--- 1/draft-ietf-perc-private-media-framework-03.txt 2017-07-03 11:13:44.155194597 -0700 +++ 2/draft-ietf-perc-private-media-framework-04.txt 2017-07-03 11:13:44.211195921 -0700 @@ -1,21 +1,21 @@ Network Working Group P. Jones Internet-Draft D. Benham Intended status: Standards Track Cisco -Expires: September 14, 2017 C. Groves +Expires: January 4, 2018 C. Groves Huawei - March 13, 2017 + July 3, 2017 A Solution Framework for Private Media in Privacy Enhanced RTP Conferencing - draft-ietf-perc-private-media-framework-03 + draft-ietf-perc-private-media-framework-04 Abstract This document describes a solution framework for ensuring that media confidentiality and integrity are maintained end-to-end within the context of a switched conferencing environment where media distribution devices are not trusted with the end-to-end media encryption keys. The solution aims to build upon existing security mechanisms defined for the real-time transport protocol (RTP). @@ -27,73 +27,79 @@ 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 http://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 September 14, 2017. + This Internet-Draft will expire on January 4, 2018. Copyright Notice Copyright (c) 2017 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 - 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 - 3. PERC Entities and Trust Model . . . . . . . . . . . . . . . . 4 + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 + 2. Conventions Used in This Document . . . . . . . . . . . . . . 4 + 3. PERC Entities and Trust Model . . . . . . . . . . . . . . . . 5 3.1. Untrusted Entities . . . . . . . . . . . . . . . . . . . 5 - 3.1.1. Media Distributor . . . . . . . . . . . . . . . . . . 5 + 3.1.1. Media Distributor . . . . . . . . . . . . . . . . . . 6 3.1.2. Call Processing . . . . . . . . . . . . . . . . . . . 6 - 3.2. Trusted Entities . . . . . . . . . . . . . . . . . . . . 6 + 3.2. Trusted Entities . . . . . . . . . . . . . . . . . . . . 7 3.2.1. Endpoint . . . . . . . . . . . . . . . . . . . . . . 7 3.2.2. Key Distributor . . . . . . . . . . . . . . . . . . . 7 4. Framework for PERC . . . . . . . . . . . . . . . . . . . . . 7 - 4.1. End-to-End and Hop-by-Hop Authenticated Encryption . . . 7 - 4.2. E2E Key Confidentiality . . . . . . . . . . . . . . . . . 8 + 4.1. End-to-End and Hop-by-Hop Authenticated Encryption . . . 8 + 4.2. E2E Key Confidentiality . . . . . . . . . . . . . . . . . 9 4.3. E2E Keys and Endpoint Operations . . . . . . . . . . . . 9 - 4.4. HBH Keys and Hop Operations . . . . . . . . . . . . . . . 9 + 4.4. HBH Keys and Hop Operations . . . . . . . . . . . . . . . 10 4.5. Key Exchange . . . . . . . . . . . . . . . . . . . . . . 10 - 4.5.1. Initial Key Exchange and Key Distributor . . . . . . 10 + 4.5.1. Initial Key Exchange and Key Distributor . . . . . . 11 4.5.2. Key Exchange during a Conference . . . . . . . . . . 11 5. Entity Trust . . . . . . . . . . . . . . . . . . . . . . . . 12 5.1. Identity Assertions . . . . . . . . . . . . . . . . . . . 12 5.2. Certificate Fingerprints in Session Signaling . . . . . . 13 5.3. Conferences Identification . . . . . . . . . . . . . . . 13 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 6.1. Third Party Attacks . . . . . . . . . . . . . . . . . . . 14 6.2. Media Distributor Attacks . . . . . . . . . . . . . . . . 14 6.2.1. Denial of service . . . . . . . . . . . . . . . . . . 14 6.2.2. Replay Attack . . . . . . . . . . . . . . . . . . . . 15 6.2.3. Delayed Playout Attack . . . . . . . . . . . . . . . 15 6.2.4. Splicing Attack . . . . . . . . . . . . . . . . . . . 15 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 9.1. Normative References . . . . . . . . . . . . . . . . . . 16 9.2. Informative References . . . . . . . . . . . . . . . . . 17 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 + Appendix A. PERC Key Inventory . . . . . . . . . . . . . . . . . 18 + A.1. DTLS-SRTP Exchange Yields HBH Keys . . . . . . . . . . . 19 + A.2. The Key Distributor Transmits the KEK (EKT Key) . . . . . 20 + A.3. Endpoints fabricate an SRTP Master Key . . . . . . . . . 20 + A.4. Who has What Key . . . . . . . . . . . . . . . . . . . . 21 + Appendix B. PERC Packet Format . . . . . . . . . . . . . . . . . 