wdiff rfc9335v1.txt rfc9335.txt

Internet Engineering Task Force (IETF) J. Uberti
Request for Comments: 9335 Clubhouse
Updates: 3711 C. Jennings
Category: Standards Track Cisco
ISSN: 2070-1721 S. Garcia Murillo
Millicast
November 2022
January 2023

Completely Encrypting RTP Header Extensions and Contributing Sources

Abstract

While the Secure Real-time Transport Protocol (SRTP) provides
confidentiality for the contents of a media packet, a significant
amount of metadata is left unprotected, including RTP header
extensions and contributing sources (CSRCs). However, this data can
be moderately sensitive in many applications. While there have been
previous attempts to protect this data, they have had limited
deployment, due to complexity as well as technical limitations.

This document updates RFC 3711, the SRTP specification, and defines
Cryptex as a new mechanism that completely encrypts header extensions
and CSRCs and uses simpler Session Description Protocol (SDP)
signaling with the goal of facilitating deployment.

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 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9335.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Problem Statement
1.2. Previous Solutions
1.3. Goals
2. Terminology
3. Design
4. SDP Considerations
5. RTP Header Processing
5.1. Sending
5.2. Receiving
6. Encryption and Decryption
6.1. Packet Structure
6.2. Encryption Procedure
6.3. Decryption Procedure
7. Backward Compatibility
8. Security Considerations
9. IANA Considerations
10. References
10.1. Normative References
10.2. Informative References
Appendix A. Test Vectors
A.1. AES-CTR
A.1.1. RTP Packet with 1-Byte One-Byte Header Extension
A.1.2. RTP Packet with 2-Byte Two-Byte Header Extension
A.1.3. RTP Packet with 1-Byte One-Byte Header Extension and CSRC
Fields
A.1.4. RTP Packet with 2-Byte Two-Byte Header Extension and CSRC
Fields
A.1.5. RTP Packet with Empty 1-Byte One-Byte Header Extension and
CSRC Fields
A.1.6. RTP Packet with Empty 2-Byte Two-Byte Header Extension and
CSRC Fields
A.2. AES-GCM
A.2.1. RTP Packet with 1-Byte One-Byte Header Extension
A.2.2. RTP Packet with 2-Byte Two-Byte Header Extension
A.2.3. RTP Packet with 1-Byte One-Byte Header Extension and CSRC
Fields
A.2.4. RTP Packet with 2-Byte Two-Byte Header Extension and CSRC
Fields
A.2.5. RTP Packet with Empty 1-Byte One-Byte Header Extension and
CSRC Fields
A.2.6. RTP Packet with Empty 2-Byte Two-Byte Header Extension and
CSRC Fields
Acknowledgements
Authors' Addresses

1. Introduction

1.1. Problem Statement

The Secure Real-time Transport Protocol (SRTP) [RFC3711] mechanism
provides message authentication for the entire RTP packet but only
encrypts the RTP payload. This has not historically been a problem,
as much of the information carried in the header has minimal
sensitivity (e.g., RTP timestamp); in addition, certain fields need
to remain as cleartext because they are used for key scheduling
(e.g., RTP synchronization source (SSRC) and sequence number).

However, as noted in [RFC6904], the security requirements can be
different for information carried in RTP header extensions, including
the per-packet sound levels defined in [RFC6464] and [RFC6465], which
are specifically noted as being sensitive in the Security
Considerations sections of those RFCs.

In addition to the contents of the header extensions, there are now
enough header extensions in active use that the header extension
identifiers themselves can provide meaningful information in terms of
determining the identity of the endpoint and/or application.
Accordingly, these identifiers can be considered a fingerprinting
issue.

Finally, the CSRCs included in RTP packets can also be sensitive,
potentially allowing a network eavesdropper to determine who was
speaking and when during an otherwise secure conference call.

