draft-ietf-perc-private-media-framework-12.txt   rfc8871.txt 
Network Working Group P. Jones Internet Engineering Task Force (IETF) P. Jones
Internet-Draft Cisco Request for Comments: 8871 Cisco
Intended status: Standards Track D. Benham Category: Standards Track D. Benham
Expires: December 7, 2019 C. Groves ISSN: 2070-1721 C. Groves
Independent Independent
June 5, 2019 January 2021
A Solution Framework for Private Media in Privacy Enhanced RTP A Solution Framework for Private Media in Privacy-Enhanced RTP
Conferencing (PERC) Conferencing (PERC)
draft-ietf-perc-private-media-framework-12
Abstract Abstract
This document describes a solution framework for ensuring that media This document describes a solution framework for ensuring that media
confidentiality and integrity are maintained end-to-end within the confidentiality and integrity are maintained end to end within the
context of a switched conferencing environment where media context of a switched conferencing environment where Media
distributors are not trusted with the end-to-end media encryption Distributors are not trusted with the end-to-end media encryption
keys. The solution builds upon existing security mechanisms defined keys. The solution builds upon existing security mechanisms defined
for the real-time transport protocol (RTP). for the Real-time Transport Protocol (RTP).
Status of This Memo Status of This Memo
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https://www.rfc-editor.org/info/rfc8871.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction
2. Conventions Used in This Document . . . . . . . . . . . . . . 4 2. Conventions Used in This Document
3. PERC Entities and Trust Model . . . . . . . . . . . . . . . . 5 3. PERC Entities and Trust Model
3.1. Untrusted Entities . . . . . . . . . . . . . . . . . . . 5 3.1. Untrusted Entities
3.1.1. Media Distributor . . . . . . . . . . . . . . . . . . 6 3.1.1. Media Distributor
3.1.2. Call Processing . . . . . . . . . . . . . . . . . . . 7 3.1.2. Call Processing
3.2. Trusted Entities . . . . . . . . . . . . . . . . . . . . 7 3.2. Trusted Entities
3.2.1. Endpoint . . . . . . . . . . . . . . . . . . . . . . 7 3.2.1. Endpoint
3.2.2. Key Distributor . . . . . . . . . . . . . . . . . . . 7 3.2.2. Key Distributor
4. Framework for PERC . . . . . . . . . . . . . . . . . . . . . 8 4. Framework for PERC
4.1. End-to-End and Hop-by-Hop Authenticated Encryption . . . 8 4.1. E2E-Authenticated and HBH-Authenticated Encryption
4.2. E2E Key Confidentiality . . . . . . . . . . . . . . . . . 9 4.2. E2E Key Confidentiality
4.3. E2E Keys and Endpoint Operations . . . . . . . . . . . . 10 4.3. E2E Keys and Endpoint Operations
4.4. HBH Keys and Per-hop Operations . . . . . . . . . . . . . 10 4.4. HBH Keys and Per-Hop Operations
4.5. Key Exchange . . . . . . . . . . . . . . . . . . . . . . 11 4.5. Key Exchange
4.5.1. Initial Key Exchange and Key Distributor . . . . . . 11 4.5.1. Initial Key Exchange and Key Distributor
4.5.2. Key Exchange during a Conference . . . . . . . . . . 13 4.5.2. Key Exchange during a Conference
5. Authentication . . . . . . . . . . . . . . . . . . . . . . . 14 5. Authentication
5.1. Identity Assertions . . . . . . . . . . . . . . . . . . . 14 5.1. Identity Assertions
5.2. Certificate Fingerprints in Session Signaling . . . . . . 14 5.2. Certificate Fingerprints in Session Signaling
5.3. Conference Identification . . . . . . . . . . . . . . . . 15 5.3. Conference Identification
6. PERC Keys . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6. PERC Keys
6.1. Key Inventory and Management Considerations . . . . . . . 15 6.1. Key Inventory and Management Considerations
6.2. DTLS-SRTP Exchange Yields HBH Keys . . . . . . . . . . . 16 6.2. DTLS-SRTP Exchange Yields HBH Keys
6.3. The Key Distributor Transmits the KEK (EKT Key) . . . . . 17 6.3. The Key Distributor Transmits the KEK (EKT Key)
6.4. Endpoints fabricate an SRTP Master Key . . . . . . . . . 18 6.4. Endpoints Fabricate an SRTP Master Key
6.5. Summary of Key Types and Entity Possession . . . . . . . 18 6.5. Summary of Key Types and Entity Possession
7. Encrypted Media Packet Format . . . . . . . . . . . . . . . . 19 7. Encrypted Media Packet Format
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20 8. Security Considerations
8.1. Third Party Attacks . . . . . . . . . . . . . . . . . . . 20 8.1. Third-Party Attacks
8.2. Media Distributor Attacks . . . . . . . . . . . . . . . . 21 8.2. Media Distributor Attacks
8.2.1. Denial of service . . . . . . . . . . . . . . . . . . 21 8.2.1. Denial of Service
8.2.2. Replay Attack . . . . . . . . . . . . . . . . . . . . 22 8.2.2. Replay Attacks
8.2.3. Delayed Playout Attack . . . . . . . . . . . . . . . 22 8.2.3. Delayed Playout Attacks
8.2.4. Splicing Attack . . . . . . . . . . . . . . . . . . . 23 8.2.4. Splicing Attacks
8.2.5. RTCP Attacks . . . . . . . . . . . . . . . . . . . . 23 8.2.5. RTCP Attacks
8.3. Key Distributor Attacks . . . . . . . . . . . . . . . . . 23 8.3. Key Distributor Attacks
8.4. Endpoint Attacks . . . . . . . . . . . . . . . . . . . . 24 8.4. Endpoint Attacks
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 9. IANA Considerations
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 25 10. References
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 25 10.1. Normative References
11.1. Normative References . . . . . . . . . . . . . . . . . . 25 10.2. Informative References
11.2. Informative References . . . . . . . . . . . . . . . . . 26 Acknowledgments
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27 Authors' Addresses
1. Introduction 1. Introduction
Switched conferencing is an increasingly popular model for multimedia Switched conferencing is an increasingly popular model for multimedia
conferences with multiple participants using a combination of audio, conferences with multiple participants using a combination of audio,
video, text, and other media types. With this model, real-time media video, text, and other media types. With this model, real-time media
flows from conference participants are not mixed, transcoded, flows from conference participants are not mixed, transcoded,
transrated, recomposed, or otherwise manipulated by a Media translated, recomposed, or otherwise manipulated by a Media
Distributor, as might be the case with a traditional media server or Distributor, as might be the case with a traditional media server or
multipoint control unit (MCU). Instead, media flows transmitted by Multipoint Control Unit (MCU). Instead, media flows transmitted by
conference participants are simply forwarded by Media Distributors to conference participants are simply forwarded by Media Distributors to
each of the other participants. Media Distributors often forward each of the other participants. Media Distributors often forward
only a subset of flows based on voice activity detection or other only a subset of flows based on voice activity detection or other
criteria. In some instances, Media Distributors may make limited criteria. In some instances, Media Distributors may make limited
modifications to RTP [RFC3550] headers, for example, but the actual modifications to RTP headers [RFC3550], for example, but the actual
media content (e.g., voice or video data) is unaltered. media content (e.g., voice or video data) is unaltered.
An advantage of switched conferencing is that Media Distributors can An advantage of switched conferencing is that Media Distributors can
be more easily deployed on general-purpose computing hardware, be more easily deployed on general-purpose computing hardware,
including virtualized environments in private and public clouds. including virtualized environments in private and public clouds.
Virtualized public cloud environments have been viewed as less secure Virtualized public cloud environments have been viewed as less
since resources are not always physically controlled by those who use secure, since resources are not always physically controlled by those
them. This document defines improved security so as to lower the who use them. This document defines improved security so as to lower
barrier to taking advantage of those environments. the barrier to taking advantage of those environments.
This document defines a solution framework wherein media privacy is This document defines a solution framework wherein media privacy is
ensured by making it impossible for a Media Distributor to gain ensured by making it impossible for a Media Distributor to gain
access to keys needed to decrypt or authenticate the actual media access to keys needed to decrypt or authenticate the actual media
content sent between conference participants. At the same time, the content sent between conference participants. At the same time, the
framework allows for the Media Distributors to modify certain RTP framework allows for the Media Distributors to modify certain RTP
headers; add, remove, encrypt, or decrypt RTP header extensions; and headers; add, remove, encrypt, or decrypt RTP header extensions; and
encrypt and decrypt RTP Control Protocol (RTCP) [RFC3550] packets. encrypt and decrypt RTP Control Protocol (RTCP) packets [RFC3550].
The framework also prevents replay attacks by authenticating each The framework also prevents replay attacks by authenticating each
packet transmitted between a given participant and the Media packet transmitted between a given participant and the Media
Distributor using a unique key per Endpoint that is independent from Distributor using a unique key per endpoint that is independent from
the key for media encryption and authentication. the key for media encryption and authentication.
This solution framework provides for enhanced privacy in RTP-based This solution framework provides for enhanced privacy in RTP-based
conferencing environments while utilizing existing security conferencing environments while utilizing existing security
procedures defined for RTP with minimal enhancements. procedures defined for RTP with minimal enhancements.
2. Conventions Used in This Document 2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
Additionally, this solution framework uses the following terms and Additionally, this solution framework uses the following terms and
acronyms: abbreviations:
End-to-End (E2E): Communications from one Endpoint through one or End-to-End (E2E): Communications from one endpoint through one or
more Media Distributors to the Endpoint at the other end. more Media Distributors to the endpoint at the other end.
Hop-by-Hop (HBH): Communications between an Endpoint and a Media Hop-by-Hop (HBH): Communications between an endpoint and a Media
Distributor or between Media Distributors. Distributor or between Media Distributors.
Trusted Endpoint (or simply Endpoint): An RTP flow terminating entity Trusted Endpoint (or simply endpoint): An RTP flow-terminating
that has possession of E2E media encryption keys and terminates E2E entity that has possession of E2E media encryption keys and
encryption. This may include embedded user conferencing equipment or terminates E2E encryption. This may include embedded user
browsers on computers, media gateways, MCUs, media recording devices conferencing equipment or browsers on computers, media gateways,
and more that are in the trusted domain for a given deployment. In MCUs, media recording devices, and more, that are in the trusted
the context of WebRTC [W3C.CR-webrtc-20180927], where control of a domain for a given deployment. In the context of WebRTC
session is divided between a JavaScript application and a browser, [W3C.CR-webrtc], where control of a session is divided between a
the browser acts as the Trusted Endpoint for purposes of this JavaScript application and a browser, the browser acts as the
framework (just as it acts as the endpoint for DTLS-SRTP [RFC5764] in Trusted Endpoint for purposes of this framework (just as it acts
one-to-one calls). as the endpoint for DTLS-SRTP [RFC5764] in one-to-one calls).