21 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 1. Introduction Switched conferencing is an increasingly popular model for multimedia conferences with multiple participants using a combination of audio, video, text, and other media types. With this model, real-time media flows from conference participants are not mixed, transcoded, transrated, recomposed, or otherwise manipulated by a Media Distributor, as might be the case with a traditional media server or multipoint control unit (MCU). Instead, media flows transmitted by @@ -228,30 +234,30 @@ 3.1.1. Media Distributor A Media Distributor forwards RTP flows between endpoints in the conference while performing per-hop authentication of each RTP packet. The Media Distributor may need access to one or more RTP headers or header extensions, potentially adding or modifying a certain subset. The Media Distributor will also relay secured messaging between the endpoints and the Key Distributor and will acquire per-hop key information from the Key Distributor. The actual - media content MUST NOT not be decryptable by a Media Distributor, so - it is untrusted to have access to the E2E media encryption keys, + media content *MUST NOT* not be decryptable by a Media Distributor, + so it is untrusted to have access to the E2E media encryption keys, which this framework's key exchange mechanisms will prevent. An endpoint's ability to join a conference hosted by a Media Distributor MUST NOT alone be interpreted as being authorized to have access to the E2E media encryption keys, as the Media Distributor does not have the ability to determine whether an endpoint is authorized. Trusted endpoint authorization is described in - [I-D.ietf-roach-perc-webrtc]. + [I-D.roach-perc-webrtc]. A Media Distributor MUST perform its role in properly forwarding media packets while taking measures to mitigate the adverse effects of denial of service attacks (refer to Section 6), etc, to a level equal to or better than traditional conferencing (i.e. non-PERC) deployments. A Media Distributor or associated conferencing infrastructure may also initiate or terminate various conference control related messaging, which is outside the scope of this framework document. @@ -324,25 +330,27 @@ This solution framework focuses on the end-to-end privacy and integrity of the participant's media by limiting access of the end- to-end key information to trusted entities. However, this framework does give a Media Distributor access to RTP headers and all or most header extensions, as well as the ability to modify a certain subset of those headers and to add header extensions. Packets received by a Media Distributor or an endpoint are authenticated hop-by-hop. To enable all of the above, this framework defines the use of two - security contexts and two associated encryption keys; an "inner" key - (E2E Key(i); i={a given endpoint}) for authenticated encryption of - RTP media between endpoints and an "outer" key (HBH Key(j); j={a - given hop}) for the hop between an endpoint and a Media Distributor - or between Media Distributor. Reference the following figure. + security contexts and two associated encryption keys: an "inner" key + (an E2E key distinct for each transmitted media flow) for + authenticated encryption of RTP media between endpoints and an + "outer" key (HBH key) known only to media distributor and the + adjacent endpoint) for the hop between an endpoint and a Media + Distributor or between Media Distributor. Reference the following + figure. +-------------+ +-------------+ | |################################| | | Media |------------------------------->| Media | | Distributor |<-------------------------------| Distributor | | X |################################| Y | | | HBH Key (XY) | | +-------------+ +-------------+ # ^ | # # ^ | # # | | # HBH HBH # | | # @@ -350,92 +358,83 @@ # | | # # | | # # |<+--#---- E2E Key (A) E2E Key (B) ---#->| | # # | | # # | | # # | v # # | v # +-------------+ +-------------+ | Endpoint A | | Endpoint B | +-------------+ +-------------+ E2E and HBH Keys Used for Authenticated Encryption - The PERC Double draft specification [I-D.ietf-perc-double] uses - standard SRTP keying material and recommended cryptographic - transform(s) to first form the inner, end-to-end SRTP cryptographic - context. That end-to-end SRTP cryptographic context MAY be used to - encrypt some RTP header extensions along with RTP media content. The - output of this is treated like an RTP packet and encrypted again - using the outer hop-by-hop cryptographic context. The endpoint - executes the entire Double operation while the Media Distributor just - performs the outer, hop-by-hop operation. + The PERC Double specification [I-D.ietf-perc-double] uses standard + SRTP keying material and recommended cryptographic transform(s) to + first form the inner, end-to-end SRTP cryptographic context. That + end-to-end SRTP cryptographic context MAY be used to encrypt some RTP + header extensions along with RTP media content. The output of this + is treated like an RTP packet and encrypted again using the outer + hop-by-hop cryptographic context. The endpoint executes the entire + Double operation while the Media Distributor just performs the outer, + hop-by-hop operation. (See Appendix A for a description of the keys + used in PERC and Appendix B for an overview of how the packet appears + on the wire.) RTCP can only be encrypted hop-by-hop, not end-to-end. This framework introduces no additional step for RTCP authenticated encryption, so the procedures needed are specified in [RFC3711] and use the same outer, hop-by-hop cryptographic context chosen in the Double operation described above. 