Internet Engineering Task Force Flemming Andreasen
MMUSIC Working Group Mark Baugher
INTERNET-DRAFT Dan Wing
EXPIRES: April 2004 Cisco Systems
October 24, 2003
Session Description Protocol Security Descriptions
for Media Streams
<draft-ietf-mmusic-sdescriptions-02.txt>
Status of this memo
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all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines a Session Description Protocol (SDP)
cryptographic attribute for media streams. The attribute describes
a cryptographic key and other parameters, which serve to configure
security for a media stream in either a single message or a
roundtrip. 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) media streams. The SDP crypto
attribute requires the services of a data security protocol to
secure the SDP message.
Table of Contents
1. Notational Conventions............................................3
2. Introduction......................................................3
3. SDP "Crypto" Attribute and Parameters.............................4
3.1 Crypto-suite....................................................5
INTERNET-DRAFT SDP Security Descriptions October 24, 2003
3.2 Key Parameters..................................................5
3.3 Session Parameters..............................................6
3.4 Example.........................................................6
4. General Use of the crypto Attribute...............................6
4.1 Use With Offer/Answer...........................................7
4.1.1 Generating the Initial Offer..............................7
4.1.2 Generating the Initial Answer.............................8
4.1.3 Offerer Processing of the Initial Answer..................9
4.1.4 Modifying the Session....................................10
4.2 Use Outside Offer/Answer: Advertising..........................10
4.3 General Backwards Compatibility Considerations.................10
5. SRTP Security Descriptions.......................................11
5.2 Crypto-suites..................................................14
5.2.1 AES_CM_128_HMAC_SHA1_80..................................14
5.2.2 AES_CM_128_HMAC_SHA1_32..................................14
5.2.3 F8_128_HMAC_SHA1_80......................................15
5.2.4 Adding new Crypto-suite Definitions......................15
5.3 Session Parameters.............................................15
5.3.1 SRC=SSRC/ROC/SEQ.........................................15
5.3.2 KDR=n....................................................18
5.3.3 UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP...................18
5.3.4 UNAUTHENTICATED_SRTP.....................................19
5.3.5 FEC_ORDER=order..........................................19
5.3.6 Window Size Hint (WSH)...................................19
5.3.7 SRTP Extension Session Parameters........................19
6. SRTP-Specific Use of the crypto Attribute........................20
6.1 Use with Offer/Answer..........................................20
6.1.1 Generating the Initial Offer.............................20
6.1.2 Generating the Initial Answer............................21
6.1.3 Offerer Processing of the Initial Answer.................22
6.1.4 Modifying the Session....................................23
6.1.5 Offer/Answer Example.....................................24
6.2 SRTP-Specific Use Outside Offer/Answer: Advertising............25
6.3 SRTP-Specific Backwards Compatibility Considerations...........25
6.4 Operation with KEYMGT= and k= lines............................26
6.5 Removal of Crypto Contexts.....................................26
7. Security Considerations..........................................26
7.1 Authentication of packets......................................27
7.2 Keystream Reuse................................................27
7.3 Signaling Authentication and Signaling Encryption..............27
8. Grammar..........................................................29
8.1 Generic "Crypto" Attribute Grammar.............................29
8.2 SRTP "Crypto" Attribute Grammar................................29
9. Open Issues......................................................30
10. IANA Considerations.............................................31
10.1 Registration of the "crypto" attribute........................31
10.2 New IANA Registries and Registration Procedures...............31
10.2.1 Security Descriptions Key Method Registry and Registration31
10.2.2 SRTP Crypto Suite Registry and Registration..............31
10.2.3 SRTP Session Parameter Registration......................32
10.3 Initial Registrations.........................................32
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11. Acknowledgements................................................32
12. Authors' Addresses..............................................33
13. Normative References............................................33
14. Informative References..........................................34
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. The abbreviation "iff" means "if and only if."
