Network Working Group D. Wing
Internet-Draft Cisco Systems
Intended status: Standards Track H. Kaplan
Expires: May 18, 2008 Acme Packet
November 15, 2007
SIP Identity using Media Path
draft-wing-sip-identity-media-01
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
The existing SIP identity mechanism (RFC4474) creates a signature
over the SIP body, including the entire SDP. As part of their normal
operation, Session Border Controllers (SBCs) and SIP Back-to-Back
User Agents (B2BUAs) modify various fields in the SDP which breaks
that signature.
This document defines a new identity mechanism which operates with
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SBCs and B2BUAs. This new identity mechanism creates a signature
over certain SIP headers and certain SDP lines, and uses both SIP
signaling and the media path to perform its identity function.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. Analysis of SIP-Identity with SBCs . . . . . . . . . . . . . . 4
3.1. SIP-Identity with SBCs . . . . . . . . . . . . . . . . . . 4
3.1.1. Validate the Signature . . . . . . . . . . . . . . . . 5
3.1.2. Destroy Existing Signature . . . . . . . . . . . . . . 6
3.1.3. Create New Identity . . . . . . . . . . . . . . . . . 6
3.1.4. Sign the New Identity . . . . . . . . . . . . . . . . 7
3.2. Transport Address as Identifier . . . . . . . . . . . . . 7
4. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Identity Media Signature . . . . . . . . . . . . . . . . . 10
4.2. Authentication Service . . . . . . . . . . . . . . . . . . 11
4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Proof of Identity Techniques . . . . . . . . . . . . . . . . . 12
5.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. DTLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.2.1. SRTP after DTLS optional . . . . . . . . . . . . . . . 13
5.3. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.3.1. ICE Public Key SDP Attribute . . . . . . . . . . . . . 13
5.3.2. New STUN attributes . . . . . . . . . . . . . . . . . 14
5.4. HIP . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. ABNF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7.1. Device Disclosure . . . . . . . . . . . . . . . . . . . . 15
8. Operational Differences from RFC4474 . . . . . . . . . . . . . 16
9. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 17
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. DTLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.2. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.3. Request without SDP . . . . . . . . . . . . . . . . . . . 20
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
13.1. Normative References . . . . . . . . . . . . . . . . . . . 21
13.2. Informational References . . . . . . . . . . . . . . . . . 22
Appendix A. ToDo List . . . . . . . . . . . . . . . . . . . . . . 23
Appendix B. Changes From Previous Versions . . . . . . . . . . . 23
B.1. Changes from 00 to 01 . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
Intellectual Property and Copyright Statements . . . . . . . . . . 25
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1. Introduction
SIP Identity [RFC4474] provides cryptographic identity for SIP
requests. It provides this protection by signing certain SIP header
fields (Contact, Date, Call-ID, CSeq, To, and From) and the SIP
message body. The SIP message body typically contains the SDP.
In cases where the originating, authenticating domain directly
connect to the terminating, validating domain, RFC4474 has
questionable value. The reason is that in such cases TLS can simply
be used for the communication between the edge proxies of each
domain, which not only provides additional security properties (e.g.,
encryption), but is also more efficient and protects all signaling
messages. The real value of RFC4474 lies in cases where SIP
signaling crosses multiple domains, belonging to different
organizations. Unfortunately, in the presence of SBCs or B2BUAs that
are in a different trust domain than the calling party or called
party, SIP Identity provides the same security properties as using
P-Asserted-Identity [RFC3325] between those same trust domains, if it
succeeds at all. In most cases it would probably fail, and force the
UAC to re-send its request without any Identity mechanism if it
wanted to establish communication.
The mechanism described in this document provides cryptographic
assurance of the endpoint's identity by signing certain SIP headers,
much like RFC4474. However, unlike RFC4474 which signs the entire
SDP, the mechanism described in this document signs only certain SDP
attributes, and not all the same SIP headers. The remote endpoint is
expected to validate the signature over the SIP headers and specified
SDP attributes, to initiate a proof of possession test over the media
path, which proves the session has been established with the "From:"
party in the SIP header. Mechanisms to perform this proof of
possession are shown using DTLS and using a small extension to ICE
[I-D.ietf-mmusic-ice]. This mechanism is also extensible, in order
to be usable by future mechanisms which need signed SDP attributes
Readers of this document are expected to be familiar with RFC4474,
"Enhancements for Authenticated Identity Management in the Session
Initiation Protocol (SIP)", which defines the Identity and Identity-
Info header fields. A future version of this document will have less
reliance on RFC4474.