4.2. E2E Key Confidentiality To ensure the confidentiality of E2E keys shared between endpoints, endpoints will make use of a common Key Encryption Key (KEK) that is known only by the trusted entities in a conference. That KEK, defined in the PERC EKT [I-D.ietf-perc-srtp-ekt-diet] as the EKTKey, - will be used to subsequently encrypt SRTP master keys used for E2E - authenticated encryption (E2E Key(i); i={a given endpoint}) of media - sent by a given endpoint. - - +----------------------+------------+-------+-------+------------+ - | Key / Entity | Endpoint A | MD X | MD Y | Endpoint B | - +----------------------+------------+-------+-------+------------+ - | KEK | Yes | No | No | Yes | - +----------------------+------------+-------+-------+------------+ - | E2E Key (i) | Yes | No | No | Yes | - +----------------------+------------+-------+-------+------------+ - | HBH Key (A<=>MD X) | Yes | Yes | No | No | - +----------------------+------------+-------+-------+------------+ - | HBH Key (B<=>MD Y) | No | No | Yes | Yes | - +----------------------+------------+---------------+------------+ - | HBH Key (MD X<=>MD Y)| No | Yes | Yes | No | - +----------------------+------------+---------------+------------+ - - Figure 2: Keys per Entity + will be used to subsequently encrypt the SRTP master key used for E2E + authenticated encryption of media sent by a given endpoint. Each + endpoint in the conference will create a random SRTP master key for + E2E authenticated encryption, thus participants in the conference + MUST keep track of the E2E keys received via the Full EKT Field for + each distinct SSRC in the conference so that it can properly decrypt + received media. Note, too, that an endpoint may change its E2E key + at any time and advertise that new key to the conference as specified + in [I-D.ietf-perc-srtp-ekt-diet]. 4.3. E2E Keys and Endpoint Operations Any given RTP media flow can be identified by its SSRC, and endpoints might send more than one at a time and change the mix of media flows transmitted during the life of a conference. Thus, endpoints MUST maintain a list of SSRCs from received RTP flows - and each SSRC's associated E2E Key(i) information. Following a - change of the KEK (i.e., EKTKey), prior E2E Key(i) information SHOULD - be retained for a period long enough to ensure that late-arriving or - out-of-order packets from other endpoints can be successfully - decrypted. The endpoint MUST discard the E2E Key(i) and KEK - information no later than when it leaves the conference. + and each SSRC's associated E2E key information. Following a change + in an E2E key, prior E2E keys SHOULD be retained by receivers for a + period long enough to ensure that late-arriving or out-of-order + packets from the endpoint can be successfully decrypted. Receiving + endpoints MUST discard old E2E keys no later than when it leaves the + conference. If there is a need to encrypt one or more RTP header extensions end- to-end, an encryption key is derived from the end-to-end SRTP master key to encrypt header extensions as per [RFC6904]. The Media Distributor will not be able use the information contained in those - header extensions encrypted with E2E keys. + header extensions encrypted with an E2E key. 4.4. HBH Keys and Hop Operations To ensure the integrity of transmitted media packets, this framework requires that every packet be authenticated hop-by-hop (HBH) between an endpoint and a Media Distributor, as well between Media Distributors. The authentication key used for hop-by-hop authentication is derived from an SRTP master key shared only on the - respective hop (HBH Key(j); j={a given hop}). Each HBH Key(j) is - distinct per hop and no two hops ever intentionally use the same SRTP - master key. + respective hop. Each HBH key is distinct per hop and no two hops + ever intentionally use the same SRTP master key. Using hop-by-hop authentication gives the Media Distributor the ability to change certain RTP