Certificate Management Service for SIP

SIP provides a mechanism for end to end encryption and integrity using S/MIME. This document extends that work to provide one specific mechanism for discovery, retrieval, and management of the certificates. It follows the Sacred Framework RFC 3760 (ADD REF) for management of the credentials. The general approach is to provide a new SIP service referred to as a Credential Server. This service allows UAs to subscribe to some other user's certificate. The certificate is delivered in a NOTIFY to the UA that subscribed. The identity of the certificate can be vouched for using the (TODO REF identity). The Credential Service can manage public certificates as well as credentials that include the user's private key. The user can install new credentials to the Credential Server using a PUBLISH. The Credential Server authenticates UAs that are changing credentials or requesting private keys using a shared secret that both the UA and Server know. Typically this will be the same shared secret that is used in Register with the Registrar for the domain. The mechanism described in this document works for both self signed certificates and certificates signed by a well known certificate authority; however, it is imagined that most UAs using this would only use self signed certificates and would use an Authentication Service as described in (TODO REF) to provide strong identity binding to the certificates.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. Certificate: An X.509 style certificate containing a public key and a list of identities in the SubjectAltName that are bound to this key. The certificates discussed in this draft are generally self signed and use the mechanisms in the (REF TODO identity) to vouch for their validity. Credential: For this document, this means the combination of a certificate and the associated private key.

Allow negotiation of E2E encrypted sessions Allow end to end encryption and integrity of SIP bodies that may be delivered in SIP signaling, such as page mode MESSAGEs or NOTIFY bodies in presence. Work for users with multiple UA devices. Provide certificate revocation mechanism

UAs discover certificates by sending a SUBSCRIBE with an event type of pkix-cert to the AOR that a certificate is desired for. This could be a SIP or tel URL. The resulting NOTIFY will contain an application/pkix-cert body which contains the certificates. The UA MUST follow the procedures in to decide if the received certificate can be used. The UA needs to cache this certificate for future use. The certificate MUST be removed form the cache if it has expired or if it is updated by a subsequent NOTIFY or if the subscription has been terminated. The NOTIFY containing a certificate must be signed by an Authentication Service as described in REF Identity. If the identity asserted by the Authentication Service does not match the identity requests, the certificates in the NOTIFY are discarded and MUST NOT be used.

UAs discover credentials by subscribing to their AOR with an event type of credential, which will result in a message containing both an application/pkix-cert body and an application/pkcs8 body that has the associated private key information for the certificate. The UA can change the user's certificate and private key by sending to the server a PUBLISH (TODO ref draft-ietf-sip-publish ) with an event type of credential that contains both an application/pkix-cert and application/pkcs8 body. The UA needs to authenticate to the Credential Server for these operations. The UA MUST use TLS to connect to the server. The UA may be configured with a specific name for the Credential Server; otherwise it defaults to the name of the domain in the User's AOR. The TLS connection MUST present a certificate that matches the expected name for the credential server, so that the UA knows it is talking to the correct server. If the certificate presented by the server does not match the expected server, the UA MUST terminate the connection and notify the User. If the UA does not do this, it may end up publishing its private key information to an attacker. The Credential Server will authenticate the UA using the usual SIP Digest mechanism, so the UA can expect to receive a SIP challenge to the SUBSCRIBE or PUBLISH messages. The application/pkix-cert body is a DER encoded X.509 certificate (TODO RFC 2585). The application/pkcs8 bodies contains a DER encoded PKCS #8 object that contains the private key. The PKCS #8 objects MUST by of type PrivateKeyInfo. The integrity and confidentiality of the PKCS #8 objects is provided by the TLS transport. The transport encoding of all the MIME bodies is binary.

