One document matched: draft-wing-media-security-requirements-05.xml
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<rfc category="info" docName="draft-wing-media-security-requirements-05"
ipr="full3978">
<front>
<title abbrev="Media Security Requirements and Analysis">Requirements and
Analysis of Media Security Key Management Protocols</title>
<author fullname="Dan Wing" initials="D." surname="Wing">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street>170 West Tasman Drive</street>
<city>San Jose</city>
<region>CA</region>
<code>95134</code>
<country>USA</country>
</postal>
<email>dwing@cisco.com</email>
</address>
</author>
<author fullname="Steffen Fries" initials="S." surname="Fries">
<organization>Siemens AG</organization>
<address>
<postal>
<street>Otto-Hahn-Ring 6</street>
<city>Munich</city>
<region>Bavaria</region>
<code>81739</code>
<country>Germany</country>
</postal>
<email>steffen.fries@siemens.com</email>
</address>
</author>
<author fullname="Hannes Tschofenig" initials="H" surname="Tschofenig">
<organization>Nokia Siemens Networks</organization>
<address>
<postal>
<street>Otto-Hahn-Ring 6</street>
<city>Munich</city>
<region>Bavaria</region>
<code>81739</code>
<country>Germany</country>
</postal>
<email>Hannes.Tschofenig@nsn.com</email>
<uri>http://www.tschofenig.com</uri>
</address>
</author>
<author fullname="Francois Audet" initials="F." surname="Audet">
<organization>Nortel</organization>
<address>
<postal>
<street>4655 Great America Parkway</street>
<city>Santa Clara</city>
<region>CA</region>
<code>95054</code>
<country>USA</country>
</postal>
<email>audet@nortel.com</email>
</address>
</author>
<author fullname="Brian Stucker" initials="B." surname="Stucker">
<organization>Nortel</organization>
<address>
<postal>
<street>2201 Lakeside</street>
<city>Richardson</city>
<region>TX</region>
<code>75082</code>
<country>USA</country>
</postal>
<email>bstucker@nortel.com</email>
<uri>http://www.linkedin.com/pub/bstucker</uri>
</address>
</author>
<date year="2007" />
<area>RAI</area>
<keyword>I-D</keyword>
<keyword>Internet-Draft</keyword>
<abstract>
<t>A number of proposals have been published to address the need of
securing media traffic. A summary of the proposals available at that
time is available in the appendix of this document. Different
assumptions, requirements, and usage environments justify every one of
them. This document aims to summarize the discussed media security
requirements. A comparison of the requirements against the individual
proposals is provided.</t>
<t>This document is discussed on the SIP mailing list,
<http://www1.ietf.org/mailman/listinfo/sip>.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>The work on media security started a long time ago where the
capability of the Session Initiation Protocol (SIP) was still at its
infancy. With the increased SIP deployment and the availability of new
SIP extensions and related protocols the need for end-to-end security
was re-evaluated. The procedure of re-evaluating prior protocol work and
design decisions is not an uncommon strategy and, to some extend,
considered necessary protocol work to ensure that the developed
protocols indeed meet the previously envisioned needs for the users in
the Internet.</t>
<t>This document aims to summarize the discussed media security
requirements, i.e., requirements for mechanisms that negotiate keys for
SRTP. The organization of this document is as follows: <xref
target="terminology"></xref> introduces terminology, <xref
target="scenarios"></xref> provides an overview about possible call
scenarios, <xref target="requirements"></xref> lists requirements for
media security, <xref target="clustering"></xref> will provide a
clustering of requirements to certain deployment environments to address
the problem that there might not be a single solution with universal
applicability and <xref target="ofs"></xref> provides out-of-scope items
and aspects for further discussion. The document concludes with a
security considerations <xref target="security"></xref>, IANA
considerations <xref target="iana"></xref> and an acknowledgement
section in <xref target="acks"></xref>. <xref
target="comparison"></xref> lists the available solution proposals and
compares them to the requirements.</t>
</section>
<section anchor="terminology" title="Terminology">
<t>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 <xref
target="RFC2119"></xref>, with the important qualification that, unless
otherwise stated, these terms apply to the design of the media security
key management protocol, not its implementation or application.</t>
<t>Additionally, the following items are used in this document:</t>
<t><list style="hanging">
<t hangText="AOR (Address-of-Record): ">A SIP or SIPS URI that
points to a domain with a location service that can map the URI to
another URI where the user might be available. Typically, the
location service is populated through registrations. An AOR is
frequently thought of as the "public address" of the user.</t>
<t hangText="SSRC: ">The 32-bit value that defines the
synchronization source, used in RTP. These are generally unique, but
collisions can occur.</t>
<t hangText="two-time pad: ">The use of the same key and the same
key index to encrypt different data. For SRTP, a two-time pad occurs
if two senders are using the same key and the same RTP SSRC
value.</t>
<t hangText="PKI">Public Key Infrastructure. Throughout this paper,
the term PKI refers to a global PKI.</t>
</list></t>
</section>
<section anchor="scenarios" title="Call Scenarios">
<t>The following subsections describe call scenarios with relevance for
media security. These call scenarios pose the most challenge to the key
management for media data in cooperation with SIP signaling.</t>
<!-- ====================================================================== -->
<section anchor="clipping"
title="Clipping Media Before Signaling Answer">
<t>Per the <xref target="RFC3264">SDP Offer/Answer Model</xref>,</t>
<t><list>
<t>"Once the offerer has sent the offer, it MUST be prepared to
receive media for any recvonly streams described by that offer. It
MUST be prepared to send and receive media for any sendrecv
streams in the offer, and send media for any sendonly streams in
the offer (of course, it cannot actually send until the peer
provides an answer with the needed address and port
information)."</t>
</list></t>
<t>To meet this requirement with SRTP, the offerer needs to know the
SRTP key for arriving media. If encrypted SRTP media arrives before
the associated SRTP key, the offerer cannot play the media -- causing
clipping.</t>
<t>For key exchange mechanisms that send the answerer's key in SDP, a
SIP provisional response <xref target="RFC3261"></xref>, such as 183
(session progress), is useful. However, the 183 messages are not
reliable unless both the calling and called end point support <xref
target="RFC3262">PRACK</xref>, use TCP across all SIP proxies,
implement Security Preconditions <xref
target="I-D.ietf-mmusic-securityprecondition"></xref>, or the both
ends implement ICE <xref target="I-D.ietf-mmusic-ice"></xref> and the
answerer implements the reliable provisional response mechanism
described in ICE. Unfortunately, there is not wide deployment of any
of these techniques and there is industry reluctance to set
requirements regarding these techniques to avoid the problem described
in this section.</t>
<t>Note that the receipt of an SDP answer is not always sufficient to
allow media to be played to the offerer. Sometimes, the offerer must
send media in order to open up firewall holes or NAT bindings before
media can be received. In this case a solution that makes the key
available before the SDP answer arrives will not help.<!-- Here additional measures as
using ICE may provide a solution space. --></t>
<t>Requirements are created due to early media, in the sense of
pre-offer/answer media, as described in <xref
target="I-D.barnes-sip-em-ps-req-sol"></xref>. Fixes to early media
might make the requirements to become obsolete, but at the time of
writing no progress has been accomplished.</t>
</section>
<!-- === -->
<section anchor="forking" title="Retargeting and Forking">
<t>In SIP, a request sent to a specific AOR but delivered to a
different AOR is called a "retarget". A typical scenario is a "call
forwarding" feature. In <xref target="retargeting_figure"></xref>
Alice sends an Invite in step 1 that is sent to Bob in step 2. Bob
responds with a redirect (SIP response code 3xx) pointing to Carol in
step 3. This redirect typically does not propagate back to Alice but
only goes to a proxy (i.e., the retargeting proxy) that sends the
original Invite to Carol in step 4.</t>
<t><figure anchor="retargeting_figure" title="Retargeting">
<artwork align="center"><![CDATA[
+-----+
|Alice|
+--+--+
|
| Invite (1)
V
+----+----+
| proxy |
++-+-----++
| ^ |
Invite (2) | | | Invite (4)
& redirect (3) | | |
V | V
++-++ ++----+
|Bob| |Carol|
+---+ +-----+
]]></artwork>
</figure></t>
<t>The mechanism used by SIP for identifying the calling party is SIP
Identity <xref target="RFC3261"></xref>. However, due to SIP
retargeting issues <xref
target="I-D.peterson-sipping-retarget"></xref>, SIP Identity can only
identify the calling party (that is, the party that initiated the SIP
request). Some key exchange mechanisms predate SIP Identity and
include their own identity mechanism. However, those built-in identity
mechanism suffer from the same SIP retargeting problem described in
the above draft. Going forward, <xref target="RFC4916">Connected
Identity</xref> allows identifying the called party. This is also
described as the 'retargeting identity' problem.</t>
<t>In SIP, 'forking' is the delivery of a request to multiple
locations. This happens when a single AOR is registered more than
once. An example of forking is when a user has a desk phone, PC
client, and mobile handset all registered with the same AOR.</t>
<t><figure anchor="forking_figure" title="Forking">
<artwork align="center"><![CDATA[
+-----+
|Alice|
+--+--+
|
| Invite
V
+-----+-----+
| proxy |
