET-DRAFT SDP Security Descriptions February, 2004
8.2 SRTP "Crypto" Attribute Grammar................................28
9. IANA Considerations..............................................29
9.1 Registration of the "crypto" attribute.........................29
9.2 New IANA Registries and Registration Procedures................29
9.2.1 Security Descriptions Key Method Registry and Registration30
9.2.2 Security Description Media Stream Transport Registry and
Registration.....................................................30
9.3 Initial Registrations..........................................30
9.3.1 Key Method...............................................30
9.3.2 SRTP Media Stream Transport..............................31
10. Acknowledgements................................................32
11. Authors' Addresses..............................................32
12. Normative References............................................32
13. Informative References..........................................33
Intellectual Property Statement.....................................35
Acknowledgement.....................................................36
1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD
NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to
be interpreted as described in [RFC2119]. The terminology in this
document conforms to [RFC2828], "Internet Security Glossary".
n^r is exponentiation where n is multiplied by itself r times; n and
r are integers. 0..k is an integer range of all integers from 0
through k inclusive.
Explanatory notes are provided in several places throughout the
document; these notes are indented two spaces from the surrounding
text.
2. Introduction
The Session Description Protocol (SDP) [SDP] describes multimedia
sessions, which can be audio, video, whiteboard, fax, modem, and
other media streams. Security services such as data origin
authentication, integrity and confidentiality are often needed for
those streams. The Secure Real-time Transport Protocol (SRTP)
[srtp] provides security services for RTP media and is signaled by
use of secure RTP transport (e.g., "RTP/SAVP" or "RTP/SAVPF") in an
SDP media (m=) line. However, there are no means within SDP itself
to configure SRTP beyond using default values. This document
specifies a new SDP attribute called "crypto", which is used to
signal and negotiate cryptographic parameters for media streams in
general, and SRTP in particular. The definition of the crypto
attribute in this document is limited to two-party unicast media
streams where each source has a unique cryptographic key; support
for multicast media streams or multipoint unicast streams is for
further study.
Andreasen, Baugher & Wing [Page 3]
INTERNET-DRAFT SDP Security Descriptions February, 2004
The crypto attribute is defined in a generic way to enable its use
with secure transports besides SRTP that can establish cryptographic
parameters with only a single message or in a single round-trip
exchange using the offer/answer model [RFC3264]. Extension to other
transports, however, is beyond the scope of this document. Each
type of secure SDP media transport needs its own specification for
the crypto-attribute parameter. These definitions are frequently
unique to the particular type of transport and must be specified in
an Internet RFC and registered with IANA according to the procedures
defined in Section 9. This document defines the security parameters
and keying material for SRTP only.
It would be self-defeating not to secure cryptographic keys and
other parameters at least as well as the data is secured. Data
security protocols such as SRTP rely upon a separate key management
system to securely establish encryption and/or authentication keys.
Key management protocols provide authenticated key establishment
(AKE) procedures to authenticate the identity of each endpoint and
protect against man-in-the-middle, reflection/replay, connection
hijacking and some denial of service attacks [skeme]. Along with
the key, an AKE protocol such as MIKEY [mikey], GDOI [GDOI], KINK
[kink], IKE [ike] or TLS securely disseminates information
describing both the key and the data-security session (for example,
whether SRTCP payloads are encrypted or unencrypted in an SRTP
session). AKE is needed because it is pointless to provide a key
over a medium where an attacker can snoop the key, alter the
definition of the key to render it useless, or change the parameters
of the security session to gain unauthorized access to session-
related information.
SDP, however, was not designed to provide AKE services, and the
media security descriptions defined in this document do not add AKE
services to SDP. This specification is no replacement for a key
management protocol or for the conveyance of key management messages
in SDP [keymgt]. The SDP security descriptions defined here are
suitable for restricted cases only where IPsec, TLS, or some other
encapsulating data-security protocol (e.g., SIP secure multiparts)
protects the SDP message. This document adds security descriptions
to those encrypted and/or authenticated SDP messages through the new
SDP "crypto" attribute, which provides the cryptographic parameters
of a media stream.
The "crypto" attribute can be adapted to any media transport, but
its precise definition is frequently unique to a particular
transport.
In Section 3, we introduce the general SDP crypto attribute, and in
Section 4 we define how it is used with and without the offer/answer
model. In Section 5, we define the crypto attribute details needed
for SRTP, and in Section 6 we define SRTP-specific use of the
attribute with and without the offer/answer model. Section 7
Andreasen, Baugher & Wing [Page 4]
INTERNET-DRAFT SDP Security Descriptions February, 2004
recites security considerations, and Section 8 gives an Augmented-
BNF grammar for the general crypto attribute as well as the SRTP-
specific use of the crypto attribute. IANA considerations are
provided in Section 9.
3. SDP "Crypto" Attribute and Parameters
A new media-level SDP attribute called "crypto" describes the
cryptographic suite, key parameters, and session parameters for the
preceding unicast media line. The "crypto" attribute MUST only
appear at the SDP media level (not the session level). The "crypto"
attribute follows the format (see Section 8.1 for the formal ABNF
grammar):
a=crypto:<tag> <crypto-suite> <key-params> [<session-params>]
The fields tag, crypto-suite, key-params, and session-params are
described in the following sub-sections. Below we show an example
of the crypto attribute for the "RTP/SAVP" transport, i.e., the
secure RTP extension to the Audio/Video Profile [srtp] (newlines
included for formatting reasons only):
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:32
The crypto-suite is AES_CM_128_HMAC_SHA1_80, key-params is defined
by the text starting with "inline:", and session-params is omitted.
3.1 Tag
The tag is a decimal number (see Section 8.1 for details) used as an
identifier for a particular crypto attribute. The tag MUST be
unique among all crypto attributes for a given media stream. It is
used with the offer/answer model (see Section 4.1) to determine
which of several offered crypto attributes were chosen by the
answerer.
In the offer/answer model, the tag is a negotiated parameter.
3.2 Crypto-suite
The crypto-suite field is an identifier (see Section 8.1 for
details) that describes the encryption and authentication algorithms
(e.g., AES_CM_128_HMAC_SHA1_80) for the transport in question. The
possible values for the crypto-suite parameter are defined within
the context of the transport, i.e., each transport defines a
separate namespace for the set of crypto-suites. For example, the
crypto-suite "AES_CM_128_HMAC_SHA1_80" defined within the context
"RTP/SAVP" transport applies to Secure RTP only; the string may be
reused for another transport (e.g., "RTP/SAVPF" [srtpf]), but a
separate definition would be needed.
Andreasen, Baugher & Wing [Page 5]
INTERNET-DRAFT SDP Security Descriptions February, 2004
In the offer/answer model, the crypto-suite is a negotiated
parameter.
3.3 Key Parameters
The key-params field provides one or more sets of keying material
for the crypto-suite in question. The field consists of a method
indicator followed by a colon, and the actual keying information as
shown below (the formal grammar is provided in Section 8.1):
key-params = <key-method> ":" <key-info>
Keying material might be provided by different means than key-
params, however this is out of the scope of this document. Only one
method is defined in this document, namely "inline", which indicates
that the actual keying material is provided in the key-info field
itself. There is a single name space for the key-method, i.e., the
key-method is transport independent. New key-methods (e.g., use of
a URL) may be defined in an IETF Standards Track RFC in the future.
Although the key-method itself may be generic, the accompanying key-
info definition is specific not only to the key-method, but also to
the transport in question. New key methods MUST be registered with
the IANA according to the procedures defined in Section 9.2.1.
Key-info is defined as a general character string (see Section 8.1
for details); further transport and key-method specific syntax and
semantics MUST be provided in an IETF RFC for each combination of
transport and key-method that wants to use it; definitions for SRTP
are provided in Section 5. Note that such definitions are provided
within the context of both a particular transport (e.g., "RTP/SAVP")
and a specific key-method (e.g., "inline"). IANA will register the
list of supported key methods for each transport.
When multiple keys are included in the key parameters, it MUST be
possible to determine which of the keys is being used in a given
media packet by a simple inspection of the media packet received; a
trial-and-error approach between the possible keys MUST NOT be
required.
