Internet DRAFT - draft-jones-perc-private-media-framework
draft-jones-perc-private-media-framework
Network Working Group P. Jones
Internet-Draft D. Benham
Intended status: Standards Track Cisco
Expires: September 22, 2016 March 21, 2016
A Solution Framework for Private Media in Privacy Enhanced RTP
Conferencing
draft-jones-perc-private-media-framework-02
Abstract
This document describes a solution framework for ensuring that media
confidentiality and integrity are maintained end-to-end within the
context of a switched conferencing environment where media
distribution devices are not trusted with the end-to-end media
encryption keys. The solution aims to build upon existing security
mechanisms defined for the real-time transport protocol (RTP).
Status of This Memo
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This Internet-Draft will expire on September 22, 2016.
Copyright Notice
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. PERC Entities and Trust Model . . . . . . . . . . . . . . . . 4
3.1. Untrusted Entities . . . . . . . . . . . . . . . . . . . 5
3.1.1. MDD . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.2. Call Processing . . . . . . . . . . . . . . . . . . . 6
3.2. Trusted Entities . . . . . . . . . . . . . . . . . . . . 6
3.2.1. Endpoint . . . . . . . . . . . . . . . . . . . . . . 6
3.2.2. KMF . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Framework for PERC . . . . . . . . . . . . . . . . . . . . . 7
4.1. End-to-End and Hop-by-Hop Authenticated Encryption . . . 7
4.2. E2E Key Confidentiality . . . . . . . . . . . . . . . . . 8
4.3. E2E Keys and Endpoint Operations . . . . . . . . . . . . 9
4.4. HBH Keys and Hop Operations . . . . . . . . . . . . . . . 9
4.5. Key Exchange . . . . . . . . . . . . . . . . . . . . . . 10
4.5.1. Initial Key Exchange and KMF . . . . . . . . . . . . 10
4.5.2. Key Exchange during a Conference . . . . . . . . . . 11
5. Entity Trust . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Identity Assertions . . . . . . . . . . . . . . . . . . . 12
5.2. Certificate Fingerprints in Session Signaling . . . . . . 12
6. Attacks on Privacy Enhanced RTP Conferencing . . . . . . . . 13
6.1. Third Party Attacks . . . . . . . . . . . . . . . . . . . 13
6.2. MDD Attacks . . . . . . . . . . . . . . . . . . . . . . . 14
6.2.1. Denial of service . . . . . . . . . . . . . . . . . . 14
6.2.2. Replay Attack . . . . . . . . . . . . . . . . . . . . 14
6.2.3. Delayed Playout Attack . . . . . . . . . . . . . . . 14
6.2.4. Splicing Attack . . . . . . . . . . . . . . . . . . . 15
7. To-Do List . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. What is Needed to Realize this Framework . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . 16
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
Switched conferencing is an increasingly popular model for multimedia
conferences with multiple participants using a combination of audio,
video, text, and other media types. With this model, real-time media
flows from conference participants are not mixed, transcoded,
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transrated, recomposed, or otherwise manipulated by a media
distribution device (MDD), as might be the case with a traditional
media server or multipoint control unit (MCU). Instead, media flows
transmitted by conference participants are simply forwarded by the
MDD to each of the other participants, often forwarding only a subset
of flows based on voice activity detection or other criteria. In
some instances, the switching MDDs may make limited modifications to
RTP [RFC3550] headers, for example, but the actual media content
(e.g., voice or video data) is unaltered.
An advantage of switched conferencing is that MDDs can be deployed on
general-purpose computing hardware. This, in turn, means that it is
possible to deploy switching MDDs in virtualized environments,
including private and public clouds. Deploying conference resources
in a cloud environment might introduce a higher security risk.
Whereas traditional conference resources were usually deployed in
private networks that were protected, cloud-based conference
resources might be viewed as less secure since they are not always
physically controlled by those who use the hardware. Additionally,
there are usually several ports open to the public in cloud
deployments, such as for remote administration, and so on.
This document defines a solution framework wherein privacy is ensured
by making it impossible for an MDD 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 switching MDD to modify certain RTP headers; add, remove,
encrypt, or decrypt RTP header extensions; and encrypt and decrypt
RTCP packets. The framework also prevents replay attacks by
authenticating each packet transmitted between a given participant
and the switching MDD by using a key that is independent from the
media encryption and authentication key(s) and is unique to the
participating endpoint and the switching MDD.
