Network Working Group P. Jones Internet-Draft D. Benham Intended status: Standards Track Cisco Expires: November 14, 2016 May 13, 2016 A Solution Framework for Private Media in Privacy Enhanced RTP Conferencing draft-ietf-perc-private-media-framework-00 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). 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 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 November 14, 2016. Copyright Notice Copyright (c) 2016 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 Jones & Benham Expires November 14, 2016 [Page 1] Internet-Draft Private Media Framework May 2016 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 3.1. Untrusted Entities . . . . . . . . . . . . . . . . . . . 5 3.1.1. MDD . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1.2. Call Processing . . . . . . . . . . . . . . . . . . . 6 3.2. Trusted Entities . . . . . . . . . . . . . . . . . . . . 6 3.2.1. Endpoint . . . . . . . . . . . . . . . . . . . . . . 6 3.2.2. KMF . . . . . . . . . . . . . . . . . . . . . . . . . 7 4. Framework for PERC . . . . . . . . . . . . . . . . . . . . . 7 4.1. End-to-End and Hop-by-Hop Authenticated Encryption . . . 7 4.2. E2E Key Confidentiality . . . . . . . . . . . . . . . . . 8 4.3. E2E Keys and Endpoint Operations . . . . . . . . . . . . 9 4.4. HBH Keys and Hop Operations . . . . . . . . . . . . . . . 9 4.5. Key Exchange . . . . . . . . . . . . . . . . . . . . . . 10 4.5.1. Initial Key Exchange and KMF . . . . . . . . . . . . 10 4.5.2. Key Exchange during a Conference . . . . . . . . . . 11 5. Entity Trust . . . . . . . . . . . . . . . . . . . . . . . . 11 5.1. Identity Assertions . . . . . . . . . . . . . . . . . . . 12 5.2. Certificate Fingerprints in Session Signaling . . . . . . 12 6. Attacks on Privacy Enhanced RTP Conferencing . . . . . . . . 12 6.1. Third Party Attacks . . . . . . . . . . . . . . . . . . . 13 6.2. MDD Attacks . . . . . . . . . . . . . . . . . . . . . . . 13 6.2.1. Denial of service . . . . . . . . . . . . . . . . . . 13 6.2.2. Replay Attack . . . . . . . . . . . . . . . . . . . . 14 6.2.3. Delayed Playout Attack . . . . . . . . . . . . . . . 14 6.2.4. Splicing Attack . . . . . . . . . . . . . . . . . . . 14 7. To-Do List . . . . . . . . . . . . . . . . . . . . . . . . . 15 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 9. Security Considerations . . . . . . . . . . . . . . . . . . . 15 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 11.1. Normative References . . . . . . . . . . . . . . . . . . 16 11.2. Informative References . . . . . . . . . . . . . . . . . 17 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 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 Jones & Benham Expires November 14, 2016 [Page 2] Internet-Draft Private Media Framework May 2016 distribution device (MDD), as might be the case with a traditional media server or multipoint control unit (MCU). Instead, media flows transmitted by conference participants are simply forwarded by the MDD to each of the other participants, often forwarding only a subset of flows based on voice activity detection or other criteria. In some instances, the MDDs may make limited modifications to RTP [RFC3550] headers, for example, but the actual media content (e.g., voice or video data) is unaltered. An advantage of switched conferencing is that media distribution devices can be more easily deployed on general-purpose computing hardware, including virtualized environments in private and public clouds. Deploying conference resources in a public cloud environment might introduce a higher security risk. Whereas traditional conference resources were usually deployed in private networks that were protected, cloud-based conference resources might be viewed as less secure since they are not always physically controlled by those who use them. Additionally, there are usually several ports open to the public in cloud deployments, such as for remote administration, and so on. This document defines a solution framework wherein media privacy is ensured by making it impossible for an media distribution device to gain access to keys needed to decrypt or authenticate the actual media content sent between conference participants. At the same time, the framework allows for the media distribution device to modify certain RTP headers; add, remove, encrypt, or decrypt RTP header extensions; and encrypt and decrypt RTCP packets. The framework also prevents replay attacks by authenticating each packet transmitted between a given participant and the media distribution device using a unique key per endpoint that is independent from the key for media encryption and authentication. A goal of this document is to define a framework for enhanced privacy in RTP-based conferencing environments while utilizing existing security procedures defined for RTP with minimal enhancements. 