++---------++
| |
Invite | | Invite
V V
+--+--+ +--+--+
|Bob-1| |Bob-2|
+-----+ +-----+
]]></artwork>
</figure></t>
<t>With forking, both Bob-1 and Bob-2 might send back SDP answers in
SIP responses. Alice will see those intermediate (18x) and final (200)
responses. It is useful for Alice to be able to associate the SIP
response with the incoming media stream. Although this association can
be done with ICE <xref target="I-D.ietf-mmusic-ice"></xref>, and ICE
is useful to make this association with RTP, it is not desirable to
require ICE to accomplish this association.</t>
<t>Forking and retargeting are often used together. For example, a
boss and secretary might have both phones ring and rollover to voice
mail if neither phone is answered.</t>
<t>To maintain security of the media traffic, only the end point that
answers the call should know the SRTP keys for the session. This is
only an issue with public key encryption and not with DH-based
approaches. For key exchange mechanisms that do not provide secure
forking or secure retargeting, one workaround is to re-key immediately
after forking or retargeting (that is, perform a re-Invite). However,
because the originator may not be aware that the call forked this
mechanism requires rekeying immediately after every session is
established. This doubles the Invite messages processed by the
network.</t>
<t>Retargeting securely introduces a more significant problem. With
retargeting, the actual recipient of the request is not the original
recipient. This means that if the offerer encrypted material (such as
the session key or the SDP) using the original recipient's public key,
the actual recipient will not be able to decrypt that material because
the recipient won't have the original recipient's private key. In some
cases, this is the intended behavior, i.e., you wanted to establish a
secure connection with a specific individual. In other cases, it is
not intended behavior (you want all voice media to be encrypted,
regardless of who answers).</t>
<t>For some forms of key management the calling party needs to know in
advance the certificate or shared secret of the called party, and
retargeting can interfere with this.</t>
<t>Further compounding this problem is a particularity of SIP that
when forking is used, there is always only one final error response
delivered to the sender of the request: the forking proxy is
responsible for choosing which final response to choose in the event
where forking results in multiple final error responses being received
by the forking proxy. This means that if a request is rejected, say
with information that the keying information was rejected and
providing the far end's credentials, it is very possible that the
rejection will never reach the sender. This problem, called the <xref
target="I-D.mahy-sipping-herfp-fix">Heterogeneous Error Response
Forking Problem (HERFP)</xref>, is difficult to solve in SIP.</t>
</section>
<!--
<section anchor="ICE4association" title="Using ICE to Associate Media and Signaling">
<t>In the absence of a technique in the key exchange to associate SIP signaling with the
media, ICE may be used. This technique does not need an external STUN server or external
TURN server; rather, what is used are ICE connectivity checks:</t>
<t>
<list style="symbols">
<t>The offer has at least one a=candidate line, matching the m/c lines</t>
<t>The answerer has to minimally support the new 'lite' mode of ICE. This means the
answerer's SDP also has an a=candidate line that matches its m/c lines. In ICE's
'lite' mode, the answerer only responds to STUN Binding Requests.</t>
<t>There are two ways the offerer will notice forking occurred:</t>
<list style="symbols">
<t>media (RTP or SRTP) arrives from different transport addresses</t>
<t>STUN connectivity checks with different STUN usernames arrive from different
transport addresses</t>
<t>multiple answers arrive in SIP signaling</t>
</list>
<t>When the offerer notices forking occurred, and the offerer needs to associate an SDP
answer with the media path, the offerer can send a STUN Binding Request to the address
specified in the SDP and perform ICE triggered checks, as specified by ICE. This
allows correlating the media path with the endpoint that generated the SDP answer.</t>
</list>
</t>
<t>[Editor's Note: Even though this describes a possible solution in a requirements
document, we listed it for further comments.]</t>
</section>
-->
<!-- === -->
<section anchor="conferencing" title="Shared Key Conferencing">
<t>For efficient scaling, large audio and video conference bridges
operate most efficiently by encrypting the current speaker once and
distributing that stream to the conference attendees. Typically,
inactive participants receive the same streams -- they hear (or see)
the active speaker(s), and the active speakers receive distinct
streams that don't include themselves. In order to maintain
confidentiality of such conferences where listeners share a common
key, all listeners must rekeyed when a listener joins or leaves a
conference.</t>
<t>An important use case for mixers/translators is a conference
bridge:</t>
<t><figure anchor="figure_centralized_keying"
title="Centralized Keying">
<artwork align="center"><![CDATA[
+----+
A --- 1 --->| |
<-- 2 ----| M |
| I |
B --- 3 --->| X |
<-- 4 ----| E |
| R |
C --- 5 --->| |
<-- 6 ----| |
+----+
]]></artwork>
</figure></t>
<t>In the figure above, 1, 3, and 5 are RTP media contributions from
Alice, Bob, and Carol, and 2, 4, and 6 are the RTP flows to those
devices carrying the 'mixed' media.</t>
<t>Several scenarios are possible:</t>
<t><list style="letters">
<t>Multiple inbound sessions: 1, 3, and 5 are distinct RTP
sessions,</t>
<t>Multiple outbound sessions: 2, 4, and 6 are distinct RTP
sessions,</t>
<t>Single inbound session: 1, 3, and 5 are just different sources
within the same RTP session,</t>
<t>Single outbound session: 2, 4, and 6 are different flows of the
same (multi-unicast) RTP session</t>
</list></t>
<t>If there are multiple inbound sessions and multiple outbound
sessions (scenarios a and b), then every keying mechanism behaves as
if the mixer were an end point and can set up a point-to-point secure
session between the participant and the mixer. This is the simplest
situation, but is computationally wasteful, since SRTP processing has
to be done independently for each participant. The use of multiple
inbound sessions (scenario a) doesn't waste computational resources,
though it does consume additional cryptographic context on the mixer
for each participant and has the advantage of non-repudiation of the
originator of the incoming stream.</t>
<t>To support a single outbound session (scenario d), the mixer has to
dictate its encryption key to the participants. Some keying mechanisms
allow the transmitter to determine its own key, and others allow the
offerer to determine the key for the offerer and answerer. Depending
on how the call is established, the offerer might be a participant
(such as a participant dialing into a conference bridge) or the
offerer might be the mixer (such as a conference bridge calling a
participant). The use of offerless Invites may help some keying
mechanisms reverse the role of offerer/answerer. A difficulty,
however, is knowing a priori if the role should be reversed for a
particular call.</t>
</section>
<section anchor="bbuas" title="B2BUA Signaling Manipulation">
<t>SRTP keying may be impacted due the presence of Back-to-Back User
Agents (B2BUA) in the signaling path. Not only does this potentially
impact the ability to exchange keying material as part of SIP
signaling, but because B2BUAs often limit the exchange of SDP, B2BUAs
can impact exchange of keying material in the media path as well.
Specifically, a number of scenarios can arise during initial call
setup that can interfere with exchanging SRTP keying material between
endpoints:</t>
<t><list style="numbers">
<t>UAC indicated support for PRACK <xref target="RFC3262"></xref>
is stripped from signaling,</t>
<t>SDP from either endpoint is not exchanged on the same message
type or message sequence in which it was sent,</t>
<t>UAC reliability extensions, such as <xref
target="RFC3262">PRACK</xref> and <xref
target="I-D.ietf-mmusic-securityprecondition">Security
Preconditions</xref> are terminated at the B2BUA itself instead of
at the intended recipient,</t>
<t>the B2BUA introduces new branches to the call flow (forking) to
network media endpoints</t>
</list></t>
<t>B2BUAs may strip support for PRACK from INVITEs in order to
simplify the types of signaling scenarios they must support when,
usually, trying to trigger network-provided early media. This impacts
SRTP keying by preventing the UAS from exchanging keying material in
the SDP answer until the next response can be sent. Even UPDATE cannot
be used to transport keying material due to limitations in <xref
target="RFC3261"></xref> requiring the answer to the offer in an
INVITE being limited to a reliable response.</t>
<t>Another not-uncommon manipulation of SIP call setup signaling is to
change the ordering in which SDP is exchanged. For example, a B2BUA
may hold onto SDP sent to it by a UAS as part of a 18x response or
UPDATE exchange and not forward that information back to the UAC until
some later point in time (typically the 200 OK to the INVITE). This
can delay key exchanges and cause clipping as a result.</t>
<t>
A less common, but observed B2BUA tactic for handling signaling
interactions during call setup, primarily for network-provided early
media, is to "fake-out" the UAC into thinking that reliability
extensions such as PRACK <xref target="RFC3262"></xref> or Resource
Management Preconditions <xref target="RFC3312"></xref> are in effect
end-to-end when they are not. This manifests itself by sending
provisional responses reliably from the perspective of the B2BUA while
stripping the extensions from INVITEs sent to the callee's UAS. It is
worth noting that such behavior is likely to be applied to Security
Preconditions <xref
target="I-D.ietf-mmusic-securityprecondition"></xref> as well for
similar reasons.
