[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]
Versions: (draft-wing-srtp-keying-eval) 00 01
02
draft-ietf-sip-media-security-requirements
Network Working Group F. Audet
Internet-Draft Nortel
Intended status: Informational D. Wing
Expires: August 3, 2007 Cisco Systems
January 30, 2007
Evaluation of SRTP Keying with SIP
draft-wing-rtpsec-keying-eval-02
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on August 3, 2007.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
Over a dozen incompatible mechanisms have been defined to key an
Secure RTP (SRTP) media stream. This document evaluates the keying
mechanisms, concentrating on their interaction with SIP features and
their security properties.
This document is discussed on the rtpsec mailing list,
<http://www.imc.org/ietf-rtpsec>.
Audet & Wing Expires August 3, 2007 [Page 1]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of Keying Mechanisms . . . . . . . . . . . . . . . . 4
3.1. Signaling Path Keying Techniques . . . . . . . . . . . . . 5
3.1.1. MIKEY-NULL . . . . . . . . . . . . . . . . . . . . . . 5
3.1.2. MIKEY-PSK . . . . . . . . . . . . . . . . . . . . . . 6
3.1.3. MIKEY-RSA . . . . . . . . . . . . . . . . . . . . . . 6
3.1.4. MIKEY-RSA-R . . . . . . . . . . . . . . . . . . . . . 6
3.1.5. MIKEY-DHSIGN . . . . . . . . . . . . . . . . . . . . . 6
3.1.6. MIKEY-DHHMAC . . . . . . . . . . . . . . . . . . . . . 7
3.1.7. MIKEY-ECIES and MIKEY-ECMQV (MIKEY-ECC) . . . . . . . 7
3.1.8. Security Descriptions with SIPS . . . . . . . . . . . 7
3.1.9. Security Descriptions with S/MIME . . . . . . . . . . 7
3.1.10. SDP-DH . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1.11. MIKEYv2 in SDP . . . . . . . . . . . . . . . . . . . . 8
3.2. Media Path Keying Technique . . . . . . . . . . . . . . . 8
3.2.1. ZRTP . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3. Signaling and Media Path Keying Techniques . . . . . . . . 9
3.3.1. EKT . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3.2. DTLS-SRTP . . . . . . . . . . . . . . . . . . . . . . 9
3.3.3. MIKEYv2 Inband . . . . . . . . . . . . . . . . . . . . 9
4. Evaluation Criteria - SIP . . . . . . . . . . . . . . . . . . 10
4.1. Secure Retargeting and Secure Forking . . . . . . . . . . 10
4.2. Clipping Media Before SDP Answer . . . . . . . . . . . . . 15
4.3. Centralized Keying . . . . . . . . . . . . . . . . . . . . 17
4.4. SSRC and ROC . . . . . . . . . . . . . . . . . . . . . . . 20
5. Evaluation Criteria - Security . . . . . . . . . . . . . . . . 22
5.1. Public Key Infrastructure . . . . . . . . . . . . . . . . 22
5.2. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . 24
5.3. Best Effort Encryption . . . . . . . . . . . . . . . . . . 25
5.4. Upgrading Algorithms . . . . . . . . . . . . . . . . . . . 28
6. Security Considerations . . . . . . . . . . . . . . . . . . . 29
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
9.1. Normative References . . . . . . . . . . . . . . . . . . . 30
9.2. Informational References . . . . . . . . . . . . . . . . . 30
Appendix A. Changelog . . . . . . . . . . . . . . . . . . . . . . 33
A.1. Changes from -01 to -02 . . . . . . . . . . . . . . . . . 33
A.2. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34
Intellectual Property and Copyright Statements . . . . . . . . . . 35
Audet & Wing Expires August 3, 2007 [Page 2]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
1. Introduction
SIP needs to operate across the world-wide public Internet and thus
needs a single, mandatory-to-implement mechanism for strongly
authenticating an endpoint. It is likely that the mechanism will be
based on RSA, Diffie-Hellman, or Digital Signature Standard (DSS) but
cannot rely on an X.509 PKI or pre-shared keys.
There are currently 13 mechanisms defined or under consideration by
the IETF to establish SRTP [RFC3711] keys between endpoints.