2. Conventions Used in This Document 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] when they appear in ALL CAPS. These words may also appear in this document in lower case as plain English words, absent their normative meanings. Additionally, this solution framework uses the following conventions, terms and acronyms: Jones & Benham Expires November 14, 2016 [Page 3] Internet-Draft Private Media Framework May 2016 E2E (End-to-End): Communications from one endpoint through one or more Media Distribution Devices to the endpoint at the other end. HBH (Hop-by-Hop): Communications between an endpoint and an Media Distribution Device or between Media Distribution Devices. Endpoint: An RTP flow terminating entity that has possession of E2E media encryption keys and terminates E2E encryption. This may include embedded user conferencing equipment or browsers on computers, media gateways, MCUs, media recording device and more that are in the trusted domain for a given deployment. MDD (Media Distribution Device): An RTP middlebox that is not allowed to to have access to E2E encryption keys. It may operate according to any of the RTP topologies [I-D.ietf-avtcore-rtp-topologies-update] per the constraints defined by the PERC system, which includes, but not limited to, having no access to RTP media unencrypted and having limits on what RTP header field it can alter. KMF (Key Management Function): An entity that is a logical function which passes keying material and related information to endpoints and Media Distribution Devices that is appropriate for each. The KMF might be co-resident with another entity trusted with E2E keying material. Conference: Two or more participants communicating via trusted endpoints to exchange RTP flows through one or more Media Distribution Devices. Third Party: Any entity that is not an Endpoint, MDD, KMF or Call Processing entity as described in this document. 3. PERC Entities and Trust Model The following figure depicts the trust relationships, direct or indirect, between entities described in the subsequent sub-sections. Note that these entities may be co-located or further divided into multiple, separate physical devices. Please note that some entities classified as untrusted in the simple, general deployment scenario used most commonly in this document might be considered trusted in other deployments. This document does not preclude such scenarios, but will keep the definitions and examples focused by only using the the simple, most general deployment scenario. Jones & Benham Expires November 14, 2016 [Page 4] Internet-Draft Private Media Framework May 2016 | +----------+ | +-----------------+ | Endpoint | | | Call Processing | +----------+ | +-----------------+ | | +----------------+ | +--------------------+ | Key Management | | | Media Distribution | | Function | | | Device | +----------------+ | +--------------------+ | Trusted | Untrusted Entities | Entities | Figure 1: Trusted and Untrusted Entities in PERC 3.1. Untrusted Entities The architecture described in this framework document enables conferencing infrastructure to be hosted in domains, such as in a cloud conferencing provider's facilities, where the trustworthiness is below the level needed to assume the privacy of participant's media will not be compromised. The conferencing infrastructure in such a domain is still trusted with reliably connecting the participants together in a conference, but not trusted with keying material needed to decrypt any of the participant's media. Entities in such lower trustworthiness domains will simply be referred to as untrusted entities from this point forward. This does not mean that they are completely untrusted as they may be trusted with most non- media related aspects of hosting a conference. 3.1.1. MDD A Media Distribution Device (MDD) forwards RTP flows between endpoints in the conference while performing per-hop authentication of each RTP packet. The MDD may need access to one or more RTP headers or header extensions, potentially adding or modifying a certain subset. The MDD will also relay secured messaging between the endpoints and the key management function and will acquire per- hop key information from the KMF. The actual media content MUST NOT not be decryptable by an MDD, 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 an MDD MUST NOT alone be interpreted as being authorized to have access to the E2E Jones & Benham Expires November 14, 2016 [Page 5] Internet-Draft Private Media Framework May 2016 media encryption keys, as the MDD does not have the ability to determine whether an endpoint is authorized. An MDD 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. pre-PERC) deployments. An MDD or associated conferencing infrastructure may also initiate or terminate various conference control related messaging, which is outside the scope of this framework document. 