| Network Working Group | P. Jones |
| Internet-Draft | Cisco |
| Intended status: Standards Track | D. Benham |
| Expires: November 1, 2019 | C. Groves |
| Independent | |
| April 30, 2019 |
A Solution Framework for Private Media in Privacy Enhanced RTP Conferencing
draft-ietf-perc-private-media-framework-10
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 distributors 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).
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 1, 2019.
Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
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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 conference participants are simply forwarded by Media Distributors to each of the other participants. Media Distributors often forward only a subset of flows based on voice activity detection or other criteria. In some instances, Media Distributors 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 Distributors can be more easily deployed on general-purpose computing hardware, including virtualized environments in private and public clouds. Virtualized public cloud environments have been viewed as less secure since resources are not always physically controlled by those who use them and since there are usually several ports open to the public. This document aims to improve security so as to lower the barrier to taking advantage of those environments.
This document defines a solution framework wherein media privacy is ensured by making it impossible for a media distributor 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 Distributors to modify certain RTP headers; add, remove, encrypt, or decrypt RTP header extensions; and encrypt and decrypt RTP Control Protocol (RTCP) [RFC3550] packets. The framework also prevents replay attacks by authenticating each packet transmitted between a given participant and the media distributor 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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
Additionally, this solution framework uses the following terms and acronyms:
End-to-End (E2E): Communications from one endpoint through one or more Media Distributors to the endpoint at the other end.
Hop-by-Hop (HBH): Communications between an endpoint and a Media Distributor or between Media Distributors.
Trusted 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. In the context of WebRTC, where control of a session is divided between a JavaScript application and a browser, the browser acts as the Trusted Endpoint for purposes of this framework (just as it acts as the endpoint for DTLS-SRTP [RFC5764] in one-to-one calls).
Media Distributor (MD): An RTP middlebox that forwards endpoint media content (e.g., voice or video data) unaltered, either a subset or all of the flows at any given time, and is never allowed to have access to E2E encryption keys. It operates according to the Selective Forwarding Middlebox RTP topologies [RFC7667] 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. The header fields that may be modified by a Media Distributor are enumerated in Section 4 of the Double cryptographic transform specification [I-D.ietf-perc-double] and chosen with respect to utility and the security considerations outlined in this document.
Key Distributor: An entity that is a logical function which distributes keying material and related information to trusted endpoints and Media Distributor(s), only that which is appropriate for each. The Key Distributor 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 Distributor.
Call Processing: All trusted endpoints in the conference connect to it by a call processing dialog, such as with the Focus defined in the Framework for Conferencing with SIP [RFC4353].
Third Party: Any entity that is not an Endpoint, Media Distributor, Key Distributor or Call Processing entity as described in this document.
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 keeps the definitions and examples focused by only using the the simple, most general deployment scenario.
|
+----------+ | +-----------------+
| Endpoint | | | Call Processing |
+----------+ | +-----------------+
|
|
+----------------+ | +--------------------+
| Key Distributor| | | Media Distributor |
+----------------+ | +--------------------+
|
Trusted | Untrusted
Entities | Entities
|
Figure 1: Trusted and Untrusted Entities in PERC
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 is not 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 are referred to as untrusted entities from this point forward.
It is important to understand that untrusted in this document does not mean an entity is not expected to function properly. Rather, it means only that the entity does not have access to the E2E media encryption keys.
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 also relays secured messaging between the endpoints and the Key Distributor and acquires per-hop key information from the Key Distributor. The actual media content must not be decryptable by a Media Distributor, as it is untrusted to have access to the E2E media encryption keys. The key exchange mechanisms specified in this framework prevents the Media Distributor from gaining access to the E2E media encryption keys.
An endpoint's ability to connect to a conference serviced by a Media Distributor does imply that the endpoint is 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. Instead, the Key Distributor is responsible for authenticating the endpoint (e.g., using WebRTC Identity [I-D.ietf-rtcweb-security-arch]) and determining its authorization to receive E2E and HBH media encryption keys.