A goal of this document is to define a framework for enhanced privacy
in RTP-based conferencing environments while utilizing existing
security procedures defined for RTP with minimal enhancements.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119] when they
appear in ALL CAPS. These words may also appear in this document in
lower case as plain English words, absent their normative meanings.
Additionally, this solution framework uses the following conventions,
terms and acronyms:
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E2E (End-to-End): Communications from one endpoint through one or
more MDDs to the endpoint at the other end.
HBH (Hop-by-Hop): Communications between an endpoint and an MDD or
between MDDs.
Endpoint: An RTP flow terminating entity that has possession of E2E
media encryption keys and terminates end-to-end (E2E) encryption.
This may include embedded user conferencing equipment or browsers on
computers, media gateways, MCUs, media recording device and more that
are in the trusted domain for a given deployment.
MDD (Media Distribution Device): An RTP middlebox that is not allowed
to to have access to E2E encryption keys. It may operate according
to any of the RTP topologies [I-D.ietf-avtcore-rtp-topologies-update]
per the constraints defined by the PERC system, which includes, but
not limited to, having no access to RTP media unencrypted and having
limits on what RTP header field it can alter.
KMF (Key Management Function): An entity that is a logical function
which passes keying material and related information to endpoints and
MDDs that is appropriate for each. The KMF might be co-resident with
another entity trusted with E2E keying material.
Conference: Two or more participants communicating via trusted
endpoints to exchange RTP flows through one or more MDDs.
Third Party: Any entity that is not an Endpoint, MDD, KMF or Call
Processing entity as described in this document.
3. PERC Entities and Trust Model
The following figure depicts the trust relationships, direct or
indirect, between entities described in the subsequent sub-sections.
Note that these entities may be co-located or further divided into
multiple, separate physical devices.
Please note that some entities classified as untrusted in the simple,
general deployment scenario used most commonly in this document might
be considered trusted in other deployments. This document does not
preclude such scenarios, but will keep the definitions and examples
focused by only using the the simple, most general deployment
scenario.
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|
+----------+ | +-----------------+
| Endpoint | | | Call Processing |
+----------+ | +-----------------+
|
|
+----------------+ | +--------------------+
| Key Management | | | Media Distribution |
| Function | | | Device |
+----------------+ | +--------------------+
|
Trusted | Untrusted
Entities | Entities
|
Figure 1: Trusted and Untrusted Entities in PERC
3.1. Untrusted Entities
The architecture described in this framework document enables
conferencing infrastructure to be hosted in domains, such as in a
cloud conferencing provider's facilities, where the trustworthiness
is below the level needed to assume the privacy of participant's
media will not be compromised. The conferencing infrastructure in
such a domain is still trusted with reliably connecting the
participants together in a conference, but not trusted with keying
material needed to decrypt any of the participant's media. Entities
in such lower trustworthiness domains will simply be referred to as
untrusted entities from this point forward. This does not mean that
they are completely untrusted as they may be trusted with most non-
media related aspects of hosting a conference.
3.1.1. MDD
An MDD forwards RTP flows between endpoints in the conference while
performing per-hop authentication of each RTP packet. The MDD may
need access to one or more RTP headers or header extensions,
potentially adding or modifying a certain subset. The MDD will also
relay secured messaging between the endpoints and the key management
function and will acquire per-hop key information from the KMF. The
actual media content MUST NOT not be decryptable by an MDD, so it is
untrusted to have access to the E2E media encryption keys, which this
framework's key exchange mechanisms will prevent.
An endpoint's ability to join a conference hosted by an MDD MUST NOT
alone be interpreted as being authorized to have access to the E2E
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media encryption keys, as the MDD does not have the ability to
determine whether an endpoint is authorized.
An MDD MUST perform its role in properly forwarding media packets
while taking measures to mitigate the adverse effects of denial of
service attacks (refer to Section Section 6), etc, to a level equal
to or better than traditional conferencing (i.e. pre-PERC)
deployments.
An MDD or associated conferencing infrastructure may also initiate or
terminate various conference control related messaging, which is
outside the scope of this framework document.