3.1.2. Call Processing The call processing function is untrusted in the simple, general deployment scenario. When a physical subset of the call processing function resides in facilities outside the trusted domain, it should not be trusted to have access to E2E key information. The call processing function may include the processing of call signaling messages, as well as the signing of those messages. It may also authenticate the endpoints for the purpose of call signaling and subsequently joining of a conference hosted through one or more MDDs. Call processing may optionally ensure the privacy of call signaling messages between itself, the endpoint, and other entities. In any deployment scenario where the call processing function is considered trusted, the call processing function MUST ensure the integrity of received messages before forwarding to other entities. 3.2. Trusted Entities From the PERC model system perspective, entities considered trusted (refer to Figure 1) can be in possession of the E2E media encryption key(s) for one or more conferences. 3.2.1. Endpoint An endpoint is considered trusted and will have access to E2E key information. While it is possible for an endpoint to be compromised, subsequently performing in undesired ways, defining endpoint resistance to compromise is outside the scope of this document. Endpoints will take measures to mitigate the adverse effects of denial of service attacks (refer to Section 6) from other entities, including from other endpoints, to a level equal to or better than traditional conference (i.e., pre-PERC) deployments. Jones & Benham Expires November 14, 2016 [Page 6] Internet-Draft Private Media Framework May 2016 3.2.2. KMF The KMF, which may be collocated with an endpoint or exist standalone, is responsible for providing key information to endpoints for both end-to-end and hop-by-hop security and for providing key information to MDDs for the hop-by-hop security. Interaction between the KMF and the call processing function may be necessary to for proper conference-to-endpoint mappings, which may or may not be satisfied by getting information directly from the endpoints or via some other means. See Section 7 for a related item in the To Do list. The KMF needs to be secured and managed in a way to prevent exploitation by an adversary, as any kind of compromise of the KMF puts the security of the conference at risk. 4. Framework for PERC The purpose for this framework is to define a means through which media privacy can be ensured when communicating within a conferencing environment consisting of one or more MDDs that only switch, hence not terminate, media. It does not otherwise attempt to hide the fact that a conference between endpoints is taking place. This framework reuses several specified RTP security technologies, including SRTP [RFC3711], PERC EKT [I-D.ietf-perc-srtp-ekt-diet], and DTLS-SRTP [RFC5764]. 4.1. End-to-End and Hop-by-Hop Authenticated Encryption This solution framework focuses on the end-to-end privacy and integrity of the participant's media by limiting access to end-to-end key information to trusted entities. However, this framework does give an MDD 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 an MDD 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 an MDD or between MDDs. Reference the following figure. Jones & Benham Expires November 14, 2016 [Page 7] Internet-Draft Private Media Framework May 2016 +--------+ HBH +-----+ HBH +-----+ HBH +--------+ | |=============| |=========| |=============| | |Endpoint|-E2E Key(A)->| MDD |-------->| MDD |------------>|Endpoint| | A |<------------| X |<--------| Y |<-E2E Key(B)-| B | | |=============| |=========| |=============| | +--------+ Key(AX) +-----+ Key(XY) +-----+ Key(YB) +--------+ 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 MDD just performs the outer, hop-by-hop operation. 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 EKT Key, 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 | MDD X | MDD Y | Endpoint B | +---------------------+------------+---------------+------------+ | KEK | Yes | No | No | Yes | +---------------------+------------+---------------+------------+ | E2E Key (i) | Yes | No | No | Yes | +---------------------+------------+---------------+------------+ | HBH Key (A<=>MDD X) | Yes | Yes | No | No | +---------------------+------------+---------------+------------+ | HBH Key (B<=>MDD Y) | No | No | Yes | Yes | +---------------------+------------+---------------+------------+ Figure 2: Keys per Entity Jones & Benham Expires November 14, 2016 [Page 8] Internet-Draft Private Media Framework May 2016 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., EKT Key), 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. 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 MDD will not be able use the information contained in those header extensions encrypted with E2E keys. See Section 7 for a related item in the To Do list. 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 an MDD, as well between MDDs. 