SIP C. Jennings
Internet-Draft Cisco Systems
Expires: August 20, 2005 J. Peterson
NeuStar, Inc.
February 19, 2005
Certificate Management Service for The Session Initiation Protocol
(SIP)
draft-ietf-sipping-certs-01
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
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RFC 3668.
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This draft defines a Credential Service that allows SIP User Agents
to use a SIP 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. The Credential Service also
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allows 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 . . . . . . . . . . . . . . . 6
5. UA Behavior with Credentials . . . . . . . . . . . . . . . . 7
6. Credential Service Behavior . . . . . . . . . . . . . . . . 8
7. Event Package Formal Definition for "certificate" . . . . . 8
7.1 Event Package Name . . . . . . . . . . . . . . . . . . . . 8
7.2 Event Package Parameters . . . . . . . . . . . . . . . . . 8
7.3 SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 9
7.4 Subscription Duration . . . . . . . . . . . . . . . . . . 9
7.5 NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . 9
7.6 Subscriber Generation of SUBSCRIBE Requests . . . . . . . 9
7.7 Notifier Processing of SUBSCRIBE Requests . . . . . . . . 9
7.8 Notifier Generation of NOTIFY Requests . . . . . . . . . . 10
7.9 Subscriber Processing of NOTIFY Requests . . . . . . . . . 10
7.10 Handling of Forked Requests . . . . . . . . . . . . . . 10
7.11 Rate of Notifications . . . . . . . . . . . . . . . . . 10
7.12 State Agents and Lists . . . . . . . . . . . . . . . . . 11
7.13 Behavior of a Proxy Server . . . . . . . . . . . . . . . 11
8. Event Package Formal Definition for "credential" . . . . . . 11
8.1 Event Package Name . . . . . . . . . . . . . . . . . . . . 11
8.2 Event Package Parameters . . . . . . . . . . . . . . . . . 11
8.3 SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 11
8.4 Subscription Duration . . . . . . . . . . . . . . . . . . 11
8.5 NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . 12
8.6 Subscriber Generation of SUBSCRIBE Requests . . . . . . . 12
8.7 Notifier Processing of SUBSCRIBE Requests . . . . . . . . 13
8.8 Notifier Generation of NOTIFY Requests . . . . . . . . . . 13
8.9 Notifier Processing of PUBLISH Requests . . . . . . . . . 13
8.10 Subscriber Processing of NOTIFY Requests . . . . . . . . 14
8.11 Handling of Forked Requests . . . . . . . . . . . . . . 14
8.12 Rate of Notifications . . . . . . . . . . . . . . . . . 14
8.13 State Agents and Lists . . . . . . . . . . . . . . . . . 14
8.14 Behavior of a Proxy Server . . . . . . . . . . . . . . . 14
9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1 Encrypted Page Mode IM Message . . . . . . . . . . . . . . 14
9.2 Setting and Retrieving UA Credentials . . . . . . . . . . 15
10. Security Considerations . . . . . . . . . . . . . . . . . . 16
10.1 Certificate Revocation . . . . . . . . . . . . . . . . . 18
10.2 Certificate Replacement . . . . . . . . . . . . . . . . 19
10.3 Trusting the Identity of a Certificate . . . . . . . . . 19
10.4 Conformity to the SACRED Framework . . . . . . . . . . . 20
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10.5 Crypto Profiles . . . . . . . . . . . . . . . . . . . . 20
10.6 User Certificate Generation . . . . . . . . . . . . . . 20
10.7 Compromised Authentication Service . . . . . . . . . . . 20
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . 21
11.1 Certificate Event Package . . . . . . . . . . . . . . . 21
11.2 Credential Event Package . . . . . . . . . . . . . . . . 22
11.3 PKCS#8 . . . . . . . . . . . . . . . . . . . . . . . . . 22
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 23
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
13.1 Normative References . . . . . . . . . . . . . . . . . . . 23
13.2 Informational References . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . 26
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1. Introduction
SIP [6] provides a mechanism [17] for end-to-end encryption and
integrity using S/MIME [16]. Several security properties of SIP
depend on S/MIME, and yet it has not been widely deployed.