header values. Which values the Media Distributor can change in the RTP header are defined in [I-D.ietf-perc-double]. RTCP can only be encrypted, giving the Media Distributor the flexibility to forward RTCP content unchanged, transmit compound RTCP packets or to initiate RTCP packets for reporting statistics or conveying other information. Performing hop- by-hop authentication for all RTP and RTCP packets also helps provide replay protection (see Section 6). @@ -456,83 +455,83 @@ Distributor, this framework utilizes a DTLS-SRTP [RFC5764] association between an endpoint and the Key Distributor. To establish this association, an endpoint will send DTLS-SRTP messages to the Media Distributor which will then forward them to the Key Distributor as defined in [I-D.ietf-perc-dtls-tunnel]. The Key Encryption Key (KEK) (i.e., EKTKey) is also conveyed by the Key Distributor over the DTLS association to endpoints via procedures defined in PERC EKT [I-D.ietf-perc-srtp-ekt-diet]. Media Distributors use DTLS-SRTP [RFC5764] directly with a peer Media - Distributor to establish HBH keys for transmitting RTP and RTCP + Distributor to establish the HBH key for transmitting RTP and RTCP packets to that peer Media Distributor. The Key Distributor does not - facilitate establishing HBH keys for use between Media Distributors. + facilitate establishing a HBH key for use between Media Distributors. 4.5.1. Initial Key Exchange and Key Distributor The procedures defined in DTLS Tunnel for PERC [I-D.ietf-perc-dtls-tunnel] establish one or more TLS tunnels between the Media Distributor and Key Distributor, making it is possible for the Media Distributor to facilitate the establishment of a secure DTLS association between each endpoint and the Key Distributor as shown the following figure. The DTLS association between endpoints and the Key Distributor will enable each endpoint to receive E2E key - information, Key Encryption Key (KEK) information (i.e., EKTKey), and - HBH key information. At the same time, the Key Distributor can + information, Key Encryption Key (KEK) information (i.e., EKT Key), + and HBH key information. At the same time, the Key Distributor can securely provide the HBH key information to the Media Distributor. The key information summarized here may include the SRTP master key, SRTP master salt, and the negotiated cryptographic transform. +-----------+ KEK info | Key | HBH Key info to to Endpoints |Distributor| Endpoints & Media Distributor +-----------+ # ^ ^ # # | | #-TLS Tunnel # | | # +-----------+ +-----------+ +-----------+ | Endpoint | DTLS | Media | DTLS | Endpoint | | KEK |<------------|Distributor|------------>| KEK | - | HBH Key(j)| to Key Dist | HBH Keys | to Key Dist | HBH Key(j)| + | HBH Key | to Key Dist | HBH Keys | to Key Dist | HBH Key | +-----------+ +-----------+ +-----------+ - Figure 3: Exchanging Key Information Between Entities + Figure 2: Exchanging Key Information Between Entities Endpoints will establish a DTLS-SRTP association over the RTP session's media ports for the purposes of key information exchange with the Key Distributor. The Media Distributor will not terminate the DTLS signaling, but will instead forward DTLS packets received from an endpoint on to the Key Distributor (and vice versa) via a tunnel established between Media Distributor and the Key Distributor. This tunnel is used to encapsulate the DTLS-SRTP signaling between the Key Distributor and endpoints will also be used to convey HBH key information from the Key Distributor to the Media Distributor, so no additional protocol or interface is required. 4.5.2. Key Exchange during a Conference Following the initial key information exchange with the Key - Distributor, endpoints will be able to encrypt media end-to-end with - their E2E Key(i), sending that E2E Key(i) to other endpoints - encrypted with KEK, and will be able to encrypt and authenticate RTP - packets using local HBH Key(j). The procedures defined do not allow - the Media Distributor to gain access to the KEK information, - preventing it from gaining access to any endpoint's E2E key and - subsequently decrypting media. + Distributor, an endpoints will be able to encrypt media end-to-end + with an E2E key, sending that E2E key to other endpoints encrypted + with the KEK, and will be able to encrypt and authenticate RTP + packets using a HBH key. The procedures defined do not allow the + Media Distributor to gain access to the KEK information, preventing + it from gaining access to any endpoint's E2E key and subsequently + decrypting media. The KEK (i.e., EKTKey) may need to change from time-to-time during the