Media Distributor (MD): An RTP middlebox that forwards Endpoint media Media Distributor (MD): An RTP middlebox that forwards endpoint
content (e.g., voice or video data) unaltered, either a subset or all media content (e.g., voice or video data) unaltered -- either a
of the flows at any given time, and is never allowed to have access subset or all of the flows at any given time -- and is never
to E2E encryption keys. It operates according to the Selective allowed to have access to E2E encryption keys. It operates
Forwarding Middlebox RTP topologies [RFC7667] per the constraints according to the Selective Forwarding Middlebox RTP topologies
defined by the PERC system, which includes, but is not limited to, [RFC7667] per the constraints defined by the Private Media in
having no access to RTP media plaintext and having limits on what RTP Privacy-Enhanced RTP Conferencing (PERC) system, which includes,
header field it can alter. The header fields that may be modified by but is not limited to, having no access to RTP media plaintext and
a Media Distributor are enumerated in Section 4 of the Double having limits on what RTP header fields it can alter. The header
cryptographic transform specification [I-D.ietf-perc-double] and fields that may be modified by a Media Distributor are enumerated
chosen with respect to utility and the security considerations in Section 4 of the double cryptographic transform specification
outlined in this document. [RFC8723] and chosen with respect to utility and the security
considerations outlined in this document.
Key Distributor: An entity that is a logical function which Key Distributor: An entity that is a logical function that
distributes keying material and related information to Trusted distributes keying material and related information to Trusted
Endpoints and Media Distributor(s), only that which is appropriate Endpoints and Media Distributor(s) -- only what is appropriate for
for each. The Key Distributor might be co-resident with another each. The Key Distributor might be co-resident with another
entity trusted with E2E keying material. entity trusted with E2E keying material.
Conference: Two or more participants communicating via Trusted Conference: Two or more participants communicating via Trusted
Endpoints to exchange RTP flows through one or more Media Endpoints to exchange RTP flows through one or more Media
Distributor. Distributors.
Call Processing: All Trusted Endpoints in the conference connect to Call Processing: All Trusted Endpoints connect to a conference via a
it by a call processing dialog, such as with the Focus defined in the call processing dialog, e.g., with the "focus" as defined in "A
Framework for Conferencing with SIP [RFC4353]. Framework for Conferencing with the Session Initiation Protocol
(SIP)" [RFC4353].
Third Party: Any entity that is not an Endpoint, Media Distributor, Third Party: Any entity that is not an endpoint, Media Distributor,
Key Distributor or Call Processing entity as described in this Key Distributor, or call processing entity as described in this
document. document.
3. PERC Entities and Trust Model 3. PERC Entities and Trust Model
The following figure depicts the trust relationships, direct or Figure 1 depicts the trust relationships, direct or indirect, between
indirect, between entities described in the subsequent sub-sections. entities described in the subsequent subsections. Note that these
Note that these entities may be co-located or further divided into entities may be co-located or further divided into multiple, separate
multiple, separate physical devices. physical devices.
Please note that some entities classified as untrusted in the simple, Please note that some entities classified as untrusted in the simple,
general deployment scenario used most commonly in this document might general deployment scenario used most commonly in this document might
be considered trusted in other deployments. This document does not be considered trusted in other deployments. This document does not
preclude such scenarios, but keeps the definitions and examples preclude such scenarios, but it keeps the definitions and examples
focused by only using the simple, most general deployment scenario. focused by only using the simple, most general deployment scenario.
| |
+----------+ | +-----------------+ +----------+ | +-----------------+
| Endpoint | | | Call Processing | | Endpoint | | | Call Processing |
+----------+ | +-----------------+ +----------+ | +-----------------+
| |
| |
+----------------+ | +--------------------+ +----------------+ | +--------------------+
| Key Distributor| | | Media Distributor | | Key Distributor| | | Media Distributor |
+----------------+ | +--------------------+ +----------------+ | +--------------------+
| |
Trusted | Untrusted Trusted | Untrusted
Entities | Entities Entities | Entities
| |
Figure 1: Trusted and Untrusted Entities in PERC Figure 1: Trusted and Untrusted Entities in PERC
3.1. Untrusted Entities 3.1. Untrusted Entities
The architecture described in this framework document enables The architecture described in this framework document enables
conferencing infrastructure to be hosted in domains, such as in a conferencing infrastructure to be hosted in domains, such as in a
cloud conferencing provider's facilities, where the trustworthiness cloud conferencing provider's facilities, where the trustworthiness
is below the level needed to assume the privacy of participant's is below the level needed to assume that the privacy of the
media is not compromised. The conferencing infrastructure in such a participant's media is not compromised. The conferencing
domain is still trusted with reliably connecting the participants infrastructure in such a domain is still trusted with reliably
together in a conference, but not trusted with keying material needed connecting the participants together in a conference but is not
to decrypt any of the participant's media. Entities in such lower trusted with keying material needed to decrypt any of the
trustworthiness domains are referred to as untrusted entities from participant's media. Entities in such less-trustworthy domains are
this point forward. referred to as untrusted entities from this point forward.
It is important to understand that untrusted in this document does It is important to understand that "untrusted" in this document does
not mean an entity is not expected to function properly. Rather, it not mean that an entity is not expected to function properly.
means only that the entity does not have access to the E2E media Rather, it means only that the entity does not have access to the E2E
encryption keys. media encryption keys.
3.1.1. Media Distributor 3.1.1. Media Distributor
A Media Distributor forwards RTP flows between Endpoints in the A Media Distributor forwards RTP flows between endpoints in the
conference while performing per-hop authentication of each RTP conference while performing per-hop authentication of each RTP
packet. The Media Distributor may need access to one or more RTP packet. The Media Distributor may need access to one or more RTP
headers or header extensions, potentially adding or modifying a headers or header extensions, potentially adding or modifying a
certain subset. The Media Distributor also relays secured messaging certain subset. The Media Distributor also relays secured messaging
between the Endpoints and the Key Distributor and acquires per-hop between the endpoints and the Key Distributor and acquires per-hop
key information from the Key Distributor. The actual media content key information from the Key Distributor. The actual media content
must not be decryptable by a Media Distributor, as it is untrusted to must not be decryptable by a Media Distributor, as it is not trusted
have access to the E2E media encryption keys. The key exchange to have access to the E2E media encryption keys. The key exchange
mechanisms specified in this framework prevent the Media Distributor mechanisms specified in this framework prevent the Media Distributor
from gaining access to the E2E media encryption keys. from gaining access to the E2E media encryption keys.
An Endpoint's ability to connect to a conference serviced by a Media An endpoint's ability to connect to a conference serviced by a Media
Distributor does imply that the Endpoint is authorized to have access Distributor implies that the endpoint is authorized to have access to
to the E2E media encryption keys, as the Media Distributor does not the E2E media encryption keys, although the Media Distributor does
have the ability to determine whether an Endpoint is authorized. not have the ability to determine whether an endpoint is authorized.
Instead, the Key Distributor is responsible for authenticating the Instead, the Key Distributor is responsible for authenticating the
Endpoint (e.g., using WebRTC Identity endpoint (e.g., using WebRTC identity assertions [RFC8827]) and
[I-D.ietf-rtcweb-security-arch]) and determining its authorization to determining its authorization to receive E2E and HBH media encryption
receive E2E and HBH media encryption keys. keys.
A Media Distributor must perform its role in properly forwarding A Media Distributor must perform its role in properly forwarding
media packets while taking measures to mitigate the adverse effects media packets while taking measures to mitigate the adverse effects
of denial of service attacks (refer to Section 8) to a level equal to of denial-of-service attacks (refer to Section 8) to a level equal to
or better than traditional conferencing (non-PERC) deployments. or better than traditional conferencing (non-PERC) deployments.
A Media Distributor or associated conferencing infrastructure may A Media Distributor or associated conferencing infrastructure may
also initiate or terminate various conference control related also initiate or terminate various messaging techniques related to
messaging, which is outside the scope of this framework document. conference control. This topic is outside the scope of this
framework document.
3.1.2. Call Processing 3.1.2. Call Processing
The call processing function is untrusted in the simple, general Call processing is untrusted in the simple, general deployment
deployment scenario. When a physical subset of the call processing scenario. When a physical subset of call processing resides in
function resides in facilities outside the trusted domain, it should facilities outside the trusted domain, it should not be trusted to
not be trusted to have access to E2E key information. have access to E2E key information.
The call processing function may include the processing of call Call processing may include the processing of call signaling
signaling messages, as well as the signing of those messages. It may messages, as well as the signing of those messages. It may also
also authenticate the Endpoints for the purpose of call signaling and authenticate the endpoints for the purpose of call signaling and of
subsequently joining of a conference hosted through one or more Media subsequently joining a conference hosted through one or more Media
Distributors. Call processing may optionally ensure the privacy of Distributors. Call processing may optionally ensure the privacy of
call signaling messages between itself, the Endpoint, and other call signaling messages between itself (call processing), the
entities. endpoint, and other entities.
3.2. Trusted Entities 3.2. Trusted Entities
From the PERC model system perspective, entities considered trusted From the PERC model system's perspective, entities considered trusted
(refer to Figure 1) can be in possession of the E2E media encryption (refer to Figure 1) can be in possession of the E2E media encryption
keys for one or more conferences. keys for one or more conferences.
3.2.1. Endpoint 3.2.1. Endpoint
An Endpoint is considered trusted and has access to E2E key An endpoint is considered trusted and has access to E2E key
information. While it is possible for an Endpoint to be compromised, information. While it is possible for an endpoint to be compromised,
subsequently performing in undesired ways, defining Endpoint subsequently performing in undesired ways, defining endpoint
resistance to compromise is outside the scope of this document. resistance to compromise is outside the scope of this document.
Endpoints take measures to mitigate the adverse effects of denial of Endpoints take measures to mitigate the adverse effects of denial-of-
service attacks (refer to Section 8) from other entities, including service attacks (refer to Section 8) from other entities, including
from other Endpoints, to a level equal to or better than traditional from other endpoints, to a level equal to or better than traditional
conference (non-PERC) deployments. conference (non-PERC) deployments.
3.2.2. Key Distributor 3.2.2. Key Distributor
The Key Distributor, which may be colocated with an Endpoint or exist The Key Distributor, which may be co-located with an endpoint or
standalone, is responsible for providing key information to Endpoints exist standalone, is responsible for providing key information to
for both end-to-end (E2E) and hop-by-hop (HBH) security and for endpoints for both E2E and HBH security and for providing key
providing key information to Media Distributors for the hop-by-hop information to Media Distributors for HBH security.
security.