</t>
<t>Finally, B2BUAs may introduce early SIP dialogs to network-provided
early media services even though no forking occurs towards the
intended callee. The impact of forking of signaling requests is
described within section <xref target="forking"></xref>.</t>
<t>The impacts of these types of signaling manipulations by B2BUAs is
currently left as an OPEN ISSUE.</t>
</section>
<section anchor="pcc" title="Policy and Media Gating Interactions">
<t>Another class of SRTP key exchange interactions that can occur is
due to policy policing and media stream gating mechanisms. These
functions are often performed by Session Border Controllers or by
firewalls. In the case of media stream gating, the flow of RTP packets
between endpoints is not authorized until a complete SDP offer/answer
exchange has taken place, commonly contingent upon the 200 OK to the
INVITE being received by the network entity controlling the media
gates. As a result, in-band keying cannot start prior to the flow of
packets being authorized. If in-band keying is used it may be possible
to detect that the RTP packet in question is part of a key exchange
and not part of any data transfer process. However, the firewalls
responsible for gating media are typically not inspecting the actual
packets received, they are simply dropping them on the floor until the
gate is opened.</t>
<t>Policy policing, which is often related to media stream gating, can
also cause potential issues. For example, if elements such as a
deep-packet inspection element were not expecting in-band SRTP key
exchanges these packets may be suppressed according to local policy
for not conforming to expected traffic profiles (specifically, not
being an SRTP packet).</t>
<t>The impacts of these types of policy and gating related
interactions is currently left as an OPEN ISSUE.</t>
</section>
</section>
<section anchor="requirements" title="Requirements">
<t><list hangIndent="7" style="format R%d:">
<t>Negotiation of SRTP keys MUST NOT cause the call setup to fail in
forked and retargeted calls where all end points are willing to use
SRTP, unless the execution of the authentication and key exchange
protocol leads to a failure (e.g., an unsuccessful authentication
attempt).</t>
<t><!-- Forking and retargeting MUST allow establishing SRTP or RTP with a
mixture of SRTP- and RTP-capable targets, such that SRTP is performed with SRTP-capable
targets and RTP targets do not cause Heterogeneous Error Response Forking Problem
(HERFP).
-->Even when some end points of a forked or retargeted call are
incapable of using SRTP, the key management protocol MUST allow the
establishment of SRTP associations with SRTP-capable endpoints and /
or RTP associations with non-SRTP-capable endpoints.</t>
<t>Forked end points MUST NOT know the SRTP key of any call
established with another forked end point.</t>
<t>The media security key management protocol MAY support the
ability to utilize an initially established security context that
was established as part of the first call setup with a remote end
point.<vspace blankLines="1" /> Specialized devices may need to
avoid public key operations or Diffie-Hellman operations as much as
possible because of the computational cost or because of the
additional call setup delay. For example, it can take a second or
two to perform a Diffie-Hellman operation in certain devices.
Examples of these specialized devices would include some handsets,
intelligent SIMs, and PSTN gateways. For the typical case because a
phone call has not yet been established, ancillary processing cycles
can be utilized to perform the PK or DH operation; for example, in a
PSTN gateway the DSP, which is not yet involved with typical DSP
operations, could be used to perform the calculation, so as to avoid
having the central host processor perform the calculation. Some
devices, such as handsets, and intelligent SIMs do not have such
ancillary processing capability.</t>
<t>The media security key management protocol SHOULD avoid clipping
media before SDP answer without requiring <xref
target="I-D.ietf-mmusic-securityprecondition">Security
Preconditions</xref>, as Security Preconditions is not widely
implemented and requires significant signaling overhead.</t>
<t>The media security key management protocol MUST provide
protection against passive attacks on the media path.</t>
<t>The media security key management protocol MUST provide
protection against passive attacks of a SIP proxy that is
legitimately routing SIP messages.</t>
<t>The media security key management protocol MUST be able to
support perfect forward secrecy (PFS). The term PFS is the property
that disclosure of the long-term secret keying material that is used
to derive an agreed ephemeral key does not compromise the secrecy of
agreed keys from earlier runs.</t>
<t>The media security key management protocol MUST support
negotiation of SRTP cipher suites without incurring per-algorithm
computational expense. This allows an offer to be built without
incurring computational expense for each algorithm.</t>
<t>If SRTP keying is performed over the media path, the keying
packets MUST NOT pass the RTP validity check defined in Appendix A.1
of <xref target="RFC3550"></xref>.</t>
<t>The media security key management protocol that utilizes
expensive cryptographic computations SHOULD offer the ability to
resume previous sessions in an efficient way.<!-- R10 is also about using the same DTLS session for RTCP and RTP.
R10 is talking about using the same PK operation (and DH) operation for your m=audio, m=video, m=application sessions.
--></t>
<t>The media security key management protocol MUST NOT require 3rd
parties to sign certificates.<vspace blankLines="1" />This
requirement points to the fact that a global PKI cannot be assumed
and opportunistic security approaches should be considered as part
of the solution.</t>
<t>The media security key management protocol SHOULD use algorithms
that allow <xref target="FIPS-140-2">FIPS 140-2</xref>
certification. <vspace blankLines="1" /> Note that the United States
Government can only purchase and use crypto implementations that
have been validated by the <xref target="FIPS-140-2">FIPS-140</xref>
process: <vspace blankLines="1" /> "The FIPS-140 standard is
applicable to all Federal agencies that use cryptographic-based
security systems to protect sensitive information in computer and
telecommunication systems, including voice systems. The adoption and
use of this standard is available to private and commercial
organizations."<xref target="cryptval"></xref> <vspace
blankLines="1" />Some commercial organizations, such as banks and
defense contractors, also require or prefer equipment which has
validated by the FIPS-140 process.</t>
<t>The media security key management protocol SHOULD be able to
associate the signaling exchange with the media traffic.<vspace
blankLines="1" />For example, if using a Diffie-Hellman keying
technique with security preconditions that forks to 20 end points,
the call initiator would get 20 provisional responses containing 20
signed Diffie-Hellman key pairs. Calculating 20 DH secrets and
validating signatures can be a difficult task depending on the
device capabilities. Hence, in the case of forking, it is not
desirable to perform a DH or PK operation with every party, but
rather only with the party that answers the call (and incur some
media clipping). To do this, the signaling and media need to be
associated so the calling party knows which key management needs to
be completed. This might be done by using the transport address
indicated in the SDP, although NATs can complicate this association.
<vspace blankLines="1" /> Allowing such an association also allows
the SDP offerer to avoid performing CPU-consuming operations (e.g.,
DH or public key operations) with attackers that have not seen the
signaling messages.</t>
<!-- <t>[Editor's Note: There are different options to achieve the association of signaling and
media, which need to be discussed. One option may be requiring the use of symmetric RTP
when applying SRTP. The only time this doesn't work is when NATs are involved. For this
case we may rely on ICE (see "Interactions with Forking" in <xref
target="I-D.ietf-mmusic-ice"/> or also the in section <xref target="ICE4association"/></t>
-->
<t>The media security key management protocol SHOULD allow to start
with RTP and then upgrade to SRTP.<!-- Dan: opportunistic probing. Hannes: Start with RTP (in case of early media) and then upgrade. --></t>
<t>The media security key management protocol SHOULD NOT introduce
new denial of service vulnerabilities.</t>
<t>The media security key management protocol SHOULD require the
adversary to have access to the data as well as the signaling path
for a successful attack to be launched. An adversary that is located
only along the data or only along the signaling path MUST NOT be
able to successfully mount an attack. A successful attack refers to
the ability for the adversary to obtain keying material to decrypt
the SRTP encrypted media traffic.</t>
<!-- <t hangText="R17:">The media security key management protocol SHOULD support the
possibility to protect non-RTP-based data traffic.
</t>
-->
<t>If two parties share an authentication infrastructure that has
3rd parties signing certificates, they SHOULD be able to make use of
it.</t>
<t>The media security key management protocol MUST provide
crypto-agility.</t>
<t>The media security key management protocol MUST protect cipher
suite negotiation against downgrading attacks.</t>
<t hangText="R21:">The media security key management protocol MUST
allow a SIP User Agent to negotiate media security parameters for
each individual session.</t>
<t>The media security key management protocol SHOULD allow
establishing SRTP keying between different call signaling protocols
(e.g., between Jabber, SIP, H.323, MGCP)</t>
<t>The media security key management protocol SHOULD support
recording of decrypted media.<vspace blankLines="1" /> Media
recording may be realized by an intermediate nodes. An example for
those intermediate nodes are devices, which could be used in banking
applications or for quality monitoring in call centers. Here, it
must be ensured, that the media security is ensured by the
intermediate nodes on the connections to the involved endpoints as
originally negotiated. The endpoints need to be informed that there
is an intermediate device and need to cooperate. A solution approach
for this is described in <xref
target="I-D.wing-sipping-srtp-key"></xref>.</t>
<t>The media security key management protocol SHOULD NOT allow end
users to determine whether their end-to-end interaction is subject
to lawful interception.</t>
<t>The media security key management protocol MUST work when there
are intermediate nodes, terminating or processing media, between the
end points.</t>
<t>The media security key management protocol MUST consider
termination of media security in a PSTN gateway.<vspace
blankLines="1" />A typical case of using media security is the one
where two entities are having a VoIP conversation over IP capable
networks. However, there are cases where the other end of the
communication is not connected to an IP capable network. In this
kind of setting, there needs to be some kind of gateway at the edge
of the IP network which converts the VoIP conversation to format
understood by the other network. An example of such gateway is a
PSTN gateway sitting at the edge of IP and PSTN networks.<vspace
blankLines="1" />If media security (e.g., SRTP protection) is
employed in this kind of gateway-setting, then media security and
the related key management needs to be terminated at the gateway.
The other network (e.g., PSTN) may have its own measures to protect
the communication, but this means that from media security point of
view the media security is not employed end-to-end between the
communicating entities.</t>
<!--
<t hangText="R9:">A solution MUST NOT expect packets to be received on the media path
until 200 OK, because the media path may be blocked by middleboxes until the 200 OK.
