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Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.
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This Internet-Draft will expire on May 15, 2008.
The existing DTLS-SRTP specification allows SRTP keys to be established between a pair of SRTP endpoints. However, when there are more than two participants in an RTP session, DTLS-SRTP is unable to provide a single key for all of the participants.
This document describes an extension to DTLS-SRTP which transports SRTP keying material from one DTLS-SRTP peer to another, so the same SRTP keying material can be used by multiple DTLS-SRTP peers. This reduces or eliminates the need to key each SRTP session individually.
1.
Introduction
2.
Notational Conventions
3.
Scenarios
3.1.
Point to Multipoint using the RFC 3550 mixer model
3.2.
Point to Multipoint using video switching MCU
4.
Protocol Description
4.1.
key_transport extension
4.2.
SRTP Key Transport Protocol
4.3.
Key Transport SSRC and RTP SSRC Collisions
4.4.
Fragmentation, Reassembly, and Retransmission
5.
Examples
6.
Security Considerations
7.
Acknowledgements
8.
IANA Considerations
9.
References
9.1.
Normative References
9.2.
Informational References
§
Author's Address
§
Intellectual Property and Copyright Statements
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Unique SRTP master keys can be established, in each direction, using DTLS-SRTP (McGrew, D. and E. Rescorla, “Datagram Transport Layer Security (DTLS) Extension to Establish Keys for Secure Real-time Transport Protocol (SRTP),” February 2009.) [I‑D.ietf‑avt‑dtls‑srtp]. Using DTLS, each peer contributes part of the keying material to generate the eventual SRTP master key.
In some scenarios after the DTLS handshake has completed, it is desirable for one peer to change its SRTP key and to transmit SRTP packets using an SRTP key that was not derived from the DTLS key exchange. These scenarios are described in Section 3 (Scenarios).
The extensions described in this document allow transporting an SRTP master key from one DTLS peer to the other. Thus, DTLS Key Transport differs from from normal DTLS-SRTP in that the SRTP master key is not derived from the TLS handshake.
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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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
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DTLS Key Transport allows mixers and video switchers to avoid having to encrypt each packet multiple times under multiple SRTP keys. DTLS Key Transport causes sharing of SRTP keys. When sharing SRTP keys with several listeners, it is imperative that the SRTP is changed when a new listener is added or a listener is removed. This is because a legitimate listener should only be able to decrypt the SRTP stream while he is listening; he should not be able to decrypt the SRTP stream prior to joining the conference or after leaving the conference. The needs for audio mixing and video switching are different, and are described below.
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In [I‑D.ietf‑avt‑topologies] (Westerlund, M. and S. Wenger, “RTP Topologies,” October 2007.), this RTP topology is called 'Topo-Mixer'. A common application of an RTP mixer is to receive one (or more) streams from speakers, each individually keyed. The mixer will then do something intelligent with those streams (e.g., select the loudest one), and sends that stream to all of the listeners. Using DTLS-SRTP, without key transport, it is computationally expensive for the mixer to encrypt the payload uniquely for each SRTP listener. The architecture of a typical mixer requires each listener's SRTP to be encrypted serially, incurring additional delay for each successive listener. This is depicted below in Figure 1 (Point to Multipoint Mixer, without DTLS Key Transport).
+-------+ +------------+
+---------+ | +--key=C->+ listener 1 |
| speaker +--key=A-->+ | +------------+
+---------+ | | +------------+
| mixer +--key=D->+ listener 2 |
+---------+ | | +------------+
| speaker +--key=B-->+ | +------------+
+---------+ | +--key=E->+ listener 3 |
+-------+ +------------+
| Figure 1: Point to Multipoint Mixer, without DTLS Key Transport |
With DTLS Key Transport, the mixer can maintain one outbound SRTP cryptographic context, and encrypt the SRTP once for all listeners. This is depicted below in Figure 2 (Point to Multipoint Mixer, with DTLS Key Transport).
+-------+ +------------+
+---------+ | +--key=C->+ listener 1 |
| speaker +--key=A-->+ | +------------+
+---------+ | | +------------+
| mixer +--key=C->+ listener 2 |
+---------+ | | +------------+
| speaker +--key=B-->+ | +------------+
+---------+ | +--key=C->+ listener 3 |
+-------+ +------------+
| Figure 2: Point to Multipoint Mixer, with DTLS Key Transport |
The mixer is aware of listeners leaving or joining, and can rekey the remaining active listeners by the mixer sending them a DTLS "new_srtp_key" message Section 4 (Protocol Description).
