Network Working Group C. Jennings Internet-Draft Cisco Systems Intended status: Standards Track J. Fischl, Ed. Expires: March 12, 2010 Skype September 8, 2009 Certificate Management Service for The Session Initiation Protocol (SIP) draft-ietf-sip-certs-09 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on March 12, 2010. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. Jennings & Fischl Expires March 12, 2010 [Page 1] Internet-Draft SIP Certificates September 2009 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. 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. Jennings & Fischl Expires March 12, 2010 [Page 2] Internet-Draft SIP Certificates September 2009 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. Event Package Parameters . . . . . . . . . . . . . . . . . 11 6.3. SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 11 6.4. Subscription Duration . . . . . . . . . . . . . . . . . . 11 6.5. NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . 11 6.6. Subscriber Generation of SUBSCRIBE Requests . . . . . . . 12 6.7. Notifier Processing of SUBSCRIBE Requests . . . . . . . . 12 6.8. Notifier Generation of NOTIFY Requests . . . . . . . . . . 12 6.9. Subscriber Processing of NOTIFY Requests . . . . . . . . . 13 6.10. Handling of Forked Requests . . . . . . . . . . . . . . . 13 6.11. Rate of Notifications . . . . . . . . . . . . . . . . . . 13 6.12. State Agents and Lists . . . . . . . . . . . . . . . . . . 13 6.13. Behavior of a Proxy Server . . . . . . . . . . . . . . . . 13 7. Event Package Formal Definition for "credential" . . . . . . . 14 7.1. Event Package Name . . . . . . . . . . . . . . . . . . . . 14 7.2. Event Package Parameters . . . . . . . . . . . . . . . . . 14 7.3. SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 14 7.4. Subscription Duration . . . . . . . . . . . . . . . . . . 14 7.5. NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . 14 7.6. Subscriber Generation of SUBSCRIBE Requests . . . . . . . 15 7.7. Notifier Processing of SUBSCRIBE Requests . . . . . . . . 15 7.8. Notifier Generation of NOTIFY Requests . . . . . . . . . . 16 7.9. Generation of PUBLISH Requests . . . . . . . . . . . . . . 16 7.10. Notifier Processing of PUBLISH Requests . . . . . . . . . 17 7.11. Subscriber Processing of NOTIFY Requests . . . . . . . . . 17 7.12. Handling of Forked Requests . . . . . . . . . . . . . . . 17 7.13. Rate of Notifications . . . . . . . . . . . . . . . . . . 17 7.14. State Agents and Lists . . . . . . . . . . . . . . . . . . 18 7.15. Behavior of a Proxy Server . . . . . . . . . . . . . . . . 18 8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 8.1. Encrypted Page Mode IM Message . . . . . . . . . . . . . . 18 8.2. Setting and Retrieving UA Credentials . . . . . . . . . . 19 9. Security Considerations . . . . . . . . . . . . . . . . . . . 20 9.1. Certificate Revocation . . . . . . . . . . . . . . . . . . 22 9.2. Certificate Replacement . . . . . . . . . . . . . . . . . 23 9.3. Trusting the Identity of a Certificate . . . . . . . . . . 23 9.4. SACRED Framework . . . . . . . . . . . . . . . . . . . . . 24 9.5. Crypto Profiles . . . . . . . . . . . . . . . . . . . . . 24 9.6. User Certificate Generation . . . . . . . . . . . . . . . 24 9.7. Compromised Authentication Service . . . . . . . . . . . . 25 Jennings & Fischl Expires March 12, 2010 [Page 3] Internet-Draft SIP Certificates September 2009 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 10.1. Certificate Event Package . . . . . . . . . . . . . . . . 26 10.2. Credential Event Package . . . . . . . . . . . . . . . . . 26 10.3. PKCS#8 . . . . . . . . . . . . . . . . . . . . . . . . . . 27 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 12.1. Normative References . . . . . . . . . . . . . . . . . . . 28 12.2. Informational References . . . . . . . . . . . . . . . . . 29 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29 Jennings & Fischl Expires March 12, 2010 [Page 4] Internet-Draft SIP Certificates September 2009 1. Introduction [RFC3261], as ammended 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 certificate 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 certificate authority can also be used with all the mechanisms in this draft, but it is expected that they are used purely as a key carrier and that their validity is not checked. Credential: For this document, credential means the combination of a certificate and the associated private key. Password Phrase: A password used to encrypt 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 SIP Identity [RFC4474] specification can be used to vouch for the Jennings & Fischl Expires March 12, 2010 [Page 5] Internet-Draft SIP Certificates September 2009 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. Jennings & Fischl Expires March 12, 2010 [Page 6] Internet-Draft SIP Certificates September 2009 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 Jennings & Fischl Expires March 12, 2010 [Page 7] Internet-Draft SIP Certificates September 2009 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 certificate authorities. In order to deploy certificates signed by well known certificate authorities, certificate 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 certificate 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 Jennings & Fischl Expires March 12, 2010 [Page 8] Internet-Draft SIP Certificates September 2009 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 Jennings & Fischl Expires March 12, 2010 [Page 9] Internet-Draft SIP Certificates September 2009 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 6.6. 