life of a conference, such as when a new participant joins or leaves a conference. Dictating if, when or how often a conference is to be re-keyed is outside the scope of this document, but this framework does accommodate re-keying during the life of a conference. - When a Key Distributor decides to rekey a conference, it transmits a + When a Key Distributor decides to re-key a conference, it transmits a specific message defined in PERC EKT [I-D.ietf-perc-srtp-ekt-diet] to each of the conference participants. The endpoint MUST create a new SRTP master key and prepare to send that key inside a Full EKT Field using the new EKTKey. Since it may take some time for all of the endpoints in conference to finish re-keying, senders MUST delay a short period of time before sending media encrypted with the new master key, but it MUST be prepared to make use of the information from a new inbound EKTKey immediately. See Section 2.2.2 of [I-D.ietf-perc-srtp-ekt-diet]. @@ -721,108 +720,314 @@ invaluable input on this document. Also, we would like to acknowledge Nermeen Ismail for serving on the initial versions of this document as a co-author. 9. References 9.1. Normative References [I-D.ietf-perc-double] Jennings, C., Jones, P., and A. Roach, "SRTP Double - Encryption Procedures", draft-ietf-perc-double-02 (work in - progress), October 2016. + Encryption Procedures", draft-ietf-perc-double-04 (work in + progress), April 2017. [I-D.ietf-perc-dtls-tunnel] Jones, P., Ellenbogen, P., and N. Ohlmeier, "DTLS Tunnel between a Media Distributor and Key Distributor to - Facilitate Key Exchange", draft-ietf-perc-dtls-tunnel-00 - (work in progress), March 2017. + Facilitate Key Exchange", draft-ietf-perc-dtls-tunnel-01 + (work in progress), April 2017. [I-D.ietf-perc-srtp-ekt-diet] Jennings, C., Mattsson, J., McGrew, D., and D. Wing, - "Encrypted Key Transport for Secure RTP", draft-ietf-perc- - srtp-ekt-diet-02 (work in progress), October 2016. + "Encrypted Key Transport for DTLS and Secure RTP", draft- + ietf-perc-srtp-ekt-diet-04 (work in progress), April 2017. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate - Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ - RFC2119, March 1997, + Requirement Levels", BCP 14, RFC 2119, + DOI 10.17487/RFC2119, March 1997, . [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, . [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, . [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, + 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, + Real-time Transport Protocol (SRTP)", RFC 6904, + DOI 10.17487/RFC6904, April 2013, . 9.2. Informative References [I-D.ietf-avtcore-rtp-topologies-update] Westerlund, M. and S. Wenger, "RTP Topologies", draft- ietf-avtcore-rtp-topologies-update-10 (work in progress), July 2015. - [I-D.ietf-roach-perc-webrtc] - Roach, A., "Using Privacy Enhanced Real-time Conferencing - (PERC) in a WebRTC Context", March 2017. - [I-D.ietf-rtcweb-security-arch] Rescorla, E., "WebRTC Security Architecture", draft-ietf- rtcweb-security-arch-12 (work in progress), June 2016. + [I-D.roach-perc-webrtc] + Roach, A., "Using Privacy Enhanced Real-time Conferencing + (PERC) in a WebRTC Context", draft-roach-perc-webrtc-00 + (work in progress), March 2017. + [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002, . [RFC4353] Rosenberg, J., "A Framework for Conferencing with the - Session Initiation Protocol (SIP)", RFC 4353, DOI - 10.17487/RFC4353, February 2006, + Session Initiation Protocol (SIP)", RFC 4353, + DOI 10.17487/RFC4353, February 2006, . [RFC4474] Peterson, J. and C. Jennings, "Enhancements for Authenticated Identity Management in the Session - Initiation Protocol (SIP)", RFC 4474, DOI 10.17487/ - RFC4474, August 2006, + Initiation Protocol (SIP)", RFC 4474, + DOI 10.17487/RFC4474, August 2006, . [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006, . [RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework for Establishing a Secure Real-time Transport Protocol (SRTP) Security Context Using Datagram Transport Layer Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May 2010, . [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, + Mixer Audio Level Indication", RFC 6464, + DOI 10.17487/RFC6464, December 2011, . +Appendix A. PERC Key Inventory + + PERC specifies the use of a number of different keys and, + understandably, it looks complicated or confusing on the surface. + This section summarizes the various keys used in the system, how they + are generated, and what purpose they serve. + + The keys are described in the order in which they would typically be + acquired. + + The