1.2. Previous Solutions

Encryption of Header Extensions in SRTP [RFC6904] was proposed in
2013 as a solution to the problem of unprotected header extension
values. However, it has not seen significant adoption and has a few
technical shortcomings.

First, the mechanism is complicated. Since it allows encryption to
be negotiated on a per-extension basis, a fair amount of signaling
logic is required. And in the SRTP layer, a somewhat complex
transform is required to allow only the selected header extension
values to be encrypted. One of the most popular SRTP implementations
had a significant bug in this area that was not detected for five
years.

Second, the mechanism only protects the header extension values and
not their identifiers or lengths. It also does not protect the
CSRCs. As noted above, this leaves a fair amount of potentially
sensitive information exposed.

Third, the mechanism bloats the header extension space. Because each
extension must be offered in both unencrypted and encrypted forms,
twice as many header extensions must be offered, which will in many
cases push implementations past the 14-extension limit for the use of
one-byte extension headers defined in [RFC8285]. Accordingly, in
many cases, implementations will need to use two-byte headers, which
are not supported well by some existing implementations.

Finally, the header extension bloat combined with the need for
backward compatibility results in additional wire overhead. Because
two-byte extension headers may not be handled well by existing
implementations, one-byte extension identifiers will need to be used
for the unencrypted (backward-compatible) forms, and two-byte for the
encrypted forms. Thus, deployment of encryption for header
extensions [RFC6904] will typically result in multiple extra bytes in
each RTP packet, compared to the present situation.

1.3. Goals

From the previous analysis, the desired properties of a solution are:

* Built on the existing SRTP framework [RFC3711] (simple to
understand)

* Built on the existing header extension framework [RFC8285] (simple
to implement)

* Protection of header extension identifiers, lengths, and values

* Protection of CSRCs when present

* Simple signaling

* Simple crypto transform and SRTP interactions

* Backward compatibility with unencrypted endpoints, if desired

* Backward compatibility with existing RTP tooling

The last point deserves further discussion. While considering other possible
solutions that would have encrypted more of the RTP header (e.g., the
number of CSRCs), lack of support on CSRCs) were considered, the inability to parse the
resultant packets with current tools was
inevitable, and the additional a generally higher level of
complexity outweighed the slight improvement in confidentiality by fixing previous in
these solutions. Hence, a new more pragmatic approach was needed taken to solve
the problem described in Section 1.1.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Design

This specification proposes a mechanism to negotiate encryption of
all RTP header extensions (ids, lengths, and values) as well as CSRC
values. It reuses the existing SRTP framework, is accordingly simple
to implement, and is backward compatible with existing RTP packet
parsing code, even when support for the mechanism has been
negotiated.

Except when explicitly stated otherwise, Cryptex reuses all the
framework procedures, transforms, and considerations described in
[RFC3711].

4. SDP Considerations

Cryptex support is indicated via a new "a=cryptex" SDP attribute
defined in this specification.

The new "a=cryptex" attribute is a property attribute as defined in
Section 5.13 of [RFC8866]; it therefore takes no value and can be
used at the session level or media level.

The presence of the "a=cryptex" attribute in the SDP (in either an
offer or an answer) indicates that the endpoint is capable of
receiving RTP packets encrypted with Cryptex, as defined below.

Once each peer has verified that the other party supports receiving
RTP packets encrypted with Cryptex, senders can unilaterally decide
whether or not to use the Cryptex mechanism on a per-packet basis.

If BUNDLE is in use as per [RFC9143] and the "a=cryptex" attribute is
present for a media line, it MUST be present for all RTP-based "m="
sections belonging to the same bundle group. This ensures that the
encrypted Media Identifier (MID) header extensions can be processed,
allowing to associate RTP streams to be associated with the correct "m=" section
in each BUNDLE group as specified in Section 9.2 of [RFC9143]. When
used with BUNDLE, this attribute is assigned to the TRANSPORT
category [RFC8859].

