Internet Engineering Task Force (IETF) P. Jones
Request for Comments: 8871
Category: Standards Track D. Benham
ISSN: 2070-1721 C. Groves
A Solution Framework for Private Media in Privacy-Enhanced RTP
This document describes a solution framework for ensuring that media
confidentiality and integrity are maintained end to end within the
context of a switched conferencing environment where Media
Distributors are not trusted with the end-to-end media encryption
keys. The solution builds upon existing security mechanisms defined
for the Real-time Transport Protocol (RTP).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8871
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) in effect on the date of
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Table of Contents 1.
Conventions Used in This Document 3.
PERC Entities and Trust Model 3.1.
Untrusted Entities 3.1.1.
Media Distributor 3.1.2.
Call Processing 3.2.
Trusted Entities 3.2.1.
Key Distributor 4.
Framework for PERC 4.1.
E2E-Authenticated and HBH-Authenticated Encryption 4.2.
E2E Key Confidentiality 4.3.
E2E Keys and Endpoint Operations 4.4.
HBH Keys and Per-Hop Operations 4.5.
Key Exchange 4.5.1.
Initial Key Exchange and Key Distributor 4.5.2.
Key Exchange during a Conference 5.
Identity Assertions 5.2.
Certificate Fingerprints in Session Signaling 5.3.
Conference Identification 6.
PERC Keys 6.1.
Key Inventory and Management Considerations 6.2.
DTLS-SRTP Exchange Yields HBH Keys 6.3.
The Key Distributor Transmits the KEK (EKT Key) 6.4.
Endpoints Fabricate an SRTP Master Key 6.5.
Summary of Key Types and Entity Possession 7.
Encrypted Media Packet Format 8.
Security Considerations 8.1.
Third-Party Attacks 8.2.
Media Distributor Attacks 8.2.1.
Denial of Service 8.2.2.
Replay Attacks 8.2.3.
Delayed Playout Attacks 8.2.4.
Splicing Attacks 8.2.5.
RTCP Attacks 8.3.
Key Distributor Attacks 8.4.
Endpoint Attacks 9.
IANA Considerations 10.
Normative References 10.2.
Switched conferencing is an increasingly popular model for multimedia
conferences with multiple participants using a combination of audio,
video, text, and other media types. With this model, real-time media
flows from conference participants are not mixed, transcoded,
translated, recomposed, or otherwise manipulated by a Media
Distributor, as might be the case with a traditional media server or
Multipoint Control Unit (MCU). Instead, media flows transmitted by
conference participants are simply forwarded by Media Distributors to
each of the other participants. Media Distributors often forward
only a subset of flows based on voice activity detection or other
criteria. In some instances, Media Distributors may make limited
modifications to RTP headers [RFC3550
], for example, but the actual
media content (e.g., voice or video data) is unaltered.
An advantage of switched conferencing is that Media Distributors can
be more easily deployed on general-purpose computing hardware,
including virtualized environments in private and public clouds.
Virtualized public cloud environments have been viewed as less
secure, since resources are not always physically controlled by those
who use them. This document defines improved security so as to lower
the barrier to taking advantage of those environments.
This document defines a solution framework wherein media privacy is
ensured by making it impossible for a Media Distributor to gain
access to keys needed to decrypt or authenticate the actual media
content sent between conference participants. At the same time, the
framework allows for the Media Distributors to modify certain RTP
headers; add, remove, encrypt, or decrypt RTP header extensions; and
encrypt and decrypt RTP Control Protocol (RTCP) packets [RFC3550
The framework also prevents replay attacks by authenticating each
packet transmitted between a given participant and the Media
Distributor using a unique key per endpoint that is independent from
the key for media encryption and authentication.
This solution framework provides for enhanced privacy in RTP-based
conferencing environments while utilizing existing security
procedures defined for RTP with minimal enhancements.
2. Conventions Used in This Document
The key words "MUST
", "MUST NOT
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in
BCP 14 [RFC2119
] when, and only when, they appear in all
capitals, as shown here.
Additionally, this solution framework uses the following terms and
End-to-End (E2E): Communications from one endpoint through one or
more Media Distributors to the endpoint at the other end.
Hop-by-Hop (HBH): Communications between an endpoint and a Media
Distributor or between Media Distributors.
Trusted Endpoint (or simply endpoint): An RTP flow-terminating
entity that has possession of E2E media encryption keys and
terminates E2E encryption. This may include embedded user
conferencing equipment or browsers on computers, media gateways,
MCUs, media recording devices, and more, that are in the trusted
domain for a given deployment. In the context of WebRTC
[W3C.CR-webrtc], where control of a session is divided between a
Trusted Endpoint for purposes of this framework (just as it acts
as the endpoint for DTLS-SRTP [RFC5764
] in one-to-one calls).
Media Distributor (MD): An RTP middlebox that forwards endpoint
media content (e.g., voice or video data) unaltered -- either a
subset or all of the flows at any given time -- and is never
allowed to have access to E2E encryption keys. It operates
according to the Selective Forwarding Middlebox RTP topologies
] per the constraints defined by the Private Media in
Privacy-Enhanced RTP Conferencing (PERC) system, which includes,
but is not limited to, having no access to RTP media plaintext and
having limits on what RTP header fields it can alter. The header
fields that may be modified by a Media Distributor are enumerated
in Section 4
of the double cryptographic transform specification
] and chosen with respect to utility and the security
considerations outlined in this document.