Although an endpoint can implement several mechanisms, these 13
mechanisms are not interoperable with each other. The mechanisms can
be broken into three general categories for exchanging SRTP keying:
exchanging keys in signaling, media, or both.
The goals of an SRTP key exchange mechanism are, in rough order:
1. Ability to deploy the mechanism across administrative boundaries,
such as on the Internet,
2. Cryptographically authenticate the endpoints,
3. Securely exchange SRTP keys,
4. Support SIP features such as retargeting and forking.
Existing key exchange mechanisms fail to meet all of these
requirements.
Two mechanisms, MIKEY and Security Descriptions, have been
standardized for SRTP key exchange. Both of these mechanisms perform
key exchange in the signaling path (SIP or RTSP).
All MIKEY modes share a common syntax (a=key-mgmt, defined in Key
Management Extensions for Session Description Protocol (SDP) and Real
Time Streaming Protocol (RTSP) [RFC4567]). The base MIKEY
specification [RFC3830] defines four MIKEY modes and additional modes
are defined in other specifications. MIKEY modes are not compatible
with each other.
The other standard mechanism, Security Descriptions, uses a different
syntax (a=crypto, defined in Security Descriptions [RFC4568]).
Several extensions to MIKEY have been proposed and several techniques
which perform some, or all, keying in the media path have been
proposed. These new techniques are also discussed in this document.
Out of scope of this document is how SIP, RTSP, and SDP messages
Audet & Wing Expires August 3, 2007 [Page 3]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
themselves are encrypted.
Call signaling (new call, end of call, call transfer, etc.) is done
in SIP, and media is sent in RTP. In the following diagram, Alice is
calling Bob. This causes Alice to emit a SIP message to her SIP
proxy, which processes the message and routes the message to Bob's
proxy which then routes it to Bob.
+---------+ SIP Invite +-------|
| Alice's +------------------>+ Bob's |
| proxy | | proxy |
+----+----+ +---+---|
^ |
SIP Invite | | SIP Invite
| V
+---+---+ +-----+
| Alice |<===================>+ Bob |
+-------+ SRTP +-----+
Figure 1: Simplified SIP Model
2. Terminology
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.
SSRC: The 32-bit value that defines the synchronization source,
used in RTP. These are generally unique, but collisions can
occur.
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.
PKI Public Key Infrastructure. Throughout this paper, the term PKI
refers to a global PKI.
3. Overview of Keying Mechanisms
Based on how the SRTP keys are exchanged, each SRTP key exchange
mechanism belongs to one general category:
Audet & Wing Expires August 3, 2007 [Page 4]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
signaling path: All the keying is carried in the call signaling
(SIP or SDP) path.
media path: All the keying is carried in the SRTP/SRTCP media
path, and no signaling whatsoever is carried in the call
signaling path.
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.
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.
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.
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.
3.1. Signaling Path Keying Techniques
3.1.1. MIKEY-NULL
MIKEY-NULL [RFC3830] 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-
Audet & Wing Expires August 3, 2007 [Page 5]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
end (e.g., by using S/MIME).
MIKEY-NULL requires one message from offerer to answerer (half a
round trip), and does not add additional media path messages.
3.1.2. MIKEY-PSK
MIKEY-PSK (pre-shared key) [RFC3830] 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.
MIKEY-PSK requires one message from offerer to answerer (half a round
trip), and does not add additional media path messages.
3.1.3. MIKEY-RSA
MIKEY-RSA [RFC3830] has the offerer encrypt the keys for both
directions using the intended answerer's public key, which is
obtained from a PKI.
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.
3.1.4. MIKEY-RSA-R
MIKEY-RSA-R An additional mode of key distribution in MIKEY: MIKEY-
RSA-R [RFC4738] 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.
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.
3.1.5. MIKEY-DHSIGN
In MIKEY-DHSIGN [RFC3830] 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.
MIKEY-DHSIGN requires one message from offerer to answerer, and one
message from answerer to offerer (full round trip), and does not add
Audet & Wing Expires August 3, 2007 [Page 6]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
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.
3.1.6. MIKEY-DHHMAC
MIKEY-DHHMAC [RFC4650] 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.
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.
3.1.7. MIKEY-ECIES and MIKEY-ECMQV (MIKEY-ECC)
ECC Algorithms For MIKEY [I-D.ietf-msec-mikey-ecc] 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) .