For SRTP, this could for example be achieved by use of Master Key
Identifiers (MKI), or <"From", "To"> values [srtp].
In the offer/answer model, the key parameter is a declarative
parameter.
3.4 Session Parameters
Session parameters are specific to a given transport and use of them
is OPTIONAL in the general framework, where they are just defined as
a general character string. If session parameters are to be used
Andreasen, Baugher & Wing [Page 6]
INTERNET-DRAFT SDP Security Descriptions February, 2004
for a given transport, then transport-specific syntax and semantics
MUST be provided in an IETF RFC; definitions for SRTP are provided
in Section 5.
In the offer/answer model, session parameters may be either
negotiated or declarative; the definition of specific session
parameters MUST indicate whether they are negotiated or declarative.
Negotiated parameters apply to data sent in both directions, whereas
declarative parameters apply only to media sent by the entity that
generated the SDP. Thus, a declarative parameter in an offer
applies to media sent by the offerer, whereas a declarative
parameter in an answer applies to media sent by the answerer.
3.5 Example
The first example shows use of the crypto attribute for the
"RTP/SAVP" media transport type (as defined in Section 4). The
"a=crypto" line is actually one long line; it is shown as two lines
due to page formatting:
v=0
o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.example.com/seminars/sdp.pdf
e=j.doe@example.com (Jane Doe)
c=IN IP4 161.44.17.12/127
t=2873397496 2873404696
m=video 51372 RTP/SAVP 31
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
m=audio 49170 RTP/SAVP 0
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
m=application 32416 udp wb
a=orient:portrait
This SDP message describes three media streams, two of which use the
"RTP/SAVP" transport. Each has a crypto attribute for the
"RTP/SAVP" transport. These secure-RTP specific descriptions are
defined in Section 5.
4. General Use of the crypto Attribute
In this section, we describe the general use of the crypto attribute
outside of any transport or key-method specific rules.
Andreasen, Baugher & Wing [Page 7]
INTERNET-DRAFT SDP Security Descriptions February, 2004
4.1 Use With Offer/Answer
The general offer/answer rules for the crypto attribute are in
addition to the rules specified in RFC 3264, which MUST be followed,
unless otherwise noted. RFC 3264 defines operation for both unicast
and multicast streams; the sections below describe operation for
two-party unicast streams only, since support for multicast streams
(and multipoint unicast streams) is for further study.
4.1.1 Generating the Initial Offer - Unicast Streams
When generating an initial offer for a unicast stream, there MUST be
one or more crypto attributes present for each media stream for
which security is desired. Each crypto attribute for a given media
stream MUST contain a unique tag.
The ordering of multiple "a=crypto" lines is significant: The most
preferred crypto line is listed first. Each crypto attribute
describes the crypto-suite, key(s) and possibly session parameters
offered for the media stream. In general, a "more preferred"
crypto-suite SHOULD be cryptographically stronger than a "less
preferred" crypto-suite.
The crypto-suite always applies to media in the directions supported
by the media stream (e.g., send and receive). The key(s), however,
apply to media in the direction from the offerer to the answerer; if
the media stream is marked as "recvonly", a key MUST still be
provided.
This is done for consistency. Also, in the case of SRTP, for
example, secure RTCP will still be flowing in both the send and
receive direction for a unidirectional stream.
The offer may include session parameters. There are no general
offer rules for the session parameters; instead, specific rules may
be provided as part of the transport specific definitions of any
session parameters.
When issuing an offer, the offerer MUST be prepared to support media
security in accordance with any of the crypto attributes included in
the offer. There are however two problems associated with this.
First of all, the offerer does not know which key the answerer will
be using for media sent to the offerer. Since media may arrive
prior to the answer, delay or clipping can occur. If this is
unacceptable to the offerer, the offerer SHOULD use a mechanism
outside the scope of this document to prevent the above problem.
For example, in SIP [RFC3261], a "security" precondition as
defined in [sprecon] could solve the above problem.
Another problem can occur when the offerer includes multiple crypto
attributes: The offerer may not be able to deduce which of the
Andreasen, Baugher & Wing [Page 8]
INTERNET-DRAFT SDP Security Descriptions February, 2004
offered crypto attributes was accepted by the answerer until the
answer is received, yet media may arrive before the answer.
If this is unacceptable to the offerer, the offerer either SHOULD
NOT include multiple crypto attributes in the offer, or a mechanism
outside the scope of this document SHOULD be used to prevent the
above problem (e.g., a "security" precondition).
4.1.2 Generating the Initial Answer - Unicast Streams
When the answerer receives the initial offer with one or more crypto
attributes for a given unicast media stream, the answerer MUST
either accept exactly one of the offered crypto attributes, or the
offered stream MUST be rejected.
If the answerer wishes to indicate support for other crypto
attributes, those can be listed by use of the SDP Simple
Capability Declaration [RFC3407] extensions.
Only crypto attributes that are valid can be accepted; valid
attributes do not violate any of the general rules defined for
security descriptions as well as any specific rules defined for the
transport and key-method in question. When selecting one of the
valid crypto attributes, the answerer SHOULD select the most
preferred crypto attribute it can support, i.e., the first valid
supported crypto attribute in the list, considering the answerer's
capabilities and security policies.
If there are one or more crypto attributes in the offer, but none of
them are valid, or none of the valid ones are supported, the offered
media stream MUST be rejected.
When an offered crypto attribute is accepted, the crypto attribute
in the answer MUST contain the following:
* The tag and crypto-suite from the accepted crypto attribute in the
offer (the same crypto-suite MUST be used in the send and receive
direction).
* The key(s) the answerer will be using for media sent to the
offerer. Note that a key MUST be provided, irrespective of any
direction attributes in the offer or answer.
Furthermore, any session parameters that are negotiated MUST be
included in the answer. Declarative session parameters provided by
the offerer are not included in the answer, however the answerer may
provide its own set of declarative session parameters.
Once the answerer has accepted one of the offered crypto attributes,
the answerer MAY begin sending media to the offerer in accordance
with the selected crypto attribute. Note however, that the offerer
Andreasen, Baugher & Wing [Page 9]
INTERNET-DRAFT SDP Security Descriptions February, 2004
may not be able to process such media packets correctly until the
answer has been received.
4.1.3 Offerer Processing of the Initial Answer - Unicast Streams
When the offerer receives the answer, the offerer MUST verify, that
one of the initially offered crypto suites and its accompanying tag
was accepted and echoed in the answer. Also, the answer MUST
include one or more keys, which will be used for media sent from the
answerer to the offerer.
If the offer contained any mandatory negotiated session parameters
(see section 5.3.6), the offerer MUST verify that said parameters
are included in the answer. If the answer contains any mandatory
declarative session parameters, the offerer MUST be able to support
those.
If any of the above fails, the negotiation MUST be deemed to have
failed.
4.1.4 Modifying the Session
Once a media stream has been established, it MAY be modified at any
time, as described in RFC 3264, Section 8. Such a modification MAY
be triggered by the security service, e.g., in order to perform a
re-keying or change the crypto-suite. If media stream security
using the general security descriptions defined here is still
desired, the crypto attribute MUST be included in these new
offer/answer exchanges. The procedures are similar to those defined
in Section 4.1.1, 4.1.2, 4.1.3 of this document, subject to the
considerations provided in RFC 3264 Section 8.
4.2 Use Outside Offer/Answer
The crypto attribute can also be used outside the context of
offer/answer where there is no negotiation of the crypto suite,
cryptographic key or session parameters. In this case, the sender
determines security parameters for the stream. Since there is no
negotiation mechanisms, the sender MUST include exactly one crypto
attribute and the receiver MUST either accept it or else SHOULD NOT
receive the associated stream. The sender SHOULD select the
security description that it deems most secure for its purposes.
4.3 General Backwards Compatibility Considerations
In the offer/answer model, it is possible that the answerer supports
a given secure transport (e.g., "RTP/SAVP") and accepts the offered
media stream, yet the answerer does not support the crypto attribute
defined in this document and hence ignores it. The offerer can
recognize this situation by seeing an accepted media stream in the
Andreasen, Baugher & Wing [Page 10]
INTERNET-DRAFT SDP Security Descriptions February, 2004
answer that does not include a crypto line. In that case, the
security negotiation defined here MUST be deemed to have failed.
