draft-ietf-avtcore-srtp-encrypted-header-ext-01.txt   draft-ietf-avtcore-srtp-encrypted-header-ext-02.txt 
AVTCORE J. Lennox AVTCORE J. Lennox
Internet-Draft Vidyo Internet-Draft Vidyo
Intended status: Standards Track October 28, 2011 Updates: 3711 (if approved) July 16, 2012
Expires: April 30, 2012 Intended status: Standards Track
Expires: January 17, 2013
Encryption of Header Extensions in the Secure Real-Time Transport Encryption of Header Extensions in the Secure Real-Time Transport
Protocol (SRTP) Protocol (SRTP)
draft-ietf-avtcore-srtp-encrypted-header-ext-01 draft-ietf-avtcore-srtp-encrypted-header-ext-02
Abstract Abstract
The Secure Real-Time Transport Protocol (SRTP) provides The Secure Real-Time Transport Protocol (SRTP) provides
authentication, but not encryption, of the headers of Real-Time authentication, but not encryption, of the headers of Real-Time
Transport Protocol (RTP) packets. However, RTP header extensions may Transport Protocol (RTP) packets. However, RTP header extensions may
carry sensitive information for which participants in multimedia carry sensitive information for which participants in multimedia
sessions want confidentiality. This document provides a mechanism, sessions want confidentiality. This document provides a mechanism,
extending the mechanisms of SRTP, to selectively encrypt RTP header extending the mechanisms of SRTP, to selectively encrypt RTP header
extensions in SRTP. extensions in SRTP.
This document updates RFC 3711, the Secure Real-Time Transport
Protocol specification, to require that all SRTP encryption
transforms specify how RTP header extensions are to be encrypted.
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 30, 2012. This Internet-Draft will expire on January 17, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Encryption Mechanism . . . . . . . . . . . . . . . . . . . . . 3 3. Encryption Mechanism . . . . . . . . . . . . . . . . . . . . . 4
3.1. Example Encryption Mask . . . . . . . . . . . . . . . . . 5 3.1. Example Encryption Mask . . . . . . . . . . . . . . . . . 5
4. Signaling (Setup) Information . . . . . . . . . . . . . . . . 6 3.2. Header Extension Keystream Generation for Existing
4.1. Backward compatibility . . . . . . . . . . . . . . . . . . 7 Encryption Transforms . . . . . . . . . . . . . . . . . . 6
3.3. Header Extension Keystream Generation for Future
Encryption Transforms . . . . . . . . . . . . . . . . . . 7
4. Signaling (Setup) Information . . . . . . . . . . . . . . . . 7
4.1. Backward compatibility . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8 5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9 8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . . 9 8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 10 Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 11
A.1. Key derivation test vectors . . . . . . . . . . . . . . . 10 A.1. Key derivation test vectors . . . . . . . . . . . . . . . 11
A.2. Header Encryption Test Vectors using AES-CM . . . . . . . 11 A.2. Header Encryption Test Vectors using AES-CM . . . . . . . 12
Appendix B. Changes From Earlier Versions . . . . . . . . . . . . 12 Appendix B. Changes From Earlier Versions . . . . . . . . . . . . 13
B.1. Changes from draft-ietf-avtcore -00 . . . . . . . . . . . 12 B.1. Changes from draft-ietf-avtcore -01 . . . . . . . . . . . 13
B.2. Changes from draft-lennox-avtcore -00 . . . . . . . . . . 13 B.2. Changes from draft-ietf-avtcore -00 . . . . . . . . . . . 14
B.3. Changes from draft-lennox-avt -02 . . . . . . . . . . . . 13 B.3. Changes from draft-lennox-avtcore -00 . . . . . . . . . . 14
B.4. Changes From Individual Submission Draft -01 . . . . . . . 13 B.4. Changes from draft-lennox-avt -02 . . . . . . . . . . . . 15
B.5. Changes From Individual Submission Draft -00 . . . . . . . 13 B.5. Changes From Individual Submission Draft -01 . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13 B.6. Changes From Individual Submission Draft -00 . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction 1. Introduction
The Secure Real-Time Transport Protocol [RFC3711] specification The Secure Real-Time Transport Protocol [RFC3711] specification
provides confidentiality, message authentication, and replay provides confidentiality, message authentication, and replay
protection for multimedia payloads sent using of the Real-Time protection for multimedia payloads sent using of the Real-Time
Protocol (RTP) [RFC3550]. However, in order to preserve RTP header Protocol (RTP) [RFC3550]. However, in order to preserve RTP header
compression efficiency, SRTP provides only authentication and replay compression efficiency, SRTP provides only authentication and replay
protection for the headers of RTP packets, not confidentiality. protection for the headers of RTP packets, not confidentiality.
