Network Working Group C. Jennings
Internet-Draft P. Jones
Intended status: Standards Track Cisco Systems
Expires: May 4, 2017 A. Roach
October 31, 2016

SRTP Double Encryption Procedures


In some conferencing scenarios, it is desirable for an intermediary to be able to manipulate some RTP parameters, while still providing strong end-to-end security guarantees. This document defines SRTP procedures that use two separate but related cryptographic contexts to provide "hop-by-hop" and "end-to-end" security guarantees. Both the end-to-end and hop-by-hop cryptographic transforms can utilize an authenticated encryption with associated data scheme or take advantage of future SRTP transforms with different properties.

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Table of Contents

1. Introduction

Cloud conferencing systems that are based on switched conferencing have a central Media Distributor device that receives media from endpoints and distributes it to other endpoints, but does not need to interpret or change the media content. For these systems, it is desirable to have one cryptographic context from the sending endpoint to the receiving endpoint that can encrypt and authenticate the media end-to-end while still allowing certain RTP header information to be changed by the Media Distributor. At the same time, a separate cryptographic context provides integrity and optional confidentiality for the media flowing between the Media Distributor and the endpoints. See the framework document that describes this concept in more detail in more detail in [I-D.jones-perc-private-media-framework].

This specification RECOMMENDS the SRTP AES-GCM transform [RFC7714] to encrypt an RTP packet for the end-to-end cryptographic context. The output of this is treated as an RTP packet and again encrypted with an SRTP transform used in the hop-by-hop cryptographic context between the endpoint and the Media Distributor. The Media Distributor decrypts and checks integrity of the hop-by-hop security. The Media Distributor MAY change some of the RTP header information that would impact the end-to-end integrity. The original value of any RTP header field that is changed is included in a new RTP header extension called the Original Header Block. The new RTP packet is encrypted with the hop-by-hop cryptographic transform before it is sent. The receiving endpoint decrypts and checks integrity using the hop-by-hop cryptographic transform and then replaces any parameters the Media Distributor changed using the information in the Original Header Block before decrypting and checking the end-to-end integrity.

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].

Terms used throughout this document include:

3. Cryptographic Contexts

This specification uses two cryptographic contexts: an inner ("end-to-end") context that is used by endpoints that originate and consume media to ensure the integrity of media end-to-end, and an outer ("hop-by-hop") context that is used between endpoints and Media Distributors to ensure the integrity of media over a single hop and to enable a Media Distributor to modify certain RTP header fields. RTCP is also encrypted using the hop-by-hop cryptographic context. The RECOMMENDED cipher for the hop-by-hop and end-to-end contexts is AES-GCM. Other combinations of SRTP ciphers that support the procedures in this document can be added to the IANA registry.

The keys and salt for these contexts are generated with the following steps:

Obviously, if the Media Distributor is to be able to modify header fields but not decrypt the payload, then it must have cryptographic context for the outer transform, but not the inner transform. This document does not define how the Media Distributor should be provisioned with this information. One possible way to provide keying material for the outer ("hop-by-hop") transform is to use [I-D.jones-perc-dtls-tunnel].

4. Original Header Block

Any SRTP packet processed following these procedures MAY contain an Original Header Block (OHB) RTP header extension.

The OHB contains the original values of any modified header fields and MUST be placed after any already-existing RTP header extensions. Placement of the OHB after any original header extensions is important so that the receiving endpoint can properly authenticate the original packet and any originally included RTP header extensions. The receiving endpoint will authenticate the original packet by restoring the modified RTP header field values and header extensions. It does this by copying the original values from the OHB and then removing the OHB extension and any other RTP header extensions that appear after the OHB extension.

The Media Distributor is only permitted to modify the extension (X) bit, payload type (PT) field, and the RTP sequence number field.

The OHB extension is either one octet in length, two octets in length, or three octets in length. The length of the OHB indicates what data is contained in the extension.