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In [I‑D.ietf‑avt‑topologies] (Westerlund, M. and S. Wenger, “RTP Topologies,” October 2007.), this RTP topology is called 'Topo-Video-switch-MCU'. Video switching works differently than audio mixing. In video switching a simple decision is made: which video stream to send to the listeners. As with audio mixing shown above, DTLS-SRTP without key transport is computationally expensive for the video switcher to encrypt the payload uniquely for each SRTP listener. The architecture of a typical video switcher requires each listener's SRTP to be encrypted serially, incurring additional delay for each successive listener. This is depicted below in Figure 3 (Point to Multipoint Video Switching, without DTLS Key Transport).
+--------- + +------------+
+---------+ | +--key=C->+ listener 1 |
| speaker +--key=A-->+selected | +------------+
+---------+ | | +------------+
| switcher +--key=D->+ listener 2 |
+---------+ | | +------------+
| speaker +--key=B-->+dropped | +------------+
+---------+ | +--key=E->+ listener 3 |
+----------+ +------------+
| Figure 3: Point to Multipoint Video Switching, without DTLS Key Transport |
With DTLS key transport, this becomes easier; in fact, the video switcher doesn't need to decrypt the SRTP at all, but just make its decision (select the stream or drop the stream) and transmit the SRTP packets to the listeners. This is depicted below in Figure 4 (Point to Multipoint Video Switching, with DTLS Key Transport).
+----------+ +------------+
+---------+ | +==key=A=>+ listener 1 |
| speaker +==key=A==>+selected | +------------+
+---------+ | | +------------+
| switcher +==key=A=>+ listener 2 |
+---------+ | | +------------+
| speaker +==key=B==>+dropped | +------------+
+---------+ | +==key=A=>+ listener 3 |
+----------+ +------------+
| Figure 4: Point to Multipoint Video Switching, with DTLS Key Transport |
The video switcher is aware of listeners leaving or joining. When such a change occurs, the video switcher SHOULD request the speaker(s) change their respective SRTP keys. This is done by the video switcher sending a DTLS "new_srtp_key_request" message. The speaker will respond with a DTLS "new_srtp_key" message, which will be relayed, by the switcher, to each of the active listeners. Failing to change the key when a listener joins (or leaves) allows a listener to decrypt SRTP traffic prior to (or after) they are authorized participants in the conference.
Discussion point: additionally, do we want to recommend the video switcher also SRTP traffic until the key is changed? Some security policies would reasonably require that, but other security policies would not require that.
When there are multiple speakers, as shown in Figure 4 (Point to Multipoint Video Switching, with DTLS Key Transport) above, each speaker transmits with his own SRTP key. That SRTP key is derived from the DTLS handshake with the switcher. Each speaker uses DTLS-SRTP Key Transport to signal the SSRC that it will use.
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This section describes the extension to the DTLS protocol for DTLS-SRTP key transport, which allows securely communicating the SRTP key to the DTLS peer.
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This document adds a new negotiated extension called "key_transport", which MUST only be requested in conjunction with the "use_srtp" extension (Section 3.2 of [I‑D.ietf‑avt‑dtls‑srtp] (McGrew, D. and E. Rescorla, “Datagram Transport Layer Security (DTLS) Extension to Establish Keys for Secure Real-time Transport Protocol (SRTP),” February 2009.)). The DTLS server indicates its support for key_transport by including key_transport in its ServerHello message. If a DTLS client includes key_transport in its ClientHello, but does not receive key_transport in the ServerHello, the DTLS client MUST NOT send DTLS packets with the srtp_key_transport content-type.
Support for the DTLS Key Transport extension is indicated in SDP with the "srtp-kt" attribute. Advertising support for the extension is necessary in SDP because in some cases it is required to establish an SRTP call. For example, a mixer may be able to only support SRTP listeners if those listeners implement DTLS Key Transport (because it lacks the CPU cycles necessary to encrypt SRTP uniquely for each listener).
A message flow showing a DTLS client and DTLS server using the key_transport extension
Client Server
ClientHello + use_srtp + key_transport
-------->
ServerHello + use_srtp + key_transport
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
SRTP packets <-------> SRTP packets
| Figure 5: Hangshake Message Flow |
After successful negotiation of the key_transport extension, the DTLS client and server MAY exchange SRTP packets, encrypted using the KDF described in [I‑D.ietf‑avt‑dtls‑srtp] (McGrew, D. and E. Rescorla, “Datagram Transport Layer Security (DTLS) Extension to Establish Keys for Secure Real-time Transport Protocol (SRTP),” February 2009.). This is normal and expected, even if Key Transport was negotiated by both sides, as neither side may (yet) have a need to alter the SRTP key. However, it is also possible that one (or both) peers will immediately send a new_srtp_key message before sending any SRTP.
