Internet DRAFT - draft-ietf-sip-certs
draft-ietf-sip-certs
Network Working Group C. Jennings
Internet-Draft Cisco Systems
Intended status: Standards Track J. Fischl, Ed.
Expires: March 25, 2011 Skype
September 21, 2010
Certificate Management Service for The Session Initiation Protocol (SIP)
draft-ietf-sip-certs-15
Abstract
This draft defines a Credential Service that allows Session
Initiation Protocol (SIP) User Agents (UAs) to use a SIP event
package to discover the certificates of other users. This mechanism
allows user agents that want to contact a given Address-of-Record
(AOR) to retrieve that AOR's certificate by subscribing to the
Credential Service, which returns an authenticated response
containing that certificate. The Credential Service also allows
users to store and retrieve their own certificates and private keys.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 25, 2011.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. UA Behavior with Certificates . . . . . . . . . . . . . . . . 9
5. UA Behavior with Credentials . . . . . . . . . . . . . . . . . 10
6. Event Package Formal Definition for "certificate" . . . . . . 11
6.1. Event Package Name . . . . . . . . . . . . . . . . . . . . 11
6.2. SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 11
6.3. Subscription Duration . . . . . . . . . . . . . . . . . . 11
6.4. NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . 11
6.5. Subscriber Generation of SUBSCRIBE Requests . . . . . . . 12
6.6. Notifier Processing of SUBSCRIBE Requests . . . . . . . . 12
6.7. Notifier Generation of NOTIFY Requests . . . . . . . . . . 12
6.8. Subscriber Processing of NOTIFY Requests . . . . . . . . . 13
6.9. Handling of Forked Requests . . . . . . . . . . . . . . . 13
6.10. Rate of Notifications . . . . . . . . . . . . . . . . . . 13
6.11. State Agents and Lists . . . . . . . . . . . . . . . . . . 13
6.12. Behavior of a Proxy Server . . . . . . . . . . . . . . . . 13
7. Event Package Formal Definition for "credential" . . . . . . . 14
7.1. Event Package Name . . . . . . . . . . . . . . . . . . . . 14
7.2. SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 14
7.3. Subscription Duration . . . . . . . . . . . . . . . . . . 14
7.4. NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . 14
7.5. Subscriber Generation of SUBSCRIBE Requests . . . . . . . 15
7.6. Notifier Processing of SUBSCRIBE Requests . . . . . . . . 15
7.7. Notifier Generation of NOTIFY Requests . . . . . . . . . . 15
7.8. Generation of PUBLISH Requests . . . . . . . . . . . . . . 16
7.9. Notifier Processing of PUBLISH Requests . . . . . . . . . 16
7.10. Subscriber Processing of NOTIFY Requests . . . . . . . . . 17
7.11. Handling of Forked Requests . . . . . . . . . . . . . . . 17
7.12. Rate of Notifications . . . . . . . . . . . . . . . . . . 17
7.13. State Agents and Lists . . . . . . . . . . . . . . . . . . 17
7.14. Behavior of a Proxy Server . . . . . . . . . . . . . . . . 18
8. Identity Signatures . . . . . . . . . . . . . . . . . . . . . 18
9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Encrypted Page Mode IM Message . . . . . . . . . . . . . . 18
9.2. Setting and Retrieving UA Credentials . . . . . . . . . . 19
10. Security Considerations . . . . . . . . . . . . . . . . . . . 20
10.1. Certificate Revocation . . . . . . . . . . . . . . . . . . 23
10.2. Certificate Replacement . . . . . . . . . . . . . . . . . 23
10.3. Trusting the Identity of a Certificate . . . . . . . . . . 23
10.3.1. Extra Assurance . . . . . . . . . . . . . . . . . . . 24
10.4. SACRED Framework . . . . . . . . . . . . . . . . . . . . . 25
10.5. Crypto Profiles . . . . . . . . . . . . . . . . . . . . . 25
10.6. User Certificate Generation . . . . . . . . . . . . . . . 26
10.7. Private Key Storage . . . . . . . . . . . . . . . . . . . 26
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10.8. Compromised Authentication Service . . . . . . . . . . . . 27
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
11.1. Certificate Event Package . . . . . . . . . . . . . . . . 28
11.2. Credential Event Package . . . . . . . . . . . . . . . . . 28
11.3. Identity Algorithm . . . . . . . . . . . . . . . . . . . . 28
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
13.1. Normative References . . . . . . . . . . . . . . . . . . . 29
13.2. Informational References . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
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1. Introduction
[RFC3261], as amended by [RFC3853], provides a mechanism for end-to-
end encryption and integrity using S/MIME [RFC3851]. Several
security properties of [RFC3261] depend on S/MIME, and yet it has not
been widely deployed. One reason is the complexity of providing a
reasonable certificate distribution infrastructure. This
specification proposes a way to address discovery, retrieval, and
management of certificates for SIP deployments. Combined with the
SIP Identity [RFC4474] specification, this specification allows users
to have certificates that are not signed by any well known
certification authority while still strongly binding the user's
identity to the certificate.