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.
3.1.8. Security Descriptions with SIPS
Security Descriptions [RFC4568] 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.
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.
3.1.9. Security Descriptions with S/MIME
This keying mechanism is identical to Section 3.1.8, except that
rather than protecting the signaling with TLS, the entire SDP is
encrypted with S/MIME.
Audet & Wing Expires August 3, 2007 [Page 7]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
3.1.10. SDP-DH
SDP Diffie-Hellman [I-D.baugher-mmusic-sdp-dh] 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 SIP-Identity
[RFC4474] and [I-D.ietf-sip-connected-identity].
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.
3.1.11. MIKEYv2 in SDP
MIKEYv2 [I-D.dondeti-msec-rtpsec-mikeyv2] 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.
Furthemore, MIKEYv2 also defines an in-band negotiation mode as an
alternative to SDP (see Section 3.3.3).
3.2. Media Path Keying Technique
3.2.1. ZRTP
ZRTP [I-D.zimmermann-avt-zrtp] 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.
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.
Audet & Wing Expires August 3, 2007 [Page 8]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
3.3. Signaling and Media Path Keying Techniques
3.3.1. EKT
EKT [I-D.mcgrew-srtp-ekt] 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.
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.
3.3.2. DTLS-SRTP
DTLS-SRTP [I-D.mcgrew-tls-srtp] exchanges public key fingerprints in
SDP [I-D.fischl-sipping-media-dtls] 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.
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.
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 TLS
[RFC4346].
3.3.3. MIKEYv2 Inband
As defined in Section 3.1.11, MIKEYv2 also defines an in-band
negotiation mode as an alternative to SDP (see Section 3.3.3). The
details are not sorted out in the draft yet on what in-band actually
means (i.e., UDP, RTP, RTCP, etc.).
Audet & Wing Expires August 3, 2007 [Page 9]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
4. Evaluation Criteria - SIP
This section considers how each keying mechanism interacts with SIP
features.
4.1. Secure Retargeting and Secure Forking
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 the figure below, Alice sends an Invite in
step 1 which 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) which sends the original Invite to
Carol in step 4.
+-----+
|Alice|
+--+--+
|
| Invite (1)
V
+----+----+
| proxy |
++-+-----++
| ^ |
Invite (2) | | | Invite (4)
& redirect (3) | | |
V | V
++-++ ++----+
|Bob| |Carol|
+---+ +-----+
Figure 2: Retargeting
Successful use of SRTP requires strongly identifying both calling
party and the called party. The mechanism used by SIP for
identifying the calling party is SIP Identity [RFC4474]. However,
due to SIP retargeting issues [I-D.peterson-sipping-retarget], 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, it is
anticipated that Connected Identity [I-D.ietf-sip-connected-identity]
may allow identifying the called party. In the list below, this is
described as the 'retargeting identity' problem.
Audet & Wing Expires August 3, 2007 [Page 10]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
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.
+-----+
|Alice|
+--+--+
|
| Invite
V
+-----+-----+
| proxy |
++---------++
| |
Invite | | Invite
V V
+--+--+ +--+--+
|Bob-1| |Bob-2|
+-----+ +-----+
Figure 3: Forking
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 [I-D.ietf-mmusic-ice], and ICE is useful to make
this association with RTP, it isn't desirable to require ICE to
accomplish this association. The table below analyzes if it is
possible for an offerer to associate the media stream with each SDP
answer, without using ICE.
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.
To maintain media security, only the endpoint that answers the call
should know the SRTP keys for the session. For key exchange
mechanisms that don't provide secure forking or secure retargeting,
one workaround is to rekey immediately after forking or retargeting.
However, because the originator may not be aware that the call forked
this mechanism requires rekeying immediately after every session is
established which causes additional signaling messages.
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
Audet & Wing Expires August 3, 2007 [Page 11]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
as the session key or the SDP) using the original recipient's public
key, the recipient will not be able to decrypt that material because
the actual 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).
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.
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-end's credentials, it is very
possible that the rejection will never reach the sender. This
problem, called the Heterogeneous Error Response Forking Problem
(HERFP) [I-D.mahy-sipping-herfp-fix] is a complicated problem to
solve in SIP.