Similar issues exist when security descriptions are used outside of
the offer/answer model.
5. SRTP Security Descriptions
In this section, we provide definitions for security descriptions
for SRTP media streams. In the next section, we define how to use
SRTP security descriptions with and without the offer/answer model.
SRTP security descriptions for a media stream MUST only be used for
media streams that use the SRTP transport (e.g., "RTP/SAVP" or
"RTP/SAVPF") in the media (m=) line and SHALL apply to that media
stream only. The following specifies rules for the "RTP/SAVP"
profile defined in [srtp], however it is expected that other secure
RTP profiles (e.g., "RTP/SAVPF") can use the same rules.
There is no assurance that an endpoint is capable of configuring its
SRTP service with a particular crypto attribute parameter, but SRTP
guarantees minimal interoperability among SRTP endpoints through the
default SRTP parameters [srtp]. More capable SRTP endpoints support
a variety of parameter values beyond the SRTP defaults and these
values can be configured by the SRTP security descriptions defined
here. An endpoint that does not support the crypto attribute will
ignore it according to the SDP. Hence the endpoint will simply
assume use of default SRTP parameters, if it supports SRTP. Such an
endpoint will not correctly process the particular media stream. By
using the Offer/Answer model, the offerer and answerer can negotiate
the crypto parameters to be used before commencement of the
multimedia session (see Section 6.1).
There are over twenty cryptographic parameters listed in the SRTP
specification. Many of these parameters have fixed values for
particular cryptographic transforms. At the time of session
establishment, moreover, there is usually no need to provide unique
settings for many of the SRTP parameters, such as salt length and
pseudo-random function (PRF). Thus, it is possible to simplify the
list of parameters by defining "cryptographic suites" that fix a set
of SRTP parameter values for the security session. This approach is
followed by the SRTP security descriptions, which uses the general
security description parameters as follows:
Andreasen, Baugher & Wing [Page 11]
INTERNET-DRAFT SDP Security Descriptions February, 2004
* crypto-suite: Identifies the encryption and authentication
transforms
* key parameter: SRTP keying material and parameters
* session parameters: The following parameters are defined:
- KDR: The SRTP Key Derivation Rate is the rate that a
pseudo-random function is applied to a master key
- UNENCRYPTED_SRTP: SRTP messages are not encrypted
- UNENCRYPTED_SRTCP: SRTCP messages are not encrypted
- UNAUTHENTICATED_SRTP: SRTP messages are not authenticated
- FEC_ORDER: Order of forward error correction (FEC)
relative to SRTP services
- WSH: Window Size Hint
- Extensions: Extension parameters can be defined
Please refer to the SRTP specification for a complete list of
parameters and their descriptions [Section 8.2, srtp]. The key
parameter, the crypto-suite, and the session parameters shown above
are described in detail in the following subsections.
5.1 SRTP Key Parameter
SRTP security descriptions define use of the "inline" key method as
described in the following. Use of any other keying method, e.g.,
URL, for SRTP security descriptions is for further study.
The "inline" type of key contains the keying material (master key
and salt) and all policy related to that master key, including how
long it can be used (lifetime) and whether or not it uses a master
key identifier (MKI) to associate an incoming SRTP packet with a
particular master key. Compliant implementations obey the policies
associated with a master key, and MUST NOT accept incoming packets
that violate the policy (e.g., after the master key lifetime has
expired).
The key parameter contains one or more cryptographic master keys,
each of which MUST be a unique cryptographically random [RFC1750]
value with respect to other master keys in the entire SDP message
(i.e., including master keys for other streams). Each key follows
the format (the formal definition is provided in Section 8.2):
"inline:" <key||salt> "|" [lifetime] "|" [MKI ":" length / FromTo]
key||salt concatenated master key and salt, base64 encoded
(see [RFC3548], Section 3)
lifetime master key lifetime (max number of SRTP or SRTCP
packets using this master key)
MKI:length MKI and length of the MKI field in SRTP packets
FromTo <"From", "To"> values, specifying the lifetime for
a master key
Andreasen, Baugher & Wing [Page 12]
INTERNET-DRAFT SDP Security Descriptions February, 2004
The following definition provides an example for
AES_CM_128_HMAC_SHA1_80:
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:4
The first field ("d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj") of the
parameter is the cryptographic master key appended with the master
salt; the two are first concatenated and then base64 encoded. The
length of the concatenated key and salt is determined by the crypto-
suite for which the key applies. If the length (after being decoded
from base64) does not match that specified for the crypto-suite, the
crypto attribute in question MUST be considered invalid. Each
master key and salt MUST be a cryptographically random number and
MUST be unique to the entire SDP message. When base64 decoding the
key and salt, padding characters (i.e., one or two "=" at the end of
the base64 encoded data) are discarded (see [RFC3548] for details).
Base64 encoding assumes that the base64 encoding input is an
integral number of octets. If a given crypto-suite requires the use
of a concatenated key and salt with a length that is not an integral
number of octets, said crypto-suite MUST define a padding scheme
that results in the base64 input being an integral number of octets.
For example, if the length defined was 250 bits, then 6 padding bits
would be needed, which could be defined to be the last 6 bits in a
256 bit input.
The second field, is the OPTIONAL lifetime of the master key as
measured in maximum number of SRTP or SRTCP packets using that
master key (i.e., the number of SRTP packets and the number of SRTCP
packets each have to be less than the lifetime). The lifetime value
MAY be written as a non-zero, positive integer or as a power of 2
(see the grammar in Section 8.2 for details). The "lifetime" value
MUST NOT exceed the maximum packet lifetime for the crypto-suite.
If the lifetime is too large or otherwise invalid then the entire
crypto attribute MUST be considered invalid. The default MAY be
implicitly signaled by omitting the lifetime value (i.e., "||").
This is convenient when the SRTP cryptographic key lifetime is the
default value. As a shortcut to avoid long decimal values, the
syntax of the lifetime allows using the literal "2^", which
indicates "two to the power of". The example above, shows a case
where the lifetime is specified as 2^20. The following example,
which is for the AES_CM_128_HMAC_SHA1_80 crypto-suite, has a default
for the lifetime field, which means that SRTP's and SRTCP's default
values will be used (see [srtp]):
inline:YUJDZGVmZ2hpSktMbW9QUXJzVHVWd3l6MTIzNDU2||1066:4
The example shows a 30-character key and concatenated salt that is
base64 encoded: The 30-character key/salt concatenation is expanded
to 40 characters by the three-in-four encoding of base64.
Andreasen, Baugher & Wing [Page 13]
INTERNET-DRAFT SDP Security Descriptions February, 2004
The third field, which is also OPTIONAL, is either the Master Key
Identifier (MKI) and its byte length, or a <"From", "To"> value.
"MKI" is the master key identifier associated with the SRTP master
key. If the MKI is given, then the length of the MKI MUST also be
given and separated from the MKI by a colon (":"). The MKI length
is the size of the MKI field in the SRTP packet, specified in bytes.
If the MKI length is not given or its value exceeds 128 (bytes),
then the entire crypto attribute MUST be considered invalid. The
substring "1:4" in the first example assigns to the key a master key
identifier of 1 that is 4 bytes long, and the second example assigns
a 4-byte master key identifier of 1066 to the key.
<"From", "To"> specifies the lifetime for a master key, expressed in
terms of the ROC and SEQ values inside whose range (including the
range ends) the master key is valid. <"From", "To"> is an
alternative to the MKI and assumes that a master key is in one-to-
one correspondence with the SRTP session key on which the <"From",
"To"> range is defined (see [srtp, Section 8.1.1] for details). The
following example illustrates the use of the <"From", "To">
parameter:
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|FT=0:0,1:0
As mentioned above, the key parameter can contain one or more master
keys. When the key parameter contains more than one master key, all
of the master keys in that key parameter MUST either include an MKI
or a <"From", "To"> value. Note that it is not permissible to mix
the two within a single key parameter (i.e., one crypto attribute);
all master keys in a given key parameter must use one or the other
(or neither). Furthermore, when using the MKI, the MKI length MUST
be the same for all keys in a given crypto attribute.