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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3. Analysis of SIP-Identity with SBCs
This section examines how SIP Identity [RFC4474] operates with
SBCs(Section 3.1) or certain back-to-back user-agents (B2BUAs), and
how SIP Identity uses transport addresses as identifiers
(Section 3.2).
3.1. SIP-Identity with SBCs
After an authorization agent signs a SIP request, the SIP request
traverses SBC(s) operated by other trust domains and finally arrives
at the destination domain. If all of those intervening SBCs support
SIP Identity, those SBCs will validate the request's signature,
destroy the existing signature, create a new identity for the
request, and sign the request's new identity. These necessary SBC
actions (described in detail below) provide the same identity
assurance as using P-Asserted-Identity between trust domains.
The functions of SBCs, and why they do what they do, is mainly
detailed in [I-D.ietf-sipping-sbc-funcs]. Regardless of the
architectural implications of their actions, the fact is their
functions are often performed as a SIP request/response traverses
across SIP domains. Other B2BUAs in the path sometimes also perform
functions which invalidate an RFC4474 signature. With regard to
RFC4474, the SBC functions which impact the signature are:
o Replacing IP addresses in SDP body parts
o Replacing the Contact URI
o Modifying the Call-ID
o Modifying the CSeq
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The following diagram shows two service providers (SP1 and SP2), and
each has an SBC at the edge of their respective networks. Each of
these SBCs would rewrite the IP addresses in the SDP. There may be
an SBC in the Enterprise as well, however that SBC would own the
appropriate Enterprise domain certificate and re-sign the request,
and thus logically appear as the Enterprise User Agent
+---------+ +---------+ +---------+ +---------+
|SP1-SBC-1|-|SP1-SBC-2|--|SP2-SBC-1|--|SP2-SBC-2|
+---+-----+ +---------+ +---------+ +-+-------+
| |
| |
+--------------+ +------+-------+
|SIP User Agent| |SIP User Agent|
| Enterprise-A | | Enterprise-B |
+--------------+ +--------------+
Figure 1: Two Service Providers with SBCs Between Two Enterprises
Enterprise-A can populate the "From:" address in its SIP requests
using E.164 telephone numbers (e.g.,
'sip:+17005551008@example.com;user=phone') or using a SIP URI (e.g.,
'sip:john.doe@example.com'). The characteristics of each choice, as
the message traverses the SBCs operated by another administrative
domain (service providers) are described below:
3.1.1. Validate the Signature
Per RFC4474, the SBC would validate the incoming SIP request. This
requires a public key operation to be performed against the SIP
request.
[[computationally expensive. Will use TLS or IPSEC instead (doesn't
require public key operation for every SIP request), or will rely on
ACLs or a dedicated link or dedicated network.]]
This creates a natural incentive for the service providers to use
transitive trust between themselves, rather than RFC4474, due to the
computational expense of the per-call public key operations on each
SIP request. For similar reasons, there is a natural incentive for
the service providers to not even validate an enterprise's RFC4474
signature but rather to rely on a contract or rely on TLS or IPSEC to
ensure the SIP signaling originated from the enterprise. Since
service providers do not allow Enterprises to be transitive domains,
they only allow From-URI domains of the enterprise, and thus do not
need a per-request 4474-style signature from the Enterprise at the
first ingress hop.
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3.1.2. Destroy Existing Signature
SBCs and B2BUAs typically modify the media transport addresses and
thus invalidate the RFC4474 signature. This prevents downstream
systems from validating original signature.
Because the original signature is invalidated by the first SBC, no
other network (SP2 nor Enterprise-B) can validate the original
signature. This means all downstream entities (in the example above,
SP2 and Enterprise-B) are relying wholly on SP1 to validate the
signature. This creates transitive trust which is undesirable - a
single bad actor or compromised system anywhere along the path can
compromise the entire identity system. Note this does not require
malicious intent within the trust domain - a weak or mis-configured
entry point into the trust chain of service providers compromises the
entire trust chain.
3.1.3. Create New Identity
In order to generate a SIP Identity signature (in the next step), the
SBC requires the private key associated with the domain of that From:
address. Because the SBC and the initiator of the SIP request are
different entities, it is unlikely the SBC will possess the
initiating domain's private key. Thus, the SBC will have to create a
new identity -- using its domain -- for the request, if it wants to
provide RFC4474.
