Internet Engineering Task Force Mark Baugher
MMUSIC Working Group Dan Wing
INTERNET-DRAFT Cisco Systems
EXPIRES: August 2003 February 24, 2003
SDP Security Descriptions for Media Streams
<draft-ietf-mmusic-sdescriptions-00.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This Internet Draft gives a cryptographic attribute to Session
Description Protocol (SDP) media streams. The attribute describes a
cryptographic key and other parameters, which serve to configure
security for a media stream. This draft also defines the SRTP
parameters for the attribute. The SDP crypto attribute requires the
services of a data security protocol to secure the SDP message.
INTERNET-DRAFT SDP Security Descriptions February 24, 2003
TABLE OF CONTENTS
1.0 Notational Conventions...........................................2
2.0 Introduction.....................................................3
3.0 SDP "Crypto" Attribute and Parameters............................4
3.1 Crypto-suite....................................................4
3.2 Application Parameter...........................................4
3.3 Key Parameter...................................................4
3.4 Session Parameters..............................................5
3.5 Examples........................................................5
4.0 RTP/SAVP (SRTP) Security Descriptions............................6
4.1 Crypto-suites...................................................7
4.1.1 AES_CM_128_HMAC_SHA1_80.....................................7
4.1.2 AES_CM_128_HMAC_SHA1_32.....................................7
4.1.3 F8_128_HMAC_SHA1_80.........................................7
4.1.4 F8_128_HMAC_SHA1_32.........................................7
4.1.5 Adding new CRYPTO-SUITE definitions.........................8
4.2 Application Parameter...........................................8
4.3 Key Parameter...................................................8
4.3.1 INLINE Usage................................................8
4.3.2 INLINE Definition...........................................9
4.4 Session Parameters.............................................10
4.4.1 SSRC=n.....................................................10
4.4.2 ROC=n......................................................11
4.4.3 KEY_DERIVATION_RATE=n......................................11
4.4.4 UNENCRYPTED................................................11
4.4.5 FEC_ORDER=order............................................12
4.4.6 UNAUTHENTICATED............................................12
5.0 Use with Offer/Answer...........................................12
5.1 Offerer Processing.............................................12
5.2 Answerer Processing............................................13
5.3 Non-RTP/SAVP Answerers.........................................13
5.4 Offer/Answer Example: Receiver Supports SRTP...................14
5.5 Offer/Answer Example: Different SRTP and SRTCP keys............14
5.6 Use of a=crypto With Active Media Streams......................15
6.0 Security Considerations.........................................16
6.1 Authentication of packets......................................16
6.1 Reuse of Keying Material ("Two-Time Pad")......................16
6.2 Signaling Authentication and Signaling Encryption..............17
7.0 Grammar.........................................................18
8.0 Acknowledgements................................................19
9.0 Authors' Addresses..............................................20
10.0 References.....................................................20
11.0 Full Copyright Statement.......................................21
1.0 Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. The
terminology conforms to [RFC2828].
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2.0 Introduction
Session Description Protocol (SDP) describes multimedia sessions,
such as Real-time Transport Protocol (RTP), white board, fax, modem
and other media sessions. Security services such as authentication
and confidentiality are often needed for these media streams. When
run under the RTP/SAVP profile, for example, an RTP stream uses the
Secure Real-time Transport Protocol (SRTP). The "RTP/SAVP"
descriptor in an SDP m=line signals the use of SRTP for a media
stream, but there are no means within SDP itself to configure SRTP
beyond using defaults values. This document specifies an SDP
attribute to signal a cryptographic parameters in addition to a key
for SRTP and other SDP media streams.
Thus, the SDP crypto attribute provides both generic and specific
security descriptions for SDP media streams that can be used for
various transports, including SRTP. In this way, the crypto
attribute can be extended to non-SRTP transports such as white
board, modem, fax, and other transports that could use various
security protocols such as IPsec or TLS. Each SDP media stream,
however, needs its own definitions that assign values to crypto-
attribute parameters. These definitions are unique to the SDP
transport and SHOULD be specified in an Internet RFC. This document
defines the parameter values for SRTP. With this document, an
application developer can describe an SRTP key and its configuration
according to application-specific needs.
It would be self-defeating, however, to not secure cryptographic
keys and other parameters at least as well as SRTP secures RTP
messages 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 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, GDOI, KINK, IKE or TLS securely disseminates
information describing both the key and the data-security session
(for example, whether SRTCP is 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 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]. SDP
media-stream security descriptions are suitable for restricted cases
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where IPsec, TLS, or some other data-security protocol protects the
SDP message.