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 (with a SIP-If-Match header field set to the current etag). This makes credential collisions both unlikely and harmless. 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, and then creating a certificate as described in Section 9.6. The UA MAY encrypt the private key with a password Jennings & Fischl Expires March 12, 2010 [Page 10] Internet-Draft SIP Certificates September 2009 phrase supplied by the user. 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 7.9. 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. 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 etag value at the time the notification was sent. Considering how infrequently credentials are updated, this hint is very likely to be the correct etag to use in the SIP-If-Match header in a SIP PUBLISH request to update the current credentials. etag-param = "etag" EQUAL token 6.3. SUBSCRIBE Bodies This package does not define any SUBSCRIBE bodies. 6.4. 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. 6.5. 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 [RFC2585]) that contains the certificate, unless an Accept header field has Jennings & Fischl Expires March 12, 2010 [Page 11] Internet-Draft SIP Certificates September 2009 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. 6.6. 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. 6.7. 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. 6.8. 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 value of the To header field in the SUBSCRIBE request. This server sending Jennings & Fischl Expires March 12, 2010 [Page 12] Internet-Draft SIP Certificates September 2009 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. 6.9. 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 9.3 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 9.1. 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. 6.10. Handling of Forked Requests This event package does not permit forked requests. At most one subscription to this event type is permitted per resource. 6.11. Rate of Notifications Notifiers SHOULD NOT generate NOTIFY requests more frequently than once per minute. 6.12. State Agents and Lists The certificate server described in this section which serves certificates is a state agent and implementations of the certificate 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. 6.13. 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 Jennings & Fischl Expires March 12, 2010 [Page 13] Internet-Draft SIP Certificates September 2009 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. 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. 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 etag value at the time the notification was sent. Considering how infrequently credentials are updated, this hint is very likely to be the correct etag to use in the SIP-If-Match header field in a SIP PUBLISH request to update the current credentials. etag-param = "etag" EQUAL token 7.3. SUBSCRIBE Bodies This package does not define any SUBSCRIBE bodies. 7.4. 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. 7.5. NOTIFY Bodies The NOTIFY MUST contain a multipart/mixed (see [RFC2046]) 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" as defined in [RFC3204]. A future extension MAY define other NOTIFY bodies. If no "Accept" Jennings & Fischl Expires March 12, 2010 [Page 14] Internet-Draft SIP Certificates September 2009 header field 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 [RFC2585]. The application/pkcs8 body contains a DER-encoded [RFC5208] 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. 7.6. 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 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. 7.7. 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 Jennings & Fischl Expires March 12, 2010 [Page 15] Internet-Draft SIP Certificates September 2009 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. 7.8. 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 request. The Notifier SHOULD include the current etag value in the "etag" Event package parameter in the NOTIFY request. The Authentication Service is applied to this 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. 7.9. 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. Jennings & Fischl Expires March 12, 2010 [Page 16] Internet-Draft SIP Certificates September 2009 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. 7.10. 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: o One of the names in the SubjectAltName of the certificate matches the authorized user making the request. 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 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. If the Subscriber submits a PUBLISH request 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. (Note that subscriptions to the certificate package are NOT terminated; each subscriber to the certificate package receives a notification with an empty body.) 7.11. 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. 7.12. Handling of Forked Requests This event package does not permit forked requests. 7.13. Rate of Notifications Notifiers SHOULD NOT generate NOTIFY requests more frequently than once per minute. Jennings & Fischl Expires March 12, 2010 [Page 17] Internet-Draft SIP Certificates September 2009 7.14. 