Internet-Draft	Completely Encrypting RTP Header Extensi	October 2021
Uberti, et al.	Expires 28 April 2022	[Page]

Workgroup:: AVTCORE
Internet-Draft:: draft-ietf-avtcore-cryptex-03
Published:: 25 October 2021
Intended Status:: Standards Track
Expires:: 28 April 2022
Authors:: J. Uberti

Clubhouse

C. Jennings

Cisco

S. Garcia Murillo

CoSMo

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 defines Cryptex as a new mechanism that completely encrypts header extensions and CSRCs and uses simpler signaling with the goal of facilitating deployment.¶

Discussion Venues

This note is to be removed before publishing as an RFC.¶

Source for this draft and an issue tracker can be found at https://github.com/juberti/cryptex.¶

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶

This Internet-Draft will expire on 28 April 2022.¶

Copyright Notice

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 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.¶

▲

1. Introduction

1.1. Problem Statement

The Secure Real-time Transport Protocol [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 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 section 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

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

Third, it 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, implementations will need to use two-byte headers in many cases, which are not supported well by some existing implementations.¶

Finally, the header extension bloat combined with the need for backwards 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 (backwards compatible) forms, and two-byte for the encrypted forms. Thus, deployment of [RFC6904] encryption for header extensions will typically result in multiple extra bytes in each RTP packet, compared to the present situation.¶

1.3. Goals

From this analysis we can state the desired properties of a solution:¶

Build on existing [RFC3711] SRTP framework (simple to understand)¶
Build on existing [RFC8285] header extension framework (simple to implement)¶
Protection of header extension ids, lengths, and values¶
Protection of CSRCs when present¶
Simple signaling¶
Simple crypto transform and SRTP interactions¶
Backward compatible with unencrypted endpoints, if desired¶
Backward compatible with existing RTP tooling¶

The last point deserves further discussion. While we considered possible solutions that would have encrypted more of the RTP header (e.g., the number of CSRCs), we felt the inability to parse the resultant packets with current tools, as well as additional complexity incurred, outweighed the slight improvement in confidentiality.¶

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 [RFC2119].¶

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.¶

4. Signaling

In order to determine whether the mechanism defined in this specification is supported, this document defines a new "a=cryptex" Session Description Protocol (SDP) [RFC4566] attribute to indicate support.¶

This attribute takes no value, and can be used at the session level or media level.¶

The presence of this attribute in the SDP (either in an offer or 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 to use the Cryptex mechanism or not.¶

The formal definition of this attribute is:¶

  Name: cryptex

  Value: None

  Usage Level: session, media

  Charset Dependent: No

  Example:

     a=cryptex

When used with BUNDLE, this attribute is assigned to the TRANSPORT category [RFC8859]. If BUNDLE is in use and the a=cryptex attribute is present for a media line, it MUST be present for all media lines belonging to the same bundle group. This ensures that the encrypted MID header extensions used to demux BUNDLE can be processed correctly.¶

5. RTP Header Processing

[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 id with value 256 MUST NOT be negotiated, as the value of this id is meant to be contained in the "appbits" of the "defined by profile" field, which are not available when using the values above.¶

If the "a=extmap-allow-mixed" attribute defined in [RFC8285] is negotiated, either one-byte or two-byte header ids can be used (with the values above), as in [RFC8285].¶

5.1. Sending

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

If the packet contains solely one-byte extension ids, 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. If the packet contains only two-byte extension ids, the header extension tag MUST be set to 0xC2DE to indicate encryption has been applied, and the 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.¶

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. The implementation MAY stop and report an error if it considers use of this specification mandatory for the RTP stream.¶

If the RTP packet passes this check, it is then decrypted according to Decryption Procedure, and passed to the 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    |    0xDE       |           length=3            | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |                  RFC 8285 header extensions                   | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |                          payload  ...                         | |
| |                               +-------------------------------+ |
| |                               | RTP padding   | RTP pad count | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
| ~                     SRTP MKI (OPTIONAL)                       ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| :                 authentication tag (RECOMMENDED)              : |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
|                                                                   |
+- Encrypted Portions*                     Authenticated Portion ---+

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

Specifically, the 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 region to encrypt, which is as shown in the section above.¶

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 Header Processing above and encrypt at the same time.¶

For AEAD ciphers (e.g., GCM), the 12-byte fixed header and the four-byte header extension header (the "defined by profile" field and the length) are considered AAD, even though they are non-contiguous in the packet if CSRCs are present.¶

6.3. Decryption Procedure

The decryption procedure is identical to that of [RFC3711] except for the region to decrypt, 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. Backwards Compatibility

This specification attempts to encrypt as much as possible without interfering with backwards 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

This specification extends SRTP by expanding the portion of the packet that is encrypted, as shown in 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 [RFC8285] header extensions¶

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 defines two new 'defined by profile' attributes, as noted in RTP Header Processing.¶

10. 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 review and suggestions.¶

11. References

11.1. 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/rfc/rfc2119>.
[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/rfc/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/rfc/rfc3711>.
[RFC4566]: Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006, <https://www.rfc-editor.org/rfc/rfc4566>.
[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/rfc/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/rfc/rfc8859>.

11.2. 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/rfc/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/rfc/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/rfc/rfc6904>.

Appendix A. Test Vectors

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

A.1. AES-CTR

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 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 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 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 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 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 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

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 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 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 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 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 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 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

Authors' Addresses

Justin Uberti

Clubhouse

Email: justin@uberti.name

Cullen Jennings

Cisco

Email: fluffy@iii.ca

Sergio Garcia Murillo

CoSMo

Email: sergio.garcia.murillo@cosmosoftware.io