Internet DRAFT - draft-wing-rtcweb-identity-media
draft-wing-rtcweb-identity-media
Network Working Group D. Wing
Internet-Draft Cisco Systems
Intended status: Standards Track H. Kaplan
Expires: September 6, 2012 Acme Packet
March 5, 2012
SIP Identity using Media Path
draft-wing-rtcweb-identity-media-00
Abstract
This document defines a new SIP identity mechanism which creates a
signature over a certain subset of SIP headers and certain subset of
SDP lines. This this mechanism works with trickling ICE candidates
and works across zero or more Session Border Controllers.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on September 6, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Wing & Kaplan Expires September 6, 2012 [Page 1]
Internet-Draft SIP Identity using Media Path March 2012
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Identity Media Signature . . . . . . . . . . . . . . . . . 6
3.2. Authentication Service . . . . . . . . . . . . . . . . . . 7
3.3. Validation . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Proof of Identity Techniques . . . . . . . . . . . . . . . . . 7
4.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. DTLS-SRTP . . . . . . . . . . . . . . . . . . . . . . . . 8
5. ABNF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6.1. Device Disclosure . . . . . . . . . . . . . . . . . . . . 9
6.2. Modification of SDP . . . . . . . . . . . . . . . . . . . 10
7. Operational Differences from RFC4474 . . . . . . . . . . . . . 10
8. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 11
9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. DTLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
12. Normative References . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Wing & Kaplan Expires September 6, 2012 [Page 2]
Internet-Draft SIP Identity using Media Path March 2012
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. This mechanism suffers from two problems. First, it
is inefficient when ICE candidates are trickled (as each update to
the ICE candidate list would have to be signed and validated).
Second, it breaks entirely if IP addresses are modified during SIP
routing, such as by a Session Border Controller.
To avoid these problems, a new mechanism is described in this
document which provides cryptographic assurance of the endpoint's
identity that works with ICE candidate trickling and works through
most B2BUAs and through most SBCs.
The mechanism described in this document signs only certain SDP
attributes (rather than all SDP attributes) and certain SIP headers.
The remote endpoint is expected to validate the signature and
initiate a cryptographic handshake over the media path, which proves
the session has been established with the "From:" party in the SIP
header. A mechanism to perform the handshake over the media path is
shown using DTLS-SRTP and TLS. This mechanism is extensible so that
techniques other than DTLS-SRTP or TLS can be used.
Readers of this document are expected to be familiar with
"Enhancements for Authenticated Identity Management in the Session
Initiation Protocol (SIP)" [RFC4474], which defines the Identity and
Identity-Info header fields that are also used by this document.
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].
3. Operation
This document can utilize an authentication proxy, a concept
originally introduced in [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.
Wing & Kaplan Expires September 6, 2012 [Page 3]
Internet-Draft SIP Identity using Media Path March 2012
o Several SIP headers and the Identity-Media header are all signed
(as detailed in Section 3.1), 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.
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. |<============= DTLS =====================>|
7. | | : | : | validated
8. | | : | : | ring phone
| | : | : | |
: :
Figure 1: Message Flow
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 DTLS. 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
Wing & Kaplan Expires September 6, 2012 [Page 4]
Internet-Draft SIP Identity using Media Path March 2012
Identity-Media header with the attributes it wants signed,
then have the authentication 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 or DTLS-SRTP) and
performs that challenge. Described in this document are
identity challenges for TLS and DTLS-SRTP.
Step 7: The TLS and DTLS-SRTP identity challenges cause the exchange
of a certificate on 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.
Wing & Kaplan Expires September 6, 2012 [Page 5]
Internet-Draft SIP Identity using Media Path March 2012
3.1. 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 4.
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
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.
Wing & Kaplan Expires September 6, 2012 [Page 6]
Internet-Draft SIP Identity using Media Path March 2012
3.2. 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 4), 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 3.1 are all signed by the authentication
service. The resulting signature is placed on the new Identity-
Media-Signature header.
3.3. 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 TLS or DTLS-SRTP identity
proof (Section 4). This causes the originating endpoint to prove
possession of its private key that corresponds to the certificate
that was signed by the remote domain's authentication service.
4. Proof of Identity Techniques
Two technique is described below -- TLS and DTLS-SRTP. Previous
versions of this document had described other techniques (ICE, HIP,
and ZRTP). Both TLS and DTLS-SRTP cryptographically prove the
identity signed by the authentication service in SIP is the same as
the identity on the media path.
Wing & Kaplan Expires September 6, 2012 [Page 7]
Internet-Draft SIP Identity using Media Path March 2012
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 3.1), 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 DTLS session and validates that the (signed)
certificate fingerprint presented in the SIP signaling matches the
certificate presented in the DTLS exchange. If they match, and the
DTLS 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.
4.1. 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. This a=fingerprint line is included in the
Identity-Media SDP attribute.
4.2. DTLS-SRTP
DTLS uses the same "fingerprint" attribute originally described for
TLS, and handled identically to TLS.
Wing & Kaplan Expires September 6, 2012 [Page 8]
Internet-Draft SIP Identity using Media Path March 2012
5. ABNF
The following figure shows the syntax of the new SIP header fields
using ABNF [RFC5234]
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 2: ABNF for new SIP headers
6. Security Considerations
[[some of RFC4474's security considerations also apply.]]
6.1. 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 -- which is exposed by DTLS-SRTP itself. 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].
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
Wing & Kaplan Expires September 6, 2012 [Page 9]
Internet-Draft SIP Identity using Media Path March 2012
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.
6.2. Modification of SDP
One issue with only signing specific SDP attributes is that a man in
the middle can modify the un-signed SDP for nefarious purposes,
beyond simply changing m=/c= lines. In particular, an attacker could
set the c= connection line used for DTLS-SRTP fingerprint to 0.0.0.0
and the m= media line to port 0, essentially disabling that offered
media session. The attacker could also add a set of c=/m= lines for
non-SRTP media, and thus make a non-SRTP offer with a perfectly valid
identity signature. Or an attacker could insert SDP capability
negotiation attributes to create a best-effort type SRTP offer, with
SRTP (rather than RTP) being the lowest preference.
This draft prevents such downgrade attacks by requiring the called UA
use DTLS-SRTP, HIP, ICE, or TLS on the media path to establish
identity. Thus, an attacker performing the attacks described above
will not successfully fool the called UA because the (intended)
victim will use DTLS-SRTP (or HIP, ICE, or TLS) on the media path,
and the attacker does not possess the private key of the legitimate
caller.
7. 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-
Wing & Kaplan Expires September 6, 2012 [Page 10]
Internet-Draft SIP Identity using Media Path March 2012
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.
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.
8. Limitations
For the identity procedure described in this document to function,
every device -- including Session Border Controllers -- on the path
MUST permit DTLS-SRTP on 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.).
9. Examples
Wing & Kaplan Expires September 6, 2012 [Page 11]
Internet-Draft SIP Identity using Media Path March 2012
9.1. 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.
(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 3: 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="
Wing & Kaplan Expires September 6, 2012 [Page 12]
Internet-Draft SIP Identity using Media Path March 2012
Figure 4: SIP Headers Inserted by Authentication Service
10. 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.
11. IANA Considerations
[[This section will be completed in a later version of this
document.]]
12. Normative References
[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.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC4916] Elwell, J., "Connected Identity in the Session Initiation
Protocol (SIP)", RFC 4916, June 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.
Wing & Kaplan Expires September 6, 2012 [Page 13]
Internet-Draft SIP Identity using Media Path March 2012
Authors' Addresses
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
Email: dwing@cisco.com
Hadriel Kaplan
Acme Packet
71 Third Ave.
Burlington, MA 01803
USA
Phone:
Fax:
Email: hkaplan@acmepacket.com
URI:
Wing & Kaplan Expires September 6, 2012 [Page 14]
