Internet Engineering Task Force (IETF) J. Lennox

Internet Engineering Task Force (IETF) J. Lennox
Request for Comments: 6904 Vidyo
Updates: 3711 April 2013
Category: Standards Track
ISSN: 2070-1721

Encryption of Header Extensions
in the Secure Real-time Transport Protocol (SRTP)

Abstract

The Secure Real-time Transport Protocol (SRTP) provides
authentication, but not encryption, of the headers of Real-time
Transport Protocol (RTP) packets. However, RTP header extensions may
carry sensitive information for which participants in multimedia
sessions want confidentiality. This document provides a mechanism,
extending the mechanisms of SRTP, to selectively encrypt RTP header
extensions in SRTP.

This document updates RFC 3711, the Secure Real-time Transport
Protocol specification, to require that all future SRTP encryption
transforms specify how RTP header extensions are to be encrypted.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6904.

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Copyright Notice

Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Encryption Mechanism . . . . . . . . . . . . . . . . . . . . 4
3.1. Example Encryption Mask . . . . . . . . . . . . . . . . . 6
3.2. Header Extension Keystream Generation for Existing
Encryption Transforms . . . . . . . . . . . . . . . . . . 7
3.3. Header Extension Keystream Generation for Future
Encryption Transforms . . . . . . . . . . . . . . . . . . 8
4. Signaling (Setup) Information . . . . . . . . . . . . . . . . 8
4.1. Backward Compatibility . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 13
A.1. Key Derivation Test Vectors . . . . . . . . . . . . . . . 13
A.2. Header Encryption Test Vectors Using AES-CM . . . . . . . 14

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1. Introduction

The Secure Real-time Transport Protocol [RFC3711] specification
provides confidentiality, message authentication, and replay
protection for multimedia payloads sent using the Real-time Protocol
(RTP) [RFC3550]. However, in order to preserve RTP header
compression efficiency, SRTP provides only authentication and replay
protection for the headers of RTP packets, not confidentiality.

For the standard portions of an RTP header, providing only
authentication and replay protection does not normally present a
problem, as the information carried in an RTP header does not provide
much information beyond that which an attacker could infer by
observing the size and timing of RTP packets. Thus, there is little
need for confidentiality of the header information.

However, the security requirements can be different for information
carried in RTP header extensions. A number of recent proposals for
header extensions using the mechanism described in "A General
Mechanism for RTP Header Extensions" [RFC5285] carry information for
which confidentiality could be desired or essential. Notably, two
recent specifications ([RFC6464] and [RFC6465]) contain information
about per-packet sound levels of the media data carried in the RTP
payload and specify that exposing this information to an eavesdropper
is unacceptable in many circumstances (as described in the Security
Considerations sections of those RFCs).

This document, therefore, defines a mechanism by which encryption can
be applied to RTP header extensions when they are transported using
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
monitoring device to be aware of RTP transmission time offsets
[RFC5450]), this mechanism can be selectively applied to a subset of
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], which are the SRTP encryption transforms
defined by Standards Track RFCs 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.

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RFC 6904 Encrypted SRTP Header Extensions April 2013

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119] and
indicate requirement levels for compliant implementations.

3. Encryption Mechanism

Encrypted header extension elements are carried in the same manner as
non-encrypted header extension elements, as defined by [RFC5285].
The one- or two-byte header of the extension elements is not
encrypted, nor is any of the header extension padding. If multiple
different header extension elements are being encrypted, they have
separate element identifier values, just as they would if they were
not encrypted. Similarly, encrypted and non-encrypted header
extension elements have separate identifier values.

Encrypted header extension elements are carried only in packets
encrypted using the Secure Real-time Transport Protocol [RFC3711].
To encrypt (or decrypt) encrypted header extension elements, an SRTP
participant first uses the SRTP key derivation algorithm, specified
in Section 4.3.1 of [RFC3711], to generate header encryption and
header salting keys, using the same pseudorandom function family as
is used for the key derivation for the SRTP session. These keys are
derived as follows:

o k_he (SRTP header encryption): <label> = 0x06, n=n_e.

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
encryption key and salting key are used as are used for the SRTP
payload. Additionally, the same master key, master salt, index, and
key_derivation_rate are used as for the SRTP payload. (Note that
since RTP headers, including header extensions, are authenticated in
SRTP, no new authentication key is needed for header extensions.)

A header extension keystream is generated for each packet containing
encrypted header extension elements. The details of how this header
extension keystream is generated depend on the encryption transform
that is used for the SRTP packet. For encryption transforms that
have been standardized as of the date of publication of this
document, see Section 3.2; for requirements for new transforms, see
Section 3.3.