The Credential Server receives credentials for users and can then provide the credentials or certificates to other user agents. The credential server receives and store credentials for users. The credentials are indexed by URI. When a UA requests a public certificate with a SUBSCRIBE, the server sends it in a NOTIFY and sends a subsequent NOTIFY any time it changes. When a credential is requested, the Server digest challenges the requesting UA to authenticate it so that the Server can verify that the UA is authorized to receive the requested credentials. When the Credential Server receives a SUBSCRIBE for a certificate, it first checks to see if it has credentials for the requested URI. If it does not it returns a response indicating the user was not found. Otherwise it sets up a subscription and forms a NOTIFY with the certificate in the body and the From header field value set to the request URI of the SUBSCRIBE. It MUST send this NOTIFY through an Authentication Service (as described in TODO REF Identity) or implement an Authentication Service itself. The Server is encouraged to keep the subscriptions active indefinitely but MAY unsubscribe at any point of time. Anytime the credentials for this URI change, the Server MUST send a new NOTIFY to any active subscriptions. When a Credential Server receives a SUBSCRIBE for a credential, the Server has to authenticate and authorize the UA and validate that adequate transport security is being used. The Server MUST digest challenge the UA to authenticate the UA and then decide if it is authorized to receive the credentials. Once the UA has authenticated to the Server, the Server can set up a subscription and send a Notify message that MUST contain the credentials. This is sent thought an Authorization Service in the same way as the certificate subscriptions. If the credential changes, the Server MUST terminate any current subscriptions and force the UA to re-authenticate. This is so that in the case of a compromised secret to retrieve the credentials, the rogue UA does not continue to receive credentials after the compromised secret has been changed. When the Credential Server receives a PUBLISH to update credentials, it MUST authenticate and authorize this is the same way it does the subscriptions for credentials. If this succeeds, the Server updates the credential for this URI and processes all the active subscriptions to this URI as described above.

SIP uses an offer/answer negotiation mechanism (REF 3264) that describes sessions using SDP that may contain keying material (ref draft-ietf-mmusic-sdescriptions) for media protocols such as SRTP (Ref 3711). This keying material needs to be protected, and SIP does this by encrypting the SDP bodies using S/MIME. If a UA receives both an unencrypted and an encrypted SDP body in an offer, it MUST consider all the SDP received as potential offers, and any encrypted SDP SHOULD be preferred to unencrypted SDP. Answers to any encrypted SDP MUST also be sent in encrypted SDP, and unencrypted SDP MUST be sent in unencrypted SDP. This is necessary so that the device receiving the answer can correctly match m lines in the SDP. If the UA that receives an encrypted offer cannot send the answer back encrypted, then it may not use these encrypted offers. In a typical call from Alice to Bob, Alice would first subscribe to Bob's certificate. If this worked, then Alice would send an Invite to Bob that contained an RTP session in unencrypted SDP and an SRTP session in encrypted SDP. Bob would select the SRTP session and send an answer with encrypted SDP selecting the SRTP session. Both Alice's and Bob's UAs would indicate to the user that a secure call had been negotiated. Alice and Bob could note this and adjust their conversation accordingly.

Applications such as presence and 911 location information result in information with significant privacy requirements being sent in SIP. Particular mime types may define special meanings when both an encrypted and unencrypted body are received but, unless otherwise specified, the UA SHOULD use the encrypted version if it can decrypt it, and ignore the unencrypted version. There is no requirement for the two versions to have the same information. For example, a page mode message could have an unencrypted version that said "I'm in the Middle East visiting people" while the encrypted version had much more sensitive information like "I'm over at Osama's house at 21.25'24"N 39.49'24"E". Depending whether the receiving device can decrypt this or not, a different message gets displayed to the receiving user.

In general, signing messages with self-signed certificates is not that useful unless some other means is used to vouch that the certificate has some meaning. If the Authentication Service is used to do this, then the Authentication Service is providing integrity across all the bodies and binding them with an identity. In this case, the additional signature becomes redundant. Because of this, it is recommended that signing bodies SHOULD NOT be done if the certificate is a self signed certificate.