Interaction between the Key Distributor and the call processing Interaction between the Key Distributor and call processing is
function is necessary for proper conference-to-Endpoint mappings. necessary for proper conference-to-endpoint mappings. This is
This is described in Section 5.3. described in Section 5.3.
The Key Distributor needs to be secured and managed in a way to The Key Distributor needs to be secured and managed in a way that
prevent exploitation by an adversary, as any kind of compromise of prevents exploitation by an adversary, as any kind of compromise of
the Key Distributor puts the security of the conference at risk. the Key Distributor puts the security of the conference at risk.
They Key Distributor needs to know which Endpoints and which Media The Key Distributor needs to know which endpoints and which Media
Distributors are authorized to participate in the conference. How Distributors are authorized to participate in the conference. How
the Key Distributor obtains this information is outside the scope of the Key Distributor obtains this information is outside the scope of
this document. However, Key Distributors MUST reject DTLS this document. However, Key Distributors MUST reject DTLS
associations with any unauthorized Endpoint or Media Distributor. associations with any unauthorized endpoint or Media Distributor.
4. Framework for PERC 4. Framework for PERC
The purpose for this framework is to define a means through which The purpose of this framework is to define a means through which
media privacy is ensured when communicating within a conferencing media privacy is ensured when communicating within a conferencing
environment consisting of one or more Media Distributors that only environment consisting of one or more Media Distributors that only
switch, hence not terminate, media. It does not otherwise attempt to switch, and hence do not terminate, media. It does not otherwise
hide the fact that a conference between Endpoints is taking place. attempt to hide the fact that a conference between endpoints is
taking place.
This framework reuses several specified RTP security technologies, This framework reuses several specified RTP security technologies,
including Secure Real-time Transport Protocol (SRTP) [RFC3711], including the Secure Real-time Transport Protocol (SRTP) [RFC3711],
Encrypted Key Transport (EKT) [I-D.ietf-perc-srtp-ekt-diet], and Encrypted Key Transport (EKT) [RFC8870], and DTLS-SRTP.
DTLS-SRTP.
4.1. End-to-End and Hop-by-Hop Authenticated Encryption 4.1. E2E-Authenticated and HBH-Authenticated Encryption
This solution framework focuses on the end-to-end privacy and This solution framework focuses on the E2E privacy and integrity of
integrity of the participant's media by limiting access to only the participant's media by limiting access to only trusted entities
trusted entities to the E2E key used for authenticated end-to-end to the E2E key used for authenticated E2E encryption. However, this
encryption. However, this framework does give a Media Distributor framework does give a Media Distributor access to RTP header fields
access to RTP headers fields and header extensions, as well as the and header extensions, as well as the ability to modify a certain
ability to modify a certain subset of the header fields and to add or subset of the header fields and to add or change header extensions.
change header extensions. Packets received by a Media Distributor or Packets received by a Media Distributor or an endpoint are
an Endpoint are authenticated hop-by-hop. authenticated hop by hop.
To enable all of the above, this framework defines the use of two To enable all of the above, this framework defines the use of two
security contexts and two associated encryption keys: an "inner" key security contexts and two associated encryption keys: an "inner" key
(an E2E key distinct for each transmitted media flow) for (a distinct E2E key for each transmitted media flow) for
authenticated encryption of RTP media between Endpoints and an authenticated encryption of RTP media between endpoints and an
"outer" key (HBH key) known only to Media Distributor or the adjacent "outer" key (a HBH key) known only to a Media Distributor or the
Endpoint for the hop between an Endpoint and a Media Distributor or adjacent endpoint for the hop between an endpoint and a Media
peer Endpoint. An Endpoint will receive one or more E2E keys from Distributor or peer endpoint. An endpoint will receive one or more
every other Endpoint in the conference that correspond to the media E2E keys from every other endpoint in the conference that correspond
flows transmitted by those other Endpoints, while HBH keys are to the media flows transmitted by those other endpoints, while HBH
derived from the DTLS-SRTP association with the Key Distributor. Two keys are derived from the DTLS-SRTP association with the Key
communicating Media Distributors use DTLS-SRTP associations directly Distributor. Two communicating Media Distributors use DTLS-SRTP
with each other to obtain the HBH keys they will use. See associations directly with each other to obtain the HBH keys they
Section 4.5 for more details on key exchange. will use. See Section 4.5 for more details on key exchange.
+-------------+ +-------------+ +-------------+ +-------------+
| |################################| | | |################################| |
| Media |------------------------ *----->| Media | | Media |------------------------ *----->| Media |
| Distributor |<----------------------*-|------| Distributor | | Distributor |<----------------------*-|------| Distributor |
| X |#####################*#|#|######| Y | | X |#####################*#|#|######| Y |
| | | | | | | | | | | | | |
+-------------+ | | | +-------------+ +-------------+ | | | +-------------+
# ^ | # HBH Key (XY) -+ | | # ^ | # # ^ | # HBH Key (XY) -+ | | # ^ | #
# | | # E2E Key (B) ---+ | # | | # # | | # E2E Key (B) ---+ | # | | #
skipping to change at page 9, line 27 skipping to change at line 391
# | | *---- HBH Key (AX) HBH Key (YB) ----* | | # # | | *---- HBH Key (AX) HBH Key (YB) ----* | | #
# | | # # | | # # | | # # | | #
# *--------- E2E Key (A) E2E Key (A) ---------* # # *--------- E2E Key (A) E2E Key (A) ---------* #
# | *------- E2E Key (B) E2E Key (B) -------* | # # | *------- E2E Key (B) E2E Key (B) -------* | #
# | | # # | | # # | | # # | | #
# | v # # | v # # | v # # | v #
+-------------+ +-------------+ +-------------+ +-------------+
| Endpoint A | | Endpoint B | | Endpoint A | | Endpoint B |
+-------------+ +-------------+ +-------------+ +-------------+
Figure 2: E2E and HBH Keys Used for Authenticated Encryption of SRTP Figure 2: E2E and HBH Keys Used for Authenticated Encryption of
Packets SRTP Packets
The Double transform [I-D.ietf-perc-double] enables Endpoints to The double transform [RFC8723] enables endpoints to perform
perform encryption using both the end-to-end and hop-by-hop contexts encryption using both the E2E and HBH contexts while still preserving
while still preserving the same overall interface as other SRTP the same overall interface as other SRTP transforms. The Media
transforms. The Media Distributor simply uses the corresponding Distributor simply uses the corresponding normal (single) AES-GCM
normal (single) AES-GCM transform, keyed with the appropriate HBH transform, keyed with the appropriate HBH keys. See Section 6.1 for
keys. See Section 6.1 for a description of the keys used in PERC and a description of the keys used in PERC and Section 7 for a diagram of
Section 7 for diagram of how encrypted RTP packets appear on the how encrypted RTP packets appear on the wire.
wire.
RTCP is only encrypted hop-by-hop, not end-to-end. This framework RTCP is only encrypted hop by hop -- not end to end. This framework
introduces no additional step for RTCP authenticated encryption, so does not provide an additional step for RTCP-authenticated
the procedures needed are specified in [RFC3711] and use the same encryption. Rather, implementations utilize the existing procedures
outer, hop-by-hop cryptographic context chosen in the Double specified in [RFC3711]; those procedures use the same outer, HBH
operation described above. For this reason, Endpoints MUST NOT send cryptographic context chosen in the double transform operation
described above. For this reason, endpoints MUST NOT send
confidential information via RTCP. confidential information via RTCP.
4.2. E2E Key Confidentiality 4.2. E2E Key Confidentiality
To ensure the confidentiality of E2E keys shared between Endpoints, To ensure the confidentiality of E2E keys shared between endpoints,
Endpoints use a common Key Encryption Key (KEK) that is known only by 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 the trusted entities in a conference. That KEK, defined in the EKT
specification [I-D.ietf-perc-srtp-ekt-diet] as the EKT Key, is used specification [RFC8870] as the EKT Key, is used to subsequently
to subsequently encrypt the SRTP master key used for E2E encrypt the SRTP master key used for E2E-authenticated encryption of
authenticated encryption of media sent by a given Endpoint. Each media sent by a given endpoint. Each endpoint in the conference
Endpoint in the conference creates an SRTP master key for E2E creates an SRTP master key for E2E-authenticated encryption and keeps
authenticated encryption and keep track of the E2E keys received via track of the E2E keys received via the Full EKT Tag for each distinct
the Full EKT Tag for each distinct synchronization source (SSRC) in synchronization source (SSRC) in the conference so that it can
the conference so that it can properly decrypt received media. An properly decrypt received media. An endpoint may change its E2E key
Endpoint may change its E2E key at any time and advertise that new at any time and advertise that new key to the conference as specified
key to the conference as specified in [I-D.ietf-perc-srtp-ekt-diet]. in [RFC8870].
4.3. E2E Keys and Endpoint Operations 4.3. E2E Keys and Endpoint Operations
Any given RTP media flow is identified by its SSRC, and an Endpoint 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 might send more than one at a time and change the mix of media flows
transmitted during the life of a conference. transmitted during the lifetime of a conference.
Thus, an Endpoint MUST maintain a list of SSRCs from received RTP Thus, an endpoint MUST maintain a list of SSRCs from received RTP
flows and each SSRC's associated E2E key information. An Endpoint flows and each SSRC's associated E2E key information. An endpoint
MUST discard old E2E keys no later than when it leaves the conference MUST discard old E2E keys no later than when it leaves the
(see Section 4.5.2). conference.
If the packet is to contain RTP header extensions, it should be noted If the packet is to contain RTP header extensions, it should be noted
that those are only encrypted HBH per [I-D.ietf-perc-double]. For that those extensions are only encrypted hop by hop per [RFC8723].
this reason, Endpoints MUST NOT transmit confidential information via For this reason, endpoints MUST NOT transmit confidential information
RTP header extensions. via RTP header extensions.
4.4. HBH Keys and Per-hop Operations 4.4. HBH Keys and Per-Hop Operations
To ensure the integrity of transmitted media packets, it is REQUIRED To ensure the integrity of transmitted media packets, it is REQUIRED
that every packet be authenticated hop-by-hop between an Endpoint and that every packet be authenticated hop by hop between an endpoint and
a Media Distributor, as well between Media Distributors. The a Media Distributor, as well as between Media Distributors. The
authentication key used for hop-by-hop authentication is derived from authentication key used for HBH authentication is derived from an
an SRTP master key shared only on the respective hop. Each HBH key SRTP master key shared only on the respective hop. Each HBH key is
is distinct per hop and no two hops ever use the same SRTP master distinct per hop, and no two hops ever use the same SRTP master key.
key.