3.1.2. Call Processing
The call processing function is untrusted in the simple, general
deployment scenario. When a physical subset of the call processing
function resides in facilities outside the trusted domain, it should
not be trusted to have access to E2E key information.
The call processing function may include the processing of call
signaling messages, as well as the signing of those messages. It may
also authenticate the endpoints for the purpose of call signaling and
subsequently joining of a conference hosted through one or more MDDs.
Call processing may optionally ensure the privacy of call signaling
messages between itself, the endpoint, and other entities.
In any deployment scenario where the call processing function is
considered trusted, the call processing function MUST ensure the
integrity of received messages before forwarding to other entities.
3.2. Trusted Entities
From the PERC model system perspective, entities considered trusted
(refer to Figure 1) can be in possession of the E2E media encryption
key(s) for one or more conferences.
3.2.1. Endpoint
An endpoint is considered trusted and will have access to E2E key
information. While it is possible for an endpoint to be compromised,
subsequently performing in undesired ways, defining endpoint
resistance to compromise is outside the scope of this document.
Endpoints will take measures to mitigate the adverse effects of
denial of service attacks (refer to Section 6) from other entities,
including from other endpoints, to a level equal to or better than
traditional conference (i.e., pre-PERC) deployments.
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3.2.2. KMF
The KMF, which may be collocated with an endpoint or exist
standalone, is responsible for providing key information to endpoints
for both end-to-end and hop-by-hop security and for providing key
information to MDDs for the hop-by-hop security.
Interaction between the KMF and the call processing function may be
necessary to for proper conference-to-endpoint mappings, which may or
may not be satisfied by getting information directly from the
endpoints or via some other means. [TO DO: Revisit this text after
design choice(s) are made between the alternatives.]
Obviously, the KMF needs to be closely managed to prevent
exploitation by an adversary, as any kind of security compromise of
the KMF puts the security of the conference at risk.
4. Framework for PERC
The purpose for this framework is to define a means through which
media privacy can be ensured when communicating within a conferencing
environment consisting of one or more MDDs that only switch, hence
not terminate, media. It does not otherwise attempt to hide the fact
that a conference between endpoints is taking place.
This framework reuses several specified RTP security technologies,
including SRTP [RFC3711], PERC EKT [I-D.jennings-perc-srtp-ekt-diet],
and DTLS-SRTP [RFC5764].
4.1. End-to-End and Hop-by-Hop Authenticated Encryption
This solution framework focuses on the end-to-end privacy and
integrity of the participant's media by limiting access to end-to-end
key information to trusted entities. However, this framework does
give an MDD access to RTP headers and all or most header extensions,
as well as the ability to modify a certain subset of those headers
and to add header extensions. Packets received by an MDD or an
endpoint are authenticated hop-by-hop.
To enable all of the above, this framework defines the use of two
security contexts and two associated encryption keys; an "inner" key
(E2E Key(i); i={a given endpoint}) for authenticated encryption of
RTP media between endpoints and an "outer" key (HBH Key(j); j={a
given hop}) for the hop between an endpoint and an MDD or between
MDDs. Reference the following figure.
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+--------+ HBH +-----+ HBH +-----+ HBH +--------+
| |=============| |=========| |=============| |
|Endpoint|-E2E Key(A)->| MDD |-------->| MDD |------------>|Endpoint|
| A |<------------| X |<--------| Y |<-E2E Key(B)-| B |
| |=============| |=========| |=============| |
+--------+ Key(AX) +-----+ Key(XY) +-----+ Key(YB) +--------+
E2E and HBH Keys Used for Authenticated Encryption
The PERC Double draft specification [I-D.jennings-perc-double] uses
standard SRTP keying material and recommended cryptographic
transform(s) to first form the inner, end-to-end SRTP cryptographic
context. That end-to-end SRTP cryptographic context MAY be used to
encrypt some RTP header extensions along with RTP media content. The
output of this is treated like an RTP packet and encrypted again
using the outer hop-by-hop cryptographic context. The endpoint
executes the entire Double operation while the MDD just performs the
outer, hop-by-hop operation.
RTCP can only be encrypted hop-by-hop, not end-to-end. This
framework introduces no additional step for RTCP authenticated
encryption, so the procedures needed are specified in [RFC3711] and
use the same outer, hop-by-hop cryptographic context chosen in the
Double operation described above.