This draft adds security descriptions to SDP messages through a new
SDP attribute named "crypto", which provides the cryptographic
parameters of a media stream. The crypto attribute MAY contain a
cryptographic key and other parameters that describe the key.
a=crypto MAY also contain "security session parameters" that are
unique to a transport.
The a=crypto parameter is applicable to all media transports, but
its value MAY be unique to a particular transport. Section 3
specifies the SDP crypto attribute generically. Section 4 defines
the crypto attribute for SRTP. Section 5 discusses use of the
crypto attribute in Offer/Answer exchanges. Section 6 recites
security considerations, and Section 7 gives an Augmented-BNF
grammar for the security descriptions.
3.0 SDP "Crypto" Attribute and Parameters
A new media-level SDP attribute called "crypto" describes the
cryptographic and security-session parameters for one or more media
entries. "a=crypto" MUST NOT appear at the SDP session level.
a=crypto:<crypto-suite> <application> <key> [<session>]
The ordering of multiple a=crypto lines is significant, and the
most-preferred is listed first; see section 5. The next sections
describes these fields in more detail.
3.1 Crypto-suite
"Crypto-suite" describes all needed information about the encryption
and authentication algorithms for the transport. The "crypto-suite"
parameter is unique to the transport.
3.2 Application Parameter
A particular transport can have multiple protocols that are secured
differently. For example, when using the RTP/SAVP transport, both
the SRTP and SRTCP protocols will be used, but the security for each
MAY be different: A longer authentication output tag might be
desired for the SRTCP control protocol than for the SRTP media
stream. The "application" parameter allows separate security
descriptions for separate protocols of a transport.
3.3 Key Parameter
The key parameter can contain an inline key descriptor, or can be a
pointer to a uri which contains the actual key:
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inline:key-descriptor
uri:absolute-uri
If the key parameter starts with the string "uri:", the URI method
is used and the value that follows is a Uniform Resource Identifier.
The URI is a resource that SHOULD be queried to obtain the
cryptographic key for the session. The format or protocols used for
the uri are beyond the scope of this document.
The INLINE method is invoked when the key parameter starts with the
string "inline:" - and the cryptographic key is encoded according
to a transport-specific syntax. Thus, the URI method is transport
generic and the INLINE method is transport specific. Section 4
describes the INLINE key-parameter syntax for RTP/SAVP, the SRTP
media transport type.
If SDP descriptions for new media-stream transports are defined in
the future, new methods MAY be defined in an Internet RFC.
3.4 Session Parameters
"Session" parameters are specific to the SDP transport and optional.
Section 4 describes the session parameters for RTP/SAVP.
3.5 Examples
The first example shows a=crypto for the RTP/SAVP transport type (as
defined in Section 4).
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 224.2.17.12/127
t=2873397496 2873404696
a=recvonly
m=video 51372 RTP/SAVP 31
a=crypto:AES_CM_128_HMAC_SHA1_80 both
inline:16/14/d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj/2^20/1:32
m=audio 49170 RTP/SAVP 0
a=crypto:AES_CM_128_HMAC_SHA1_32 srtp
inline:16/14/NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj/2^20/1:32
a=crypto:AES_CM_128_HMAC_SHA1_80 srtcp
inline:16/14/ ZkBkQythOTg3NjU0MSEzMDMyMT01NDg5N2RlRkF/2^20/1:32
m=application 32416 udp wb
a=orient:portrait
This SDP message describes three "recvonly" media streams, two of
which use the RTP/SAVP transport. The first a=crypto line appears
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in the m=video media entry; it is associated with the RTP/SAVP
transport of the m=video line and uses the SRTP default crypto-
suite, "AES_CM_128_HMAC_SHA1_80," and its key parameter carries the
SRTP master key data and descriptors inline. The m=video a=crypto
attribute applies to both SRTP and SRTCP. The m=audio media entry
uses the "crypto-suite=AES_CM_128_HMAC_SHA1_32," having a short 32-
bit tag for SRTP, and it uses AES_CM_128_HMAC_SHA1_80 for SRTCP.
The RTP/SAVP-specific descriptions are defined in the next section.
4.0 RTP/SAVP (SRTP) Security Descriptions
The generic security descriptions of the preceding section need
parameter values for specific media transports; this section defines
the crypto attribute values and parameters for the RTP/SAVP (SRTP)
transport. SRTP services for a media stream MUST be signaled
through the presence of an RTP/SAVP transport descriptor in the m=
line and SHALL apply only to that media entry.