2. Introduction
The Session Description Protocol (SDP) describes multimedia
sessions, which can be audio, video, whiteboard, fax, modem, and
other media sessions. Security services such as data origin
authentication, integrity and confidentiality are often needed for
media streams. The Secure Real-time Transport Protocol (SRTP)
[srtp] provides such security services and is signaled by use of the
"RTP/SAVP" transport 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 crypto attribute is defined in a generic way to enable its use
with secure transports besides SRTP that need to signal and
negotiate cryptographic parameters, e.g. IPsec [ipsec], S/MIME
[s/mime], or TLS [tls], if and only if such parameters can either be
advertised in a single message, or negotiated in a single round-trip
by use of the offer/answer model [RFC3264]. Such extensions,
however, are 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 10. 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 SRTP secures RTP packets or
IPsec secures IP packets. Data security protocols such as SRTP rely
upon a separate key management system to securely establish
encryption and/or authentication keys. Key management protocols
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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 [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 that follow 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
"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 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. A list of
open issues is provided in Section 9 and IANA considerations are
provided in Section 10.
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 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 a formal ABNF grammar):
a=crypto:<crypto-suite> <key-params> *<session-params>
The fields crypto-suite, key-params, and session-param are described
in the following sub-sections. Below we show an example of the
crypto attribute for the "RTP/SAVP" transport, i.e. SRTP (newlines
included for formatting reasons only):
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a=crypto:AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:32
SRC=/721/13
The crypto-suite is AES_CM_128_HMAC_SHA1_80, key-params is defined
by the line starting with "inline:", and there is a single session-
param named "SRC".
3.1 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 of the
"RTP/SAVP" transport applies to Secure RTP only; the string may be
reused for another transport, however a separate definition would be
needed.
3.2 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 (a formal grammar is provided in Section 8.1):
key-params = <key-method> ":" <key-info>
Keying material may be provided by different means. One method is
defined in this document, namely "inline", which indicates that the
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 RFC in the future, in which case they may be used
with any transport, provided the definitions for that transport
support use of the new key-method. New key methods MUST be
registered with the IANA according to the procedures defined in
Section 10.2.1.
Key-info is here just 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").
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When multiple keys are included in the key parameters, it MUST be
possible to determine which key is being used 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.
3.3 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
for a given transport, then key-method and transport-specific syntax
and semantics MUST be provided in an IETF RFC for each transport
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 specific key-method (e.g. "inline") and a particular
transport (e.g. "RTP/SAVP").
3.4 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, although it is shown as two lines in this
document 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:AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
m=audio 49170 RTP/SAVP 0
a=crypto: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 RTP/SAVP-specific descriptions are defined in the
Section 5.
4. General Use of the crypto Attribute
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In this section, we describe the general use of the crypto attribute
outside of any transport or key-method specific rules.
4.1 Use With Offer/Answer
In this section, we define the general rules for use of the crypto
attribute with the offer/answer [RFC3264] model. These rules are in
addition to the rules specified in RFC 3264, which MUST be followed,
unless otherwise noted.
4.1.1 Generating the Initial Offer
4.1.1.1 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. 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 stronger
cryptographically than a "less preferred" crypto suite.
The crypto-suite always applies to media in all directions supported
by the media stream (e.g. send and receive).
The key(s) 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 for example
SRTP, secure RTCP will still be flowing in both the send and
receive direction for a unidirectional stream.
There are no general offer/answer rules for the session parameters;
instead, specific rules are provided as part of the transport and
key-method 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; the answerer may or may not
choose the same key as the offerer chose in his sending direction
(in fact, the answerer SHOULD NOT use the same key as explained in
Section 4.1.2.1). Since media may arrive prior to the answer, delay
or clipping may 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.
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For example, a "security" precondition [RFC3312] could be defined
to solve the above problem.
Another problem can occur when the offerer includes multiple crypto
attributes, since the offerer may not be able to deduce which of the
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).
4.1.1.2 Multicast Streams
The rules for multicast streams are similar to those for unicast
streams, except as noted below:
* In order to ensure that all participants use the same crypto
parameters, there MUST be exactly one crypto attribute per media
stream.
* The key(s) provided apply to media in all directions supported by
the media stream, as opposed to just the sending direction.
4.1.2 Generating the Initial Answer
4.1.2.1 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, i.e. do not violate any of
general rules defined for security descriptions as well as any
specific rules defined for the transport and key method in question
can be accepted. 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 is 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.
The crypto attribute in the answer MUST contain the following:
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* The 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.
There are no general offer/answer rules for the session parameters;
instead, specific rules are provided as part of the transport and
key-method specific definitions of any 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
may not be able to process such media packets correctly until the
answer has been received.