</t>
This requirement was recently proposed and we did not want to
consider it. -->
</list></t>
</section>
<section anchor="clustering" title="Requirements Classification">
<t>An adversary might be located along <list style="numbers">
<t hangText="(1)">the media path,</t>
<t hangText="(2)">the signaling path,</t>
<t hangText="(3)">the media and the signaling path.</t>
</list></t>
<t>An attacker that can solely be located along the signaling path, and
does not have access to media, is not considered (ref item 2).</t>
<t>Furthermore, it is reasonable to consider the capabilities of the
adversary. We also have different types of adversaries, namely <list
style="letters">
<t hangText="(a)">active adversary</t>
<t hangText="(b)">passive adversary</t>
</list></t>
<t>Note that the adversary model for (a) and (b) also assumes the
attacker being able to control SIP signaling entities.</t>
<t>With respect to item (a) an adversary may need to be active with
regard to the key exchange relevant information traveling along the data
or the signaling path.</t>
<t>Some of the deployment variants of the media security key management
proposals under considerations do not provide protection against
man-in-the-middle adversaries under certain conditions, for example when
SIP signaling entities are compromised, when a global PKI is missing or
pre-shared secrets are not exchanged between the end points prior to the
protocol exchange.</t>
<t>Based on the above-mentioned considerations the following
classifications can be made: <list style="hanging">
<t hangText="Class I:"><vspace blankLines="1" /> Passive attack on
the signaling and the data path sufficient to reveal the content of
the media traffic. <vspace blankLines="1" /></t>
<t hangText="Class II:"><vspace blankLines="1" /> Active attack on
the signaling path and passive attack on the data path to reveal the
content of the media traffic. <vspace blankLines="1" /></t>
<t hangText="Class III:"><vspace blankLines="1" /> Active attack on
the signaling and the data path necessary to reveal the content of
the media traffic. <vspace blankLines="1" /></t>
<t hangText="Class IV:"><vspace blankLines="1" /> Active attack is
required and will be detected by the end points when adversary
tampers with the messages.</t>
</list></t>
<t>For example, Security Descriptions falls into Class I since the
adversary needs to learn the Security Descriptions key by processing a
signaling message at a SIP proxy (assuming that the adversary is in
control of the SIP proxy). Subsequent media traffic can be decrypted
with the help of the learned key.</t>
<t>As another example, DTLS-RTP falls into Class III when DTLS is used a
public key based ciphersuite with self-signed certificates and without
SIP Identity. An adversary would have to modify the fingerprint that is
sent along the signaling path and subsequently to modify the
certificates carried in the DTLS handshake that travel along the media
path.</t>
<t>An attack is not successful when SIP Identity is used, the adversary
is not between the SIP UA and its Authentication Service (or at the
Authentication Service), both end points are able to verify the digital
signature (of the SIP Identity) and are able to validate the
corresponding certificates.</t>
</section>
<section anchor="security" title="Security Considerations">
<t>This document lists requirements for securing media traffic. As such,
it addresses security throughout the document.</t>
</section>
<section anchor="iana" title="IANA Considerations">
<t>This document does not require actions by IANA.</t>
</section>
<section anchor="acks" title="Acknowledgements">
<t>The authors would like to thank the participants of the two RTPSEC
BoFs and the members of the RTPSEC mailing list. Further thanks to the
following individuals for their specific suggestions which improved this
document: Flemming Andreasen, Richard Barnes, Mark Baugher, Wolfgang
Buecker, Werner Dittmann, Lakshminath Dondeti, John Elwell, Martin
Euchner, Hans-Heinrich Grusdt, Christer Holmberg, Guenther Horn, Peter
Howard, Leo Huang, Dragan Ignjatic, Cullen Jennings, Alan Johnston, Vesa
Lehtovirta, Matt Lepinski, David McGrew, David Oran, Colin Perkins, Eric
Raymond, Peter Schneider, Eric Rescorla, Srinath Thiruvengadam, Dave
Ward, and Dan York.</t>
<t>Thanks also to Dragan Ignjatic (and our co-author, Steffen Fries) for
their excellent <xref target="I-D.ietf-msec-mikey-applicability">MIKEY
modes</xref> document, which formed the basis for the MIKEY
comparisons.</t>
</section>
</middle>
<back>
<references title="Normative References">
&RFC2119;
&RFC3261;
&RFC3262;
&RFC3264;
&RFC3711;
<reference anchor="FIPS-140-2"
target="http://csrc.nist.gov/publications/fips/fips140-2/fips1402.pdf">
<front>
<title>Security Requirements for Cryptographic Modules</title>
<author fullname="NIST">
<organization>NIST</organization>
</author>
<date day="13" month="June" year="2005" />
</front>
</reference>
<reference anchor="cryptval"
target="http://csrc.nist.gov/cryptval/140-2APP.htm">
<front>
<title>Cryptographic Module Validation Program</title>
<author fullname="NIST">
<organization>NIST</organization>
</author>
<date day="19" month="December" year="2006" />
</front>
</reference>
</references>
<references title="Informative References">
&I-D.ietf-mmusic-securityprecondition;
&RFC3312;
&RFC3550;
&I-D.ietf-mmusic-ice;
&I-D.peterson-sipping-retarget;
&RFC4474;
&I-D.barnes-sip-em-ps-req-sol;
&I-D.wing-sipping-srtp-key;
&rfc4568;
&rfc4650;
&I-D.ietf-msec-mikey-ecc;
&rfc4738;
&I-D.ietf-sip-certs;
&I-D.mahy-sipping-herfp-fix;
&rfc3830;
&rfc4492;
&rfc3388;
&rfc4346;
&rfc4916;
&I-D.fischl-sipping-media-dtls;
&I-D.ietf-msec-mikey-applicability;
&I-D.zimmermann-avt-zrtp;
&I-D.baugher-mmusic-sdp-dh;
&I-D.mcgrew-srtp-ekt;
&rfc4771;
&I-D.jennings-sipping-multipart;
&I-D.mcgrew-tls-srtp;
&I-D.dondeti-msec-rtpsec-mikeyv2;
&I-D.ietf-mmusic-sdp-capability-negotiation;
</references>
<section anchor="comparison" title="Overview of Keying Mechanisms">
<t>Based on how the SRTP keys are exchanged, each SRTP key exchange
mechanism belongs to one general category:</t>
<t><list>
<t><list style="hanging">
<t hangText="signaling path:">All the keying is carried in the
call signaling (SIP or SDP) path.</t>
<t hangText="media path:">All the keying is carried in the
SRTP/SRTCP media path, and no signaling whatsoever is carried in
the call signaling path.</t>
<t hangText="signaling and media path:">Parts of the keying are
carried in the SRTP/SRTCP media path, and parts are carried in
the call signaling (SIP or SDP) path.</t>
</list></t>
</list></t>
<t>One of the significant benefits of SRTP over other end-to-end
encryption mechanisms, such as for example IPsec, is that SRTP is
bandwidth efficient and SRTP retains the header of RTP packets.
Bandwidth efficiency is vital for VoIP in many scenarios where access
bandwidth is limited or expensive, and retaining the RTP header is
important for troubleshooting packet loss, delay, and jitter.</t>
<t>Related to SRTP's characteristics is a goal that any SRTP keying
mechanism to also be efficient and not cause additional call setup
delay. Contributors to additional call setup delay include network or
database operations: retrieval of certificates and additional SIP or
media path messages, and computational overhead of establishing keys or
validating certificates.</t>
<t>When examining the choice between keying in the signaling path,
keying in the media path, or keying in both paths, it is important to
realize the media path is generally 'faster' than the SIP signaling
path. The SIP signaling path has computational elements involved which
parse and route SIP messages. The media path, on the other hand, does
not normally have computational elements involved, and even when
computational elements such as firewalls are involved, they cause very
little additional delay. Thus, the media path can be useful for
exchanging several messages to establish SRTP keys. A disadvantage of
keying over the media path is that interworking different key exchange
requires the interworking function be in the media path, rather than
just in the signaling path; in practice this involvement is probably
unavoidable anyway.</t>
<section title="Signaling Path Keying Techniques">
<section title="MIKEY-NULL">
<t><xref target="RFC3830">MIKEY-NULL</xref> has the offerer indicate
the SRTP keys for both directions. The key is sent unencrypted in
SDP, which means the SDP must be encrypted hop-by-hop (e.g., by
using TLS (SIPS)) or end-to-end (e.g., by using S/MIME).</t>
<t>MIKEY-NULL requires one message from offerer to answerer (half a
round trip), and does not add additional media path messages.</t>
</section>
<section title="MIKEY-PSK">
<t>MIKEY-PSK (pre-shared key) <xref target="RFC3830"></xref>
requires that all endpoints share one common key. MIKEY-PSK has the
offerer encrypt the SRTP keys for both directions using this
pre-shared key.</t>
<t>MIKEY-PSK requires one message from offerer to answerer (half a
round trip), and does not add additional media path messages.</t>
</section>
<section title="MIKEY-RSA">
<t><xref target="RFC3830">MIKEY-RSA</xref> has the offerer encrypt
the keys for both directions using the intended answerer's public
key, which is obtained from a PKI.</t>
<t>MIKEY-RSA requires one message from offerer to answerer (half a
round trip), and does not add additional media path messages.
MIKEY-RSA requires the offerer to obtain the intended answerer's
certificate.</t>
</section>
<section title="MIKEY-RSA-R">
<t>MIKEY-RSA-R <xref target="RFC4738">An additional mode of key
distribution in MIKEY: MIKEY-RSA-R</xref> is essentially the same as
MIKEY-RSA but reverses the role of the offerer and the answerer with
regards to providing the keys. That is, the answerer encrypts the
keys for both directions using the offerer's public key. Both the
offerer and answerer validate each other's public keys using a PKI.
