SIP                                                          C. Jennings
Internet-Draft                                             Cisco Systems
Expires: April 17, 2005                                      J. Peterson
                                                           NeuStar, Inc.
                                                        October 17, 2004

                 Certificate Management Service for SIP

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

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Copyright Notice

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


   This draft defines a Credential Service that uses a SIP subscribe/
   notify mechanism to discover other users' certificates and
   credentials and be notified about changes to these certificates.
   Other user agents that want to contact that AOR can retrieve these
   certificates from the server.  The result is that widespread
   deployment of S/MIME in SIP is possible, because no extra expense or
   effort is required of the end user.

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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Conventions  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.   Goals  . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.   UA Discovering Certificates  . . . . . . . . . . . . . . . .   4
   5.   UA Discovering and Publishing Credentials  . . . . . . . . .   5
   6.   Credential Server Behavior . . . . . . . . . . . . . . . . .   5
   7.   Negotiation of Secure Session  . . . . . . . . . . . . . . .   6
   8.   Encrypting Bodies of SIP Messages  . . . . . . . . . . . . .   8
   9.   Signing Bodies of SIP Messages . . . . . . . . . . . . . . .   8
   10.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1   Encrypted Page Mode IM Message . . . . . . . . . . . . .   8
     10.2   SRTP Phone Call  . . . . . . . . . . . . . . . . . . . .   9
     10.3   Setting and Retrieving UA Credentials  . . . . . . . . .  10
   11.  Security Considerations  . . . . . . . . . . . . . . . . . .  11
     11.1   Trusting the Identity of a Certificate . . . . . . . . .  12
     11.2   Conformity to the SACRED Framework . . . . . . . . . . .  13
     11.3   Crypto Profiles  . . . . . . . . . . . . . . . . . . . .  13
   12.  IANA . . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     12.1   Certificate Event Package  . . . . . . . . . . . . . . .  13
     12.2   Credential Event Package . . . . . . . . . . . . . . . .  14
     12.3   PKCS #8  . . . . . . . . . . . . . . . . . . . . . . . .  14
   13.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  15
   14.  References . . . . . . . . . . . . . . . . . . . . . . . . .  15
   14.1   Normative References . . . . . . . . . . . . . . . . . . .  15
   14.2   Informational References . . . . . . . . . . . . . . . . .  16
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  17
        Intellectual Property and Copyright Statements . . . . . . .  18

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1.  Introduction

   SIP provides a mechanism for end to end encryption and integrity
   using S/MIME, and several security properties of SIP depend on S/
   MIME.  S/MIME has not been widely implemented or deployed due to the
   complexity of providing a reasonable certificate distribution
   infrastructure.  This document proposes a way to address certificate
   discovery, retrieval, and management for SIP deployments.  It follows
   the Sacred Framework RFC 3760 [7] for management of the credentials.
   Combined with the Identity [2] work, this work 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.  This mechanism allows UAs such as IP phones to enroll
   and get their credentials without any more configuration information
   than they commonly have today, without any extra effort or key clicks
   by the end user, and without any extra expense for the end user.
   This mechanism also lets the UA discover and retrieve the public
   certificate for any other user and find out about certificate

   The general approach is to provide a new SIP service referred to as a
   Credential Server that allows UAs to subscribe to some other user's
   certificate.  The certificate is delivered in a SIP NOTIFY to the UA
   that subscribes.  The identity of the certificate can be vouched for
   using Identity [2] work, which uses the domain's certificate to sign
   that the NOTIFY really is from the Credential server for the request
   user in that domain.  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 SIP PUBLISH.  The Credential Server authenticates UAs that
   are changing credentials or requesting private keys using a shared
   secret that both the UA and the 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 [2] to provide strong identity binding of a
   SIP AOR to the certificates.

   Previous versions of this work proposed using HTTP instead of SIP for
   communicating with the Credential Server.  The key difference with
   using SIP is that a certificate can be revoked by sending a new
   NOTIFY; in the HTTP based scheme, the certificates were cached for a
   predefined period of time, typically one day, so that a revocation
   could take effect only after the cache had expired.  The earlier

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   version also did not deal with the SACRED problem and allowed several
   devices with the same AOR all to have different private keys.  This
   resulted in very large SIP messages and was looking fairly unwieldy;
   so now, the UAs for one AOR share private keying material and use the
   SACRED framework to move it between devices.

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 Identity work [2] to vouch for their

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

3.  Goals

   This work allows S/MIME to be used to meet the following goals and

   o  Not require any efforts (zero clicks) on behalf of an end user to
      acquire and start using end to end security.
   o  Not require any extra cost on a per user basis in deploying
      systems that support this.  It does require that the domain have a
      web server style certificate that is either self signed or signed
      by a well known CA.
   o  Allow negotiation of E2E encrypted SRTP 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
      NOTIFY bodies in presence.
   o  Work for users with multiple UA devices.
   o  Provide a certificate revocation mechanism.

