Internet Engineering Task Force Flemming Andreasen
MMUSIC Working Group Mark Baugher
INTERNET-DRAFT Dan Wing
EXPIRES: December 2003 Cisco Systems
June 27, 2003
SDP Security Descriptions for Media Streams
<draft-ietf-mmusic-sdescriptions-01.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 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. This document defines the Secure Real-
time Transport Protocol (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 June 20, 2003
TABLE OF CONTENTS
1. Notational Conventions............................................2
2. Introduction......................................................3
3. SDP "Crypto" Attribute and Parameters.............................4
3.1 Crypto-suite....................................................4
3.2 Key Parameter...................................................4
3.4 Session Parameters..............................................5
3.5 Examples........................................................5
4. 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 Adding new CRYPTO-SUITE definitions.........................8
4.2 Key-param Parameter.............................................8
4.2.1 Key Usage...................................................8
4.2.2 INLINE Definition...........................................8
4.3 Session Parameters.............................................10
4.3.1 SRC=/SSRC/ROC/SEQ..........................................10
4.3.2 KEY_DERIVATION_RATE=n......................................11
4.3.3 UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP.....................11
4.3.4 FEC_ORDER=order............................................11
4.3.5 UNAUTHENTICATED_SRTP.......................................12
5. Use with Offer/Answer............................................12
5.1 Generating the Offer...........................................12
5.2 Answerer Processing............................................12
5.4 Non-RTP/SAVP Answerers.........................................14
5.4 Offer/Answer Example: Receiver Supports SRTP...................14
5.7 Use of a=crypto With Active Media Streams......................15
5.8 Operation with KEYMGT and Key lines............................15
6. Security Considerations..........................................15
6.1 Authentication of packets......................................16
6.1 Key-stream Reuse...............................................16
6.2 Signaling Authentication and Signaling Encryption..............17
7. SRTP "Crypto" Attribute Grammar..................................18
8. Open Issues......................................................19
9. Acknowledgements.................................................19
10. Authors' Addresses..............................................19
11. Normative References............................................20
Intellectual Property Statement.....................................21
Acknowledgement.....................................................22
1. 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. 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
these media streams. The Secure Real-time Transport Protocol (SRTP)
can be used to provide such security services, and use of it can be
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 defaults values. This document specifies a new
SDP attribute called "crypto", which is used to signal and negotiate
cryptographic parameters for SRTP.
The crypto attribute might be extended to non-SRTP transports such
as whiteboard, modem, fax, and other transports that could use
various security protocols such as IPsec or TLS. These extensions,
however, are beyond the scope of this document. Each type of SDP
media stream needs its own definitions that assign values to its
crypto-attribute parameters. These definitions are unique to the
particular SDP transport and SHOULD be specified in an Internet RFC.
This document defines the parameter values for SRTP.
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
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 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 are suitable for restricted
cases where IPsec, TLS, or some other encapsulating data-security
protocol (e.g. SIP secure multiparts) protects the SDP message.
This draft adds security descriptions to those encrypted and/or
authenticated SDP messages through the "crypto" attribute, which
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provides the cryptographic parameters of a media stream. The "
crypto" attribute could be adapted to any media transport, but its
definition is unique to a particular transport. In Section 3, we
introduce the SDP crypto attribute, and in Section 4, we define the
crypto attribute details needed for SRTP. In Section 5, we specify
how to use the crypto attribute for SRTP streams with the
Offer/Answer model [RFC3264]. Section 6 recites security
considerations, and Section 7 gives an Augmented-BNF grammar for the
SRTP security descriptions provided for the crypto attribute. A
list of open issues is provided in Section 8.
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
proceeding media line. The "crypto" attribute MUST only appear at
the SDP media level (not the session level). The "crypto" attribute
is defined by the following ABNF [RFC2234]:
"a=crypto:" crypto-suite SP key-param *(SP session-param)
where "SP" is the space character (see [RFC2234]); the fields
crypto-suite, key-param, and session-param are described in Section
3.1, 3.2, and 3.3.