4.2. E2E Key Confidentiality
To ensure the confidentiality of E2E keys shared between endpoints,
endpoints will make use of a common Key Encryption Key (KEK) that is
known only by the trusted entities in a conference. That KEK,
defined in the PERC EKT [I-D.jennings-perc-srtp-ekt-diet] as the EKT
Key, will be used to subsequently encrypt SRTP master keys used for
E2E authenticated encryption (E2E Key(i); i={a given endpoint}) of
media sent by a given endpoint.
+---------------------+------------+-------+-------+------------+
| Key / Entity | Endpoint A | MDD X | MDD Y | Endpoint B |
+---------------------+------------+---------------+------------+
| KEK | Yes | No | No | Yes |
+---------------------+------------+---------------+------------+
| E2E Key (i) | Yes | No | No | Yes |
+---------------------+------------+---------------+------------+
| HBH Key (A<=>MDD X) | Yes | Yes | No | No |
+---------------------+------------+---------------+------------+
| HBH Key (B<=>MDD Y) | No | No | Yes | Yes |
+---------------------+------------+---------------+------------+
Figure 2: Keys per Entity
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4.3. E2E Keys and Endpoint Operations
Any given RTP media flow can be identified by its SSRC, and endpoints
might send more than one at a time and change the mix of media flows
transmitted during the life of a conference.
Thus, endpoints MUST maintain a list of SSRCs from received RTP flows
and each SSRC's associated E2E Key(i) information. Following a
change of the KEK (i.e., EKT Key), prior E2E Key(i) information
SHOULD be retained just long enough to ensure that late-arriving or
out-of-order packets can be successfully decrypted and rendered.
[NOTE: Perhaps a separate best practices document can recommend
durations after some real world testing?] The endpoint SHOULD
discard the E2E Key(i) and KEK information when it leaves the
conference.
If there is a need to encrypt one or more RTP header extensions end-
to-end, an encryption key is derived from the end-to-end SRTP master
key to encrypt header extensions as per [RFC6904]. The MDD will not
be able use the information contained in those header extensions with
E2E encryption. [TO DO: Add a list to this doc of RTP Header
Extensions that are off limits to - the MUST NOTs - be E2E
encrypted.]
4.4. HBH Keys and Hop Operations
To ensure the integrity of transmitted media packets, this framework
requires that every packet be authenticated hop-by-hop (HBH) between
an endpoint and an MDD, as well between MDDs. The authentication key
used for hop-by-hop authentication is derived from an SRTP master key
shared only on the respective hop (HBH Key(j); j={a given hop}).
Each HBH Key(j) is distinct per hop and no two hops ever
intentionally use the same SRTP master key.
Using hop-by-hop authentication gives the MDD the ability to change
certain RTP header values. Which values the MDD can change in the
RTP header are defined in [I-D.jennings-perc-double]. RTCP can only
be encrtpted, giving the MDD the flexibility to forward RTCP content
unchanged, transmit compound RTCP packets or to initiate RTCP packets
for reporting statistics or conveying other information. Performing
hop-by-hop authentication for all RTP and RTCP packets also helps
provide replay protection (see Section 6).
If there is a need to encrypt one or more RTP header extensions hop-
by-hop, an encryption key is derived from the hop-by-hop SRTP master
key to encrypt header extensions as per [RFC6904]. This will still
give the switching MDD visibility into header extensions, such as the
one used to determine audio level [RFC6464] of conference
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participants. Note that when RTP header extensions are encrypted,
all hops - in the untrusted domain at least - will need to decrypt
and re-encrypt these encrypted header extensions.
4.5. Key Exchange
To facilitate key exchange required to establish or generate an E2E
key and a HBH key for an endpoint and the same HBH key for the MDD,
this framework utilizes a DTLS-SRTP [RFC5764] association between an
endpoint and the KMF. To establish this association, an endpoint
will send DTLS-SRTP messages to the MDD which will then forward them
to the MDD as defined in DTLS Tunnel for PERC
[I-D.jones-perc-dtls-tunnel]. The KEK (i.e., EKT Key) is also
conveyed by the KMF over the DTLS association to endpoints via
procedures defined in PERC EKT [I-D.jennings-perc-srtp-ekt-diet].