5.2 Crypto-suites
The SRTP crypto-suites define the encryption and authentication
transforms to be used for the SRTP media stream. The SRTP
specification has defined three crypto-suites, which are described
further in the following subsections in the context of the SRTP
security descriptions. The table below provides an overview of the
crypto-suites and their parameters:
Andreasen, Baugher & Wing [Page 14]
INTERNET-DRAFT SDP Security Descriptions February, 2004
+---------------------+-------------+--------------+---------------+
| |AES_CM_128_ | AES_CM_128_ | F8_128_ |
| |HMAC_SHA1_80 | HMAC_SHA1_32 | HMAC_SHA1_80 |
+---------------------+-------------+--------------+---------------+
| Master key length | 128 bits | 128 bits | 128 bits |
| Salt value | 112 bits | 112 bits | 112 bits |
| Default lifetime | 2^31 packets| 2^31 packets | 2^31 packets |
| Cipher | AES Counter | AES Counter | F8 |
| | Mode | Mode | |
| Encryption key | 128 bits | 128 bits | 128 bits |
| MAC | HMAC-SHA1 | HMAC-SHA1 | HMAC-SHA1 |
| Authentication tag | 80 bits | 32 bits | 80 bits |
| SRTP auth. key | 160 bits | 160 bits | 160 bits |
| SRTCP auth. key | 160 bits | 160 bits | 160 bits |
+---------------------+-------------+--------------+---------------+
5.2.1 AES_CM_128_HMAC_SHA1_80
AES_CM_128_HMAC_SHA1_80 is the SRTP default AES Counter Mode cipher
and HMAC-SHA1 message authentication having an 80-bit authentication
tag. The master-key length is 128 bits and has a default lifetime
of a maximum of 2^31 SRTP packets or SRTCP packets, whichever comes
first [Page 39, srtp].
Technically, SRTP allows 2^48 SRTP packets or 2^31 SRTCP packets,
whichever comes first. SRTP security descriptions, however,
simplify the parameters to share a single upper bound of 2^31
packets. It is RECOMMENDED that automated key management allow
easy and efficient rekeying at intervals far smaller than 2^31
packets given today's media rates or even HDTV media rates.
The SRTP and SRTCP encryption key lengths are 128 bits. The SRTP
and SRTCP authentication key lengths are 160 bits (see Security
Considerations in Section 7). The master salt value is 112 bits in
length and the session salt value is 112 bits in length. The
pseudo-random function (PRF) is the default SRTP pseudo-random
function that uses AES Counter Mode with a 128-bit key length.
The length of the base64 decoded key and salt value for this crypto-
suite MUST be 30 characters, i.e., 240 bits; otherwise the crypto
attribute is considered invalid.
5.2.2 AES_CM_128_HMAC_SHA1_32
This crypto-suite is identical to AES_CM_128_HMAC_SHA1_80 except
that the authentication tag is 32 bits.
The length of the base64-decoded key and salt value for this crypto-
suite MUST be 30 characters, i.e., 240 bits; otherwise the crypto
attribute is considered invalid.
Andreasen, Baugher & Wing [Page 15]
INTERNET-DRAFT SDP Security Descriptions February, 2004
5.2.3 F8_128_HMAC_SHA1_80
This crypto-suite is identical to AES_CM_128_HMAC_SHA1_80 except the
cipher is F8 [srtp].
The length of the base64 decoded key and salt value for this crypto-
suite MUST be 30 characters, i.e. 240 bits; otherwise the crypto
attribute is considered invalid.
5.2.4 Adding new Crypto-suite Definitions
If new transforms are added to SRTP, new definitions for those
transforms SHOULD be given for the SRTP security descriptions and
published in an IETF RFC. Sections 5.2.1 through 5.2.3 illustrate
how to define crypto-suite values for particular cryptographic
transforms. Any new crypto-suites MUST be registered with IANA
following the procedures in section 9.
5.3 Session Parameters
SRTP security descriptions define a set of "session" parameters,
which OPTIONALLY may be used to override SRTP session defaults for
the SRTP and SRTCP streams. These parameters configure an RTP
session for SRTP services. The session parameters provide session-
specific information to establish the SRTP cryptographic context.
5.3.1 KDR=n
KDR specifies the Key Derivation Rate, as described in section 4.3.1
of [srtp].
The value n MUST be an integer in the set {1,2,...,24}, which
denotes a power of 2 from 2^1 to 2^24, inclusive. The SRTP key
derivation rate controls how frequently a new session key is derived
from an SRTP master key [srtp]. When the key derivation rate is not
specified (i.e., the KDR parameter is omitted), a single initial key
derivation is performed [srtp].
In the offer/answer model, KDR is a declarative parameter.
5.3.2 UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP
SRTP and SRTCP packet payloads are encrypted by default. The
UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP session parameters modify the
default behavior of the crypto-suites with which they are used:
Andreasen, Baugher & Wing [Page 16]
INTERNET-DRAFT SDP Security Descriptions February, 2004
* UNENCRYPTED_SRTCP signals that the SRTCP packet payloads are not
encrypted.
* UNENCRYPTED_SRTP signals that the SRTP packet payloads are not
encrypted.
In the offer/answer model, these parameters are negotiated.
5.3.3 UNAUTHENTICATED_SRTP
SRTP and SRTCP packet payloads are authenticated by default. The
UNAUTHENTICATED_SRTP session parameter signals that SRTP messages
are not authenticated. Use of UNAUTHENTICATED_SRTP is NOT
RECOMMENDED (see Security Considerations).
The SRTP specification requires use of message authentication for
SRTCP, but not for SRTP [srtp].
In the offer/answer model, this parameter is negotiated.
5.3.4 FEC_ORDER=order
FEC_ORDER signals the use of forward error correction for the RTP
packets [rfc2733]. The forward error correction values for "order"
are FEC_SRTP, SRTP_FEC, or SPLIT [mikey]. FEC_SRTP signals that FEC
is applied before SRTP processing by the sender of the SRTP media
and after SRTP processing by the receiver of the SRTP media;
FEC_SRTP is the default. SRTP_FEC is the reverse processing. SPLIT
signals that the sender performs SRTP encryption, followed by FEC
processing, followed by SRTP authentication; processing is reversed
on the receiver.
In the offer/answer model, FEC_ORDER is a declarative parameter.
5.3.5 Window Size Hint (WSH)
SRTP defines the SRTP-WINDOW-SIZE [srtp, section 3.3.2] parameter to
protect against replay attacks. The minimum value is 64 [srtp],
however this value may be considered too low for some applications
(e.g., video).
The Window Size Hint (WSH) session parameter provides a hint for how
big this window should be to work satisfactorily (e.g., based on
sender knowledge of number of packets per second). However, there
might be enough information given in SDP attributes like
"a=maxprate" and the bandwidth modifiers to allow a receiver to
derive the parameter satisfactorily. Consequently, this value is
only considered a hint to the receiver of the SDP which MAY choose
to ignore the value provided.
Andreasen, Baugher & Wing [Page 17]
INTERNET-DRAFT SDP Security Descriptions February, 2004
In the offer/answer model, WSH is a declarative parameter.
5.3.6 Defining New SRTP Session Parameters
New SRTP session parameters for the SRTP security descriptions can
be defined in an IETF RFC and registered with IANA according to the
registration procedures defined in Section 9.
New SRTP session parameters are by default mandatory. A newly-
defined SRTP session parameter that is prefixed with the dash
character ("-") however is considered optional and MAY be ignored.
If an SDP crypto attribute is received with an unknown session
parameter that is not prefixed with a "-" character, that crypto
attribute MUST be considered invalid.
5.4 SRTP Crypto Context Initialization
In addition to the various SRTP parameters defined above, there are
three pieces of information that are critical to the operation of
the default SRTP ciphers:
* SSRC: Synchronization source
* ROC: Roll-over counter for a given SSRC
* SEQ: Sequence number for a given SSRC
In a unicast session, as defined here, there are three constraints
on these values.