The new identity creates difficulties with downstream whitelists,
call routing, and reputation systems. For example, downstream
systems may sometimes see the identity +14085551212@example.com when
the request is routed through certain SBCs, and may see a different
identity, +14085551212@example.net, when the request is routed
through different SBCs. Such different routing might be done due to
network outages or for cost savings (e.g., evening rates). Due to
these different identities, the domain name no longer has much
meaning -- for E.164 telephone numbers, the user-part becomes the
entire identity. In some sense that's appropriate; if the user-part
is truly a global-scope E.164 number, then the domain portion is
essentially meaningless. It might as well have been a tel-URI,
except that making it a SIP-URI is more common and allows rfc4474 to
be used.
A SBC might receive an identity containing an E.164 phone number or
containing a SIP URI. When forming a new identity, the SBC performs
different steps for each of those cases:
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E.164 telephone numbers:
The SBC would remove the original domain name and substitute its
own domain on the right-hand side.
SIP URIs:
Unlike E.164 telephone numbers, the SBC is not able to simply
substitute its domain name for the enterprise's domain name due to
user-part name collisions. There is only one unappealing
solution: use the so-called escape-hack from email. For example,
the SBC could rewrite sip:john.doe@example.com to
sip:john.doe%example.com@sp1.example.net.
3.1.4. Sign the New Identity
Finally, the SBC will sign the new identity, using the private key
associated with the new identity. This private key is known to the
SBC (because it is the SBC's domain name). This function incurs a
public key operation for that SIP request.
3.2. Transport Address as Identifier
SIP Identity signs the SDP so that the IP address (contained in the
SDP) provides identity. From [RFC4474]:
"This mechanism also provides a signature over the bodies of SIP
requests. The most important reason for doing so is to protect
Session Description Protocol (SDP) bodies carried in SIP requests.
There is little purpose in establishing the identity of the user
that originated a SIP request if this assurance is not coupled
with a comparable assurance over the media descriptors."
RFC4474 ties an identifier (IP address) with the identity (SIP
"From:" address), which is counter to ongoing efforts in the IETF to
split identifiers from identity (e.g., [I-D.ietf-hip-base],
[I-D.farinacci-lisp], [I-D.carpenter-idloc-map-cons],
[I-D.whittle-ivip-arch]).
The presence of media relays (e.g., TURN [I-D.ietf-behave-turn]),
which may be used by an attacker, means that relying exclusively on
such identifiers is risky. For example, if an attacker were able to
cause re-use of the (signed) transport address within the replay
window, the attacker can successfully impersonate the identity.
Additionally, RFC4474's tying of identities to IP address causes a
failure in certain NAT scenarios when the source transport address of
arriving media does not agree with the SDP. While not written down
in a standard at this time, if both endpoints are using ICE
[I-D.ietf-mmusic-ice], the ICE username and password (sent in SDP and
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signed by RFC4474) can be reliably used to establish identity.
However, if either endpoint does not support ICE, the arriving media
will be considered fraudulent because the arriving media does not
match with the RFC4474-signed SDP.
4. Operation
The operation of SIP-Identity-Media is similar to RFC4474 and uses
authentication service proxies much like RFC4474. The basic steps
are:
o A new header, Identity-Media, is created containing the names of
certain SDP attributes from SDP bodyparts, and containing a hash
of non-SDP bodyparts.
o Several SIP headers and the Identity-Media header are all signed
(as detailed in Section 4.1), and the result is placed in
Identity-Media-Signature.
o The receiving domain validates the signature, and if the request
is an invitation to establish a media channel, performs a proof of
identity validation using DTLS, TLS, ICE, or HIP over the media
path.
The following figure shows how the Authentication Service and the
media validation is performed. The figure assumes the endpoints
themselves perform the media validation.
: Service :
Enterprise-A : Provider-1 : Enterprise-B
: :
Auth. : B2BUA or : Auth.
Endpoint-A Service : SBC : Service Endpoint-B
| | : | : | |
1. |--INVITE->| : | : | |
2. | sign : | : | |
3. | |-INVITE-->|-INVITE-->| |
4. | | : | : validate |
5. | | : | : |-------->|
6. |<=========tls, dtls, ice, or hip=========>|
7. | | : | : | validated
8. | | : | : | ring phone
| | : | : | |
: :
Figure 2: Message Flow
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Step 1: Originating endpoint prepares to send an INVITE and chooses
the identity-challenge technique it supports, and indicates
that in the SDP it generates. Described in this document
are identity challenges for TLS, DTLS, ICE, and HIP. It
then sends the INVITE to its local SIP proxy.