While Endpoints also perform HBH authentication, the ability of the While endpoints also perform HBH authentication, the ability of the
Endpoints to reconstruct the original RTP header also enables the endpoints to reconstruct the original RTP header also enables the
Endpoints to authenticate RTP packets E2E. This design yields endpoints to authenticate RTP packets end to end. This design yields
flexibility to the Media Distributor to change certain RTP header flexibility to the Media Distributor to change certain RTP header
values as packets are forwarded. Which values the Media Distributor values as packets are forwarded. Values that the Media Distributor
can change in the RTP header are defined in [I-D.ietf-perc-double]. can change in the RTP header are defined in [RFC8723]. RTCP can only
RTCP can only be encrypted hop-by-hop, giving the Media Distributor be encrypted hop by hop, giving the Media Distributor the flexibility
the flexibility to forward RTCP content unchanged, transmit compound to (1) forward RTCP content unchanged, (2) transmit compound RTCP
RTCP packets or to initiate RTCP packets for reporting statistics or packets, (3) initiate RTCP packets for reporting statistics, or
conveying other information. Performing hop-by-hop authentication (4) convey other information. Performing HBH authentication for all
for all RTP and RTCP packets also helps provide replay protection RTP and RTCP packets also helps provide replay protection (see
(see Section 8). The use of the replay protection mechanism Section 8). The use of the replay protection mechanism specified in
specified in Section 3.3.2 of [RFC3711] is REQUIRED at each hop. 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- 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 by hop, the endpoint derives an encryption key from the HBH SRTP
master key to encrypt header extensions as per [RFC6904]. This still master key to encrypt header extensions as per [RFC6904]. This still
gives the Media Distributor visibility into header extensions, such gives the Media Distributor visibility into header extensions, such
as the one used to determine audio level [RFC6464] of conference as the one used to determine the audio level [RFC6464] of conference
participants. Note that when RTP header extensions are encrypted, participants. Note that when RTP header extensions are encrypted,
all hops need to decrypt and re-encrypt these encrypted header all hops need to decrypt and re-encrypt these encrypted header
extensions. Please refer to Sections 5.1 through 5.3 of extensions. Please refer to Sections 5.1, 5.2, and 5.3 of [RFC8723]
[I-D.ietf-perc-double] for procedures to perform RTP header extension for procedures to perform RTP header extension encryption and
encryption and decryption. decryption.
4.5. Key Exchange 4.5. Key Exchange
In brief, the keys used by any given Endpoints are determined in the In brief, the keys used by any given endpoints are determined as
following way: follows:
o The HBH keys that the Endpoint uses to send and receive SRTP media * The HBH keys that the endpoint uses to send and receive SRTP media
are derived from a DTLS handshake that the Endpoint performs with are derived from a DTLS handshake that the endpoint performs with
the Key Distributor (following normal DTLS-SRTP procedures). the Key Distributor (following normal DTLS-SRTP procedures).
o The E2E key that an Endpoint uses to send SRTP media can either be * The E2E key that an endpoint uses to send SRTP media can be either
set from the DTLS-SRTP association with the Key Distributor or set from the DTLS-SRTP association with the Key Distributor or
chosen by the Endpoint. It is then distributed to other Endpoints chosen by the endpoint. It is then distributed to other endpoints
in a Full EKT Tag, encrypted under an EKT Key provided to the in a Full EKT Tag, encrypted under an EKT Key provided to the
client by the Key Distributor within the DTLS channel they client by the Key Distributor within the DTLS channel they
negotiated. Note that an Endpoint MAY create a different E2E key negotiated. Note that an endpoint MAY create a different E2E key
per media flow, where a media flow is identified by its SSRC per media flow, where a media flow is identified by its SSRC
value. value.
o Each E2E key that an Endpoint uses to receive SRTP media is set by * Each E2E key that an endpoint uses to receive SRTP media is set by
receiving a Full EKT Tag from another Endpoint. receiving a Full EKT Tag from another endpoint.
o The HBH keys used between two Media Distributors is derived from * The HBH keys used between two Media Distributors are derived via
DTLS-SRTP procedures employed directly between them. DTLS-SRTP procedures employed directly between them.
4.5.1. Initial Key Exchange and Key Distributor 4.5.1. Initial Key Exchange and Key Distributor
The Media Distributor maintains a tunnel with the Key Distributor The Media Distributor maintains a tunnel with the Key Distributor
(e.g., using [I-D.ietf-perc-dtls-tunnel]), making it possible for the (e.g., using the tunnel protocol defined in [PERC-DTLS]), making it
Media Distributor to facilitate the establishment of a secure DTLS possible for the Media Distributor to facilitate the establishment of
association between each Endpoint and the Key Distributor as shown a secure DTLS association between each endpoint and the Key
the following figure. The DTLS association between Endpoints and the Distributor as shown in Figure 3. The DTLS association between
Key Distributor enables each Endpoint to generate E2E and HBH keys endpoints and the Key Distributor enables each endpoint to generate
and receive the KEK. At the same time, the Key Distributor securely E2E and HBH keys and receive the KEK. At the same time, the Key
provides the HBH key information to the Media Distributor. The key Distributor securely provides the HBH key information to the Media
information summarized here may include the SRTP master key, SRTP Distributor. The key information summarized here may include the
master salt, and the negotiated cryptographic transform. SRTP master key, the SRTP master salt, and the negotiated
cryptographic transform.
+-----------+ +-----------+
KEK info | Key | HBH Key info to KEK info | Key | HBH Key info to
to Endpoints |Distributor| Endpoints & Media Distributor to Endpoints |Distributor| Endpoints & Media Distributor
+-----------+ +-----------+
# ^ ^ # # ^ ^ #
# | | #--- Tunnel # | | #--- Tunnel
# | | # # | | #
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+ +-----------+
| Endpoint | DTLS | Media | DTLS | Endpoint | | Endpoint | DTLS | Media | DTLS | Endpoint |
| KEK |<------------|Distributor|------------>| KEK | | KEK |<------------|Distributor|------------>| KEK |
| HBH Key | to Key Dist | HBH Keys | to Key Dist | HBH Key | | HBH Key | to Key Dist | HBH Keys | to Key Dist | HBH Key |
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+ +-----------+
Figure 3: Exchanging Key Information Between Entities Figure 3: Exchanging Key Information between Entities
In addition to the secure tunnel between the Media Distributor and In addition to the secure tunnel between the Media Distributor and
the Key Distributor, there are two additional types of security the Key Distributor, there are two additional types of security
associations utilized as a part of the key exchange as discussed in associations utilized as a part of the key exchange, as discussed in
the following paragraphs. One is a DTLS-SRTP association between an the following paragraphs. One is a DTLS-SRTP association between an
Endpoint and the Key Distributor (with packets passing through the endpoint and the Key Distributor (with packets passing through the
Media Distributor) and the other is a DTLS-SRTP association between Media Distributor), and the other is a DTLS-SRTP association between
peer Media Distributors. peer Media Distributors.
Endpoints establish a DTLS-SRTP association over the RTP session with Endpoints establish a DTLS-SRTP association over the RTP session with
the Media Distributor and its media ports for the purposes of key the Media Distributor and its media ports for the purposes of key
information exchange with the Key Distributor. The Media Distributor information exchange with the Key Distributor. The Media Distributor
does not terminate the DTLS signaling, but instead forwards DTLS does not terminate the DTLS signaling but instead forwards DTLS
packets received from an Endpoint on to the Key Distributor (and vice packets received from an endpoint on to the Key Distributor (and vice
versa) via a tunnel established between Media Distributor and the Key versa) via a tunnel established between the Media Distributor and the
Distributor. Key Distributor.
In establishing the DTLS association between Endpoints and the Key When establishing the DTLS association between endpoints and the Key
Distributor, the Endpoint MUST act as the DTLS client and the Key Distributor, the endpoint MUST act as the DTLS client, and the Key
Distributor MUST act as the DTLS server. The KEK is conveyed by the Distributor MUST act as the DTLS server. The KEK is conveyed by the
Key Distributor over the DTLS association to Endpoints via procedures Key Distributor over the DTLS association to endpoints via procedures
defined in EKT [I-D.ietf-perc-srtp-ekt-diet] via the EKTKey message. defined in EKT [RFC8870] via the EKTKey message.
The Key Distributor MUST NOT establish DTLS-SRTP associations with The Key Distributor MUST NOT establish DTLS-SRTP associations with
Endpoints without first authenticating the Media Distributor endpoints without first authenticating the Media Distributor
tunneling the DTLS-SRTP packets from the Endpoint. tunneling the DTLS-SRTP packets from the endpoint.
Note that following DTLS-SRTP procedures for the Note that following DTLS-SRTP procedures for the cipher defined in
[I-D.ietf-perc-double] cipher, the Endpoint generates both E2E and [RFC8723], the endpoint generates both E2E and HBH encryption keys
HBH encryption keys and salt values. Endpoints MUST either use the and salt values. Endpoints MUST either use the DTLS-SRTP-generated
DTLS-SRTP generated E2E key for transmission or generate a fresh E2E E2E key for transmission or generate a fresh E2E key. In either
key. In either case, the generated SRTP master salt for E2E case, the generated SRTP master salt for E2E encryption MUST be
encryption MUST be replaced with the salt value provided by the Key replaced with the salt value provided by the Key Distributor via the
Distributor via the EKTKey message. That is because every Endpoint EKTKey message. That is because every endpoint in the conference
in the conference uses the same SRTP master salt. The Endpoint only uses the same SRTP master salt. The endpoint only transmits the SRTP
transmits the SRTP master key (not the salt) used for E2E encryption master key (not the salt) used for E2E encryption to other endpoints
to other Endpoints in RTP/RTCP packets per in RTP/RTCP packets per [RFC8870].
[I-D.ietf-perc-srtp-ekt-diet].
Media Distributors use DTLS-SRTP directly with a peer Media Media Distributors use DTLS-SRTP directly with a peer Media
Distributor to establish the HBH key 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 packets to that peer Media Distributor. The Key Distributor does not
facilitate establishing a HBH key for use between Media Distributors. facilitate establishing a HBH key for use between Media Distributors.
4.5.2. Key Exchange during a Conference 4.5.2. Key Exchange during a Conference
Following the initial key information exchange with the Key Following the initial key information exchange with the Key
Distributor, an Endpoint is able to encrypt media end-to-end with an 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 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 KEK, and is able to encrypt and authenticate RTP packets using a HBH
key. The procedures defined do not allow the Media Distributor to key. This framework does not allow the Media Distributor to gain
gain access to the KEK information, preventing it from gaining access access to the KEK information, preventing it from gaining access to
to any Endpoint's E2E key and subsequently decrypting media. any endpoint's E2E key and subsequently decrypting media.