Both endpoints can change the Cryptex support status by modifying the
session as specified in Section 8 of [RFC3264]. Generating
subsequent SDP offers and answers MUST use the same procedures for
including the "a=cryptex" attribute as the ones on the initial offer
and answer.

5. RTP Header Processing

A General Mechanism for RTP Header Extensions [RFC8285] defines two
values for the "defined by profile" field for carrying one-byte and
two-byte header extensions. In order to allow a receiver to
determine if an incoming RTP packet is using the encryption scheme in
this specification, two new values are defined:

* 0xC0DE for the encrypted version of the one-byte header extensions
(instead of 0xBEDE).

* 0xC2DE for the encrypted versions of the two-byte header
extensions (instead of 0x100).

In the case of using two-byte header extensions, the extension
identifier with value 256 MUST NOT be negotiated, as the value of
this identifier is meant to be contained in the "appbits" of the
"defined by profile" field, which are is not available when using the
values above.

Note that as per [RFC8285], it is not possible to mix one-byte and
two-byte headers on the same RTP packet. Mixing one-byte and two-
byte headers on the same RTP stream requires negotiation of the
"extmap-allow-mixed" SDP attribute as defined in Section 4.1.2 6 of
[RFC8285].

Peers MAY negotiate both Cryptex and the Encryption of Header
Extensions mechanism defined in [RFC6904] via SDP offer/answer as
described in Section 4, and if both mechanisms are supported, either
one can be used for any given packet. However, if a packet is
encrypted with Cryptex, it MUST NOT also use header extension
encryption [RFC6904], and vice versa.

If one of the peers has advertised both the ability to receive
cryptex both
Cryptex and the ability to receive header extensions encrypted as per [RFC6904] in the SDP
exchange, it is RECOMMENDED that the other peer use Cryptex rather
than the mechanism in [RFC6904] when sending RTP packets so that all
the header extensions and CSRCS are encrypted unless encrypted. However, if there is
a compelling reason to use the mechanism in [RFC6904] (e.g., a need
for some header extensions to be sent in the clear so that so they
are processable by RTP middleboxes) in which case, it middleboxes), the other peer SHOULD use the
mechanism in [RFC6904] instead.

5.1. Sending

When the mechanism defined by this specification has been negotiated,
sending an RTP packet that has any CSRCs or contains any header
extensions [RFC8285] follows the steps below. This mechanism MUST
NOT be used with header extensions other than the variety described
in [RFC8285].

If the RTP packet contains one-byte headers, the 16-bit RTP header
extension tag MUST be set to 0xC0DE to indicate that the encryption
has been applied and the one-byte framing is being used. Otherwise, If the RTP
packet contains two-byte headers, the header extension tag MUST be
set to 0xC2DE to indicate encryption has been applied and the two-byte two-
byte framing is being used.

If the packet contains CSRCs but no header extensions, an empty
extension block consisting of the 0xC0DE tag and a 16-bit length
field set to zero (explicitly permitted by [RFC3550]) MUST be
appended, and the X bit MUST be set to 1 to indicate an extension
block is present. This is necessary to provide the receiver an
indication that the CSRCs in the packet are encrypted.

The RTP packet MUST then be encrypted as described in Encryption
Procedure. Section 6.2
("Encryption Procedure").

5.2. Receiving

When receiving an RTP packet that contains header extensions, the
"defined by profile" field MUST be checked to ensure the payload is
formatted according to this specification. If the field does not
match one of the values defined above, the implementation MUST
instead handle it according to the specification that defines that
value.

Alternatively, if the implementation considers the use of this
specification mandatory and the "defined by profile" field does not
match one of the values defined above, it MUST stop the processing of
the RTP packet and report an error for the RTP stream.

If the RTP packet passes this check, it is then decrypted according
to Decryption Procedure as
described in Section 6.3 ("Decryption Procedure") and passed to the
next layer to process the packet and its extensions. In the event
that a zero-length extension block was added as indicated above, it
can be left as is and will be processed normally.