For the standard portions of an RTP header, this does not normally For the standard portions of an RTP header, this does not normally
present a problem, as the information carried in an RTP header does present a problem, as the information carried in an RTP header does
not provide much information beyond that which an attacker could not provide much information beyond that which an attacker could
infer by observing the size and timing of RTP packets. Thus, there infer by observing the size and timing of RTP packets. Thus, there
is little need for confidentiality of the header information. is little need for confidentiality of the header information.
However, this is not necessarily true for information carried in RTP However, this is not necessarily true for information carried in RTP
header extensions. A number of recent proposals for header header extensions. A number of recent proposals for header
extensions using the General Mechanism for RTP Header Extensions extensions using the General Mechanism for RTP Header Extensions
[RFC5285] carry information for which confidentiality could be [RFC5285] carry information for which confidentiality could be
desired or essential. Notably, two recent drafts desired or essential. Notably, two recent specifications ([RFC6464]
([I-D.ietf-avtext-client-to-mixer-audio-level] and and [RFC6465]) carry information about per-packet sound levels of the
[I-D.ietf-avtext-mixer-to-client-audio-level]) carry information media data carried in the RTP payload, and exposing this to an
about per-packet sound levels of the media data carried in the RTP eavesdropper is unacceptable in many circumstances.
payload, and exposing this to an eavesdropper may be unacceptable in
many circumstances.
This document, therefore, defines a mechanism by which encryption can This document, therefore, defines a mechanism by which encryption can
be applied to RTP header extensions when they are transported using be applied to RTP header extensions when they are transported using
SRTP. As an RTP sender may wish some extension information to be SRTP. As an RTP sender may wish some extension information to be
sent in the clear (for example, it may be useful for a network sent in the clear (for example, it may be useful for a network
monitoring device to be aware of RTP transmission time offsets monitoring device to be aware of RTP transmission time offsets
[RFC5450]), this mechanism can be selectively applied to a subset of [RFC5450]), this mechanism can be selectively applied to a subset of
the header extension elements carried in an SRTP packet. the header extension elements carried in an SRTP packet.
The mechanism defined by this document encrypts packets' header
extensions using the same cryptographic algorithms and parameters as
are used to encrypt the packets' RTP payloads. This document defines
how this is done for the encryption transforms defined in [RFC3711],
[RFC5669], and [RFC6188], the SRTP encryption transforms defined by
standards-track IETF documents at the time of this writing. It also
updates [RFC3711], to indicate that specifications of future SRTP
encryption transforms must define how header extension encryption is
to be performed.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119] and document are to be interpreted as described in RFC 2119 [RFC2119] and
indicate requirement levels for compliant implementations. indicate requirement levels for compliant implementations.
3. Encryption Mechanism 3. Encryption Mechanism
Encrypted header extension elements are carried in the same manner as Encrypted header extension elements are carried in the same manner as
skipping to change at page 4, line 25 skipping to change at page 4, line 32
using the same pseudo-random function family as are used for the key using the same pseudo-random function family as are used for the key
derivation for the SRTP session. These keys are derived as follows: derivation for the SRTP session. These keys are derived as follows:
o k_he (SRTP header encryption): <label> = 0x06, n=n_e. o k_he (SRTP header encryption): <label> = 0x06, n=n_e.
o k_hs (SRTP header salting key): <label> = 0x07, n=n_s. o k_hs (SRTP header salting key): <label> = 0x07, n=n_s.
where n_e and n_s are from the cryptographic context: the same size where n_e and n_s are from the cryptographic context: the same size
encryption key and salting key are used as are used for the SRTP encryption key and salting key are used as are used for the SRTP
payload. (Note that since RTP headers, including extension headers, payload. (Note that since RTP headers, including extension headers,
are authenticated in SRTP, no new authentication key is needed for are authenticated in SRTP, no new authentication key is needed for
extension headers.) extension headers.)
For SRTP encryption transforms that operate by generating a A header extension keystream is generated for each packet containing
keystream, a header keystream is generated for each packet containing encrypted header extension elements. The details of how this header
an encrypted header, using the same encryption transform and extension keystream is generated depend on the encryption transform
Initialization Vector (IV) as is used for the SRTP payload, except that is used for the SRTP packet. For encryption transforms that
that the SRTP encryption and salting keys k_e and k_s are replaced by have been standardized as of the publication of this document, see
the SRTP header encryption and header salting keys k_he and k_hs, Section 3.2; for requirements for new transforms, see Section 3.3.
respectively.