The KEK may need to change from time-to-time during the life of a The KEK may need to change from time to time during the lifetime of a
conference, such as when a new participant joins or leaves a conference, such as when a new participant joins or leaves a
conference. Dictating if, when or how often a conference is to be conference. Dictating if, when, or how often a conference is to be
re-keyed is outside the scope of this document, but this framework rekeyed is outside the scope of this document, but this framework
does accommodate re-keying during the life of a conference. does accommodate rekeying during the lifetime of a conference.
When a Key Distributor decides to re-key a conference, it transmits a When a Key Distributor decides to rekey a conference, it transmits a
new EKTKey message containing the new EKT Key to each of the new EKTKey message containing the new EKT Key to each of the
conference participants. Upon receipt of the new EKT Key, the conference participants. Upon receipt of the new EKT Key, the
Endpoint MUST create a new SRTP master key and prepare to send that 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 key inside a FullEKTField using the new EKT Key as per Section 4.5 of
of [I-D.ietf-perc-srtp-ekt-diet]. In order to allow time for all [RFC8870]. In order to allow time for all endpoints in the
Endpoints in the conference to receive the new keys, the sender conference to receive the new keys, the sender should follow the
should follow the recommendations in Section 4.7 of [I-D.ietf-perc- recommendations in Section 4.6 of [RFC8870]. On receiving a new EKT
srtp-ekt-diet]. On receiving a new EKT Key, Endpoints MUST be Key, endpoints MUST be prepared to decrypt EKT Tags using the new
prepared to decrypt EKT tags using the new key. The EKT SPI field is key. The EKT Security Parameter Index (SPI) field is used to
used to differentiate between tags encrypted with the old and new differentiate between EKT Tags encrypted with the old and new keys.
keys.
After re-keying, an Endpoint SHOULD retain prior SRTP master keys and After rekeying, an endpoint SHOULD retain prior SRTP master keys and
EKT Key for a period of time sufficient for the purpose of ensuring EKT Keys for a period of time sufficient for the purpose of ensuring
it can decrypt late-arriving or out-of-order packets or packets sent that it can decrypt late-arriving or out-of-order packets or packets
by other Endpoints that used the prior keys for a period of time 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 after rekeying began. An endpoint MAY retain old keys until the end
of the conference. of the conference.
Endpoints MAY follow the procedures in section 5.2 of [RFC5764] to Endpoints MAY follow the procedures in Section 5.2 of [RFC5764] to
renegotiate HBH keys as desired. If new HBH keys are generated, the renegotiate HBH keys as desired. If new HBH keys are generated, the
new keys are also delivered to the Media Distributor following the new keys are also delivered to the Media Distributor following the
procedures defined in [I-D.ietf-perc-dtls-tunnel] as one possible procedures defined in [PERC-DTLS] as one possible method.
method.
Endpoints MAY change the E2E encryption key used at any time. An At any time, endpoints MAY change the E2E encryption key being used.
Endpoint MUST generate a new E2E encryption key whenever it receives An endpoint MUST generate a new E2E encryption key whenever it
a new EKT Key. After switching to a new key, the new key is conveyed receives a new EKT Key. After switching to a new key, the new key is
to other Endpoints in the conference in RTP/RTCP packets per conveyed to other endpoints in the conference in RTP/RTCP packets per
[I-D.ietf-perc-srtp-ekt-diet]. [RFC8870].
5. Authentication 5. Authentication
It is important to this solution framework that the entities can It is important that entities can validate the authenticity of other
validate the authenticity of other entities, especially the Key entities, especially the Key Distributor and endpoints. Details on
Distributor and Endpoints. The details of this are outside the scope this topic are outside the scope of this specification, but a few
of specification but a few possibilities are discussed in the possibilities are discussed in the following sections. The critical
following sections. The critical requirements are that an Endpoint requirements are that (1) an endpoint can verify that it is connected
can verify it is connected to the correct Key Distributor for the to the correct Key Distributor for the conference and (2) the Key
conference and the Key Distributor can verify the Endpoint is the Distributor can verify that the endpoint is the correct endpoint for
correct Endpoint for the conference. the conference.
Two possible approaches to solve this are Identity Assertions and Two possible approaches to resolve this situation are identity
Certificate Fingerprints. assertions and certificate fingerprints.
5.1. Identity Assertions 5.1. Identity Assertions
WebRTC Identity assertion [I-D.ietf-rtcweb-security-arch] is used to A WebRTC identity assertion [RFC8827] is used to bind the identity of
bind the identity of the user of the Endpoint to the fingerprint of the user of the endpoint to the fingerprint of the DTLS-SRTP
the DTLS-SRTP certificate used for the call. This certificate is certificate used for the call. This certificate is unique for a
unique for a given call and a conference. This allows the Key given call and a conference. This certificate is unique for a given
Distributor to ensure that only authorized users participate in the call and a conference, allowing the Key Distributor to ensure that
conference. Similarly the Key Distributor can create a WebRTC only authorized users participate in the conference. Similarly, the
Identity assertion to bind the fingerprint of the unique certificate Key Distributor can create a WebRTC identity assertion to bind the
used by the Key Distributor for this conference so that the Endpoint fingerprint of the unique certificate used by the Key Distributor for
can validate it is talking to the correct Key Distributor. Such a this conference so that the endpoint can verify that it is talking to
setup requires an Identity Provider (Idp) trusted by the Endpoints the correct Key Distributor. Such a setup requires an Identity
and the Key Distributor. Provider (IdP) trusted by the endpoints and the Key Distributor.
5.2. Certificate Fingerprints in Session Signaling 5.2. Certificate Fingerprints in Session Signaling
Entities managing session signaling are generally assumed to be Entities managing session signaling are generally assumed to be
untrusted in the PERC framework. However, there are some deployment untrusted in the PERC framework. However, there are some deployment
scenarios where parts of the session signaling may be assumed scenarios where parts of the session signaling may be assumed
trustworthy for the purposes of exchanging, in a manner that can be trustworthy for the purposes of exchanging, in a manner that can be
authenticated, the fingerprint of an entity's certificate. authenticated, the fingerprint of an entity's certificate.
As a concrete example, SIP [RFC3261] and Session Description Protocol As a concrete example, SIP [RFC3261] and the Session Description
(SDP) [RFC4566] can be used to convey the fingerprint information per Protocol (SDP) [RFC4566] can be used to convey the fingerprint
[RFC5763]. An Endpoint's SIP User Agent would send an INVITE message information per [RFC5763]. An endpoint's SIP User Agent would send
containing SDP for the media session along with the Endpoint's an INVITE message containing SDP for the media session along with the
certificate fingerprint, which can be signed using the procedures endpoint's certificate fingerprint, which can be signed using the
described in [RFC8224] for the benefit of forwarding the message to procedures described in [RFC8224] for the benefit of forwarding the
other entities by the Focus [RFC4353]. Other entities can verify the message to other entities by the focus [RFC4353]. Other entities can
fingerprints match the certificates found in the DTLS-SRTP verify that the fingerprints match the certificates found in the
connections to find the identity of the far end of the DTLS-SRTP DTLS-SRTP connections to find the identity of the far end of the
connection and verify that is the authorized entity. DTLS-SRTP connection and verify that it is the authorized entity.
Ultimately, if using session signaling, an Endpoint's certificate Ultimately, if using session signaling, an endpoint's certificate
fingerprint would need to be securely mapped to a user and conveyed 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 to the Key Distributor so that it can check that the user in question
authorized. Similarly, the Key Distributor's certificate fingerprint is authorized. Similarly, the Key Distributor's certificate
can be conveyed to an Endpoint in a manner that can be authenticated fingerprint can be conveyed to an endpoint in a manner that can be
as being an authorized Key Distributor for this conference. authenticated as being an authorized Key Distributor for this
conference.
5.3. Conference Identification 5.3. Conference Identification
The Key Distributor needs to know what Endpoints are being added to a The Key Distributor needs to know what endpoints are being added to a
given conference. Thus, the Key Distributor and the Media given conference. Thus, the Key Distributor and the Media
Distributor need to know Endpoint-to-conference mappings, which is Distributor need to know endpoint-to-conference mappings, which are
enabled by exchanging a conference-specific unique identifier defined enabled by exchanging a conference-specific unique identifier as
in [I-D.ietf-perc-dtls-tunnel]. How this unique identifier is described in [PERC-DTLS]. How this unique identifier is assigned is
assigned is outside the scope of this document. outside the scope of this document.
6. PERC Keys 6. PERC Keys
This section describes the various keys employed by PERC. This section describes the various keys employed by PERC.
6.1. Key Inventory and Management Considerations 6.1. Key Inventory and Management Considerations
This section summarizes the several different keys used in the PERC This section summarizes the several different keys used in the PERC
framework, how they are generated, and what purpose they serve. framework, how they are generated, and what purpose they serve.
The keys are described in the order in which they would typically be The keys are described in the order in which they would typically be
acquired. acquired.
The various keys used in PERC are shown in Figure 4 below. The various keys used in PERC are shown in Table 1 below.
+-----------+----------------------------------------------------+ +===========+=============================================+
| Key | Description | | Key | Description |
+-----------+----------------------------------------------------+ +===========+=============================================+
| HBH Key | SRTP master key used to encrypt media hop-by-hop. | | HBH Key | SRTP master key used to encrypt media hop |
+-----------+----------------------------------------------------+ | | by hop. |
| KEK | Key shared by all Endpoints and used to encrypt | +-----------+---------------------------------------------+
| (EKT Key) | each Endpoint's E2E SRTP master key so receiving | | KEK | Key shared by all endpoints and used to |
| | Endpoints can decrypt media. | | (EKT Key) | encrypt each endpoint's E2E SRTP master key |
+-----------+----------------------------------------------------+ | | so receiving endpoints can decrypt media. |
| E2E Key | SRTP master key used to encrypt media end-to-end. | +-----------+---------------------------------------------+
+-----------+----------------------------------------------------+ | E2E Key | SRTP master key used to encrypt media end |
| | to end. |
+-----------+---------------------------------------------+
Figure 4: Key Inventory Table 1: Key Inventory
While the number of key types is very small, it should be understood 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 that the actual number of distinct keys can be large as the
conference grows in size. conference grows in size.
As an example, with 1,000 participants in a conference, there would 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 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 same master salt. Each of those keys is passed through the Key
defined in [RFC3711] to produce the actual encryption and Derivation Function (KDF) as defined in [RFC3711] to produce the
authentication keys. actual encryption and authentication keys.