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. Using hop-by-hop authentication gives the MDD the ability to change certain RTP header values. Which values the MDD can change in the RTP header are defined in [I-D.ietf-perc-double]. RTCP can only be encrypted, giving the MDD 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). If there is a need to encrypt one or more RTP header extensions hop- by-hop, an encryption key is derived from the hop-by-hop SRTP master key to encrypt header extensions as per [RFC6904]. This will still give the switching MDD visibility into header extensions, such as the one used to determine audio level [RFC6464] of conference participants. Note that when RTP header extensions are encrypted, all hops - in the untrusted domain at least - will need to decrypt and re-encrypt these encrypted header extensions. Jones & Benham Expires November 14, 2016 [Page 9] Internet-Draft Private Media Framework May 2016 4.5. Key Exchange To facilitate key exchange required to establish or generate an E2E key and a HBH key for an endpoint and the same HBH key for the MDD, this framework utilizes a DTLS-SRTP [RFC5764] association between an endpoint and the KMF. To establish this association, an endpoint will send DTLS-SRTP messages to the MDD which will then forward them to the MDD as defined in [I-D.jones-perc-dtls-tunnel]. The Key Encryption Key (KEK) (i.e., EKT Key) is also conveyed by the KMF over the DTLS association to endpoints via procedures defined in PERC EKT [I-D.ietf-perc-srtp-ekt-diet]. MDDs use DTLS-SRTP [RFC5764] directly with a peer MDD to establish HBH keys for transmitting RTP and RTCP packets that peer MDD. The KMF does not facilitate establishing HBH keys for use between MDDs. 4.5.1. Initial Key Exchange and KMF The procedures defined in DTLS Tunnel for PERC [I-D.jones-perc-dtls-tunnel] establish one or more DTLS tunnels between the MDD and KMF, making it is possible for the MDD to facilitate the establishment of a secure DTLS association between each endpoint and the KMF as shown the following figure. The DTLS association between endpoints and the KMF will enable each endpoint to receive E2E key information, Key Encryption Key (KEK) information (i.e., EKT Key), and HBH key information. At the same time, the KMF can securely provide the HBH key information to the MDD. The key information summarized here may include the SRTP master key, SRTP master salt, and the negotiated cryptographic transform. KEK info +---------+ HBH Key info to endpoints | KMF | to endpoints & MDD +---------+ | ^ ^ | | | | |-DTLS Tunnel | | | | +-----------+ +---------+ +-----------+ | Endpoint | DTLS | MDD | DTLS | Endpoint | | KEK |<--------| |-------->| KEK | | HBH Key(j)| to KMF | HBH Keys| to KMF | HBH Key(j)| +-----------+ +---------+ +-----------+ Figure 3: 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 Jones & Benham Expires November 14, 2016 [Page 10] Internet-Draft Private Media Framework May 2016 with the KMF. The MDD will not terminate the DTLS signaling, but will instead forward DTLS packets received from an endpoint on to the KMF (and vice versa) via a tunnel established between MDD and the KMF. This tunnel used to encapsulate the DTLS-SRTP signaling between the KMF and endpoints will also be used to convey HBH key information from the KMF to the MDD, so no additional protocol or interface is required. 4.5.2. Key Exchange during a Conference Following the initial key information exchange with the KMF, 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 MDD 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., EKT Key) 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 KMF decides to rekey 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 EKT Key. 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 EKT Key immediately. See Section 2.2.2 of [I-D.ietf-perc-srtp-ekt-diet]. 5. Entity Trust It is important to this solution framework that the entities can trust and validate the authenticity of other entities, especially the KMF and endpoints. The details of this are outside the scope of specification but a few possibilities are discussed in the following sections. The key requirements is that endpoints can verify they are connected to the correct KMF for the conference and the KMF can verify the endpoints are the correct endpoints for the conference. Two possible approaches to solve this are Identity Assertions and Certificate Fingerprints. Jones & Benham Expires November 14, 2016 [Page 11] Internet-Draft Private Media Framework May 2016 5.1. Identity Assertions WebRTC Identity assertion (EDITOR NOTE: add I-D reference) can be used to bind the identity of the user of the endpoint to the fingerprint of the DTLS-SRTP certificate used for the call. This certificate is unique for a given call and a conference. This allows the KMF to ensure that only authorized users participate in the conference. Similarly