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 8) 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.
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 Media Distributors. Call processing may optionally ensure the privacy of call signaling messages between itself, the endpoint, and other entities.
From the PERC model system perspective, entities considered trusted (refer to Figure 1) can be in possession of the E2E media encryption keys for one or more conferences.
An endpoint is considered trusted and has 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 take measures to mitigate the adverse effects of denial of service attacks (refer to Section 8) from other entities, including from other endpoints, to a level equal to or better than traditional conference (i.e., non-PERC) deployments.
The Key Distributor, which may be colocated with an endpoint or exist standalone, is responsible for providing key information to endpoints for both end-to-end (E2E) and hop-by-hop (HBH) security and for providing key information to Media Distributors for the hop-by-hop security.
Interaction between the Key Distributor and the call processing function is necessary for proper conference-to-endpoint mappings. This is described in Section 5.3.
The Key Distributor needs to be secured and managed in a way to prevent exploitation by an adversary, as any kind of compromise of the Key Distributor puts the security of the conference at risk.
The purpose for this framework is to define a means through which media privacy is ensured when communicating within a conferencing environment consisting of one or more Media Distributors 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 Secure Real-time Transport Protocol (SRTP) [RFC3711], Encrypted Key Transport (EKT) [I-D.ietf-perc-srtp-ekt-diet], and DTLS-SRTP [RFC5764].
This solution framework focuses on the end-to-end privacy and integrity of the participant's media by limiting access to only trusted entities to the E2E key used for authenticated end-to-end encryption. 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 (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.
+-------------+ +-------------+ | |################################| | | Media |------------------------ *----->| Media | | Distributor |<----------------------*-|------| Distributor | | X |#####################*#|#|######| Y | | | | | | | | +-------------+ | | | +-------------+ # ^ | # HBH Key (XY) -+ | | # ^ | # # | | # E2E Key (B) ---+ | # | | # # | | # E2E Key (A) -----+ # | | # # | | # # | | # # | | # # | | # # | | *---- HBH Key (AX) HBH Key (YB) ----* | | # # | | # # | | # # *--------- E2E Key (A) E2E Key (A) ---------* # # | *------- E2E Key (B) E2E Key (B) -------* | # # | | # # | | # # | v # # | v # +-------------+ +-------------+ | Endpoint A | | Endpoint B | +-------------+ +-------------+
Figure 2: E2E and HBH Keys Used for Authenticated Encryption of SRTP Packets
The Double transform [I-D.ietf-perc-double] enables endpoints to perform encryption using both the end-to-end and hop-by-hop contexts while still preserving the same overall interface as other SRTP transforms. The Media Distributor simply uses the corresponding normal (single) AES-GCM transform, keyed with the appropriate HBH keys. See Section 6.1 for a description of the keys used in PERC and Section 7 for diagram of how encrypted RTP packets appear on the wire.
RTCP is only 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.
To ensure the confidentiality of E2E keys shared between endpoints, endpoints use a common Key Encryption Key (KEK) that is known only by the trusted entities in a conference. That KEK, defined in the EKT [I-D.ietf-perc-srtp-ekt-diet] specification as the EKT Key, is 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 creates an SRTP master key for E2E authenticated encryption and keep track of the E2E keys received via the Full EKT Tag for each distinct synchronization source (SSRC) in the conference so that it can properly decrypt received media. 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].
Any given RTP media flow is identified by its SSRC, and an endpoint might send more than one at a time and change the mix of media flows transmitted during the life of a conference.
Thus, an endpoint MUST maintain a list of SSRCs from received RTP flows and each SSRC's associated E2E key information. An endpoint MUST discard old E2E keys no later than when it leaves the conference (see Section 4.5.2).
If there is a need to encrypt one or more RTP header extensions end-to-end, the endpoint derives an encryption key from the E2E SRTP master key to encrypt header extensions as per [RFC6904]. The Media Distributor is unable use the information contained in those header extensions encrypted with an E2E key.