6. Encryption and Decryption

6.1. Packet Structure

When this mechanism is active, the SRTP packet is protected as
follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
|V=2|P|X| CC |M| PT | sequence number | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| timestamp | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| synchronization source (SSRC) identifier | |
+>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
| | contributing source (CSRC) identifiers | |
| | .... | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
X | 0xC0 or 0xC2 | 0xDE | length | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | RFC 8285 header extensions | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | payload ... | |
| | +-------------------------------+ |
| | | RTP padding | RTP pad count | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
| ~ SRTP MKI Master Key Identifier (MKI) (OPTIONAL) ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| : authentication tag (RECOMMENDED) : |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+- Encrypted Portion* Portion Authenticated Portion ---+

Figure 1: A Protected SRTP Packet

Note that, as required by [RFC8285], the 4 bytes at the start of the
extension block are not encrypted.

Specifically, the encrypted portion Encrypted Portion MUST include any CSRC
identifiers, any RTP header extension (except for the first 4 bytes),
and the RTP payload.

6.2. Encryption Procedure

The encryption procedure is identical to that of [RFC3711] except for
the Encrypted Portion of the SRTP packet. The plaintext input to the
cipher is as follows:

Plaintext = CSRC identifiers (if used) || header extension data ||
RTP payload || RTP padding (if used) || RTP pad count (if used). used)

Here "header extension data" refers to the content of the RTP
extension field, excluding the first four bytes (the extension header
[RFC8285]). The first 4 * CSRC count (CC) bytes of the ciphertext
are placed in the CSRC field of the RTP header. The remainder of the
ciphertext is the RTP payload of the encrypted packet.

To minimize changes to surrounding code, the encryption mechanism can
choose to replace a "defined by profile" field from [RFC8285] with
its counterpart defined in RTP Section 5 ("RTP Header Processing above Processing") and
encrypt at the same time.

For Authenticated Encryption with Associated Data (AEAD) ciphers
(e.g., Galois/Counter Mode (GCM)), AES-GCM), the 12-byte fixed header and the four-byte header
extension header (the "defined by profile" field and the length) are
considered additional authenticated data (AAD), even though they are
non-contiguous in the packet if CSRCs are present.

Associated Data: fixed header || extension header (if X=1)

Here "fixed header" refers to the 12-byte fixed portion of the RTP
header, and "extension header" refers to the four-byte extension
header [RFC8285] ("defined by profile" and extension length).

Implementations can rearrange a packet so that the AAD and plaintext
are contiguous by swapping the order of the extension header and the
CSRC identifiers, resulting in an intermediate representation of the
form shown in Figure 2. After encryption, the CSRCs (now encrypted)
and extension header would need to be swapped back to their original
positions. A similar operation can be done when decrypting to create
contiguous ciphertext and AAD inputs.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
|V=2|P|X| CC |M| PT | sequence number | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| timestamp | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| synchronization source (SSRC) identifier | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| 0xC0 or 0xC2 | 0xDE | length | |
+>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+<+
| | contributing source (CSRC) identifiers | |
| | .... | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | RFC 8285 header extensions | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | payload ... | |
| | +-------------------------------+ |
| | | RTP padding | RTP pad count | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+- Plaintext Input AAD Input ---+

Figure 2: An RTP Packet Transformed to Make Cryptex Cipher Inputs
Contiguous

Note that this intermediate representation is only displayed as
reference for implementations and is not meant to be sent on the
wire.

6.3. Decryption Procedure

The decryption procedure is identical to that of [RFC3711] except for
the Encrypted Portion of the SRTP packet, which is as shown in the
section above.

To minimize changes to surrounding code, the decryption mechanism can
choose to replace the "defined by profile" field with its no-
encryption counterpart from [RFC8285] and decrypt at the same time.

7. Backward Compatibility

This specification attempts to encrypt as much as possible without
interfering with backward compatibility for systems that expect a
certain structure from an RTPv2 packet, including systems that
perform demultiplexing based on packet headers. Accordingly, the
first two bytes of the RTP packet are not encrypted.