various keys used in PERC are shown in Figure 3 below. + + +-----------+----------------------------------------------------+ + | Key | Description | + +-----------+----------------------------------------------------+ + | KEK | Key shared by all endpoints and used to encrypt | + | (EKT Key) | each endpoint's SRTP master key so receiving | + | | endpoints can decrypt media. | + +-----------+----------------------------------------------------+ + | HBH Key | Key used to encrypt media hop-by-hop. | + +-----------+----------------------------------------------------+ + | E2E Key | Key used to encrypt media end-to-end. | + +-----------+----------------------------------------------------+ + + Figure 3: Key Inventory + + As you can see, the number key types is very small. However, what + can be challenging is keeping track of all of the distinct E2E keys + as the conference grows in size. With 1,000 participants in a + conference, there will be 1,000 distinct SRTP master keys, all of + which share the same master salt. Each of those keys are passed + through the KDF defined in [RFC3711] to produce the actual encryption + and authentication keys. Complicating key management is the fact + that the KEK can change and, when it does, the endpoints generate new + SRTP master keys. And, of course, there is a new SRTP master salt to + go with those keys. Endpoints have to retain old keys for a period + of time to ensure they can properly decrypt late-arriving or out-of- + order packets. + + The time required to retain old keys (either EKT Keys or SRTP master + keys) is not specified, but they should be retained at least for the + period of time required to re-key the conference or handle late- + arriving or out-of-order packets. A period of 60s should be + considered a generous retention period, but endpoints may keep old + keys on hand until the end of the conference. + + Or more detailed explanation of each of the keys follows. + +A.1. DTLS-SRTP Exchange Yields HBH Keys + + The first set of keys acquired are for hop-by-hop encryption and + decryption. Assuming the use of Double [I-D.ietf-perc-double], the + endpoint would perform DTLS-SRTP exchange with the key distributor + and receive a key that is, in fact, "double" the size that is needed. + Per the Double specification, the E2E part is the first half of the + key, so the endpoint will just discard that information in PERC. It + is not used. The second half of the key material is for HBH + operations, so that half of the key (corresponding to the least + significant bits) is assigned internally as the HBH key. + + The media distributor doesn't perform DTLS-SRTP, but it is at this + point that the key distributor will inform the media distributor of + the HBH key value via the tunnel protocol + ([I-D.ietf-perc-dtls-tunnel]). The key distributor will send the + least significant bits corresponding to the half of the keying + material determined through DTLS-SRTP with the endpoint to the media + distributor via the tunnel protocol. There is a salt generated along + with the HBH key. The salt is also longer than needed for HBH + operations, thus only the least significant bits of the required + length (i.e., half of the generated salt material) are sent to the + media distributor via the tunnel protocol. + + No two endpoints will have the same HBH key, thus the media + distributor must keep track each distinct HBH key (and the + corresponding salt) and use it only for the specified hop. + + This key is also used for HBH encryption of RTCP. RTCP is not end- + to-end encrypted in PERC. + +A.2. The Key Distributor Transmits the KEK (EKT Key) + + Via the aforementioned DTLS-SRTP association, the key distributor + will send the endpoint the KEK (i.e., EKT Key per + [I-D.ietf-perc-srtp-ekt-diet]). This key is known only to the key + distributor and endpoints. This key is the most important to protect + since having knowledge of this key (and the SRTP master salt + transmitted as a part of the same message) will allow an entity to + decrypt any media packet in the conference. + + Note that the key distributor can send any number of EKT Keys to + endpoints. This can be used to re-key the entire conference. Each + key is identified by a "Security Parameter Index" (SPI) value. + Endpoints should expect that a conference might be re-keyed when a + new participant joins a conference or when a participant leaves a + conference in order to protect the confidentiality of the + conversation before and after such events. + + The SRTP master salt to be used by the endpoint is transmitted along + with the EKT Key. All endpoints in the