the KMF can create a WeBRTC Identity assertion bound the fingerprint of the unique certificate used by the KMF for this conference so that the endpoint can validate it is talking to the correct KMF. 5.2. Certificate Fingerprints in Session Signaling Entities managing session signaling are generally assumed to be untrusted in the PERC framework. However, there are some deployment scenarios where parts of the session signaling may be assumed trustworthy for the purposes of exchanging, in a manner that can be authenticated, the fingerprint of an entity's certificate. As a concrete example, SIP [RFC3261] and SDP [RFC4566] can be used to convey the fingerprint information per [RFC5763]. An endpoint's SIP User Agent would send an INVITE message containing SDP for the media session along with the endpoint's certificate fingerprint, which can be signed using the procedures described in [RFC4474] for the benefit of forwarding the message to other entities. Other entities can now verify the fingerprints match the certificates found in the DTLS-SRTP connections to find the identity of the far end of the DTLS-SRTP connection and check that is the authorized entity. Ultimately, if using session signaling, an endpoint's certificate fingerprint would need to be securely mapped to a user and conveyed to the KMF so that it can check that that user is authorized. Similarly, the KMF's certificate fingerprint can be conveyed to endpoint in a manner that can be authenticated as being an authorized KMF for this conference. 6. Attacks on Privacy Enhanced RTP Conferencing This framework, and the individual protocols defined to support it, must take care to not increase the exposure to Denial of Service (DoS) attacks by untrusted or third-party entities and should take measures to mitigate, where possible, more serious DoS attacks from on-path and off-path attackers. The following section enumerates the kind of attacks that will be considered in the development of this framework's solution. Jones & Benham Expires November 14, 2016 [Page 12] Internet-Draft Private Media Framework May 2016 6.1. Third Party Attacks On-path attacks are mitigated by HBH integrity protection and encryption. The integrity protection mitigates packet modification and encryption makes selective blocking of packets harder, but not impossible. Off-path attackers may try connecting to different PERC entities and send specifically crafted packets. A successful attacker might be able to get the MDD to forward such packets. If not making use of HBH authentication on the MDD, such an attack could only be detected in the receiving endpoints where the forged packets would finally be dropped. Another potential attack is a third party claiming to be an MDD, fooling endpoints in to sending packets to the false MDD instead of the correct one. The deceived sending endpoints could incorrectly assuming their packets have been delivered to endpoints when they in fact have not. Further, the false MDD may cascade to another legitimate MDD creating a false version of the real conference. This attack can be mitigated by the false MDD not being authenticated by the KMF during PERC Tunnel establishment. Without the tunnel in place, endpoints will not establish secure associations with the KMF and receive the KEK, causing the conference to not proceed. 6.2. MDD Attacks The MDD can attack the session in a number of possible ways. 6.2.1. Denial of service Any modification of the end-to-end authenticated data will result in the receiving endpoint getting an integrity failure when performing authentication on the received packet. The MDD can also attempt to perform resource consumption attacks on the receiving endpoint. One such attack would be to insert random SSRC/CSRC values in any RTP packet with an inband key-distribution message attached (i.e., Full EKT Field). Since such a message would trigger the receiver to form a new cryptographic context, the MDD can attempt to consume the receiving endpoints resources. Another denial of service attack is where the MDD rewrites the PT field to indicate a different codec. The effect of this attack is that any payload packetized and encoded according to one RTP payload format is then processed using another payload format and codec. Assuming that the implementation is robust to random input, it is Jones & Benham Expires November 14, 2016 [Page 13] Internet-Draft Private Media Framework May 2016 unlikely to cause crashes in the receiving software/hardware. However, it is not unlikely that such rewriting will cause severe media degradation. For audio formats, this attack is likely to cause highly disturbing audio and/or can be damaging to hearing and playout equipment. 