In addition, this specification provides a mechanism that allows SIP
User Agents such as IP phones to enroll and get their credentials
without any more configuration information than they commonly have
today. The end user expends no extra effort.
2. Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Certificate: A PKIX [RFC5280] style certificate containing a public
key and a list of identities in the SubjectAltName that are bound
to this key. The certificates discussed in this draft are
generally self-signed and use the mechanisms in the SIP Identity
[RFC4474] specification to vouch for their validity. Certificates
that are signed by a certification authority can also be used with
all the mechanisms in this draft, however, they need not be
validated by the receiver (although the receiver can validate them
for extra assurance; see Section 10.3.1).
Credential: For this document, credential means the combination of a
certificate and the associated private key.
Password Phrase: A password used to encrypt and decrypt a PKCS#8
private key.
3. Overview
The general approach is to provide a new SIP service referred to as a
"credential service" that allows SIP User Agents (UAs) to subscribe
to other users' certificates using a new SIP event package [RFC3265].
The certificate is delivered to the subscribing UA in a corresponding
SIP NOTIFY request. An Authentication Service as described in the
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SIP Identity [RFC4474] specification can be used to vouch for the
identity of the sender of the certificate by using the sender's proxy
domain certificate to sign the NOTIFY request. The Authentication
Service is vouching that the sender is allowed to populate the SIP
From header field value. The sender of the message is vouching that
this is an appropriate certificate for the user identified in the SIP
from header field value. The credential service can manage public
certificates as well as the user's private keys. Users can update
their credentials, as stored on the credential service, using a SIP
PUBLISH [RFC3903] request. The UA authenticates to the credential
service using a shared secret when a UA is updating a credential.
Typically the shared secret will be the same one that is used by the
UA to authenticate a REGISTER request with the Registrar for the
domain (usually with SIP Digest Authentication).
The following figure shows Bob publishing his credentials from one of
his User Agents (e.g. his laptop software client), retrieving his
credentials from another of his User Agents (e.g. his mobile phone),
and then Alice retrieving Bob's certificate and sending a message to
Bob. SIP 200-class responses are omitted from the diagram to make the
figure easier to understand.
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example.com domain
------------------
Alice Proxy Auth Cred Bob1 Bob2
| | | | TLS Handshake | |
| [ Bob generates ] |<--------------------->|
| [ credentials and ] | PUBLISH (credential) |
| [ publishes them ] |<----------------------|
| | | | Digest Challenge |
| | | |---------------------->|
| | | | PUBLISH + Digest |
| | | |<----------------------|
| | | | |
| | | | time passes... |
| | | | |
| | | | TLS Handshake |
| [ Bob later gets ] |<---------------->|
| [ back his own ] | SUBSCRIBE |
| [ credentials ] | (credential) |
| [ at another ] |<-----------------|
| [ User Agent ] | SUBSCRIBE+Digest |
| | | |<-----------------|
| | | | NOTIFY |
| | | |----------------->|
| | | | Bob Decrypts key |
| | | | |
| | | | |
| SUBSCRIBE (certificate) | Alice fetches |
|---------->|----->|----->| Bob's cert |
| | |NOTIFY| |
| NOTIFY+Identity |<-----| |
|<----------+------| | Alice uses cert |
| | | | to encrypt |
| MESSAGE | | | message to Bob |
|---------->|------+------+----------------->|
Bob's UA (Bob2) does a TLS [RFC5246] handshake with the credential
server to authenticate that the UA is connected to the correct
credential server. Then Bob's UA publishes his newly created or
updated credentials. The credential server digest challenges the UA
to authenticate that the UA knows Bob's shared secret. Once the UA
is authenticated, the credential server stores Bob's credentials.
Another of Bob's User Agents (Bob1) wants to fetch its current
credentials. It does a TLS [RFC5246] handshake with the credential
server to authenticate that the UA is connected to the correct
credential server. Then Bob's UA subscribes for the credentials.
The credential server digest challenges the UA to authenticate that
the UA knows Bob's shared secret. Once the UA is authenticated, the
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credential server sends a NOTIFY that contains Bob's credentials.
The private key portion of the credential may have been encrypted
with a secret that only Bob's UA (and not the credential server)
knows. In this case, once Bob's UA decrypts the private key it will
be ready to go. Typically Bob's UA would do this when it first
registered on the network.
Some time later Alice decides that she wishes to discover Bob's
certificate so that she can send him an encrypted message or so that
she can verify the signature on a message from Bob. Alice's UA sends
a SUBSCRIBE message to Bob's AOR. The proxy in Bob's domain routes
this to the credential server via an "authentication service" as
defined in [RFC4474]. The credential server returns a NOTIFY that
contains Bob's public certificate in the body. This is routed
through an authentication service that signs that this message really
can validly claim to be from the AOR "sip:bob@example.com". Alice's
UA receives the certificate and can use it to encrypt a message to
Bob.
It is critical to understand that the only way that Alice can trust
that the certificate really is the one for Bob and that the NOTIFY
has not been spoofed is for Alice to check that the Identity
[RFC4474] header field value is correct.