Step 2: Originating endpoint's authentication service creates a new
header, Identity-Media, containing certain attribute names
from the SDP (e.g., "a=fingerprint", "a=ice-pub-key"). The
authentication service then creates a signature over certain
SIP headers (e.g., From, To) and this new Identity-Media
header. The resulting signature is inserted into the new
Identity-Media-Signature header. An Identity-Info header is
added, pointing to this domain's certificate. The INVITE,
with these additional headers, is forwarded to the next
administrative domain.
[NOTE: alternatively, we could allow the UAC to create the
Identity- Media header with the attributes it wants signed,
then have the auth server sign them and insert the signature
header - this would be more flexible]
Step 3: The next administrative domain has an SBC (or B2BUA). The
SBC modifies or rewrites certain SDP fields. Most typically
an SBC will modify the "m" and "c" lines. These
modifications do not break the signature, so long as the SBC
doesn't remove the headers Identity-Media, Identity-Media-
Signature, or Identity-Info, and do not remove or alter the
signed attributes from the SDP.
Step 4: The terminating endpoint's authentication service receives
the INVITE. It validates that the signature contained in
the Identity-Media-Signature header, and validates that the
signing certificate is owned by the originating domain from
step 2. This validation is done by using the certificate
pointed to in the Identity-Info header, which MUST match the
domain in the From: address.
Step 5: If the validation was successful, the terminating endpoint's
authentication service forwards the INVITE to the endpoint.
Step 6: The terminating endpoint chooses a compatible identity-
challenge technique from the INVITE (TLS, DTLS, ICE, or
HIP), and performs that challenge. Described in this
document are identity challenges for TLS, DTLS, ICE, and
HIP.
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Step 7: All of the identity challenges (TLS, DTLS, ICE, and HIP)
cause the exchange of either a certificate or a public key
in the media path. The terminating endpoint compares the
certificate or public key with the fingerprint in the
(signed) Identity-Media header (originally created in step
2). If they match, the terminating endpoint completes the
identity challenge exchange. After completion, the
originating endpoint has proven (to the terminating
endpoint) that the originating endpoint knows the private
key associated with the certificate (or public key) signed
in step 2. The terminating endpoint has now validated the
identity of the originating endpoint.
Step 8: The terminating endpoint can reliably and honestly indicate
calling party information ("caller-id") and ring the phone.
4.1. Identity Media Signature
In RFC4474, a signature is formed over some SIP headers and over the
entire body (which most typically contains SDP). In this
specification, some SIP headers are signed but only specific SDP
attributes that provide cryptographic identity are signed (e.g.,
"a=fingerprint" and its value). The specific SDP attributes that are
signed depends on which cryptographic identity technique(s) is used;
see section Section 5.
The SIP headers that are signed, for the signature placed into the
Identity-Media-Signature header are:
o The AoR of the UA sending the message, or addr-spec of the From
header field (referred to occasionally here as the 'identity
field').
o The addr-spec component of the To header field, which is the AoR
to which the request is being sent.
o The SIP method.
o [NOTE: Contact, CSeq and Call-Id not included]
o The Date header field, with exactly one space each for each SP and
the weekday and month items case set as shown in the BNF in
RFC3261. RFC3261 specifies that the BNF for weekday and month is
a choice amongst a set of tokens. The RFC2234 rules for the BNF
specify that tokens are case sensitive. However, when used to
construct the canonical string defined here, the first letter of
each week and month MUST be capitalized, and the remaining two
letters must be lowercase. This matches the capitalization
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provided in the definition of each token. All requests that use
the Identity-Media mechanism MUST contain a Date header.
o The Identity-Media header field value.
The hash is formed of these elements:
digest-string = addr-spec "|" addr-spec "|"
Method "|" SIP-date "|"
attrib-bodyhash-list
The first addr-spec MUST be taken from the From header field value,
the second addr-spec MUST be taken from the To header field value.
The Identity-Info header points to where the authentication service's
certificate can be retrieved from.
4.2. Authentication Service
The authentication service examines the SIP message body to build the
Identity-Media header. For each message body found, in the order
found:
o if the body part is application/sdp, the authentication service
retrieves only the cryptographic attributes from the SDP (as
described in Section 5), and appends that information to the
Identity-Media header.
o otherwise, for all other body parts, the body part is hashed using
SHA-1, and the first 96 bytes are appended to the Identity-Media
header using "BPH=".