The AES-CM and AES-f8 transforms defined in [RFC3711] both operate in
this keystream mode, as do the AES_192_CM and AES_256_CM transforms
defined in [RFC6188]. For other transforms (for example,
Authenticated Encryption with Associated Data (AEAD) cryptographic
transforms, such as AES_GCM and AES_CCM [I-D.ietf-avt-srtp-aes-gcm])
their usage of header extensions MUST be specified explicitly. (As
of this writing, it is believed that it will be sufficient for SRTP
packets protected with AEAD transforms to use a CM transform with
equivalent algorithms and key lengths for their encrypted headers;
however, this guidance is not normative.)
Once the header keystream is generated, the SRTP participant then
computes an encryption mask for the header extension, identifying the
portions of the header extension that are, or are to be, encrypted.
This encryption mask corresponds to the entire payload of each header
extension element that is encrypted. It does not include any non-
encrypted header extension elements, any extension element headers,
or any padding octets. The encryption mask has all-bits-1 octets
(i.e., hexadecimal 0xff) for header extension octets which are to be
encrypted, and all-bits-0 octets for header extension octets which
are not to be.
For those octets indicated in the encryption mask, the SRTP Once the header extension keystream is generated, the SRTP
participant bitwise exclusive-ors the header extension with the participant then computes an encryption mask for the header
keystream to produce the ciphertext version of the header extension. extension, identifying the portions of the header extension that are,
Those octets not indicated in the encryption mask are left or are to be, encrypted. This encryption mask corresponds to the
unmodified. Thus, conceptually, the encryption mask is logically entire payload of each header extension element that is encrypted.
ANDed with the keystream to produce a masked keystream. The sender It does not include any non-encrypted header extension elements, any
and receiver MUST use the same encryption mask. The set of extension extension element headers, or any padding octets. The encryption
mask has all-bits-1 octets (i.e., hexadecimal 0xff) for header
extension octets which are to be encrypted, and all-bits-0 octets for
header extension octets which are not to be. The set of extension
elements to be encrypted is communicated between the sender and the elements to be encrypted is communicated between the sender and the
receiver using the signaling mechanisms described in Section 4. receiver using the signaling mechanisms described in Section 4.
The SRTP participant bitwise-ANDs the encryption mask with the
keystream to produce a masked keystream. It then bitwise exclusive-
ors the header extension with this masked keystream to produce the
ciphertext version of the header extension. (Thus, octets indicated
as all-bits-1 in the encrypted mask are encrypted, whereas those
indicated as all-bits-0 are not.)
The header extension encryption process does not include the "defined
by profile" or "length" fields of the header extension, only the
field that [RFC3550] Section 5.3.1 calls "header extension" proper,
starting with the first [RFC5285] ID and length. Thus, both the
encryption mask and the keystream begin at this point.
The SRTP authentication tag is computed across the encrypted header The SRTP authentication tag is computed across the encrypted header
extension, i.e., the data that is actually transmitted on the wire. extension, i.e., the data that is actually transmitted on the wire.
Thus, header extension encryption MUST be done before the Thus, header extension encryption MUST be done before the
authentication tag is computed, and authentication tag validation authentication tag is computed, and authentication tag validation
MUST be done on the encrypted header extensions. For receivers, MUST be done on the encrypted header extensions. For receivers,
header extension decryption SHOULD be done only after the receiver header extension decryption SHOULD be done only after the receiver
has validated the packet's message authentication tag, and the has validated the packet's message authentication tag, and the
receiver MUST NOT take any actions based on decrypted headers that receiver MUST NOT take any actions based on decrypted headers that
could affect the security or proper functioning of the system, prior could affect the security or proper functioning of the system, prior
to validating the authentication tag. to validating the authentication tag.