Key Distributor: An entity that is a logical function that
distributes keying material and related information to Trusted
Endpoints and Media Distributor(s) -- only what is appropriate for
each. The Key Distributor might be co-resident with another
entity trusted with E2E keying material.
Conference: Two or more participants communicating via Trusted
Endpoints to exchange RTP flows through one or more Media
Call Processing: All Trusted Endpoints connect to a conference via a
call processing dialog, e.g., with the "focus" as defined in "A
Framework for Conferencing with the Session Initiation Protocol
Third Party: Any entity that is not an endpoint, Media Distributor,
Key Distributor, or call processing entity as described in this
3. PERC Entities and Trust Model
Figure 1 depicts the trust relationships, direct or indirect, between
entities described in the subsequent subsections. Note that these
entities may be co-located or further divided into multiple, separate
Please note that some entities classified as untrusted in the simple,
general deployment scenario used most commonly in this document might
be considered trusted in other deployments. This document does not
preclude such scenarios, but it keeps the definitions and examples
focused by only using the simple, most general deployment scenario.
+----------+ | +-----------------+
| Endpoint | | | Call Processing |
+----------+ | +-----------------+
+----------------+ | +--------------------+
| Key Distributor| | | Media Distributor |
+----------------+ | +--------------------+
Trusted | Untrusted
Entities | Entities
Figure 1: Trusted and Untrusted Entities in PERC
3.1. Untrusted Entities
The architecture described in this framework document enables
conferencing infrastructure to be hosted in domains, such as in a
cloud conferencing provider's facilities, where the trustworthiness
is below the level needed to assume that the privacy of the
participant's media is not compromised. The conferencing
infrastructure in such a domain is still trusted with reliably
connecting the participants together in a conference but is not
trusted with keying material needed to decrypt any of the
participant's media. Entities in such less-trustworthy domains are
referred to as untrusted entities from this point forward.
It is important to understand that "untrusted" in this document does
not mean that an entity is not expected to function properly.
Rather, it means only that the entity does not have access to the E2E
media encryption keys.
3.1.1. Media Distributor
A Media Distributor forwards RTP flows between endpoints in the
conference while performing per-hop authentication of each RTP
packet. The Media Distributor may need access to one or more RTP
headers or header extensions, potentially adding or modifying a
certain subset. The Media Distributor also relays secured messaging
between the endpoints and the Key Distributor and acquires per-hop
key information from the Key Distributor. The actual media content
must not be decryptable by a Media Distributor, as it is not trusted
to have access to the E2E media encryption keys. The key exchange
mechanisms specified in this framework prevent the Media Distributor
from gaining access to the E2E media encryption keys.
An endpoint's ability to connect to a conference serviced by a Media
Distributor implies that the endpoint is authorized to have access to
the E2E media encryption keys, although the Media Distributor does
not have the ability to determine whether an endpoint is authorized.
Instead, the Key Distributor is responsible for authenticating the
endpoint (e.g., using WebRTC identity assertions [RFC8827
determining its authorization to receive E2E and HBH media encryption
A Media Distributor must perform its role in properly forwarding
media packets while taking measures to mitigate the adverse effects
of denial-of-service attacks (refer to Section 8
) to a level equal to
or better than traditional conferencing (non-PERC) deployments.
A Media Distributor or associated conferencing infrastructure may
also initiate or terminate various messaging techniques related to
conference control. This topic is outside the scope of this
3.1.2. Call Processing
Call processing is untrusted in the simple, general deployment
scenario. When a physical subset of call processing resides in
facilities outside the trusted domain, it should not be trusted to
have access to E2E key information.
Call processing may include the processing of call signaling
messages, as well as the signing of those messages. It may also
authenticate the endpoints for the purpose of call signaling and of
subsequently joining a conference hosted through one or more Media
Distributors. Call processing may optionally ensure the privacy of
call signaling messages between itself (call processing), the
endpoint, and other entities.
3.2. Trusted Entities
From the PERC model system's perspective, entities considered trusted
(refer to Figure 1) can be in possession of the E2E media encryption
keys for one or more conferences.
An endpoint is considered trusted and has access to E2E key
information. While it is possible for an endpoint to be compromised,
subsequently performing in undesired ways, defining endpoint
resistance to compromise is outside the scope of this document.
Endpoints take measures to mitigate the adverse effects of denial-of-
service attacks (refer to Section 8
) from other entities, including
from other endpoints, to a level equal to or better than traditional
conference (non-PERC) deployments.
3.2.2. Key Distributor
The Key Distributor, which may be co-located with an endpoint or
exist standalone, is responsible for providing key information to
endpoints for both E2E and HBH security and for providing key
information to Media Distributors for HBH security.
Interaction between the Key Distributor and call processing is
necessary for proper conference-to-endpoint mappings. This is
described in Section 5.3
The Key Distributor needs to be secured and managed in a way that
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 needs to know which endpoints and which Media
Distributors are authorized to participate in the conference. How
the Key Distributor obtains this information is outside the scope of
this document. However, Key Distributors MUST
associations with any unauthorized endpoint or Media Distributor.