4.1.2.2 Multicast Streams
The rules for multicast streams are similar to those for unicast
streams, except as noted below:
* The crypto-suite in the answer MUST be the same as the one in the
offer (unless the offered media stream is rejected). Since no
more than one crypto attribute can be offered for a multicast
stream, this is satisfied trivially.
* The key(s) provided apply to media in all directions supported by
the media stream, as opposed to just the sending direction.
Consequently, the key(s) in the answer MUST be the same as the
key(s) in the offer.
4.1.3 Offerer Processing of the Initial Answer
4.1.3.1 Unicast Streams
When the offerer receives the answer, the offerer MUST verify, that
exactly one of the offered crypto attributes was accepted.
Otherwise, the offerer MUST consider the offer/answer negotiation to
have failed for that stream.
The key(s) included in the answer are the key(s) that will be used
for media sent from the answerer to the offerer and hence the
offerer MUST use those key(s) to process media received; the key(s)
might not be the same as the key(s) used by the offerer for sending
media to the answerer.
There are no general offer/answer rules for the session parameters;
instead, specific rules are provided as part of the transport and
key-method specific definitions of any session parameters.
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4.1.3.2 Multicast Streams
When the offerer receives the answer, the offerer MUST verify, that
the offered crypto attribute and key(s) were accepted and echoed in
the answer. Otherwise, the offerer MUST consider the offer/answer
negotiation to have failed for that stream for *that answerer* and
hence the answerer is not considered a participant in that media
stream. If there are other participants in the multimedia session,
the session may continue unaffected by this particular answerer's
failure.
There are no general offer/answer rules for the session parameters;
instead, specific rules are provided as part of the transport and
key-method specific definitions of any session parameters.
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 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 subject to the considerations provided in RFC
3264 Section 8.
4.2 Use Outside Offer/Answer: Advertising
The crypto attribute can also be used outside the context of
offer/answer. For example, when using the Session Announcement
Protocol (SAP) [RFC2974], there is no negotiation of the media
streams described by the SDP; instead media streams are simply
advertised.
The crypto attribute defined here can be used in such environments
where the crypto parameters are advertised in a single message
rather than being negotiated in a roundtrip (an offer and an
answer), albeit with certain restrictions:
* There MUST be exactly one crypto attribute.
There are no general rules for the session parameters; instead,
specific rules for advertising session parameters are provided as
part of the transport and key-method specific definitions of any
session parameters.
4.3 General Backwards Compatibility Considerations
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It is possible that the answerer supports a given secure transport
and accepts the offered media stream, yet the answerer does not
support the crypto attribute defined here. The offerer can
recognize this situation by seeing an accepted 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.
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 "RTP/SAVP" transport in the media (m=)
line and SHALL apply to that media stream only.
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 (per [SDPnew]) and hence, if it supports SRTP, it will
simply assume use of default SRTP parameters. 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:
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* crypto-suite: Identifies the encryption and authentication
transforms
* key parameter: SRTP keying material and parameters
* session parameters: The following parameters are defined:
- SRC: An <SSRC, ROC, SEQ> triple
- 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 sections.
5.1.1.1 SRTP Key Parameter
SRTP security descriptions define use of the "inline" key method as
described in the following. Use of any other keying method for SRTP
security descriptions is for further study.
The "inline" type of key contains the keying material and all policy
relating to that 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 key lifetime has expired).
The key parameter contains a semi-colon separated list of
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 in the list follows the format (a
formal definition is provided in Section 8.2):
"inline:" <key salt> "|" [<lifetime] "|" [MKI:length / FromTo]
key||salt concatenated key and salt, base64 encoded
lifetime key lifetime (number of packets)
MKI:length MKI and length of the MKI field in SRTP packets.
FromTo <"From", "To"> values, specifying the lifetime for
a master key.
The following definition provides an example for
AES_CM_128_HMAC_SHA1_80:
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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
entire crypto attribute MUST be considered invalid and an "invalid
key/salt" condition SHOULD be logged. Each master key and salt MUST
be a cryptographically random number and MUST be unique to the SDP
message.
The second field, is the OPTIONAL lifetime of the master key as
measured in maximum total number of packets using that key. 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 and an
"invalid lifetime" condition SHOULD be logged. 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 the SRTP's and SRTCP's default
values will be used (2^31):
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.