3.1. Example Encryption Mask 3.1. Example Encryption Mask
If a sender wished to send a header extension containing an encrypted If a sender wished to send a header extension containing an encrypted
SMPTE timecode [RFC5484] with ID 1, a plaintext transmission time SMPTE timecode [RFC5484] with ID 1, a plaintext transmission time
offset [RFC5450] with ID 2, an encrypted audio level indication offset [RFC5450] with ID 2, an encrypted audio level indication
[I-D.ietf-avtext-client-to-mixer-audio-level] with ID 3, and an [RFC6464] with ID 3, and an encrypted NTP Timestamp [RFC6051] with ID
encrypted NTP Timestamp [RFC6051] with ID 4, the plaintext RTP header 4, the plaintext RTP header extension might look like this:
extension might look like this:
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=1 | len=7 | SMTPE timecode (long form) | | ID=1 | len=7 | SMTPE timecode (long form) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SMTPE timecode (continued) | | SMTPE timecode (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SMTPE (cont'd)| ID=2 | len=2 | toffset | | SMTPE (cont'd)| ID=2 | len=2 | toffset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at page 6, line 45 skipping to change at page 6, line 28
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1| |1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|0 0 0 0 0 0 0 0| |1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|1 1 1 1 1 1 1 1|0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 Figure 2
In the mask, the octets corresponding to the payloads of the In the mask, the octets corresponding to the payloads of the
encrypted header extension elements are set to all-1 values, and encrypted header extension elements are set to all-1 values, and
octets corresponding to non-encrypted elements, element headers, and octets corresponding to non-encrypted header extension elements,
header extension padding are set to all-0 values. element headers, and header extension padding are set to all-0
values.
3.2. Header Extension Keystream Generation for Existing Encryption
Transforms
For the AES-CM and AES-f8 transforms [RFC3711], the SEED-CTR
transform [RFC5669], and the AES_192_CM and AES_256_CM transforms
[RFC6188], the header extension keystream SHALL be generated for each
packet containing encrypted header extension elements, using the same
encryption transform and Initialization Vector (IV) as is used for
that packet's SRTP payload, except that the SRTP encryption and
salting keys k_e and k_s are replaced by the SRTP header encryption
and header salting keys k_he and k_hs, defined above, respectively.
For the SEED-CCM and SEED-GCM transforms [RFC5669], the header
extension keystream SHALL be generated using the algorithm specified
above for the SEED-CTR algorithm. (Because the AEAD transform used
on the payload in these algorithms includes the RTP header, including
the RTP header extension, in its Associated Authenticated Data (AAD),
counter-mode encryption for the header extension is believed to be of
equivalent cryptographic strength to the CCM and GCM transforms.)
For the NULL encryption transform [RFC3711], the header extension
keystream SHALL be all-zero.
3.3. Header Extension Keystream Generation for Future Encryption
Transforms
When new SRTP encryption transforms are defined, this document
updates [RFC3711] as follows: in addition to the rules specified in
Section 6 of RFC 3711, the standard track RFC defining the new
transform MUST specify how the encryption transform is to be used
with header extension encryption.
It is RECOMMENDED that new transformations follow the same mechanisms
as are defined in Section 3.2, if these are applicable and are
believed to be cryptographically adequate for the transform in
question.
4. Signaling (Setup) Information 4. Signaling (Setup) Information
Encrypted header extension elements are signaled in the SDP extmap Encrypted header extension elements are signaled in the SDP extmap
attribute, using the URI "urn:ietf:params:rtp-hdrext:encrypt", attribute, using the URI "urn:ietf:params:rtp-hdrext:encrypt",
followed by the URI of the header extension element being encrypted followed by the URI of the header extension element being encrypted
as well as any extensionattributes that extension normally takes. as well as any extensionattributes that extension normally takes.
Figure 3 gives a formal Augmented Backus-Naur Form (ABNF) [RFC5234]
showing this grammar extension.
enc-extensionattributes = extensionname [SP extensionattributes]
extensionattributes /= enc-extensionattributes
Figure 3: Syntax of the "encrypt" extensionattributes
Thus, for example, to signal an SRTP session using encrypted SMPTE Thus, for example, to signal an SRTP session using encrypted SMPTE
timecodes [RFC5484], while simultaneously signaling plaintext timecodes [RFC5484], while simultaneously signaling plaintext
transmission time offsets [RFC5450], an SDP document could contain transmission time offsets [RFC5450], an SDP document could contain
(line breaks added for formatting): (line breaks added for formatting):
m=audio 49170 RTP/SAVP 0 m=audio 49170 RTP/SAVP 0
a=crypto:1 AES_CM_128_HMAC_SHA1_32 \ a=crypto:1 AES_CM_128_HMAC_SHA1_32 \
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=extmap:1 urn:ietf:params:rtp-hdrext:encrypt \ a=extmap:1 urn:ietf:params:rtp-hdrext:encrypt \
urn:ietf:params:rtp-hdrext:smpte-tc 25@600/24 urn:ietf:params:rtp-hdrext:smpte-tc 25@600/24
a=extmap:2 urn:ietf:params:rtp-hdrext:toffset a=extmap:2 urn:ietf:params:rtp-hdrext:toffset
Figure 3 Figure 4
This example uses SDP Security Descriptions [RFC4568] for SRTP This example uses SDP Security Descriptions [RFC4568] for SRTP
keying, but this is merely for illustration; any SRTP keying keying, but this is merely for illustration; any SRTP keying
mechanism to establish session keys will work. mechanism to establish session keys will work.