There is no assurance that a receiver is capable of configuring its
SRTP service with a particular crypto attribute parameter, but SRTP
guarantees minimal interoperability among SRTP systems through the
default SRTP parameters [srtp]. More capable SRTP receivers support
a variety of parameter values beyond the SRTP defaults and can be
configured by the crypto attribute. A receiver that does not
recognize a=crypto and assumes default SRTP parameters might receive
a stream that uses non-default parameters, which will cause that
receiver to fail. An Offer/Answer capabilities exchange, however,
allows sender and receiver to agree on parameters before
commencement of the multimedia session (see Section 5.0).
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, however, there is usually no need to provide unique
settings for many of the SRTP parameters. Thus, it is possible to
simplify the list of parameters in "cryptographic suites" that fix a
set of SRTP parameter values for the security session. The list of
SRTP parameters, including the crypto-suite parameter for SDP
a=crypto follows.
SDP SRTP Parameter Description
------------------ -----------
CRYPTO-SUITE Encryption and authentication transforms
SSRC SSRC of the sender of the SDP message
ROC Roll-over counter
KEY_DERIVATION_RATE Rate that the pseudo-random function
is applied to a key
UNENCRYPTED Protocol messages are not encrypted
UNAUTHENTICATED SRTP messages are not authenticated
FEC_ORDER Order of forward error correction (FEC)
relative to SRTP services
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Please refer to the SRTP specification for a complete list of
parameters and their descriptions [Section 8.2, srtp]. The CRYPTO-
SUITE and the five session parameters shown in the table above are
described in the following sections. If a receiver cannot recognize
a parameter or value, then the receiver MUST NOT participate in the
media stream and SHOULD log an "invalid name" condition unless the
receiver is participating in an Offer/Answer exchange (Section 5).
4.1 Crypto-suites
A crypto-suite value appears as the first parameter in a=crypto. The
CRYPTO-SUITE value MAY be different for SRTP and SRTCP as described
in Section 4.2. If a receiver does not support the particular
crypto-suite, then the receiver MUST NOT participate in the media
stream and SHOULD log an "unrecognized crypto-suite" condition
unless the receiver is participating in an Offer/Answer exchange
(Section 5). RTP/SAVP has four crypto-suites as described below.
4.1.1 AES_CM_128_HMAC_SHA1_80
This is the SRTP default AES Counter Mode cipher and HMAC-SHA1
message authentication having a 80-bit authentication tag. The
encryption and authentication key lengths are 128 bits. The master
salt value is 112 bits and the session salt value is 112 bits. The
PRF is the default SRTP pseudo-random function that uses AES Counter
Mode with a 128-bit key length.
4.1.2 AES_CM_128_HMAC_SHA1_32
The SRTP AES Counter Mode cipher is used with HMAC-SHA1 message
authentication having an 32-bit authentication tag. The encryption
and authentication key lengths are 128 bits. The master salt value
is 112 bits and the session salt value is 112 bits. These values
apply to SRTP and to SRTCP. The PRF is the default SRTP pseudo-
random function that uses AES Counter Mode with a 128-bit key
length.
4.1.3 F8_128_HMAC_SHA1_80
The SRTP f8 cipher is used with HMAC-SHA1 message authentication
having a 80-bit authentication tag. The encryption and
authentication key lengths are 128 bits. The master salt value is
112 bits and the session salt value is 112 bits. The PRF is the
default SRTP pseudo-random function that uses AES Counter Mode with
a 128-bit key length.
4.1.4 F8_128_HMAC_SHA1_32
The SRTP f8 cipher is used with HMAC-SHA1 message authentication
having a 32-bit authentication tag. The encryption and
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authentication key lengths are 128 bits. The master salt value is
112 bits and the session salt value is 112 bits. The PRF is the
default SRTP pseudo-random function that uses AES Counter Mode with
a 128-bit key length.
4.1.5 Adding new CRYPTO-SUITE definitions
If new transforms are added to SRTP, new definitions for those
transforms SHOULD be given for the SDP crypto attribute and
published in an Internet RFC. Sections 4.1.1 through 4.1.4
illustrate how to define CRYPTO-SUITE values for particular
cryptographic transforms. New definitions MAY be added to existing
transforms, moreover, to augment or modify definitions 4.1.1 through
4.1.4.
4.2 Application Parameter
The "application" parameter indicates if this a=crypto line applies
to only secure RTP, only secure RTCP, or to both secure RTP and
RTCP. The values for this are "srtp", "srtcp", or "both". If a
receiver finds an "srtp" a=crypto without a corresponding "srtcp"
a=crypto, or vice versa, it MUST NOT participate in the media stream
and SHOULD log an "missing crypto attribute" condition.