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 if it exceeds 128 (bytes), then
the entire crypto attribute MUST be considered invalid and an
"invalid MKI length" condition SHOULD be logged. 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 key identifier of 1066 to the key.
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<"From", "To"> specifies the lifetime for a master key, expressed in
terms of the ROC and SEQ values inside whose range (including the
range end-points) 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. 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 MUST either include an MKI or a <"From", "To">
value. Note that it is not permissible to mix and match use of 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.
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 below are
described in the context of the SRTP security descriptions.
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 [srtp]. 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 SRTP authentication key is 32 bits and the SRTCP
authentication key is 80 bits.
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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.
5.2.3 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 guidelines in section 10.
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 and are described in the following.
5.3.1 SRC=SSRC/ROC/SEQ
The SRTP cryptographic context for a given SRTP session is
identified by the synchronization source (SSRC). Furthermore,
associated with a cryptographic context is the SRTP packet index
which is derived from the RTP sequence number (SEQ) and a rollover
counter (ROC). The SSRC and SEQ are included in the SRTP packets,
however they are not included in standard SDP (for various reasons).
The ROC is neither included in the SRTP packets nor standard SDP but
is instead derived algorithmically based on the total number of
packets sent. This presents a couple of challenges:
* If the master key is shared between two or more session
participants, SSRC collisions MUST be avoided; SSRC collision
detection and resolution is not an acceptable alternative as this
can lead to the two-time pad problem [srtp].
* If a participant joins an ongoing session (where the ROC is non-
zero), the participant needs to learn the ROC somehow.
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* If the initial sequence number is close to the maximum sequence
number and the initial SRTP packets are lost, the receiver may not
update his ROC correctly.
* When joining a multicast RTP session with multiple participants, a
separate crypto context needs to be established for each
participant (SSRC). Even if the same master key is used by all
participants, the ROC for each still needs to be learned somehow.
The SRC session parameter provides information to establish the SRTP
cryptographic context. It contains information about one or more of
the following:
* SSRC: Synchronization source
* ROC: Roll-over counter
* SEQ: Sequence number
The ROC and sequence number are typically only needed for sessions
already in progress (as when rekeying or when joining a multicast
session).
Zero or more SRC parameters MAY appear in a crypto attribute. When
more than one SRC parameter is present, each of them MUST contain a
distinct SSRC value. Each SRC parameter defines a separate SRTP
crypto context (see section 3.2 of [srtp]) that SHALL share the
master key and salt defined by one or more inline key parameters.
The total number of all packets that are encrypted by keys derived
from this master key MUST NOT exceed the lifetime of the inline key.
The SRTP crypto contexts so defined SHALL also have a common
definition for the crypto-suite and all other crypto parameters.
SSRC is the RTP SSRC that is associated with the crypto context, and
is an integer in the range of 0..2^32-1. If an SSRC value is
invalid, the entire crypto attribute line MUST be considered invalid
and an "invalid SSRC" condition SHOULD be logged. If an SSRC value
collides with an SSRC for an existing participant in the session,
the entire crypto attribute line MUST be considered invalid and an
"SSRC collision" condition SHOULD be logged.
OPEN ISSUE: It would be nice to have a way of indicating this
condition in an answer SDP, but we quickly end up duplicating the
RTP collision detection and resolution, which we don't want to.
ROC is the SRTP rollover counter (ROC) in the range of 0..2^32-1 and
is zero by default. Typically the ROC value is specified as a non-
zero value for an ongoing SRTP stream in which the ROC has cycled
one or more times [srtp]. The receiver of the SDP message SHOULD
refresh the ROC value before joining an ongoing session. Depending
on the nature of the session control, the late-joining receiver
might need to refresh its ROC value through a unicast exchange or
through receipt of a multicast or unicast message containing a ROC
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SRTP description. If the ROC is greater than 2^32-1, then the
entire crypto attribute line MUST be considered invalid and an
"invalid ROC" condition SHOULD be logged.
SEQ is the SRTP sequence number (SEQ), which MUST be in the range of
0..2^16-1. SRTP uses the RTP sequence number and the ROC to compute
the packet index [srtp]. For this reason, the initial SEQ SHOULD be
in the range of 0..2^15-1 to avoid an ambiguity when packets are
lost at the start of the session. At the start of a session, an
SSRC source that randomly selects a high sequence-number value can
put the receiver in an ambiguous situation: If initial packets are
lost in transit up to the point that the sequence number wraps
(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].