MDDs use DTLS-SRTP [RFC5764] directly with a peer MDD to establish
HBH keys for transmitting RTP and RTCP packets that peer MDD. The
KMF does not facilitate establishing HBH keys for use between MDDs.
4.5.1. Initial Key Exchange and KMF
The procedures defined in DTLS Tunnel for PERC
[I-D.jones-perc-dtls-tunnel] establish one or more DTLS tunnels
between the MDD and KMF, making it is possible for the MDD to
facilitate the establishment of a secure DTLS association between
each endpoint and the KMF as shown the following figure. The DTLS
association between endpoints and the KMF will enable each endpoint
to receive E2E key information, Key Encryption Key (KEK) information
(i.e., EKT Key), and HBH key information. At the same time, the KMF
can securely provide the HBH key information to the MDD. The key
information summarized here may include the SRTP master key, SRTP
master salt, and the negotiated cryptographic transform.
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KEK info +---------+ HBH Key info
to endpoints | KMF | to endpoints & MDD
+---------+
| ^ ^ |
| | | |-DTLS Tunnel
| | | |
+-----------+ +---------+ +-----------+
| Endpoint | DTLS | MDD | DTLS | Endpoint |
| KEK |<--------| |-------->| KEK |
| HBH Key(j)| to KMF | HBH Keys| to KMF | HBH Key(j)|
+-----------+ +---------+ +-----------+
Figure 3: Exchanging Key Information Between Entities
Endpoints will establish a DTLS-SRTP association over the RTP
session's media ports for the purposes of key information exchange
with the KMF. The MDD will not terminate the DTLS signaling, but
will instead forward DTLS packets received from an endpoint on to the
KMF (and vice versa) via a tunnel established between MDD and the
KMF. This tunnel used to encapsulate the DTLS-SRTP signaling between
the KMF and endpoints will also be used to convey HBH key information
from the KMF to the MDD, so no additional protocol or interface is
required.
4.5.2. Key Exchange during a Conference
Following the initial key information exchange with the KMF,
endpoints will be able to encrypt media end-to-end with their E2E
Key(i), sending that E2E Key(i) to other endpoints encrypted with
KEK, and will be able to encrypt and authenticate RTP packets using
local HBH Key(j). The procedures defined do not allow the MDD to
gain access to the KEK information, preventing it from gaining access
to any endpoint's E2E key and subsequently decrypting media.
The KEK (i.e., EKT Key) may need to change from time-to-time during
the life of a conference, such as when a new participant joins or
leaves a conference. Dictating if, when or how often a conference is
to be re-keyed is outside the scope of this document, but this
framework does accommodate re-keying during the life of a conference.
When a KMF decides to rekey a conference, it transmits a specific
message defined in PERC EKT [I-D.jennings-perc-srtp-ekt-diet] to each
of the conference participants. The endpoint MUST create a new SRTP
master key and prepare to send that key inside a Full EKT Field using
the new EKT Key. Since it may take some time for all of the endpoints
in conference to finish re-keying, senders SHOULD delay a short
period of time before sending media encrypted with the new master
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key, but it MUST be prepared to make use of the information from a
new inbound EKT Key immediately. [TO DO: Either need to pick a short
delay period for endpoints to use per above, defer to a future best
practices document or consider having the KMF manage the delay period
given it knows the size of a given conference.]
5. Entity Trust
It is important to this solution framework that the entities can
trust and validate the authenticity of other entities, especially the
KMF and endpoints. The details of this are outside the scope of
specification but a few possibilities are discussed in the following
sections. The key requirements is that endpoints can verify they are
connected to the correct KMF for the conference and the KMF can
verify the endpoints are the correct endpoints for the conference.
Two possible approaches to solve this are Identity Assertions and
Certificate Fingerprints.
5.1. Identity Assertions
WebRTC (EDIT: add reference) Identity assertion can be used to bind
the identity of the user of the endpoint to the fingerprint of the
DTLS-SRTP certificate used for the call. This certificate is unique
for a given call and a conference. This allows the KMF to ensure
that only authorized users participate in the conference. Similarly
the KMF can create a WeBRTC Identity assertion bound the fingerprint
of the unique certificate used by the KMF for this conference so that
the endpoint can validate it is talking to the correct KMF.