4. Framework for PERC
The purpose of this framework is to define a means through which
media privacy is ensured when communicating within a conferencing
environment consisting of one or more Media Distributors that only
switch, and hence do not terminate, media. It does not otherwise
attempt to hide the fact that a conference between endpoints is
This framework reuses several specified RTP security technologies,
including the Secure Real-time Transport Protocol (SRTP) [RFC3711
Encrypted Key Transport (EKT) [RFC8870
], and DTLS-SRTP.
4.1. E2E-Authenticated and HBH-Authenticated Encryption
This solution framework focuses on the E2E privacy and integrity of
the participant's media by limiting access to only trusted entities
to the E2E key used for authenticated E2E encryption. However, this
framework does give a Media Distributor access to RTP header fields
and header extensions, as well as the ability to modify a certain
subset of the header fields and to add or change header extensions.
Packets received by a Media Distributor or an endpoint are
authenticated hop by hop.
To enable all of the above, this framework defines the use of two
security contexts and two associated encryption keys: an "inner" key
(a distinct E2E key for each transmitted media flow) for
authenticated encryption of RTP media between endpoints and an
"outer" key (a HBH key) known only to a Media Distributor or the
adjacent endpoint for the hop between an endpoint and a Media
Distributor or peer endpoint. An endpoint will receive one or more
E2E keys from every other endpoint in the conference that correspond
to the media flows transmitted by those other endpoints, while HBH
keys are derived from the DTLS-SRTP association with the Key
Distributor. Two communicating Media Distributors use DTLS-SRTP
associations directly with each other to obtain the HBH keys they
will use. See Section 4.5
for more details on key exchange.
| |################################| |
| Media |------------------------ *----->| Media |
| Distributor |<----------------------*-|------| Distributor |
| X |#####################*#|#|######| Y |
| | | | | | |
+-------------+ | | | +-------------+
# ^ | # HBH Key (XY) -+ | | # ^ | #
# | | # E2E Key (B) ---+ | # | | #
# | | # E2E Key (A) -----+ # | | #
# | | # # | | #
# | | # # | | #
# | | *---- HBH Key (AX) HBH Key (YB) ----* | | #
# | | # # | | #
# *--------- E2E Key (A) E2E Key (A) ---------* #
# | *------- E2E Key (B) E2E Key (B) -------* | #
# | | # # | | #
# | v # # | v #
| Endpoint A | | Endpoint B |
Figure 2: E2E and HBH Keys Used for Authenticated Encryption of
The double transform [RFC8723
] enables endpoints to perform
encryption using both the E2E and HBH contexts while still preserving
the same overall interface as other SRTP transforms. The Media
Distributor simply uses the corresponding normal (single) AES-GCM
transform, keyed with the appropriate HBH keys. See Section 6.1
a description of the keys used in PERC and Section 7
for a diagram of
how encrypted RTP packets appear on the wire.
RTCP is only encrypted hop by hop -- not end to end. This framework
does not provide an additional step for RTCP-authenticated
encryption. Rather, implementations utilize the existing procedures
specified in [RFC3711
]; those procedures use the same outer, HBH
cryptographic context chosen in the double transform operation
described above. For this reason, endpoints MUST NOT
confidential information via RTCP.
4.2. E2E Key Confidentiality
To ensure the confidentiality of E2E keys shared between endpoints,
endpoints use a common Key Encryption Key (KEK) that is known only by
the trusted entities in a conference. That KEK, defined in the EKT
] as the EKT Key, is used to subsequently
encrypt the SRTP master key used for E2E-authenticated encryption of
media sent by a given endpoint. Each endpoint in the conference
creates an SRTP master key for E2E-authenticated encryption and keeps
track of the E2E keys received via the Full EKT Tag for each distinct
synchronization source (SSRC) in the conference so that it can
properly decrypt received media. An endpoint may change its E2E key
at any time and advertise that new key to the conference as specified
4.3. E2E Keys and Endpoint Operations
Any given RTP media flow is identified by its SSRC, and an endpoint
might send more than one at a time and change the mix of media flows
transmitted during the lifetime of a conference.
Thus, an endpoint MUST
maintain a list of SSRCs from received RTP
flows and each SSRC's associated E2E key information. An endpoint MUST
discard old E2E keys no later than when it leaves the
If the packet is to contain RTP header extensions, it should be noted
that those extensions are only encrypted hop by hop per [RFC8723
For this reason, endpoints MUST NOT
transmit confidential information
via RTP header extensions.
4.4. HBH Keys and Per-Hop Operations
To ensure the integrity of transmitted media packets, it is REQUIRED
that every packet be authenticated hop by hop between an endpoint and
a Media Distributor, as well as between Media Distributors. The
authentication key used for HBH authentication is derived from an
SRTP master key shared only on the respective hop. Each HBH key is
distinct per hop, and no two hops ever use the same SRTP master key.
While endpoints also perform HBH authentication, the ability of the
endpoints to reconstruct the original RTP header also enables the
endpoints to authenticate RTP packets end to end. This design yields
flexibility to the Media Distributor to change certain RTP header
values as packets are forwarded. Values that the Media Distributor
can change in the RTP header are defined in [RFC8723
]. RTCP can only
be encrypted hop by hop, giving the Media Distributor the flexibility
to (1) forward RTCP content unchanged, (2) transmit compound RTCP
packets, (3) initiate RTCP packets for reporting statistics, or
(4) convey other information. Performing HBH authentication for all
RTP and RTCP packets also helps provide replay protection (see Section 8
). The use of the replay protection mechanism specified in
Section 3.3.2 of [RFC3711
] is REQUIRED
at each hop.