The first constraint is on the SSRC, which makes an SRTP keystream
be unique from other participants. As explained in SRTP, the
keystream MUST NOT be reused on two or more different pieces of
plaintext. Keystream reuse makes the ciphertext vulnerable. One
vulnerability is that known-plaintext fields in one stream can
expose portions of the reused keystream and this could further
expose more plaintext in other streams. Since all current SRTP
encryption transforms use keystreams, key sharing is a general
problem [srtp]. SRTP mitigates this problem by including the SSRC
of the sender in the keystream. But SRTP does not solve this
problem in its entirety because Real-time Transport Protocol has
SSRC collisions, which are very rare [rtp], but quite possible.
During a collision, two or more SSRCs that share a master key will
have identical keystreams for overlapping portions of the RTP
sequence-number space. SRTP Security Descriptions avoids keystream
reuse by making unique master keys REQUIRED for the sender and
receiver of the security description. Thus, the first constraint is
satisfied.
Also note, that there is a second problem with SSRC collisions:
The SSRC is used to identify the crypto context and thereby the
cipher, key, ROC, etc. to process incoming packets. In case of
SSRC collisions, crypto context identification becomes ambiguous
Andreasen, Baugher & Wing [Page 18]
INTERNET-DRAFT SDP Security Descriptions February, 2004
and correct packet processing may not occur. Furthermore, if an
RTCP BYE packet is to be sent for a colliding SSRC, that packet
may also have to be secured. In a (unicast) point-to-multipoint
scenario, this can be problematic for the same reasons, i.e., it
is not known which of the possible crypto contexts to use. Note
that these problems are not unique to the SDP security
descriptions; any use of SRTP needs to consider them.
The second constraint is that the ROC MUST be zero at the time that
each SSRC commences sending packets. Thus, there is no concept of a
"late joiner" in SRTP security descriptions, which are constrained
to be unicast and pairwise. The ROC and SEQ form a "packet index"
in the default SRTP transforms and the ROC is consistently set to
zero at session commencement, according to this document.
The third constraint is that the initial value of SEQ SHOULD be
chosen to be within the range of 0..2^15-1; this avoids an ambiguity
when packets are lost at the start of the session. If at the start
of a session, an SSRC source might randomly select a high sequence-
number value and put the receiver in an ambiguous situation: If
initial packets are lost in transit up to the point that the
sequence number wraps (i.e., exceeds 2^16-1), then the receiver
might not recognize that its ROC needs to be incremented. By
restricting the initial SEQ to the range of 0..2^15-1, SRTP packet-
index determination will find the correct ROC value, unless all of
the first 2^15 packets are lost (which seems, if not impossible,
then rather unlikely). See Section 3.3.1 of the SRTP specification
regarding packet-index determination [srtp].
The packet index, therefore, depends on the SSRC, the SEQ of an
incoming packet and the ROC, which is an SRTP crypto context
variable. Thus, SRTP has a big security dependency on SSRC
uniqueness. This fact might lead one to consider establishing the
SSRC by an entity that keeps these values from colliding. One
problem with this approach, however, is that the SSRC belongs to the
transport (RTP or SRTP) and not to the signaling. It would be an
imposition on RTP and SRTP to require that the SSRC be read and
written by an external system such as SDP.
Given the above constraints, unicast SRTP crypto contexts can be
established without the need to negotiate SSRC values in the SRTP
security descriptions. Instead, an approach called "late binding"
is RECOMMENDED by this specification. When a packet arrives, the
SSRC that is contained in it can be bound to the crypto context at
the time of session commencement (i.e., SRTP packet arrival) rather
than at the time of session signaling (i.e., receipt of an SDP).
With the arrival of the packet containing the SSRC, all the data
items needed for the SRTP crypto context are held by the receiver
(note that the ROC value by definition is zero; if non-zero values
were to be supported, additional signaling would be required). In
Andreasen, Baugher & Wing [Page 19]
INTERNET-DRAFT SDP Security Descriptions February, 2004
other words, the crypto context for a secure RTP session using late
binding is initially identified by the SDP as:
<*, address, port>
where '*' is a wildcard SSRC, "address" is the local receive address
from the "c=" line, and "port" is the local receive port from the
"m=" line. When the first packet arrives with ssrcX in its SSRC
field, the crypto context
<ssrcX, address, port>
is instantiated subject to the following constraints:
* Media packets are authenticated: Authentication MUST succeed;
otherwise, the crypto context is not instantiated.
* Media packets are not authenticated: Crypto context is
automatically instantiated.
It should be noted, that use of late binding when there is no
authentication of the SRTP media packets is subject to numerous
security attacks and consequently it is NOT RECOMMENDED (of course,
this can be said for unauthenticated SRTP in general).
Note that use of late binding without authentication results in
local state being created as a result of receiving a packet from
any unknown SSRC. UNAUTHENTICATED_SRTP, therefore is NOT
RECOMMENDED because it invites easy denial-of-service attack. In
contrast, late binding with authentication does not suffer from
this weakness.
With the constraints and procedures described above, it is not
necessary to explicitly signal the SSRC, ROC and SEQ for a unicast
SRTP session.
5.5 Removal of Crypto Contexts
The mechanism defined above addresses the issue of creating crypto
contexts, however in practice, session participants may want to
remove crypto contexts prior to session termination. Since a crypto
context contains information that can not automatically be recovered
(e.g., ROC), it is important that the sender and receiver agree on
when a crypto context can be removed, and perhaps more importantly
when it cannot.
Even when late binding is used for a unicast stream, the ROC is
lost and cannot be recovered automatically (unless it is zero)
once the crypto context is removed.
Andreasen, Baugher & Wing [Page 20]
INTERNET-DRAFT SDP Security Descriptions February, 2004
We resolve this problem as follows. When SRTP security descriptions
are being used, crypto contexts removal MUST follow the same rules
as SSRC removal from the member table [RFC 3550]; note that this can
happen as the result of an SRTCP BYE packet or a simple time-out due
to inactivity. Inactive session participants that wish to ensure
their crypto contexts are not timed out MUST thus send SRTCP packets
at regular intervals.
6. SRTP-Specific Use of the crypto Attribute
Section 4 describes general use of the crypto attribute, and this
section completes it by describing SRTP-specific use.
6.1 Use with Offer/Answer
In this section, we describe how the SRTP security descriptions are
used with the offer/answer model to negotiate cryptographic
capabilities and communicate SRTP master keys. The rules defined
below complement the general offer/answer rules defined in Section
4.1, which MUST be followed, unless otherwise specified. Note that
the rules below define unicast operation only; support for multicast
and multipoint unicast streams is for further study.
6.1.1 Generating the Initial Offer - Unicast Streams
When the initial offer is generated, the offerer MUST follow the
steps in Section 4.1.1 as well as the following steps.
For each unicast media line (m=) using the secure RTP transport
where the offerer wants to specify cryptographic parameters, the
offerer MUST provide at least one valid SRTP security description
("a=crypto" line), as defined in Section 5.
The offerer MAY include one or more other SRTP session parameters as
defined in Section 5.3. Note however, that if any SRTP session
parameters are included that are not known to the answerer, but are
nonetheless mandatory (see Section 5.3.6), the negotiation will fail
if the answerer does not support them.
6.1.2 Generating the Initial Answer - Unicast Streams
When the initial answer is generated, the answerer MUST follow the
steps in Section 4.1.2 as well as the following steps.
For each unicast media line which uses the secure RTP transport and
contains one or more "a=crypto" lines in the offer, the answerer
MUST either accept one (and only one) of the crypto lines for that
media stream, or it MUST reject the media stream. Only "a=crypto"
lines that are considered valid SRTP security descriptions as
defined in Section 5 can be accepted. Furthermore, all parameters
(crypto-suite, key parameter, and mandatory session parameters) MUST
Andreasen, Baugher & Wing [Page 21]
INTERNET-DRAFT SDP Security Descriptions February, 2004
be acceptable to the answerer in order for the offered media stream
to be accepted.
When the answerer accepts an SRTP unicast media stream with a crypto
line, the answerer MUST include one or more master keys appropriate
for the selected crypto algorithm; the master key(s) included in the
answer MUST be different from those in the offer.
When the master key(s) are not shared between the offerer and
answerer, SSRC collisions between the offerer and answerer will
not lead to keystream reuse, and hence SSRC collisions do not
necessarily have to be prevented.