Complicating key management is the fact that the KEK can change and, Complicating key management is the fact that the KEK can change and,
when it does, the Endpoints generate new SRTP master keys that are when it does, the endpoints generate new SRTP master keys that are
associated with a new EKT SPI. Endpoints might retain old keys for a associated with a new EKT SPI. Endpoints might retain old keys for a
period of time to ensure they can properly decrypt late-arriving or period of time to ensure that they can properly decrypt late-arriving
out-of-order packets, which means the number of keys held during that or out-of-order packets, which means that the number of keys held
period of time might substantially more. during that period of time might be substantially higher.
A more detailed explanation of each of the keys follows. A more detailed explanation of each of the keys follows.
6.2. DTLS-SRTP Exchange Yields HBH Keys 6.2. DTLS-SRTP Exchange Yields HBH Keys
The first set of keys acquired are for hop-by-hop encryption and The first set of keys acquired are for HBH encryption and decryption.
decryption. Per the Double [I-D.ietf-perc-double] procedures, the Per the double transform procedures [RFC8723], the endpoint performs
Endpoint performs DTLS-SRTP exchange with the Key Distributor and a DTLS-SRTP exchange with the Key Distributor and receives a key that
receives a key that is, in fact, "double" the size that is needed. is, in fact, "double" the size that is needed. The E2E part is the
The end-to-end part is the first half of the key, so the Endpoint first half of the key, so the endpoint discards that information when
discards that information when generating its own key. The second generating its own key. The second half of the keying material is
half of the key material is for hop-by-hop operations, so that half for HBH operations, so that half of the key (corresponding to the
of the key (corresponding to the least significant bits) is assigned least significant bits) is assigned internally as the HBH key.
internally as the HBH key.
The Key Distributor informs the Media Distributor of the HBH key. The Key Distributor informs the Media Distributor of the HBH key.
Specifically, the Key Distributor sends the least significant bits Specifically, the Key Distributor sends the least significant bits
corresponding to the half of the keying material determined through corresponding to the half of the keying material determined through
DTLS-SRTP with the Endpoint to the Media Distributor. A salt value 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 is generated along with the HBH key. The salt is also longer than
needed for hop-by-hop operations, thus only the least significant needed for HBH operations; thus, only the least significant bits of
bits of the required length (half of the generated salt material) are the required length (half of the generated salt material) are sent to
sent to the Media Distributor. One way to transmit this key and salt the Media Distributor. One way to transmit this key and salt
information is via the tunnel protocol defined in information is via the tunnel protocol defined in [PERC-DTLS].
[I-D.ietf-perc-dtls-tunnel].
No two Endpoints have the same HBH key, thus the Media Distributor No two endpoints have the same HBH key; thus, the Media Distributor
MUST keep track each distinct HBH key (and the corresponding salt) MUST keep track of each distinct HBH key (and the corresponding salt)
and use it only for the specified hop. and use it only for the specified hop.
The HBH key is also used for hop-by-hop encryption of RTCP. RTCP is The HBH key is also used for HBH encryption of RTCP. RTCP is not
not end-to-end encrypted in PERC. E2E-encrypted in PERC.
6.3. The Key Distributor Transmits the KEK (EKT Key) 6.3. The Key Distributor Transmits the KEK (EKT Key)
Via the aforementioned DTLS-SRTP association, the Key Distributor The Key Distributor sends the KEK (the EKT Key per [RFC8870]) to the
sends the Endpoint the KEK (EKT Key per endpoint via the aforementioned DTLS-SRTP association. This key is
[I-D.ietf-perc-srtp-ekt-diet]). This key is known only to the Key known only to the Key Distributor and endpoints; it is the most
Distributor and Endpoints. This key is the most important to protect important entity to protect, since having knowledge of this key (and
since having knowledge of this key (and the SRTP master salt the SRTP master salt transmitted as a part of the same message)
transmitted as a part of the same message) allows an entity to allows an entity to decrypt any media packet in the conference.
decrypt any media packet in the conference.
Note that the Key Distributor can send any number of EKT Keys to 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 endpoints. This information is used to rekey the entire conference.
is identified by a "Security Parameter Index" (SPI) value. Endpoints Each key is identified by an SPI value. Endpoints MUST expect that a
MUST expect that a conference might be re-keyed when a new conference might be rekeyed when a new participant joins a conference
participant joins a conference or when a participant leaves a or when a participant leaves a conference, in order to protect the
conference in order to protect the confidentiality of the confidentiality of the conversation before and after such events.
conversation before and after such events.
The SRTP master salt to be used by the Endpoint is transmitted along 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 with the EKT Key. All endpoints in the conference utilize the same
SRTP master salt that corresponds with a given EKT Key. SRTP master salt that corresponds with a given EKT Key.
The Full EKT Tag in media packets is encrypted using a cipher 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 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 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 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]). EKT Tag data as per [RFC8870]).
The Media Distributor is not given the KEK. The KEK is not given to the Media Distributor.
6.4. Endpoints fabricate an SRTP Master Key 6.4. Endpoints Fabricate an SRTP Master Key
As stated earlier, the E2E key determined via DTLS-SRTP MAY be As stated earlier, the E2E key determined via DTLS-SRTP MAY be
discarded in favor of a locally-generated E2E SRTP master key. While discarded in favor of a locally generated E2E SRTP master key. While
the DTLS-SRTP-derived SRTP master key can be used initially, the the DTLS-SRTP-derived SRTP master key can be used initially, the
Endpoint might choose to change the SRTP master key periodically and 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. 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 A locally generated SRTP master key is used along with the master
salt transmitted to the Endpoint from the Key Distributor via the salt transmitted to the endpoint from the Key Distributor via the
EKTKey message to encrypt media end-to-end. EKTKey message to encrypt media end to end.
Since the Media Distributor is not involved in E2E functions, it does 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, not create this key, nor does it have access to any endpoint's E2E
too, that even the Key Distributor is unaware of the locally- key. Note, too, that even the Key Distributor is unaware of the
generated E2E keys used by each Endpoint. locally generated E2E keys used by each endpoint.
The Endpoint transmits its E2E key to other Endpoints in the The endpoint transmits its E2E key to other endpoints in the
conference by periodically including it in SRTP packets in a Full EKT 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 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 Key provided by the Key Distributor. The master salt is not
transmitted, though, since all Endpoints receive the same master salt transmitted, though, since all endpoints receive the same master salt
via the EKTKey message from the Key Distributor. The recommended via the EKTKey message from the Key Distributor. The recommended
frequency with which an Endpoint transmits its SRTP master key is frequency with which an endpoint transmits its SRTP master key is
specified in [I-D.ietf-perc-srtp-ekt-diet]. specified in [RFC8870].
6.5. Summary of Key Types and Entity Possession 6.5. Summary of Key Types and Entity Possession
All Endpoints have knowledge of the KEK. All endpoints have knowledge of the KEK.
Every HBH key is distinct for a given Endpoint, thus Endpoint A and 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. Since HBH Endpoint B do not have knowledge of the other's HBH key. Since HBH
keys are derived from a DTLS-SRTP association, there is at most one keys are derived from a DTLS-SRTP association, there is at most one
HBH key per Endpoint, (The only exception is where the DTLS-SRTP HBH key per endpoint. (The only exception is where the DTLS-SRTP
association might be rekeyed per Section 5.2 of [RFC5764] and a new association might be rekeyed per Section 5.2 of [RFC5764] and a new
key is created to replace the former key.) key is created to replace the former key.)
Each Endpoint generates its own E2E key (SRTP master key), thus there Each endpoint generates its own E2E key (SRTP master key); thus,
is a distinct E2E key per endpoint. This key is transmitted there is a distinct E2E key per endpoint. This key is transmitted
(encrypted) via the Full EKT Tag to other Endpoints. Endpoints that (encrypted) via the Full EKT Tag to other endpoints. Endpoints that
receive media from a given transmitting Endpoint gain knowledge of receive media from a given transmitting endpoint gain knowledge of
the transmitter's E2E key via the Full EKT Tag. the transmitter's E2E key via the Full EKT Tag.
To summarize the various keys and which entity is in possession of a Table 2 summarizes the various keys and which entity is in possession
given key, refer to Figure 5. of a given key.
+----------------------+------------+-------+-------+------------+ +=======================+============+======+======+============+
| Key / Entity | Endpoint A | MD X | MD Y | Endpoint B | | Key/Entity | Endpoint A | MD X | MD Y | Endpoint B |
+----------------------+------------+-------+-------+------------+ +=======================+============+======+======+============+
| KEK (EKT Key) | Yes | No | No | Yes | | KEK (EKT Key) | Yes | No | No | Yes |
+----------------------+------------+-------+-------+------------+ +-----------------------+------------+------+------+------------+
| E2E Key (A and B) | Yes | No | No | Yes | | E2E Key (A and B) | Yes | No | No | Yes |
+----------------------+------------+-------+-------+------------+ +-----------------------+------------+------+------+------------+
| HBH Key (A<=>MD X) | Yes | Yes | No | No | | HBH Key (A<=>MD X) | Yes | Yes | No | No |
+----------------------+------------+-------+-------+------------+ +-----------------------+------------+------+------+------------+
| HBH Key (B<=>MD Y) | No | No | Yes | Yes | | HBH Key (B<=>MD Y) | No | No | Yes | Yes |
+----------------------+------------+---------------+------------+ +-----------------------+------------+------+------+------------+
| HBH Key (MD X<=>MD Y)| No | Yes | Yes | No | | HBH Key (MD X<=>MD Y) | No | Yes | Yes | No |
+----------------------+------------+---------------+------------+ +-----------------------+------------+------+------+------------+
Figure 5: Keys Types and Entity Possession Table 2: Key Types and Entity Possession
7. Encrypted Media Packet Format 7. Encrypted Media Packet Format
Figure 6 presents a complete picture of what an encrypted media Figure 4 presents a complete picture of what an encrypted media
packet per this framework looks like when transmitted over the wire. packet per this framework looks like when transmitted over the wire.
The packet format shown is encrypted using the Double cryptographic The packet format shown in the figure is encrypted using the double
transform with an EKT Tag appended to the end. cryptographic transform with an EKT Tag appended to the end.