The mechanism described in this document works for both self-signed
certificates and certificates signed by well known certification
authorities. In order to deploy certificates signed by well known
certification authorities, certification authorities would have to
support adding SIP URIs to the SubjectAltName of the certificates
they generate. This is something which has been rarely implemented
by commercial certification authorities. However, most UAs would
only use self-signed certificates and would use an Authentication
Service as described in [RFC4474] to provide a strong binding of an
AOR to the certificates.
The mechanisms described in this draft allow for three different
styles of deployment:
1. Deployments where the credential server only stores certificates
and does not store any private key information. If the
deployment had users with multiple devices, some other scheme
(perhaps even manual provisioning) would be used to get the right
private keys onto all the devices that a user uses.
2. Deployments where the credential server stores certificates and
also stores an encrypted version of the private keys. The
credential server would not know or need the password phrase for
decrypting the private key. The credential server would help
move the private keys between devices but the user would need to
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enter a password phrase on each device to allow that device to
decrypt (and encrypt) the private key information.
3. Deployments where the credential server generates and stores the
certificates and private keys. Deployments such as these may not
use password phrases. Consequently, the private keys are not
encrypted inside the PKCS#8 objects. This style of deployment
would often have the credential server, instead of the devices,
create the credentials.
4. UA Behavior with Certificates
When a User Agent wishes to discover some other user's certificate it
subscribes to the "certificate" SIP event package as described in
Section 6 to get the certificate. While the subscription is active,
if the certificate is updated, the Subscriber will receive the
updated certificate in a notification.
The Subscriber needs to decide how long it is willing to trust that
the certificate it receives is still valid. If the certificate is
revoked before it expires, the Notifier will send a notification with
an empty body to indicate that the certificate is no longer valid.
If the certificate is renewed before it expires, the Notifier will
send a notification with a body containing the new certificate. Note
that the Subscriber might not receive the notification if an attacker
blocks this traffic. The amount of time that the Subscriber caches a
certificate SHOULD be configurable. A default of one day is
RECOMMENDED.
Note that the actual duration of the subscription is unrelated to the
caching time or validity time of the corresponding certificate.
Allowing subscriptions to persist after a certificate is no longer
valid ensures that Subscribers receive the replacement certificate in
a timely fashion. The Notifier could return an immediate
notification with the certificate in response to subscribe and then
immediately terminate subscription, setting the reason parameter to
"probation". The Subscriber will have to periodically poll the
Notifier to verify validity of the certificate.
If the UA uses a cached certificate in a request and receives a 437
(Unsupported Certificate) response, it SHOULD remove the certificate
it used from the cache, attempt to fetch the certificate again. If
the certificate is changed, then the UA SHOULD retry the original
request again with the new certificate. This situation usually
indicates that the certificate was recently updated, and that the
Subscriber has not received a corresponding notification. If the
certificate fetched is the same as the one that was previously in the
cache, then the UA SHOULD NOT try the request again. This situation
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can happened when the request was retargeted to a different user than
the original request. The 437 response is defined in [RFC4474].
Note: A UA that has a presence list MAY want to subscribe to the
certificates of all the presentities in the list when the UA
subscribes to their presence, so that when the user wishes to
contact a presentity, the UA will already have the appropriate
certificate. Future specifications might consider the possibility
of retrieving the certificates along with the presence documents.
The details of how a UA deals with receiving encrypted messages is
outside the scope of this specification. It is worth noting that if
Charlie's UAS receives a request that is encrypted to Bob, it would
be valid and legal for that UA to send a 302 redirecting the call to
Bob.
5. UA Behavior with Credentials
UAs discover their own credentials by subscribing to their AOR with
an event type of credential as described in Section 7. After a UA
registers, it SHOULD retrieve its credentials by subscribing to them
as described in Section 6.5.
When a UA discovers its credential, the private key information might
be encrypted with a password phrase. The UA SHOULD request that the
user enter the password phrase on the device, and the UA MAY cache
this password phrase for future use.
There are several different cases in which a UA should generate a new
credential:
o If the UA receives a NOTIFY with no body for the credential
package.
o If the certificate has expired.
o If the certificate's notAfter date is within the next 600 seconds,
the UA SHOULD attempt to create replacement credentials. The UA
does this by waiting a random amount of time between 0 and 300
seconds. If no new credentials have been received in that time,
the UA creates new credentials to replace the expiring ones and
sends them in a PUBLISH request following the rules for modifying
event state from Section 4.4 of [RFC3903].
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 as described in [RFC4086] and then creating a
certificate as described in Section 10.6. The UA MAY encrypt the
private key with a password phrase supplied by the user as specified
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in Section 10.5. Next, the UA updates the user's credential by
sending a PUBLISH [RFC3903] request with the credentials or just the
certificate as described in Section 7.8.
If a UA wishes to revoke the existing certificate without publishing
a new one, it MUST send a PUBLISH with an empty body to the
credential server.