conference utilize the same + SRTP master salt that corresponds with a given EKT Key. + + The EKT Field in media packets is encrypted using a cipher specified + via the EKTKey message (e.g., AES Key Wrap with a 128-bit key). This + cipher is different than the cipher used to protect media and is only + used to encrypt the endpoint's SRTP master key (and other EKT Field + data as per [I-D.ietf-perc-srtp-ekt-diet]). + + The media distributor is not given the KEK (i.e., EKT Key). + +A.3. Endpoints fabricate an SRTP Master Key + + As stated earlier, the E2E key determined via DTLS-SRTP is discarded. + While it could have been used, the fact that endpoints may need to + change the SRTP master key periodically or are forced to change the + SRTP master key as a result of the EKT key changing means using it + has only limited utility. To reduce complexity, PERC prescribes that + endpoints manufacturer random SRTP master keys locally to be used for + E2E encryption. + + This locally-generated SRTP master key is used along with the master + salt transmitted to the endpoint from the key distributor via the + EKTKey message to encrypt media end-to-end. + + Since the media distributor is not involved in E2E functions, it will + not create this key nor have access to any endpoint's E2E key. Note, + too, that even the key distributor is unaware of the locally- + generated E2E keys used by each endpoint. + + The endpoint will transmit its E2E key to other endpoints in the + conference by periodically including it in SRTP packets in a Full EKT + Field. When placed in the Full EKT Field, it is encrypted using the + EKT Key provided by the key distributor. The master salt is not + transmitted, though, since all endpoints will have received the same + master salt via the EKTKey message. The recommended frequency with + which an endpoint transmits its SRTP master key is specified in + [I-D.ietf-perc-srtp-ekt-diet]. + +A.4. Who has What Key + + All endpoints have knowledge of the KEK. + + Every HBH key is distinct for a given endpoint, thus Endpoint A and + endpoint B do not have knowledge of the other's HBH key. + + Each endpoint generates its own E2E Key (SRTP master key), thus the + key distinct per endpoint. This key is transmitted (encrypted) via + the EKT Field to other endpoints. Endpoints that receive media from + a given transmitting endpoint will therefore have knowledge of the + transmitter's E2E key. + + To summarize the various keys and which entity is in possession of a + given key, refer to Figure 4. + + +----------------------+------------+-------+-------+------------+ + | Key / Entity | Endpoint A | MD X | MD Y | Endpoint B | + +----------------------+------------+-------+-------+------------+ + | KEK | Yes | No | No | Yes | + +----------------------+------------+-------+-------+------------+ + | E2E Key (A and B) | Yes | No | No | Yes | + +----------------------+------------+-------+-------+------------+ + | HBH Key (A<=>MD X) | Yes | Yes | No | No | + +----------------------+------------+-------+-------+------------+ + | HBH Key (B<=>MD Y) | No | No | Yes | Yes | + +----------------------+------------+---------------+------------+ + | HBH Key (MD X<=>MD Y)| No | Yes | Yes | No | + +----------------------+------------+---------------+------------+ + + Figure 4: Keys per Entity + +Appendix B. PERC Packet Format + + Figure 5 presents a complete picture of what a PERC packet looks like + when transmitted over the wire. + + 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 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + A |V=2|P|X| CC |M| PT | sequence number | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + A | timestamp | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + A | synchronization source (SSRC) identifier | + +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ + A | contributing source (CSRC) identifiers | + A | .... | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + A | RTP extension (OPTIONAL) | + A | (including the OHB) | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + C : : + C : Ciphertext Payload : + C : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + R : : + R : EKT Field : + R : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + C = Ciphertext (encrypted and authenticated) + A = Associated Data (authenticated only) + R = neither encrypted nor authenticated, added + after Authenticated Encryption completed + + Figure 5: PERC Packet Format + Authors' Addresses Paul E. Jones Cisco 7025 Kit Creek Rd. Research Triangle Park, North Carolina 27709 USA Phone: +1 919 476 2048 Email: paulej@packetizer.com