0 1 2 3 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 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 |V=2|P|X| CC |M| PT | sequence number | IO
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IO +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IO
| timestamp | IO | timestamp | IO
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IO +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IO
| synchronization source (SSRC) identifier | IO | synchronization source (SSRC) identifier | IO
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ IO +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ IO
skipping to change at page 20, line 32 skipping to change at line 881
O +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O O +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O
O | | E2E authentication tag | |O O | | E2E authentication tag | |O
O | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |O O | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |O
O | | OHB ... | |O O | | OHB ... | |O
+>| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |+ +>| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |+
| | | HBH authentication tag | || | | | HBH authentication tag | ||
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ || | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ||
| | | EKT Tag ... | R || | | | EKT Tag ... | R ||
| | +-+-+-+-+-+-+-+-+-+ | || | | +-+-+-+-+-+-+-+-+-+ | ||
| | +- Neither encrypted nor authenticated; || | | +- Neither encrypted nor authenticated; ||
| | appended after Double is performed || | | appended after the double transform ||
| | || | | is performed ||
| | || | | ||
| +- E2E Encrypted Portion E2E Authenticated Portion ---+| | +- E2E-Encrypted Portion E2E-Authenticated Portion ---+|
| | | |
+--- HBH Encrypted Portion HBH Authenticated Portion ----+ +--- HBH-Encrypted Portion HBH-Authenticated Portion ----+
I = Inner (E2E) encryption / authentication I = Inner (E2E) encryption/authentication
O = Outer (HBH) encryption / authentication O = Outer (HBH) encryption/authentication
Figure 6: Encrypted Media Packet Format Figure 4: Encrypted Media Packet Format
8. Security Considerations 8. Security Considerations
8.1. Third Party Attacks 8.1. Third-Party Attacks
Third party attacks are attacks attempted by an adversary that is not Third-party attacks are attacks attempted by an adversary that is not
supposed to have access to keying material or is otherwise not an supposed to have access to keying material or is otherwise not an
authorized participant in the conference. authorized participant in the conference.
On-path attacks are mitigated by hop-by-hop integrity protection and On-path attacks are mitigated by HBH integrity protection and
encryption. The integrity protection mitigates packet modification encryption. The integrity protection mitigates packet modification.
and encryption makes selective blocking of packets harder, but not Encryption makes selective blocking of packets harder, but not
impossible. impossible.
Off-path attackers could try connecting to different PERC entities to Off-path attackers could try connecting to different PERC entities to
send specifically crafted packets with an aim of forcing the receiver send specifically crafted packets with an aim of forcing the receiver
to forward or render bogus media packets. Endpoints and Media to forward or render bogus media packets. Endpoints and Media
Distributors mitigate such an attack by performing hop-by-hop Distributors mitigate such an attack by performing HBH authentication
authentication and discarding packets that fail authentication. and discarding packets that fail authentication.
Another attack vector is a third party claiming to be a Media Another attack vector is a third party claiming to be a Media
Distributor, fooling Endpoints into sending packets to the false Distributor, fooling endpoints into sending packets to the false
Media Distributor instead of the correct one. The deceived sending Media Distributor instead of the correct one. The deceived sending
Endpoints could incorrectly assume their packets have been delivered endpoints could incorrectly assume that their packets have been
to Endpoints when they in fact have not. While this attack is delivered to endpoints when they in fact have not. While this attack
possible, the result is a simple denial of service with no leakage of is possible, the result is a simple denial of service with no leakage
confidential information, since the false Media Distributor would not of confidential information, since the false Media Distributor would
have access to either HBH or E2E encryption keys. not have access to either HBH or E2E encryption keys.
A third party could cause a denial-of-service by transmitting many A third party could cause a denial of service by transmitting many
bogus or replayed packets toward receiving devices that ultimately bogus or replayed packets toward receiving devices and ultimately
degrade conference or device performance. Therefore, implementations degrading conference or device performance. Therefore,
might wish to devise mechanisms to safeguard against such implementations might wish to devise mechanisms to safeguard against
illegitimate packets, such as utilizing rate-limiting or performing such illegitimate packets, such as utilizing rate-limiting or
basic sanity-checks on packets (e.g., looking at packet length or performing basic sanity checks on packets (e.g., looking at packet
expected sequence number ranges) before performing more expensive length or expected sequence number ranges), before performing
decryption operations. decryption operations that are more expensive.
Use of mutual DTLS authentication (as required by DTLS-SRTP) also The use of mutual DTLS authentication (as required by DTLS-SRTP) also
helps to prevent a denial-of-service attack by preventing a false helps to prevent a denial-of-service attack by preventing a false
Endpoint or false Media Distributor from successfully participating endpoint or false Media Distributor from successfully participating
as a perceived valid media sender that could otherwise carry out an as a perceived valid media sender that could otherwise carry out an
on-path attack. When mutual authentication fails, a receiving on-path attack. When mutual authentication fails, a receiving
Endpoint would know that it could safely discard media packets endpoint would know that it could safely discard media packets
received from the Endpoint without inspection. received from the endpoint without inspection.
8.2. Media Distributor Attacks 8.2. Media Distributor Attacks
A malicious or compromised Media Distributor can attack the session A malicious or compromised Media Distributor can attack the session
in a number of possible ways. in a number of possible ways, as discussed below.
8.2.1. Denial of service 8.2.1. Denial of Service
A simple form of attack is discarding received packets that should be A simple form of attack is discarding received packets that should be
forwarded. This solution framework does not introduce any mitigation forwarded. This solution framework does not provide any mitigation
for Media Distributors that fail to forward media packets. mechanisms for Media Distributors that fail to forward media packets.
Another form of attack is modifying received packets before Another form of attack is modifying received packets before
forwarding. With this solution framework, any modification of the forwarding. With this solution framework, any modification of the
end-to-end authenticated data results in the receiving Endpoint E2E-authenticated data results in the receiving endpoint getting an
getting an integrity failure when performing authentication on the integrity failure when performing authentication on the received
received packet. packet.
The Media Distributor can also attempt to perform resource The Media Distributor can also attempt to perform resource
consumption attacks on the receiving Endpoint. One such attack would consumption attacks on the receiving endpoint. One such attack would
be to insert random SSRC/CSRC values in any RTP packet along with a be to insert random SSRC/CSRC values in any RTP packet along with a
Full EKT Tag. Since such a message would trigger the receiver to Full EKT Tag. Since such a message would trigger the receiver to
form a new cryptographic context, the Media Distributor can attempt form a new cryptographic context, the Media Distributor can attempt
to consume the receiving Endpoint's resources. While E2E to consume the receiving endpoint's resources. While E2E
authentication would fail and the cryptographic context would be authentication would fail and the cryptographic context would be
destroyed, the key derivation operation would nonetheless consume destroyed, the key derivation operation would nonetheless consume
some computational resources. While resource consumption attacks some computational resources. While resource consumption attacks
cannot be mitigated entirely, rate-limiting packets might help reduce cannot be mitigated entirely, rate-limiting packets might help reduce
the impact of such attacks. the impact of such attacks.
8.2.2. Replay Attack 8.2.2. Replay Attacks
A replay attack is when an already received packet from a previous A replay attack is an attack where an already-received packet from a
point in the RTP stream is replayed as new packet. This could, for previous point in the RTP stream is replayed as a new packet. This
example, allow a Media Distributor to transmit a sequence of packets could, for example, allow a Media Distributor to transmit a sequence
identified as a user saying "yes", instead of the "no" the user of packets identified as a user saying "yes", instead of the "no" the
actually said. user actually said.
A replay attack is mitigated by the requirement to implement replay A replay attack is mitigated by the requirement to implement replay
protection as described in Section 3.3.2 of [RFC3711]. End-to-end protection as described in Section 3.3.2 of [RFC3711]. E2E replay
replay protection MUST be provided for the duration of the protection MUST be provided for the duration of the conference.
conference.
8.2.3. Delayed Playout Attack 8.2.3. Delayed Playout Attacks
A delayed playout attack is one where media is received and held by a A delayed playout attack is an attack where media is received and
Media Distributor and then forwarded to Endpoints at a later point in held by a Media Distributor and then forwarded to endpoints at a
time. later point in time.
This attack is possible even if E2E replay protection is in place. This attack is possible even if E2E replay protection is in place.
Because the Media Distributor is allowed to select a subset of Because the Media Distributor is allowed to select a subset of
streams and not forward the rest to a receiver, such as in forwarding 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 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 E2E packet sequence. The problem here is that a Media Distributor
can select to not deliver a particular stream for a while. can choose to not deliver a particular stream for a while.
While the Media Distributor can purposely stop forwarding media While the Media Distributor can purposely stop forwarding media
flows, it can also select an arbitrary starting point to resume flows, it can also select an arbitrary starting point to resume
forwarding those media flow, perhaps forwarding old packets rather forwarding those media flows, perhaps forwarding old packets rather
than current packets. As a consequence, what the media source sent than current packets. As a consequence, what the media source sent
can be substantially delayed at the receiver with the receiver can be substantially delayed at the receiver with the receiver
believing that newly arriving packets are delayed only by transport believing that newly arriving packets are delayed only by transport
delay when the packets may actually be minutes or hours old. delay when the packets may actually be minutes or hours old.
While this attack cannot be eliminated entirely, its effectiveness While this attack cannot be eliminated entirely, its effectiveness
can be reduced by re-keying the conference periodically since can be reduced by rekeying the conference periodically, since
significantly-delayed media encrypted with expired keys would not be significantly delayed media encrypted with expired keys would not be
decrypted by Endpoints. decrypted by endpoints.
8.2.4. Splicing Attack 8.2.4. Splicing Attacks
A splicing attack is an attack where a Media Distributor receiving A splicing attack is an attack where a Media Distributor receiving
multiple media sources splices one media stream into the other. If multiple media sources splices one media stream into the other. If
the Media Distributor were able to change the SSRC without the the Media Distributor were able to change the SSRC without the
receiver having any method for verifying the original source ID, then receiver having any method for verifying the original source ID, then
the Media Distributor could first deliver stream A and then later the Media Distributor could first deliver stream A and then later
forward stream B under the same SSRC as stream A was previously forward stream B under the same SSRC that stream A was previously
using. By including the SSRC in the integrity check for each packet, using. By including the SSRC in the integrity check for each packet
both HBH and E2E, PERC prevents splicing attacks. -- both HBH and E2E -- PERC prevents splicing attacks.
8.2.5. RTCP Attacks 8.2.5. RTCP Attacks
PERC does not provide end-to-end protection of RTCP messages. This PERC does not provide E2E protection of RTCP messages. This allows a
allows a compromised Media Distributor to impact any message that compromised Media Distributor to impact any message that might be
might be transmitted via RTCP, including media statistics, picture transmitted via RTCP, including media statistics, picture requests,
requests, or loss indication. It is also possible for a compromised or loss indication. It is also possible for a compromised Media
Media Distributor to forge requests, such as requests to the Endpoint Distributor to forge requests, such as requests to the endpoint to
to send a new picture. Such requests can consume significant send a new picture. Such requests can consume significant bandwidth
bandwidth and impair conference performance. and impair conference performance.