MIKEY-RSA-R also enables sending certificates in the MIKEY
message.</t>
<t>MIKEY-RSA-R requires one message from offerer to answer, and one
message from answerer to offerer (full round trip), and does not add
additional media path messages. MIKEY-RSA-R requires the offerer
validate the answerer's certificate.</t>
</section>
<section title="MIKEY-DHSIGN">
<t><xref target="RFC3830">In MIKEY-DHSIGN</xref> the offerer and
answerer derive the key from a Diffie-Hellman exchange. In order to
prevent an active man-in-the-middle the DH exchange itself is signed
using each endpoint's private key and the associated public keys are
validated using a PKI.</t>
<t>MIKEY-DHSIGN requires one message from offerer to answerer, and
one message from answerer to offerer (full round trip), and does not
add additional media path messages. MIKEY-DHSIGN requires the
offerer and answerer to validate each other's certificates.
MIKEY-DHSIGN also enables sending the answerer's certificate in the
MIKEY message.</t>
</section>
<section title="MIKEY-DHHMAC">
<t><xref target="RFC4650">MIKEY-DHHMAC</xref> uses a pre-shared
secret to HMAC the Diffie-Hellman exchange, essentially combining
aspects of MIKEY-PSK with MIKEY-DHSIGN, but without MIKEY-DHSIGN's
need for a PKI to authenticate the Diffie-Hellman exchange.</t>
<t>MIKEY-DHHMAC requires one message from offerer to answerer, and
one message from answerer to offerer (full round trip), and does not
add additional media path messages.</t>
</section>
<section title="MIKEY-ECIES and MIKEY-ECMQV (MIKEY-ECC)">
<t><xref target="I-D.ietf-msec-mikey-ecc">ECC Algorithms For
MIKEY</xref> describes how ECC can be used with MIKEY-RSA (using
ECDSA signature) and with MIKEY-DHSIGN (using a new DH-Group code),
and also defines two new ECC-based algorithms, Elliptic Curve
Integrated Encryption Scheme (ECIES) and Elliptic Curve
Menezes-Qu-Vanstone (ECMQV) .</t>
<t>For the purposes of this paper, the ECDSA signature, MIKEY-ECIES,
and MIKEY-ECMQV function exactly like MIKEY-RSA, and the new
DH-Group code function exactly like MIKEY-DHSIGN. Therefore these
ECC mechanisms aren't discussed separately in this paper.</t>
</section>
<section anchor="sdesc" title="Security Descriptions with SIPS">
<t><xref target="RFC4568">Security Descriptions</xref> has each side
indicate the key it will use for transmitting SRTP media, and the
keys are sent in the clear in SDP. Security Descriptions relies on
hop-by-hop (TLS via "SIPS:") encryption to protect the keys
exchanged in signaling.</t>
<t>Security Descriptions requires one message from offerer to
answerer, and one message from answerer to offerer (full round
trip), and does not add additional media path messages.</t>
</section>
<section title="Security Descriptions with S/MIME">
<t>This keying mechanism is identical to <xref
target="sdesc"></xref>, except that rather than protecting the
signaling with TLS, the entire SDP is encrypted with S/MIME.</t>
</section>
<section title="SDP-DH (expired)">
<t><xref target="I-D.baugher-mmusic-sdp-dh">SDP
Diffie-Hellman</xref> exchanges Diffie-Hellman messages in the
signaling path to establish session keys. To protect against active
man-in-the-middle attacks, the Diffie-Hellman exchange needs to be
protected with S/MIME, SIPS, or <xref
target="RFC4474">SIP-Identity</xref> and <xref
target="RFC4474"></xref>.</t>
<t>SDP-DH requires one message from offerer to answerer, and one
message from answerer to offerer (full round trip), and does not add
additional media path messages.</t>
</section>
<section anchor="mikey2-sdp" title="MIKEYv2 in SDP (expired)">
<t><xref target="I-D.dondeti-msec-rtpsec-mikeyv2">MIKEYv2</xref>
adds mode negotiation to MIKEYv1 and removes the time
synchronization requirement. It therefore now takes 2 round-trips to
complete. In the first round trip, the communicating parties learn
each other's identities, agree on a MIKEY mode, crypto algorithm,
SRTP policy, and exchanges nonces for replay protection. In the
second round trip, they negotiate unicast and/or group SRTP context
for SRTP and/or SRTCP.</t>
<t>Furthemore, MIKEYv2 also defines an in-band negotiation mode as
an alternative to SDP (see <xref
target="mikey2-inband"></xref>).</t>
</section>
</section>
<section title="Media Path Keying Technique">
<t></t>
<section title="ZRTP">
<t><xref target="I-D.zimmermann-avt-zrtp">ZRTP</xref> does not
exchange information in the signaling path (although it's possible
for endpoints to indicate support for ZRTP with "a=zrtp" in the
initial Offer). In ZRTP the keys are exchanged entirely in the media
path using a Diffie-Hellman exchange. The advantage to this
mechanism is that the signaling channel is used only for call setup
and the media channel is used to establish an encrypted channel --
much like encryption devices on the PSTN. ZRTP uses voice
authentication of its Diffie-Hellman exchange by having each person
read digits to the other person. Subsequent sessions with the same
ZRTP endpoint can be authenticated using the stored hash of the
previously negotiated key rather than voice authentication.</t>
<t>ZRTP uses 4 media path messages (Hello, Commit, DHPart1, and
DHPart2) to establish the SRTP key, and 3 media path confirmation
messages. The first 4 are sent as RTP packets (using RTP header
extensions), and the last 3 are sent in conjunction with SRTP media
packets (again as SRTP header extensions). Note that unencrypted RTP
is being exchanged until the SRTP keys are established.</t>
</section>
</section>
<section title="Signaling and Media Path Keying Techniques">
<t></t>
<section title="EKT">
<t><xref target="I-D.mcgrew-srtp-ekt">EKT</xref> relies on another
SRTP key exchange protocol, such as Security Descriptions or MIKEY,
for bootstrapping. In the initial phase, each member of a conference
uses an SRTP key exchange protocol to establish a common key
encryption key (KEK). Each member may use the KEK to securely
transport its SRTP master key and current SRTP rollover counter
(ROC), via RTCP, to the other participants in the session.</t>
<t>EKT requires the offerer to send some parameters (EKT_Cipher,
KEK, and security parameter index (SPI)) via the bootstrapping
protocol such as Security Descriptions or MIKEY. Each answerer sends
an SRTCP message which contains the answerer's SRTP Master Key,
rollover counter, and the SRTP sequence number. Rekeying is done by
sending a new SRTCP message. For reliable transport, multiple RTCP
messages need to be sent.</t>
</section>
<section anchor="dtls-srtp" title="DTLS-SRTP">
<t><xref target="I-D.mcgrew-tls-srtp">DTLS-SRTP</xref> exchanges
public key fingerprints in SDP <xref
target="I-D.fischl-sipping-media-dtls"></xref> and then establishes
a DTLS session over the media channel. The endpoints use the DTLS
handshake to agree on crypto suites and establish SRTP session keys.
SRTP packets are then exchanged between the endpoints.</t>
<t>DTLS-SRTP requires one message from offerer to answerer (half
round trip), and, if the offerer wishes to correlate the SDP answer
with the endpoint, requires one message from answer to offerer (full
round trip). DTLS-SRTP uses 4 media path messages to establish the
SRTP key.</t>
<t>This paper assumes DTLS will use TLS_RSA_WITH_3DES_EDE_CBC_SHA as
its cipher suite, which is the mandatory-to-implement cipher suite
in <xref target="RFC4346">TLS</xref>.</t>
</section>
<section anchor="mikey2-inband" title="MIKEYv2 Inband (expired)">
<t>As defined in <xref target="mikey2-sdp"></xref>, MIKEYv2 also
defines an in-band negotiation mode as an alternative to SDP (see
<xref target="mikey2-inband"></xref>). The details are not sorted
out in the draft yet on what in-band actually means (i.e., UDP, RTP,
RTCP, etc.).</t>
</section>
</section>
</section>
<section title="Evaluation Criteria - SIP">
<t>This section considers how each keying mechanism interacts with SIP
features.</t>
<section anchor="retargeting"
title="Secure Retargeting and Secure Forking">
<t></t>
<t>Retargeting and forking of signaling requests is described within
section <xref target="forking"></xref>. The following builds upon this
description.</t>
<t>The following list compares the behavior of secure forking,
answering association, two-time pads, and secure retargeting for each
keying mechanism.</t>
<t><list>
<t><list style="hanging">
<t hangText="MIKEY-NULL">Secure Forking: No, all AORs see
offerer's and answerer's keys. Answer is associated with media
by the SSRC in MIKEY. Additionally, a two-time pad occurs if
two branches choose the same 32-bit SSRC and transmit SRTP
packets.<vspace blankLines="1" />Secure Retargeting: No, all
targets see offerer's and answerer's keys. Suffers from
retargeting identity problem.</t>
<t hangText="MIKEY-PSK"><vspace blankLines="0" />Secure
Forking: No, all AORs see offerer's and answerer's keys.
Answer is associated with media by the SSRC in MIKEY. Note
that all AORs must share the same pre-shared key in order for
forking to work at all with MIKEY-PSK. Additionally, a
two-time pad occurs if two branches choose the same 32-bit
SSRC and transmit SRTP packets.<vspace blankLines="1" />Secure
Retargeting: Not secure. For retargeting to work, the final
target must possess the correct PSK. As this is likely in
scenarios were the call is targeted to another device
belonging to the same user (forking), it is very unlikely that
other users will possess that PSK and be able to successfully
answer that call.</t>
<t hangText="MIKEY-RSA"><vspace blankLines="0" />Secure
Forking: No, all AORs see offerer's and answerer's keys.