For example, A SIP request with three bodyparts: text/plain,
application/sdp, and image/jpg, the Identity-Media attribute would
contain a bodypart hash of the text/plain part, certain SDP attribute
lines from the application/sdp bodypart (a=fingerprint in this
example), and a bodypart hash of the image/jpg bodypart:
Identity-Media: BPH="e32je3j23cjek3dz","a=fingerprint",
BPH="8fj289r3i892381c"
This Identity-Media header, along with the headers and portions of
headers described in Section 4.1 are all signed by the authentication
service. The resulting signature is placed on the new Identity-
Media-Signature header.
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4.3. Validation
The validation service can be performed by the remote endpoint itself
or by a device acting on behalf of the endpoint. The validation
service first checks the signature in the Identity-Media-Signature
field. If this is valid, the endpoint (or its validation service
operating on its behalf) then initiates a DTLS, TLS, ICE, or HIP
identity proof (Section 5). This causes the originating endpoint to
prove possession of its private key that corresponds to the
certificate (or public key) that was signed by the remote domain's
authentication service.
5. Proof of Identity Techniques
Four techniques are described below, TLS, DTLS, ICE, and HIP. Each
provides a means to cryptographically prove the identity signed by
the authentication service in SIP is the same as the identity on the
media path.
Each of these techniques work similarly -- a fingerprint of the
certificate (or, with ICE, the public key itself) is included in the
SDP. The authentication service creates a new Identity-Media header
and places into that header those SDP attribute names associated with
that technique. The authentication service then creates a signature
over specific SIP headers (see Section 4.1), and places that
signature into the new Identity-Media-Signature header. The SIP
request is then sent outside of the originating domain.
The receiving domain validates the Identity-Media-Signature. If
successful, the SIP request is forwarded to the end system. The end
system initiates a TLS, DTLS, ICE, or HIP session and validates that
the (signed) certificate fingerprint presented in the SIP signaling
matches the certificate presented in the TLS, DTLS, ICE, or HIP
exchange. If they match, and the TLS, DTLS, ICE, or HIP exchange
completes successfully, the local endpoint has validated the identity
of the remote endpoint.
Note: Due to SIP forking, the calling party may receive many
identity challenges, each incurring a public key operation to prove
identity. Mechanisms to deal with this are for future study.
5.1. TLS
TLS uses the "fingerprint" attribute to provide a hash of the
certificate in the SDP. The fingerprint attribute is defined by
[RFC4572] for TLS.
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5.2. DTLS
DTLS uses the same "fingerprint" attribute originally described for
TLS. The syntax is described in [I-D.ietf-sip-dtls-srtp-framework].
5.2.1. SRTP after DTLS optional
[[Discussion Point: Is there interest in having identity without
SRTP??]]
DTLS is only necessary to prove identity with DTLS; SRTP [RFC3711]
does not need to be used afterwards. Obviously, using SRTP provides
significant benefits over continuing to use RTP, because an attacker
can inject bogus RTP after a successful validation of identity which
is quite undesirable. The SDP for doing RTP after a DTLS exchange
might be signaled in SDP by using "RTP/AVP" rather than "RTP/SAVP"
(lines folded for readability):
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 3456 RTP/AVP 0 18
a=fingerprint:SHA-1
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
Of course, it would be desirable to more clearly indicate this
somehow in SDP. The example above collides with non-standard, but
deployed, "best-effort" media encryption mechanisms. SDP Capability
Negotiation [I-D.ietf-mmusic-sdp-capability-negotiation] might be a
useful consideration for this functionality.
5.3. ICE
ICE doesn't have inherent support for public/private keys. If public
keys were sent with other ICE attributes, there can be a real risk of
an ICE connectivity check exceeding the MTU. ICE lacks a mechanism
to fragment such large messages. It is also bandwidth inefficient to
send multiple ICE connectivity checks containing public keys, either
as retransmissions or with multiple candidates. Thus, for ICE, the
public key is sent in SDP and the public key's fingerprint is
exchanged on the media path -- opposite of TLS, DTLS, and HIP.
5.3.1. ICE Public Key SDP Attribute
The offerer includes its public key, which it will use for the
subsequent PK-CHALLENGE and PK-RESPONSE, in its SDP. The syntax is a
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BASE64-encoded version of the endpoint's public key.
The new attribute is called "ice-pub-key", which may appear on the
session level, media level, or both.
5.3.2. New STUN attributes
Two new STUN attributes are defined to carry the plaintext challenge
and the encrypted response.