6. Event Package Formal Definition for "certificate"
6.1. Event Package Name
This document defines a SIP Event Package as defined in [RFC3265].
The event-package token name for this package is:
certificate
6.2. SUBSCRIBE Bodies
This package does not define any SUBSCRIBE bodies.
6.3. Subscription Duration
Subscriptions to this event package can range from no time to weeks.
Subscriptions in days are more typical and are RECOMMENDED. The
default subscription duration for this event package is one day.
The credential service is encouraged to keep the subscriptions active
for AORs that are communicating frequently, but the credential
service MAY terminate the subscription at any point in time.
6.4. NOTIFY Bodies
The body of a NOTIFY request for this package MUST either be empty or
contain an application/pkix-cert body (as defined in [RFC2585]) that
contains the certificate, unless an Accept header field has
negotiated some other type. The Content-Disposition MUST be set to
"signal" as defined in [RFC3204].
A future extension MAY define other NOTIFY bodies. If no "Accept"
header field is present in the SUBSCRIBE, the body type defined in
this document MUST be assumed.
Implementations which generate large notifications are reminded to
follow the message size restrictions for unreliable transports
articulated in Section 18.1.1 of SIP.
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6.5. Subscriber Generation of SUBSCRIBE Requests
A UA discovers a certificate by sending a SUBSCRIBE request with an
event type of "certificate" to the AOR for which a certificate is
desired. In general, the UA stays subscribed to the certificate for
as long as it plans to use and cache the certificate, so that the UA
can be notified about changes or revocations to the certificate.
Subscriber User Agents will typically subscribe to certificate
information for a period of hours or days, and automatically attempt
to re-subscribe just before the subscription is completely expired.
When a user de-registers from a device (logoff, power down of a
mobile device, etc.), subscribers SHOULD unsubscribe by sending a
SUBSCRIBE request with an Expires header field of zero.
6.6. Notifier Processing of SUBSCRIBE Requests
When a SIP credential server receives a SUBSCRIBE request with the
certificate event-type, it is not necessary to authenticate the
subscription request. The Notifier MAY limit the duration of the
subscription to an administrator-defined period of time. The
duration of the subscription does not correspond in any way to the
period for which the certificate will be valid.
When the credential server receives a SUBSCRIBE request for a
certificate, it first checks to see if it has credentials for the
requested URI. If it does not have a certificate, it returns a
NOTIFY request with an empty message body.
6.7. Notifier Generation of NOTIFY Requests
Immediately after a subscription is accepted, the Notifier MUST send
a NOTIFY with the current certificate, or an empty body if no
certificate is available for the target user. In either case it
forms a NOTIFY with the From header field value set to the value of
the To header field in the SUBSCRIBE request. This server sending
the NOTIFY needs either to implement an Authentication Service (as
described in SIP Identity [RFC4474]) or else the server needs to be
set up such that the NOTIFY request will be sent through an
Authentication Service. Sending the NOTIFY request through the
Authentication Service requires the SUBSCRIBE request to have been
routed through the Authentication Service, since the NOTIFY is sent
within the dialog formed by the subscription.
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6.8. Subscriber Processing of NOTIFY Requests
The resulting NOTIFY will contain an application/pkix-cert body that
contains the requested certificate. The UA MUST follow the
procedures in Section 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 [RFC4474]. If the identity asserted by the
Authentication Service does not match the AOR that the UA subscribed
to, the certificate in the NOTIFY is discarded and MUST NOT be used.
6.9. Handling of Forked Requests
This event package does not permit forked requests. At most one
subscription to this event type is permitted per resource.
6.10. Rate of Notifications
Notifiers SHOULD NOT generate NOTIFY requests more frequently than
once per minute.
6.11. State Agents and Lists
The credential server described in this section which serves
certificates is a state agent as defined in [RFC3265] and
implementations of the credential server MUST be implemented as a
state agent.
Implementers MUST NOT use the event list extension [RFC4662] with
this event type. It is not possible to make such an approach work,
because the Authentication service would have to simultaneously
assert several different identities.
6.12. Behavior of a Proxy Server
There are no additional requirements on a SIP Proxy, other than to
transparently forward the SUBSCRIBE and NOTIFY requests as required
in SIP. This specification describes the Proxy, Authentication
service, and credential service as three separate services, but it is
certainly possible to build a single SIP network element that
performs all of these services at the same time.
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7. Event Package Formal Definition for "credential"
7.1. Event Package Name
This document defines a SIP Event Package as defined in [RFC3265].
The event-package token name for this package is:
credential
7.2. SUBSCRIBE Bodies
This package does not define any SUBSCRIBE bodies.
7.3. Subscription Duration
Subscriptions to this event package can range from hours to one week.
Subscriptions in days are more typical and are RECOMMENDED. The
default subscription duration for this event package is one day.
The credential service SHOULD keep subscriptions active for UAs that
are currently registered.
7.4. NOTIFY Bodies
An implementation compliant to this specification MUST support the
multipart/mixed type (see [RFC2046]). This allows a notification to
contain multiple resource documents including at a minimum the
application/pkix-cert body with the certificate and an application/
pkcs8 body that has the associated private key information for the
certificate. The application/pkcs8 media type is defined in
[RFC5958].
