Internet Engineering Task Force J. Arkko MMUSIC Working Group E. Carrara INTERNET-DRAFT F. Lindholm Expires: August 2002 M. Naslund K. Norrman Ericsson February, 2002 Key Management Extensions for SDP and RTSP
<draft-ietf-mmusic-kmgmt-ext-02.txt><draft-ietf-mmusic-kmgmt-ext-03.txt> Status of this memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Abstract This document defines general extensions for SDP and RTSP to carry the security information needed by a key management protocol, in order to secure the media. These extensions are presented as a framework, to be used by one or more key management protocols. As such, its use is meaningful only when it is completed by the key management protocol in use. General guidelines are also given on how the framework should be used together with SIP and RTSP. TABLE OF CONTENTS 1. Introduction.................................................. 2 1.1. Notational Conventions...................................... 3 2. Extensions to SDP and RTSP.................................... 3 2.1. SDP Extensions.............................................. 4 2.2. RTSP Extensions............................................. 45 3. Usage with SIP and RTSP....................................... 5 3.1. General SDP processing...................................... 56 3.2. SIP usage................................................... 6 3.3. RTSP usage.................................................. 67 3.4. Example scenarios........................................... 7 4. Security Considerations....................................... 9Adding a Key management protocol..............................10 5. IANA Considerations...........................................10Security Considerations.......................................10 6. Conclusions...................................................10IANA Considerations...........................................11 7. Acknowledgments...............................................10Conclusions...................................................11 8. Author's Addresses............................................10Acknowledgments...............................................11 9. References....................................................11Author's Addresses............................................11 10. References...................................................12 1. Introduction There has recently been work to define a security framework for the protection of real-time applications running over RTP, [SRTP]. However, a security protocol needs a key management infrastructure to exchange keys and security parameters, managing and refreshing keys, etc. A key management protocol is executed prior to the security protocol execution. The key management protocol's main goal is to, in a secure and reliable way, establish a so called security association for the security protocol. This includes one or several cryptographic keys and a set of necessary parameters for the security protocol, e.g., cipher or authentication algorithm to be used. The key management protocol has similarities with, e.g., SIP and RTSP in the sense that it negotiates necessary information in order to be able to setup the session. The focus in the following sections is to describe SDP attribute extensions and RTSP header extensions to support key management, and a possible integration within SIP and RTSP. A framework is therefore described in the following, that will need to be accompanied by one or more key management protocols. Some of the motivations to create a framework with the possibility to include the key management in the session establishment are: * Just as the codec information is a description of how to encode and decode the audio (or video) stream, the key management data is a description of how to encrypt and decrypt the data. * The possibility to negotiate the security for the entire multimedia session at the same time. * The knowledge of the media at the session establishment makes it easy to tie the key management to the multimedia sessions. * This approach may be more efficient than setting up the security later, as that approach might force extra roundtrips, possibly also a separate set-up for each stream, hence implying more delay to the actual setup of the media session. Currently in SDP [SDP],[SDPnew], one field exists to transport keys, i.e. the "key=" field. However, this is not enough for a key management protocol. The approach here is to use and extend the SDP description to transport the key management offer/answer and also to associate it with the media sessions. SIP uses the offer/answer model [OAM] whereby extensions to SDP will be enough. An extra RTSP header is also defined. 1.1. Notational Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119. 2. Extensions to SDP and RTSP This section describes common attributes that are to be included in an SDP description or in an RTSP header when an integrated key management protocol is used. All attribute values MUST follow the general SDP or RTSP guideline. For the SDP description, the key management attributes may be defined at session level (i.e. before the media descriptor lines) and/or at media level. If the key management attributes are defined at media level, they will only apply to that specific media. If the key management attributes are defined at both session and media level, the media level definition overrides the session level definition for that specific media. The extensions were defined in a way to: * give a minimal impact on current SDP implementations, i.e. only minimal modifications MUST be built into an existing SDP stack in order to support the key management. * make it easy to use another key management protocol, or a new version, without having to redefine the attributes or add new ones. The following set of attributes have been identified as necessary to support: * key management protocol identifier, to indicate the key management protocol used ("keymgmt-prtcl")("keymgmt-prtcl"). * key management raw data field, to transport the key management protocol data ("keymgmt-data")("keymgmt-data"). The key management protocol data contains the necessary information to establish the security protocol, e.g., keys and cryptographic parameters. All parameters and keys are protected by the key management. Note that if the key management protocol fails, e.g., the receiver does not accept any of the proposed security parameters, or simply does not understand the key management protocol, the security setup will fail. Consequently, it is impossible to establish a secure session. This is very similar to the normal SIP/SDP behavior; if the sender only supports two codecs, and the receiver do not support any of them, it will be problematic to set up a session. * in the case of SDP, an extra (optional) authentication attribute to be able to tie the key management data to the surrounding media definitions ("key-extra-auth"). 