5.3.2.1. PK-CHALLENGE
This is sent in a STUN Binding Request, and contains the bits to be
encrypted by the private key. Up to 256 bits can be included in the
challenge. When a STUN Binding Request is received containing this
attribute, the contents of the PK-CHALLENGE are encrypted using the
private key, and the result is included in the PK-RESPONSE attribute
of the Binding Response.
The PK-CHALLENGE MUST be the same for each candidate address that is
being tested for connectivity. If this requirement is not followed,
the peer will incur a public key operation for every ICE connectivity
check, which is not reasonable or necessary.
5.3.2.2. PK-RESPONSE
This is sent in a STUN Binding Response from the offerer to the
answerer, and contains the encrypted result of the PK-CHALLENGE.
5.4. HIP
In [I-D.tschofenig-hiprg-host-identities], a new attribute "key-
mgmt:host-identity-tag" is defined which contains the hash of the
public key used in the subsequent HIP exchange. This can be utilized
and signed exactly like the "fingerprint" attribute for TLS or DTLS.
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6. ABNF
The following figure shows the syntax of the new SIP header fields
using ABNF [RFC4234]
identity-media = "Identity-Media" HCOLON
attrib-bodyhash-list
attrib-bodyhash-list = attrib-bodyhash *(COMMA attrib-bodyhash)
attrib-bodyhash = quoted-attrib | quoted-bodyparthash
quoted-attribute = DQUOTE attribute DQUOTE ; SDP "a=" line
quoted-bodyhash = "BPH" EQUAL DQUOTE bodyparthash DQUOTE
bodyparthash = 32HEXDIG
identity-media-sig = "Identity-Media-Signature" HCOLON
signature
signature = DQUOT 32HEXDIG DQUOT
Identity-Info = "Identity-Info" HCOLON ident-info
*( SEMI ident-info-params )
ident-info = LAQUOT absoluteURI RAQUOT
ident-info-params = ident-info-alg / ident-info-extension
ident-info-alg = "alg" EQUAL token
ident-info-extension = generic-param
Figure 6: ABNF for new SIP headers
The following figure shows the syntax of the new SDP attribute
containing the ICE public key. This is used only by endpoints
implementing the ICE proof of identity technique (Section 5.3).
ice-pub-key = token ; BASE64 encoded public key
Figure 7: ABNF for new SDP attribute
7. Security Considerations
[[some of RFC4474's security considerations also apply.]]
7.1. Device Disclosure
Although the mechanism described in this paper allows SBCs to be used
with a cryptographic identity scheme, it does expose the identity of
the user's certificate. If a unique certificate is installed on each
user's device, the remote party will be able to discern which device
is terminating the call. This problem is more pronounced when SIP
retargeting occurs in conjunction with Connected Identity [RFC4916].
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If this isn't desired, there are two solutions:
o All devices under the control of the user will need to have the
same certificate (and associated private key) installed on them.
o The device needs to manufacture a new self-signed certificate (or
public key) for each call, and populate the appropriate SDP
attributes with that certificate (or public key). This is
possible because the identity service described in this paper does
not require the same certificate or public key to be used on every
call.
8. Operational Differences from RFC4474
RFC4474 imposes one public key operation for the authentication
service and one for validation. If Connected Identity [RFC4916] is
used, only one additional public key operation is necessary for the
header signature validation; the expense of the DTLS, TLS, or ICE
public key operation has already been incurred by both parties and is
not repeated.
RFC4474 includes the Contact URI in the signed headers. That is not
required by this mechanism because it adds no security property, and
will fail validation when crossing SBCs and B2BUA's. It is of
dubious security value because Via/Record-Route can be inserted for
response interception regardless, and some requests don't contain a
Contact anyway (e.g., MESSAGE). It does not provide any replay/
copy-paste protection either, for the same reasons.
RFC4474 includes the CSeq in the signed headers. That is not
required by this mechanism because it adds little security, and will
fail validation when crossing SBCs and B2BUA's in some cases. It is
of little security value because it provides no protection from cut-
paste attack for different targets, and although it would prevent
replay attack within the same session, since the media key-related
SDP portions are signed anyway, replaying the request will not do
anything useful.
RFC4474 includes the Call-Id in the signed headers. That is not
required by this mechanism because it adds little security, and will
fail validation when crossing SBCs and B2BUA's in some cases. It is
of little security value because it provides no protection from cut-
paste attack for different targets, and although it would prevent
replay attack for the same target, since the media key-related SDP
portions are signed anyway, replaying the request will not do
anything useful.