Declarative session parameters may be added to the answer as usual,
however the answerer SHOULD NOT add any mandatory session parameter
(see Section 5.3.6) that might be unknown to the offerer.
If the answerer cannot find any valid crypto line that it supports,
or if its configured policy prohibits any cryptographic key
parameter (e.g., key length) or cryptographic session parameter
(e.g., KDR, FEC_ORDER), it MUST reject the media stream, unless it
is able to successfully negotiate use of SRTP by other means outside
the scope of this document (e.g., by use of MIKEY [mikey]).
6.1.3 Offerer Processing of the Initial Answer - Unicast Streams
When the offerer receives the answer, it MUST perform the steps in
Section 4.1.3 as well as the following steps for each SRTP media
stream it offered with one or more crypto lines in it.
If the media stream was accepted and it contains a crypto line, it
MUST be checked that the crypto line is valid according to the
constraints specified in Section 5.
If the offerer either does not support or is not willing to honor
one or more of the SRTP parameters in the answer, the offerer MUST
consider the crypto line invalid.
If the crypto line is not valid, or the offerer's configured policy
prohibits any cryptographic key parameter (e.g. key length) or
cryptographic session parameter, the SRTP security negotiation MUST
be deemed to have failed.
6.1.4 Modifying the Session
When a media stream using the SRTP security descriptions has been
established, and a new offer/answer exchange is performed, the
offerer and answerer MUST follow the steps in Section 4.1.4 as well
as the following steps.
Andreasen, Baugher & Wing [Page 22]
INTERNET-DRAFT SDP Security Descriptions February, 2004
When modifying the session, all negotiated aspects of the SRTP media
stream can be modified. For example, a new crypto suite can be used
or a new master key can be established. As described in RFC 3264,
when doing a new offer/answer exchange there will be a window of
time, where the offerer and the answerer must be prepared to receive
media according to both the old and the new offer/answer exchange.
This requirement applies here as well, however the following should
be noted:
* When authentication is not being used, it may not be possible for
either the offerer or the answerer to determine if a given packet
is encrypted according to the old or new offer/answer exchange.
RFC 3264 defines a couple of techniques to address this problem,
e.g., changing the payload types used and/or the transport
addresses. Note however that a change in transport addresses may
have an impact on Quality of Service as well as firewall and NAT
traversal. The SRTP security descriptions offers two other ways
of dealing with this; use the MKI (which adds a few bytes to each
SRTP packet) or the <"From","To"> mechanism (which doesn't add
bytes to each SRTP packet) as described in Section 5.1. For
further details on MKI and "<"From","To">, please refer to [srtp].
* If the answerer changes its master key, the offerer will not be
able to process packets secured via this master key until the
answer is received.
As noted in Section 4.1.1, this could for example be addressed by
using a security "precondition" [sprecon].
Finally note, that if the new offer is rejected, the old crypto
parameters remain in place.
6.1.5 Offer/Answer Example
In this example, the offerer supports two crypto suites (F8 and
AES). The a=crypto line is actually one long line, although it is
shown as two lines in this document due to page formatting.
Andreasen, Baugher & Wing [Page 23]
INTERNET-DRAFT SDP Security Descriptions February, 2004
Offerer sends:
v=0
o=sam 2890844526 2890842807 IN IP4 10.47.16.5
s=SRTP Discussion
i=A discussion of Secure RTP
u=http://www.example.com/seminars/srtp.pdf
e=marge@example.com (Marge Simpson)
c=IN IP4 168.2.17.12
t=2873397496 2873404696
m=audio 49170 RTP/SAVP 0
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
a=crypto:2 F8_128_HMAC_SHA1_80
inline:MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm|2^20|1:4;
inline:QUJjZGVmMTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5|2^20|2:4
FEC_ORDER=FEC_SRTP
Answerer replies:
v=0
o=jill 25690844 8070842634 IN IP4 10.47.16.5
s=SRTP Discussion
i=A discussion of Secure RTP
u=http://www.example.com/seminars/srtp.pdf
e=homer@example.com (Homer Simpson)
c=IN IP4 168.2.17.11
t=2873397526 2873405696
m=audio 32640 RTP/SAVP 0
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
In this case, the session would use the AES_CM_128_HMAC_SHA1_80
crypto suite for the RTP and RTCP traffic.
6.2 SRTP-Specific Use Outside Offer/Answer
These are the same as Section 4.2.
6.3 Support for SIP Forking
As mentioned earlier, the security descriptions defined here do not
support multicast media streams or multipoint unicast streams.
However, in the SIP protocol, it is possible to receive several
answers to a single offer due to the use of forking (see [SIP]).
Receiving multiple answers leads to a couple of problems for the
SRTP security descriptions:
Andreasen, Baugher & Wing [Page 24]
INTERNET-DRAFT SDP Security Descriptions February, 2004
* Different answerers may choose different ciphers, keys, etc.,
however there is no way for the offerer to associate a particular
incoming media packet with a particular answer.
* Two or more answerers may pick the same SSRC and hence the SSRC
collision problems mentioned earlier may arise.
As stated earlier, the above point-to-multipoint cases are outside
the scope of the SDP security descriptions. However, there is a way
of supporting SIP forking: Change the multipoint scenario resulting
from SIP forking into multiple two-party unicast cases. This is
done as follows:
For each answer received beyond the initial answer, issue a new
offer to that particular answerer using a new receive transport
address (IP address and port); note that this requires support for
the SIP UPDATE method [RFC 3313]. Also, to ensure that two media
sessions are not inadvertently established prior to the UPDATE being
processed by one of them, use security preconditions [sprecon].
Finally, note that all the answerers will know the key(s) being
proposed by the initial offer. If the offerer wants to ensure
security with respect to other answerers, a new offer/answer
exchange with a new key needs to be performed with the first
answerer as well.
6.4 SRTP-Specific Backwards Compatibility Considerations
It is possible that the answerer supports the SRTP transport and
accepts the offered media stream, yet it does not support the crypto
attribute defined here. The offerer can recognize this situation by
seeing an accepted SRTP media stream in the answer that does not
include a crypto line. In that case, the security negotiation
defined here MUST be deemed to have failed.
Also, if a media stream with a given SRTP transport (e.g.,
"RTP/SAVP") is sent to a device that does not support SRTP, that
media stream will be rejected.
6.5 Operation with KEYMGT= and k= lines
An offer MAY include both "a=crypto" and "a=keymgt" lines [keymgt].
Per SDP rules, the answerer will ignore attribute lines that it does
not understand. If the answerer supports both "a=crypto" and
"a=keymgt", the answer MUST include either "a=crypto" or "a=keymgt"
but not both, as including both is undefined.
An offer MAY include both "a=crypto" and "k=" lines [SDP]. Per SDP
rules, the answerer will ignore attribute lines it does not
understand. If the answerer supports both "a=crypto" and "k=", the
Andreasen, Baugher & Wing [Page 25]
INTERNET-DRAFT SDP Security Descriptions February, 2004
answer MUST include either "a=crypto" or "k=" but not both, as
including both is undefined.
7. Security Considerations
Like all SDP messages, SDP messages containing security
descriptions, are conveyed in an encapsulating application protocol
(e.g., SIP, MGCP, etc.). It is the responsibility of the
encapsulating protocol to ensure the protection of the SDP security
descriptions. Therefore, the application protocol SHOULD either
invoke its own security mechanisms (e.g., secure multiparts) or
alternatively utilize a lower-layer security service (e.g., TLS, or
IPSec). This security service SHOULD provide strong message
authentication and packet-payload encryption as well as effective
replay protection.
7.1 Authentication of packets
Security descriptions as defined herein signal security services for
RTP packets. RTP messages are vulnerable to a variety of attacks
such as replay and forging. To limit these attacks, SRTP message
integrity mechanisms SHOULD be used (SRTP replay protection is
always enabled).