8.3. Key Distributor Attacks 8.3. Key Distributor Attacks
As stated in Section 3.2.2, the Key Distributor needs to be secured As stated in Section 3.2.2, the Key Distributor needs to be secured,
since exploiting the Key Server can allow an adversary to gain access since exploiting the Key Server can allow an adversary to gain access
to the keying material for one or more conferences. Having access to to the keying material for one or more conferences. Having access to
that keying material would then allow the adversary to decrypt media that keying material would then allow the adversary to decrypt media
sent from any Endpoint in the conference. sent from any endpoint in the conference.
As a first line of defense, the Key Distributor authenticates every As a first line of defense, the Key Distributor authenticates every
security association, both associations with Endpoints and Media security association -- associations with both endpoints and Media
Distributors. The Key Distributor knows which entities are Distributors. The Key Distributor knows which entities are
authorized to have access to which keys and inspection of authorized to have access to which keys, and inspection of
certificates will substantially reduce the risk of providing keys to certificates will substantially reduce the risk of providing keys to
an adversary. an adversary.
Both physical and network access to the Key Distributor should be Both physical and network access to the Key Distributor should be
severely restricted. This may be more difficult to achieve when the severely restricted. This may be more difficult to achieve when the
Key Distributor is embedded within and Endpoint, for example. Key Distributor is embedded within, for example, an endpoint.
Nonetheless, consideration should be given to shielding the Key Nonetheless, consideration should be given to shielding the Key
Distributor from unauthorized access or any access that is not Distributor from unauthorized access or any access that is not
strictly necessary for the support of an ongoing conference. strictly necessary for the support of an ongoing conference.
Consideration should be given to whether access to the keying Consideration should be given to whether access to the keying
material will be needed beyond the conclusion of a conference. If material will be needed beyond the conclusion of a conference. If
not needed, the Key Distributor's policy should be to destroy the not needed, the Key Distributor's policy should be to destroy the
keying material once the conference concludes or when keying material keying material once the conference concludes or when keying material
changes during the course of the conference. If keying material is changes during the course of the conference. If keying material is
needed beyond the lifetime of the conference, further consideration needed beyond the lifetime of the conference, further consideration
should be given to protecting keying material from future exposure. should be given to protecting keying material from future exposure.
While it might be obvious, it is worth stating to avoid any doubt While it might seem obvious, it is worth making this point, to avoid
that if an adversary were to record the media packets transmitted any doubt that if an adversary were to record the media packets
during a conference and then gain unauthorized access to the keying transmitted during a conference and then gain unauthorized access to
material left unsecured on the Key Distributor even years later, the the keying material left unsecured on the Key Distributor even years
adversary could decrypt the content every packet transmitted during later, the adversary could decrypt the content of every packet
the conference. transmitted during the conference.
8.4. Endpoint Attacks 8.4. Endpoint Attacks
A Trusted Endpoint is so named because conference confidentiality A Trusted Endpoint is so named because conference confidentiality
relies heavily on the security and integrity of the Endpoint. If an relies heavily on the security and integrity of the endpoint. If an
adversary successfully exploits a vulnerability in an Endpoint, it adversary successfully exploits a vulnerability in an endpoint, it
might be possible for the adversary to obtain all of the keying might be possible for the adversary to obtain all of the keying
material used in the conference. With that keying material, an material used in the conference. With that keying material, an
adversary could decrypt any of the media flows received from any adversary could decrypt any of the media flows received from any
other Endpoint, either in real-time or at a later point in time other endpoint, either in real time or at a later point in time
(assuming the adversary makes a copy of the media packets). (assuming that the adversary makes a copy of the media packets).
Additionally, if an adversary successfully exploits an Endpoint, the Additionally, if an adversary successfully exploits an endpoint, the
adversary could inject media into the conference. One way an adversary could inject media into the conference. For example, an
adversary could do that is by manipulating the RTP or SRTP software adversary could manipulate the RTP or SRTP software to transmit
to transmit whatever media the adversary wishes to send. This could whatever media the adversary wishes to send. This could involve the
involve re-use of the Endpoint's SSRC, a new SSRC, or the SSRC value reuse of the compromised endpoint's SSRC or, since all conference
of an existing endpoint. This is made possible since all conference participants share the same KEK, the use of a new SSRC or the SSRC
participants share the same KEK. Only a single SRTP cipher suite value of another endpoint. Only a single SRTP cipher suite defined
defined provides source authentication properties that allow an provides source authentication properties that allow an endpoint to
endpoint to cryptographically assert that it sent a particular E2E cryptographically assert that it sent a particular E2E-protected
protected packet (namely, TESLA [RFC4383]), and its usage is packet (namely, Timed Efficient Stream Loss-Tolerant Authentication
presently not defined for PERC. The suite defined in PERC only (TESLA) [RFC4383]), and its usage is presently not defined for PERC.
allows an Endpoint to determine that whoever sent a packet had The suite defined in PERC only allows an endpoint to determine that
received the KEK. whoever sent a packet had received the KEK.
However, attacks on the endpoint are not limited to the PERC-specific However, attacks on the endpoint are not limited to the PERC-specific
software within the Endpoint. An attacker could inject media or software within the endpoint. An attacker could inject media or
record media by manipulating the software that sits between the PERC- record media by manipulating the software that sits between the PERC-
enabled application and the hardware microphone of video camera, for enabled application and the hardware microphone of a video camera,
example. Likewise, an attacker could potentially access confidential for example. Likewise, an attacker could potentially access
media by accessing memory, cache, disk storage, etc. if the endpoint confidential media by accessing memory, cache, disk storage, etc. if
is no secured. the endpoint is not secured.
9. IANA Considerations 9. IANA Considerations
There are no IANA considerations for this document. This document has no IANA actions.
10. Acknowledgments
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.
11. References
11.1. Normative References
[I-D.ietf-perc-double] 10. References
Jennings, C., Jones, P., Barnes, R., and A. Roach, "SRTP
Double Encryption Procedures", draft-ietf-perc-double-10
(work in progress), October 2018.
[I-D.ietf-perc-srtp-ekt-diet] 10.1. Normative References
Jennings, C., Mattsson, J., McGrew, D., Wing, D., and F.
Andreasen, "Encrypted Key Transport for DTLS and Secure
RTP", draft-ietf-perc-srtp-ekt-diet-09 (work in progress),
October 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>. July 2003, <https://www.rfc-editor.org/info/rfc3550>.
skipping to change at page 26, line 14 skipping to change at line 1127
[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure [RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904, Real-time Transport Protocol (SRTP)", RFC 6904,
DOI 10.17487/RFC6904, April 2013, DOI 10.17487/RFC6904, April 2013,
<https://www.rfc-editor.org/info/rfc6904>. <https://www.rfc-editor.org/info/rfc6904>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
11.2. Informative References [RFC8723] Jennings, C., Jones, P., Barnes, R., and A.B. Roach,
"Double Encryption Procedures for the Secure Real-Time
Transport Protocol (SRTP)", RFC 8723,
DOI 10.17487/RFC8723, April 2020,
<https://www.rfc-editor.org/info/rfc8723>.
[I-D.ietf-perc-dtls-tunnel] [RFC8870] Jennings, C., Mattsson, J., McGrew, D., Wing, D., and F.
Jones, P., Ellenbogen, P., and N. Ohlmeier, "DTLS Tunnel Andreasen, "Encrypted Key Transport for DTLS and Secure
between a Media Distributor and Key Distributor to RTP", RFC 8870, DOI 10.17487/RFC8870, January 2021,
Facilitate Key Exchange", draft-ietf-perc-dtls-tunnel-05 <https://www.rfc-editor.org/info/rfc8870>.
(work in progress), April 2019.
[I-D.ietf-rtcweb-security-arch] 10.2. Informative References
Rescorla, E., "WebRTC Security Architecture", draft-ietf-
rtcweb-security-arch-18 (work in progress), February 2019. [PERC-DTLS]
Jones, P. E., Ellenbogen, P. M., and N. H. Ohlmeier, "DTLS
Tunnel between a Media Distributor and Key Distributor to
Facilitate Key Exchange", Work in Progress, Internet-
Draft, draft-ietf-perc-dtls-tunnel-06, 16 October 2019,
<https://tools.ietf.org/html/draft-ietf-perc-dtls-tunnel-
06>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261, Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002, DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>. <https://www.rfc-editor.org/info/rfc3261>.
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the [RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353, Session Initiation Protocol (SIP)", RFC 4353,
DOI 10.17487/RFC4353, February 2006, DOI 10.17487/RFC4353, February 2006,
skipping to change at page 27, line 27 skipping to change at line 1197
[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667, [RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015, DOI 10.17487/RFC7667, November 2015,
<https://www.rfc-editor.org/info/rfc7667>. <https://www.rfc-editor.org/info/rfc7667>.
[RFC8224] Peterson, J., Jennings, C., Rescorla, E., and C. Wendt, [RFC8224] Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session "Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 8224, Initiation Protocol (SIP)", RFC 8224,
DOI 10.17487/RFC8224, February 2018, DOI 10.17487/RFC8224, February 2018,
<https://www.rfc-editor.org/info/rfc8224>. <https://www.rfc-editor.org/info/rfc8224>.
[W3C.CR-webrtc-20180927] [RFC8827] Rescorla, E., "WebRTC Security Architecture", RFC 8827,
Bergkvist, A., Burnett, D., Jennings, C., Narayanan, A., DOI 10.17487/RFC8827, January 2021,
Aboba, B., Brandstetter, T., and J. Bruaroey, "WebRTC 1.0: <https://www.rfc-editor.org/info/rfc8827>.
Real-time Communication Between Browsers", World Wide Web
Consortium CR CR-webrtc-20180927, September 2018, [W3C.CR-webrtc]
<https://www.w3.org/TR/2018/CR-webrtc-20180927>. Jennings, C., Boström, H., and J-I. Bruaroey, "WebRTC 1.0:
Real-time Communication Between Browsers", W3C Proposed
Recommendation, <https://www.w3.org/TR/webrtc/>.
Acknowledgments
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 draft
versions of this document as a coauthor. 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 (Section 8).
Authors' Addresses Authors' Addresses
Paul E. Jones Paul E. Jones
Cisco Cisco
7025 Kit Creek Rd. 7025 Kit Creek Rd.
Research Triangle Park, North Carolina 27709 Research Triangle Park, North Carolina 27709
USA United States of America
Phone: +1 919 476 2048 Phone: +1 919 476 2048
Email: paulej@packetizer.com Email: paulej@packetizer.com
David Benham David Benham
Independent Independent
California
United States of America
Email: dabenham@gmail.com Email: dabenham@gmail.com
Christian Groves Christian Groves
Independent Independent
Melbourne Melbourne
Australia Australia
Email: cngroves.std@gmail.com Email: cngroves.std@gmail.com
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