This specification also attempts to reuse the key scheduling from
SRTP, which depends on the RTP packet sequence number and SSRC
identifier. Accordingly, these values are also not encrypted.

8. Security Considerations

All security considerations in Section 9 of [RFC3711] are applicable
to this specification, except specification; the exception is Section 9.4. Confidentiality 9.4, because
confidentiality of the RTP Header which is the purpose of this
specification.

The risks of using weak or NULL authentication with SRTP, described
in Section 9.5 of [RFC3711], apply to encrypted header extensions as
well.

This specification extends SRTP by expanding the Encrypted Portion of
the RTP packet, as shown in Packet Structure. Section 6.1 ("Packet Structure"). It
does not change how SRTP authentication works in any way. Given that
more of the packet is being encrypted than before, this is
necessarily an improvement.

The RTP fields that are left unencrypted (see rationale above) are as
follows:

* RTP version

* padding bit

* extension bit

* number of CSRCs

* marker bit

* payload type

* sequence number

* timestamp

* SSRC identifier

* number of header extensions [RFC8285]

These values contain a fixed set (i.e., one that won't be changed by
extensions) of information that, at present, is observed to have low
sensitivity. In the event any of these values need to be encrypted,
SRTP is likely the wrong protocol to use and a fully encapsulating
protocol such as DTLS is preferred (with its attendant per-packet
overhead).

9. IANA Considerations

This document updates the "attribute-name (formerly "att-field")"
subregistry of the "Session Description Protocol (SDP) Parameters"
registry (see Section 8.2.4 of [RFC8866]). Specifically, it adds the
SDP "a=cryptex" attribute for use at both the media level and the
session level.

Contact name: IETF AVT Working Group or IESG if the AVT Working
Group is closed

Contact email address: avt@ietf.org

Attribute name: cryptex

Attribute syntax: This attribute takes no values.

Attribute semantics: N/A

Attribute value: N/A

Usage level: session, media

Charset dependent: No

Purpose: The presence of this attribute in the SDP indicates that
the endpoint is capable of receiving RTP packets encrypted with
Cryptex as described in this document.

O/A procedures: SDP O/A procedures are described in Section 4 of
this document.

Mux Category: TRANSPORT

10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<https://www.rfc-editor.org/info/rfc3264>.

[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.

[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8285] Singer, D., Desineni, H., and R. Even, Ed., "A General
Mechanism for RTP Header Extensions", RFC 8285,
DOI 10.17487/RFC8285, October 2017,
<https://www.rfc-editor.org/info/rfc8285>.

[RFC8859] Nandakumar, S., "A Framework for Session Description
Protocol (SDP) Attributes When Multiplexing", RFC 8859,
DOI 10.17487/RFC8859, January 2021,
<https://www.rfc-editor.org/info/rfc8859>.

[RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
Session Description Protocol", RFC 8866,
DOI 10.17487/RFC8866, January 2021,
<https://www.rfc-editor.org/info/rfc8866>.

[RFC9143] Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", RFC 9143,
DOI 10.17487/RFC9143, February 2022,
<https://www.rfc-editor.org/info/rfc9143>.

10.2. Informative References

[RFC6464] Lennox, J., Ed., Ivov, E., and E. Marocco, "A Real-time
Transport Protocol (RTP) Header Extension for Client-to-
Mixer Audio Level Indication", RFC 6464,
DOI 10.17487/RFC6464, December 2011,
<https://www.rfc-editor.org/info/rfc6464>.

[RFC6465] Ivov, E., Ed., Marocco, E., Ed., and J. Lennox, "A Real-
time Transport Protocol (RTP) Header Extension for Mixer-
to-Client Audio Level Indication", RFC 6465,
DOI 10.17487/RFC6465, December 2011,
<https://www.rfc-editor.org/info/rfc6465>.