The absence of an Accept header in the SUBSCRIBE indicates support
for multipart/mixed and the content types application/pkix-cert and
application/pkcs8. If an Accept header is present, these types MUST
be included, in additional to any other types supported by the
client.
The application/pkix-cert body is a DER encoded X.509v3 certificate
[RFC2585]. The application/pkcs8 body contains a DER-encoded
[RFC5958] 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.
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7.5. 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 directly connect to the server
acting as the credential service or to a server that is authoritative
for the domain of the credential service. The UA MUST NOT connect
through an intermediate proxy to the credential service. The UA may
be configured with a specific name for the credential service;
otherwise normal SIP routing is used. As described in RFC 3261, the
TLS connection needs to present a certificate that matches the
expected name of the server to which the connection was formed, so
that the UA knows it is talking to the correct server. Failing to do
this may result in the UA publishing its private key information to
an attacker. The credential service will authenticate the UA using
the usual SIP Digest mechanism, so the UA can expect to receive a SIP
challenge to the SUBSCRIBE or PUBLISH requests.
7.6. Notifier Processing of SUBSCRIBE Requests
When a credential service receives a SUBSCRIBE for a credential, the
credential service has to authenticate and authorize the UA and
validate that adequate transport security is being used. Only a UA
that can authenticate as being able to register as the AOR is
authorized to receive the credentials for that AOR. The Credential
Service MUST digest challenge the UA to authenticate the UA and then
decide if it is authorized to receive the credentials. If
authentication is successful, the Notifier MAY limit the duration of
the subscription to an administrator-defined period of time. The
duration of the subscription MUST NOT be larger than the length of
time for which the certificate is still valid. The Expires header
field SHOULD be set so that it is not longer than the notAfter date
in the certificate.
7.7. Notifier Generation of NOTIFY Requests
Once the UA has authenticated with the credential service and the
subscription is accepted, the credential service MUST immediately
send a Notify request. The Authentication Service is applied to this
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NOTIFY request in the same way as the certificate subscriptions. If
the credential is revoked, the credential service MUST terminate any
current subscriptions and force the UA to re-authenticate by sending
a NOTIFY with its Subscription-State header field set to "terminated"
and a reason parameter of "deactivated". (This causes a Subscriber
to retry the subscription immediately.) This is so that if a secret
for retrieving the credentials gets compromised, the rogue UA will
not continue to receive credentials after the compromised secret has
been changed.
Any time the credentials for this URI change, the credential service
MUST send a new NOTIFY to any active subscriptions with the new
credentials.
The notification MUST be sent over TLS so that it is integrity
protected and the TLS needs to be directly connected between the UA
and the credential service with no intermediaries.
7.8. Generation of PUBLISH Requests
A user agent SHOULD be configurable to control whether it publishes
the credential for a user or just the user's certificate.
When publishing just a certificate, the body contains an application/
pkix-cert. When publishing a credential, the body contains a
multipart/mixed containing both an application/pkix-cert and an
application/pkcs8 body.
When the UA sends the PUBLISH [RFC3903] request, it needs to do the
following:
o The UA MUST use TLS to directly connect to the server acting as
the credential service or to a server that is authoritative for
the domain of the credential service. The UA MUST NOT connect
through an intermediate proxy to the credential service.
o The Expires header field value in the PUBLISH request SHOULD be
set to match the time for which the certificate is valid.
o If the certificate includes Basic Constraints, it SHOULD set the
cA flag to false.
7.9. 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 the authorization succeeds, then
the credential service MUST perform the following check on the
certificate:
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o The notBefore validity time MUST NOT be in the future.
o The notAfter validity time MUST be in the future.
o If a cA Basic Constraint flag is set in the certificate, it is set
to false.
If all of these succeed, the credential service updates the
credential for this URI, processes all the active certificates and
credential subscriptions to this URI, and generates a NOTIFY request
with the new credential or certificate. Note the SubjectAltName
SHOULD NOT be checked as that would restrict which certificates could
be used and offers no additional security guarantees.
If the Subscriber submits a PUBLISH request with no body and
Expires=0, this revokes the current credentials. Watchers of these
credentials will receive update with no body indicating that they
MUST stop any previously stored credentials. Note that subscriptions
to the certificate package are NOT terminated; each subscriber to the
certificate package receives a notification with an empty body.
7.10. 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, it MUST send a 437 (Unsupported
Certificate) response. Later if the user provides a new password
phrase for the private key, the UA can subscribe to the credentials
again and attempt to decrypt with the new password phrase.
7.11. Handling of Forked Requests
This event package does not permit forked requests.
7.12. Rate of Notifications
Notifiers SHOULD NOT generate NOTIFY requests more frequently than
once per minute.
7.13. State Agents and Lists
The credential server described in this section which serves
credentials is a state agent and implementations of the credential
server MUST be implemented as a state agent.