If the OHB is one octet in length, it contains the original PT field value. In this case, the OHB has this form:

 0 1 2 3 4 5 6 7
|R|     PT      |

Note that "R" indicates a reserved bit that MUST be set to zero when sending a packet and ignored upon receipt.

If the OHB is two octets in length, it contains the original RTP packet sequence number. In this case, the OHB has this form:

 0                   1 
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 
|        Sequence Number        |

If the OHB is three octets in length, it contains the original PT field value and RTP packet sequence number. In this case, the OHB has this form:

 0                   1                   2
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
|R|     PT      |        Sequence Number        |

If a Media Distributor modifies an original RTP header value, the Media Distributor MUST include the OHB extension to reflect the changed value, setting the X bit in the RTP header to 1 if no header extensions were originally present. If another Media Distributor along the media path makes additional changes to the RTP header and any original value is already present in the OHB, the Media Distributor must extend the OHB by adding the changed value to the OHB. To properly preserve original RTP header values, a Media Distributor MUST NOT change a value already present in the OHB extension.

5. RTP Operations

5.1. Encrypting a Packet

To encrypt a packet, the endpoint encrypts the packet using the inner cryptographic context and then encrypts using the outer cryptographic context. The processes is as follows:

5.2. Relaying a Packet

The Media Distributor does not have a notion of outer or inner cryptographic contexts. Rather, the Media Distributor has a single cryptographic context. The cryptographic transform and key used to decrypt a packet and any encrypted RTP header extensions would be the same as those used in the endpoint's outer cryptographic context.

In order to modify a packet, the Media Distributor decrypts the packet, modifies the packet, updates the OHB with any modifications not already present in the OHB, and re-encrypts the packet using the cryptographic context used for next hop.

5.3. Decrypting a Packet

To decrypt a packet, the endpoint first decrypts and verifies using the outer cryptographic context, then uses the OHB to reconstruct the original packet, which it decrypts and verifies with the inner cryptographic context.

Once the packet has been successfully decrypted, the application needs to be careful about which information it uses to get the correct behavior. The application MUST use only the information found in the synthetic SRTP packet and MUST NOT use the other data that was in the outer SRTP packet with the following exceptions:

If any of the following RTP headers extensions are found in the outer SRTP packet, they MAY be used:

6. RTCP Operations

Unlike RTP, which is encrypted both hop-by-hop and end-to-end using two separate cryptographic contexts, RTCP is encrypted using only the outer (HBH) cryptographic context. The procedures for RTCP encryption are specified in [RFC3711] and this document introduces no additional steps.

7. Recommended Inner and Outer Cryptographic Transforms

This specification recommends and defines AES-GCM as both the inner and outer cryptographic transforms, identified as DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM and DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM. These transforms provide for authenticated encryption and will consume additional processing time double-encrypting for HBH and E2E. However, the approach is secure and simple, and is thus viewed as an acceptable trade-off in processing efficiency.

Note that names for the cryptographic transforms are of the form DOUBLE_(inner transform)_(outer transform).

While this document only defines a profile based on AES-GCM, it is possible for future documents to define further profiles with different inner and outer transforms in this same framework. For example, if a new SRTP transform was defined that encrypts some or all of the RTP header, it would be reasonable for systems to have the option of using that for the outer transform. Similarly, if a new transform was defined that provided only integrity, that would also be reasonable to use for the HBH as the payload data is already encrypted by the E2E.

The AES-GCM cryptographic transform introduces an additional 16 octets to the length of the packet. When using AES-GCM for both the inner and outer cryptographic transforms, the total additional length is 32 octets. If no other header extensions are present in the packet and the OHB is introduced, that will consume an additional 8 octets. If other extensions are already present, the OHB will consume up to 4 additional octets.

8. Security Considerations

To summarize what is encrypted and authenticated, we will refer to all the RTP fields and headers created by the sender and before the pay load as the initial envelope and the RTP payload information with the media as the payload. Any additional headers added by the Media Distributor are referred to as the extra envelope. The sender uses the E2E key to encrypts the payload and authenticate the payload + initial envelope which using an AEAD cipher results in a slight longer new payload. Then the sender uses the HBH key to encrypt the new payload and authenticate the initial envelope and new payload.