The following list compares the behavior of secure forking, answering
association, two-time pads, and secure retargeting for each keying
mechanism.
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.
Secure Retargeting: No, all targets see offerer's and
answerer's keys. Suffers from retargeting identity problem.
MIKEY-PSK
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.
Secure Retargeting: Not secure. For retargeting to work, the
Audet & Wing Expires August 3, 2007 [Page 12]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
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.
MIKEY-RSA
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.
Secure Retargeting: No.
MIKEY-RSA-R
Secure Forking: Yes. Answer is associated with media by the
SSRC in MIKEY.
Secure Retargeting: Yes.
MIKEY-DHSIGN
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.
Secure Retargeting: Yes, each target negotiates unique keys
with the offerer for both directions.
MIKEYv2 in SDP
The behavior will depend on which mode is picked.
MIKEY-DHHMAC
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.
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.
Security Descriptions with SIPS
Secure Forking: No. Each forked endpoint sees the offerer's
key. Answer is not associated with media.
Audet & Wing Expires August 3, 2007 [Page 13]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
Secure Retargeting: No. Each target sees the offerer's key.
Security Descriptions with S/MIME
Secure Forking: No. Each forked endpoint sees the offerer's
key. Answer is not associated with media.
Secure Retargeting: No. Each target sees the offerer's key.
Suffers from retargeting identity problem.
SDP-DH
Secure Forking: Yes. Each forked endpoint calculates a unique
SRTP key. Answer is not associated with media.
Secure Retargeting: Yes. The final target calculates a unique
SRTP key.
ZRTP
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.
Secure Retargeting: Yes. The final target calculates a unique
SRTP key.
EKT
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.
Secure Retargeting: Inherited from the bootstrapping mechanism
(the specific MIKEY mode or Security Descriptions).
DTLS-SRTP
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.
Secure Retargeting: Yes. The final target calculates a unique
SRTP key.
MIKEYv2 Inband
The behavior will depend on which mode is picked.
Audet & Wing Expires August 3, 2007 [Page 14]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
4.2. Clipping Media Before SDP Answer
Per the SDP Offer/Answer Model [RFC3264],
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).
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 and violating the above MUST requirement.s
For key exchange mechanisms which send the answerer's key in SDP, a
SIP provisional response [RFC3261] such as 183 (session progress) is
useful. However the 183 messages aren't reliable unless both the
calling and called endpoint support PRACK [RFC3262], use TCP across
all SIP proxies, implement Security Preconditions
[I-D.ietf-mmusic-securityprecondition], or the both ends implement
ICE [I-D.ietf-mmusic-ice] and the answerer implements the reliable
provisional response mechanism described in ICE. However, there is
not wide deployment of any of these techniques and there is industry
reluctance to requiring these techniques as solutions to avoid the
problem described in this section.
Furthermore, the problem 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.
The following list compares the behavior of clipping before SDP
answer for each keying mechanism.
MIKEY-NULL
Not clipped. The offerer provides the answerer's keys.
MIKEY-PSK
Not clipped. The offerer provides the answerer's keys.
Audet & Wing Expires August 3, 2007 [Page 15]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
MIKEY-RSA
Not clipped. The offerer provides the answerer's keys.
MIKEY-RSA-R
Clipped. The answer contains the answerer's encryption key.
MIKEY-DHSIGN
Clipped. The answer contains the answerer's Diffie-Hellman
response.
MIKEY-DHHMAC
Clipped. The answer contains the answerer's Diffie-Hellman
response.
MIKEYv2 in SDP
The behavior will depend on which mode is picked.
Security Descriptions with SIPS
Clipped. The answer contains the answerer's encryption key.
Security Descriptions with S/MIME
Clipped. The answer contains the answerer's encryption key.
SDP-DH
Clipped. The answer contains the answerer's Diffie-Hellman
response.
ZRTP
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.
EKT
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.
Note: RTCP needs to work, in the answerer-to-offerer
direction, before the offerer can decrypt SRTP media.
DTLS-SRTP
Not clipped. Keys are exchanged in the media path without
relying on the signaling path.
Audet & Wing Expires August 3, 2007 [Page 16]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
MIKEYv2 Inband
Not clipped. Keys are exchanged in the media path without
relying on the signaling path.