Answer is associated with media by the SSRC in MIKEY. Note
that all AORs must share the same private key in order for
forking to work at all with MIKEY-RSA. Additionally, a
two-time pad occurs if two branches choose the same 32-bit
SSRC and transmit SRTP packets.<vspace blankLines="1" />Secure
Retargeting: No.</t>
<t hangText="MIKEY-RSA-R"><vspace blankLines="0" />Secure
Forking: Yes. Answer is associated with media by the SSRC in
MIKEY.<vspace blankLines="1" />Secure Retargeting: Yes.</t>
<t hangText="MIKEY-DHSIGN"><vspace blankLines="0" />Secure
Forking: Yes, each forked endpoint negotiates unique keys with
the offerer for both directions. Answer is associated with
media by the SSRC in MIKEY.<vspace blankLines="1" />Secure
Retargeting: Yes, each target negotiates unique keys with the
offerer for both directions.</t>
<t hangText="MIKEYv2 in SDP"><vspace blankLines="0" />The
behavior will depend on which mode is picked.</t>
<t hangText="MIKEY-DHHMAC"><vspace blankLines="0" />Secure
Forking: Yes, each forked endpoint negotiates unique keys with
the offerer for both directions. Answer is associated with
media by the SSRC in MIKEY.<vspace blankLines="1" />Secure
Retargeting: Yes, each target negotiates unique keys with the
offerer for both directions. Note that for the keys to be
meaningful, it would require the PSK to be the same for all
the potential intermediaries, which would only happen within a
single domain.</t>
<t hangText="Security Descriptions with SIPS"><vspace
blankLines="0" />Secure Forking: No. Each forked endpoint sees
the offerer's key. Answer is not associated with media.<vspace
blankLines="1" />Secure Retargeting: No. Each target sees the
offerer's key.</t>
<t hangText="Security Descriptions with S/MIME"><vspace
blankLines="0" />Secure Forking: No. Each forked endpoint sees
the offerer's key. Answer is not associated with media.<vspace
blankLines="1" />Secure Retargeting: No. Each target sees the
offerer's key. Suffers from retargeting identity problem.</t>
<t hangText="SDP-DH"><vspace blankLines="0" />Secure Forking:
Yes. Each forked endpoint calculates a unique SRTP key. Answer
is not associated with media.<vspace blankLines="1" />Secure
Retargeting: Yes. The final target calculates a unique SRTP
key.</t>
<t hangText="ZRTP"><vspace blankLines="0" />Secure Forking:
Yes. Each forked endpoint calculates a unique SRTP key. As
ZRTP isn't signaled in SDP, there is no association of the
answer with media.<vspace blankLines="1" />Secure Retargeting:
Yes. The final target calculates a unique SRTP key.</t>
<t hangText="EKT"><vspace blankLines="0" />Secure Forking:
Inherited from the bootstrapping mechanism (the specific MIKEY
mode or Security Descriptions). Answer is associated with
media by the SPI in the EKT protocol. Answer is associated
with media by the SPI in the EKT protocol.<vspace
blankLines="1" />Secure Retargeting: Inherited from the
bootstrapping mechanism (the specific MIKEY mode or Security
Descriptions).</t>
<t hangText="DTLS-SRTP"><vspace blankLines="0" />Secure
Forking: Yes. Each forked endpoint calculates a unique SRTP
key. Answer is associated with media by the certificate
fingerprint in signaling and certificate in the media
path.<vspace blankLines="1" /> Secure Retargeting: Yes. The
final target calculates a unique SRTP key.</t>
<t hangText="MIKEYv2 Inband"><vspace blankLines="0" />The
behavior will depend on which mode is picked.</t>
</list></t>
</list></t>
</section>
<section title="Clipping Media Before SDP Answer">
<t>Clipping media before receiving the signaling answer is described
within section <xref target="clipping"></xref>. The following builds
upon this description.</t>
<t>Furthermore, the problem of clipping gets compounded when forking
is used. For example, if using a Diffie-Hellman keying technique with
security preconditions that forks to 20 endpoints, the call initiator
would get 20 provisional responses containing 20 signed Diffie-Hellman
half keys. Calculating 20 DH secrets and validating signatures can be
a difficult task depending on the device capabilities.</t>
<t>The following list compares the behavior of clipping before SDP
answer for each keying mechanism.</t>
<t><list>
<t><list style="hanging">
<t hangText="MIKEY-NULL"><vspace blankLines="0" />Not clipped.
The offerer provides the answerer's keys.</t>
<t hangText="MIKEY-PSK"><vspace blankLines="0" />Not clipped.
The offerer provides the answerer's keys.</t>
<t hangText="MIKEY-RSA"><vspace blankLines="0" />Not clipped.
The offerer provides the answerer's keys.</t>
<t hangText="MIKEY-RSA-R"><vspace blankLines="0" />Clipped.
The answer contains the answerer's encryption key.</t>
<t hangText="MIKEY-DHSIGN"><vspace blankLines="0" />Clipped.
The answer contains the answerer's Diffie-Hellman
response.</t>
<t hangText="MIKEY-DHHMAC"><vspace blankLines="0" />Clipped.
The answer contains the answerer's Diffie-Hellman
response.</t>
<t hangText="MIKEYv2 in SDP"><vspace blankLines="0" />The
behavior will depend on which mode is picked.</t>
<t hangText="Security Descriptions with SIPS"><vspace
blankLines="0" />Clipped. The answer contains the answerer's
encryption key.</t>
<t hangText="Security Descriptions with S/MIME"><vspace
blankLines="0" />Clipped. The answer contains the answerer's
encryption key.</t>
<t hangText="SDP-DH"><vspace blankLines="0" />Clipped. The
answer contains the answerer's Diffie-Hellman response.</t>
<t hangText="ZRTP"><vspace blankLines="0" />Not clipped
because the session intially uses RTP. While RTP is flowing,
both ends negotiate SRTP keys in the media path and then
switch to using SRTP.</t>
<t hangText="EKT"><vspace blankLines="0" />Not clipped, as
long as the first RTCP packet (containing the answerer's key)
is not lost in transit. The answerer sends its encryption key
in RTCP, which arrives at the same time (or before) the first
SRTP packet encrypted with that key.<list>
<t>Note: RTCP needs to work, in the answerer-to-offerer
direction, before the offerer can decrypt SRTP media.</t>
</list></t>
<t hangText="DTLS-SRTP"><vspace blankLines="0" />Not clipped.
Keys are exchanged in the media path without relying on the
signaling path.</t>
<t hangText="MIKEYv2 Inband"><vspace blankLines="0" />Not
clipped. Keys are exchanged in the media path without relying
on the signaling path.</t>
</list></t>
</list></t>
</section>
<section title="Centralized Keying">
<t>Centralized keying is described within section <xref
target="conferencing"></xref>. The following builds upon this
description.</t>
<t>The following list describes how each keying mechanism behaves with
centralized keying (scenario d) and rekeying.<list>
<t><list style="hanging">
<t hangText="MIKEY-NULL"><vspace blankLines="0" />Keying: Yes,
if offerer is the mixer. No, if offerer is the participant
(end user).<vspace blankLines="1" />Rekeying: Yes, via
re-Invite</t>
<t hangText="MIKEY-PSK"><vspace blankLines="0" />Keying: Yes,
if offerer is the mixer. No, if offerer is the participant
(end user).<vspace blankLines="1" />Rekeying: Yes, with a
re-Invite</t>
<t hangText="MIKEY-RSA"><vspace blankLines="0" />Keying: Yes,
if offerer is the mixer. No, if offerer is the participant
(end user).<vspace blankLines="1" />Rekeying: Yes, with a
re-Invite</t>
<t hangText="MIKEY-RSA-R"><vspace blankLines="0" />Keying: No,
if offerer is the mixer. Yes, if offerer is the participant
(end user).<vspace blankLines="1" />Rekeying: n/a</t>
<t hangText="MIKEY-DHSIGN"><vspace blankLines="0" />Keying:
No; a group-key Diffie-Hellman protocol is not
supported.<vspace blankLines="1" />Rekeying: n/a</t>
<t hangText="MIKEY-DHHMAC"><vspace blankLines="0" />Keying:
No; a group-key Diffie-Hellman protocol is not
supported.<vspace blankLines="1" />Rekeying: n/a</t>
<t hangText="MIKEYv2 in SDP"><vspace blankLines="0" />The
behavior will depend on which mode is picked.</t>
<t hangText="Security Descriptions with SIPS"><vspace
blankLines="0" />Keying: Yes, if offerer is the mixer. Yes, if
offerer is the participant.<vspace blankLines="1" />Rekeying:
Yes, with a Re-Invite.</t>
<t hangText="Security Descriptions with S/MIME"><vspace
blankLines="0" />Keying: Yes, if offerer is the mixer. Yes, if
offerer is the participant.<vspace blankLines="1" />Rekeying:
Yes, with a Re-Invite.</t>
<t hangText="SDP-DH"><vspace blankLines="0" />Keying: No; a
group-key Diffie-Hellman protocol is not supported.<vspace
blankLines="1" />Rekeying: n/a</t>
<t hangText="ZRTP"><vspace blankLines="0" />Keying: No; a
group-key Diffie-Hellman protocol is not supported.<vspace
blankLines="1" />Rekeying: n/a</t>
<t hangText="EKT"><vspace blankLines="0" />Keying: Yes. After
bootstrapping a KEK using Security Descriptions or MIKEY, each
member originating an SRTP stream can send its SRTP master
key, sequence number and ROC via RTCP.<vspace
blankLines="1" />Rekeying: Yes. EKT supports each sender to
transmit its SRTP master key to the group via RTCP packets.