4.3 Key Parameter
If the "key" parameter has a "uri:" descriptor, the value is a
Uniform Resource Identifier value as described in Section 3. When
key-parameter has an "inline:" descriptor, the value contains a
cryptographic key that MUST be a unique random value with respect to
other "inline:" values in the SDP message.
4.3.1 INLINE Usage
The "inline:" descriptor is applicable to SDP media-entries having a
"recvonly," "sendonly" or "sendrecv" direction attribute. In
general, the source of data will generate the master key to protect
its data, but this is a matter of local policy and application
preference. Multicast applications, for example, often will use a
third-party provider of a master key. Thus, when the inline key is
used, it SHOULD be used for a recvonly media-entry or for the
received stream of sendrecv media-entry. The inline key MAY be used
for a sendonly media-entry or for streams that are sent and received
on a sendrecv media-entry. The following paragraphs add detail to
these inline-key recommendations for recvonly, sendonly, and
sendrecv media entries.
In the recvonly case, the inline SRTP master key SHOULD be used to
derive keys [SRTP] to decrypt/authenticate incoming SRTP messages.
When the a=crypto "application" parameter is set to "both," the
receiver also derives keys from the same master key to
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decrypt/authenticate incoming SRTCP messages. When that receiver
sends RTP Receiver Reports for the incoming SRTP stream, it SHOULD
derive keys from the same master key to encrypt/authenticate
outgoing SRTCP messages for that SRTP stream.
In the sendonly case, the inline SRTP master key SHOULD be used to
derive keys [SRTP] to encrypt and authenticate outgoing SRTP
messages. When the a=crypto "application" parameter is set to
"both," the sender also derives keys from the same SRTP master key
to encrypt and authenticate outgoing SRTCP message. When that
sender sends RTP Sender Reports for the outgoing SRTP stream, it
SHOULD derive keys from the same master key to encrypt/authenticate
outgoing SRTCP messages for the outgoing SRTP stream.
In the sendrecv case, the inline SRTP master key SHOULD be used as
in the recvonly case described above but MAY also be used as in the
sendonly case.
4.3.2 INLINE Definition
If the identifier is "inline", the key-descriptor MUST have the
following format.
key_length/salt_length/BASE64(key||salt)/lifetime/MKI:MKI_length
The "key_length" is the integer length of the SRTP master key in
bytes, and "salt_length" is the integer length of the master salt in
bytes. If their sum is less than the sum of the lengths of the
master key and salt of the crypto suite, then the receiver MUST NOT
participate in the media stream and SHOULD log a "key length too
short" condition. If their sum is greater than the crypto-suite sum,
then bytes are truncated from the right (i.e. "little end"). The
key_length and salt_length MUST appear in the "inline" encoding. For
example,
inline:16/14/d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj/2^20/1:32 (1)
has a key_length of 16 and a salt_length of 14.
The third part of the "inline" encoding is the cryptographic master
key appended with the master salt ("||" denotes concatenation).
Each master key and salt MUST be a random number and MUST be unique
to the SDP message. Both are concatenated and then base-64 encoded.
If the length of the concatenated keys (after being decoded from
base 64) does not equal or exceed the sum of the key_length and
salt_length, the receiver MUST NOT participate in the media stream
and SHOULD log a "inline encoding too short" condition. For
example,
inline:16/8/YUJDZGVmZ2hpSktMbW9QUXJzVHVWd3l6//1066:32 (2)
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has a key_length of 16, a salt_length of 8, and a 32-character key
and concatenated salt that is base-64 encoded: The 24-character
key/salt concatenation is expanded to 32 characters by the three-in-
four encoding of base 64.
The fourth part of the of the "inline" encoding is the OPTIONAL
lifetime of the master key as measured in number of packets
encrypted or authenticated with that key. The lifetime value MAY be
written as an non-zero, positive integer or as a power of 2, and is
indicated with "2^"; see the ABNF in Section 7 for details. The
default value is 2^48, which is 2^48 packets encrypted with a master
key according to the SRTP standard [srtp]. The "lifetime" value
MUST NOT exceed the maximum packets lifetime for the crypto-suite
(e.g. 2^48 for AES Counter Mode with a 128-bit key). If lifetime is
too large or otherwise invalid, then the receiver MUST NOT
participate in the media stream and SHOULD log an "invalid lifetime"
condition. The default MAY be implicitly signaled by having no
described value for lifetime (i.e. "//"). This is convenient when
the srtp crypto_key lifetime is allowed to default. Trailing
slashes ("/") MUST follow the master key and lifetime; otherwise,
the receiver MUST NOT participate in the media stream and SHOULD log
an "invalid inline encoding" condition. Example (1), above, shows a
case where the lifetime is specified as 2^20 while example (2) shows
an empty lifetime that implicitly uses the SRTP default value of
2^48.