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A new protocol is defined, called the srtp_key_transport protocol which uses srtp_key_transport content-type which consists of the following message types:
- new_srtp_key_request:
- request that the DTLS peer choose a new key. Valid responses are new_srtp_key and new_srtp_key_error.
- new_srtp_key:
- contains the new SRTP keying material. This message is sent by a DTLS endpoint that wants to change its SRTP key. This does not change any cryptographic parameters (a new DTLS handshake is necessary for that), but only the DTLS key for the associated SRTP session. This message includes the SSRC that will be used for this key, which allows listeners to establish one SRTP crypto-context per speaker (necessary for the video switching scenario). The key chosen MUST be cryptographically random [RFC4086] (Eastlake, D., Schiller, J., and S. Crocker, “Randomness Requirements for Security,” June 2005.).
- new_key_activate:
- indicates receiver is prepared to receive SRTP packets encrypted with the new key.
- new_srtp_key_failure:
- indicates a failure.
At any time, the DTLS client or DTLS server MAY send a key_transport message, as shown in Figure 6 (New Key Message Flow). The sender of the new_srtp_key message MAY immediately start transmitting SRTP packets with this new key. However, to account for loss of the new_srtp_key message it is RECOMMENDED that the sender wait before changing to the new SRTP key until it receives the new_key_activate message or it times out waiting for the new_key_activate_message. The duration of this timeout may vary depending on the sensitivity of the content (e.g., 1 second or 10 seconds). In any case, the new_srtp_key message is retransmitted until acknowledged by receipt of a new_key_activate message.
Client / Server Server / Client
[new_srtp_key_request] -------->
<-------- new_srtp_key
new_srtp_key_activiate -------->
| Figure 6: New Key Message Flow |
The following figure shows the state machine for the protocol.
receive new_srtp_key_request from peer
or decide to choose new SRTP key
|
|
send V
new_srtp_key +---------------+ timeout
+---------| Communicate |--------+
| | Key | |
+-------->| | |
+---------------+ |
| ^ |
receive | | +----------------+
new_key_activate| +---| send SRTP using|
| | new SRTP key |
+----------------+ +----------------+
| send SRTP using|
| new SRTP key |
+----------------+
|
V
done
| Figure 7: Key Transport protocol state machine |
Using the syntax described in TLS (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.) [RFC4346], the following structures are used:
enum {
new_srtp_key_request(0),
new_srtp_key(1),
new_srtp_key_activate(2),
new_srtp_key_failure(128),
(255)
} SRTPKeyTransportType;
struct {
SRTPKeyTransportType keytrans_type;
uint24 length;
uint16 message_seq;
uint24 fragment_offset;
uint24 fragment_length;
select (SRTPKeyTransportType) {
case new_srtp_key_request: NewSRTPKeyRequest;
case new_srtp_key: NewSRTPKey;
case new_srtp_key_activate: NewSRTPKeyActivate;
case new_srtp_key_failure: NewSRTPKeyFailure;
};
} KeyTransport;
struct {
uint random<64>; // additional entropy for peer
} NewSRTPKeyRequest;
struct {
uint32 ssrc; // SSRC used for this keys
uint key<16..32>; // The last change_cipher_spec
uint auth_tag<4..10> // decides the length of key
// and auth_tag
uint random<64>; // additional entropy for peer
} NewSRTPKey;
struct {
uint random<64>; // additional entropy for peer
} NewSRTPKeyActivate;
struct { } NewSRTPKeyFailure;
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Per [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.), if an RTP source notices an RTP SSRC collision, it is required to change its SSRC. If it has negotiated support for DTLS-SRTP Key Transport, it then MUST also send a new_srtp_key message indicating the new SSRC. As part of this procedure, it does not need to also change its SRTP key.
The communication of the SSRC used for each key is necessary if there are multiple speakers in the video switching scenario. However, because a speaker is not able to determine if their audio or their video is being switched, a speaker MUST always adhere indicate a change in SSRC by following the procedure in this section for any SRTP stream (audio, video, or other).
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Much like the DTLS handshake itself, this extension also needs to handle fragmentation and reassembly (to send a large key) and retransmission (to account for packet loss). Using the same technique as DTLS's handshake, the message_seq, fragment_offset, fragment_length are used.
When transmitting the key transport message, the sender divides the message into a series of N contiguous data ranges. These ranges MUST NOT be larger than the maximum handshake fragment size and MUST jointly contain the entire key transport message. The ranges SHOULD NOT overlap. The sender then creates N key transport messages, all with the same message_seq value as the original key transport message. Each new message is labelled with the fragment_offset (the number of bytes contained in previous fragments) and the fragment_length (the length of this fragment). The length field in all messages is the same as the length field of the original message. An unfragmented message is a degenerate case with fragment_offset=0 and fragment_length=length.