It is not necessary to signal SEQ and ROC at the start of the SRTP
session if the receivers do not join the session late, which is
typical in IP telephony, multimedia client/server, and similar
applications. Large-scale multicast applications, however, will
sometimes have late joiners to the session and MAY choose to use the
SRC session parameter to set the SEQ and the ROC. The SSRC MAY also
be initialized in the SRC parameter; this can for example be useful
to establish the crypto contexts (in particular the ROC) for all the
session participants.
Like SEQ and ROC, SSRC is OPTIONAL (unless there are multiple SRC
parameters in which case it is mandatory) and often need not be
signaled. If the master key is not shared among senders for their
encryption services, then SSRC uniqueness is NOT REQUIRED (see
Section 7.2) and the SSRC need not be signaled. In this way, each
master key is used for encryption by exactly one sender and used for
decryption by one or more receivers: In this case, there is no risk
of keystream reuse for the crypto-suite ciphers of Section 5.2.1,
5.2.2, and 5.2.3.
The SRTP crypto context can be established for the SRTP session
address in the connection (c=) line and the port in the media (m=)
line (or rtpmap) without having specified an SSRC value in the SRTP
security descriptions. This is called "late binding" by this
specification. If late binding is used, then when a packet arrives,
the SSRC that is contained in it can be bound to the crypto context
at the time of session commencement rather than at the time of
session signaling. With the arrival of the packet containing the
SSRC, all the data items (except the ROC if it is non-zero) needed
for the SRTP crypto context are held by the receiver. In other
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words, the crypto context for an RTP/SAVP session using late binding
is initially identified by the SDP as:
<*, address, port>
where '*' is a wildcard SSRC, "address" is from the "c=" line, and
"port" is 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). Endpoints
that do not wish to subject themselves to such security risks can
either signal the SSRC by out-of-band mechanisms (as defined here),
or ensure that only authenticated SRTP is being used.
5.3.2 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 {0,1,2,...,24}, which
denotes a power of 2 from 2^0 to 2^24, inclusive. The SRTP key
derivation rate controls how frequently a new session key is derived
from an SRTP master key [srtp]. The default value is 0, which
causes the key derivation function to be invoked exactly once (since
2^0 is 1).
5.3.3 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:
* UNENCRYPTED_SRTCP signals that the SRTCP packet payloads are not
encrypted.
* UNENCRYPTED_SRTP signals that the SRTP packet payloads are not
encrypted.
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5.3.4 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].
5.3.5 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.
5.3.6 Window Size Hint (WSH)
SRTP defines the SRTP-WINDOW-SIZE [SRTP, section 3.3.2] parameter to
protect against replay attacks. The minimum value, per [srtp], is
64, 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.
5.3.7 SRTP Extension 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 10.
SRTP extension session parameters are by default mandatory. An SRTP
extension session parameter that is prefixed with the dash character
("-") however is considered optional and MAY be ignored. If a SDP
is received with an unknown mandatory session parameter in a crypto
attribute, that crypto attribute MUST be considered invalid and a
"unknown session parameter" condition SHOULD be logged.
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6. SRTP-Specific Use of the crypto Attribute
In this section, we describe the SRTP-specific use of the crypto
attribute.
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.
6.1.1 Generating the Initial Offer
6.1.1.1 Unicast Streams
When the initial offer is generated, the offerer MUST follow the
steps in Section 4.1.1.1 as well as the following steps.
For each unicast media line (m=) using the "RTP/SAVP" 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 SRTP session parameters as
defined in Section 5.3. Note however, that if any extension SRTP
session parameters are included, the negotiation will fail if the
answerer does not support them.
6.1.1.2 Multicast Streams
When the initial offer is generated, the offerer MUST follow the
steps in Section 4.1.1.2 as well as the following steps.
For each multicast media line (m=) using the "RTP/SAVP" 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. Furthermore, the
<"From", "To"> parameter in the key parameter MUST NOT be used,
unless the media stream is marked as "recvonly".
The <"From", "To"> value is SSRC specific, and hence will only
work when there is a single sender in the multicast case, i.e. all
invited participants only receive media.
The offerer MAY include one or more SRTP session parameters as
defined in Section 5.3. Note however, that if any extension SRTP
session parameters are included, the negotiation will fail if the
answerer does not support them.