7.2 Keystream Reuse
SRTP security descriptions signal configuration parameters for SRTP
sessions. Misconfigured SRTP sessions are vulnerable to attacks on
their encryption services when running the crypto suites defined in
Sections 5.2.1, 5.2.2, and 5.2.3. An SRTP encryption service is
"misconfigured" when two or more media streams are encrypted using
the same AES keystream. When senders and receivers share derived
session keys, SRTP requires that the SSRCs of session participants
serve to make their corresponding keystreams unique, which is
violated in the case of SSRC collision: SRTP SSRC collision
drastically weakens SRTP or SRTCP payload encryption during the time
that identical keystreams were used [srtp]. An attacker, for
example, might collect SRTP and SRTCP messages and await a
collision. This attack on the AES-CM and AES-f8 encryption is
avoided entirely when each media stream has its own unique master
key in both the send and receive direction. This specification
restricts use of SDP security description to unicast point-to-point
streams so that keys are not shared between SRTP hosts, and the
master keys used in the send and receive direction for a given media
stream are unique.
7.3 Signaling Authentication and Signaling Encryption
There is no reason to incur the complexity and computational expense
of SRTP, however, when its key establishment is exposed to
unauthorized parties. In most cases, the SRTP crypto attribute and
Andreasen, Baugher & Wing [Page 26]
INTERNET-DRAFT SDP Security Descriptions February, 2004
its parameters are vulnerable to denial of service attacks when they
are carried in an unauthenticated SDP message. In some cases, the
integrity or confidentiality of the RTP stream can be compromised.
For example, if an attacker sets UNENCRYPTED for the SRTP stream in
an offer, this could result in the answerer not decrypting the
encrypted SRTP messages. In the worst case, the answerer might
itself send unencrypted SRTP and leave its data exposed to snooping.
Thus, MIME secure multiparts, IPsec, TLS, or some other data
security service SHOULD be used to provide message authentication
for the encapsulating protocol that carries the SDP messages having
a crypto attribute (a=crypto). Furthermore, encryption of the
encapsulating payload SHOULD be used because a master key parameter
(inline) appears in the message. Failure to encrypt the SDP message
containing an inline SRTP master key renders the SRTP authentication
or encryption service useless in practically all circumstances.
Failure to authenticate an SDP message that carries SRTP parameters
renders the SRTP authentication or encryption service useless in
most practical applications.
When the communication path of the SDP message is routed through
intermediate systems that inspect parts of the SDP message, security
protocols such as IPsec or TLS SHOULD NOT be used for encrypting
and/or authenticating the security description. In the case of
intermediate-system processing of a message containing SDP security
descriptions, the "a=crypto" attributes SHOULD be protected end-to-
end so that the intermediate system can neither modify the security
description nor access the keying material. Network or transport
security protocols that terminate at each intermediate system,
therefore, SHOULD NOT be used for protecting SDP security
descriptions. A security protocol SHOULD allow the security
descriptions to be encrypted and authenticated end-to-end
independently of the portions of the SDP message that any
intermediate system modifies or inspects: MIME secure multiparts
are RECOMMENDED for the protection of SDP messages that are
processed by intermediate systems.
When the SDP parameters cannot be carried in an encrypted and
authenticated SDP message, it is RECOMMENDED that a key management
protocol be used instead of the security descriptions defined here
(a=crypto). The proposed SDP key-mgmt extension [keymgt] allows
authentication and encryption of the key management protocol data
independently of the SDP message that carries it. The security of
the SDP SRTP attribute, however, is as good as the data security
protocol that protects the SDP message. For example, if an IPSec
security association exists between the SDP source and destination
endpoints, then this solution is more secure than use of the key-
mgmt statement in an unauthenticated SDP message, which is
vulnerable to tampering.
Andreasen, Baugher & Wing [Page 27]
INTERNET-DRAFT SDP Security Descriptions February, 2004
8. Grammar
In this section we first provide the ABNF grammar for the generic
crypto attribute, and then we provide the ABNF grammar for the SRTP
specific use of the crypto attribute.
8.1 Generic "Crypto" Attribute Grammar
The ABNF grammar for the crypto attribute is defined below:
"a=crypto:" tag 1*WSP crypto-suite 1*WSP key-params
*(1*WSP session-param)
tag = 1*ALPHANUM
crypto-suite = 1*(ALPHA / DIGIT / "_")
key-params = key-param *(";" key-param)
key-param = key-method ":" key-info
key-method = "inline" / key-method-ext
key-method-ext = 1*(ALPHA / DIGIT / "_")
key-info = %x21-3A / %x3C-7E ; visible (printing) characters
; except semi-colon
session-param = 1*(VCHAR) ; visible (printing) characters
where WSP, ALPHA, DIGIT, and VCHAR are defined in [RFC2234].
8.2 SRTP "Crypto" Attribute Grammar
This section provides an Augmented BNF [RFC2234] grammar for the
SRTP-specific use of the SDP crypto attribute:
crypto-suite = srtp-crypto-suite
key-method = srtp-key-method
key-info = srtp-key-info
session-param = srtp-session-param
srtp-crypto-suite = "AES_CM_128_HMAC_SHA1_32" /
"F8_128_HMAC_SHA1_32" /
"AES_CM_128_HMAC_SHA1_80" /
srtp-crypto-suite-ext
srtp-key-method = "inline"
srtp-key-info = key-salt "|" [lifetime] "|" [mki / FromTo]
key-salt = 1*(base64) ; binary key and salt values
; concatenated together, and then
; base64 encoded [section 6.8 of
; RFC2046]
lifetime = ["2^"] 1*(DIGIT) ; see section 5.1 for "2^"
mki = mki-value ":" mki-length
Andreasen, Baugher & Wing [Page 28]
INTERNET-DRAFT SDP Security Descriptions February, 2004
mki-value = 1*DIGIT
mki-length = 1*3DIGIT ; range 1..128.
FromTo = "FT=" ftval "," ftval
ftval = roc ":" seq ; packet index expressed in terms
; of ROC and SEQ.
roc = 1*DIGIT ; range 0..2^32-1
seq = 1*DIGIT ; range 0..2^16-1
srtp-session-param = kdr /
"UNENCRYPTED_SRTP" /
"UNENCRYPTED_SRTCP" /
"UNAUTHENTICATED_SRTP" /
fec-order /
wsh /
srtp-session-extension
kdr = "KDR=" 1*2(DIGIT) ; range 0..24, power of two
fec-order = "FEC_ORDER=" fec-type
fec-type = "FEC_SRTP" / "SRTP_FEC" / "SPLIT"
wsh = "WSH=" 2*DIGIT ; minimum value is 64
base64 = ALPHA / DIGIT / "+" / "/" / "="
srtp-crypto-suite-ext = 1*(ALPHA / DIGIT / "_")
srtp-session-extension = ["-"] 1*(VCHAR) ;visible chars [RFC2234]
; first character must not be dash ("-")
9. IANA Considerations
9.1 Registration of the "crypto" attribute
The IANA is hereby requested to register a new SDP attribute as
follows:
Attribute name: crypto
Long form name: Security description cryptographic attribute
for media streams
Type of attribute: Media-level
Subject to charset: No
Purpose: Security descriptions
Appropriate values: See Section 3
9.2 New IANA Registries and Registration Procedures
The following sub-sections define a new IANA registry with
associated sub-registries to be used for the SDP security
descriptions. The IANA is hereby requested to create an SDP
Security Description registry as shown below and further described
in the following sections:
Andreasen, Baugher & Wing [Page 29]
INTERNET-DRAFT SDP Security Descriptions February, 2004
SDP Security Descriptions
|
+- Key Methods (described in 9.2.1)
|
+- Media Stream Transports (described in 9.2.2)
|
+- Transport1 (e.g. SRTP)
| |
| +- Supported Key Methods (e.g. inline)
| |
| +- crypto suites
| |
| +- session parameters
|
+- Transport2
: :
9.2.1Security Descriptions Key Method Registry and Registration
The IANA is hereby requested to create a new subregistry for SDP
security description key methods. An IANA key method registration
MUST be documented in an IETF Standards Track RFC and it MUST
provide the name of the key method in accordance with the grammar
for key-method-ext defined in Section 8.1.
9.2.2Security Description Media Stream Transport Registry and
Registration
The IANA is hereby requested to create a new subregistry for SDP
security description Media Stream Transports. An IANA media stream
transport registration MUST be documented in an RFC in accordance
with the procedures defined in Section 3 and 4 of this document.