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The mechanism described in this document has the following advantages
over RFC4474:
o Only the edge network needs to create signatures on SIP requests
-- not every intervening SBC,
o The original cryptographically-provable identity is preserved
across any number of SBCs, B2BUA's, etc.
o SBCs, B2BUA's, and other "middle-boxes" in intermediate domains do
not need to be upgraded or changed in order for the originating
and terminating domains to use this new mechanism.
9. Limitations
For the identity procedure described in this document to function,
every device -- including Session Border Controllers -- on the path
MUST permit DTLS, TLS, ICE, or HIP messages to be exchanged in the
media path. Further, those devices MUST NOT interfere with the
signed SDP attributes or the new SIP headers necessary for Identity
Media to operate.
For the technique described in this document to function, all on-path
SIP elements -- SBCs, B2BUAs, and SIP proxies -- MUST NOT interfere
with the signed headers. The identity mechanism described in this
document is not harmed if on-path SIP elements alter the SDP (e.g.,
by deleting non-signed attributes, connection addresses, etc.).
10. Examples
10.1. DTLS
This example shows how two a=fingerprint lines in SDP would populate
the Identity-Media SIP header field. The following is an example of
an INVITE created by the endpoint.
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(lines folded for readability)
INVITE sip:bob@biloxi.example.org SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
To: Bob <sip:bob@biloxi.example.org>
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Max-Forwards: 70
Date: Thu, 21 Feb 2002 13:02:03 GMT
Contact: <sip:alice@pc33.atlanta.example.com>
Content-Type: application/sdp
Content-Length: 147
v=0
o=- 6418913922105372816 2105372818 IN IP4 192.0.2.1
s=example2
c=IN IP4 192.0.2.1
t=0 0
m=audio 54113 RTP/SAVP 0
a=fingerprint:SHA-1
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
m=video 54115 RTP/SAVP 0
a=fingerprint:SHA-1
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
Figure 8: Example with DTLS
The SIP proxy performing the Media Identity authentication service
would then insert the following three SIP headers into the message.
The Identity-Media header contains all of the SDP attribute lines
that are signed and the Identity-Media header contains the signature
of all of the relevant SIP headers and of the Identity-Media header.
Lines are folded for readability:
Identity-Info: <https://atlanta.example.com/atlanta.cer>
;alg=rsa-sha1
Identity-Media: "a=fingerprint","a=fingerprint"
Identity-Media-Signature:
"ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa
ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn
FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U="
Figure 9: SIP Headers Inserted by Authentication Service
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10.2. ICE
With ICE, the public key is exchanged in the signaling path (in SDP)
rather than in the media path (as is done with TLS, DTLS, and HIP).
This is the INVITE as it left the SIP user agent (lines folded for
readability):
INVITE sip:bob@biloxi.example.org SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
To: Bob <sip:bob@biloxi.example.org>
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Max-Forwards: 70
Date: Thu, 21 Feb 2002 13:02:03 GMT
Contact: <sip:alice@pc33.atlanta.example.com>
Content-Type: application/sdp
Content-Length: 147
v=0
o=- 6418913922105372816 2105372818 IN IP4 192.0.2.1
s=example2
c=IN IP4 192.0.2.1
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
a=pub-key:ejfiwj289ceucuezeceEJFjefkcjeiquiefekureickejfeefe
uirujejfecejejejkfeJJCEIUQQIEFJCQUCJCEQUURIE09dnjkeefjek
m=audio 54113 RTP/AVP 0
a=candidate:1 1 UDP 2130706431 192.0.2.1 54113 typ host
Figure 10: Example with ICE
The SIP proxy performing the Media Identity authentication service
would then insert the following three SIP headers into the message.
The Identity-Media header contains the ICE public key attribute and
the Identity-Media header contains the signature of all of the
relevant SIP headers and of the Identity-Media header (lines are
folded for readability):
Identity-Info: <https://atlanta.example.com/atlanta.cer>
;alg=rsa-sha1
Identity-Media: "a=pub-key"
Identity-Media-Signature:
"jjsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI+p6q5TOQXHMmz6uEo3svJsSH49th8qc
efQBbHC00VMZr2k+t6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0Ssjcd
VcunyaZucyRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Jcqe="
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Figure 11: Headers Inserted by Authentication Service
10.3. Request without SDP
This example shows how a SIP request without SDP is signed.
Message as sent by the UAC (lines folded for readability)
MESSAGE sip:user2@example.com SIP/2.0
Via: SIP/2.0/TCP user1pc.example.com;branch=z9hG4bK776sgdkse
Max-Forwards: 70
From: sip:user1@example.com;tag=49583
To: sip:user2@example.com
Call-ID: asd88asd77a@1.2.3.4
CSeq: 1 MESSAGE
Content-Type: text/plain
Content-Length: 18
Watson, come here.