The MKI value is OPTIONAL as is its specified bit length (see
Section 7). "MKI" is the master key index 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 bits, and MUST be a positive multiple of 8. If the
MKI_length is not given or if the value exceeds 128, then the
receiver MUST NOT participate in the media stream and SHOULD log an
"invalid MKI_length" condition. If the value of the MKI is larger
than allowed by MKI_length, then the receiver MUST NOT participate
in the media stream and SHOULD log an "invalid MKI" condition. The
substring "1:32" in example (1) assigns to the key a key index of 1
that is 32 bits long, and example (2) assigns a 32-bit key index of
1066 to the key.
4.4 Session Parameters
The "session" parameters are OPTIONAL and MAY override SRTP session
defaults for the SRTP and SRTCP streams. These parameters configure
an RTP session for SRTP services.
4.4.1 SSRC=n
The value n is an integer in the range of 0..2^32-1 for the RTP SSRC
parameter, which is undefined by default. This is the RTP SSRC of
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the sender of the SDP message. If n is invalid, the receiver MUST
NOT participate in the media stream but SHOULD log an "invalid SSRC"
condition.
SSRC MAY be specified when the setting of the "application"
parameter is "srtp" or "both." Otherwise the receiver MUST NOT
participate in the media stream and SHOULD log an "invalid session
parameter" condition.
4.4.2 ROC=n
The value "n" is an integer in the range of 0..2^32-1 for the SRTP
rollover counter (ROC), which is zero by default. The ROC MAY be
set to a non-zero value for an ongoing RTP/SAVP stream in which the
SRTP ROC has cycled one or more times [srtp]. The receiver of the
SDP message SHOULD refresh the ROC value before joining a session
"late." How "late" is defined depends on the rate of the particular
RTP stream and the time that has elapsed since its commencement.
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 SDP message. If n is
invalid, then the receiver MUST NOT participate in the media stream
but SHOULD log an "invalid ROC" condition.
ROC MAY be specified when the setting of the "application" parameter
is "srtp" or "both." Otherwise the receiver MUST NOT participate in
the media stream and SHOULD log an "invalid session parameter"
condition.
4.4.3 KEY_DERIVATION_RATE=n
The value n may be an integer in the set {1,2,4,...,2^24}, i.e. a
power of 2 between 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.
Key_Derivation_Rate MAY be specified when the "application"
parameter setting is "srtp" or "both". Otherwise the receiver MUST
NOT participate in the media stream and SHOULD log an "invalid
session parameter" condition.
4.4.4 UNENCRYPTED
This indicates that the SRTP or SRTCP stream is not encrypted. SRTP
and SRTCP messages are encrypted by default.
UNENCRYPTED MAY be specified for "srtp", "srtcp", or "both". If the
a=crypto "application" setting is "both," then both the SRTP and
SRTCP streams are unencrypted.
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4.4.5 FEC_ORDER=order
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 on the sender and after SRTP processing on
the receiver; FEC_SRTP is the default. SRTP_FEC is the reverse
processing. SPLIT signals that SRTP encryption occurs on the
sender, followed by FEC processing, followed by SRTP authentication;
processing is reversed on the receiver. If the receiver cannot
recognize the order value, then the receiver MUST NOT participate in
the media stream but SHOULD log an "invalid FEC_ORDER" condition.
If specified, it MUST only be specified with "srtp" or "both."
Otherwise the receiver MUST NOT participate in the media stream and
SHOULD log an "invalid session parameter" condition.
4.4.6 UNAUTHENTICATED
This parameter signals that SRTP messages are not authenticated.
SRTP authenticates SRTP messages by default (see Security
Considerations).
If specified, it MUST only be specified with "srtp", or "both" since
it applies only to the SRTP stream: Authentication is mandatory for
secure RTCP. If UNAUTHENTICATED appears in an a=crypto with an
"srtcp" application parameter, the receiver MUST NOT participate in
the media stream and SHOULD log an "invalid session parameter"
condition.
5.0 Use with Offer/Answer
A receiver that does not recognize a=crypto and assumes default SRTP
parameters might receive a stream that uses non-default parameters,
which will cause that receiver to fail. An Offer/Answer
capabilities exchange, however, allows sender and receiver to agree
on parameters before commencement of the multimedia session. The
sender and receiver use an Offer/Answer exchange [RFC3264] to match
cryptographic capabilities.
This section discusses Offer/Answer exchanges only for the RTP/SAVP
(SRTP). A future revision of this document will consider
Offer/Answer exchanges for security descriptions in general.