5.2. Certificate Fingerprints in Session Signaling
Entities managing session signaling are generally assumed to be
untrusted in the PERC framework. However, there are some deployment
scenarios where parts of the session signaling may be assumed
trustworthy for the purposes of exchanging, in a manner that can be
authenticated, the fingerprint of an entity's certificate.
As a concrete example, SIP [RFC3261] and SDP [RFC4566] can be used to
convey the fingerprint information per [RFC5763]. An endpoint's SIP
User Agent would send an INVITE message containing SDP for the media
session along with the endpoint's certificate fingerprint, which can
be signed using the procedures described in [RFC4474] for the benefit
of forwarding the message to other entities. Other entities can now
verify the fingerprints match the certificates found in the DTLS-SRTP
connections to find the identity of the far end of the DTLS-SRTP
connection and check that is the authorized entity.
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Ultimately, if using session signaling, an endpoint's certificate
fingerprint would need to be securely mapped to a user and conveyed
to the KMF so that it can check that that user is authorized.
Similarly, the KMF's certificate fingerprint can be conveyed to
endpoint in a manner that can be authenticated as being an authorized
KMF for this conference.
6. Attacks on Privacy Enhanced RTP Conferencing
This framework, and the individual protocols defined to support it,
must take care to not increase the exposure to Denial of Service
(DoS) attacks by untrusted or third-party entities and should take
measures to mitigate, where possible, more serious DoS attacks from
on-path and off-path attackers.
The following section enumerates the kind of attacks that will be
considered in the development of this framework's solution.
6.1. Third Party Attacks
On-path attacks are mitigated by HBH integrity protection and
encryption. The integrity protection mitigates packet modification
and encryption makes selective blocking of packets harder, but not
impossible.
Off-path attackers may try connecting to different PERC entities and
send specifically crafted packets. A successful attacker might be
able to get the MDD to forward such packets. If not making use of
HBH authentication on the MDD, such an attack could only be detected
in the receiving endpoints where the forged packets would finally be
dropped.
Another potential attack is a third party claiming to be an MDD,
fooling endpoints in to sending packets to the false MDD instead of
the correct one. The deceived sending endpoints could incorrectly
assuming their packets have been delivered to endpoints when they in
fact have not. Further, the false MDD may cascade to another
legitimate MDD creating a false version of the real conference. This
attack is mitigated since false MDD would not be authenicated by the
KMF and be able tunnel the DTLS-SRTP signaling to/from the KMF.
Since it cannot tunnel the DTLS-SRTP signaling, endpoints will not
get the KEK and the conference will not go on. [TO DO: Include the
exchange valid MDD certficates with the KMF in this doc or not?]
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6.2. MDD Attacks
The MDD can attack the session in a number of possible ways.
6.2.1. Denial of service
Any modification of the end-to-end authenticated data will result in
the receiving endpoint getting an integrity failure when performing
authentication on the received packet.
The MDD can also attempt to perform resource consumption attacks on
the receiving endpoint. One such attack would be to insert random
SSRC/CSRC values in any RTP packet with an inband key-distribution
message attached (i.e., Full EKT Field). Since such a message would
trigger the receiver to form a new cryptographic context, the MDD can
attempt to consume the receiving endpoints resources.
Another denial of service attack is where the MDD rewrites the PT
field to indicate a different codec. The effect of this attack is
that any payload packetized and encoded according to one RTP payload
format is then processed using another payload format and codec.
Assuming that the implementation is robust to random input, it is
unlikely to cause crashes in the receiving software/hardware.
However, it is not unlikely that such rewriting will cause severe
media degradation.
For audio formats, this attack is likely to cause highly disturbing
audio and/or can be damaging to hearing and playout equipment.
6.2.2. Replay Attack
Replay attack is when an already received packets from a previous
point in the RTP stream is replayed as new packet. This could, for
example, allow an MDD to transmit a sequence of packets identified as
a user saying "yes", instead of the "no" the user actually said.
The mitigation for a replay attack is to prevent old packets beyond a
small-to-modest jitter and network re-ordering sized window to be
rejected. End-to-end replay protection MUST be provided for the
whole duration of the conference.
6.2.3. Delayed Playout Attack
The delayed playout attack is a variant of the replay attack. This
attack is possible even if E2E replay protection is in place.