Thus, EKT supports each originator of an SRTP stream to rekey
at any time.</t>
<t hangText="DTLS-SRTP"><vspace blankLines="0" />Keying: Yes,
because with the assumed cipher suite,
TLS_RSA_WITH_3DES_EDE_CBC_SHA, each end indicates its SRTP
key.<vspace blankLines="1" />Rekeying: via DTLS in the media
path.</t>
<t hangText="MIKEYv2 Inband"><vspace blankLines="0" />The
behavior will depend on which mode is picked.</t>
</list></t>
</list></t>
</section>
<section title="SSRC and ROC">
<t>In SRTP, a cryptographic context is defined as the SSRC,
destination network address, and destination transport port number.
Whereas RTP, a flow is defined as the destination network address and
destination transport port number. This results in a problem -- how to
communicate the SSRC so that the SSRC can be used for the
cryptographic context.</t>
<t>Two approaches have emerged for this communication. One, used by
all MIKEY modes, is to communicate the SSRCs to the peer in the MIKEY
exchange. Another, used by Security Descriptions, is to use "late
bindng" -- that is, any new packet containing a previously-unseen SSRC
(which arrives at the same destination network address and destination
transport port number) will create a new cryptographic context.
Another approach, common amongst techniques with media-path SRTP key
establishment, is to require a handshake over that media path before
SRTP packets are sent. MIKEY's approach changes RTP's SSRC collision
detection behavior by requiring RTP to pre-establish the SSRC values
for each session.</t>
<t>Another related issue is that SRTP introduces a rollover counter
(ROC), which records how many times the SRTP sequence number has
rolled over. As the sequence number is used for SRTP's default
ciphers, it is important that all endpoints know the value of the ROC.
The ROC starts at 0 at the beginning of a session.</t>
<t>Some keying mechanisms cause a two-time pad to occur if two
endpoints of a forked call have an SSRC collision.</t>
<t>Note: A proposal has been made to send the ROC value on every Nth
SRTP packet<xref target="RFC4771"></xref>. This proposal has not yet
been incorporated into this document.</t>
<t>The following list examines handling of SSRC and ROC:</t>
<t><list>
<t><list style="hanging">
<t hangText="MIKEY-NULL"><vspace blankLines="0" />Each
endpoint indicates a set of SSRCs and the ROC for SRTP packets
it transmits.</t>
<t hangText="MIKEY-PSK"><vspace blankLines="0" />Each endpoint
indicates a set of SSRCs and the ROC for SRTP packets it
transmits.</t>
<t hangText="MIKEY-RSA"><vspace blankLines="0" />Each endpoint
indicates a set of SSRCs and the ROC for SRTP packets it
transmits.</t>
<t hangText="MIKEY-RSA-R"><vspace blankLines="0" />Each
endpoint indicates a set of SSRCs and the ROC for SRTP packets
it transmits.</t>
<t hangText="MIKEY-DHSIGN"><vspace blankLines="0" />Each
endpoint indicates a set of SSRCs and the ROC for SRTP packets
it transmits.</t>
<t hangText="MIKEY-DHHMAC"><vspace blankLines="0" />Each
endpoint indicates a set of SSRCs and the ROC for SRTP packets
it transmits.</t>
<t hangText="MIKEYv2 in SDP"><vspace blankLines="0" />Each
endpoint indicates a set of SSRCs and the ROC for SRTP packets
it transmits.</t>
<t hangText="Security Descriptions with SIPS"><vspace
blankLines="0" />Neither SSRC nor ROC are signaled. SSRC 'late
binding' is used.</t>
<t hangText="Security Descriptions with S/MIME"><vspace
blankLines="0" />Neither SSRC nor ROC are signaled. SSRC 'late
binding' is used.</t>
<t hangText="SDP-DH"><vspace blankLines="0" />Neither SSRC nor
ROC are signaled. SSRC 'late binding' is used.</t>
<t hangText="ZRTP"><vspace blankLines="0" />Neither SSRC nor
ROC are signaled. SSRC 'late binding' is used.</t>
<t hangText="EKT"><vspace blankLines="0" />The SSRC of the
SRTCP packet containing an EKT update corresponds to the SRTP
master key and other parameters within that packet.</t>
<t hangText="DTLS-SRTP"><vspace blankLines="0" />Neither SSRC
nor ROC are signaled. SSRC 'late binding' is used.</t>
<t hangText="MIKEYv2 Inband"><vspace blankLines="0" />Each
endpoint indicates a set of SSRCs and the ROC for SRTP packets
it transmits.</t>
</list></t>
</list></t>
</section>
</section>
<section title="Evaluation Criteria - Security">
<t>This section evaluates each keying mechanism on the basis of their
security properties.</t>
<section title="Public Key Infrastructure">
<t>There are two aspects of PKI requirements -- one aspect is if PKI
is necessary in order for the mechanism to function at all, the other
is if PKI is used to authenticate a certificate. With interactive
communications it is desirable to avoid fetching certificates that
delay call setup; rather it is preferable to fetch or validate
certificates in such a way that call setup isn't delayed. For example,
a certificate can be validated while the phone is ringing or can be
validated while ring-back tones are being played or even while the
called party is answering the phone and saying "hello".</t>
<t hangText="Avoids PKI:">SRTP key exchange mechanisms that require a
global PKI to operate are gated on the deployment of a common PKI
available to both endpoints. This means that no media security is
achievable until such a PKI exists. For SIP, something like <xref
target="I-D.ietf-sip-certs">sip-certs</xref> might be used to obtain
the certificate of a peer.</t>
<t><list>
<t>Note: Even if SIP CERTs was deployed, the <xref
target="retargeting">retargeting problem</xref> would still
prevent successful deployment of keying techniques which require
the offerer to obtain the actual target's public key.</t>
</list></t>
<t>The following list compares the PKI requirements of each keying
mechanism, both if a PKI is required for the key exchange itself, and
if PKI is only used to authenticate the certificate supplied in
signaling.</t>
<t><list>
<t><list style="hanging">
<t hangText="MIKEY-NULL"><vspace blankLines="0" />PKI not
used.</t>
<t hangText="MIKEY-PSK"><vspace blankLines="0" />PKI not used;
rather, all endpoints must have some way to exchange
per-endpoint or per-system pre-shared keys.</t>
<t hangText="MIKEY-RSA"><vspace blankLines="0" />The offerer
obtains the intended answerer's public key before initiating
the call. This public key is used to encrypt the SRTP keys.
There is no defined mechanism for the offerer to obtain the
answerer's public key, although <xref
target="I-D.ietf-sip-certs"></xref> might be viable in the
future.</t>
<t hangText="MIKEY-RSA-R"><vspace blankLines="0" />The offer
contains the offerer's public key. The answerer uses that
public key to encrypt the SRTP keys that will be used by the
offerer and the answerer. A PKI is necessary to validate the
certificates.</t>
<t hangText="MIKEY-DHSIGN"><vspace blankLines="0" />PKI is
used to authenticate the public key that is included in the
MIKEY message, by walking the CA trust chain.</t>
<t hangText="MIKEY-DHHMAC"><vspace blankLines="0" />PKI not
used; rather, all endpoints must have some way to exchange
per-endpoint or per-system pre-shared keys.</t>
<t hangText="MIKEYv2 in SDP"><vspace blankLines="0" />The
behavior will depend on which mode is picked.</t>
<t hangText="Security Descriptions with SIPS"><vspace
blankLines="0" />PKI not used.</t>
<t hangText="Security Descriptions with S/MIME"><vspace
blankLines="0" />PKI is needed for S/MIME. The offerer must
obtain the intended target's public key and encrypt their SDP
with that key. The answerer must obtain the offerer's public
key and encrypt their SDP with that key.</t>
<t hangText="SDP-DH"><vspace blankLines="0" />PKI not
used.</t>
<t hangText="ZRTP"><vspace blankLines="0" />PKI not used.</t>
<t hangText="EKT"><vspace blankLines="0" />PKI not used by EKT
itself, but might be used by the EKT bootstrapping keying
mechanism (such as certain MIKEY modes).</t>
<t hangText="DTLS-SRTP"><vspace blankLines="0" />Remote
party's certificate is sent in media path, and a fingerprint
of the same certificate is sent in the signaling path.</t>
<t hangText="MIKEYv2 Inband"><vspace blankLines="0" />The
behavior will depend on which mode is picked.</t>
</list></t>
</list></t>
</section>
<section title="Perfect Forward Secrecy">
<t>In the context of SRTP, Perfect Forward Secrecy is the property
that SRTP session keys that protected a previous session are not
compromised if the static keys belonging to the endpoints are
compromised. That is, if someone were to record your encrypted session
content and later acquires either party's private key, that encrypted
session content would be safe from decryption if your key exchange
mechanism had perfect forward secrecy.</t>
<t>The following list describes how each key exchange mechanism
provides PFS.</t>
<t><list>
<t><list style="hanging">
<t hangText="MIKEY-NULL"><vspace blankLines="0" />No PFS.</t>
<t hangText="MIKEY-PSK"><vspace blankLines="0" />No PFS.</t>
<t hangText="MIKEY-RSA"><vspace blankLines="0" />No PFS.</t>
<t hangText="MIKEY-RSA-R"><vspace blankLines="0" />No PFS.</t>
<t hangText="MIKEY-DHSIGN"><vspace blankLines="0" />PFS is
provided with the Diffie-Hellman exchange.</t>
<t hangText="MIKEY-DHHMAC"><vspace blankLines="0" />PFS is
provided with the Diffie-Hellman exchange.</t>