4.3. Centralized Keying
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.
An important use case for mixers/translators is a conference bridge:
+----+
A --- 1 --->| |
<-- 2 ----| M |
| I |
B --- 3 --->| X |
<-- 4 ----| E |
| R |
C --- 5 --->| |
<-- 6 ----| |
+----+
Figure 4: Centralized Keying
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.
Several scenarios are possible:
a. Multiple inbound sessions: 1, 3, and 5 are distinct RTP
sessions,
b. Multiple outbound sessions: 2, 4, and 6 are distinct RTP
sessions,
c. Single inbound session: 1, 3, and 5 are just different sources
within the same RTP session,
d. Single outbound session: 2, 4, and 6 are different flows of the
same (multi-unicast) RTP session
Audet & Wing Expires August 3, 2007 [Page 17]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
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 endpoint 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.
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.
The following list describes how each keying mechanism behaves with
centralized keying (scenario d) and rekeying.
MIKEY-NULL
Keying: Yes, if offerer is the mixer. No, if offerer is the
participant (end user).
Rekeying: Yes, via re-Invite
MIKEY-PSK
Keying: Yes, if offerer is the mixer. No, if offerer is the
participant (end user).
Rekeying: Yes, with a re-Invite
MIKEY-RSA
Keying: Yes, if offerer is the mixer. No, if offerer is the
participant (end user).
Rekeying: Yes, with a re-Invite
Audet & Wing Expires August 3, 2007 [Page 18]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
MIKEY-RSA-R
Keying: No, if offerer is the mixer. Yes, if offerer is the
participant (end user).
Rekeying: n/a
MIKEY-DHSIGN
Keying: No; a group-key Diffie-Hellman protocol is not
supported.
Rekeying: n/a
MIKEY-DHHMAC
Keying: No; a group-key Diffie-Hellman protocol is not
supported.
Rekeying: n/a
MIKEYv2 in SDP
The behavior will depend on which mode is picked.
Security Descriptions with SIPS
Keying: Yes, if offerer is the mixer. Yes, if offerer is the
participant.
Rekeying: Yes, with a Re-Invite.
Security Descriptions with S/MIME
Keying: Yes, if offerer is the mixer. Yes, if offerer is the
participant.
Rekeying: Yes, with a Re-Invite.
SDP-DH
Keying: No; a group-key Diffie-Hellman protocol is not
supported.
Rekeying: n/a
ZRTP
Keying: No; a group-key Diffie-Hellman protocol is not
supported.
Rekeying: n/a
Audet & Wing Expires August 3, 2007 [Page 19]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
EKT
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.
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.
DTLS-SRTP
Keying: Yes, because with the assumed cipher suite,
TLS_RSA_WITH_3DES_EDE_CBC_SHA, each end indicates its SRTP key.
Rekeying: via DTLS in the media path.
MIKEYv2 Inband
The behavior will depend on which mode is picked.
4.4. SSRC and ROC
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.
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.
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.
Some keying mechanisms cause a two-time pad to occur if two endpoints
of a forked call have an SSRC collision.
Audet & Wing Expires August 3, 2007 [Page 20]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
Note: A proposal has been made to send the ROC value on every Nth
SRTP packet[RFC4771]. This proposal has not yet been incorporated
into this document.
The following list examines handling of SSRC and ROC:
MIKEY-NULL
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
MIKEY-PSK
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
MIKEY-RSA
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
MIKEY-RSA-R
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
MIKEY-DHSIGN
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
MIKEY-DHHMAC
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
MIKEYv2 in SDP
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
Security Descriptions with SIPS
Neither SSRC nor ROC are signaled. SSRC 'late binding' is
used.
Security Descriptions with S/MIME
Neither SSRC nor ROC are signaled. SSRC 'late binding' is
used.
SDP-DH
Neither SSRC nor ROC are signaled. SSRC 'late binding' is
used.
Audet & Wing Expires August 3, 2007 [Page 21]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
ZRTP
Neither SSRC nor ROC are signaled. SSRC 'late binding' is
used.
EKT
The SSRC of the SRTCP packet containing an EKT update
corresponds to the SRTP master key and other parameters within
that packet.
DTLS-SRTP
Neither SSRC nor ROC are signaled. SSRC 'late binding' is
used.