Figure 12: Example with no SDP
The authentication service would add the following headers to the
above message:
Identity-Info: <https://atlanta.example.com/atlanta.cer>
;alg=rsa-sha1
Identity-Media:
BPH="MZr2k+t6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxA"
Identity-Media-Signature:
"diOPoQZYOy2wrVghuhcsMbHWUSFxI+p6q5TOQXHMmz6uEo3svJsSH49th8qcjjsR
bHC00VMZr2k+t6efQBVmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB09JcVc
unyaZucyRlBYYQTLqWzJ+KVhPKbfU/pryhVnqeSsjcd="
Figure 13: added headers
11. Acknowledgements
The mechanism described in this paper is derived from Jon Peterson
and Cullen Jennings' [RFC4474], which was formerly a document of the
SIP working group.
Thanks to Hans Persson for his suggestions which improved this
document.
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12. IANA Considerations
This document will add new IANA registrations for its new STUN
attributes.
[[This section will be completed in a later version of this
document.]]
13. References
13.1. Normative References
[I-D.ietf-behave-turn]
Rosenberg, J., "Traversal Using Relays around NAT (TURN):
Relay Extensions to Session Traversal Utilities for NAT
(STUN)", draft-ietf-behave-turn-04 (work in progress),
July 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[I-D.ietf-sip-dtls-srtp-framework]
Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing an SRTP Security Context using DTLS",
draft-ietf-sip-dtls-srtp-framework-00 (work in progress),
November 2007.
[RFC4572] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
Description Protocol (SDP)", RFC 4572, July 2006.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[RFC4916] Elwell, J., "Connected Identity in the Session Initiation
Protocol (SIP)", RFC 4916, June 2007.
[I-D.tschofenig-hiprg-host-identities]
Tschofenig, H., "Interaction between SIP and HIP",
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draft-tschofenig-hiprg-host-identities-05 (work in
progress), June 2007.
[I-D.ietf-hip-base]
Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
"Host Identity Protocol", draft-ietf-hip-base-10 (work in
progress), October 2007.
[I-D.farinacci-lisp]
Farinacci, D., "Locator/ID Separation Protocol (LISP)",
draft-farinacci-lisp-05 (work in progress), November 2007.
[I-D.carpenter-idloc-map-cons]
Carpenter, B., "General Identifier-Locator Mapping
Considerations", draft-carpenter-idloc-map-cons-01 (work
in progress), June 2007.
[I-D.whittle-ivip-arch]
Whittle, R., "Ivip (Internet Vastly Improved Plumbing)
Architecture", draft-whittle-ivip-arch-00 (work in
progress), July 2007.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-19 (work in progress), October 2007.
13.2. Informational References
[I-D.ietf-sipping-sbc-funcs]
Hautakorpi, J., "Requirements from SIP (Session Initiation
Protocol) Session Border Control Deployments",
draft-ietf-sipping-sbc-funcs-03 (work in progress),
April 2007.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
November 2002.
[I-D.ietf-mmusic-sdp-capability-negotiation]
Andreasen, F., "SDP Capability Negotiation",
draft-ietf-mmusic-sdp-capability-negotiation-07 (work in
progress), October 2007.
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Appendix A. ToDo List
o Add Table-2 of RFC3261
o re-use RFC4474 response code for failures, or invent new ones?
o describe what occurs if both SIP-Identity-Media and SIP-Identity
are both used?
Appendix B. Changes From Previous Versions
B.1. Changes from 00 to 01
o Removed "Contact" header from signature. SBCs need to change it.
o Removed "Call ID" header from signature. This header often
contains an IP address, so many SBCs change it.
o Removed "CSeq" header from signature. This header is sometimes
changed by SBCs and B2BUA's.
o include SDP attribute names in Identity-Media signature. This
allows any attribute to be signed.
o Old "Identity-Fingerprints" header renamed to "Identity-Media",
and only attribute names are listed in it, not attribute values.
o Old "Identity-Media" header renamed to "Identity-Media-Signature".
o Described how to sign SIP requests without an SDP body part, and
with a mix of SDP and non-SDP bodyparts.
Authors' Addresses
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
Email: dwing@cisco.com
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Hadriel Kaplan
Acme Packet
71 Third Ave.
Burlington, MA 01803
USA
Phone:
Fax:
Email: hkaplan@acmepacket.com
URI:
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