The extmap SDP attribute is defined in [RFC5285] as being either a The extmap SDP attribute is defined in [RFC5285] as being either a
session or media attribute. If the extmap for an encrypted header session or media attribute. If the extmap for an encrypted header
extension is specified as a media attribute, it MUST only be extension is specified as a media attribute, it MUST only be
specified for media which use SRTP-based RTP profiles. If such an specified for media which use SRTP-based RTP profiles. If such an
extmap is specified as a session attribute, there MUST be at least extmap is specified as a session attribute, there MUST be at least
one media in the SDP session which uses an SRTP-based RTP profile; one media in the SDP session which uses an SRTP-based RTP profile;
the session-level extmap applies to all the SRTP-based media in the the session-level extmap applies to all the SRTP-based media in the
session, and MUST be ignored for all other (non-SRTP or non-RTP) session, and MUST be ignored for all other (non-SRTP or non-RTP)
media. media.
The "urn:ietf:params:rtp-hdrext:encrypt" extension MUST NOT be
recursively applied to itself.
4.1. Backward compatibility 4.1. Backward compatibility
Following the procedures in [RFC5285], an SDP endpoint which does not Following the procedures in [RFC5285], an SDP endpoint which does not
understand the "urn:ietf:params:rtp-hdrext:encrypt" extension URI understand the "urn:ietf:params:rtp-hdrext:encrypt" extension URI
will ignore the extension, and (for SDP offer/answer) negotiate not will ignore the extension, and (for SDP offer/answer) negotiate not
to use it. to use it.
In a negotiated session (whether using offer/answer or some other In a negotiated session (whether using offer/answer or some other
means), best-effort encryption of a header extension element is means), best-effort encryption of a header extension element is
possible: an endpoint MAY offer the same header extension element possible: an endpoint MAY offer the same header extension element
both encrypted and unencrypted. Receivers which understand header both encrypted and unencrypted. Receivers which understand header
extension encryption SHOULD choose the encrypted form and mark the extension encryption SHOULD choose the encrypted form and mark the
unencrypted form "inactive", unless they have an explicit reason to unencrypted form "inactive", unless they have an explicit reason to
prefer the unencrypted form. (Note that, as always, users of best- prefer the unencrypted form. (Note that, as always, users of best-
effort encryption MUST be cautious of bid-down attacks, and ensure, effort encryption MUST be cautious of bid-down attacks, where a man-
for example, that signaling is integrity-protected.) in-the-middle attacker removes a higher-security option, forcing
endpoints to negotiate a lower-security one. Appropriate
countermeasures depend on the signaling protocol in use, but users
can ensure, for example, that signaling is integrity-protected.)
5. Security Considerations 5. Security Considerations
The security properties of header extension elements protected by the The security properties of header extension elements protected by the
mechanism in this document are equivalent to those for SRTP payloads. mechanism in this document are equivalent to those for SRTP payloads.
The mechanism defined in this document does not provide The mechanism defined in this document does not provide
confidentiality about which header extension elements are used for a confidentiality about which header extension elements are used for a
given SRTP packet, only for the content of those header extension given SRTP packet, only for the content of those header extension
elements. This appears to be in the spirit of SRTP itself, which elements. This appears to be in the spirit of SRTP itself, which
does not encrypt RTP headers. If this is a concern, an alternate does not encrypt RTP headers. If this is a concern, an alternate
mechanism would be needed to provide confidentiality. mechanism would be needed to provide confidentiality.
For the two-byte-header form of header extension elements (0x100x), For the two-byte-header form of header extension elements (0x100x),
this mechanism does not provide any protection to zero-length header this mechanism does not provide any protection to zero-length header
extension elements (for which their presence or absence is the only extension elements (for which their presence or absence is the only
information they carry). It also does not provide any protection for information they carry). It also does not provide any protection for
the two-byte-headers' app bits (field 256, the lowest four bits of the two-byte-headers' app bits (field 256, the lowest four bits of
the "defined by profile" field). Neither of these features are used the "defined by profile" field). Neither of these features are
in for one-byte-header form of header extension elements (0xBEDE), so present in for one-byte-header form of header extension elements
these limitations do not apply in that case. (0xBEDE), so these limitations do not apply in that case.
This mechanism cannot protect RTP header extensions which do not use
the mechanism defined in [RFC5285].