4.  UA Discovering Certificates

   UAs discover certificates by sending a SUBSCRIBE with an event type
   of pkix-cert to the AOR for which a certificate is desired.  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 from the cache if it has

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   expired, 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 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 the server a
   PUBLISH[3] with an event type of credential that contains both an
   application/pkix-cert and an 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 so, 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
   [10].  The application/pkcs8 bodies contains a DER encoded PKCS #8
   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 transport.  The transport encoding
   of all the MIME bodies is binary.

6.  Credential Server Behavior

   The Credential Server stores credentials for users and can provide
   the credentials or certificates to other user agents.  The
   credentials are indexed by a URI that corresponds to the AOR of the
   user.  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

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   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 Identity [2]) or implement an
   Authentication Service itself.  The Server is encouraged to keep the
   subscriptions active for AORs that are communicating frequently but
   MAY unsubscribe at any point of time.  Any time the credentials for
   this URI change or get revoked, 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 with the Server, the Server can set up
   a subscription and send a Notify message that MUST contain the
   credentials.  This NOTIFY message 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
   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.

   When the Credential Server receives a PUBLISH to update credentials,
   it MUST authenticate and authorize this 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.

7.  Negotiation of Secure Session

   SIP uses an offer/answer negotiation mechanism[15] that describes
   sessions using SDP that may contain keying material, described in
   [13], for media protocols such as SRTP [14].  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 offer in an
   multipart/alternative body, it interprets these as it would a normal
   multipart alternative as defined in RFC 2046 [16], which means it

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   picks the last alternative that it can support.  Any bodies that
   cannot be decrypted are treated as unsupportable.  The sending UA
   should generally put encrypted offers after unencrypted ones, since
   encrypted ones are preferred.  The UA constructs the answer to the
   offer as it normally would and may include both encrypted and
   unencrypted versions of the answer using multipart/alternative.  The
   only wrinkle here is that if the UA sent multiple bodies with an
   offer, it needs to be able to match the answer (or answers) to the
   offer that was chosen.

   The UA that made the offer can uniquely identify the various MIME
   bodies using a MIME Content-ID header.  However, the UA sending the
   answers needs to provide the label of the Content-ID in the response.
   Solutions were considered that put the Content-ID identifier in a SIP
   Header, a MIME header, or an SDP attribute.  Since the issue here is
   fundamentally about providing information that is all at the MIME
   level about the relation between one set of multipart/alternatives
   and the other MIME body that is being sent, the best solution seems
   to involve passing this tag at the MIME level.  A new MIME header
   called "Content-Related-To" updates RFC 2045 with:

    rid := "Content-Related-To" ":" msg-id

   and adds "[rid CRLF]" to the Identity headers.

   The identifier supplied in the Content-Related-To header must be a
   valid Content-ID from a previous MIME message that this body is
   related to.

   The UA looks at the multipart/alternatives and selects the best one
   it can use.  It MUST include a Content-Related-To in the MIME for the
   answer that copies the tag from the related Content-ID header of the
   offer body it has chosen to use.

   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 that the call was
   secure and adjust their conversation accordingly.

   If one of the UASes does not support multipart MIME, then the that
   UAS will return an error response that the multipart body type is not
   supported and may be ineligible to receive this call.  For a wide
   variety of reasons, SIP UAs need to be able to receive multipart MIME

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   to interoperate well.

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 encrypted
   and unencrypted bodies 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 that the two
   versions 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".  Which message will get displayed to the receiving user
   will depend on whether or not the receiving device can decrypt the

9.  Signing Bodies of SIP Messages

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

10.  Examples

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

10.1  Encrypted Page Mode IM Message

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

    Event: certificate

   The credential server responds with the certificate in a NOTIFY.

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    NOTIFY  SIP/2.0
    Subscription-State: active; expires=7200
    From: <>;tag=1234
    Identity: "12dsfsdk2389403823cbed"
    Event: certificate
    Content-Type: application/pkix-cert

    < certificate data >

   Next Alice sends a SIP MESSAGE message to Bob and can encrypt the
   body using Bob's public key as shown below.  Thought outside the
   scope of this document, it is worth noting that IM messages often
   have common plain text like "Hi" and that setting up symmetric keys
   for extended session mode IM conversations will likely be more
   efficient as well as less likely to compromise the asymmetric key in
   the certificate.

    Content-Type: application/pkcs7-mime

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

10.2  SRTP Phone Call

   In this example, Alice calls Bob and offers both an RTP and an SRTP
   session.  The SDP for the SRTP session contains the SRTP keying
   material and is encrypted with S/MIME.  If Alice does not already
   have Bob's public key from previous communications, she fetches Bob's
   public key from Bob's credential server in the same way as shown in
   the previous example.

   Alice sends an INVITE to Bob that offers two alternative SDP bodies,
   one of which is encrypted and contains the SRTP keying information.
   The encrypted version is preferred so it comes second and both
   contain Content-ID headers.