The ordering of multiple "a=crypto" lines is significant: The most-
preferred crypto line is listed first; see section 5 for details. We
now describe the crypto fields in more detail.
3.1 Crypto-suite
The crypto-suite field describes all needed information about the
encryption and authentication algorithms for the RTP/SAVP transport.
The ABNF grammar for crypto-suite is:
crypto-suite = VCHAR
where VCHAR is defined in [RFC2234]. The possible values for the
crypto-suite parameter is unique to the transport.
3.2 Key Parameter
The key-param field MUST either contain an inline key descriptor,
or it MUST be a pointer to a uri which contains the actual key. The
ABNF grammar for key-param is:
key-param = inline-key / uri-key
inline-key = "inline:" key-descriptor
key-descriptor = VCHAR
uri-key = "uri:" absolute-uri
absolute-uri = VCHAR
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where VCHAR is defined in [RFC2234].
If the key parameter starts with the string "uri:", the URI method
is used and the value that follows MUST be 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, however it is
RECOMMENDED that such protocols provide both integrity and
confidentiality.
The INLINE method is invoked when the key parameter starts with the
string "inline:"; the cryptographic key is encoded according to a
transport-specific syntax subject to the general format provided
above. 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).
3.4 Session Parameters
The session parameters are specific to the SDP media stream
transport and are OPTIONAL. The ABNF grammar for session-param is:
session-param = VCHAR
where VCHAR is defined in [RFC2234]. Section 4 describes the session
parameters for RTP/SAVP.
3.5 Example
The first example shows use of the crypto attribute for the RTP/SAVP
media transport type (as defined in Section 4). The a=crypto line
is actually one long line, 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:d/16/14/d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj/2^20/1:32
m=audio 49170 RTP/SAVP 0
a=crypto:AES_CM_128_HMAC_SHA1_32
inline:d/16/14/NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj/2^20/1:32
m=application 32416 udp wb
a=orient:portrait
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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
next section.
4. RTP/SAVP (SRTP) Security Descriptions
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 crypto attribute defined in this
document. 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 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, 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.
SRTP Crypto Parameter Description
--------------------- -----------
CRYPTO-SUITE Encryption and authentication transforms
INLINE SRTP and associated parameters
SRC An <SSRC, ROC, SEQ> triple
KEY_DERIVATION_RATE Rate that the 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
Table 4-1: SRTP Crypto-suite, Key and Session Parameters
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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, the key parameter, and the 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
attribute. 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 three 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 an 80-bit authentication tag. The
master-key length is 128 bits and has a lifetime of a maximum of
2^48 SRTP packets or 2^31 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). 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 a 32-bit authentication tag for SRTP packets;
SRTCP uses an 80-bit tag. The master-key length is 128 bits and has
a lifetime of a maximum of 2^48 SRTP packets or 2^31 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). 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.
4.1.3 F8_128_HMAC_SHA1_80
The SRTP f8 cipher is used with HMAC-SHA1 message authentication
having a 80-bit authentication tag. The master-key length is 128
bits and has a lifetime of a maximum of 2^48 SRTP packets or 2^31
SRTCP packets, whichever comes first [srtp]. The SRTP and SRTCP
encryption key lengths are 128 bits. The SRTP and SRTCP
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authentication key lengths are 160 bits (see Security
Considerations). 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 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 4.1.3
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
4.1.3. For example, if AES_CM_128_HMAC_SHA1_80 were desired that
used a 160-bit master key, then a new crypto-suite MUST be defined
having a new name that is identical to AES_CM_128_HMAC_SHA1_80 but
with the size of the master key defined to be 160 bits instead of
128 bits.
4.2 Key-param Parameter
If the key-param parameter has a "uri:" descriptor, the value is a
Uniform Resource Identifier value as described in Section 3. When
the key-param parameter has an "inline:" descriptor, the value
contains a cryptographic master key that MUST be a unique
cryptographically random [RFC1750] value with respect to other
"inline:" values in the SDP message.