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After the header extension 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. (For an example of this procedure, see
Section 3.1.) 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 that are to be encrypted and all-bits-0 octets for
header extension octets that are not to be encrypted. The set of
extension elements to be encrypted is communicated between the sender
and the receiver using the signaling mechanisms described in
Section 4.

This encryption mask is computed separately for every packet that
carries a header extension. Based on the non-encrypted portions of
the headers and the signaled list of encrypted extension elements, a
receiver can always determine the correct encryption mask for any
encrypted header extension.

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 Section 5.3.1 of [RFC3550] calls "header extension"
proper, starting with the first [RFC5285] ID and length. Thus, both
the encryption mask and the keystream begin at this point.

This header extension encryption process could, equivalently, be
computed by considering the encryption mask as a mixture of the
encrypted and unencrypted headers, i.e., as

EncryptedHeader = (Encrypt(Key, Plaintext) AND MASK) OR
(Plaintext AND (NOT MASK))

where Encrypt is the encryption function, MASK is the encryption
mask, and AND, OR, and NOT are bitwise operations. This formulation
of the encryption process might be preferred by implementations for
which encryption is performed by a separate module and cannot be
modified easily.

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The SRTP authentication tag is computed across the encrypted header
extension, i.e., the data that is actually transmitted on the wire.
Thus, header extension encryption MUST be done before the
authentication tag is computed, and authentication tag validation
MUST be done on the encrypted header extensions. For receivers,
header extension decryption SHOULD be done only after the receiver
has validated the packet's message authentication tag, and the
receiver MUST NOT take any actions based on decrypted headers, prior
to validating the authentication tag, that could affect the security
or proper functioning of the system.

3.1. Example Encryption Mask

If a sender wished to send a header extension containing an encrypted
SMPTE timecode [RFC5484] with ID 1, a plaintext transmission time
offset [RFC5450] with ID 2, an encrypted audio level indication
[RFC6464] with ID 3, and an encrypted NTP timestamp [RFC6051] with ID
4, the plaintext RTP header extension might look like this:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=1 | len=7 | SMTPE timecode (long form) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SMTPE timecode (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SMTPE (cont'd)| ID=2 | len=2 | toffset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| toffset (ct'd)| ID=3 | len=0 | audio level | ID=4 | len=6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP timestamp (Variant B) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP timestamp (Variant B, cont'd) | padding = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: Structure of Plaintext Example Header Extension

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The corresponding encryption mask would then be:

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 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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0|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|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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Encryption Mask for Example Header Extension

In the mask, the octets corresponding to the payloads of the
encrypted header extension elements are set to all-1 values, and the
octets corresponding to non-encrypted header extension elements,
element headers, and header extension padding are set to all-zero
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 are 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, respectively, as defined
above.

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 Authenticated
Encryption with Associated Data (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.)

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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 Standards 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 of this document if they are applicable
and are believed to be cryptographically adequate for the transform
in question.

4. Signaling (Setup) Information

Encrypted header extension elements are signaled in the Session
Description Protocol (SDP) extmap attribute using the URI
"urn:ietf:params:rtp-hdrext:encrypt" followed by the URI of the
header extension element being encrypted, as well as any
extensionattributes that extension normally takes. Figure 3 gives a
formal Augmented Backus-Naur Form (ABNF) [RFC5234] showing this
grammar extension, extending the grammar defined in [RFC5285].

enc-extensionname = %x75.72.6e.3a.69.65.74.66.3a.70.61.72.61.6d.73.3a
%x72.74.70.2d.68.64.72.65.78.74.3a.65.6e.63.72.79.70.74
; "urn:ietf:params:rtp-hdrext:encrypt" in lower case

extmap =/ mapentry SP enc-extensionname SP extensionname
[SP extensionattributes]

; extmap, mapentry, extensionname, and extensionattributes
; are defined in [RFC5285]

Figure 3: Syntax of the "encrypt" extmap

Thus, for example, to signal an SRTP session using encrypted SMPTE
timecodes [RFC5484], while simultaneously signaling plaintext
transmission time offsets [RFC5450], an SDP document could contain
the text shown in Figure 4 (line breaks have been added for
formatting).

