SIP C. Jennings
Internet-Draft Cisco Systems
Expires: November 14, 2004 J. Peterson
NeuStar, Inc.
May 16, 2004
Certificate Management Service for SIP
draft-jennings-sipping-certs-03
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
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
sipping@ietf.org 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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
validity.
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
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 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
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): draft- TODO
Person & email address to contact for further information:
Cullen Jennings <fluffy@cisco.com>
12.2 Credential Event Package
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): draft- TODO
Person & email address to contact for further information:
Cullen Jennings <fluffy@cisco.com>
12.3 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: 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 <fluffy@cisco.com>
Intended usage: COMMON
Author/Change controller:
Cullen Jennings <fluffy@cisco.com>
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
2004.
Authors' Addresses
Cullen Jennings
Cisco Systems
170 West Tasman Drive
MS: SJC-21/2
San Jose, CA 95134
USA
Phone: +1 408 902-3341
EMail: fluffy@cisco.com
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Jon Peterson
NeuStar, Inc.
1800 Sutter St
Suite 570
Concord, CA 94520
US
Phone: +1 925/363-8720
EMail: jon.peterson@neustar.biz
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