MIKEYv2 Inband
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
5. Evaluation Criteria - Security
This section evaluates each keying mechanism on the basis of their
security properties.
5.1. Public Key Infrastructure
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".
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 sip-certs [I-D.ietf-sip-certs] might
be used to obtain the certificate of a peer.
Note: Even if Sip-certs was deployed, the retargeting problem
(Section 4.1) would still prevent successful deployment of keying
techniques which require the offerer to obtain the actual target's
public key.
The following list compares the PKI requirements of each keying
mechanism, both if a PKI is required for the key exchange itself, and
Audet & Wing Expires August 3, 2007 [Page 22]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
if PKI is only used to authenticate the certificate supplied in
signaling.
MIKEY-NULL
PKI not used.
MIKEY-PSK
PKI not used; rather, all endpoints must have some way to
exchange per-endpoint or per-system pre-shared keys.
MIKEY-RSA
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 [I-D.ietf-sip-certs]
might be viable in the future.
MIKEY-RSA-R
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.
MIKEY-DHSIGN
PKI is used to authenticate the public key that is included in
the MIKEY message, by walking the CA trust chain.
MIKEY-DHHMAC
PKI not used; rather, all endpoints must have some way to
exchange per-endpoint or per-system pre-shared keys.
MIKEYv2 in SDP
The behavior will depend on which mode is picked.
Security Descriptions with SIPS
PKI not used.
Security Descriptions with S/MIME
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.
Audet & Wing Expires August 3, 2007 [Page 23]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
SDP-DH
PKI not used.
ZRTP
PKI not used.
EKT
PKI not used by EKT itself, but might be used by the EKT
bootstrapping keying mechanism (such as certain MIKEY modes).
DTLS-SRTP
Remote party's certificate is sent in media path, and a
fingerprint of the same certificate is sent in the signaling
path.
MIKEYv2 Inband
The behavior will depend on which mode is picked.
5.2. Perfect Forward Secrecy
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.
The following list describes how each key exchange mechanism provides
PFS.
MIKEY-NULL
No PFS.
MIKEY-PSK
No PFS.
MIKEY-RSA
No PFS.
MIKEY-RSA-R
No PFS.
Audet & Wing Expires August 3, 2007 [Page 24]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
MIKEY-DHSIGN
PFS is provided with the Diffie-Hellman exchange.
MIKEY-DHHMAC
PFS is provided with the Diffie-Hellman exchange.
MIKEYv2 in SDP
The behavior will depend on which mode is picked.
Security Descriptions with SIPS
No PFS.
Security Descriptions with S/MIME
No PFS.
SDP-DH
PFS is provided with the Diffie-Hellman exchange.
ZRTP
PFS is provided with the Diffie-Hellman exchange.
EKT
No PFS.
DTLS-SRTP
PFS is achieved if the negotiated cipher suite includes an
exponential or discrete-logarithmic key exchange (such as
Diffie-Hellman or Elliptic Curve Diffie-Hellman [RFC4492]).
MIKEYv2 Inband
The behavior will depend on which mode is picked.
5.3. Best Effort Encryption
Note: With the ongoing efforts in SDP Capability Negotiation
[I-D.ietf-mmusic-sdp-capability-negotiation], the conclusions
reached in this section may no longer be accurate.
With best effort encryption, SRTP is used with endpoints that support
SRTP, otherwise RTP is used.
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.
Audet & Wing Expires August 3, 2007 [Page 25]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
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.
Currently, several techniques are commonly considered as candidates
to provide opportunistic encryption:
multipart/alternative
[I-D.jennings-sipping-multipart] 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 Heterogeneous Error
Response Forking Problem (HERFP) [I-D.mahy-sipping-herfp-fix]
causes problems if such non-multipart-capable endpoints were
involved in the forking.
SDP Grouping
A new SDP grouping mechanism (following the idea introduced in
[RFC3388]) 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.
session attribute
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 SRTP [RFC3711].
Probing
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.
The following list compares the availability of best effort
encryption for each keying mechanism.
Audet & Wing Expires August 3, 2007 [Page 26]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
MIKEY-NULL
No best effort encryption.
MIKEY-PSK
No best effort encryption.
MIKEY-RSA
No best effort encryption.
MIKEY-RSA-R
No best effort encryption.