Implementers MUST NOT use the event list extension [RFC4662] with
this event type.
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7.14. Behavior of a Proxy Server
The behavior is identical to behavior described for certificate
subscriptions described in Section 6.12.
8. Identity Signatures
The [RFC4474] Authentication service defined an signature algorithm
based on SHA-1 called rsa-sha1. This specification adds an signature
algorithm that is roughly the same but based on SHA-256 and called
rsa-sha256.
When using the rsa-sha256 algorithm, the signature MUST be computed
in exactly the same way as described in section 9 of [RFC4474] with
the exception that instead of using sha1WithRSAEncryption, the
computation is done using sha256WithRSAEncryption as described in
[RFC5754].
Implementations of this specification MUST implement both rsa-sha1
and rsa-sha256. The IANA registration for rsa-sha256 is defined in
Section 11.3.
9. Examples
In all these examples, large parts of the messages are omitted to
highlight what is relevant to this draft. The lines in the examples
that are prefixed by $ represent encrypted blocks of data.
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
...
Event: certificate
The credential service responds with the certificate in a NOTIFY.
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NOTIFY alice@atlanta.example.com SIP/2.0
Subscription-State: active; expires=7200
....
From: <sip:bob@biloxi.example.com>;tag=1234
Identity: ".... stuff removed ...."
Identity-Info: <https://atlanta.example.com/cert>;alg=rsa-sha256
....
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.
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 certificate and private key
to the credential service, it sends a PUBLISH request like the one
below. This must be sent over a TLS connection directly to the
domain of the credential service. The credential service presents a
certificate where the SubjectAltName contains an entry that matches
the domain name in the request line of the PUBLISH request and digest
challenges the request to authenticate her.
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PUBLISH sips:alice@atlanta.example.com SIP/2.0
...
Event: credential
Content-Type: multipart/mixed;boundary=boundary
Content-Disposition: signal
--boundary
Content-ID: 123
Content-Type: application/pkix-cert
< Public certificate for Alice >
--boundary
Content-ID: 456
Content-Type: application/pkcs8
< Private Key for Alice >
--boundary
If one of Alice's UAs subscribes to the credential event, the UA will
be digest challenged, and the NOTIFY will include a body similar to
the one in the PUBLISH section above.
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.
In this figure, authC refers to authentication and authZ refers to
authorization.
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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 credential server. The UA authenticated the
Server using the certificates from the TLS handshake. The Server
authenticated the UA using a digest style challenge with a shared
secret.
The UA, labeled Bob, wishes to request its credentials from the
server. First it forms a TLS connection to the Server, which
provides integrity and privacy protection and also authenticates the
server to Bob's UA. Next the UA requests its credentials using a
SUBSCRIBE request. The Server digest challenges this to authenticate
Bob's UA. The server and Bob's UA have a shared secret that is used
for this. If the authentication is successful, the server sends the
credentials to Bob's UA. The private key in the credentials may have
been encrypted using a shared secret that the server does not know.
A similar process would be used for Bob's UA to publish new
credentials to the server. Bob's UA would send a PUBLISH request
containing the new credentials. When this happened, all the other
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UAs that were subscribed to Bob's credentials would receive a NOTIFY
with the new credentials.
Alice wishes to find Bob's certificate and sends a SUBSCRIBE to the
server. The server sends the response in a NOTIFY. This does not
need to be sent over a privacy or integrity protected channel, as the
Authentication service described in [RFC4474] provides integrity
protection of this information and signs it with the certificate for
the domain.
This whole scheme is highly dependent on trusting the operators of
the credential service and trusting that the credential service will
not be compromised. The security of all the users will be
compromised if the credential service is compromised.
Note: There has been significant discussion of the topic of
avoiding deployments in which the credential servers store the
private keys, even in some encrypted form that the credential
server does not know how to decrypt. Various schemes were
considered to avoid this but they all result in either moving the
problem to some other server, which does not seem to make the
problem any better, or having a different credential for each
device. For some deployments where each user has only one device
this is fine but for deployments with multiple devices, it would
require that when Alice went to contact Bob, Alice would have to
provide messages encrypted for all of Bob's devices. The sipping
working group did consider this architecture and decided it was
not appropriate due both to the information it revealed about the
devices and users and the amount of signaling required to make it
work.
This specification requires that TLS be used for the SIP
communications to place and retrieve a UA's private key. This
provides security in two ways:
1. Confidentiality is provided for the digest authentication
exchange, thus protecting it from dictionary attacks.
2. Confidentiality is provided for the private key, thus protecting
it from being exposed to passive attackers.
In order to prevent man-in-the-middle attacks, TLS clients MUST check
that the SubjectAltName of the certificate for the server they
connected to exactly matches the server they were trying to connect
to. The connection must be directly connected to the correct server
or any intermediaries in the TLS path can compromise the certificate
and instead provide a certificate for which the attacker knows the
private key. This may lead the UA that relies on this compromised
certificate to lose confidential information. Failing to use TLS or
selecting a poor cipher suite (such as NULL encryption) may result in
credentials, including private keys, being sent unencrypted over the
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network and will render the whole system useless.