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    INVITE SIP/2.0
    Content-Type: multipart/alternative;boundary=boundary

    Content-ID: 123
    Content-Type: application/sdp
    Content-Disposition: session

    < SDP offer for ordinary RTP only >
    Content-ID: 456
    Content-Type: application/pkcs7-mime
    Content-Disposition: session

    $ Content-Type: application/sdp
    $ < encrypted SDP with key for SRTP >

   If Bob's UA does not have Alice's public key, Bob's UA would fetch it
   as shown in the previous example.  Assuming that Bob's UA supported
   encryption, it would select the second alternative offer and
   construct an appropriate answer.  The 200 includes the MIME
   Content-Related-To header that indicates which alternative MIME body
   was chosen.

    200 OK
    Content-ID: 789
    Content-Related-To: 456
    Content-Type: application/pkcs7-mime
    Content-Disposition: session

    $ Content-Type: application/sdp
    $ < encrypted SDP with key for SRTP >

10.3  Setting and Retrieving UA Credentials

   When Alice's UA wishes to publish Alice's public and private keys to
   the Credential Server, it sends a PUBLISH message like the one below.
   This must be sent over a TLS connection in which the other end of the
   connection presents a certificate that matches the Credential Server
   for Alice and digest challenges the message to authenticate her.

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    Content-Type: multipart/mixed;boundary=boundary

    Content-ID: 123
    Content-Type: application/pkix-cert
    Content-Disposition: session

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

    < Private Key for Alice >

   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.

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 security of all the users will be completely
   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 including
   private key being sent unencrypted over the network and will render
   the whole system useless.  Implementations really must use TLS or
   there is no point in implementing any of this.  The TLS encryption
   used to pass the private key in a PUBLISH or SUBSCRIBE operation
   should use a cipher algorithm and key length equivalent to or greater
   than the private key being transmitted.  Likewise, the authentication
   of the user submitting the private key should be equivalent to or
   higher than the key being transmitted.

   The correct checking of chained certificates as specified in TLS [11]
   is critical for the client to authenticate the server.  If the client
   does not authenticate that it is talking to the correct credential
   server, a man in the middle attack is possible.  If a UA receives a
   certificate in a TLS connection that it cannot chain back to a well
   known CA but is otherwise valid (i.e.  a self signed certificate), it
   MAY ask the user if it wishes to allow this certificate to be used,

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   and if so, add it to the store of trusted certificates for TLS
   connections and for verifying Identity headers.  When asking if a
   certificate can be used, the device SHOULD provide the SHA-1
   fingerprint of the certificate to the user.  Since the users seldom
   check the fingerprint of a certificate, accepting the certificate
   results in a leap of faith that the TLS connection was not made to an
   attacker when the certificate was accepted.  Subsequent connection to
   the same server are secure as long as the initial connection where
   the certificate was accepted was not compromised.

   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, so they will not end up using the old invalid

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.

   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 Identity
      [2] work 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 both
      stop using the certificate and terminate the subscription 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 list.
   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

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   that the 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 the
   mechanism described in 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 Framework
   [7].  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 the TLS session.  The TLS
   sessions form a strong session key that is used to protect the
   credentials being exchanged.

11.3  Crypto Profiles

   Credential Servers SHOULD implement the server name indication
   extensions in RFC 3546 [8] 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

   The MIME Content-Related-To header does not require any IANA actions.

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-ietf-sipping-certs

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

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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-ietf-sipping-certs

   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: draft-ietf-sipping-certs

   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

   Many thanks to Eric Rescorla, Jim Schaad, Rohan Mahy and significant
   help and discussion on this and many others for useful comments
   including Kumiko Ono, Peter Gutmann, Russ Housley, Magnus Nystrom,
   Paul Hoffman, Dan Wing, Mike Hammer, and Jason Fischl.

14.  References

14.1  Normative References

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

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   [2]   Peterson, J. and C. Jennings, "Enhancements for Authenticated
         Identity Management in the Session Initiation Protocol (SIP)",
         draft-ietf-sip-identity-03 (work in progress), September 2004.

   [3]   Niemi, A., "Session Initiation Protocol (SIP) Extension for
         Event State Publication", draft-ietf-sip-publish-04 (work in
         progress), May 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.

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

   [11]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
         2246, January 1999.

14.2  Informational References

   [12]  Gutmann, P., "Internet X.509 Public Key Infrastructure
         Operational Protocols: Certificate  Store Access via HTTP",
         draft-ietf-pkix-certstore-http-08 (work in progress), August

   [13]  Andreasen, F., Baugher, M. and D. Wing, "Session Description
         Protocol Security Descriptions for Media Streams",
         draft-ietf-mmusic-sdescriptions-07 (work in progress), July

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   [14]  Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K.
         Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
         3711, March 2004.

   [15]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264, June 2002.

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

Authors' Addresses

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

   Phone: +1 408 902-3341

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

   Phone: +1 925/363-8720

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