MIKEY-DHSIGN
No best effort encryption.
MIKEY-DHHMAC
No best effort encryption.
MIKEYv2 in SDP
No best effort encryption.
Security Descriptions with SIPS
No best effort encryption.
Security Descriptions with S/MIME
No best effort encryption.
SDP-DH
No best effort encryption.
ZRTP
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 [I-D.zimmermann-avt-zrtp]). Current
implementations of ZRTP use probing.
EKT
No best effort encryption.
DTLS-SRTP
No best effort encryption.
MIKEY Inband
No best effort encryption.
Audet & Wing Expires August 3, 2007 [Page 27]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
5.4. Upgrading Algorithms
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.
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.
The following list describes the CPU effort involved in using each
key exchange technique.
MIKEY-NULL
No significant computaional expense.
MIKEY-PSK
No significant computational expense.
MIKEY-RSA
For each offered SRTP crypto suite, the offerer has to perform
RSA operation to encrypt the TGK
MIKEY-RSA-R
For each offered SRTP crypto suite, the offerer has to perform
public key operation to sign the MIKEY message.
MIKEY-DHSIGN
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.
MIKEY-DHHMAC
For each offered SRTP crypto suite, the offerer has to perform
Diffie-Hellman operation.
Audet & Wing Expires August 3, 2007 [Page 28]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
MIKEYv2 in SDP
The behavior will depend on which mode is picked.
Security Descriptions with SIPS
No significant computational expense.
Security Descriptions with S/MIME
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.
SDP-DH
For each offered SRTP crypto suite, the offerer has to perform
a Diffie-Hellman operation.
ZRTP
The offerer has no additional computational expense at all, as
the offer contains no information about ZRTP or might contain
"a=zrtp".
EKT
The offerer's Computational expense depends entirely on the EKT
bootstrapping mechanism selected (one or more MIKEY modes or
Security Descriptions).
DTLS-SRTP
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.
MIKEYv2 Inband
The behavior will depend on which mode is picked.
6. Security Considerations
This entire document discusses security.
7. Acknowledgements
Special thanks to Steffen Fries and Dragan Ignjatic for their
excellent MIKEY comparison document
[I-D.ietf-msec-mikey-applicability].
Thanks also to Cullen Jennings, David Oran, David McGrew, Mark
Baugher, Flemming Andreasen, Eric Raymond, Dave Ward, Leo Huang, Eric
Audet & Wing Expires August 3, 2007 [Page 29]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
Rescorla, Lakshminath Dondeti, Steffen Fries, Alan Johnston, Dragan
Ignjatic and John Elwell for their assistance with this document.
8. IANA Considerations
This document does not add new IANA registrations.
9. References
9.1. Normative References
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
9.2. Informational References
[RFC4567] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
Carrara, "Key Management Extensions for Session
Description Protocol (SDP) and Real Time Streaming
Protocol (RTSP)", RFC 4567, July 2006.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.
[I-D.ietf-mmusic-securityprecondition]
Andreasen, F. and D. Wing, "Security Preconditions for
Session Description Protocol (SDP) Media Streams",
draft-ietf-mmusic-securityprecondition-03 (work in
progress), October 2006.
[RFC4650] Euchner, M., "HMAC-Authenticated Diffie-Hellman for
Multimedia Internet KEYing (MIKEY)", RFC 4650,
September 2006.
[I-D.ietf-msec-mikey-ecc]
Audet & Wing Expires August 3, 2007 [Page 30]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
Milne, A., "ECC Algorithms for MIKEY",
draft-ietf-msec-mikey-ecc-01 (work in progress),
October 2006.
[RFC4738] Ignjatic, D., Dondeti, L., Audet, F., and P. Lin, "MIKEY-
RSA-R: An Additional Mode of Key Distribution in
Multimedia Internet KEYing (MIKEY)", RFC 4738,
November 2006.
[I-D.ietf-sip-certs]
Jennings, C., "Certificate Management Service for The
Session Initiation Protocol (SIP)",
draft-ietf-sip-certs-02 (work in progress), October 2006.
[I-D.mahy-sipping-herfp-fix]
Mahy, R., "A Solution to the Heterogeneous Error Response
Forking Problem (HERFP) in the Session Initiation
Protocol (SIP)", draft-mahy-sipping-herfp-fix-01 (work in
progress), March 2006.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, June 2002.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, May 2006.