To ensure the integrity of transmitted media packets, it is REQUIRED that every packet be authenticated hop-by-hop 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. Each HBH key is distinct per hop and no two hops ever use the same SRTP master key.
While endpoints also perform HBH authentication, the ability of the endpoints to reconstruct the original RTP header also enables the endpoints to authenticate RTP packets E2E. This design yields flexibility to the Media Distributor to change certain RTP header values as packets are forwarded. Which values the Media Distributor can change in the RTP header are defined in [I-D.ietf-perc-double]. RTCP can only be encrypted hop-by-hop, 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 8). The use of the replay protection mechanism specified in Section 3.3.2 of [RFC3711] is REQUIRED at each hop.
If there is a need to encrypt one or more RTP header extensions hop-by-hop, the endpoint derives an encryption key from the HBH SRTP master key to encrypt header extensions as per [RFC6904]. This still gives the Media Distributor 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 need to decrypt and re-encrypt these encrypted header extensions.
In brief, the keys used by any given endpoints are determined in the following way:
The Media Distributor maintains a tunnel with the Key Distributor (e.g., using [I-D.ietf-perc-dtls-tunnel]), making it 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 enables each endpoint to generate E2E and HBH keys and receive the Key Encryption Key (KEK) (i.e., EKT Key). At the same time, the Key Distributor securely provides 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
+-----------+
# ^ ^ #
# | | #--- Tunnel
# | | #
+-----------+ +-----------+ +-----------+
| Endpoint | DTLS | Media | DTLS | Endpoint |
| KEK |<------------|Distributor|------------>| KEK |
| HBH Key | to Key Dist | HBH Keys | to Key Dist | HBH Key |
+-----------+ +-----------+ +-----------+
Figure 3: Exchanging Key Information Between Entities
In addition to the secure tunnel between the Media Distributor and the Key Distributor, there are two additional types of security associations utilized as a part of the key exchange as discussed in the following paragraphs. One is a DTLS-SRTP association between an endpoint and the Key Distributor (with packets passing through the Media Distributor) and the other is a DTLS-SRTP association between peer Media Distributors.
Endpoints establish a DTLS-SRTP [RFC5764] association over the RTP session's media ports for the purposes of key information exchange with the Key Distributor. The Media Distributor does not terminate the DTLS signaling, but instead forwards 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.
In establishing the DTLS association between endpoints and the Key Distributor, the endpoint MUST act as the DTLS client and the Key Distributor MUST act as the DTLS server. The Key Encryption Key (KEK) (i.e., EKT Key) is conveyed by the Key Distributor over the DTLS association to endpoints via procedures defined in EKT [I-D.ietf-perc-srtp-ekt-diet] via the EKTKey message.
The Key Distributor MUST NOT establish DTLS-SRTP associations with endpoints without first authenticating the Media Distributor tunneling the DTLS-SRTP packets from the endpoint.
Note that following DTLS-SRTP procedures for the [I-D.ietf-perc-double] cipher, the endpoint generates both E2E and HBH encryption keys and salt values. Endpoints MUST either use the DTLS-SRTP generated E2E key for transmission or generate a fresh E2E key. In either case, the generated SRTP master salt for E2E encryption MUST be replaced with the salt value provided by the Key Distributor via the EKTKey message. That is because every endpoint in the conference uses the same SRTP master salt. The endpoint only transmits the SRTP master key (not the salt) used for E2E encryption to other endpoints in RTP/RTCP packets per [I-D.ietf-perc-srtp-ekt-diet].
Media Distributors use DTLS-SRTP [RFC5764] directly with a peer Media Distributor to establish the HBH key for transmitting RTP and RTCP packets to that peer Media Distributor. The Key Distributor does not facilitate establishing a HBH key for use between Media Distributors.