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RFC 6904 Encrypted SRTP Header Extensions April 2013

m=audio 49170 RTP/SAVP 0
a=crypto:1 AES_CM_128_HMAC_SHA1_32 \
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=extmap:1 urn:ietf:params:rtp-hdrext:encrypt \
urn:ietf:params:rtp-hdrext:smpte-tc 25@600/24
a=extmap:2 urn:ietf:params:rtp-hdrext:toffset

Figure 4: Sample SDP Document Offering Encrypted Headers

This example uses SDP security descriptions [RFC4568] for SRTP
keying, but this is merely for illustration. Any SRTP keying
mechanism to establish session keys will work.

The extmap SDP attribute is defined in [RFC5285] as being either a
session or media attribute. If the extmap for an encrypted header
extension is specified as a media attribute, it MUST be specified
only for media that use SRTP-based RTP profiles. If such an extmap
is specified as a session attribute, there MUST be at least one media
in the SDP session that uses an SRTP-based RTP profile. 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) media.

The "urn:ietf:params:rtp-hdrext:encrypt" extension MUST NOT be
recursively applied to itself.

4.1. Backward Compatibility

Following the procedures in [RFC5285], an SDP endpoint that does not
understand the "urn:ietf:params:rtp-hdrext:encrypt" extension URI
will ignore the extension and, for SDP offer/answer, will negotiate
not to use it.

For backward compatibility with endpoints that do not implement this
specification, in a negotiated session (whether using offer/answer or
some other means), best-effort encryption of a header extension
element is possible: an endpoint MAY offer the same header extension
element both encrypted and unencrypted. An offerer MUST offer only
best-effort negotiation when lack of confidentiality would be
acceptable in the backward-compatible case. Answerers (or equivalent
peers in a negotiation) that understand header extension encryption
SHOULD choose the encrypted form of the offered header extension
element and mark the unencrypted form "inactive", unless they have an
explicit reason to prefer the unencrypted form. In all cases,
answerers MUST NOT negotiate the use of, and senders MUST NOT send,
both encrypted and unencrypted forms of the same header extension.

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Note that, as always, users of best-effort encryption MUST be
cautious of bid-down attacks, where a man-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

The security properties of header extension elements protected by the
mechanism in this document are equivalent to those for SRTP payloads.

The mechanism defined in this document does not provide
confidentiality about which header extension elements are used for a
given SRTP packet, only for the content of those header extension
elements. This appears to be in the spirit of SRTP itself, which
does not encrypt RTP headers. If this is a concern, an alternate
mechanism would be needed to provide confidentiality.

For the two-byte-header form of header extension elements (0x100N,
where "N" is the appbits field), this mechanism does not provide any
protection to zero-length header extension elements (for which their
presence or absence is the only information they carry). It also
does not provide any protection for the appbits (field 256, the
lowest four bits of the "defined by profile" field) of the two-byte
headers. Neither of these features is present in the one-byte-header
form of header extension elements (0xBEDE), so these limitations do
not apply in that case.

This mechanism cannot protect RTP header extensions that do not use
the mechanism defined in [RFC5285].

This document does not specify the circumstances in which extension
header encryption should be used. Documents defining specific header
extension elements should provide guidance on when encryption is
appropriate for these elements.

If a middlebox does not have access to the SRTP authentication keys,
it has no way to verify the authenticity of unencrypted RTP header
extension elements (or the unencrypted RTP header), even though it
can monitor them. Therefore, such middleboxes MUST treat such
headers as untrusted and potentially generated by an attacker, in the
same way as they treat 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.)

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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.

The dangers of using weak or NULL authentication with SRTP, described
in Section 9.5 of [RFC3711], apply to encrypted header extensions as
well. In particular, since some header extension elements will have
some easily guessed plaintext bits, strong authentication is REQUIRED
if an attacker setting such bits could have a meaningful effect on
the behavior of the system.

The technique defined in this document can be applied only to
encryption transforms that work by generating a pseudorandom
keystream and bitwise exclusive-ORing it with the plaintext, such as
CTR or f8. It will not work with ECB, CBC, or any other encryption
method that does not use a keystream.

6. IANA Considerations

This document defines a new extension URI to the RTP Compact Header
Extensions subregistry of the Real-Time Transport Protocol (RTP)
Parameters registry, according to the following data:

Extension URI: urn:ietf:params:rtp-hdrext:encrypt
Description: Encrypted header extension element
Contact: [email protected]
Reference: RFC 6904

7. Acknowledgments

Thanks to Benoit Claise, Roni Even, Stephen Farrell, Kevin Igoe, Joel
Jaeggli, David McGrew, David Singer, Robert Sparks, Magnus
Westerlund, Qin Wu, and Felix Wyss for their comments and suggestions
in the development of this specification.