If there is a need to encrypt one or more RTP header extensions hop
by hop, the endpoint derives an encryption key from the HBH SRTP
master key to encrypt header extensions as per [RFC6904
]. This still
gives the Media Distributor visibility into header extensions, such
as the one used to determine the audio level [RFC6464
] of conference
participants. Note that when RTP header extensions are encrypted,
all hops need to decrypt and re-encrypt these encrypted header
extensions. Please refer to Sections 5.1
, and 5.3
for procedures to perform RTP header extension encryption and
4.5. Key Exchange
In brief, the keys used by any given endpoints are determined as
* The HBH keys that the endpoint uses to send and receive SRTP media
are derived from a DTLS handshake that the endpoint performs with
the Key Distributor (following normal DTLS-SRTP procedures).
* 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
chosen by the endpoint. It is then distributed to other endpoints
in a Full EKT Tag, encrypted under an EKT Key provided to the
client by the Key Distributor within the DTLS channel they
negotiated. Note that an endpoint MAY
create a different E2E key
per media flow, where a media flow is identified by its SSRC
* Each E2E key that an endpoint uses to receive SRTP media is set by
receiving a Full EKT Tag from another endpoint.
* The HBH keys used between two Media Distributors are derived via
DTLS-SRTP procedures employed directly between them.
4.5.1. Initial Key Exchange and Key Distributor
The Media Distributor maintains a tunnel with the Key Distributor
(e.g., using the tunnel protocol defined in [PERC-DTLS]), making it
possible for the Media Distributor to facilitate the establishment of
a secure DTLS association between each endpoint and the Key
Distributor as shown in Figure 3. The DTLS association between
endpoints and the Key Distributor enables each endpoint to generate
E2E and HBH keys and receive the KEK. At the same time, the Key
Distributor securely provides the HBH key information to the Media
Distributor. The key information summarized here may include the
SRTP master key, the SRTP master salt, and the negotiated
KEK info | Key | HBH Key info to
to Endpoints |Distributor| Endpoints & Media Distributor
# ^ ^ #
# | | #--- Tunnel
# | | #
+-----------+ +-----------+ +-----------+
| Endpoint | DTLS | Media | DTLS | Endpoint |
| KEK |<------------|Distributor|------------>| KEK |
| HBH Key | to Key Dist | HBH Keys | to Key Dist | HBH Key |
+-----------+ +-----------+ +-----------+
Figure 3: Exchanging Key Information between Entities
In addition to the secure tunnel between the Media Distributor and
the Key Distributor, there are two additional types of security
associations utilized as a part of the key exchange, as discussed in
the following paragraphs. One is a DTLS-SRTP association between an
endpoint and the Key Distributor (with packets passing through the
Media Distributor), and the other is a DTLS-SRTP association between
peer Media Distributors.
Endpoints establish a DTLS-SRTP association over the RTP session with
the Media Distributor and its media ports for the purposes of key
information exchange with the Key Distributor. The Media Distributor
does not terminate the DTLS signaling but instead forwards DTLS
packets received from an endpoint on to the Key Distributor (and vice
versa) via a tunnel established between the Media Distributor and the
When establishing the DTLS association between endpoints and the Key
Distributor, the endpoint MUST
act as the DTLS client, and the Key
act as the DTLS server. The KEK is conveyed by the
Key Distributor over the DTLS association to endpoints via procedures
defined in EKT [RFC8870
] via the EKTKey message.
The Key Distributor MUST NOT
establish DTLS-SRTP associations with
endpoints without first authenticating the Media Distributor
tunneling the DTLS-SRTP packets from the endpoint.
Note that following DTLS-SRTP procedures for the cipher defined in
], the endpoint generates both E2E and HBH encryption keys
and salt values. Endpoints MUST
either use the DTLS-SRTP-generated
E2E key for transmission or generate a fresh E2E key. In either
case, the generated SRTP master salt for E2E encryption MUST
replaced with the salt value provided by the Key Distributor via the
EKTKey message. That is because every endpoint in the conference
uses the same SRTP master salt. The endpoint only transmits the SRTP
master key (not the salt) used for E2E encryption to other endpoints
in RTP/RTCP packets per [RFC8870
Media Distributors use DTLS-SRTP directly with a peer Media
Distributor to establish the HBH key for transmitting RTP and RTCP
packets to that peer Media Distributor. The Key Distributor does not
facilitate establishing a HBH key for use between Media Distributors.
4.5.2. Key Exchange during a Conference
Following the initial key information exchange with the Key
Distributor, an endpoint is able to encrypt media end to end with an
E2E key, sending that E2E key to other endpoints encrypted with the
KEK, and is able to encrypt and authenticate RTP packets using a HBH
key. This framework does not allow the Media Distributor to gain
access to the KEK information, preventing it from gaining access to
any endpoint's E2E key and subsequently decrypting media.
The KEK 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. Dictating if, when, or how often a conference is to be
rekeyed is outside the scope of this document, but this framework
does accommodate rekeying during the lifetime of a conference.