Following the initial key information exchange with the Key Distributor, an endpoint is able to encrypt media end-to-end with an E2E key, sending that E2E key to other endpoints encrypted with the KEK, and is 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., 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 Key Distributor decides to re-key a conference, it transmits a new EKTKey message containing the new EKT Key [I-D.ietf-perc-srtp-ekt-diet] to each of the conference participants. Upon receipt of the new EKT Key, 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 as per Section 4.5 of [I-D.ietf-perc-srtp-ekt-diet]. In order to allow time for all endpoints in the conference to receive the new keys, the sender should follow the recommendations in Section 4.7 of [I-D.ietf-perc-srtp-ekt-diet]. On receiving a new EKT Key, endpoints MUST be prepared to decrypt EKT tags using the new key. The EKT SPI field is used to differentiate between tags encrypted with the old and new keys.
After re-keying, an endpoint SHOULD retain prior SRTP master keys and EKT Key for a period of time sufficient for the purpose of ensuring it can decrypt late-arriving or out-of-order packets or packets sent by other endpoints that used the prior keys for a period of time after re-keying began. An endpoint MAY retain old keys until the end of the conference.
Endpoints MAY follow the procedures in section 5.2 of [RFC5764] to renegotiate HBH keys as desired. If new HBH keys are generated, the new keys are also delivered to the Media Distributor following the procedures defined in [I-D.ietf-perc-dtls-tunnel] as one possible method.
Endpoints MAY change the E2E encryption key used at any time. Endpoints MUST generate a new E2E encryption key whenever it receives a new EKT Key. After switching to a new key, the new key is conveyed to other endpoints in the conference in RTP/RTCP packets per [I-D.ietf-perc-srtp-ekt-diet].
It is important to this solution framework that the entities can validate the authenticity of other entities, especially the Key Distributor 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 are that an endpoint can verify it is connected to the correct Key Distributor for the conference and the Key Distributor can verify the endpoint is the correct endpoint for the conference.
Two possible approaches to solve this are Identity Assertions and Certificate Fingerprints.
WebRTC Identity assertion [I-D.ietf-rtcweb-security-arch] is 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 Key Distributor to ensure that only authorized users participate in the conference. Similarly the Key Distributor can create a WebRTC Identity assertion to bind the fingerprint of the unique certificate used by the Key Distributor for this conference so that the endpoint can validate it is talking to the correct Key Distributor. Such a setup requires an Identity Provider (Idp) trusted by the endpoints and the Key Distributor.
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 Session Description Protocol (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 [RFC8224] for the benefit of forwarding the message to other entities by the Focus [RFC4353]. Other entities can 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 verify 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 Key Distributor so that it can check that that user is authorized. Similarly, the Key Distributor's certificate fingerprint can be conveyed to endpoint in a manner that can be authenticated as being an authorized Key Distributor for this conference.
The Key Distributor needs to know what endpoints are being added to a given conference. Thus, the Key Distributor and the Media Distributor need to know endpoint-to-conference mappings, which is enabled by exchanging a conference-specific unique identifier defined in [I-D.ietf-perc-dtls-tunnel]. How this unique identifier is assigned is outside the scope of this document.
This section describes the various keys employed by PERC.
This section summarizes the several different keys used in the PERC framework, 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 4 below.
+-----------+----------------------------------------------------+ | Key | Description | +-----------+----------------------------------------------------+ | KEK | Key shared by all endpoints and used to encrypt | | (EKT Key) | each endpoint's E2E SRTP master key so receiving | | | endpoints can decrypt media. | +-----------+----------------------------------------------------+ | HBH Key | SRTP master key used to encrypt media hop-by-hop. | +-----------+----------------------------------------------------+ | E2E Key | SRTP master key used to encrypt media end-to-end. | +-----------+----------------------------------------------------+
Figure 4: Key Inventory
While the number of key types is very small, it should be understood that the actual number of distinct keys can be large as the conference grows in size.
As an example, with 1,000 participants in a conference, there would be at least 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 that are associated with a new EKT SPI. Endpoints have to retain old keys for a period of time to ensure they can properly decrypt late-arriving or out-of-order packets.
A more detailed explanation of each of the keys follows.