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6.1.2 Generating the Initial Answer
6.1.2.1 Unicast Streams
When the initial answer is generated, the answerer MUST follow the
steps in Section 4.1.2.1 as well as the following steps.
For each unicast media line using the "RTP/SAVP" transport that
contains one or more "a=crypto" lines in the offer, the answerer
MUST either accept 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 session parameters) MUST be acceptable to the
answerer in order for the offered media stream to be accepted.
When the answerer accepts an "RTP/SAVP" unicast media stream with a
crypto line, the answerer indicates acceptance by including its own
"a=crypto" line in the answer. The answer crypto line MUST include
at least the selected SRTP crypto-suite and one or more master keys
appropriate for the selected crypto algorithm; the master key(s)
included in the answer SHOULD be different from those in the offer.
If the master key(s) are not shared between the offerer and
answerer, SSRC collisions are acceptable, which simplifies the
overall operation.
Session parameters MAY be included in the answer as well; any
session parameters included in the answer are independent of session
parameters included in the offer. Use of extension SRTP session
parameters SHOULD be avoided unless it is known that the offerer
supports these.
If the answerer cannot find any valid crypto line that it supports,
or 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 "RTP/SAVP" by other means outside the
scope of this document (e.g., by use of MIKEY [mikey]).
6.1.2.2 Multicast Streams
When the initial answer is generated, the answerer MUST follow the
steps in Section 4.1.2.2 as well as the following steps.
For each multicast media stream using the "RTP/SAVP" transport that
contains an "a=crypto" line in the offer, the answerer MUST either
accept the first crypto line for that media stream (note that there
should only be one crypto line), or it MUST reject the media stream.
The crypto line MUST only be accepted if it is considered a valid
SRTP security description as defined in Section 5. Furthermore, all
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parameters (crypto-suite, key parameter, and session parameters)
MUST be acceptable to the answerer in order for the offered media
stream to be accepted.
When the answerer accepts an "RTP/SAVP" multicast media stream with
a crypto line, the answerer indicates acceptance by repeating the
crypto line from the offer in the answer, except for the session
parameters which SHOULD be excluded.
There is only a single view of a multicast stream (unlike
unicast), and hence there is no reason to repeat optional
parameters that cannot change anyway.
OPEN ISSUE: It is not clear that all session parameters should be
excluded from the answer. In particular, we may want to allow for
inclusion of the SRC parameter, as this would enable a new-comer
to instantiate crypto-contexts for other participants in a
multicast conference, provided the conference is using a shared
key. If each sender uses a unique key, something else would be
needed (e.g. an offer/answer exchange with each participant or an
entirely different mechanism).
If the answerer cannot find any valid crypto line that it supports,
or 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.
It should be noted, that multicast streams with more than one sender
that are negotiated by use of this mechanism will be using the same
master key for sending and receiving and hence SSRC collisions must
be avoided. The mechanism defined here does not provide a way to
avoid such SSRC collisions for multicast streams, and hence means
outside of the scope of this document are needed to ensure that SSRC
collisions are avoided. Examples of how this can be achieved
include a centralized controller supplying unique SSRCs to the
session participants or a separate protocol that can ensure SSRC
uniqueness prior to sending any SRTP packets.
6.1.3 Offerer Processing of the Initial Answer
6.1.3.1 Unicast Streams
When the offerer receives the answer, it MUST perform the steps in
Section 4.1.3.1 as well as the following steps for each "RTP/SAVP"
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.
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If the crypto line contains any SRTP session parameters, those
parameters define SRTP behavior for media sent from the answerer to
the offerer. If the offerer either does not support or is not
willing to honor one or more of the SRTP session 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.
6.1.3.2 Multicast Streams
When the offerer receives the answer, it MUST perform the steps in
Section 4.1.3.2 as well as the following steps for each "RTP/SAVP"
media stream it offered with a crypto line 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 crypto line includes any
session parameters, those are simply ignored.
OPEN ISSUE: As noted in Section 6.1.2.2, it may make sense to
allow for some session parameters, e.g. SRC, to be included.
If the crypto line is not valid, the SRTP security negotiation MUST
be deemed to have failed for that particular answerer.
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.
Unicast Streams:
* The offerer SHOULD include the ROC and SEQ (unless both are made
available to the answerer by other means); this enables the
answerer to establish the complete crypto context in case he
currently does not have the ROC.