[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904,
DOI 10.17487/RFC6904, April 2013,
<https://www.rfc-editor.org/info/rfc6904>.

[RFC7714] McGrew, D. and K. Igoe, "AES-GCM Authenticated Encryption
in the Secure Real-time Transport Protocol (SRTP)",
RFC 7714, DOI 10.17487/RFC7714, December 2015,
<https://www.rfc-editor.org/info/rfc7714>.

Appendix A. Test Vectors

All values are in hexadecimal and represented in network order (big
endian).

A.1. AES-CTR

The following subsections list the test vectors for using cryptex Cryptex
with AES-CTR as per [RFC3711].

Common values are organized as follows:

Rollover Counter: 00000000
Master Key: e1f97a0d3e018be0d64fa32c06de4139
Master Salt: 0ec675ad498afeebb6960b3aabe6
Crypto Suite: AES_CM_128_HMAC_SHA1_80
Session Key: c61e7a93744f39ee10734afe3ff7a087
Session Salt: 30cbbc08863d8c85d49db34a9ae1
Authentication Key: cebe321f6ff7716b6fd4ab49af256a156d38baa4

A.1.1. RTP Packet with 1-Byte One-Byte Header Extension

RTP Packet:

900f1235
decafbad
cafebabe
bede0001
51000200
abababab
abababab
abababab
abababab

Encrypted RTP Packet:

900f1235
decafbad
cafebabe
c0de0001
eb923652
51c3e036
f8de27e9
c27ee3e0
b4651d9f
bc4218a7
0244522f
34a5

A.1.2. RTP Packet with 2-Byte Two-Byte Header Extension

RTP Packet:

900f1236
decafbad
cafebabe
10000001
05020002
abababab
abababab
abababab
abababab

Encrypted RTP Packet:

900f1236
decafbad
cafebabe
c2de0001
4ed9cc4e
6a712b30
96c5ca77
339d4204
ce0d7739
6cab6958
5fbce381
94a5

A.1.3. RTP Packet with 1-Byte One-Byte Header Extension and CSRC Fields

RTP Packet:

920f1238
decafbad
cafebabe
0001e240
0000b26e
bede0001
51000200
abababab
abababab
abababab
abababab

Encrypted RTP Packet:

920f1238
decafbad
cafebabe
8bb6e12b
5cff16dd
c0de0001
92838c8c
09e58393
e1de3a9a
74734d67
45671338
c3acf11d
a2df8423
bee0

A.1.4. RTP Packet with 2-Byte Two-Byte Header Extension and CSRC Fields

RTP Packet:

920f1239
decafbad
cafebabe
0001e240
0000b26e
10000001
05020002
abababab
abababab
abababab
abababab

Encrypted RTP Packet:

920f1239
decafbad
cafebabe
f70e513e
b90b9b25
c2de0001
bbed4848
faa64466
5f3d7f34
125914e9
f4d0ae92
3c6f479b
95a0f7b5
3133

A.1.5. RTP Packet with Empty 1-Byte One-Byte Header Extension and CSRC Fields

RTP Packet:

920f123a
decafbad
cafebabe
0001e240
0000b26e
bede0000
abababab
abababab
abababab
abababab

Encrypted RTP Packet:

920f123a
decafbad
cafebabe
7130b6ab
fe2ab0e3
c0de0000
e3d9f64b
25c9e74c
b4cf8e43
fb92e378
1c2c0cea
b6b3a499
a14c

A.1.6. RTP Packet with Empty 2-Byte Two-Byte Header Extension and CSRC Fields

RTP Packet:

920f123b
decafbad
cafebabe
0001e240
0000b26e
10000000
abababab
abababab
abababab
abababab

Encrypted RTP Packet:

920f123b
decafbad
cafebabe
cbf24c12
4330e1c8
c2de0000
599dd45b
c9d687b6
03e8b59d
771fd38e
88b170e0
cd31e125
eabe

A.2. AES-GCM

The following subsections list the test vectors for using cryptex Cryptex
with AES-GCM as per [RFC7714].

