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Versions: (draft-ietf-sipping-certs) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 6072

Network Working Group                                        C. Jennings
Internet-Draft                                             Cisco Systems
Expires: October 7, 2008                                  J. Fischl, Ed.
                                                 CounterPath Corporation
                                                           April 5, 2008


Certificate Management Service for The Session Initiation Protocol (SIP)
                        draft-ietf-sip-certs-06

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
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   This Internet-Draft will expire on October 7, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   This draft defines a Credential Service that allows Session
   Initiation Protocol (SIP) User Agents (UAs) to use a SIP event
   package to discover the certificates of other users.  This mechanism
   allows user agents that want to contact a given Address-of-Record
   (AOR) to retrieve that AOR's certificate by subscribing to the
   Credential Service, which returns an authenticated response
   containing that certificate.  The Credential Service also allows



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   users to store and retrieve their own certificates and private keys.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  UA Behavior with Certificates  . . . . . . . . . . . . . . . .  8
   5.  UA Behavior with Credentials . . . . . . . . . . . . . . . . .  9
   6.  Event Package Formal Definition for "certificate"  . . . . . . 10
     6.1.  Event Package Name . . . . . . . . . . . . . . . . . . . . 10
     6.2.  Event Package Parameters . . . . . . . . . . . . . . . . . 10
     6.3.  SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 10
     6.4.  Subscription Duration  . . . . . . . . . . . . . . . . . . 10
     6.5.  NOTIFY Bodies  . . . . . . . . . . . . . . . . . . . . . . 10
     6.6.  Subscriber Generation of SUBSCRIBE Requests  . . . . . . . 11
     6.7.  Notifier Processing of SUBSCRIBE Requests  . . . . . . . . 11
     6.8.  Notifier Generation of NOTIFY Requests . . . . . . . . . . 11
     6.9.  Subscriber Processing of NOTIFY Requests . . . . . . . . . 12
     6.10. Handling of Forked Requests  . . . . . . . . . . . . . . . 12
     6.11. Rate of Notifications  . . . . . . . . . . . . . . . . . . 12
     6.12. State Agents and Lists . . . . . . . . . . . . . . . . . . 12
     6.13. Behavior of a Proxy Server . . . . . . . . . . . . . . . . 12
   7.  Event Package Formal Definition for "credential" . . . . . . . 12
     7.1.  Event Package Name . . . . . . . . . . . . . . . . . . . . 13
     7.2.  Event Package Parameters . . . . . . . . . . . . . . . . . 13
     7.3.  SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 13
     7.4.  Subscription Duration  . . . . . . . . . . . . . . . . . . 13
     7.5.  NOTIFY Bodies  . . . . . . . . . . . . . . . . . . . . . . 13
     7.6.  Subscriber Generation of SUBSCRIBE Requests  . . . . . . . 14
     7.7.  Notifier Processing of SUBSCRIBE Requests  . . . . . . . . 14
     7.8.  Notifier Generation of NOTIFY Requests . . . . . . . . . . 14
     7.9.  Generation of PUBLISH Requests . . . . . . . . . . . . . . 15
     7.10. Notifier Processing of PUBLISH Requests  . . . . . . . . . 15
     7.11. Subscriber Processing of NOTIFY Requests . . . . . . . . . 16
     7.12. Handling of Forked Requests  . . . . . . . . . . . . . . . 16
     7.13. Rate of Notifications  . . . . . . . . . . . . . . . . . . 16
     7.14. State Agents and Lists . . . . . . . . . . . . . . . . . . 16
     7.15. Behavior of a Proxy Server . . . . . . . . . . . . . . . . 16
   8.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     8.1.  Encrypted Page Mode IM Message . . . . . . . . . . . . . . 17
     8.2.  Setting and Retrieving UA Credentials  . . . . . . . . . . 18
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
     9.1.  Certificate Revocation . . . . . . . . . . . . . . . . . . 21
     9.2.  Certificate Replacement  . . . . . . . . . . . . . . . . . 21
     9.3.  Trusting the Identity of a Certificate . . . . . . . . . . 21
     9.4.  SACRED Framework . . . . . . . . . . . . . . . . . . . . . 22



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     9.5.  Crypto Profiles  . . . . . . . . . . . . . . . . . . . . . 23
     9.6.  User Certificate Generation  . . . . . . . . . . . . . . . 23
     9.7.  Compromised Authentication Service . . . . . . . . . . . . 23
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
     10.1. Certificate Event Package  . . . . . . . . . . . . . . . . 24
     10.2. Credential Event Package . . . . . . . . . . . . . . . . . 24
     10.3. PKCS#8 . . . . . . . . . . . . . . . . . . . . . . . . . . 25
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 26
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 27
     12.2. Informational References . . . . . . . . . . . . . . . . . 28
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
   Intellectual Property and Copyright Statements . . . . . . . . . . 30






































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1.  Introduction

   SIP [RFC3261] provides a mechanism [RFC3853] for end-to-end
   encryption and integrity using S/MIME [RFC3851].  Several security
   properties of SIP depend on S/MIME, and yet it has not been widely
   deployed.  One reason is the complexity of providing a reasonable
   certificate distribution infrastructure.  This specification proposes
   a way to address discovery, retrieval, and management of certificates
   for SIP deployments.  Combined with the SIP Identity [RFC4474]
   specification, this specification allows users to have certificates
   that are not signed by any well known certificate authority while
   still strongly binding the user's identity to the certificate.

   In addition, this specification provides a mechanism that allows SIP
   User Agents such as IP phones to enroll and get their credentials
   without any more configuration information than they commonly have
   today.  The end user expends no extra effort.


2.  Definitions

   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].

   Certificate: A PKIX [RFC3280] style certificate containing a public
      key and a list of identities in the SubjectAltName that are bound
      to this key.  The certificates discussed in this draft are
      generally self signed and use the mechanisms in the SIP Identity
      [RFC4474] specification to vouch for their validity.  Certificates
      that are signed by a certificate authority can also be used with
      all the mechanisms in this draft, but it is expected that they are
      used purely as a key carrier and that their validity is not
      checked.
   Credential: For this document, credential means the combination of a
      certificate and the associated private key.
   password phrase: A password used to encrypt a PKCS#8 private key.


3.  Overview

   The general approach is to provide a new SIP service referred to as a
   "credential service" that allows SIP User Agents (UAs) to subscribe
   to other users' certificates using a new SIP event package [RFC3265].
   The certificate is delivered to the subscribing UA in a corresponding
   SIP NOTIFY request.  The identity of the certificate can be vouched
   for using the Authentication Service from the SIP Identity [RFC4474]
   specification, which uses the domain's certificate to sign the NOTIFY



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   request.  The credential service can manage public certificates as
   well as the user's private keys.  Users can update their credentials,
   as stored on the credential service, using a SIP PUBLISH [RFC3903]
   request.  The UA authenticates to the credential service using a
   shared secret when a UA is updating a credential.  Typically the
   shared secret will be the same one that is used by the UA to
   authenticate a REGISTER request with the Registrar for the domain
   (usually with SIP Digest Authentication).

   The following figure shows Bob publishing his credentials from one of
   his User Agents (e.g. his laptop software client), retrieving his
   credentials from another of his User Agents (e.g. his mobile phone),
   and then Alice retrieving Bob's certificate and sending a message to
   Bob. SIP 200-class responses are omitted from the diagram to make the
   figure easier to understand.




































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                example.com domain
                ------------------
    Alice       Proxy  Auth   Cred               Bob1  Bob2
      |           |      |      | TLS Handshake    |    |
      |  [ Bob generates   ]    |<--------------------->|
      |  [ credentials and ]    | PUBLISH (credential)  |
      |  [ publishes them  ]    |<----------------------|
      |           |      |      | Digest Challenge      |
      |           |      |      |---------------------->|
      |           |      |      | PUBLISH + Digest      |
      |           |      |      |<----------------------|
      |           |      |      |                  |
      |           |      |      | time passes...   |
      |           |      |      |                  |
      |           |      |      | TLS Handshake    |
      |   [ Bob later gets ]    |<---------------->|
      |   [ back his own   ]    | SUBSCRIBE        |
      |   [ credentials    ]    | (credential)     |
      |   [ at another     ]    |<-----------------|
      |   [ User Agent     ]    | SUBSCRIBE+Digest |
      |           |      |      |<-----------------|
      |           |      |      | NOTIFY           |
      |           |      |      |----------------->|
      |           |      |      | Bob Decrypts key |
      |           |      |      |                  |
      |           |      |      |                  |
      | SUBSCRIBE (certificate) |    Alice fetches |
      |---------->|----->|----->|    Bob's cert    |
      |           |      |NOTIFY|                  |
      | NOTIFY+Identity  |<-----|                  |
      |<----------+------|      |  Alice uses cert |
      |           |      |      |  to encrypt      |
      | MESSAGE   |      |      |  message to Bob  |
      |---------->|------+------+----------------->|

   Bob's UA (Bob2) does a TLS [RFC4366] handshake with the credential
   server to authenticate that the UA is connected to the correct
   credential server.  Then Bob's UA publishes his newly created or
   updated credentials.  The credential server digest challenges the UA
   to authenticate that the UA knows Bob's shared secret.  Once the UA
   is authenticated, the credential server stores Bob's credentials.

   Another of Bob's User Agents (Bob1) wants to fetch its current
   credentials.  It does a TLS [RFC4366] handshake with the credential
   server to authenticate that the UA is connected to the correct
   credential server.  Then Bob's UA subscribes for the credentials.
   The credential server digest challenges the UA to authenticate that
   the UA knows Bob's shared secret.  Once the UA is authenticated, the



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   credential server sends a NOTIFY that contains Bob's credentials.
   The private key portion of the credential may have been encrypted
   with a secret that only Bob's UA (and not the credential server)
   knows.  In this case, once Bob's UA decrypts the private key it will
   be ready to go.  Typically Bob's UA would do this when it first
   registered on the network.

   Some time later Alice decides that she wishes to discover Bob's
   certificate so that she can send him an encrypted message or so that
   she can verify the signature on a message from Bob. Alice's UA sends
   a SUBSCRIBE message to Bob's AOR.  The proxy in Bob's domain routes
   this to the credential server via an "authentication service" as
   defined in [RFC4474].  The credential server returns a NOTIFY that
   contains Bob's public certificate in the body.  This is routed
   through an authentication service that signs that this message really
   can validly claim to be from the AOR "sip:bob@example.com".  Alice's
   UA receives the certificate and can use it to encrypt a message to
   Bob.

   It is critical to understand that the only way that Alice can trust
   that the certificate really is the one for Bob and that the NOTIFY
   has not been spoofed is for Alice to check that the Identity
   [RFC4474] header field value is correct.

   The mechanism described in this document works for both self signed
   certificates and certificates signed by well known certificate
   authorities.  However, most UAs would only use self signed
   certificates and would use an Authentication Service as described in
   [RFC4474] to provide a strong binding of an AOR to the certificates.

   The mechanisms described in this draft allow for three different
   styles of deployment:

   1.  Deployments where the credential server only stores certificates
       and does not store any private key information.  If the
       deployment had users with multiple devices, some other scheme
       (perhaps even manual provisioning) would be used to get the right
       private keys onto all the devices that a user uses.
   2.  Deployments where the credential server stores certificates and
       also stores an encrypted version of the private keys.  The
       credential server would not know or need the password phrase for
       decrypting the private key.  The credential server would help
       move the private keys between devices but the user would need to
       enter a password phrase on each device to allow that device to
       decrypt (and encrypt) the private key information.
   3.  Deployments where the credential server stores the certificates
       and private keys and also knows the password phrase for
       decrypting the private keys.  Deployments such as these may not



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       even use password phrases, in which case the private keys are not
       encrypted inside the PKCS#8 objects.  This style of deployment
       would often have the credential server, instead of the devices,
       create the credentials.


4.  UA Behavior with Certificates

   When a User Agent wishes to discover some other user's certificate it
   subscribes to the "certificate" SIP event package as described in
   Section 6 to get the certificate.  While the subscription is active,
   if the certificate is updated, the Subscriber will receive the
   updated certificate in a notification.

   The Subscriber needs to decide how long it is willing to trust that
   the certificate it receives is still valid.  If the certificate is
   revoked before it expires, the Notifier will send a notification with
   an empty body to indicate that the certificate is no longer valid.
   However, the Subscriber might not receive the notification if an
   attacker blocks this traffic.  The amount of time that the Subscriber
   caches a certificate SHOULD be configurable.  A default of one day is
   RECOMMENDED.

   Note that the actual duration of the subscription is unrelated to the
   caching time or validity time of the corresponding certificate.
   Allowing subscriptions to persist after a certificate is no longer
   valid ensures that Subscribers receive the replacement certificate in
   a timely fashion.  In some cases, the Notifier will not allow
   unauthenticated subscriptions to persist.  The Notifier could return
   an immediate notification with the certificate in response to
   subscribe and then immediately terminate subscription, setting the
   reason parameter to "probation".  The Subscriber will have to
   periodically poll the Notifier to verify validity of the certificate.

   If the UA uses a cached certificate in a request and receives a 437
   (Unsupported Certificate) response, it SHOULD remove the certificate
   it used from the cache, attempt to fetch the certificate again.  If
   the certificate is changed, then the UA SHOULD retry the original
   request again with the new certificate.  This situation usually
   indicates that the certificate was recently updated, and that the
   Subscriber has not received a corresponding notification.  If the
   certificate fetched is the same as the one that was previously in the
   cache, then the UA SHOULD NOT try the request again.  This situation
   can happened when the request was retargeted to a different user than
   the original request.  The 437 response is defined in [RFC4474].






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      Note: A UA that has a presence list MAY want to subscribe to the
      certificates of all the presentities in the list when the UA
      subscribes to their presence, so that when the user wishes to
      contact a presentity, the UA will already have the appropriate
      certificate.  Future specifications might consider the possibility
      of retrieving the certificates along with the presence documents.

   The details of how a UA deals with receiving encrypted messages is
   outside the scope of this specification.  It is worth noting that if
   Charlie's UAS receives a request that is encrypted to Bob, it would
   be valid and legal for that UA to send a 302 redirecting the call to
   Bob.


5.  UA Behavior with Credentials

   UAs discover their own credentials by subscribing to their AOR with
   an event type of credential as described in Section 7.  After a UA
   registers, it SHOULD retrieve its credentials by subscribing to them
   as described in Section 6.6.

   When a UA discovers its credential, the private key information might
   be encrypted with a password phrase.  The UA SHOULD request that the
   user enter the password phrase on the device, and the UA MAY cache
   this password phrase for future use.

   There are several different cases in which a UA should generate a new
   credential:
   o  If the UA receives a NOTIFY with no body for the credential
      package.
   o  If the certificate has expired.
   o  If the certificate's notAfter date is within the next 600 seconds,
      the UA SHOULD attempt to create replacement credentials.  The UA
      does this by waiting a random amount of time between 0 and 300
      seconds.  If no new credentials have been received in that time,
      the UA creates new credentials to replace the expiring ones and
      sends them in a PUBLISH request (with a SIP-If-Match header field
      set to the current etag).  This makes credential collisions both
      unlikely and harmless.
   o  If the user of the device has indicated via the user interface
      that they wish to revoke the current certificate and issue a new
      one.
   Credentials are created by creating a new key pair which will require
   appropriate randomness, and then creating a certificate as described
   in Section 9.6.  The UA MAY encrypt the private key with a password
   phrase supplied by the user.  Next, the UA updates the user's
   credential by sending a PUBLISH [RFC3903] request with the
   credentials or just the certificate as described in Section 7.9.



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   If a UA wishes to revoke the existing certificate without publishing
   a new one, it MUST send a PUBLISH with an empty body to the
   credential server.


6.  Event Package Formal Definition for "certificate"

6.1.  Event Package Name

   This document defines a SIP Event Package as defined in [RFC3265].
   The event-package token name for this package is:

          certificate

6.2.  Event Package Parameters

   This package defines the "etag" Event header parameter which is valid
   only in NOTIFY requests.  It contains a token which represents the
   SIP etag value at the time the notification was sent.  Considering
   how infrequently credentials are updated, this hint is very likely to
   be the correct etag to use in the SIP-If-Match header in a SIP
   PUBLISH request to update the current credentials.

6.3.  SUBSCRIBE Bodies

   This package does not define any SUBSCRIBE bodies.

6.4.  Subscription Duration

   Subscriptions to this event package can range from no time to weeks.
   Subscriptions in days are more typical and are RECOMMENDED.  The
   default subscription duration for this event package is one day.

   The credential service is encouraged to keep the subscriptions active
   for AORs that are communicating frequently, but the credential
   service MAY terminate the subscription at any point in time.

6.5.  NOTIFY Bodies

   The body of a NOTIFY request for this package MUST either be empty or
   contain an application/pkix-cert body (as defined in [RFC2585]) that
   contains the certificate, unless an Accept header field has
   negotiated some other type.  The Content-Disposition MUST be set to
   "signal" as defined in [RFC3204].

   A future extension MAY define other NOTIFY bodies.  If no "Accept"
   header field is present in the SUBSCRIBE, the body type defined in
   this document MUST be assumed.



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   Implementations which generate large notifications are reminded to
   follow the message size restrictions for unreliable transports
   articulated in Section 18.1.1 of SIP.

6.6.  Subscriber Generation of SUBSCRIBE Requests

   A UA discovers a certificate by sending a SUBSCRIBE request with an
   event type of "certificate" to the AOR for which a certificate is
   desired.  In general, the UA stays subscribed to the certificate for
   as long as it plans to use and cache the certificate, so that the UA
   can be notified about changes or revocations to the certificate.

   Subscriber User Agents will typically subscribe to certificate
   information for a period of hours or days, and automatically attempt
   to re-subscribe just before the subscription is completely expired.

   When a user de-registers from a device (logoff, power down of a
   mobile device, etc.), subscribers SHOULD unsubscribe by sending a
   SUBSCRIBE request with an Expires header field of zero.

6.7.  Notifier Processing of SUBSCRIBE Requests

   When a SIP credential server receives a SUBSCRIBE request with the
   certificate event-type, it is not necessary to authenticate the
   subscription request.  The Notifier MAY limit the duration of the
   subscription to an administrator-defined period of time.  The
   duration of the subscription does not correspond in any way to the
   period for which the certificate will be valid.

   When the credential server receives a SUBSCRIBE request for a
   certificate, it first checks to see if it has credentials for the
   requested URI.  If it does not have a certificate, it returns a
   NOTIFY request with an empty message body.

6.8.  Notifier Generation of NOTIFY Requests

   Immediately after a subscription is accepted, the Notifier MUST send
   a NOTIFY with the current certificate, or an empty body if no
   certificate is available for the target user.  In either case it
   forms a NOTIFY with the From header field value set to the value of
   the To header field in the SUBSCRIBE request.  This server sending
   the NOTIFY needs either to implement an Authentication Service (as
   described in SIP Identity [RFC4474]) or else the server needs to be
   set up such that the NOTIFY request will be sent through an
   Authentication Service.  Sending the NOTIFY request through the
   Authentication Service requires the SUBSCRIBE request to have been
   routed through the Authentication Service, since the NOTIFY is sent
   within the dialog formed by the subscription.



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6.9.  Subscriber Processing of NOTIFY Requests

   The resulting NOTIFY will contain an application/pkix-cert body that
   contains the requested certificate.  The UA MUST follow the
   procedures in Section 9.3 to decide if the received certificate can
   be used.  The UA needs to cache this certificate for future use.  The
   maximum length of time it should be cached for is discussed in
   Section 9.1.  The certificate MUST be removed from the cache if the
   certificate has been revoked (if a NOTIFY with an empty body is
   received), or if it is updated by a subsequent NOTIFY.  The UA MUST
   check that the NOTIFY is correctly signed by an Authentication
   Service as described in [RFC4474].  If the identity asserted by the
   Authentication Service does not match the AOR that the UA subscribed
   to, the certificate in the NOTIFY is discarded and MUST NOT be used.

6.10.  Handling of Forked Requests

   This event package does not permit forked requests.  At most one
   subscription to this event type is permitted per resource.

6.11.  Rate of Notifications

   Notifiers SHOULD NOT generate NOTIFY requests more frequently than
   once per minute.

6.12.  State Agents and Lists

   The certificate server described in this section which serves
   certificates is a state agent and implementations of the certificate
   server MUST be implemented as a state agent.

   Implementers MUST NOT use the event list extension [RFC4662] with
   this event type.  It is not possible to make such an approach work,
   because the Authentication service would have to simultaneously
   assert several different identities.

6.13.  Behavior of a Proxy Server

   There are no additional requirements on a SIP Proxy, other than to
   transparently forward the SUBSCRIBE and NOTIFY requests as required
   in SIP.  This specification describes the Proxy, Authentication
   service, and credential service as three separate services, but it is
   certainly possible to build a single SIP network element that
   performs all of these services at the same time.


7.  Event Package Formal Definition for "credential"




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7.1.  Event Package Name

   This document defines a SIP Event Package as defined in [RFC3265].
   The event-package token name for this package is:

         credential

7.2.  Event Package Parameters

   This package defines the "etag" Event header parameter which is valid
   only in NOTIFY requests.  It contains a token which represents the
   SIP etag value at the time the notification was sent.  Considering
   how infrequently credentials are updated, this hint is very likely to
   be the correct etag to use in the SIP-If-Match header field in a SIP
   PUBLISH request to update the current credentials.

       etag-param = "etag" EQUAL token

7.3.  SUBSCRIBE Bodies

   This package does not define any SUBSCRIBE bodies.

7.4.  Subscription Duration

   Subscriptions to this event package can range from hours to one week.
   Subscriptions in days are more typical and are RECOMMENDED.  The
   default subscription duration for this event package is one day.

   The credential service SHOULD keep subscriptions active for UAs that
   are currently registered.

7.5.  NOTIFY Bodies

   The NOTIFY MUST contain a multipart/mixed (see [RFC2046]) body that
   contains both an application/pkix-cert body with the certificate and
   an application/pkcs8 body that has the associated private key
   information for the certificate.  The Content-Disposition MUST be set
   to "signal" as defined in [RFC3204].

   A future extension MAY define other NOTIFY bodies.  If no "Accept"
   header field is present in the SUBSCRIBE, the body type defined in
   this document MUST be assumed.

   The application/pkix-cert body is a DER encoded X.509v3 certificate
   [RFC2585].  The application/pkcs8 body contains a DER-encoded PKCS#8
   [PKCS.8.1993] object that contains the private key.  The PKCS#8
   objects MUST be of type PrivateKeyInfo.  The integrity and
   confidentiality of the PKCS#8 objects is provided by the TLS



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   transport.  The transport encoding of all the MIME bodies is binary.

7.6.  Subscriber Generation of SUBSCRIBE Requests

   A Subscriber User Agent will subscribe to its credential information
   for a period of hours or days and will automatically attempt to re-
   subscribe before the subscription has completely expired.

   The Subscriber SHOULD subscribe to its credentials whenever a new
   user becomes associated with the device (a new login).  The
   subscriber SHOULD also renew its subscription immediately after a
   reboot, or when the subscriber's network connectivity has just been
   re-established.

   The UA needs to authenticate with the credential service for these
   operations.  The UA MUST use TLS to connect to the server.  The UA
   may be configured with a specific name for the credential service;
   otherwise normal SIP routing is used.  As described in RFC 3261, the
   TLS connection needs to present a certificate that matches the
   expected name of the server to which the connection was formed, so
   that the UA knows it is talking to the correct server.  Failing to do
   this may result in the UA publishing its private key information to
   an attacker.  The credential service will authenticate the UA using
   the usual SIP Digest mechanism, so the UA can expect to receive a SIP
   challenge to the SUBSCRIBE or PUBLISH requests.

7.7.  Notifier Processing of SUBSCRIBE Requests

   When a credential service receives a SUBSCRIBE for a credential, the
   credential service has to authenticate and authorize the UA and
   validate that adequate transport security is being used.  Only a UA
   that can authenticate as being able to register as the AOR is
   authorized to receive the credentials for that AOR.  The credential
   Service MUST digest challenge the UA to authenticate the UA and then
   decide if it is authorized to receive the credentials.  If
   authentication is successful, the Notifier MAY limit the duration of
   the subscription to an administrator-defined period of time.  The
   duration of the subscription MUST NOT be larger than the length of
   time for which the certificate is still valid.  The Expires header
   field SHOULD be set so that it is not longer than the notAfter date
   in the certificate.

7.8.  Notifier Generation of NOTIFY Requests

   Once the UA has authenticated with the credential service and the
   subscription is accepted, the credential service MUST immediately
   send a Notify request.  The Notifier SHOULD include the current etag
   value in the "etag" Event package parameter in the NOTIFY request.



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   The Authentication Service is applied to this NOTIFY request in the
   same way as the certificate subscriptions.  If the credential is
   revoked, the credential service MUST terminate any current
   subscriptions and force the UA to re-authenticate by sending a NOTIFY
   with its Subscription-State header field set to "terminated" and a
   reason parameter of "deactivated".  (This causes a Subscriber to
   retry the subscription immediately.)  This is so that if a secret for
   retrieving the credentials gets compromised, the rogue UA will not
   continue to receive credentials after the compromised secret has been
   changed.

   Any time the credentials for this URI change, the credential service
   MUST send a new NOTIFY to any active subscriptions with the new
   credentials.

7.9.  Generation of PUBLISH Requests

   A user agent SHOULD be configurable to control whether it publishes
   the credential for a user or just the user's certificate.

   When publishing just a certificate, the body contains an application/
   pkix-cert.  When publishing a credential, the body contains a
   multipart/mixed containing both an application/pkix-cert and an
   application/pkcs8 body.

   When the UA sends the PUBLISH [RFC3903] request, it needs to do the
   following:
   o  The Expires header field value in the PUBLISH request SHOULD be
      set to match the time for which the certificate is valid.
   o  If the certificate includes Basic Constraints, it SHOULD set the
      CA flag to false.
   o  The PUBLISH request SHOULD include a SIP-If-Match header field
      with the previous etag from the subscription.  This prevents
      multiple User Agents for the same AOR from publishing conflicting
      credentials.  Note that UAs replace credentials that are about to
      expire at a random time (described in Section 5), reducing the
      chance of publishing conflicting credentials even without using
      the etag.

7.10.  Notifier Processing of PUBLISH Requests

   When the credential service receives a PUBLISH to update credentials,
   it MUST authenticate and authorize this request the same way as for
   subscriptions for credentials.  If the authorization succeeds, then
   the credential service MUST perform the following check on the
   certificate:





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   o  One of the names in the SubjectAltName of the certificate matches
      the authorized user making the request.
   o  The notBefore validity time MUST NOT be in the future.
   o  The notAfter validity time MUST be in the future.
   o  If a CA Basic Constraint is set in the certificate, it is set to
      false.
   If all of these succeed, the credential service updates the
   credential for this URI, processes all the active certificates and
   credential subscriptions to this URI, and generates a NOTIFY request
   with the new credential or certificate.

   If the Subscriber submits a PUBLISH request with no body, this
   revokes the current credentials and causes all subscriptions to the
   credential package to be deactivated as described in the previous
   section.  (Note that subscriptions to the certificate package are NOT
   terminated; each subscriber to the certificate package receives a
   notification with an empty body.)

7.11.  Subscriber Processing of NOTIFY Requests

   When the UA receives a valid NOTIFY request, it should replace its
   existing credentials with the new received ones.  If the UA cannot
   decrypt the PKCS#8 object, it MUST send a 437 (Unsupported
   Certificate) response.  Later if the user provides a new password
   phrase for the private key, the UA can subscribe to the credentials
   again and attempt to decrypt with the new password phrase.

7.12.  Handling of Forked Requests

   This event package does not permit forked requests.

7.13.  Rate of Notifications

   Notifiers SHOULD NOT generate NOTIFY requests more frequently than
   once per minute.

7.14.  State Agents and Lists

   The credential server described in this section which serves
   credentials is a state agent and implementations of the credential
   server MUST be implemented as a state agent.

   Implementers MUST NOT use the event list extension [RFC4662] with
   this event type.

7.15.  Behavior of a Proxy Server

   The behavior is identical to behavior described for certificate



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   subscriptions described in Section 6.13.


8.  Examples

   In all these examples, large parts of the messages are omitted to
   highlight what is relevant to this draft.  The lines in the examples
   that are prefixed by $ represent encrypted blocks of data.

8.1.  Encrypted Page Mode IM Message

   In this example, Alice sends Bob an encrypted page mode instant
   message.  Alice does not already have Bob's public key from previous
   communications, so she fetches Bob's public key from Bob's credential
   service:


   SUBSCRIBE sip:bob@biloxi.example.com SIP/2.0
   ...
   Event: certificate

   The credential service responds with the certificate in a NOTIFY.


   NOTIFY alice@atlanta.example.com  SIP/2.0
   Subscription-State: active; expires=7200
   ....
   From: <sip:bob@biloxi.example.com>;tag=1234
   Identity: "NJguAbpmYXjnlxFmlOkumMI+MZXjB2iV/NW5xsFQqzD/p4yiovrJBqhd3T
              ZkegnsmoHryzk9gTBH7Gj/erixEFIf82o3Anmb+CIbrgdl03gGaD6ICvkp
              VqoMXZZjdvSpycyHOhh1cmUx3b9Vr3pZuEh+cB01pbMQ8B1ch++iMjw="
   Identity-Info: <https://atlanta.example.com/cert>;alg=rsa-sha1
   ....
   Event: certificate
   Content-Type: application/pkix-cert
   Content-Disposition: signal

   < certificate data >

   Next, Alice sends a SIP MESSAGE message to Bob and can encrypt the
   body using Bob's public key as shown below.










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    MESSAGE sip:bob@biloxi.example.com SIP/2.0
    ...
    Content-Type: application/pkcs7-mime
    Content-Disposition: render

    $ Content-Type: text/plain
    $
    $ < encrypted version of "Hello" >

8.2.  Setting and Retrieving UA Credentials

   When Alice's UA wishes to publish Alice's public and private keys to
   the credential service, it sends a PUBLISH request like the one
   below.  This must be sent over a TLS connection directly to the
   domain of the credential service.  The credential service presents a
   certificate where the subjectAltName contains an entry that matches
   the domain name in the request line of the PUBLISH request and digest
   challenges the request to authenticate her.


    PUBLISH sips:alice@atlanta.example.com SIP/2.0
    ...
    Event: credential
    Content-Type: multipart/mixed;boundary=boundary
    Content-Disposition: signal

    --boundary
    Content-ID: 123
    Content-Type: application/pkix-cert


    < Public certificate for Alice >
    --boundary
    Content-ID: 456
    Content-Type: application/pkcs8

    < Private Key for Alice >
    --boundary

   If one of Alice's UAs subscribes to the credential event, the UA will
   be digest challenged, and the NOTIFY will include a body similar to
   the one in the PUBLISH section above.


9.  Security Considerations

   The high level message flow from a security point of view is
   summarized in the following figure.  The 200 responses are removed



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   from the figure as they do not have much to do with the overall
   security.

   In this figure, authC refers to authentication and authZ refers to
   authorization.

   Alice     Server              Bob UA
    |           | TLS Handshake    | 1) Client authC/Z server
    |           |<---------------->|
    |           | PUBLISH          | 2) Client sends request
    |           |<-----------------|    (write credential)
    |           | Digest Challenge | 3) Server challenges client
    |           |----------------->|
    |           | PUBLISH + Digest | 4) Server authC/Z client
    |           |<-----------------|
    |           |      time...     |
    |           |                  |
    |           | TLS Handshake    | 5) Client authC/Z server
    |           |<---------------->|
    |           | SUBSCRIBE        | 6) Client sends request
    |           |<-----------------|    (read credential)
    |           | Digest Challenge | 7) Server challenges client
    |           |----------------->|
    |           | SUBSCRIBE+Digest | 8) Server authC/Z client
    |           |<-----------------|
    |           | NOTIFY           | 9) Server returns credential
    |           |----------------->|
    |           |
    | SUBSCRIBE |   10) Client requests certificate
    |---------->|
    |           |
    |NOTIFY+AUTH|   11) Server returns user's certificate and signs that
    |<----------|       it is valid using certificate for the domain
    |           |

   When the UA, labeled Bob, first created a credential for Bob, it
   would store this on the credential server.  The UA authenticated the
   Server using the certificates from the TLS handshake.  The Server
   authenticated the UA using a digest style challenge with a shared
   secret.

   The UA, labeled Bob, wishes to request its credentials from the
   server.  First it forms a TLS connection to the Server, which
   provides integrity and privacy protection and also authenticates the
   server to Bob's UA.  Next the UA requests its credentials using a
   SUBSCRIBE request.  The Server digest challenges this to authenticate
   Bob's UA.  The server and Bob's UA have a shared secret that is used
   for this.  If the authentication is successful, the server sends the



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   credentials to Bob's UA.  The private key in the credentials may have
   been encrypted using a shared secret that the server does not know.

   A similar process would be used for Bob's UA to publish new
   credentials to the server.  Bob's UA would send a PUBLISH request
   containing the new credentials.  When this happened, all the other
   UAs that were subscribed to Bob's credentials would receive a NOTIFY
   with the new credentials.

   Alice wishes to find Bob's certificate and sends a SUBSCRIBE to the
   server.  The server sends the response in a NOTIFY.  This does not
   need to be sent over a privacy or integrity protected channel, as the
   Authentication service described in [RFC4474] provides integrity
   protection of this information and signs it with the certificate for
   the domain.

   This whole scheme is highly dependent on trusting the operators of
   the credential service and trusting that the credential service will
   not be compromised.  The security of all the users will be
   compromised if the credential service is compromised.

      Note: There has been significant discussion of the topic of
      avoiding deployments in which the credential servers store the
      private keys, even in some encrypted form that the credential
      server does not know how to decrypt.  Various schemes were
      considered to avoid this but they all result in either moving the
      problem to some other server, which does not seem to make the
      problem any better, or having a different credential for each
      device.  For some deployments where each user has only one device
      this is fine but for deployments with multiple devices, it would
      require that when Alice went to contact Bob, Alice would have to
      provide messages encrypted for all of Bob's devices.  The sipping
      working group did consider this architecture and decided it was
      not appropriate due both to the information it revealed about the
      devices and users and the amount of signaling required to make it
      work.

   This specification requires that TLS be used for the SIP
   communications to place and retrieve a UA's private key.  This
   provides security in two ways:
   1.  Confidentiality is provided for the digest authentication
       exchange, thus protecting it from dictionary attacks.
   2.  Confidentiality is provided for the private key, thus protecting
       it from being exposed to passive attackers.
   In order to prevent man-in-the-middle attacks, TLS clients MUST check
   that the SubjectAltName of the certificate for the server they
   connected to exactly matches the server they were trying to connect
   to.  Failing to use TLS or selecting a poor cipher suite (such as



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   NULL encryption) may result in credentials, including private keys,
   being sent unencrypted over the network and will render the whole
   system useless.

   The correct checking of chained certificates as specified in TLS
   [RFC4366] is critical for the client to authenticate the server.  If
   the client does not authenticate that it is talking to the correct
   credential service, a man in the middle attack is possible.

9.1.  Certificate Revocation

   If a particular credential needs to be revoked, the new credential is
   simply published to the credential service.  Every device with a copy
   of the old credential or certificate in its cache will have a
   subscription and will rapidly (order of seconds) be notified and
   replace its cache.  Clients that are not subscribed will subscribe
   when they next need to use the certificate and will get the new
   certificate.

   It is possible that an attacker could mount a DOS attack such that
   the UA that had cached a certificate did not receive the NOTIFY with
   its revocation.  To protect against this attack, the UA needs to
   limit how long it caches certificates.  After this time, the UA would
   invalidate the cached information even though no NOTIFY had ever been
   received due to the attacker blocking it.

   The duration of this cached information is in some ways similar to a
   device deciding how often to check a CRL list.  For many
   applications, a default time of 1 day is suggested, but for some
   applications it may be desirable to set the time to zero so that no
   certificates are cached at all and the credential is checked for
   validity every time the certificate is used.

9.2.  Certificate Replacement

   The UAs in the system replace the certificates close to the time that
   the certificates would expire.  If a UA has used the same key pair to
   encrypt a very large volume of traffic, the UA MAY choose to replace
   the credential with a new one before the normal expiration.

9.3.  Trusting the Identity of a Certificate

   When a UA wishes to discover the certificate for
   sip:alice@example.com, the UA subscribes to the certificate for
   alice@example.com and receives a certificate in the body of a SIP
   NOTIFY request.  The term original URI is used to describe the URI
   that was in the To header field value of the SUBSCRIBE request.  So
   in this case the original URI would be sip:alice@example.com.



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   If the certificate is signed by a trusted CA, and one of the names in
   the SubjectAltName matches the original URI, then this certificate
   MAY be used but only for exactly the original URI and not for other
   identities found in the SubjectAltName.  Otherwise, there are several
   steps the UA MUST perform before using this certificate.
   o  The From header field in the NOTIFY request MUST match the
      original URI that was subscribed to.
   o  The UA MUST check the Identity header field as described in the
      Identity [RFC4474] specification to validate that bodies have not
      been tampered with and that an Authentication Service has
      validated this From header field.
   o  The UA MUST check the validity time of the certificate and stop
      using the certificate if it is invalid.  (Implementations are
      reminded to verify both the notBefore and notAfter validity
      times.)
   o  The certificate MAY have several names in the SubjectAltName but
      the UA MUST only use this certificate when it needs the
      certificate for the identity asserted by the Authentication
      Service in the NOTIFY.  This means that the certificate should
      only be indexed in the certificate cache by the AOR that the
      Authentication Service asserted and not by the value of all the
      identities found in the SubjectAltName list.
   These steps result in a chain of bindings that result in a trusted
   binding between the original AOR that was subscribed to and a public
   key.  The original AOR is forced to match the From.  The
   Authentication Service validates that this request did come from the
   identity claimed in the From header field value and that the bodies
   in the request that carry the certificate have not been tampered
   with.  The certificate in the body contains the public key for the
   identity.  Only the UA that can authenticate as this AOR, or devices
   with access to the private key of the domain, can tamper with this
   body.  This stops other users from being able to provide a false
   public key.  This chain of assertion from original URI, to From, to
   body, to public key is critical to the security of the mechanism
   described in this specification.  If any of the steps above are not
   followed, this chain of security will be broken and the system will
   not work.

9.4.  SACRED Framework

   This specification includes a mechanism that allows end users to
   share the same credentials across different end-user devices.  This
   mechanism is based on the one presented in the SACRED Framework
   [RFC3760].  While this mechanism is fully described in this document,
   the requirements and background are more thoroughly discussed in
   [RFC3760].

   Specifically, Section 7.6, Section 7.7 and Section 7.10 follow the



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   cTLS architecture described in section 4.2.2 of [RFC3760].  The
   client authenticates the server using the server's TLS certificate.
   The server authenticates the client using a SIP digest transaction
   inside the TLS session.  The TLS sessions form a strong session key
   that is used to protect the credentials being exchanged.

9.5.  Crypto Profiles

   Credential services SHOULD implement the server name indication
   extensions in [RFC4366] and they MUST support a TLS profile of
   TLS_RSA_WITH_AES_128_CBC_SHA as described in [RFC3268] as a profile
   of TLS_RSA_WITH_3DES_EDE_CBC_SHA.

   The PKCS#8 in the clients MUST implement PBES2 with a key derivation
   algorithm of PBKDF2 using HMAC with SHA1 and an encryption algorithm
   of DES-EDE2-CBC-Pad as defined in [RFC2898].  It is RECOMMENDED that
   this profile be used when using PKCS#8.  A different passphrase
   SHOULD be used for the PKCS#8 encryption than is used for server
   authentication.

9.6.  User Certificate Generation

   The certificates should be consistent with [RFC3280].  A
   signatureAlgorithm of sha1WithRSAEncryption MUST be implemented.  The
   Issuers SHOULD be the same as the subject.  Given the ease of issuing
   new certificates with this system, the Validity can be relatively
   short.  A Validity of one year or less is RECOMMENDED.  The
   subjectAltName must have a URI type that is set to the SIP URL
   corresponding to the user AOR.  It MAY be desirable to put some
   randomness into the length of time for which the certificates are
   valid so that it does not become necessary to renew all the
   certificates in the system at the same time.

   It is worth noting that a UA can discover the current time by looking
   at the Date header field value in the 200 response to a REGISTER
   request.

9.7.  Compromised Authentication Service

   One of this worst attacks against this system would be if the
   Authentication Service were compromised.  This attack is somewhat
   analogous to a CA being compromised in traditional PKI systems.  The
   attacker could make a fake certificate for which it knows the private
   key, use it to receive any traffic for a given use, and then re-
   encrypt that traffic with the correct key and forward the
   communication to the intended receiver.  The attacker would thus
   become a man in the middle in the communications.




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   There is not too much that can be done to protect against this.  A UA
   MAY subscribe to its own certificate under some other identity to try
   to detect whether the credential server is handing out the correct
   certificates.  It will be difficult to do this in a way that does not
   allow the credential server to recognize the user's UA.

   The UA MAY also save the fingerprints of the cached certificates and
   warn users when the certificates change significantly before their
   expiry date.

   The UA MAY also allow the user to see the fingerprints for the cached
   certificates so that they can be verified by some other out of band
   means.


10.  IANA Considerations

   This specification defines two new event packages that IANA is
   requested to add the registry at:
      http://www.iana.org/assignments/sip-events
   It also defines a new mime type that IANA is requested to add to the
   registry at:
      http://www.iana.org/assignments/media-types/application

10.1.  Certificate Event Package


   To: ietf-sip-events@iana.org
   Subject: Registration of new SIP event package

   Package Name: certificate

   Is this registration for a Template Package: No

   Published Specification(s): This document

   New Event header parameters: This package defines no
                                new parameters

   Person & email address to contact for further information:
     Cullen Jennings <fluffy@cisco.com>

10.2.  Credential Event Package








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   To: ietf-sip-events@iana.org
   Subject: Registration of new SIP event package

   Package Name: credential

   Is this registration for a Template Package: No

   Published Specification(s): This document

   New Event header parameters: "etag"

   Person & email address to contact for further information:
     Cullen Jennings <fluffy@cisco.com>

10.3.  PKCS#8




































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   To: ietf-types@iana.org
   Subject: Registration of MIME media type application/pkcs8

   MIME media type name: application

   MIME subtype name: pkcs8

   Required parameters: None

   Optional parameters: None

   Encoding considerations: binary

   Security considerations: Carries a cryptographic private key

   Interoperability considerations:
        The PKCS#8 object inside this MIME type MUST be DER-encoded

   Published specification:
        RSA Laboratories, "Private-Key Information Syntax Standard,
        Version 1.2", PKCS 8, November 1993.

   Applications which use this media type: Any MIME-compliant transport

   Additional information:
     Magic number(s): None
     File extension(s): .p8
     Macintosh File Type Code(s): none

   Person & email address to contact for further information:
     Cullen Jennings <fluffy@cisco.com>

   Intended usage: COMMON

   Author/Change controller:
     the IESG


11.  Acknowledgments

   Many thanks to Eric Rescorla, Jim Schaad, Rohan Mahy for significant
   help and discussion.  Many others provided useful comments, including
   Kumiko Ono, Peter Gutmann, Russ Housley, Yaron Pdut, Aki Niemi,
   Magnus Nystrom, Paul Hoffman, Adina Simu, Dan Wing, Mike Hammer and
   Lyndsay Campbell.  Rohan Mahy, John Elwell, and Jonathan Rosenberg
   provided detailed review and text.





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12.  References

12.1.  Normative References

   [PKCS.8.1993]
              RSA Laboratories, "Private-Key Information Syntax
              Standard, Version 1.2", PKCS 8, November 1993.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2585]  Housley, R. and P. Hoffman, "Internet X.509 Public Key
              Infrastructure Operational Protocols: FTP and HTTP",
              RFC 2585, May 1999.

   [RFC3204]  Zimmerer, E., Peterson, J., Vemuri, A., Ong, L., Audet,
              F., Watson, M., and M. Zonoun, "MIME media types for ISUP
              and QSIG Objects", RFC 3204, December 2001.

   [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.

   [RFC3265]  Roach, A., "Session Initiation Protocol (SIP)-Specific
              Event Notification", RFC 3265, June 2002.

   [RFC3268]  Chown, P., "Advanced Encryption Standard (AES)
              Ciphersuites for Transport Layer Security (TLS)",
              RFC 3268, June 2002.

   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [RFC3903]  Niemi, A., "Session Initiation Protocol (SIP) Extension
              for Event State Publication", RFC 3903, October 2004.

   [RFC4366]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, April 2006.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for



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              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC2898]  Kaliski, B., "PKCS #5: Password-Based Cryptography
              Specification Version 2.0", RFC 2898, September 2000.

              This reference is normative.  The mechanisms used in this
              specification from RFC2898 are stable and sutable for use
              in a standards track specification.  RFC2898 has been used
              as a normative reference in several prior standards track
              documents including RFC3185, RFC3370, RFC3962, and
              RFC4656.

12.2.  Informational References

   [RFC3760]  Gustafson, D., Just, M., and M. Nystrom, "Securely
              Available Credentials (SACRED) - Credential Server
              Framework", RFC 3760, April 2004.

   [RFC3851]  Ramsdell, B., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Message Specification",
              RFC 3851, July 2004.

   [RFC3853]  Peterson, J., "S/MIME Advanced Encryption Standard (AES)
              Requirement for the Session Initiation Protocol (SIP)",
              RFC 3853, July 2004.

   [RFC4662]  Roach, A., Rosenberg, J., and B. Campbell, "A Session
              Initiation Protocol (SIP) Event Notification Extension for
              Resource Lists", RFC 4662, August 2006.





















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Authors' Addresses

   Cullen Jennings
   Cisco Systems
   170 West Tasman Drive
   MS: SJC-21/2
   San Jose, CA  95134
   USA

   Phone: +1 408 421-9990
   Email: fluffy@cisco.com


   Jason Fischl (editor)
   CounterPath Corporation
   Suite 300
   One Bentall Centre
   505 Burrard Street
   Vancouver, BC  V7X 1M3
   Canada

   Phone: +1 604 320-3344
   Email: jason@counterpath.com
   URI:   http://www.counterpath.com



























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