Multicast Streams:
* When the media stream is "recvonly", the offerer SHOULD include
the ROC and SEQ (unless both are made available to the answerer by
other means); this enables the answerer to establish the complete
crypto context in case he currently does not have the ROC.
It should be noted, that the mechanism defined here does not provide
a way to communicate the ROC for multiple senders, which may be
needed in some multicast scenarios, e.g. conferencing. If
renegotiation is needed, a separate mechanism, such as [GDOI], will
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be needed for this. These methods are beyond the scope of this
document.
OPEN ISSUE: As noted in Section 6.1.2.2, it is not clear that we
couldn't do that with the SRC parameter.
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 5.1.1.1. 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 4.1.1.1, this could for example be addressed
by defining a security "precondition" [RFC3312]
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.
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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:AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP SRC=//49126
a=crypto:F8_128_HMAC_SHA1_80
inline:MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm|2^20|1:4
FEC_ORDER=FEC_SRTP SRC=//49126
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:AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
SRC=/721/13
In this case, the session would use the AES_CM_128_HMAC_SHA1_80
crypto suite for the RTP and RTCP traffic. The answerer is also
specifying both its current rollover counter (721), and sequence
number (13).
6.2 SRTP-Specific Use Outside Offer/Answer: Advertising
The SRTP security descriptions can be used outside the context of
offer/answer as described in Section 4.2. In those cases, the
general rules defined in Section 4.2 as well as the SRTP-specific
rule defined below MUST be followed:
* If any SRTP session parameters are included, they MUST be
supported by the recipient of the SDP; otherwise, the recipient
MUST NOT join the SRTP session.
6.3 SRTP-Specific Backwards Compatibility Considerations
It is possible that the answerer supports the "RTP/SAVP" transport
and accepts the offered media stream, yet it does not support the
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crypto attribute defined here. The offerer can recognize this
situation by seeing an accepted "RTP/SAVP" 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 transport set to "RTP/SAVP" is sent to
a device that does not support "RTP/SAVP", that media stream will be
rejected.
6.4 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 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 [SDPnew]. Per
SDP rules, the answerer will ignore attribute lines it does not
understand. If the answerer supports both "a=crypto" and "k=", the
answer MUST include either "a=crypto" or "k=" but not both, as
including both is undefined.
6.5 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 and SEQ), 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 once the crypt context
is removed.
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.
7. Security Considerations
Like all SDP messages, SDP messages containing security
descriptions, are conveyed in an encapsulating application protocol
(e.g. SIP, MGCP, RTSP, SAP, etc.). It is the responsibility of the
encapsulating protocol to ensure the protection of the SDP security
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descriptions. Therefore, the application protocol SHOULD either
invoke its own security mechanisms to do so, or alternatively
utilize a lower-layer security service (e.g. TLS, 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). Source authentication (i.e. data-origin
authentication) of unicast SRTP messages SHOULD be performed [srtp].
7.2 Keystream Reuse
Security descriptions as defined herein 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, as this document
RECOMMENDS (see Section 6.1.2.1), i.e. keys are not shared between
multiple media streams, and the keys used in the send and receive
direction for a given media stream are unique.
SRTP multicast operation requires that each host-sender have a
unique SRTP keystream. This can be accomplished by ensuring that
each sender be allocated a unique key or by ensuring that the SSRC
of each sender will not collide. Since SSRC collision might occur,
the latter condition is avoided when all SSRCs are assigned by a
central authority such as a 3rd-party key server [srtp]. The
RECOMMENDED approach of this document is to allocate a different
master key for each host-participant of an SRTP session.
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
its parameters are vulnerable to denial of service attacks when they
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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, 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 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 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.
There are practical cases, however, where SDP security is not end-
to-end: If there is a third-party provider between the sender and
receiver, then the data-security session might not be end-to-end.
That is, one possible configuration might have an IPsec or TLS
connection between the sender of the SDP message and the provider,
such as a VoIP service provider, with a second secure connection
between the provider and the receiver:
signaling controller---(network-b)---signaling controller
| |
(network a) (network c)
| |
sender----------------(SRTP bearer)--------------receiver
where all of link a, b, and c are encrypted with TLS or IPsec.