5.1 Offerer Processing
On each SDP media line (m=) where the <transport> is "RTP/SAVP", the
offerer MAY follow that media line with at least one a=crypto line.
The lines are specified in preference order, with the most preferred
listed first. The offerer determines the crypto parameters based on
its capabilities and its security policies.
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To specify a list of preferred crypto suites for RTP, RTCP, or both,
the offerer includes separate a=crypto lines, in preference order.
Each offer is grouped. If separate rtp and rtcp keys are wanted,
the srtp a=crypto line MUST be sent first, and both the RTP and RTCP
keys MUST always be sent, even if the endpoint is recvonly.
The offerer obtains keying material or the uri pointing to a
keyserver by means that are outside the scope of this specification
(e.g. the offerer could generate the material or request the
material from a third party).
5.2 Answerer Processing
For each SDP media line where the <transport> is RTP/SAVP and the
stream is bi-directional stream will be created, the answer MUST
include a media line with its <transport> set to RTP/SAVP in order
to accept the offer; otherwise, the offer is rejected for that media
entry.
The answerer examines the a=crypto lines, in order. If the a=crypto
line indicates the application is rtp, and the line immediately
following indicates the application is rtcp, the receiver groups
these two lines together; otherwise, this group is ignored as it is
syntactically invalid. If the a=crypto line indicates the
application is "both", it is grouped by itself.
After grouping, the answerer selects the first group of a=crypto
that it supports, considering the answerer's capabilities and its
security policies.
After selecting one group, the answerer obtains keys appropriate for
the selected crypto algorithm(s). The key MUST have the same key
length and salt length as the offer.
To set up the bi-directional media with <transport> set to RTP/SAVP,
the answerer includes one or two a=crypto lines after the media
line. If the offer indicated separate keys for RTP and RTCP, the
answer MUST do the same.
5.3 Non-RTP/SAVP Answerers
If a media line with <transport> set to RTP/SAVP is sent to a device
that doesn't suppport RTP/SAVP, that media line will not be
processed.
If an offerer wants to interoperate with such a device, albeit
without the benefits of SRTP or SRTCP, the offerer MUST include an
additional media line with <transport> set to RTP/AVP, and other
values in that line MUST match the values of the associated
"RTP/SAVP" media line. This second media line would be specified
after all of the attribute (a=) lines for the RTP/SAVP media
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5.4 Offer/Answer Example: Receiver Supports SRTP
In this example, the offerer supports two crypto suites (F8 and
AES). When presented with multiple "a=crypto" lines for an "m="
line, the answerer will chose the first crypto suite that it
supports, and the answerer MUST reply with only one "a=crypto" line
per "m=" line
Offerer transmits:
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 224.2.17.12/127
t=2873397496 2873404696
a=recvonly
m=audio 49170 RTP/SAVP 0
a=crypto:AES_CM_128_HMAC_SHA1_80 both
inline:16/14/WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz/20/1:32
a=crypto:F8_128_HMAC_SHA1_80 both
inline:16/14/MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm/20/1:32
Answerer transmits:
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 224.2.17.11/128
t=2873397526 2873405696
a=sendonly
m=audio 32640 RTP/SAVP 0
a=crypto:F8_128_HMAC_SHA1_80 both
inline:16/14/MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm/20/1:32
In this case, the session would use the F8_128_HMAC_SHA1_80 crypto
suite for the RTP and RTCP traffic it generates.
5.5 Offer/Answer Example: Different SRTP and SRTCP keys
In this example, the offerer requests use of one crypto suite for
SRTP (AES) and a different crypto suite for RTCP.
Offerer transmits:
v=0
o=sam 2890844526 2890842807 IN IP4 10.47.16.5
s=SRTP Discussion
i=A discussion of Secure RTP
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u=http://www.example.com/seminars/srtp.pdf
e=marge@example.com (Marge Simpson)
c=IN IP4 224.2.17.12/127
t=2873397496 2873404696
a=recvonly
m=audio 49170 RTP/SAVP 0
a=crypto:AES_CM_128_HMAC_SHA1_80 rtp
inline:16/14/WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz/20/1:32
a=crypto:F8_128_HMAC_SHA1_80 rtcp
inline:16/14/MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm/20/1:32
Answerer transmits:
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 224.2.17.11/128
t=2873397526 2873405696
a=sendonly
m=audio 32640 RTP/SAVP 0
a=crypto:AES_CM_128_HMAC_SHA1_80 rtp
inline:16/14/8NxJiu9zZW1jdGwgKCkgewkyMjA7fQp9CnVubTnC/20/1:32
a=crypto:F8_128_HMAC_SHA1_80 rtcp
16/14/c2VtZ2V0KSkgewogICAgc3V/20/1:32
5.6 Use of a=crypto With Active Media Streams
When an active SRTP session is rekeyed, this is indicated by sending
a new SDP. Rekey MUST NOT be done with an Offer/Answer exchange,
but rather as a unidirectional SDP transmission.