4.2.1 Key Usage
The "inline" type of key contains the keying material and all policy
relating to that key, including how it can be used (for encryption,
decryption, or both encryption and decryption), how long it can be
used (lifetime) and whether or not it uses a master key index
(master key index or MKI) to associate an incoming SRTP packet with
a 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,
for example).
4.2.2 INLINE Definition
If the identifier is "inline", the key-param descriptor has the
format described in Section 7 (Grammar) and contains the following
fields:
use key use indicator
key_length key length
salt_length salt length
key||salt concatenated key and salt, BASE64-encoded
lifetime key lifetime (number of packets)
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MKI:length MKI and length of the MKI field in SRTP packets.
The "use" indicator defines usage as one of three values which are
all provided from the perspective of the recipient of the SDP: "d"
means the key is used for decryption only, "e" means the key is used
for encryption only, and "b" means the key is used for both
encryption and decryption. If the crypto suite uses the same key
for both encryption and decryption, "b" MUST be specified.
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. Their sum MUST be exactly equal to the length of the
concatenated master key and salt provided in the fourth field. The
key_length and salt_length MUST appear in the "inline" encoding. For
example,
inline:d/16/14/d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj/2^20/1:4
is a decryption key with a key length of 16 and a salt length of 14.
The fourth part of the "inline" encoding is the cryptographic master
key appended with the master salt. Each master key and salt MUST be
a cryptographically random number and MUST be unique to the SDP
message. Both are concatenated and then base-64 encoded. If the
length of the concatenated key and salt (after being decoded from
base 64) does not equal the sum of the key_length and salt_length
indicated, the receiver MUST NOT use this crypto attribute line for
the media stream and SHOULD log a "inline encoding too short"
condition. For example,
inline:d/16/8/YUJDZGVmZ2hpSktMbW9QUXJzVHVWd3l6//1066:4
describes a decryption key with 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 fifth part of the of the "inline" encoding is the OPTIONAL
lifetime of the master key as measured in number of packets using
that key. The lifetime value MAY be written as an non-zero,
positive integer or as a power of 2, see the ABNF in Section 7 for
details. The "lifetime" value MUST NOT exceed the maximum packet
lifetime for the crypto-suite. If lifetime is too large or
otherwise invalid, then the receiver MUST NOT use this crypto
attribute line for 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.
The first example, above, shows a case where the lifetime is
specified as 2^20 while the second example shows an empty lifetime,
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which means the SRTP default value of 2^48 will be used with
UNENCRYPTED_SRTCP and 2^31 will be used otherwise.
The MKI and its byte length are OPTIONAL (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 bytes. If the
MKI_length is not given or if the value exceeds 128, then the
receiver MUST NOT use this crypto attribute line for 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 use this crypto attribute line for the media
stream and SHOULD log an "invalid MKI" condition. The substring
"1:4" in the first example assigns to the key a master key index of
1 that is 4 bytes long, and the second example assigns a 4-byte key
index of 1066 to the key.
4.3 Session Parameters
The "session" parameters are OPTIONAL and serve to override SRTP
session defaults for the SRTP and SRTCP streams. These parameters
configure an RTP session for SRTP services.
4.3.1 SRC=SSRC/ROC/SEQ
The SRC session parameter provides information to establish the SRTP
cryptographic context. 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. 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
an INLINE parameter. 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 OPTIONAL provided that either a ROC or a SEQ appear in the
SRC parameter. SSRC 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 that is associated with the inline key. If the SSRC value is
invalid, the receiver MUST NOT use this crypto attribute line for
the media stream but SHOULD log an "invalid SSRC" condition. If
SSRC is specified and an SRTP packet is received with a different
SSRC value, the receiver SHOULD discard the packet and log an error.