In this case, the third-party provider has access to the contents of
the SRTP descriptions in the SDP message. SDP key-mgmt statement,
however, allows true end-to-end security that is independent of the
service provider, who often needs access to some parts of the SDP
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message to render its services. The SRTP attribute SHOULD NOT be
used when end-to-end authentication or confidentiality is needed but
the SDP message is not secured end-to-end (such as the above example
where a third-party provider maintains the security associations
with the endpoints for the SDP message).
8. Grammar
8.1 Generic "Crypto" Attribute Grammar
The ABNF grammar for the crypto attribute is defined below:
"a=crypto:" crypto-suite 1*WSP key-params *(1*WSP session-param)
crypto-suite = 1*(ALPHA / DIGIT / "_")
key-params = key-param *(";" key-params)
key-param = key-method ":" key-info
key-method = "inline" | key-method-ext
key-method-ext = 1*(ALPHA / DIGIT / "_")
key-info = %x21-3A / %x3B-7E ; visible (printing) characters
; except semi-colon
session-param = 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 5.1.1.1 for "2^"
mki = mki-value ":" mki-length
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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.
srtp-session-param = src /
kdr /
"UNENCRYPTED_SRTP" /
"UNENCRYPTED_SRTCP" /
"UNAUTHENTICATED_SRTP" /
fec-order /
wsh /
srtp-session-extension
src = "SRC=" [ssrc] "/" [roc] "/" [seq]
ssrc = 1*DIGIT ; range 0..2^32-1
roc = 1*DIGIT ; range 0..2^32-1
seq = 1*DIGIT ; range 0..2^16-1
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. Open Issues
The following is a list of open issues in this document:
* The use of security descriptions, and in particular SRTP security
descriptions, with multicast streams where offer/answer is being
used is not well understood and requires further consideration.
* The security descriptions do not deal with hierarchically encoded
streams (or at least they have not been considered).
* The current mechanism does not allow for a key to be specified as
being an encryption or decryption key or both; instead this is
inferred from the context (e.g. unicast offer). Should there be a
mechanism to allow a key to be tagged as an encryption, decryption
or both key ?
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10. IANA Considerations
10.1 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
10.2 New IANA Registries and Registration Procedures
The following sub-sections define several new IANA registries to be
used for the security descriptions. It is suggested that the
following registry structure be used for these:
Security Descriptions
|
+- Key Methods (described in 10.2.1)
|
+- Media Stream Transports
|
+- SRTP
|
+- SRTP crypto suites (described in Section 10.2.2)
|
+- SRTP session parameters (described in Section 10.2.3)
10.2.1 Security Descriptions Key Method Registry and Registration
The IANA is hereby requested to create a new registry for SDP
security description key methods. An IANA key method registration
MUST be documented in an IETF 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.
10.2.2 SRTP Crypto Suite Registry and Registration
The IANA is hereby requested to create a new registry for SRTP
crypto suites. An IANA crypto suite registration MUST indicate the
crypto suite name in accordance with the grammar for srtp-crypto-
suite-ext defined in Section 8.2.
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The semantics of the crypto suite MUST be described in an IETF RFC,
including the semantics of the "inline" key-method and any special
semantics of parameters.
10.2.3 SRTP Session Parameter Registration
The IANA is hereby requested to create a new registry for SRTP
session parameters. 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.
10.3 Initial Registrations
The following security descriptions key methods are hereby
registered:
inline
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 following SRTP session parameters are hereby registered:
SRC
KDR
UNENCRYPTED_SRTP
UNENCRYPTED_SRTCP
UNAUTHENTICATED_SRTP
FEC_ORDER
WSH
The ABNF for all of the above is already included in the ABNF
section of this document.
11. 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 David McGrew, Mats Naslund, Mike
Thomas, Elisabetta Cararra, Brian Weis, Dave Oran, Bill Foster, Earl
Carter, Matt Hammer and Dave Singer. These people shared
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observations, identified errors and made suggestions for improving
the specification. 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. Jonathan Rosenberg suggested reducing
the complexity by specifying only one security parameter for each
media stream.
12. 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
+1-408-853-4418
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134 USA
dwing@cisco.com
+1-408-902-3348
13. 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.
[SDPnew] 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.
Andreasen, Baugher & Wing [Page 33]
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[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.
14. 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.
[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.
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[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 .
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Andreasen, Baugher & Wing [Page 36]