Certainly, 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. It follows the Sacred Framework RFC 3760 [7]
for management of the credentials. Combined with the SIP Identity
[2] 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. This mechanism 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 RFC 2119 [5].
Certificate: An X.509v3 [15] 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 [2]
specification to vouch for their validity.
Credential: For this document, credential means the combination of a
certificate and the associated 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
some other user's certificate using a new SIP event package [4]. 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 [2]
specification, which uses the domain's certificate to sign the NOTIFY
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 [3] 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
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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 (ex: his laptop software client), retrieving his
credentials from another of his User Agents (ex: 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.
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 [11] 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
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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 one of Bob's User Agents (Bob1) wants to fetch its current
credentials. It does a TLS [11] 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
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. 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 authorization service.
The credential server returns a NOFITY 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.
The mechanism described in this document works for both self signed
certificates and certificates signed by well known certificate
authorities; 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 a strong binding of an AOR to
the certificates.
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 7 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
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caches a certificate SHOULD be configurable. A default of one day is
RECOMMENDED.
Note that the actual duration of the subscription is orthogonal to
the caching time or validity time of the corresponding certificate.
Allowing subscriptions to persist after a certificate is not 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 493
(Undecipherable) response, it SHOULD remove the certificate it used
from the cache, attempt to fetch the certificate again, and retry the
original request again. This situation usually indicates that the
certificate was recently updated, and that the Subscriber has not
received a corresponding notification.
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.
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 8. After a UA
registers, it SHOULD retrieve its credentials.
There are several different cases in which a UA should generate a new
credential:
o If the UA receives a NOTIFY for the credential package with no
body.
o If the certificate has expired.
o If the certificate is within ten minutes of expiring, the UA
SHOULD attempt to create replacement credentials. The UA does
this by waiting a random amount of time between 0 and 600 seconds.
If no new credentials was 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 set to the current
entity-tag). This makes credential collisions both unlikely and
harmless.
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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, creating a certificate as described in
Section 10.6, and encrypting the private key with a pass phrase
supplied by the user. Then the UA can change the user's credential
by sending the server a PUBLISH[3] with an event type of credential
that contains a multipart/mixed containing both an
application/pkix-cert and an application/pkcs8 body. The PUBLISH
request MUST include a SIP-If-Match header field with the previous
entity-tag know to the subscriber. This prevents multiple User
Agents for the same AOR from publishing conflicting credentials.
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. Credential Service Behavior
The Credential Service stores credentials for users and can provide
the credentials to other user agents belonging to the same user, and
certificates to any user agent. 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 the UA the certificate
in a NOTIFY and sends a subsequent NOTIFY any time the certificate
changes. When a credential is requested, the Credential Service
digest challenges the requesting UA to authenticate it so that the
Credential Service can verify that the UA is authorized to receive
the requested credentials. When a credential is published, the
Credential Service digest challenges the requesting UA to
authenticate it so that the Credential Service can verify that the UA
is authorized to change the credentials. This behavior is defined in
Section 7 and Section 8.
7. Event Package Formal Definition for "certificate"
7.1 Event Package Name
This document defines a SIP Event Package as defined in RFC 3265 [4].
The event-package token name for this package is:
certificate
7.2 Event Package Parameters
This package does not define any event package parameters.
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7.3 SUBSCRIBE Bodies
This package does not define any SUBSCRIBE bodies.
7.4 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.
7.5 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 [10]) that
contains the certificate, unless an Accept header has negotiated some
other type. The Content-Disposition MUST be set to "signal".
A future extension MAY define other NOTIFY bodies. If no "Accept"
header is present in the SUBSCRIBE, the body type defined in this
document MUST be assumed.
Implementations which generate large notifications are reminded to
follow the message size restrictions for unreliable transports
articulated in Section 18.1.1 of SIP.
7.6 Subscriber Generation of SUBSCRIBE Requests
UAs discover certificates by sending a SUBSCRIBE 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 message with an Expires header of zero.
7.7 Notifier Processing of SUBSCRIBE Requests
When a SIP Credential Server receives a SUBSCRIBE request with the
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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 Service receives a SUBSCRIBE 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.