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.

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RFC 6904 Encrypted SRTP Header Extensions April 2013

[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
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
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
RTP", RFC 6188, March 2011.

8.2. Informative References

[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.

[RFC5450] Singer, D. and H. Desineni, "Transmission Time Offsets in
RTP Streams", RFC 5450, March 2009.

[RFC5484] Singer, D., "Associating Time-Codes with RTP Streams", RFC
5484, March 2009.

[RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
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.

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Appendix A. Test Vectors

A.1. Key Derivation Test Vectors

This section provides test data for the header extension key
derivation function, using AES-128 in Counter Mode. (The algorithms
and keys used are the same as those for the test vectors in Appendix
B.3 of [RFC3711].)

The inputs to the key derivation function are the 16-octet master key
and the 14-octet master salt:

master key: E1F97A0D3E018BE0D64FA32C06DE4139

master salt: 0EC675AD498AFEEBB6960B3AABE6

Following [RFC3711], the input block for AES-CM is generated by
exclusive-ORing the master salt with the concatenation of the
encryption key label 0x06 with (index DIV kdr), then padding on the
right with two null octets, which implements the multiply-by-2^16
operation (see Section 4.3.3 of [RFC3711]). The resulting value is
then AES-CM-encrypted using the master key to get the cipher key.

index DIV kdr: 000000000000
label: 06
master salt: 0EC675AD498AFEEBB6960B3AABE6
--------------------------------------------------
XOR: 0EC675AD498AFEEDB6960B3AABE6 (x, PRF input)

x*2^16: 0EC675AD498AFEEDB6960B3AABE60000 (AES-CM input)

hdr. cipher key: 549752054D6FB708622C4A2E596A1B93 (AES-CM output)

Next, we show how the cipher salt is generated. The input block for
AES-CM is generated by exclusive-ORing the master salt with the
concatenation of the encryption salt label. That value is padded and
encrypted as above.

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index DIV kdr: 000000000000
label: 07
master salt: 0EC675AD498AFEEBB6960B3AABE6

--------------------------------------------------
XOR: 0EC675AD498AFEECB6960B3AABE6 (x, PRF input)

x*2^16: 0EC675AD498AFEECB6960B3AABE60000 (AES-CM input)

AB01818174C40D39A3781F7C2D270733 (AES-CM ouptut)

hdr. cipher salt: AB01818174C40D39A3781F7C2D27

A.2. Header Encryption Test Vectors Using AES-CM

This section provides test vectors for the encryption of a header
extension using the AES_CM cryptographic transform.

The header extension is encrypted using the header cipher key and
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 Salt: AB01818174C40D39A3781F7C2D27

SSRC: CAFEBABE
Rollover Counter: 00000000
Sequence Number: 1234
----------------------------------------------
Init. Counter: AB018181BE3AB787A3781F7C3F130000

The SRTP session was negotiated to indicate that header extension ID
values 1, 3, and 4 are encrypted.

In hexadecimal, the header extension being encrypted is as follows
(spaces have been added to show the internal structure of the header
extension):

17 414273A475262748 22 0000C8 30 8E 46 55996386B395FB 00

This header extension is 24 bytes long. (Its values are intended to
represent plausible values of the header extension elements shown in
Section 3.1, but their specific meaning is not important for the
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.

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RFC 6904 Encrypted SRTP Header Extensions April 2013

In hexadecimal, the corresponding encryption mask selecting the
bodies of header extensions 1, 2, and 4 (corresponding to the mask in
Figure 2) is:

00 FFFFFFFFFFFFFFFF 00 000000 00 FF 00 FFFFFFFFFFFFFF 00

Finally, we compute the keystream from the session key and the
initial counter, apply the mask to the keystream, and then exclusive-
OR the keystream with the plaintext:

Initial keystream: 1E19C8E1D481C779549ED1617AAA1B7A
FC0D933AE7ED6CC8
Mask (hex): 00FFFFFFFFFFFFFFFF0000000000FF00
FFFFFFFFFFFFFF00
Masked keystream: 0019C8E1D481C7795400000000001B00
FC0D933AE7ED6C00
Plaintext: 17414273A475262748220000C8308E46
55996386B395FB00
Ciphertext: 17588A9270F4E15E1C220000C8309546
A994F0BC54789700

Author's Address

Jonathan Lennox
Vidyo, Inc.
433 Hackensack Avenue
Seventh Floor
Hackensack, NJ 07601
US

EMail: [email protected]

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