When a Key Distributor decides to rekey a conference, it transmits a
new EKTKey message containing the new EKT Key to each of the
conference participants. Upon receipt of the new EKT Key, the
create a new SRTP master key and prepare to send that
key inside a FullEKTField using the new EKT Key as per Section 4.5
]. In order to allow time for all endpoints in the
conference to receive the new keys, the sender should follow the
recommendations in Section 4.6 of [RFC8870
]. On receiving a new EKT
Key, endpoints MUST
be prepared to decrypt EKT Tags using the new
key. The EKT Security Parameter Index (SPI) field is used to
differentiate between EKT Tags encrypted with the old and new keys.
After rekeying, an endpoint SHOULD
retain prior SRTP master keys and
EKT Keys for a period of time sufficient for the purpose of ensuring
that it can decrypt late-arriving or out-of-order packets or packets
sent by other endpoints that used the prior keys for a period of time
after rekeying began. An endpoint MAY
retain old keys until the end
of the conference.
follow the procedures in Section 5.2
renegotiate HBH keys as desired. If new HBH keys are generated, the
new keys are also delivered to the Media Distributor following the
procedures defined in [PERC-DTLS] as one possible method.
At any time, endpoints MAY
change the E2E encryption key being used.
An endpoint MUST
generate a new E2E encryption key whenever it
receives a new EKT Key. After switching to a new key, the new key is
conveyed to other endpoints in the conference in RTP/RTCP packets per
It is important that entities can validate the authenticity of other
entities, especially the Key Distributor and endpoints. Details on
this topic are outside the scope of this specification, but a few
possibilities are discussed in the following sections. The critical
requirements are that (1) an endpoint can verify that it is connected
to the correct Key Distributor for the conference and (2) the Key
Distributor can verify that the endpoint is the correct endpoint for
Two possible approaches to resolve this situation are identity
assertions and certificate fingerprints.
5.1. Identity Assertions
A WebRTC identity assertion [RFC8827
] is used to bind the identity of
the user of the endpoint to the fingerprint of the DTLS-SRTP
certificate used for the call. This certificate is unique for a
given call and a conference. This certificate is unique for a given
call and a conference, allowing the Key Distributor to ensure that
only authorized users participate in the conference. Similarly, the
Key Distributor can create a WebRTC identity assertion to bind the
fingerprint of the unique certificate used by the Key Distributor for
this conference so that the endpoint can verify that it is talking to
the correct Key Distributor. Such a setup requires an Identity
Provider (IdP) trusted by the endpoints and the Key Distributor.
5.2. Certificate Fingerprints in Session Signaling
Entities managing session signaling are generally assumed to be
untrusted in the PERC framework. However, there are some deployment
scenarios where parts of the session signaling may be assumed
trustworthy for the purposes of exchanging, in a manner that can be
authenticated, the fingerprint of an entity's certificate.
As a concrete example, SIP [RFC3261
] and the Session Description
Protocol (SDP) [RFC4566
] can be used to convey the fingerprint
information per [RFC5763
]. An endpoint's SIP User Agent would send
an INVITE message containing SDP for the media session along with the
endpoint's certificate fingerprint, which can be signed using the
procedures described in [RFC8224
] for the benefit of forwarding the
message to other entities by the focus [RFC4353
]. Other entities can
verify that the fingerprints match the certificates found in the
DTLS-SRTP connections to find the identity of the far end of the
DTLS-SRTP connection and verify that it is the authorized entity.
Ultimately, if using session signaling, an endpoint's certificate
fingerprint would need to be securely mapped to a user and conveyed
to the Key Distributor so that it can check that the user in question
is authorized. Similarly, the Key Distributor's certificate
fingerprint can be conveyed to an endpoint in a manner that can be
authenticated as being an authorized Key Distributor for this
5.3. Conference Identification
The Key Distributor needs to know what endpoints are being added to a
given conference. Thus, the Key Distributor and the Media
Distributor need to know endpoint-to-conference mappings, which are
enabled by exchanging a conference-specific unique identifier as
described in [PERC-DTLS]. How this unique identifier is assigned is
outside the scope of this document.
6. PERC Keys
This section describes the various keys employed by PERC.
6.1. Key Inventory and Management Considerations
This section summarizes the several different keys used in the PERC
framework, how they are generated, and what purpose they serve.
The keys are described in the order in which they would typically be
The various keys used in PERC are shown in Table 1 below.
| Key | Description |
| HBH Key | SRTP master key used to encrypt media hop |
| | by hop. |
| KEK | Key shared by all endpoints and used to |
| (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. |
Table 1: Key Inventory
While the number of key types is very small, it should be understood
that the actual number of distinct keys can be large as the
conference grows in size.
As an example, with 1,000 participants in a conference, there would
be at least 1,000 distinct SRTP master keys, all of which share the
same master salt. Each of those keys is passed through the Key
Derivation Function (KDF) as defined in [RFC3711
] to produce the
actual encryption and authentication keys.
Complicating key management is the fact that the KEK can change and,
when it does, the endpoints generate new SRTP master keys that are
associated with a new EKT SPI. Endpoints might retain old keys for a
period of time to ensure that they can properly decrypt late-arriving
or out-of-order packets, which means that the number of keys held
during that period of time might be substantially higher.
A more detailed explanation of each of the keys follows.
6.2. DTLS-SRTP Exchange Yields HBH Keys
The first set of keys acquired are for HBH encryption and decryption.