When the Offerer needs to rekey, the Offerer MUST only send a=crypto
lines which match a=crypto lines it had received in the Answer.
When an SRTP or SRTCP transmitter needs to rekey, the only new
values permitted in the a=crypto line(s) are the new key and new
salt -- other cryptographic parameters MUST NOT be changed.
If the Answerer selected a=crypto lines using the "inline" method,
the exact same a=crypto line(s) as agreed to in the Answer MUST be
sent and a new new inline key MUST be sent.
If the Answer selected a=crypto lines using the "uri" method, the
sender MAY transmit the same uri, and the recipient MUST re-fetch
the uri.
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6.0 Security Considerations
One needs to define SDP security descriptions for a specific SDP
media transport for a=crypto to be useful. The definitions SHOULD
be specified in an Internet RFC, which has security implications
that MUST be considered in the RFC. This section considers the SRTP
descriptions for the RTP/SAVP transport as specified in this
Internet Draft.
6.1 Authentication of packets
RTP messages are vulnerable to a variety of attacks such as replay
and forging. To limit these attacks, SRTP message integrity and
anti-replay mechanisms SHOULD be used. Source authentication of
unicast SRTP messages SHOULD be performed [srtp]. Note that SRTP
source-message authentication does not authenticate the IP-address
of the SRTP source, but ensures that the SRTP message that the SRTP
receiver had received is exactly what the SRTP sender had sent.
Source authentication of multicast SRTP messages is today non-
standard and hence for further study. Use of the UNAUTHENTICATED
parameter therefore, is NOT RECOMMENDED. SRTP supports this setting,
however, for voice applications where authentication is implicit in
the application [srtp]. In general, applications SHOULD NOT set
UNAUTHENTICATED.
6.1 Reuse of Keying Material ("Two-Time Pad")
Misconfigured SRTP sessions, moreover, are vulnerable to attacks on
their encryption services when running crypto suites of Sections
4.1.1, 4.1.2 and 4.1.3. An SRTP encryption service is "mis-
configured" 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
make them unique, which is violated in the case of SSRC collision:
RTP SSRC collision reveals SRTP or SRTCP plaintext 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, as this document requires (Section 4.2). There is risk of
attack, however, when an SDP media stream has an "a=sendrecv"
direction attribute and a pair of senders are sharing a master key
for their encryption (i.e. a weaker condition than sharing a master
key). It is RECOMMENDED, therefore, that a sendrecv stream have two
SRTP master keys, one for each directional stream. By implication,
the SDP message that describes the sendrecv stream MUST NOT be a
multicast media stream that provides inline keys from multiple
receivers. For the same reason, the risk recurs when a media stream
has an "a=sendonly" direction attribute in an multicast SDP message.
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SRTP multicast operation requires that each host-sender have a
unique SRTP master key. This can be accomplished by ensuring that
each receiver be allocated a unique key or by ensuring that the SSRC
of each receiver is unique. Since SSRC collision might occur, the
latter condition is not possible unless all SSRCs are assigned by a
central authority such as a 3rd-party key server [srtp]. This is
for further study. The RECOMMENDED approach of this document is to
allocate a different master key for each host-participant of an SRTP
session.
6.2 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 attribute and 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 set 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.
IPsec, TLS, S/MIME or some other data security service SHOULD be
used to provide message authentication for SDP messages that carry
the SRTP attribute. Message encryption SHOULD be used when a master
key parameter 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. The proposed SDP key-mgmt statement [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 endpoint, its
signaling controller, and the 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,
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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 is provided the contents of
the SRTP attribute 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 message to render its services. The SRTP attribute
MUST 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).
7.0 Grammar
This section provides an Augmented BNF grammar for the SRTP profile
of the SDP crypto-attribute. ABNF is defined in [RFC2234]. The
rule names <att-field> and <att-value> are from SDP [RFC2327].