<t hangText="MIKEYv2 in SDP"><vspace blankLines="0" />The
behavior will depend on which mode is picked.</t>
<t hangText="Security Descriptions with SIPS"><vspace
blankLines="0" />No PFS.</t>
<t hangText="Security Descriptions with S/MIME"><vspace
blankLines="0" />No PFS.</t>
<t hangText="SDP-DH"><vspace blankLines="0" />PFS is provided
with the Diffie-Hellman exchange.</t>
<t hangText="ZRTP"><vspace blankLines="0" />PFS is provided
with the Diffie-Hellman exchange.</t>
<t hangText="EKT"><vspace blankLines="0" />No PFS.</t>
<t hangText="DTLS-SRTP"><vspace blankLines="0" />PFS is
achieved if the negotiated cipher suite includes an
exponential or discrete-logarithmic key exchange (such as
Diffie-Hellman or <xref target="RFC4492">Elliptic Curve
Diffie-Hellman</xref>).</t>
<t hangText="MIKEYv2 Inband"><vspace blankLines="0" />The
behavior will depend on which mode is picked.</t>
</list></t>
</list></t>
</section>
<section title="Best Effort Encryption">
<t><vspace blankLines="1" /> <list>
<t>Note: With the ongoing efforts in <xref
target="I-D.ietf-mmusic-sdp-capability-negotiation">SDP Capability
Negotiation</xref>, the conclusions reached in this section may no
longer be accurate.</t>
</list> <vspace blankLines="1" /></t>
<t>With best effort encryption, SRTP is used with endpoints that
support SRTP, otherwise RTP is used.</t>
<t>SIP needs a backwards-compatible best effort encryption in order
for SRTP to work successfully with SIP retargeting and forking when
there is a mix of forked or retargeted devices that support SRTP and
don't support SRTP.</t>
<t><list>
<t>Consider the case of Bob, with a phone that only does RTP and a
voice mail system that supports SRTP and RTP. If Alice calls Bob
with an SRTP offer, Bob's RTP-only phone will reject the media
stream (with an empty "m=" line) because Bob's phone doesn't
understand SRTP (RTP/SAVP). Alice's phone will see this rejected
media stream and may terminate the entire call (BYE) and
re-initiate the call as RTP-only, or Alice's phone may decide to
continue with call setup with the SRTP-capable leg (the voice mail
system). If Alice's phone decided to re-initiate the call as
RTP-only, and Bob doesn't answer his phone, Alice will then leave
voice mail using only RTP, rather than SRTP as expected.</t>
</list>Currently, several techniques are commonly considered as
candidates to provide opportunistic encryption:</t>
<t><list style="hanging">
<t hangText="multipart/alternative"><vspace blankLines="0" />
<xref target="I-D.jennings-sipping-multipart"></xref> describes
how to form a multipart/alternative body part in SIP. The
significant issues with this technique are (1) that multipart MIME
is incompatible with existing SIP proxies, firewalls, Session
Border Controllers, and endpoints and (2) when forking, the <xref
target="I-D.mahy-sipping-herfp-fix">Heterogeneous Error Response
Forking Problem (HERFP)</xref> causes problems if such
non-multipart-capable endpoints were involved in the forking.</t>
<t hangText="SDP Grouping"><vspace blankLines="0" />A new SDP
grouping mechanism (following the idea introduced in <xref
target="RFC3388"></xref>) has been discussed which would allow a
media line to indicate RTP/AVP and another media line to indicate
RTP/SAVP, allowing non-SRTP-aware endpoints to choose RTP/AVP and
SRTP-aware endpoints to choose RTP/SAVP. As of this writing, this
SDP grouping mechanism has not been published as an Internet
Draft.</t>
<t hangText="session attribute"><vspace blankLines="0" />With this
technique, the endpoints signal their desire to do SRTP by
signaling RTP (RTP/AVP), and using an attribute ("a=") in the SDP.
This technique is entirely backwards compatible with
non-SRTP-aware endpoints, but doesn't use the RTP/SAVP protocol
registered by <xref target="RFC3711">SRTP</xref>.</t>
<t hangText="Probing"><vspace blankLines="0" />With this
technique, the endpoints first establish an RTP session using RTP
(RTP/AVP). The endpoints send probe messages, over the media path,
to determine if the remote endpoint supports their keying
technique.</t>
</list>The following list compares the availability of best effort
encryption for each keying mechanism.</t>
<t><list>
<t><list style="hanging">
<t hangText="MIKEY-NULL"><vspace blankLines="0" />No best
effort encryption.</t>
<t hangText="MIKEY-PSK"><vspace blankLines="0" />No best
effort encryption.</t>
<t hangText="MIKEY-RSA"><vspace blankLines="0" />No best
effort encryption.</t>
<t hangText="MIKEY-RSA-R"><vspace blankLines="0" />No best
effort encryption.</t>
<t hangText="MIKEY-DHSIGN"><vspace blankLines="0" />No best
effort encryption.</t>
<t hangText="MIKEY-DHHMAC"><vspace blankLines="0" />No best
effort encryption.</t>
<t hangText="MIKEYv2 in SDP"><vspace blankLines="0" />No best
effort encryption.</t>
<t hangText="Security Descriptions with SIPS"><vspace
blankLines="0" />No best effort encryption.</t>
<t hangText="Security Descriptions with S/MIME"><vspace
blankLines="0" />No best effort encryption.</t>
<t hangText="SDP-DH"><vspace blankLines="0" />No best effort
encryption.</t>
<t hangText="ZRTP"><vspace blankLines="0" />Best effort
encryption is done by probing (sending RTP messages with
header extensions) or by session attribute (see "a=zrtp",
defined in section 10 of <xref
target="I-D.zimmermann-avt-zrtp"></xref>). Current
implementations of ZRTP use probing.</t>
<t hangText="EKT"><vspace blankLines="0" />No best effort
encryption.</t>
<t hangText="DTLS-SRTP"><vspace blankLines="0" />No best
effort encryption.</t>
<t hangText="MIKEY Inband"><vspace blankLines="0" />No best
effort encryption.</t>
</list></t>
</list></t>
</section>
<section title="Upgrading Algorithms">
<t>It is necessary to allow upgrading SRTP encryption and hash
algorithms, as well as upgrading the cryptographic functions used for
the key exchange mechanism. With SIP's offer/answer model, this can be
computionally expensive because the offer needs to contain all
combinations of the key exchange mechanisms (all MIKEY modes, Security
Descriptions) and all SRTP cryptographic suites (AES-128, AES-256) and
all SRTP cryptographic hash functions (SHA-1, SHA-256) that the
offerer supports. In order to do this, the offerer has to expend CPU
resources to build an offer containing all of this information which
becomes computationally prohibitive.</t>
<t>Thus, it is important to keep the offerer's CPU impact fixed so
that offering multiple new SRTP encryption and hash functions incurs
no additional expense.</t>
<t>The following list describes the CPU effort involved in using each
key exchange technique.</t>
<t><list>
<t><list style="hanging">
<t hangText="MIKEY-NULL"><vspace blankLines="0" />No
significant computaional expense.</t>
<t hangText="MIKEY-PSK"><vspace blankLines="0" />No
significant computational expense.</t>
<t hangText="MIKEY-RSA"><vspace blankLines="0" />For each
offered SRTP crypto suite, the offerer has to perform RSA
operation to encrypt the TGK</t>
<t hangText="MIKEY-RSA-R"><vspace blankLines="0" />For each
offered SRTP crypto suite, the offerer has to perform public
key operation to sign the MIKEY message.</t>
<t hangText="MIKEY-DHSIGN"><vspace blankLines="0" />For each
offered SRTP crypto suite, the offerer has to perform
Diffie-Hellman operation, and a public key operation to sign
the Diffie-Hellman output.</t>
<t hangText="MIKEY-DHHMAC"><vspace blankLines="0" />For each
offered SRTP crypto suite, the offerer has to perform
Diffie-Hellman operation.</t>
<t hangText="MIKEYv2 in SDP"><vspace blankLines="0" />The
behavior will depend on which mode is picked.</t>
<t hangText="Security Descriptions with SIPS"><vspace
blankLines="0" />No significant computational expense.</t>
<t hangText="Security Descriptions with S/MIME"><vspace
blankLines="0" />S/MIME requires the offerer and the answerer
to encrypt the SDP with the other's public key, and to decrypt
the received SDP with their own private key.</t>
<t hangText="SDP-DH"><vspace blankLines="0" />For each offered
SRTP crypto suite, the offerer has to perform a Diffie-Hellman
operation.</t>
<t hangText="ZRTP"><vspace blankLines="0" />The offerer has no
additional computational expense at all, as the offer contains
no information about ZRTP or might contain "a=zrtp".</t>
<t hangText="EKT"><vspace blankLines="0" />The offerer's
Computational expense depends entirely on the EKT
bootstrapping mechanism selected (one or more MIKEY modes or
Security Descriptions).</t>
<t hangText="DTLS-SRTP"><vspace blankLines="0" />The offerer
has no additional computational expense at all, as the offer
contains only a fingerprint of the certificate that will be
presented in the DTLS exchange.</t>
<t hangText="MIKEYv2 Inband"><vspace blankLines="0" />The
behavior will depend on which mode is picked.</t>
</list></t>
</list></t>
</section>
</section>
<section anchor="ofs" title="Out-of-Scope">
<t>Discussions concluded that key management for shared-key encryption
of conferencing is outside the scope of this document. As the priority
is point-to-point unicast SRTP session keying, resolving shared-key SRTP
session keying is deferred to later and left as an item for future
investigations.</t>
</section>
</back>
</rfc>| PAFTECH AB 2003-2026 | 2026-04-24 04:37:29 |