Per the double transform procedures [RFC8723
], the endpoint performs
a DTLS-SRTP exchange with the Key Distributor and receives a key that
is, in fact, "double" the size that is needed. The E2E part is the
first half of the key, so the endpoint discards that information when
generating its own key. The second half of the keying material is
for HBH operations, so that half of the key (corresponding to the
least significant bits) is assigned internally as the HBH key.
The Key Distributor informs the Media Distributor of the HBH key.
Specifically, the Key Distributor sends the least significant bits
corresponding to the half of the keying material determined through
DTLS-SRTP with the endpoint to the Media Distributor. A salt value
is generated along with the HBH key. The salt is also longer than
needed for HBH operations; thus, only the least significant bits of
the required length (half of the generated salt material) are sent to
the Media Distributor. One way to transmit this key and salt
information is via the tunnel protocol defined in [PERC-DTLS].
No two endpoints have the same HBH key; thus, the Media Distributor MUST
keep track of each distinct HBH key (and the corresponding salt)
and use it only for the specified hop.
The HBH key is also used for HBH encryption of RTCP. RTCP is not
E2E-encrypted in PERC.
6.3. The Key Distributor Transmits the KEK (EKT Key)
The Key Distributor sends the KEK (the EKT Key per [RFC8870
]) to the
endpoint via the aforementioned DTLS-SRTP association. This key is
known only to the Key Distributor and endpoints; it is the most
important entity to protect, since having knowledge of this key (and
the SRTP master salt transmitted as a part of the same message)
allows an entity to decrypt any media packet in the conference.
Note that the Key Distributor can send any number of EKT Keys to
endpoints. This information is used to rekey the entire conference.
Each key is identified by an SPI value. Endpoints MUST
expect that a
conference might be rekeyed when a new participant joins a conference
or when a participant leaves a conference, in order to protect the
confidentiality of the conversation before and after such events.
The SRTP master salt to be used by the endpoint is transmitted along
with the EKT Key. All endpoints in the conference utilize the same
SRTP master salt that corresponds with a given EKT Key.
The Full EKT Tag in media packets is encrypted using a cipher
specified via the EKTKey message (e.g., AES Key Wrap with a 128-bit
key). This cipher is different than the cipher used to protect media
and is only used to encrypt the endpoint's SRTP master key (and other
EKT Tag data as per [RFC8870
The KEK is not given to the Media Distributor.
6.4. Endpoints Fabricate an SRTP Master Key
As stated earlier, the E2E key determined via DTLS-SRTP MAY
discarded in favor of a locally generated E2E SRTP master key. While
the DTLS-SRTP-derived SRTP master key can be used initially, the
endpoint might choose to change the SRTP master key periodically and MUST
change the SRTP master key as a result of the EKT Key changing.
A locally generated SRTP master key is used along with the master
salt transmitted to the endpoint from the Key Distributor via the
EKTKey message to encrypt media end to end.
Since the Media Distributor is not involved in E2E functions, it does
not create this key, nor does it have access to any endpoint's E2E
key. Note, too, that even the Key Distributor is unaware of the
locally generated E2E keys used by each endpoint.
The endpoint transmits its E2E key to other endpoints in the
conference by periodically including it in SRTP packets in a Full EKT
Tag. When placed in the Full EKT Tag, it is encrypted using the EKT
Key provided by the Key Distributor. The master salt is not
transmitted, though, since all endpoints receive the same master salt
via the EKTKey message from the Key Distributor. The recommended
frequency with which an endpoint transmits its SRTP master key is
specified in [RFC8870
6.5. Summary of Key Types and Entity Possession
All endpoints have knowledge of the KEK.
Every HBH key is distinct for a given endpoint; thus, Endpoint A and
Endpoint B do not have knowledge of the other's HBH key. Since HBH
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
association might be rekeyed per Section 5.2
] and a new
key is created to replace the former key.)
Each endpoint generates its own E2E key (SRTP master key); thus,
there is a distinct E2E key per endpoint. This key is transmitted
(encrypted) via the Full EKT Tag to other endpoints. Endpoints that
receive media from a given transmitting endpoint gain knowledge of
the transmitter's E2E key via the Full EKT Tag.
Table 2 summarizes the various keys and which entity is in possession
of a given key.
| Key/Entity | Endpoint A | MD X | MD Y | Endpoint B |
| KEK (EKT Key) | Yes | No | No | Yes |
| E2E Key (A and B) | Yes | No | No | Yes |
| HBH Key (A<=>MD X) | Yes | Yes | No | No |
| HBH Key (B<=>MD Y) | No | No | Yes | Yes |
| HBH Key (MD X<=>MD Y) | No | Yes | Yes | No |
Table 2: Key Types and Entity Possession
7. Encrypted Media Packet Format
Figure 4 presents a complete picture of what an encrypted media
packet per this framework looks like when transmitted over the wire.