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. 7.15. Behavior of a Proxy Server The behavior is identical to behavior described for certificate subscriptions described in Section 6.13. 8. 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. 8.1. 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. NOTIFY alice@atlanta.example.com SIP/2.0 Subscription-State: active; expires=7200 .... From: ;tag=1234 Identity: "NJguAbpmYXjnlxFmlOkumMI+MZXjB2iV/NW5xsFQqzD/p4yiovrJBqhd3T ZkegnsmoHryzk9gTBH7Gj/erixEFIf82o3Anmb+CIbrgdl03gGaD6ICvkp VqoMXZZjdvSpycyHOhh1cmUx3b9Vr3pZuEh+cB01pbMQ8B1ch++iMjw=" Identity-Info: ;alg=rsa-sha1 .... Event: certificate Content-Type: application/pkix-cert Content-Disposition: signal < certificate data > Jennings & Fischl Expires March 12, 2010 [Page 18] Internet-Draft SIP Certificates September 2009 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" > 8.2. 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 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. 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. Jennings & Fischl Expires March 12, 2010 [Page 19] Internet-Draft SIP Certificates September 2009 9. 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. 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 Jennings & Fischl Expires March 12, 2010 [Page 20] Internet-Draft SIP Certificates September 2009 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 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. Jennings & Fischl Expires March 12, 2010 [Page 21] Internet-Draft SIP Certificates September 2009 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 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. 9.1. 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. Jennings & Fischl Expires March 12, 2010 [Page 22] Internet-Draft SIP Certificates September 2009 9.2. 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. 9.3. 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 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 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 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 Jennings & Fischl Expires March 12, 2010 [Page 23] Internet-Draft SIP Certificates September 2009 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. 9.4. 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 7.6, Section 7.7 and Section 7.10 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. 9.5. Crypto Profiles Credential services SHOULD implement the server name indication extensions in [RFC5246] and they MUST support a TLS profile of TLS_RSA_WITH_AES_128_CBC_SHA as described in [RFC5246] as a profile of TLS_RSA_WITH_3DES_EDE_CBC_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 [RFC2898]. It is RECOMMENDED that this profile be used when using PKCS#8. A different passphrase SHOULD be used for the PKCS#8 encryption than is used for server authentication. 9.6. User Certificate Generation The certificates should be consistent with [RFC5280]. 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. Jennings & Fischl Expires March 12, 2010 [Page 24] Internet-Draft SIP Certificates September 2009 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. 9.7. 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 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 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. 10. 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 Jennings & Fischl Expires March 12, 2010 [Page 25] Internet-Draft SIP Certificates September 2009 10.1. 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 10.2. 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 Jennings & Fischl Expires March 12, 2010 [Page 26] Internet-Draft SIP Certificates September 2009 10.3. 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: binary Security considerations: Carries a cryptographic private key Interoperability considerations: The PKCS#8 object inside this MIME type MUST be DER-encoded Published specification: RSA Laboratories, "Private-Key Information Syntax Standard, Version 1.2", PKCS 8, November 1993. 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 Intended usage: COMMON Author/Change controller: the IESG 11. 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, Yaron Pdut, Aki Niemi, Magnus Nystrom, Paul Hoffman, Adina Simu, Dan Wing, Mike Hammer and Lyndsay Campbell. Rohan Mahy, John Elwell, and Jonathan Rosenberg provided detailed review and text. Jennings & Fischl Expires March 12, 2010 [Page 27] Internet-Draft SIP Certificates September 2009 12. References 12.1. 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. [RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography Specification Version 2.0", RFC 2898, September 2000. [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. [RFC5208] Kaliski, B., "Public-Key Cryptography Standards (PKCS) #8: Private-Key Information Syntax Specification Version 1.2", RFC 5208, May 2008. [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. Jennings & Fischl Expires March 12, 2010 [Page 28] Internet-Draft SIP Certificates September 2009 12.2. Informational References [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. Authors' Addresses Cullen Jennings Cisco Systems 170 West Tasman Drive MS: SJC-21/2 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@skypelabs.com Jennings & Fischl Expires March 12, 2010 [Page 29]