The registration MUST provide the name of the transport and a list
of supported key methods.
In addition, each new media stream transport registry must contain a
crypto-suite registry and a session parameter registry as well as
IANA instructions for how to populate these registries.
9.3 Initial Registrations
9.3.1 Key Method
The following security descriptions key methods are hereby
registered:
inline
Andreasen, Baugher & Wing [Page 30]
INTERNET-DRAFT SDP Security Descriptions February, 2004
9.3.2 SRTP Media Stream Transport
The IANA is hereby requested to create an SDP Security Description
Media Stream Transport subregistry for "SRTP". The key methods
supported is "inline". The reference for the SDP security
description for SRTP is this document.
9.3.2.1 SRTP Crypto Suite Registry and Registration
The IANA is hereby requested to create a new subregistry for SRTP
crypto suites under the SRTP transport of the SDP Security
Descriptions. An IANA SRTP crypto suite registration MUST indicate
the crypto suite name in accordance with the grammar for srtp-
crypto-suite-ext defined in Section 8.2.
The semantics of the SRTP crypto suite MUST be described in an IETF
RFC, including the semantics of the "inline" key-method and any
special semantics of parameters.
The following SRTP crypto suites are hereby registered:
AES_CM_128_HMAC_SHA1_80
AES_CM_128_HMAC_SHA1_32
F8_128_HMAC_SHA1_80
The reference for these crypto-suites is provided in this document.
9.3.2.2 SRTP Session Parameter Registration
The IANA is hereby requested to create a new subregistry for SRTP
session parameters under the SRTP transport of the SDP Security
Descriptions. An IANA SRTP session parameter registration MUST
indicate the session parameter name (srtp-session-extension as
defined in Section 8.2); the name MUST NOT begin with the dash
character ("-").
The semantics of the parameter MUST be described in an IETF RFC. If
values can be assigned to the parameter, then the format and
possible values that can be assigned MUST be described in the IETF
RFC as well. Also, it MUST be specified whether the parameter is
declarative or negotiated in the offer/answer model.
The following SRTP session parameters are hereby registered:
SRC
KDR
UNENCRYPTED_SRTP
UNENCRYPTED_SRTCP
UNAUTHENTICATED_SRTP
FEC_ORDER
WSH
Andreasen, Baugher & Wing [Page 31]
INTERNET-DRAFT SDP Security Descriptions February, 2004
The reference for these parameters is this document.
10. Acknowledgements
This document is a product of the IETF MMUSIC working group and has
benefited from comments from its participants. This document also
benefited from discussions with Elisabetta Cararra, Earl Carter,
Bill Foster, Matt Hammer, Cullen Jennings, Paul Kyzivat, David
McGrew, Mats Naslund, Dave Oran, Jonathan Rosenberg, Dave Singer,
Mike Thomas, Brian Weis, and Magnus Westerlund. These people shared
observations, identified errors and made suggestions for improving
the specification. Magnus provided many useful comments and Mats
made several valuable suggestions on parameters and syntax that are
in the current draft. Dave Oran and Mike Thomas encouraged us to
bring this work to the IETF for standardization. David McGrew
suggested the conservative approach of requiring unique master keys
for each unicast SDP media stream as followed in this document.
Paul Kyzivat suggested how to handle SIP forking. Jonathan
Rosenberg suggested reducing the complexity by specifying only one
security parameter for each media stream.
11. Authors' Addresses
Flemming Andreasen
Cisco Systems, Inc.
499 Thornall Street, 8th Floor
Edison, New Jersey 08837 USA
fandreas@cisco.com
Mark Baugher
5510 SW Orchid Street
Portland, Oregon 97219 USA
mbaugher@cisco.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134 USA
dwing@cisco.com
12. Normative References
[RFC3550] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC 3550,
July 2003, http://www.ietf.org/rfc/rfc3550.txt.
[RFC2234] D. Crocker, P. Overell, "Augmented BNF for Syntax
Specifications: ABNF," RFC 2234, November 1997,
http://www.ietf.org/rfc/rfc2234.txt.
Andreasen, Baugher & Wing [Page 32]
INTERNET-DRAFT SDP Security Descriptions February, 2004
[SDP] M. Handley, V. Jacobson, C. Perkins, "SDP: Session Description
Protocol", Work in Progress.
[RFC2733] J. Rosenberg, H. Schulzrinne, "An RTP Payload Format for
Generic Forward Error Correction", RFC 2733, December 1999,
http://www.ietf.org/rfc/rfc2733.txt.
[RFC2828] R. Shirey, "Internet Security Glossary", RFC 2828, May
2000, http://www.ietf.org/rfc/rfc2828.txt.
[RFC3264] J. Rosenberg, H. Schulzrinne, "An Offer/Answer Model with
the Session Description Protocol (SDP)", RFC 3264, June 2202,
http://www.ietf.org/rfc/rfc3264.txt.
[srtp] M. Baugher, R. Blom, E. Carrara, D. McGrew, M. Naslund, K.
Norrman, D. Oran, "The Secure Real-time Transport Protocol", Work in
Progress.
[RFC1750] D. Eastlake 3rd, S. Crocker, J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994,
http://www.ietf.org/rfc/rfc1750.txt.
[RFC3548] S. Josefsson, "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
13. Informative References
[RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple
Capability Declaration", RFC 3407, October 2002,
http://www.ietf.org/rfc/rfc3407.txt.
[Bellovin] Steven M. Bellovin, "Problem Areas for the IP Security
Protocols," in Proceedings of the Sixth Usenix Unix Security
Symposium, pp. 1-16, San Jose, CA, July 1996.
[GDOI] M. Baugher, B. Weis, T. Hardjono, H. Harney, "The Group
Domain of Interpretation", RFC 3547, July 2003,
http://www.ietf.org/rfc/rfc3547.txt.
[kink] M. Thomas, J. Vilhuber, "Kerberized Internet Negotiation of
Keys (KINK)", Work in Progress.
[ike] D. Harkins, D. Carrel, "The Internet Key Exchange (IKE)", RFC
2409, November 1998, http://www.ietf.org/rfc/rfc2409.txt.
[ipsec] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998,
http://www.ietf.org/rfc/rfc2401.txt.
Andreasen, Baugher & Wing [Page 33]
INTERNET-DRAFT SDP Security Descriptions February, 2004
[s/mime] Ramsdell B., "S/MIME Version 3 Message Specification", RFC
2633, June 1999, http://www.ietf.org/rfc/rfc2633.txt.
[tls] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999, http://www.ietf.org/rfc/rfc2246.txt.
[keymgt] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman,
"Key Management Extensions for SDP and RTSP", Work in Progress.
[mikey] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman,
"MIKEY: Multimedia Internet KEYing", Work in Progress.
[RFC2045] N. Freed, N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies", RFC
2045, November 1996, http://www.ietf.org/rfc/rfc2045.txt.
[RFC2104] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2014, November 1997,
http://www.ietf.org/rfc/rfc2104.txt.
[skeme] H. Krawczyk, "SKEME: A Versatile Secure Key Exchange
Mechanism for the Internet", ISOC Secure Networks and Distributed
Systems Symposium, San Diego, 1996.
[RFC3312] G. Camarillo, W. Marshall, J. Rosenberg, "Integration of
Resource Management and Session Initiation Protocol (SIP)", RFC
3312, October 2002, http://www.ietf.org/rfc/rfc3312.txt.
[RFC2974] M. Handley, C. Perkins, E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000,
http://www.ietf.org/rfc/rfc2974.txt.
[srtpf] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
RTCP-based Feedback (RTP/SAVPF)", work in progress, October 2003.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[sprecon] Andreasen, F., Baugher, M., and D. Wing, "Security
Preconditions for Session Description Protocol Media Streams", work
in progress, February 2004.
Andreasen, Baugher & Wing [Page 34]
INTERNET-DRAFT SDP Security Descriptions February, 2004
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances
of licenses to be made available, or the result of an attempt made
to obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification
can be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement
Copyright(C) The Internet Society 2004. All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Andreasen, Baugher & Wing [Page 35]
INTERNET-DRAFT SDP Security Descriptions February, 2004
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Andreasen, Baugher & Wing [Page 36]