6.2.2. Replay Attack Replay attack is when an already received packets from a previous point in the RTP stream is replayed as new packet. This could, for example, allow an MDD to transmit a sequence of packets identified as a user saying "yes", instead of the "no" the user actually said. The mitigation for a replay attack is to prevent old packets beyond a small-to-modest jitter and network re-ordering sized window to be rejected. End-to-end replay protection MUST be provided for the whole duration of the conference. 6.2.3. Delayed Playout Attack The delayed playout attack is a variant of the replay attack. This attack is possible even if E2E replay protection is in place. However, due to fact that the MDD is allowed to select a sub-set of streams and not forward the rest to a receiver, such as in forwarding only the most active speakers, the receiver has to accept gaps in the E2E packet sequence. The issue with this is that an MDD can select to not deliver a particular stream for a while. Within the window from last packet forwarded to the receiver and the latest received by the MDD, the MDD can select an arbitrary starting point when resuming forwarding packets. Thus what the media source said can be substantially delayed at the receiver with the receiver believing that it is what was said just now, and only delayed due to transport delay. 6.2.4. Splicing Attack The splicing attack is an attack where an MDD receiving multiple media sources splices one media stream into the other. If the MDD is able to change the SSRC without the receiver having any method for verifying the original source ID, then the MDD could first deliver stream A and then later forward stream B under the same SSRC as stream A was previously using. Not allowing the MDD to change the SSRC mitigates this attack. Jones & Benham Expires November 14, 2016 [Page 14] Internet-Draft Private Media Framework May 2016 7. To-Do List o The mapping of endpoints-to-conference identifiers may need to be conveyed in the framework. Need Revisit this text after a design choice is made between alternatives. o Consider adding a list of RTP header extensions that should/must not be E2E encrypted. o Endpoints, KMF and MDD provider must securely convey their respective certificate information directly or indirectly via some means, such as via an identity service provider. o If as in "Double" draft, the ROC value is no longer in the clear and associated with the "outer" protection scheme, we may need to require that the MDD maintain a separate ROC value for each SSRC sent to each endpoint. This ROC value should start at 0 regardless of the sequence number in that first packet sent to an endpoint. o Investigate adding ability to enable the transmission of one-way media from a non-trusted device (e.g., announcements). One possible solution is to have the KMF send an "ekt_key" message that is explicitly labeled for receive-only and giving that to announcement servers. A possible approach would be to define EKT Keys with a SPI > 32000, for example, for this purpose and endpoints should only use those EKT Keys to decrypt Full EKT Fields received from such transmitters. Endpoints would never send media with EKT Keys with those SPI values. 8. IANA Considerations There are no IANA considerations for this document. 9. Security Considerations [TBD] 10. Acknowledgments The authors would like to thank Mo Zanaty and Christian Oien for 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. Jones & Benham Expires November 14, 2016 [Page 15] Internet-Draft Private Media Framework May 2016 11. References 11.1. Normative References [I-D.ietf-perc-double] Jennings, C., Jones, P., and A. Roach, "SRTP Double Encryption Procedures", draft-ietf-perc-double-00 (work in progress), May 2016. [I-D.ietf-perc-srtp-ekt-diet] Mattsson, J., McGrew, D., Wing, D., and C. Jennings, "Encrypted Key Transport for Secure RTP", draft-ietf-perc- srtp-ekt-diet-00 (work in progress), May 2016. [I-D.jones-perc-dtls-tunnel] Jones, P., "DTLS Tunnel between Media Distribution Device and Key Management Function to Facilitate Key Exchange", draft-jones-perc-dtls-tunnel-02 (work in progress), March 2016. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 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, . [RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure Real-time Transport Protocol (SRTP)", RFC 6904, DOI 10.17487/RFC6904, April 2013, . Jones & Benham Expires November 14, 2016 [Page 16] Internet-Draft Private Media Framework May 2016 11.2. 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. [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, . [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, . [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, . Authors' Addresses Paul Jones Cisco 7025 Kit Creek Rd. Research Triangle Park, North Carolina 27709 USA Phone: +1 919 476 2048 Email: paulej@packetizer.com Jones & Benham Expires November 14, 2016 [Page 17] Internet-Draft Private Media Framework May 2016 David Benham Cisco 170 West Tasman Drive San Jose, California 95134 USA Email: dbenham@cisco.com Jones & Benham Expires November 14, 2016 [Page 18]