ROC is OPTIONAL provided that either an SSRC or a SEQ appear in the
SRC parameter. ROC is an integer in the range of 0..2^32-1 for the
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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 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 SDP message containing a ROC SRTP description. If ROC is
greater than 2^32-1, then the receiver MUST NOT use this crypto
attribute line for the media stream but SHOULD log an "invalid ROC"
condition.
SEQ is OPTIONAL provided that either an SSRC or a ROC appear in the
SRC parameter. SEQ is an integer in the range of 0..2^16-1 for the
SRTP sequence number (SEQ). SRTP uses the RTP sequence number (and
the ROC) to compute the packet index [srtp]. At the start of a
session, an SSRC 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. See also section 3.3.1 of [srtp]. If SEQ
is greater than 2^16-1, then the receiver MUST NOT use this crypto
attribute line for the media stream but SHOULD log an "invalid SEQ"
condition.
4.3.2 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).
4.3.3 UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP
UNENCRYPTED_SRTCP indicates that the SRTCP packet payloads are not
encrypted. UNENCRYPTED_SRTP indicates that the SRTP packet payloads
are not encrypted. SRTP and SRTCP packet payloads are encrypted by
default.
4.3.4 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
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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 use this
crypto attribute line for the media stream but SHOULD log an
"invalid FEC_ORDER" condition.
4.3.5 UNAUTHENTICATED_SRTP
This parameter signals that SRTP messages are not authenticated.
SRTP authenticates SRTP messages by default. Use of
UNAUTHENTICATED_SRTP is NOT RECOMMENDED (see Security
Considerations).
5. Use with Offer/Answer
The Offer/Answer model [RFC 3264] enables parties that wish to
engage in a multimedia session to negotiate the media stream
parameters that will be used for the multimedia session. In this
section, we detail how the security descriptions defined for SRTP
are used with the offer/answer model to negotiate cryptographic
capabilities and communicate SRTP master keys.
5.1 Generating the Offer
For each SDP media line (m=) using the "RTP/SAVP" transport where
the offerer wants to specify cryptographic parameters, the offerer
MUST provide at least one "a=crypto" line. When multiple crypto
lines are provided, the a=crypto 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.
The offerer obtains keying material for "inline", or obtains the uri
pointing to a key server. The mechanism to generate or obtain a key
is outside the scope of this specification.
5.2 Answerer Processing
For each SDP media line using the "RTP/SAVP" transport that contains
an "a=crypto" line in the offer, the answerer MUST either accept
exactly one of the crypto lines for that media stream, or it MUST
reject the media stream as described in RFC 3264. Note that if
there are no "a=crypto" lines for the media stream in the offer,
then the answerer MUST skip the following steps and instead use the
default SRTP/SRTCP parameters for that media stream (note that the
endpoint will then need to somehow obtain the correct master key as
well). When the answerer accepts an "RTP/SAVP" media stream with a
crypto line, the answerer MUST include exactly one "a=crypto" line
in the answer. The answer crypto line MUST include at least the
selected crypto-suite and a key-param parameter.
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To determine if the answerer can accept any of the provided
"a=crypto" lines, the answerer examines the crypto lines in order.
If an "a=crypto" line does not satisfy the constraints provided in
Section 4, that crypto line is deemed invalid and MUST be discarded.
The answerer selects the first valid crypto line that it supports,
considering the answerer's capabilities and security policies. 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. SSRC,
ROC, 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]).
After selecting a single crypto line, the answerer generates a
master key appropriate for the selected crypto algorithm(s), unless
the offered master key was specified to apply to both encryption and
decryption, in which case the offered master key MUST be used
instead. If the offered master key was for decryption, then the
answerer MUST use it to decrypt any incoming packets; the key
provided in the answer MUST also be marked as being for decryption,
since the answerer will be using it when encrypting it's packets.
Similarly, if the offered key was for encryption, then the answerer
MUST use it to encrypt any packets it sends and the key it provides
in its answer MUST be used to decrypt any incoming packets. The
master key in the answer 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 "a=crypto" line after its media line with
transport set to RTP/SAVP.