The first set of keys acquired are for hop-by-hop encryption and decryption. Per the Double [I-D.ietf-perc-double] procedures, the endpoint performs DTLS-SRTP exchange with the key distributor and receives a key that is, in fact, "double" the size that is needed. The end-to-end part is the first half of the key, so the endpoint discards that information when generating its own key. The second half of the key material is for hop-by-hop operations, so that half of the key (corresponding to the least significant bits) is assigned internally as the HBH key.
The Key Distributor informs the Media Distributor of the HBH key. Specifically, the Key Distributor sends the least significant bits corresponding to the half of the keying material determined through DTLS-SRTP with the endpoint to the Media Distributor. A salt value is generated along with the HBH key. The salt is also longer than needed for hop-by-hop 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. One way to transmit this key and salt information is via the tunnel protocol defined in [I-D.ietf-perc-dtls-tunnel].
No two endpoints 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.
The HBH key is also used for hop-by-hop encryption of RTCP. RTCP is not end-to-end encrypted in PERC.
Via the aforementioned DTLS-SRTP association, the Key Distributor sends 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) allows 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 is used to re-key the entire conference. Each key is identified by a "Security Parameter Index" (SPI) value. Endpoints MUST 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 Full EKT Tag 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 Tag data as per [I-D.ietf-perc-srtp-ekt-diet]).
The media distributor is not given the KEK (i.e., EKT Key).
As stated earlier, the E2E key determined via DTLS-SRTP MAY be discarded in favor of a locally-generated E2E SRTP master key. While the DTLS-SRTP-derived SRTP master key can be used initially, the endpoint might choose to change the SRTP master key periodically and MUST change the SRTP master key as a result of the EKT key changing.
A 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 does 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 transmits its E2E key to other endpoints in the conference by periodically including it in SRTP packets in a Full EKT Tag. When placed in the Full EKT Tag, it is encrypted using the EKT Key provided by the key distributor. The master salt is not transmitted, though, since all endpoints receive the same master salt via the EKTKey message from the Key Distributor. The recommended frequency with which an endpoint transmits its SRTP master key is specified in [I-D.ietf-perc-srtp-ekt-diet].
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 distinct per endpoint. This key is transmitted (encrypted) via the Full EKT Tag to other endpoints. Endpoints that receive media from a given transmitting endpoint gains knowledge of the transmitter's E2E key via the Full EKT Tag.
To summarize the various keys and which entity is in possession of a given key, refer to Figure 5.
+----------------------+------------+-------+-------+------------+ | 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 5: Keys Types and Entity Possession
Figure 6 presents a complete picture of what an encrypted media packet per this framework looks like when transmitted over the wire. The packet format shown is encrypted using the Double cryptographic transform with an EKT Tag appended to the end.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<++
|V=2|P|X| CC |M| PT | sequence number | IO
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IO
| timestamp | IO
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IO
| synchronization source (SSRC) identifier | IO
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ IO
| contributing source (CSRC) identifiers | IO
| .... | IO
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O
| RTP extension (OPTIONAL) ... | |O
+>+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O
O I | payload ... | IO
O I | +-------------------------------+ IO
O I | | RTP padding | RTP pad count | IO
O +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O
O | | E2E authentication tag | |O
O | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |O
O | | OHB ... | |O
+>| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |+
| | | HBH authentication tag | ||
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ||
| | | EKT Tag ... | R ||
| | +-+-+-+-+-+-+-+-+-+ | ||
| | +- Neither encrypted nor authenticated; ||
| | appended after Double is performed ||
| | ||
| | ||
| +- E2E Encrypted Portion E2E Authenticated Portion ---+|
| |
+--- HBH Encrypted Portion HBH Authenticated Portion ----+
Figure 6: Encrypted Media Packet Format
Third party attacks are attacks attempted by an adversary that is not supposed to have access to keying material or is otherwise not an authorized participant in the conference.
On-path attacks are mitigated by hop-by-hop integrity protection and encryption. The integrity protection mitigates packet modification and encryption makes selective blocking of packets harder, but not impossible.