7.8 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 AOR of the
resource being subscribed to. This server sending the NOTIFY needs
either to implement an Authentication Service (as described in SIP
Identity [2]) or else the server needs to be set up such that the
NOFIFY will be sent through an Authentication Service. Sending the
NOTIFY through the the Authentication Service requires the SUBSCRIBE
to have been routed through the Authentication Service, since the
NOTIFY is sent within the dialog formed by the SUBSCRIBE.
7.9 Subscriber Processing of NOTIFY Requests
The resulting NOTIFY will contain an application/pkix-cert body which
contains the requested certificate. The UA MUST follow the
procedures in Section 10.3 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 10.1 . 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 [2]. 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.
7.10 Handling of Forked Requests
This event package does not permit forked requests. At most one
subscription to this event type is permitted per resource.
7.11 Rate of Notifications
Notifiers SHOULD NOT generate NOTIFY requests more frequently than
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once per minute.
7.12 State Agents and Lists
Implementers MUST NOT implement state agents for this event type.
Likewise, implementations MUST NOT use the event list extension [19]
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.
7.13 Behavior of a Proxy Server
There are no additional requirements on a SIP Proxy, other than to
transparently forward the SUBSCRIBE and NOTIFY methods 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.
8. Event Package Formal Definition for "credential"
8.1 Event Package Name
This document defines a SIP Event Package as defined in RFC 3265 [4].
The event-package token name for this package is:
credential
8.2 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 entity-tag value at the time the notification was sent.
Considering how infrequently credentials are updated, this hint is
very likely to be the correct entity-tag to use in the SIP-If-Match
header in a SIP PUBLISH request to update the current credentials.
etag-param = "etag" EQUAL token
8.3 SUBSCRIBE Bodies
This package does not define any SUBSCRIBE bodies.
8.4 Subscription Duration
Subscriptions to this event package can range from hours to one week.
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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.
8.5 NOTIFY Bodies
The NOTIFY MUST contain a multipart/mixed (see [14]) 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".
A future extension MAY define other NOTIFY bodies. If no "Accept"
header 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
[10]. The application/pkcs8 body contains a DER-encoded PKCS#8 [1]
object that contains the private key. 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.
8.6 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 messages.
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8.7 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
should be set appropriately.
8.8 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 message. The Notifier SHOULD include the current
entity-tag value in the "etag" Event package parameter in the NOTIFY
request. The Authentication Service is applied to this NOTIFY
message 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 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.
8.9 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 this succeeds, the Credential
Service updates the credential for this URI, processes all the active
certificate and credential subscriptions to this URI, and generates a
NOTIFY request with the new credential or certificate. If the
Subscriber submits a PUBLISH requests 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.
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(Note that subscriptions to the certificate package are NOT
terminated, each subscriber to the certificate package receives a
notification with an empty body.)
8.10 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 with the private key, it MUST send a 493
Undecipherable response. Later if the user provides a new pass
phrase for the private key, the UA can subscribe to the credentials
again and attempt to decrypt with the new pass phrase.
8.11 Handling of Forked Requests
This event package does not permit forked requests.
8.12 Rate of Notifications
Notifiers SHOULD NOT generate NOTIFY requests more frequently than
once per minute.
8.13 State Agents and Lists
Implementers MUST NOT implement state agents for this event type.
Likewise, implementations MUST NOT use the event list extension [19]
with this event type.
8.14 Behavior of a Proxy Server
The behavior is identical to behavior described for certificate
subscriptions described in Section 7.13.
9. 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.
9.1 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
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...
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: "12dsfsdk2389403823cbed"
Identity-Info: sips:billoxi.example.com
....
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. Although outside the
scope of this document, it is worth noting that instant messages
often have common plain text like "Hi", so that setting up symmetric
keys for extended session mode IM conversations will likely increase
efficiency, as well as reducing the likelihood of compromising the
asymmetric key in the certificate.
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" >
9.2 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 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 Service for Alice and digest challenges the message to
authenticate her.
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PUBLISH sips:alice@atlanta.example.com SIP/2.0
...
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.
10. Security Considerations
The high level message flow from a security point of view is
summarized in the following figure. The 200 responses are removed
from the figure as they do not have much to do with the overall
security.
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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 the server. The UA authenticated the Server
using the certificates from the TLS handshake while 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 with provides
integrity and privacy protection and also authenticates the server to
Bob's UA. Next the UA requests its credentials using a SUBSCRIBE.