5.3 Offerer Processing of the Answer
When the offerer receives the answer, it MUST perform the following
steps for each "RTP/SAVP" media stream it offerered 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 media line is valid according to the
constraints specified in Section 4. Furthermore, the offerer MUST
validate, that the crypto-suite selected was one of the offered
crypto-suites for the media stream. If any of these checks fails,
the security negotiation defined here MUST be deemed to have failed.
It is possible that the answerer supports the "RTP/SAVP" transport
and accepts the offered media stream, yet it does not support the
crypto attribute defined here. The offerer can recognize this
situation by seeing an accepted "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.
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5.4 Non-RTP/SAVP Answerers
If a media stream with transport set to "RTP/SAVP" is sent to a
device that doesn't support "RTP/SAVP", that media stream will be
rejected.
In some cases, it is desirable to send SRTP when possible, but allow
use of RTP if SRTP isn't supported by a remote device. However, it
is ambiguous to send an extra media line with transport set to
"RTP/AVP" and with the same port as the "RTP/SAVP" line. Thus, an
offerer MUST NOT specify multiple media lines with the same port.
Instead, such interoperability is obtained by first sending an offer
with transport set to "RTP/SAVP". If that media line is rejected by
the answerer, the offerer can, if its policy permits, send a new
offer with transport set to "RTP/AVP". Also, the SDP extensions
defined in RFC 3407 [RFC3407] can be used by both the offerer and
answerer to indicate capabilities above and beyond what is being
negotiated for the media stream. Another offer/answer exchange will
then be needed to negotiate use of any of these latent capabilities.
5.4 Offer/Answer Example: Receiver Supports SRTP
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.
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:d/16/14/WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz/2^20/1:32
FEC_ORDER=FEC_SRTP SRC=17174//49126
a=crypto:F8_128_HMAC_SHA1_80
inline:d/16/14/MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm/2^20/1:32
FEC_ORDER=FEC_SRTP SRC=17174//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)
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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:d/16/14/PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR/2^20/1:32
SRC=88131/721/13
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 initial SSRC (88131), rollover counter (721),
and rollover counter (13).
5.7 Use of a=crypto With Active Media Streams
When an active SRTP session is rekeyed, this is indicated by sending
a new SDP. Rekeying is done following the rules described for a
normal Offer/Answer exchange. The Answerer can take this
opportunity to rekey the traffic it is sending, if the Answerer
desires. During rekeying, the session parameters cannot be changed
and MUST NOT be specified in the Offer or the Answer.
When the Offerer needs to rekey, the offerer MUST send an "a=crypto"
line with same crypto-suite, key length, and salt length that was
previously accepted by the Answerer.
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 answerer selected "a=crypto" lines using the "uri" method,
the sender MAY transmit the same uri, and the recipient MUST fetch
the new key using the uri.
5.8 Operation with KEYMGT and Key lines
An Offer MAY include both a=crypto and a=keymgt lines [keymgt]. Per
SDP rules, the Answerer will ignore attribute lines it doesn't
understand. If the Answerer supports both a=crypto and [keymgt],
the Answer MUST include either a=crypto or [keymgt], as including
both is undefined.
6. 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
descriptions. Therefore, that protocol should either invoke its own
security mechanisms to do so, or alternatively utilize a lower-layer
security service (e.g. TLS, IPSEC) where that service is deemed
adequate for protecting the encapsulating protocol itself. Where
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the encapsulating protocol is used in both a hop-by-hop and end-to-
end context (e.g. SIP), an end-to-end security service SHOULD be
provided by that protocol for all sensitive information including
SDP's security parameters. This service SHOULD provide strong
message authentication and packet-payload encryption as well as
effective replay protection. As an example, SIP with S/MIME bodies
satisfies these requirements.