The correct checking of chained certificates as specified in TLS
[RFC5246] 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
applications, a default time of 1 day is suggested, but for some
applications it may be desirable to set the time to zero so that no
certificates are cached at all and the credential is checked for
validity every time the certificate is used.
The UA MUST NOT cache the certificates for a period longer than that
of the subscription duration. This is to avoid the UA using invalid
cached credentials when the notifier of the new credentials has been
prevented from updating the UA.
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 before the normal expiration.
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
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NOTIFY request. The term original URI is used to describe the URI
that was in the To header field value of the SUBSCRIBE request. So
in this case the original URI would be sip:alice@example.com.
If the certificate is signed by a trusted certification authority,
and one of the names in the SubjectAltName matches the original URI,
then this certificate MAY be used but only for exactly the original
URI and not for other identities found in the SubjectAltName.
Otherwise, there are several steps the UA MUST perform before using
this certificate.
o The From header field in the NOTIFY request MUST match the
original URI that was subscribed to.
o The UA MUST check the Identity header field as described in the
Identity [RFC4474] specification to validate that bodies have not
been tampered with and that an Authentication Service has
validated this From header field.
o The UA MUST check the validity time of the certificate and stop
using the certificate if it is invalid. (Implementations are
reminded to verify both the notBefore and notAfter validity
times.)
o The certificate MAY have several names in the SubjectAltName but
the UA MUST only use this certificate when it needs the
certificate for the identity asserted by the Authentication
Service in the NOTIFY. This means that the certificate should
only be indexed in the certificate cache by the AOR that the
Authentication Service asserted and not by the value of all the
identities found in the SubjectAltName list.
These steps result in a chain of bindings that result in a trusted
binding between the original AOR that was subscribed to and a public
key. The original AOR is forced to match the From. The
Authentication Service validates that this request did come from the
identity claimed in the From header field value and that the bodies
in the request that carry the certificate have not been tampered
with. The certificate in the body contains the public key for the
identity. Only the UA that can authenticate as this AOR, or devices
with access to the private key of the domain, can tamper with this
body. This stops other users from being able to provide a false
public key. This chain of assertion from original URI, to From, to
body, to public key is critical to the security of the mechanism
described in this specification. If any of the steps above are not
followed, this chain of security will be broken and the system will
not work.
10.3.1. Extra Assurance
Although the certificates used with this document need not be
validatable to a trust anchor via PKIX [RFC5280] procedures,
certificates which can be validated may also be distributed via this
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mechanism. Such certificates potentially offer an additional level
of security because they can be used with the secure (and partially
isolated) certification authority user verification and key issuance
toolset, rather than depending on the security of generic SIP
implementations.
When a relying party receives a certificate which is not self-signed,
it MAY attempt to validate it using the rules in Section 6 of
[RFC5280]. If the certificate validates successfully and the names
correctly match the user's AOR (see Section 10.6), then the
implementation SHOULD provide some indication that the certificate
has been validated with an external authority. In general, failure
to validate a certificate via this mechanism SHOULD NOT be used as a
reason to reject the certificate. However, if the certificate is
revoked, then the implementation SHOULD reject it.
10.4. SACRED Framework
This specification includes a mechanism that allows end users to
share the same credentials across different end-user devices. This
mechanism is based on the one presented in the SACRED Framework
[RFC3760]. While this mechanism is fully described in this document,
the requirements and background are more thoroughly discussed in
[RFC3760].
Specifically, Section 7.5, Section 7.6 and Section 7.9 follow the
cTLS architecture described in section 4.2.2 of [RFC3760]. The
client authenticates the server using the server's TLS certificate.
The server authenticates the client using a SIP digest transaction
inside the TLS session. The TLS sessions form a strong session key
that is used to protect the credentials being exchanged.
10.5. Crypto Profiles
Credential Services SHOULD implement the server name indication
extensions in [RFC4366]. As specified in [RFC5246], Credential
Services MUST support the TLS cipher suite
TLS_RSA_WITH_AES_128_CBC_SHA. In addition, they MUST support the TLS
cipher suite TLS_RSA_WITH_AES_128_CBC_SHA256 as specified in
[RFC5246]. If additional cipher suites are supported, then
implementations MUST NOT negotiate a cipher suite that employs NULL
encryption, integrity, or authentication algorithms.
Implementations of TLS typically support multiple versions of the
Transport Layer Security protocol as well as the older Secure Sockets
Layer (SSL) protocol. Because of known security vulnerabilities,
clients and servers MUST NOT request, offer, or use SSL 2.0. See
Appendix E.2 of [RFC5246]for further details.
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The PKCS#8 in the clients MUST implement PBES2 with a key derivation
algorithm of PBKDF2 using HMAC with SHA-256 [RFC5754] and an
encryption algorithm of id-aes128-wrap-pad as defined in [RFC5649].