The packet format shown in the figure is encrypted using the double
cryptographic transform with an EKT Tag appended to the end.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|V=2|P|X| CC |M| PT | sequence number | IO
| timestamp | IO
| synchronization source (SSRC) identifier | IO
| contributing source (CSRC) identifiers | IO
| .... | IO
| RTP extension (OPTIONAL
) ... | |O
O I | payload ... | IO
O I | +-------------------------------+ IO
O I | | RTP padding | RTP pad count | IO
O | | E2E authentication tag | |O
O | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |O
O | | OHB ... | |O
+>| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |+
| | | HBH authentication tag | ||
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ||
| | | EKT Tag ... | R ||
| | +-+-+-+-+-+-+-+-+-+ | ||
| | +- Neither encrypted nor authenticated; ||
| | appended after the double transform ||
| | is performed ||
| | ||
| +- E2E-Encrypted Portion E2E-Authenticated Portion ---+|
+--- HBH-Encrypted Portion HBH-Authenticated Portion ----+
I = Inner (E2E) encryption/authentication
O = Outer (HBH) encryption/authentication
Figure 4: Encrypted Media Packet Format
8. Security Considerations
8.1. Third-Party Attacks
Third-party attacks are attacks attempted by an adversary that is not
supposed to have access to keying material or is otherwise not an
authorized participant in the conference.
On-path attacks are mitigated by HBH integrity protection and
encryption. The integrity protection mitigates packet modification.
Encryption makes selective blocking of packets harder, but not
Off-path attackers could try connecting to different PERC entities to
send specifically crafted packets with an aim of forcing the receiver
to forward or render bogus media packets. Endpoints and Media
Distributors mitigate such an attack by performing HBH authentication
and discarding packets that fail authentication.
Another attack vector is a third party claiming to be a Media
Distributor, fooling endpoints into sending packets to the false
Media Distributor instead of the correct one. The deceived sending
endpoints could incorrectly assume that their packets have been
delivered to endpoints when they in fact have not. While this attack
is possible, the result is a simple denial of service with no leakage
of confidential information, since the false Media Distributor would
not have access to either HBH or E2E encryption keys.
A third party could cause a denial of service by transmitting many
bogus or replayed packets toward receiving devices and ultimately
degrading conference or device performance. Therefore,
implementations might wish to devise mechanisms to safeguard against
such illegitimate packets, such as utilizing rate-limiting or
performing basic sanity checks on packets (e.g., looking at packet
length or expected sequence number ranges), before performing
decryption operations that are more expensive.
The use of mutual DTLS authentication (as required by DTLS-SRTP) also
helps to prevent a denial-of-service attack by preventing a false
endpoint or false Media Distributor from successfully participating
as a perceived valid media sender that could otherwise carry out an
on-path attack. When mutual authentication fails, a receiving
endpoint would know that it could safely discard media packets
received from the endpoint without inspection.
8.2. Media Distributor Attacks
A malicious or compromised Media Distributor can attack the session
in a number of possible ways, as discussed below.
8.2.1. Denial of Service
A simple form of attack is discarding received packets that should be
forwarded. This solution framework does not provide any mitigation
mechanisms for Media Distributors that fail to forward media packets.
Another form of attack is modifying received packets before
forwarding. With this solution framework, any modification of the
E2E-authenticated data results in the receiving endpoint getting an
integrity failure when performing authentication on the received
The Media Distributor can also attempt to perform resource
consumption attacks on the receiving endpoint. One such attack would
be to insert random SSRC/CSRC values in any RTP packet along with a
Full EKT Tag. Since such a message would trigger the receiver to
form a new cryptographic context, the Media Distributor can attempt
to consume the receiving endpoint's resources. While E2E
authentication would fail and the cryptographic context would be
destroyed, the key derivation operation would nonetheless consume
some computational resources. While resource consumption attacks
cannot be mitigated entirely, rate-limiting packets might help reduce
the impact of such attacks.
8.2.2. Replay Attacks
A replay attack is an attack where an already-received packet from a
previous point in the RTP stream is replayed as a new packet. This
could, for example, allow a Media Distributor to transmit a sequence
of packets identified as a user saying "yes", instead of the "no" the
user actually said.
A replay attack is mitigated by the requirement to implement replay
protection as described in Section 3.3.2 of [RFC3711
]. E2E replay
be provided for the duration of the conference.
8.2.3. Delayed Playout Attacks
A delayed playout attack is an attack where media is received and
held by a Media Distributor and then forwarded to endpoints at a
later point in time.
This attack is possible even if E2E replay protection is in place.
Because the Media Distributor is allowed to select a subset of
streams and not forward the rest to a receiver, such as in forwarding
only the most active speakers, the receiver has to accept gaps in the
E2E packet sequence. The problem here is that a Media Distributor
can choose to not deliver a particular stream for a while.
While the Media Distributor can purposely stop forwarding media
flows, it can also select an arbitrary starting point to resume
forwarding those media flows, perhaps forwarding old packets rather
than current packets. As a consequence, what the media source sent
can be substantially delayed at the receiver with the receiver
believing that newly arriving packets are delayed only by transport
delay when the packets may actually be minutes or hours old.
While this attack cannot be eliminated entirely, its effectiveness
can be reduced by rekeying the conference periodically, since
significantly delayed media encrypted with expired keys would not be
decrypted by endpoints.
8.2.4. Splicing Attacks
A splicing attack is an attack where a Media Distributor receiving
multiple media sources splices one media stream into the other. If
the Media Distributor were able to change the SSRC without the
receiver having any method for verifying the original source ID, then
the Media Distributor could first deliver stream A and then later
forward stream B under the same SSRC that stream A was previously
using. By including the SSRC in the integrity check for each packet
-- both HBH and E2E -- PERC prevents splicing attacks.