Common values are organized as follows:

Rollover Counter: 00000000
Master Key: 000102030405060708090a0b0c0d0e0f
Master Salt: a0a1a2a3a4a5a6a7a8a9aaab
Crypto Suite: AEAD_AES_128_GCM
Session Key: 077c6143cb221bc355ff23d5f984a16e
Session Salt: 9af3e95364ebac9c99c5a7c4

A.2.1. RTP Packet with 1-Byte One-Byte Header Extension

RTP Packet:

900f1235
decafbad
cafebabe
bede0001
51000200
abababab
abababab
abababab
abababab

Encrypted RTP Packet:

900f1235
decafbad
cafebabe
c0de0001
39972dc9
572c4d99
e8fc355d
e743fb2e
94f9d8ff
54e72f41
93bbc5c7
4ffab0fa
9fa0fbeb

A.2.2. RTP Packet with 2-Byte Two-Byte Header Extension

RTP Packet:

900f1236
decafbad
cafebabe
10000001
05020002
abababab
abababab
abababab
abababab

Encrypted RTP Packet:

900f1236
decafbad
cafebabe
c2de0001
bb75a4c5
45cd1f41
3bdb7daa
2b1e3263
de313667
c9632490
81b35a65
f5cb6c88
b394235f

A.2.3. RTP Packet with 1-Byte One-Byte Header Extension and CSRC Fields

RTP Packet:

920f1238
decafbad
cafebabe
0001e240
0000b26e
bede0001
51000200
abababab
abababab
abababab
abababab

Encrypted RTP Packet:

920f1238
decafbad
cafebabe
63bbccc4
a7f695c4
c0de0001
8ad7c71f
ac70a80c
92866b4c
6ba98546
ef913586
e95ffaaf
fe956885
bb0647a8
bc094ac8

A.2.4. RTP Packet with 2-Byte Two-Byte Header Extension and CSRC Fields

RTP Packet:

920f1239
decafbad
cafebabe
0001e240
0000b26e
10000001
05020002
abababab
abababab
abababab
abababab

Encrypted RTP Packet:

920f1239
decafbad
cafebabe
3680524f
8d312b00
c2de0001
c78d1200
38422bc1
11a7187a
18246f98
0c059cc6
bc9df8b6
26394eca
344e4b05
d80fea83

A.2.5. RTP Packet with Empty 1-Byte One-Byte Header Extension and CSRC Fields

RTP Packet:

920f123a
decafbad
cafebabe
0001e240
0000b26e
bede0000
abababab
abababab
abababab
abababab

Encrypted RTP Packet:

920f123a
decafbad
cafebabe
15b6bb43
37906fff
c0de0000
b7b96453
7a2b03ab
7ba5389c
e9331712
6b5d974d
f30c6884
dcb651c5
e120c1da

A.2.6. RTP Packet with Empty 2-Byte Two-Byte Header Extension and CSRC Fields

RTP Packet:

920f123b
decafbad
cafebabe
0001e240
0000b26e
10000000
abababab
abababab
abababab
abababab

Encrypted RTP Packet:

920f123b
decafbad
cafebabe
dcb38c9e
48bf95f4
c2de0000
61ee432c
f9203170
76613258
d3ce4236
c06ac429
681ad084
13512dc9
8b5207d8

Acknowledgements

The authors wish to thank Lennart Grahl for pointing out many of the
issues with the existing header encryption mechanism, as well as
suggestions for this proposal. Thanks also to Jonathan Lennox, Inaki
Castillo, and Bernard Aboba for their reviews and suggestions.

Authors' Addresses

Justin Uberti
Clubhouse
Email: justin@uberti.name

Cullen Jennings
Cisco
Email: fluffy@iii.ca

Sergio Garcia Murillo
Millicast
Email: sergio.garcia.murillo@cosmosoftware.io