Some pre-standard deployments of this specification used DES-EDE2-
CBC-Pad as defined in [RFC2898] so, for some implementations, it may
be desirable to also support that algorithm. A different passphrase
SHOULD be used for the PKCS#8 encryption than is used for
authentication of the client. It is important to choose an
sufficiently strong passphrases. Specific advice on the passphrase
can be found in section 6 of [RFC5958].
10.6. User Certificate Generation
The certificates need to be consistent with [RFC5280]. The
sha1WithRSAEncryption and sha256WithRSAEncryption algorithm for the
signatureAlgorithm 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.
When creating a new key pair for a certificate, it is critical to
have appropriate randomness as described in [RFC4086]. This can be
challenging on some embedded devices such as some IP Phones and
implementors should pay particular attention to this point.
It is worth noting that a UA can discover the current time by looking
at the Date header field value in the 200 response to a REGISTER
request.
10.7. Private Key Storage
The protection afforded private keys is a critical security factor.
On a small scale, failure of devices to protect the private keys will
permit an attacker to masquerade as the user or decrypt their
personal information. As noted in the SACRED Framework, when stored
on an end user device, such as a diskette or hard drive, credentials
SHOULD NOT be in the clear. It is RECOMMENDED that private keys be
stored securely in the device, more specifically encrypting them
using tamper-resistant hardware encryption and exposing them only
when required: for example, the private key is decrypted when needed
to generate a digital signature, and re-encrypted immediately to
limit exposure in the RAM for a short period of time. Some
implementations may limit access to private keys by prompting users
for a PIN prior to allowing access to the private key.
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On the server side, the protection of unencrypted PKCS#8 objects is
equally important. A server failing to protect the private keys
would be catastrophic as attackers with access to unencrypted PKCS#8
object could masquerade as any user whose private key was not
encrypted. Therefore, it is also recommended that the private keys
be stored securely in the server, more specifically encrypting them
using tamper-resistant hardware encryption and exposing them only
when required.
FIPS 140-2 [FIPS-140-2] provides useful guidance on secure storage.
10.8. Compromised Authentication Service
One of this worst attacks against this system would be if the
Authentication Service were compromised. This attack is somewhat
analogous to a certification authority 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 significantly before their
expiry date.
The UA MAY also allow the user to see the fingerprints for the cached
certificates so that they can be verified by some other out of band
means.
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
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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>
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
Person & email address to contact for further information:
Cullen Jennings <fluffy@cisco.com>
11.3. Identity Algorithm
IANA will add the following entry to the "Identity-Info Algorithm
Parameter Values" registry. Note to RFC Editor: Please replace
RFCAAAA with the number for this RFC.
'alg' Parameter Name Reference
---------------------- ---------
rsa-sha256 [RFCAAAA]
12. Acknowledgments
Many thanks to Eric Rescorla, Russ Housley, Jim Schaad, Rohan Mahy
and Sean Turner for significant help, discussion, and text. Many
others provided useful comments and text, including Kumiko Ono, Peter
Gutmann, Yaron Pdut, Aki Niemi, Magnus Nystrom, Paul Hoffman, Adina
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Simu, Dan Wing, Mike Hammer, Pasi Eronen, Alexey Melnikov, Tim Polk,
John Elwell, Jonathan Rosenberg and Lyndsay Campbell.
13. References
13.1. Normative References
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP",
RFC 2585, May 1999.
[RFC3204] Zimmerer, E., Peterson, J., Vemuri, A., Ong, L., Audet,
F., Watson, M., and M. Zonoun, "MIME media types for ISUP
and QSIG Objects", RFC 3204, December 2001.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension
for Event State Publication", RFC 3903, October 2004.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
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[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
[RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
Message Syntax", RFC 5754, January 2010.
[RFC5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard
(AES) Key Wrap with Padding Algorithm", RFC 5649,
September 2009.
[RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958,
August 2010.
13.2. Informational References
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, September 2000.
[RFC3760] Gustafson, D., Just, M., and M. Nystrom, "Securely
Available Credentials (SACRED) - Credential Server
Framework", RFC 3760, April 2004.
[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
[RFC3853] Peterson, J., "S/MIME Advanced Encryption Standard (AES)
Requirement for the Session Initiation Protocol (SIP)",
RFC 3853, July 2004.
[RFC4662] Roach, A., Campbell, B., and J. Rosenberg, "A Session
Initiation Protocol (SIP) Event Notification Extension for
Resource Lists", RFC 4662, August 2006.
[FIPS-140-2]
NIST, "Security Requirements for Cryptographic Modules",
June 2005, <http://csrc.nist.gov/publications/fips/
fips140-2/fips1402.pdf>.
Jennings & Fischl Expires March 25, 2011 [Page 30]
Internet-Draft SIP Certificates September 2010
Authors' Addresses
Cullen Jennings
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
Phone: +1 408 421-9990
Email: fluffy@cisco.com
Jason Fischl (editor)
Skype
2145 Hamilton Ave.
San Jose, CA 95125
USA
Phone: +1-415-202-5192
Email: jason.fischl@skype.net
Jennings & Fischl Expires March 25, 2011 [Page 31]