Off-path attackers could try connecting to different PERC entities and send specifically crafted packets. Endpoints and Media Distributors mitigate such an attack by performing hop-by-hop authentication and discarding packets that fail authentication.
Another attack vector is a third party claiming to be a Media Distributor, fooling endpoints into sending packets to the false Media Distributor instead of the correct one. The deceived sending endpoints could incorrectly assume their packets have been delivered to endpoints when they in fact have not. While this attack is possible, the result is a simple denial of service with no leakage of confidential information, since the false Media Distributor would not have access to either HBH or E2E encryption keys.
Use of mutual DTLS authentication (as required by DTLS-SRTP) ensures that a false endpoint or false Media Distributor cannot interact with a legitimate Media Distributor or endpoint. While confidentiality would not be compromised by failing to implement mutual authentication, employing it helps mitigate against denial of service attacks wherein a false entity sends a stream of packets that the would force a legitimate entity to spend time attempting to decrypt.
A third party could cause a denial-of-service by transmitting many bogus or replayed packets toward receiving devices that ultimately degrade conference or device performance. Therefore, implementations might wish to devise mechanisms to safeguard against such illegitimate packets, such as utilizing rate-limiting or performing basic sanity-checks on packets (e.g., looking at packet length or expected sequence number ranges) before performing more expensive decryption operations.
A malicious or compromised Media Distributor can attack the session in a number of possible ways.
A simple form of attack is discarding received packets that should be forwarded. This solution framework does not introduce any mitigation for Media Distributors that fail to forward media packets.
Another form of attack is modifying received packets before forwarding. With this solution framework, any modification of the end-to-end authenticated data results in the receiving endpoint getting an integrity failure when performing authentication on the received packet.
The Media Distributor 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 Tag). Since such a message would trigger the receiver to form a new cryptographic context, the Media Distributor can attempt to consume the receiving endpoints resources. While E2E authentication would fail and the cryptographic context would be destroyed, the key derivation operation would nonetheless consume some computational resources. While resource consumption attacks cannot be mitigated entirely, rate-limiting packets might help reduce the impact of such attacks.
A replay attack is when an already received packet from a previous point in the RTP stream is replayed as new packet. This could, for example, allow a Media Distributor to transmit a sequence of packets identified as a user saying "yes", instead of the "no" the user actually said.
A replay attack is mitigated by the requirement to implement replay protection as described in Section 3.3.2 of [RFC3711]. End-to-end replay protection MUST be provided for the duration of the conference.
A delayed playout attack is one where media is received and held by a media distributor and then forwarded to endpoints at a later point in time.
This attack is possible even if E2E replay protection is in place. However, due to fact that the Media Distributor is allowed to select a subset 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 a Media Distributor 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 Media Distributor, the Media Distributor 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.
While this attack cannot be eliminated entirely, its effectiveness can be reduced by re-keying the conference periodically since significantly-delayed media encrypted with expired keys would not be decrypted by endpoints.
A splicing attack is an attack where a Media Distributor receiving multiple media sources splices one media stream into the other. If the Media Distributor were able to change the SSRC without the receiver having any method for verifying the original source ID, then the Media Distributor could first deliver stream A and then later forward stream B under the same SSRC as stream A was previously using. By including the SSRC in the integrity check for each packet, both HBH and E2E, PERC prevents splicing attacks.
PERC does not provide end-to-end protection of RTCP messages. This allows a compromised Media Distributor to impact any message that might be transmitted via RTCP, including media statistics, picture requests, or loss indication. It is also possible for a compromised Media Distributor to forge requests, such as requests to the endpoint to send a new picture. Such requests can consume significant bandwidth and impair conference performance.
There are no IANA considerations for this document.
The authors would like to thank Mo Zanaty, Christian Oien, and Richard Barnes 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. We would also like to acknowledge John Mattsson, Mats Naslund, and Magnus Westerlund for providing some of the text in the document, including much of the original text in the security considerations section.