8.2.5. RTCP Attacks
PERC does not provide E2E protection of RTCP messages. This allows a
compromised Media Distributor to impact any message that might be
transmitted via RTCP, including media statistics, picture requests,
or loss indication. It is also possible for a compromised Media
Distributor to forge requests, such as requests to the endpoint to
send a new picture. Such requests can consume significant bandwidth
and impair conference performance.
8.3. Key Distributor Attacks
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
to the keying material for one or more conferences. Having access to
that keying material would then allow the adversary to decrypt media
sent from any endpoint in the conference.
As a first line of defense, the Key Distributor authenticates every
security association -- associations with both endpoints and Media
Distributors. The Key Distributor knows which entities are
authorized to have access to which keys, and inspection of
certificates will substantially reduce the risk of providing keys to
Both physical and network access to the Key Distributor should be
severely restricted. This may be more difficult to achieve when the
Key Distributor is embedded within, for example, an endpoint.
Nonetheless, consideration should be given to shielding the Key
Distributor from unauthorized access or any access that is not
strictly necessary for the support of an ongoing conference.
Consideration should be given to whether access to the keying
material will be needed beyond the conclusion of a conference. If
not needed, the Key Distributor's policy should be to destroy the
keying material once the conference concludes or when keying material
changes during the course of the conference. If keying material is
needed beyond the lifetime of the conference, further consideration
should be given to protecting keying material from future exposure.
While it might seem obvious, it is worth making this point, to avoid
any doubt that if an adversary were to record the media packets
transmitted during a conference and then gain unauthorized access to
the keying material left unsecured on the Key Distributor even years
later, the adversary could decrypt the content of every packet
transmitted during the conference.
8.4. Endpoint Attacks
A Trusted Endpoint is so named because conference confidentiality
relies heavily on the security and integrity of the endpoint. If an
adversary successfully exploits a vulnerability in an endpoint, it
might be possible for the adversary to obtain all of the keying
material used in the conference. With that keying material, an
adversary could decrypt any of the media flows received from any
other endpoint, either in real time or at a later point in time
(assuming that the adversary makes a copy of the media packets).
Additionally, if an adversary successfully exploits an endpoint, the
adversary could inject media into the conference. For example, an
adversary could manipulate the RTP or SRTP software to transmit
whatever media the adversary wishes to send. This could involve the
reuse of the compromised endpoint's SSRC or, since all conference
participants share the same KEK, the use of a new SSRC or the SSRC
value of another endpoint. Only a single SRTP cipher suite defined
provides source authentication properties that allow an endpoint to
cryptographically assert that it sent a particular E2E-protected
packet (namely, Timed Efficient Stream Loss-Tolerant Authentication
]), and its usage is presently not defined for PERC.
The suite defined in PERC only allows an endpoint to determine that
whoever sent a packet had received the KEK.
However, attacks on the endpoint are not limited to the PERC-specific
software within the endpoint. An attacker could inject media or
record media by manipulating the software that sits between the PERC-
enabled application and the hardware microphone of a video camera,
for example. Likewise, an attacker could potentially access
confidential media by accessing memory, cache, disk storage, etc. if
the endpoint is not secured.
9. IANA Considerations
This document has no IANA actions.
10.1. Normative References
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550
, DOI 10.17487/RFC3550
July 2003, <https://www.rfc-editor.org/info/rfc3550
] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711
, DOI 10.17487/RFC3711
, March 2004,
] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904
, April 2013,
] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119
Key Words", BCP 14, RFC 8174
, DOI 10.17487/RFC8174
May 2017, <https://www.rfc-editor.org/info/rfc8174
] Jennings, C., Jones, P., Barnes, R., and A.B. Roach,
"Double Encryption Procedures for the Secure Real-Time
Transport Protocol (SRTP)", RFC 8723
, April 2020,
] Jennings, C., Mattsson, J., McGrew, D., Wing, D., and F.
Andreasen, "Encrypted Key Transport for DTLS and Secure
RTP", RFC 8870
, DOI 10.17487/RFC8870
, January 2021,
10.2. Informative References
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,
] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261
, June 2002,
] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353
, February 2006,
] Baugher, M. and E. Carrara, "The Use of Timed Efficient
Stream Loss-Tolerant Authentication (TESLA) in the Secure
Real-time Transport Protocol (SRTP)", RFC 4383
, February 2006,
] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566
, DOI 10.17487/RFC4566
July 2006, <https://www.rfc-editor.org/info/rfc4566
] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763
, DOI 10.17487/RFC5763
] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764
, May 2010,
] Lennox, J., Ed., Ivov, E., and E. Marocco, "A Real-time
Transport Protocol (RTP) Header Extension for Client-to-
Mixer Audio Level Indication", RFC 6464
, December 2011,
] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667
, November 2015,
] Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 8224
, February 2018,
] Rescorla, E., "WebRTC Security Architecture", RFC 8827
, January 2021,
Jennings, C., Boström, H., and J-I. Bruaroey, "WebRTC 1.0:
Real-time Communication Between Browsers", W3C Proposed
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
Paul E. Jones
7025 Kit Creek Rd.
Research Triangle Park, North Carolina 27709
United States of America
Phone: +1 919 476 2048
United States of America