[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[I-D.fischl-sipping-media-dtls]
Fischl, J., "Datagram Transport Layer Security (DTLS)
Protocol for Protection of Media Traffic Established with
the Session Initiation Protocol",
draft-fischl-sipping-media-dtls-01 (work in progress),
Audet & Wing Expires August 3, 2007 [Page 31]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
June 2006.
[I-D.ietf-msec-mikey-applicability]
Fries, S. and D. Ignjatic, "On the applicability of
various MIKEY modes and extensions",
draft-ietf-msec-mikey-applicability-03 (work in progress),
December 2006.
[I-D.zimmermann-avt-zrtp]
Zimmermann, P., "ZRTP: Extensions to RTP for Diffie-
Hellman Key Agreement for SRTP",
draft-zimmermann-avt-zrtp-02 (work in progress),
October 2006.
[I-D.baugher-mmusic-sdp-dh]
Baugher, M. and D. McGrew, "Diffie-Hellman Exchanges for
Multimedia Sessions", draft-baugher-mmusic-sdp-dh-00 (work
in progress), February 2006.
[I-D.mcgrew-srtp-ekt]
McGrew, D., "Encrypted Key Transport for Secure RTP",
draft-mcgrew-srtp-ekt-01 (work in progress), June 2006.
[RFC4771] Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
Transform Carrying Roll-Over Counter for the Secure Real-
time Transport Protocol (SRTP)", RFC 4771, January 2007.
[I-D.peterson-sipping-retarget]
Peterson, J., "Retargeting and Security in SIP: A
Framework and Requirements",
draft-peterson-sipping-retarget-00 (work in progress),
February 2005.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Methodology for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-13 (work in progress), January 2007.
[I-D.ietf-sip-connected-identity]
Elwell, J., "Connected Identity in the Session Initiation
Protocol (SIP)", draft-ietf-sip-connected-identity-04
(work in progress), January 2007.
[I-D.jennings-sipping-multipart]
Wing, D. and C. Jennings, "Session Initiation Protocol
(SIP) Offer/Answer with Multipart Alternative",
draft-jennings-sipping-multipart-02 (work in progress),
Audet & Wing Expires August 3, 2007 [Page 32]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
March 2006.
[I-D.mcgrew-tls-srtp]
Rescorla, E. and D. McGrew, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for Secure
Real-time Transport Protocol (SRTP)",
draft-mcgrew-tls-srtp-00 (work in progress), June 2006.
[I-D.dondeti-msec-rtpsec-mikeyv2]
Dondeti, L., "MIKEYv2: SRTP Key Management using MIKEY,
revisited", draft-dondeti-msec-rtpsec-mikeyv2-00 (work in
progress), June 2006.
[I-D.ietf-mmusic-sdp-capability-negotiation]
Andreasen, F., "SDP Capability Negotiation",
draft-ietf-mmusic-sdp-capability-negotiation-01 (work in
progress), January 2007.
Appendix A. Changelog
Note to RFC Editor: this appendix should be removed prior to
publication.
A.1. Changes from -01 to -02
o Removed DTLS-RTP
o Added note about SDP Capability Negotiation and its impact on
best-effort SRTP
A.2. Changes from -00 to -01
o Added MIKEYv2 as part of the main proposals.
o Removed retargeting as a problem for best-effort encryption's
multipart/alternative
o "Opportunistic encryption" to "best-effort encryption"
o Added 'Upgrading Algorithm' section
o Separate analysis of 'Security Descriptions with SIPS' and
'Security Descriptions with S/MIME'
Audet & Wing Expires August 3, 2007 [Page 33]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
Authors' Addresses
Francois Audet
Nortel
4655 Great America Parkway
Santa Clara, CA 95054
USA
Email: audet@nortel.com
Dan Wing
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
Email: dwing@cisco.com
Audet & Wing Expires August 3, 2007 [Page 34]
Internet-Draft Evaluation of SRTP Keying with SIP January 2007
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
Audet & Wing Expires August 3, 2007 [Page 35]
Html markup produced by rfcmarkup 1.129c, available from
https://tools.ietf.org/tools/rfcmarkup/