This document does not specify the circumstances in which extension This document does not specify the circumstances in which extension
header encryption should be used. Documents defining specific header header encryption should be used. Documents defining specific header
extension elements should provide guidance on when encryption is extension elements should provide guidance on when encryption is
appropriate for these elements. appropriate for these elements.
If a middlebox does not have access to the SRTP authentication keys, If a middlebox does not have access to the SRTP authentication keys,
it has no way to verify the authenticity of unencrypted RTP header it has no way to verify the authenticity of unencrypted RTP header
extension elements (or the unencrypted RTP header), even though it extension elements (or the unencrypted RTP header), even though it
can monitor them. Therefore, such middleboxes MUST treat such can monitor them. Therefore, such middleboxes MUST treat such
headers as untrusted and potentially generated by an attacker. headers as untrusted and potentially generated by an attacker, in the
same way as unauthenticated traffic. (This does not mean that
middleboxes cannot view and interpret such traffic, of course, only
that appropriate skepticism needs to be maintained about the results
of such interpretation.).
There is no mechanism defined to protect header extensions with
different algorithms or encryption keys than are used to protect the
RTP payloads. In particular, it is not possible to provide
confidentiality for a header extension while leaving the payload in
cleartext.
6. IANA Considerations 6. IANA Considerations
This document defines a new extension URI to the RTP Compact Header This document defines a new extension URI to the RTP Compact Header
Extensions subregistry of the Real-Time Transport Protocol (RTP) Extensions subregistry of the Real-Time Transport Protocol (RTP)
Parameters registry, according to the following data: Parameters registry, according to the following data:
Extension URI: urn:ietf:params:rtp-hdrext:encrypt Extension URI: urn:ietf:params:rtp-hdrext:encrypt
Description: Encrypted extension header element Description: Encrypted extension header element
Contact: jonathan@vidyo.com Contact: jonathan@vidyo.com
Reference: RFC XXXX Reference: RFC XXXX
(Note to the RFC-Editor: please replace "XXXX" with the number of (Note to the RFC-Editor: please replace "XXXX" with the number of
this document prior to publication as an RFC.) this document prior to publication as an RFC.)
7. Acknowledgments 7. Acknowledgments
Thanks to Roni Even, Kevin Igoe, David McGrew, David Singer, Qin Wu, Thanks to Roni Even, Kevin Igoe, David McGrew, Magnus Westerlund,
and Felix Wyss for their comments and suggestions in the development David Singer, Qin Wu, and Felix Wyss for their comments and
of this specification. suggestions in the development of this specification.
8. References 8. References
8.1. Normative References 8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003. Applications", STD 64, RFC 3550, July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004. RFC 3711, March 2004.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP [RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, July 2008. Header Extensions", RFC 5285, July 2008.
[RFC5669] Yoon, S., Kim, J., Park, H., Jeong, H., and Y. Won, "The
SEED Cipher Algorithm and Its Use with the Secure Real-
Time Transport Protocol (SRTP)", RFC 5669, August 2010.
[RFC6188] McGrew, D., "The Use of AES-192 and AES-256 in Secure [RFC6188] McGrew, D., "The Use of AES-192 and AES-256 in Secure
RTP", RFC 6188, March 2011. RTP", RFC 6188, March 2011.
8.2. Informative References 8.2. Informative References
[I-D.ietf-avt-srtp-aes-gcm]
McGrew, D., "AES-GCM and AES-CCM Authenticated Encryption
in Secure RTP (SRTP)", draft-ietf-avt-srtp-aes-gcm-01
(work in progress), January 2011.
[I-D.ietf-avtext-client-to-mixer-audio-level]
Lennox, J., Ivov, E., and E. Marocco, "A Real-Time
Transport Protocol (RTP) Header Extension for Client-to-
Mixer Audio Level Indication",
draft-ietf-avtext-client-to-mixer-audio-level-05 (work in
progress), September 2011.
[I-D.ietf-avtext-mixer-to-client-audio-level]
Ivov, E., Marocco, E., and J. Lennox, "A Real-Time
Transport Protocol (RTP) Header Extension for Mixer-to-
Client Audio Level Indication",
draft-ietf-avtext-mixer-to-client-audio-level-05 (work in
progress), September 2011.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006. Streams", RFC 4568, July 2006.
[RFC5450] Singer, D. and H. Desineni, "Transmission Time Offsets in [RFC5450] Singer, D. and H. Desineni, "Transmission Time Offsets in
RTP Streams", RFC 5450, March 2009. RTP Streams", RFC 5450, March 2009.
[RFC5484] Singer, D., "Associating Time-Codes with RTP Streams", [RFC5484] Singer, D., "Associating Time-Codes with RTP Streams",
RFC 5484, March 2009. RFC 5484, March 2009.
[RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP [RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
Flows", RFC 6051, November 2010. Flows", RFC 6051, November 2010.
[RFC6464] Lennox, J., Ivov, E., and E. Marocco, "A Real-time
Transport Protocol (RTP) Header Extension for Client-to-
Mixer Audio Level Indication", RFC 6464, December 2011.
[RFC6465] Ivov, E., Marocco, E., and J. Lennox, "A Real-time
Transport Protocol (RTP) Header Extension for Mixer-to-
Client Audio Level Indication", RFC 6465, December 2011.
Appendix A. Test Vectors Appendix A. Test Vectors
A.1. Key derivation test vectors A.1. Key derivation test vectors
This section provides test data for the header extension key This section provides test data for the header extension key
derivation function, using AES-128 in Counter Mode. (The algorithms derivation function, using AES-128 in Counter Mode. (The algorithms
and keys used are the same as those for the the test vectors in and keys used are the same as those for the the test vectors in
Appendix B.3 of [RFC3711].) Appendix B.3 of [RFC3711].)
The inputs to the key derivation function are the 16 octet master key The inputs to the key derivation function are the 16 octet master key
skipping to change at page 11, line 24 skipping to change at page 12, line 28
AB01818174C40D39A3781F7C2D270733 (AES-CM ouptut) AB01818174C40D39A3781F7C2D270733 (AES-CM ouptut)
hdr. cipher salt: AB01818174C40D39A3781F7C2D27 hdr. cipher salt: AB01818174C40D39A3781F7C2D27
A.2. Header Encryption Test Vectors using AES-CM A.2. Header Encryption Test Vectors using AES-CM
This section provides test vectors for the encryption of a header This section provides test vectors for the encryption of a header
extension, using the AES_CM cryptographic transform. extension, using the AES_CM cryptographic transform.
The header extension element is encrypted using the header cipher key The header extension is encrypted using the header cipher key and
and header cipher salt computed in Appendix A.1. header cipher salt computed in Appendix A.1. The header extension is
carried in an SRTP-encrypted RTP packet with SSRC 0xCAFEBABE,
sequence number 0x1234, and an all-zero rollover counter.
Session Key: 549752054D6FB708622C4A2E596A1B93 Session Key: 549752054D6FB708622C4A2E596A1B93
Session Salt: AB01818174C40D39A3781F7C2D27 Session Salt: AB01818174C40D39A3781F7C2D27
SSRC: CAFEBABE SSRC: CAFEBABE
Rollover Counter: 00000000 Rollover Counter: 00000000
Sequence Number: 1234 Sequence Number: 1234
---------------------------------------------- ----------------------------------------------
Init. Counter: AB018181BE3AB787A3781F7C3F130000 Init. Counter: AB018181BE3AB787A3781F7C3F130000
The RTP session was negotiated to indicate that header extension ID The SRTP session was negotiated to indicate that header extension ID
values 1, 3 and 4 are encrypted. values 1, 3 and 4 are encrypted.
In hexidecimal, the header extension being encrypted is (spaces added In hexadecimal, the header extension being encrypted is (spaces added
to show the internal structure of the header extension): to show the internal structure of the header extension):
17 414273A475262748 22 0000C8 30 8E 46 55996386B395FB 00 17 414273A475262748 22 0000C8 30 8E 46 55996386B395FB 00
This header extension is 24 bytes long. (Its values are intended to This header extension is 24 bytes long. (Its values are intended to
represent plausible values of the header extension elements shown in represent plausible values of the header extension elements shown in
Section 3.1, but their specific meaning is not important for the Section 3.1, but their specific meaning is not important for the
example.) example.) The header extension "defined by profile" and "length"
fields, which in this case are BEDE 0006 in hexadecimal, are not
included in the encryption process.