When a DTLS implementation receives a key transport message fragment, it MUST buffer it until it has the entire key transport message. DTLS implementations MUST be able to handle overlapping fragment ranges. This allows senders to retransmit key transport messages with smaller fragment sizes during path MTU discovery.
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In some scenarios, a device cannot successfully establish an SRTP session without DTLS-SRTP Key Transport. The following two examples show
Using the SDP syntax described in [I‑D.ietf‑mmusic‑sdp‑capability‑negotiation] (Andreasen, F., “SDP Capability Negotiation,” March 2010.), the following figure shows an offerer that requires DTLS Key Transport in order to set up this call as an SRTP call, otherwise it can set up this call as an RTP call. This is indicated by the ",2" on the "a=pcfg" line. If the answerer does not understand "a=dtls-srtp" but does understand DTLS-SRTP and [I‑D.ietf‑mmusic‑sdp‑capability‑negotiation] (Andreasen, F., “SDP Capability Negotiation,” March 2010.), this can cannot be established using DTLS-SRTP; however, it can be established using RTP.
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/AVP 0 18
a=tcap:1 RTP/SAVP
a=acap:1 SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=acap:2 dtls-srtp
a=pcfg:1 t=1 a=1,2
| Figure 8: Example SDP offer showing Key Transport is required |
Using the SDP syntax described in [I‑D.ietf‑mmusic‑sdp‑capability‑negotiation] (Andreasen, F., “SDP Capability Negotiation,” March 2010.), the following figure shows an offerer that indicates support for DTLS Key Transport but does not require DTLS Key Transport in order to set up this call as an SRTP call. This is indicated by the ",[2]" on the "a=pcfg" line. If the answerer does not understand "a=dtls-srtp" but does understand DTLS-SRTP and [I‑D.ietf‑mmusic‑sdp‑capability‑negotiation] (Andreasen, F., “SDP Capability Negotiation,” March 2010.), this call can still be established using DTLS-SRTP.
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 53456 RTP/AVP 0 18
a=tcap:1 RTP/SAVP
a=acap:1 SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=acap:2 dtls-srtp
a=pcfg:1 t=1 a=1,[2]
| Figure 9: Example SDP offer showing Key Transport is optional |
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In the point-to-multipoint scenario, Section 3.1 (Point to Multipoint using the RFC 3550 mixer model), all of the listeners know the key being used by the mixer. Any of those listeners could create SRTP packets that are encrypted with this same key, and send those SRTP packets to other listeners. In order to reduce the vulnerability to this threat, it is RECOMMENDED that the source transport address of received SRTP packets be discarded if they do not match the source transport address of the associated DTLS-SRTP session. Additionally, the network SHOULD prevent IP address spoofing [RFC2827] (Ferguson, P. and D. Senie, “Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing,” May 2000.).
In the
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Thanks to David McGrew for his improvements to this document and to the underlying protocol.
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Register new SDP attribute "dtls-kt", new TLS content-type "key_transport" (26?).
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| [I-D.ietf-avt-dtls-srtp] | McGrew, D. and E. Rescorla, “Datagram Transport Layer Security (DTLS) Extension to Establish Keys for Secure Real-time Transport Protocol (SRTP),” draft-ietf-avt-dtls-srtp-07 (work in progress), February 2009 (TXT). |
| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| [RFC4346] | Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” RFC 4346, April 2006 (TXT). |
| [RFC2827] | Ferguson, P. and D. Senie, “Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing,” BCP 38, RFC 2827, May 2000 (TXT). |
| [I-D.ietf-mmusic-sdp-capability-negotiation] | Andreasen, F., “SDP Capability Negotiation,” draft-ietf-mmusic-sdp-capability-negotiation-13 (work in progress), March 2010 (TXT). |
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| [I-D.ietf-avt-topologies] | Westerlund, M. and S. Wenger, “RTP Topologies,” draft-ietf-avt-topologies-07 (work in progress), October 2007 (TXT). |
| [RFC3550] | Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” STD 64, RFC 3550, July 2003 (TXT, PS, PDF). |
| [RFC4086] | Eastlake, D., Schiller, J., and S. Crocker, “Randomness Requirements for Security,” BCP 106, RFC 4086, June 2005 (TXT). |
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| Dan Wing | |
| Cisco Systems, Inc. | |
| 170 West Tasman Drive | |
| San Jose, CA 95134 | |
| USA | |
| Email: | dwing@cisco.com |
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