att-field = "crypto"
att-value = cipher application
application = cipher-both / cipher-srtp / cipher-srtcp
cipher-both = key-parameter *[SP sess-par-both]
cipher-srtp = key-parameter *[SP sess-par-srtp]
cipher-srtcp = key-parameter *[SP sess-par-srtcp]
key-parameter = method-inline / method-uri
cipher = "AES_CM_128_HMAC_SHA1_32" /
"F8_128_HMAC_SHA1_32" /
"AES_CM_128_HMAC_SHA1_80"
sess-par-both = roc / ; Roll-Over Counter
kdr / ; Key Derivation Rate
"UNENCRYPTED"
sess-par-srtp = ssrc / fec-order
sess-par-srtcp = "UNAUTHENTICATED"
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method-inline = "inline:" key-info
method-uri = "uri:<" absoluteURI ">" ; absoluteURI defined in
; [RFC2396]
key-info = "/" key-length "/" salt-length "/" key-salt
"/" [lifetime] "/" [mki]
key-length = 1*(DIGIT) ; length in bytes
salt-length = 1*(DIGIT) ; length in bytes
key = 1*(base64)
salt = 1*(base64)
key-salt = key salt ; key and salt concatenated and
; then base64 encoded [section
; 6.8 of RFC2045]
lifetime = ["2^"] 1*(DIGIT)
mki = "/" mki-length ":" mki-value
mki-length = 1*3(DIGIT) ; MUST be multiple of 8 and
; MUST not exceed 128
mki-value = 1*(base64)
roc = "ROC:" 1*(DIGIT)
kdr = "KDR:" 1*(DIGIT)
ssrc = "SSRC:" ssrc-value
ssrc-value = 1*(DIGIT) *["," ssrc-value]
fec-order = "FEC:" 1*(DIGIT)
base64 = ALPHA / DIGIT / "+" / "/" / "="
8.0 Acknowledgements
This document benefited from discussions with Flemming Andreasen,
David McGrew, Mats Naslund, Mike Thomas, Elisabetta Cararra, Brian
Weis, Dave Oran, Bill Foster, Earl Carter, Matt Hammer and Dave
Singer. These people shared 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 recommended the generic approach to the
SDP media-stream security descriptions that is followed in this
draft. Flemming Andreasen suggested some changes to an earlier
draft that greatly simplify this document. David McGrew suggested
the conservative approach of using unique master keys for each SDP
media stream as followed in this document. Jonathan Rosenberg
suggested reducing the complexity by specifying only one security
parameter for each media stream.
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9.0 Authors' Addresses
Mark Baugher
5510 SW Orchid Street
Portland, Oregon
mbaugher@rdrop.com
+1-408-853-4418
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134 USA
dwing@cisco.com
+1 408 525 5314
10.0 References
[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.
[keymgt] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman,
"Key Management Extensions for SDP and RTSP", June 2002,
http://search.ietf.org/internet-drafts/draft-ietf-mmusic-kmgmt-ext-
06.txt, Work in Progress
[mikey] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman,
"MIKEY: Multimedia Internet KEYing", July 2002,
http://search.ietf.org/internet-drafts/draft-ietf-msec-mikey-06.txt,
Work in Progress
[RFC1889] H. Schulzrinne, S. Casner, R. Fredrick, V. Jacobson, "RTP:
A Transport Protocol for Real-Time Applications", January 1996,
http://www.ietf.org/rfc/rfc1889.txt
[RFC2045] N. Freed, N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
November 1996, http://www.ietf.org/rfc/rfc2045.txt
[RFC2104] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", November 1997,
http://www.ietf.org/rfc/rfc2104.txt
[RFC2234] D. Crocker, P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", November 1997,
http://www.ietf.org/rfc/rfc2234.txt
[RFC2327] M. Handley, V. Jacobson, "SDP: Session Description
Protocol", April 1998, http://www.ietf.org/rfc/rfc2327.txt
Baugher & Wing [Page 20]
INTERNET-DRAFT SDP Security Descriptions February 24, 2003
[RFC2828] R. Shirey, "Internet Security Glossary", May 2000,
http://www.ietf.org/rfc/rfc2828.txt
[RFC2396] T. Berners-Lee, R. Fielding, L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", August 1998,
http://www.ietf.org/rfc/rfc2396.txt
[RFC3264] "J. Rosenberg, H. Schulzrinne, "An Offer/Answer Model with
the Session Description Protocol (SDP)", June 2202,
http://www.ietf.org/rfc/rfc3264.txt
[skeme] H. Krawczyk, "SKEME: A Versatile Secure Key Exchange
Mechanism for the Internet", ISOC Secure Networks and Distributed
Systems Symposium, San Diego, 1996.
[srtp] M. Baugher, R. Blom, E. Carrara, D. McGrew, M. Naslund, K.
Norrman, D. Oran, "The Secure Real-time Transport Protocol", June
2002, http://search.ietf.org/internet-drafts/draft-ietf-avt-srtp-
05.txt, Work in Progress
11.0 Full Copyright Statement
"Copyright (C) The Internet Society 2003. 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.
Baugher & Wing [Page 21]