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 sever sends the 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. The SUBSCRIBE would be a PUBLISH and
there would be no NOTIFY. When this happened, all the other UAs that
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were subscribed to Bob's credentials would receive a new 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 NOTIFY. This does not need
to be sent over a privacy or integrity protected channel, as the
Authentication service described in [2] 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.
This specification requires the TLS session to be used for SIP
communications to the Credential Service. As specified in RFC 3261,
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 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 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
Service, a man in the middle attack is possible.
10.1 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
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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 the the credential is checked
for validity every time the certificate is used.
10.2 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.
10.3 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 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
that was subscribed to.
o The UA MUST check the Identity header as described in the Identity
[2] specification 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 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 message did come from the
identity claimed in the From and that the bodies and the From have
not been tampered with. The certificate in the body contains the
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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.
10.4 Conformity to the SACRED Framework
This specification 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.
10.5 Crypto Profiles
Credential Services 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_EDE_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 RFC 2898 [12]. It is RECOMMENDED
that this profile be used when using PKCS#8.
10.6 User Certificate Generation
The certificates should be consistent with RFC 3280 [13]. 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.
10.7 Compromised Authentication Service
One of this worst attacks against this system is if the
Authentication Service were compromised. This attack is somewhat
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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.
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 resulting in a system that
was secure other than an leap of faith when the certificate first
arrives or is changed.
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.
11. 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.
11.1 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>
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11.2 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): This document
New Event header parameters: "etag"
Person & email address to contact for further information:
Cullen Jennings <fluffy@cisco.com>
11.3 PKCS#8
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: The PKCS#8 object inside this MIME type
MUST be DER-encoded.
This MIME type was designed for use with
protocols which can carry binary-encoded
data. Protocols which do not carry binary
data (which have line length or
character-set restrictions for example)
MUST use a reversible transfer encoding
(such as base64) to carry this MIME type.
Protocols which carry binary data SHOULD
use a transfer encoding of "binary".
Security considerations: Carries a cryptographic private key
Interoperability considerations: None
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Published specification: See reference to PKCS#8 [1]
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
12. 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, Magnus Nystrom, Paul
Hoffman, Dan Wing, Mike Hammer, Lyndsay Campbell, and Jason Fischl.
Rohan Mahy and Jonathan Rosenberg provided detailed review and text.
13. References
13.1 Normative References
[1] RSA Laboratories, "Private-Key Information Syntax Standard,
Version 1.2", PKCS 8, November 1993.
[2] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
draft-ietf-sip-identity-04 (work in progress), February 2005.
[3] Niemi, A., "Session Initiation Protocol (SIP) Extension for
Event State Publication", RFC 3903, October 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:
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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.
[12] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, September 2000.
[13] 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.
[14] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, November
1996.
[15] International Telecommunications Union, "Information technology
- Open Systems Interconnection - The Directory: Public-key and
attribute certificate frameworks", ITU-T Recommendation X.509,
ISO Standard 9594-8, March 2000.
13.2 Informational References
[16] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851, July
2004.
[17] Peterson, J., "S/MIME Advanced Encryption Standard (AES)
Requirement for the Session Initiation Protocol (SIP)", RFC
3853, July 2004.
[18] Gutmann, P., "Internet X.509 Public Key Infrastructure
Operational Protocols: Certificate Store Access via HTTP",
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draft-ietf-pkix-certstore-http-08 (work in progress), August
2004.
[19] Roach, A., Rosenberg, J. and B. Campbell, "A Session Initiation
Protocol (SIP) Event Notification Extension for Resource
Lists", draft-ietf-simple-event-list-07 (work in progress),
January 2005.
Authors' Addresses
Cullen Jennings
Cisco Systems
170 West Tasman Drive
MS: SJC-21/2
San Jose, CA 95134
USA
Phone: +1 408 921-9990
EMail: fluffy@cisco.com
Jon Peterson
NeuStar, Inc.
1800 Sutter St
Suite 570
Concord, CA 94520
US
Phone: +1 925/363-8720
EMail: jon.peterson@neustar.biz
URI: http://www.neustar.biz/
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