In hexidecimal, the corresponding encryption mask selecting the In hexadecimal, the corresponding encryption mask selecting the
bodies of header extensions 1, 2, and 4 (corresponding to the mask in bodies of header extensions 1, 2, and 4 (corresponding to the mask in
Figure 2 is: Figure 2) is:
00 FFFFFFFFFFFFFFFF 00 000000 00 FF 00 FFFFFFFFFFFFFF 00 00 FFFFFFFFFFFFFFFF 00 000000 00 FF 00 FFFFFFFFFFFFFF 00
Finally, we compute the keystream from the session key and the Finally, we compute the keystream from the session key and the
initial counter, apply the mask to the keystream, and then xor the initial counter, apply the mask to the keystream, and then xor the
keystream with the plaintext: keystream with the plaintext:
Initial keystream: 1E19C8E1D481C779549ED1617AAA1B7A Initial keystream: 1E19C8E1D481C779549ED1617AAA1B7A
FC0D933AE7ED6CC8 FC0D933AE7ED6CC8
Mask (Hex): 00FFFFFFFFFFFFFFFF0000000000FF00 Mask (Hex): 00FFFFFFFFFFFFFFFF0000000000FF00
skipping to change at page 12, line 27 skipping to change at page 13, line 34
Plaintext: 17414273A475262748220000C8308E46 Plaintext: 17414273A475262748220000C8308E46
55996386B395FB00 55996386B395FB00
Ciphertext: 17588A9270F4E15E1C220000C8309546 Ciphertext: 17588A9270F4E15E1C220000C8309546
A994F0BC54789700 A994F0BC54789700
Appendix B. Changes From Earlier Versions Appendix B. Changes From Earlier Versions
Note to the RFC-Editor: please remove this section prior to Note to the RFC-Editor: please remove this section prior to
publication as an RFC. publication as an RFC.
B.1. Changes from draft-ietf-avtcore -00 B.1. Changes from draft-ietf-avtcore -01
Made the draft update RFC 3711, and added a section specifying that
all future SRTP encryption transforms must specify how header
extension encryption is to be done.
Explicitly distinguished the processing of existing encryption
transforms from future ones.
Clarified description of the process by which the encryption mask is
applied, and that encryption does not apply to the header extension
"defined by profile" or "length" fields.
Defined how header extension encryption is to be done with the SEED
algorithms defined in RFC 5669, and with the NULL algorithm.
Added ABNF grammar for the SDP syntax.
Clarified that header extension encryption must not be applied to
itself.
Expanded discussion of bid-down attacks.
Pointed out that this mechanism can't protect non-RFC5285 header
extensions, and that there's no way to give different protection to
header extensions than to payloads.
Updated references to now-published RFCs.
Editorial clarifications.
Added Magnus Westerlund to the Acknowledgments.
B.2. Changes from draft-ietf-avtcore -00
Clarified usage of Key Derivation Algorithm Clarified usage of Key Derivation Algorithm
Provided non-normative guidance for how to use this mechanism with Provided non-normative guidance for how to use this mechanism with
Authenticated Encryption with Associated Data (AEAD) transforms. Authenticated Encryption with Associated Data (AEAD) transforms.
Corrected SMPTE Timecode header extension element in example header Corrected SMPTE Timecode header extension element in example header
extension (it's eight bytes, not sixteen). Added an NTP timestamp to extension (it's eight bytes, not sixteen). Added an NTP timestamp to
the example to fill it back out to original size. the example to fill it back out to original size.
skipping to change at page 13, line 5 skipping to change at page 14, line 46
Added a note to the security considerations about the dangers for Added a note to the security considerations about the dangers for
middleboxes observing unencrypted headers (both header extension middleboxes observing unencrypted headers (both header extension
elements and RTP headers) without being able to verify the elements and RTP headers) without being able to verify the
authentication keys. authentication keys.
Added test vectors. Added test vectors.
Added acknowledgments section. Added acknowledgments section.
B.2. Changes from draft-lennox-avtcore -00 B.3. Changes from draft-lennox-avtcore -00
o Published as working group item. o Published as working group item.
o Added discussion of limitations when used with the two-byte-header o Added discussion of limitations when used with the two-byte-header
form of header extension elements. form of header extension elements.
o Added open issue about how to use this mechanism with o Added open issue about how to use this mechanism with
Authenticated Encryption with Associated Data (AEAD) transforms. Authenticated Encryption with Associated Data (AEAD) transforms.
o Updated references. o Updated references.
B.3. Changes from draft-lennox-avt -02 B.4. Changes from draft-lennox-avt -02
o Retargeted at AVTCORE working group. o Retargeted at AVTCORE working group.
o Updated references. o Updated references.
B.4. Changes From Individual Submission Draft -01 B.5. Changes From Individual Submission Draft -01
o Minor editorial changes. o Minor editorial changes.
B.5. Changes From Individual Submission Draft -00 B.6. Changes From Individual Submission Draft -00
o Clarified description of encryption mask creation. o Clarified description of encryption mask creation.
o Added example encryption mask. o Added example encryption mask.
o Editorial changes. o Editorial changes.
Author's Address Author's Address
Jonathan Lennox Jonathan Lennox
Vidyo, Inc. Vidyo, Inc.
433 Hackensack Avenue 433 Hackensack Avenue
 End of changes. 39 change blocks. 
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