However, due to fact that the MDD is allowed to select a sub-set of
streams and not forward the rest to a receiver, such as in forwarding
only the most active speakers, the receiver has to accept gaps in the
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E2E packet sequence. The issue with this is that an MDD can select
to not deliver a particular stream for a while.
Within the window from last packet forwarded to the receiver and the
latest received by the MDD, the MDD can select an arbitrary starting
point when resuming forwarding packets. Thus what the media source
said can be substantially delayed at the receiver with the receiver
believing that it is what was said just now, and only delayed due to
transport delay.
6.2.4. Splicing Attack
The splicing attack is an attack where an MDD receiving multiple
media sources splices one media stream into the other. If the MDD is
able to change the SSRC without the receiver having any method for
verifying the original source ID, then the MDD could first deliver
stream A and then later forward stream B under the same SSRC as
stream A was previously using. Not allowing the MDD to change the
SSRC mitigates this attack.
7. To-Do List
7.1. What is Needed to Realize this Framework
o Endpoints and KMF must securely convey their respective
certificate information directly or indirectly via some other
means or identity service provider.
o If as in "Double" draft, the ROC value is no longer in the clear
and associated with the "outer" protection scheme, we may need to
require that the MDD maintain a separate ROC value for each SSRC
sent to each separate endpoint. This ROC value should start at 0
regardless of the sequence number in that first packet sent to an
endpoint. [EDIT: Do we document this in this framework or in
Double draft?]
o Investigate adding ability to enable the transmission of one-way
media from a non-trusted device (e.g., announcements). One
possible solution is to have the KMF send an "ekt_key" message
that is explicitly labeled for receive-only and giving that to
announcement servers. As opposed to modifying the EKT spec for
this PERC-specific need, we could say in the framework that EKT
Keys with a SPI > 32000, say, are intended for this purpose and
trusted endpoints should only use those EKT Keys to decrypt Full
EKT Fields received from such transmitters. Thus, trusted
endpoints would never send media with EKT Keys having those SPI
values.
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8. IANA Considerations
There are no IANA considerations for this document.
9. Security Considerations
[TBD]
10. Acknowledgments
The authors would like to thank Mo Zanaty and Christian Oien for
invaluable input on this document. Also, we would like to
acknowledge Nermeen Ismail for serving on the initial versions of
this document as a co-author.
11. References
11.1. Normative References
[I-D.jennings-perc-double]
Jennings, C., Jones, P., and A. Roach, "SRTP Double
Encryption Procedures", draft-jennings-perc-double-01
(work in progress), March 2016.
[I-D.jennings-perc-srtp-ekt-diet]
Mattsson, J., McGrew, D., Wing, D., Andreasen, F., and C.
Jennings, "Encrypted Key Transport for Secure RTP", March
2016.
[I-D.jones-perc-dtls-tunnel]
Jones, P., "DTLS Tunnel between Media Distribution Device
and Key Management Function to Facilitate Key Exchange",
draft-jones-perc-dtls-tunnel-02 (work in progress),
March 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
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[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<http://www.rfc-editor.org/info/rfc3711>.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, DOI
10.17487/RFC5764, May 2010,
<http://www.rfc-editor.org/info/rfc5764>.
[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904, DOI
10.17487/RFC6904, April 2013,
<http://www.rfc-editor.org/info/rfc6904>.
11.2. Informative References
[I-D.ietf-avtcore-rtp-topologies-update]
Westerlund, M. and S. Wenger, "RTP Topologies", draft-
ietf-avtcore-rtp-topologies-update-10 (work in progress),
July 2015.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, DOI 10.17487/
RFC4474, August 2006,
<http://www.rfc-editor.org/info/rfc4474>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
2010, <http://www.rfc-editor.org/info/rfc5763>.
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[RFC6464] Lennox, J., Ed., Ivov, E., and E. Marocco, "A Real-time
Transport Protocol (RTP) Header Extension for Client-to-
Mixer Audio Level Indication", RFC 6464, DOI 10.17487/
RFC6464, December 2011,
<http://www.rfc-editor.org/info/rfc6464>.
Authors' Addresses
Paul Jones
Cisco
7025 Kit Creek Rd.
Research Triangle Park, North Carolina 27709
USA
Phone: +1 919 476 2048
Email: paulej@packetizer.com
David Benham
Cisco
170 West Tasman Drive
San Jose, California 95134
USA
Email: dbenham@cisco.com
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