The Media Distributor has the HBH key so it can check the authentication of the received packet across the initial envelope and payload data but it can't decrypt the payload as it does not have the E2E key. It can add extra envelope information. It then authenticates the initial plus extra envelope information plus payload with a HBH key. This HBH for the outgoing packet is typically different than the HBH key for the incoming packet.

The receiver can check the authentication of the initial and extra envelope information. This, along with the OBH, is used to construct a synthetic packet that is should be identical to one the sender created and the receiver can check that it is identical and then decrypt the original payload.

The end result is that if the authentications succeed, the receiver knows exactly what the original sender sent, as well as exactly which modifications were made by the Media Distributor.

It is obviously critical that the intermediary has only the outer transform parameters and not the inner transform parameters. We rely on an external key management protocol to assure this property.

Modifications by the intermediary result in the recipient getting two values for changed parameters (original and modified). The recipient will have to choose which to use; there is risk in using either that depends on the session setup.

The security properties for both the inner and outer key holders are the same as the security properties of classic SRTP.

9. IANA Considerations

9.1. RTP Header Extension

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

Extension URI: urn:ietf:params:rtp-hdrext:ohb

Description: Original Header Block

Contact: Cullen Jennings <>

Reference: RFCXXXX

Note to RFC Editor: Replace RFCXXXX with the RFC number of this specification.


We request IANA to add the following values to defines a DTLS-SRTP "SRTP Protection Profile" defined in [RFC5764].

Value Profile Reference

Note to IANA: Please assign value RFCXXXX and update table to point at this RFC for these values.

The SRTP transform parameters for each of these protection are:

    cipher:                 AES_128_GCM then AES_128_GCM 
    cipher_key_length:      256 bits
    cipher_salt_length:     192 bits
    aead_auth_tag_length:   32 octets
    auth_function:          NULL
    auth_key_length:        N/A
    auth_tag_length:        N/A
    maximum lifetime:       at most 2^31 SRTCP packets and
                            at most 2^48 SRTP packets

    cipher:                 AES_256_GCM then AES_256_GCM 
    cipher_key_length:      512 bits
    cipher_salt_length:     192 bits
    aead_auth_tag_length:   32 octets
    auth_function:          NULL
    auth_key_length:        N/A
    auth_tag_length:        N/A
    maximum lifetime:       at most 2^31 SRTCP packets and
                            at most 2^48 SRTP packets

The first half of the key and salt is used for the inner (E2E) transform and the second half is used for the outer (HBH) transform.

10. Acknowledgments

Many thanks to review from Suhas Nandakumar, David Benham, Magnus Westerlund and significant text from Richard Barnes.

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.
[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.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July 2008.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer Security (DTLS) Extension to Establish Keys for the Secure Real-time Transport Protocol (SRTP)", RFC 5764, DOI 10.17487/RFC5764, May 2010.
[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure Real-time Transport Protocol (SRTP)", RFC 6904, DOI 10.17487/RFC6904, April 2013.
[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.

11.2. Informative References

[I-D.jones-perc-dtls-tunnel] Jones, P., "A DTLS Tunnel between Media Distributor and Key Distributor to Facilitate Key Exchange", Internet-Draft draft-jones-perc-dtls-tunnel-03, July 2016.
[I-D.jones-perc-private-media-framework] Jones, P. and D. Benham, "A Solution Framework for Private Media in Privacy Enhanced RTP Conferencing", Internet-Draft draft-jones-perc-private-media-framework-02, March 2016.
[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, DOI 10.17487/RFC6465, December 2011.

Authors' Addresses

Cullen Jennings Cisco Systems EMail:
Paul E. Jones Cisco Systems EMail:
Adam Roach Mozilla EMail: