SIP                                                          C. Jennings
Internet-Draft                                             Cisco Systems
Expires: November 14, 2004                                   J. Peterson
                                                           NeuStar, Inc.
                                                            May 16, 2004

                 Certificate Management Service for SIP

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   This Internet-Draft will expire on November 14, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   This draft defines a Credential Service in SIP that uses a subscribe/
   notify mechanism to discover other users' certificates and
   credentials and be notified about changes to the certificates.

   This is a very early draft and is being discussed on the mailing list.

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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Conventions  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.   Goals  . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.   UA Discovering Certificates  . . . . . . . . . . . . . . . .   4
   5.   UA Discovering and Publishing Credentials  . . . . . . . . .   4
   6.   Credential Server Behavior . . . . . . . . . . . . . . . . .   5
   7.   Negotiation of Secure Session  . . . . . . . . . . . . . . .   6
   8.   Encrypting Bodies of SIP messages  . . . . . . . . . . . . .   6
   9.   Signing Bodies of SIP message  . . . . . . . . . . . . . . .   6
   10.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   10.1 Get Credential for UA  . . . . . . . . . . . . . . . . . . .   7
   10.2 Encrypted Page Mode IM Message . . . . . . . . . . . . . . .   7
   10.3 SRTP Phone Call  . . . . . . . . . . . . . . . . . . . . . .   7
   11.  Security Considerations  . . . . . . . . . . . . . . . . . .   7
   11.1 Trusting the Identity of a Certificate . . . . . . . . . . .   7
   11.2 Conformity to the SACRED Framework . . . . . . . . . . . . .   8
   12.  IANA . . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   12.1 Certificate Event Package  . . . . . . . . . . . . . . . . .   9
   12.2 Credential Event Package . . . . . . . . . . . . . . . . . .   9
   12.3 PKCS #8  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   13.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  10
        Normative References . . . . . . . . . . . . . . . . . . . .  10
        Informational References . . . . . . . . . . . . . . . . . .  11
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  11
        Intellectual Property and Copyright Statements . . . . . . .  13

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1. Introduction

   SIP provides a mechanism for end to end encryption and integrity
   using S/MIME. This document extends that work to provide one specific
   mechanism for discovery, retrieval, and management of the
   certificates. It follows the Sacred Framework RFC 3760 (ADD REF) for
   management of the credentials.

   The general approach is to provide a new SIP service referred to as a
   Credential Server. This service allows UAs to subscribe to some other
   user's certificate. The certificate is delivered in a NOTIFY to the
   UA that subscribed. The identity of the certificate can be vouched
   for using the (TODO REF identity). The Credential Service can manage
   public certificates as well as credentials that include the user's
   private key. The user can install new credentials to the Credential
   Server using a PUBLISH. The Credential Server authenticates UAs that
   are changing credentials or requesting private keys using a shared
   secret that both the UA and Server know. Typically this will be the
   same shared secret that is used in Register with the Registrar for
   the domain.

   The mechanism described in this document works for both self signed
   certificates and certificates signed by a well known certificate
   authority; however, it is imagined that most UAs using this would
   only use self signed certificates and would use an Authentication
   Service as described in (TODO REF) to provide strong identity binding
   to the certificates.

2. Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [5].

   Certificate: An X.509 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 (REF TODO identity) to vouch for their

   Credential: For this document, this means the combination of a
   certificate and the associated private key.

3. Goals

   o  Allow negotiation of E2E encrypted sessions
   o  Allow end to end encryption and integrity of SIP bodies that may
      be delivered in SIP signaling, such as page mode MESSAGEs or

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      NOTIFY bodies in presence.
   o  Work for users with multiple UA devices.
   o  Provide certificate revocation mechanism

4. UA Discovering Certificates

   UAs discover certificates by sending a SUBSCRIBE with an event type
   of pkix-cert to the AOR that a certificate is desired for. This could
   be a SIP or tel URL. The resulting NOTIFY will contain an
   application/pkix-cert body which contains the certificates. The UA
   MUST follow the procedures in Section 11.1 to decide if the received
   certificate can be used. The UA needs to cache this certificate for
   future use. The certificate MUST be removed form the cache if it has
   expired or if it is updated by a subsequent NOTIFY or if the
   subscription has been terminated. The NOTIFY containing a certificate
   must be signed by an Authentication Service as described in REF
   Identity. If the identity asserted by the Authentication Service does
   not match the identity requests, the certificates in the NOTIFY are
   discarded and MUST NOT be used.

5. UA Discovering and Publishing Credentials

   UAs discover credentials by subscribing to their AOR with an event
   type of credential, which will result in a message containing both an
   application/pkix-cert body and an application/pkcs8 body that has the
   associated private key information for the certificate. The UA can
   change the user's certificate and private key by sending to the
   server a PUBLISH (TODO ref draft-ietf-sip-publish ) with an event
   type of credential that contains both an application/pkix-cert and
   application/pkcs8 body.

   The UA needs to authenticate to the Credential Server 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 Server;
   otherwise it defaults to the name of the domain in the User's AOR.
   The TLS connection MUST present a certificate that matches the
   expected name for the credential server, so that the UA knows it is
   talking to the correct server. If the certificate presented by the
   server does not match the expected server, the UA MUST terminate the
   connection and notify the User. If the UA does not do this, it may
   end up publishing its private key information to an attacker. The
   Credential Server 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 messages.

   The application/pkix-cert body is a DER encoded X.509 certificate
   (TODO RFC 2585). The application/pkcs8 bodies contains a DER encoded
   PKCS #8 object that contains the private key. The PKCS #8 objects

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   MUST by of type PrivateKeyInfo. The integrity and confidentiality of
   the PKCS #8 objects is provided by the TLS transport. The transport
   encoding of all the MIME bodies is binary.

6. Credential Server Behavior

   The Credential Server receives credentials for users and can then
   provide the credentials or certificates to other user agents. The
   credential server receives and store credentials for users. The
   credentials are indexed by URI. When a UA requests a public
   certificate with a SUBSCRIBE, the server sends it in a NOTIFY and
   sends a subsequent NOTIFY any time it changes. When a credential is
   requested, the Server digest challenges the requesting UA to
   authenticate it so that the Server can verify that the UA is
   authorized to receive the requested credentials.

   When the Credential Server receives a SUBSCRIBE for a certificate, it
   first checks to see if it has credentials for the requested URI. If
   it does not it returns a response indicating the user was not found.
   Otherwise it sets up a subscription and forms a NOTIFY with the
   certificate in the body and the From header field value set to the
   request URI of the SUBSCRIBE. It MUST send this NOTIFY through an
   Authentication Service (as described in TODO REF Identity) or
   implement an Authentication Service itself. The Server is encouraged
   to keep the subscriptions active indefinitely but MAY unsubscribe at
   any point of time. Anytime the credentials for this URI change, the
   Server MUST send a new NOTIFY to any active subscriptions.

   When a Credential Server receives a SUBSCRIBE for a credential, the
   Server has to authenticate and authorize the UA and validate that
   adequate transport security is being used. The Server MUST digest
   challenge the UA to authenticate the UA and then decide if it is
   authorized to receive the credentials.

   Once the UA has authenticated to the Server, the Server can set up a
   subscription and send a Notify message that MUST contain the
   credentials. This is sent thought an Authorization Service in the
   same way as the certificate subscriptions. If the credential changes,
   the Server MUST terminate any current subscriptions and force the UA
   to re-authenticate. This is so that in the case of a compromised
   secret to retrieve the credentials, the rogue UA does not continue to
   receive credentials after the compromised secret has been changed.

   When the Credential Server receives a PUBLISH to update credentials,
   it MUST authenticate and authorize this is the same way it does the
   subscriptions for credentials. If this succeeds, the Server updates
   the credential for this URI and processes all the active
   subscriptions to this URI as described above.

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7. Negotiation of Secure Session

   SIP uses an offer/answer negotiation mechanism (REF 3264) that
   describes sessions using SDP that may contain keying material (ref
   draft-ietf-mmusic-sdescriptions) for media protocols such as SRTP
   (Ref 3711). This keying material needs to be protected, and SIP does
   this by encrypting the SDP bodies using S/MIME.

   If a UA receives both an unencrypted and an encrypted SDP body in an
   offer, it MUST consider all the SDP received as potential offers, and
   any encrypted SDP SHOULD be preferred to unencrypted SDP. Answers to
   any encrypted SDP MUST also be sent in encrypted SDP, and unencrypted
   SDP MUST be sent in unencrypted SDP. This is necessary so that the
   device receiving the answer can correctly match m lines in the SDP.
   If the UA that receives an encrypted offer cannot send the answer
   back encrypted, then it may not use these encrypted offers.

   In a typical call from Alice to Bob, Alice would first subscribe to
   Bob's certificate. If this worked, then Alice would send an Invite to
   Bob that contained an RTP session in unencrypted SDP and an SRTP
   session in encrypted SDP. Bob would select the SRTP session and send
   an answer with encrypted SDP selecting the SRTP session. Both Alice's
   and Bob's UAs would indicate to the user that a secure call had been
   negotiated. Alice and Bob could note this and adjust their
   conversation accordingly.

8. Encrypting Bodies of SIP messages

   Applications such as presence and 911 location information result in
   information with significant privacy requirements being sent in SIP.
   Particular mime types may define special meanings when both an
   encrypted and unencrypted body are received but, unless otherwise
   specified, the UA SHOULD use the encrypted version if it can decrypt
   it, and ignore the unencrypted version. There is no requirement for
   the two versions to have the same information. For example, a page
   mode message could have an unencrypted version that said "I'm in the
   Middle East visiting people" while the encrypted version had much
   more sensitive information like "I'm over at Osama's house at
   21.25'24"N 39.49'24"E". Depending whether the receiving device can
   decrypt this or not, a different message gets displayed to the
   receiving user.

9. Signing Bodies of SIP message

   In general, signing messages with self-signed certificates is not
   that useful unless some other means is used to vouch that the
   certificate has some meaning. If the Authentication Service is used
   to do this, then the Authentication Service is providing integrity

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   across all the bodies and binding them with an identity. In this
   case, the additional signature becomes redundant. Because of this, it
   is recommended that signing bodies SHOULD NOT be done if the
   certificate is a self signed certificate.

10. Examples

10.1 Get Credential for UA

10.2 Encrypted Page Mode IM Message

10.3 SRTP Phone Call

11. Security Considerations

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

   This work requires the TLS session to be used for communications to
   the Credential Server. Failing to use TLS or selecting a poor cipher
   suite (such as NULL encryption) will result in credentials being sent
   unencrypted over the network and render the whole system useless.
   Implementation really must use TLS or there is no point in
   implementing any of this. In addition, the correct checking of
   chained certificates as specified in the TLS RFC (TODO REF) is
   critical for the client to authenticate the server.

   If a particular credential needs to be revoked, the new credential is
   simply published to the Credential Server. Every device keeping this
   current in its cache will have a subscription to the credential and
   will rapidly (order of seconds) be notified and replace its cache.
   Clients that are not subscribed will subscribe and get the new
   certificate and do not end up using the old invalid certificate.

11.1 Trusting the Identity of a Certificate

   When a UA wishes to discover the certificate for, the UA subscribes to the certificate for and receives a certificate in the body of a SIP
   Notify message. The term original URI is used to describe the
   original URI that was subscribed to.

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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 in the NOTIFY message MUST match the original URI.
   o  The UA MUST check the Identity header as described in the REF TODO
      Identity draft to validate that bodies have not been tampered with
      and that an Authentication Service has validated this From header.
   o  The UA MUST check the validity time of the certificate and stop
      using the certificate once it is invalid.
   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 in the Original URI. This means that
      the certificate should only be indexed in the certificate cache by
      the value of the original URI, not by the value of all the
      identities found in the SubjectAltName.
   These steps result in a chain of bindings that result in a trusted
   binding between the original URI and a public key. The Original URI
   is forced to match the From. The Authentication Service validates
   that this message did come from the identity claimed in the From and
   that bodies and From 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 user can tamper with this body so the owner
   of the identity can provide a false public key but other users
   cannot. This chain of assertion from original URI, to From, to body,
   to public key is critical to the security of this document. If any of
   the steps above are not followed, this chain of security will be
   broken and the system will not work.

11.2 Conformity to the SACRED Framework

   This work uses the security design outlined in the SACRED (REF TODO)
   Framework. Specifically it follows the cTLS architecture described in
   section 4.2.2 of RFC 3760. The client authenticates the server using
   the server's TLS certificate. The server authenticates the client
   using a SIP digest transaction inside of the TLS session. The TLS
   sessions form a strong session key used to protect the credentials
   being exchanged.

   Credential Servers SHOULD implement the server name indication
   extensions in RFC 3546 TODO REF and they MUST support a TLS profile
   of TLS_RSA_WITH_AES_128_CBC_SHA as described in RFC 3268 [9] and a
   profile of TLS_RSA_WITH_3DES_CBC_SHA.

12. IANA

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12.1 Certificate Event Package

   Subject: Registration of new SIP event package

   Package Name: certificate

   Is this registration for a Template Package: No

   Published Specification(s): draft- TODO

   Person & email address to contact for further information:
     Cullen Jennings <>

12.2 Credential Event Package

   Subject: Registration of new SIP event package

   Package Name: credential

   Is this registration for a Template Package: No

   Published Specification(s): draft- TODO

   Person & email address to contact for further information:
     Cullen Jennings <>

12.3 PKCS #8

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   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: will be binary for 8-bit transports

   Security considerations: Carries a cryptographic private key

   Interoperability considerations: None

   Published specification: TODO

   Applications which use this media type: Any MIME-complaint 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 <>

   Intended usage: COMMON

   Author/Change controller:
     Cullen Jennings <>

13. Acknowledgments

Normative References

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

   [2]  Peterson, J., "Enhancements for Authenticated Identity
        Management in the Session Initiation Protocol (SIP)",
        draft-ietf-sip-identity-02 (work in progress), May 2004.

   [3]  Niemi, A., "Session Initiation Protocol (SIP) Extension for
        Event State Publication", draft-ietf-sip-publish-03 (work in

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        progress), Febuary 2004.

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

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

   [6]  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.

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

   [8]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J. and T.
        Wright, "Transport Layer Security (TLS) Extensions", RFC 3546,
        June 2003.

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

Informational References

   [10]  Gutmann, P., "Internet X.509 Public Key Infrastructure
         Operational Protocols: Certificate  Store Access via HTTP",
         draft-ietf-pkix-certstore-http-06 (work in progress), April

Authors' Addresses

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

   Phone: +1 408 902-3341

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   Jon Peterson
   NeuStar, Inc.
   1800 Sutter St
   Suite 570
   Concord, CA  94520

   Phone: +1 925/363-8720

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