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
mechanisms SHOULD be used (SRTP replay protection is always
enabled). 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. But
even in multicast sessions, SRTP packet authentication ensures that
the packet originated from a member of the secure group. Use of the
UNAUTHENTICATED_SRTP parameter, therefore, is NOT RECOMMENDED.
6.1 Key-stream Reuse
Misconfigured SRTP sessions, moreover, are vulnerable to attacks on
their encryption services when running the crypto suites defined in
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 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, as this document
RECOMMENDS (Section 4.2).
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]. 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.
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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 crypto 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 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.
IPsec, TLS, encapsulating-protocol security 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 because 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 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 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,
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.
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In this case, the third-party provider gets 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 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).
7. SRTP "Crypto" Attribute Grammar
This section provides an Augmented BNF grammar for the SRTP profile
of the SDP crypto attribute. ABNF is defined in [RFC2234].
key-param = method-inline / method-uri
crypto-suite = "AES_CM_128_HMAC_SHA1_32" /
"F8_128_HMAC_SHA1_32" /
"AES_CM_128_HMAC_SHA1_80"
method-inline = "inline:" key-info *(SP session-param)
method-uri = "uri:<" absoluteURI ">" ; absoluteURI defined in
; [RFC2396]
key-info = key-use "/" key-length "/" salt-length "/" key-salt
"/" [lifetime] "/" [mki]
key-use = "d" / "e" / "b" ; decrypt, encrypt, or both
key-length = 1*DIGIT
salt-length = 1*DIGIT
key-salt = 1*(base64) ; binary key and salt values
; concatenated together, and then
; base64 encoded [section 6.8 of
; RFC2046]
lifetime = ["2^"] 1*(DIGIT)
mki = mki-length ":" mki-value
mki-length = 1*DIGIT ; real value is 2^mki-length, max 128
mki-value = 1*DIGIT
session-param = src /
kdr /
"UNENCRYPTED_SRTP" /
"UNENCRYPTED_SRTCP" /
"UNAUTHENTICATED_SRTP" /
fec-order
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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*(DIGIT)
fec-order = "FEC_ORDER=" fec-type
fec-type = "FEC_SRTP" / "SRTP_FEC" / "SPLIT"
base64 = ALPHA / DIGIT / "+" / "/" / "="
8. Open Issues
The following is a list of open issues in this document:
* The crypto attribute can be used with or without offer/answer,
however, details on usage outside of offer/answer are missing.
* The offer/answer procedures need to be expanded to better describe
unidirectional and inactive streams, unicast versus multicast, as
well as additional detail for some of the session parameters.
9. Acknowledgements
This document 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 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 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.
10. 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
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Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134 USA
dwing@cisco.com
+1-408-902-3348
11. Normative References
[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.
[RFC2234] D. Crocker, P. Overell, "Augmented BNF for Syntax
Specifications: ABNF," November 1997,
http://www.ietf.org/rfc/rfc2234.txt.
[SDPnew] M. Handley, V. Jacobson, C. Perkins, "SDP: Session
Description Protocol,", Work in Progress.
[RFC2828] R. Shirey, "Internet Security Glossary", May 2000,
http://www.ietf.org/rfc/rfc2828.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
[srtp] M. Baugher, R. Blom, E. Carrara, D. McGrew, M. Naslund, K.
Norrman, D. Oran, "The Secure Real-time Transport Protocol", May
2003, http://search.ietf.org/internet-drafts/draft-ietf-avt-srtp-
08.txt, Work in Progress
[RFC1750] D. Eastlake 3rd, S. Crocker, J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
12. Informative References
[RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple
Capability Declaration", RFC 3407, October 2002.
[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", February 2003,
http://search.ietf.org/internet-drafts/draft-ietf-mmusic-kmgmt-ext-
07.txt, Work in Progress.
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[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.
[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.
[skeme] H. Krawczyk, "SKEME: A Versatile Secure Key Exchange
Mechanism for the Internet", ISOC Secure Networks and Distributed
Systems Symposium, San Diego, 1996.
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