This whole scheme is highly dependent on trusting the operators of the credential server and trusting that the Credential Server will not be compromised. The complete security of all the users will be compromised if the Credential Server is compromised. This work requires the TLS session to be used for communications to the Credential Server. Failing to use TLS or selecting a poor cipher suite (such as NULL encryption) will result in credentials being sent unencrypted over the network and render the whole system useless. Implementation really must use TLS or there is no point in implementing any of this. In addition, the correct checking of chained certificates as specified in the TLS RFC (TODO REF) is critical for the client to authenticate the server. If a particular credential needs to be revoked, the new credential is simply published to the Credential Server. Every device keeping this current in its cache will have a subscription to the credential and will rapidly (order of seconds) be notified and replace its cache. Clients that are not subscribed will subscribe and get the new certificate and do not end up using the old invalid certificate.

When a UA wishes to discover the certificate for sip:alice@example.com, the UA subscribes to the certificate for alice@example.com and receives a certificate in the body of a SIP Notify message. The term original URI is used to describe the original URI that was subscribed to. If the certificate is signed by a trusted CA, and one of the names in the SubjectAltName matches the original URI, then this certificate MAY be used but only for exactly the Original URI and not for other identities found in the SubjectAltName. Otherwise, there are several steps the UA MUST perform before using this certificate. The From header in the NOTIFY message MUST match the original URI. The UA MUST check the Identity header as described in the REF TODO Identity draft to validate that bodies have not been tampered with and that an Authentication Service has validated this From header. The UA MUST check the validity time of the certificate and stop using the certificate once it is invalid. The certificate MAY have several names in the SubjectAltName but the UA MUST only use this certificate when it needs the certificate for the identity in the Original URI. This means that the certificate should only be indexed in the certificate cache by the value of the original URI, not by the value of all the identities found in the SubjectAltName. These steps result in a chain of bindings that result in a trusted binding between the original URI and a public key. The Original URI is forced to match the From. The Authentication Service validates that this message did come from the identity claimed in the From and that bodies and From have not been tampered with. The certificate in the body contains the public key for the identity. Only the UA that can authenticate as this user can tamper with this body so the owner of the identity can provide a false public key but other users cannot. This chain of assertion from original URI, to From, to body, to public key is critical to the security of this document. If any of the steps above are not followed, this chain of security will be broken and the system will not work.

This work uses the security design outlined in the SACRED (REF TODO) Framework. Specifically it follows the cTLS architecture described in section 4.2.2 of RFC 3760. The client authenticates the server using the server's TLS certificate. The server authenticates the client using a SIP digest transaction inside of the TLS session. The TLS sessions form a strong session key used to protect the credentials being exchanged.

Credential Servers SHOULD implement the server name indication extensions in RFC 3546 TODO REF and they MUST support a TLS profile of TLS_RSA_WITH_AES_128_CBC_SHA as described in RFC 3268 and a profile of TLS_RSA_WITH_3DES_CBC_SHA.

To: ietf-sip-events@iana.org Subject: Registration of new SIP event package Package Name: certificate Is this registration for a Template Package: No Published Specification(s): draft- TODO Person & email address to contact for further information: Cullen Jennings <fluffy@cisco.com>

To: ietf-sip-events@iana.org Subject: Registration of new SIP event package Package Name: credential Is this registration for a Template Package: No Published Specification(s): draft- TODO Person & email address to contact for further information: Cullen Jennings <fluffy@cisco.com>

To: ietf-types@iana.org Subject: Registration of MIME media type application/pkcs8 MIME media type name: application MIME subtype name: pkcs8 Required parameters: None Optional parameters: None Encoding considerations: will be binary for 8-bit transports Security considerations: Carries a cryptographic private key Interoperability considerations: None Published specification: TODO Applications which use this media type: Any MIME-complaint transport Additional information: Magic number(s): None File extension(s): .p8 Macintosh File Type Code(s): none Person & email address to contact for further information: Cullen Jennings <fluffy@cisco.com> Intended usage: COMMON Author/Change controller: Cullen Jennings <fluffy@cisco.com>