2.1. SDP Extensions This section provides an Augmented Backus-Naur Form (ABNF) grammar (as used in [SDP])[SDPnew]) for the key management extensions to SDP. Note that the new definitions are compliant with the definition of an attribute field, i.e. attribute = (att-field ":" att-value) | att-field Three new attributes are defined, keymgmt-prtcl, keymgmt-data, and key-extra-auth. keymgmt-prtcl = "keymgmt-prot:" prtcl-name prtcl-name = 1*(safe)non-ws-string ; e.g. "MIKEY" keymgmt-data = "keymgmt-data:" byte-string key-extra-auth = "keymgmt-auth:" byte-string where safenon-ws-string and byte-string are as defined in SDP [SDP].[SDPnew]. 2.2. RTSP Extensions To support the three different attributes described, the following RTSP header is defined: KeyMgmt = "KeyMgmt" ":" [stream-url] protocol data stream-url = "url" "=" url ";" protocol = "Prot" "=" token ";" data = "Data" "=" quoted-string url, token and quoted-string are as defined in the RTSP specification [RTSP]. The url indicates the stream URL, which the parameters correspond to. The KeyMgmt header should be possible to use in both request and response messages of the following methods: * DESCRIBE * ANNOUNCE * SETUP * PLAY * RECORD * SET_PARAMETER * GET_PARAMETER * OPTIONS 3. Usage with SIP and RTSP This section gives recommendations of how/when to include the defined key management attributes when SIP and/or RTSP are used together with SDP. Some general requirements MUST be set on the key management protocol if it has to be suitable to work together with SIP and RTSP: * It MUST be possible to execute the key management protocol in at most one roundtrip in case the answerer accepts the offer. * The protocol MUST return, to SDP/RTSP, a valid answer whether the provided offer was accepted or not. * There MUST be a possibility for the key management protocol to tie the media sessions to the negotiated parameters (i.e. an interface between the key management and the SDP and RTSP implementation MUST exist). Today, the MIKEY protocol has adopted the key management extensions to work together with SIP and RTSP. Other protocols may use the described attributes and header, e.g. Kerberos.Kerberos [KERB]. 3.1. General SDP processing When an SDP message is created, the following procedure should be applied: * The "keymgmt-prot" attribute is filled in with the identifier of the key management protocol used (e.g. MIKEY or Kerberos). * The "keymgmt-data" attribute is filled in by using the key management protocol interface to obtain key management data. The data may e.g. be a MIKEY message or Kerberos ticket. * If used, the "keymgmt-auth" attribute is filled in by using the key management protocol interface to obtain key management data. Note that what data from SDP are supposed to be provided to the interface MUST be specified by the key management protocol itself. A received SDP message that contains the key management attributes SHOULD process these attributes in the following manner: * Detect the key management protocol used by parsing the "keymgmt- prot" attribute. * Extract the key management data from the "keymgmt-data" attribute and call the key management interface with the extracted data. Note that depending on key management protocol, some extra parameters might of course be requested, such as the source/destination network address/port(s) for the specified media. * Depending on the outcome of the key management processing (i.e. whether it was accepted or not), SDP processing can proceed according to normal processing (e.g. according to the offer/answer model, see also Section 3.2.). * If the optional "keymgmt-auth" attribute is included, this is processed as the "keymgmt-data". 3.2. SIP usage The offerer should include the key management data within an offer that contains the media description it should apply to. The answerer MUST check with the key management protocol if the attribute values are valid, and then obtain from the key management the data to include in the answer. If the offer is not accepted, the answerer returns a notification message and the offerer may go out with a new (different) offer, depending on the local security policy. Re-keying should be handled as a new offer, i.e. a re-INVITE should be sent with the new proposed parameters. The answerer treats this as a new offer where the key management is the issue of change. 3.3. RTSP usage RTSP does not use the offer/answer model, as SIP does. This causes some problems as it is not possible (without abusing RTSP) to send back an answer to the server (as the server will in most cases be the one initiating the security parameter exchange). To solve this, a new header has been introduced (Section 2.2). The processing of a key management header in RTSP should be done analogous of the SDP message processing. The only differences will be that the url has been introduced and that there is no corresponding parameter for the extra authentication. The url should be processed as a standard RTSP stream url, i.e. to identify the session. It is however not mandatory to use. The initial key management message from a server should be sent to the client using SDP. When responding to this, the client uses the new RTSP header to send back an answer (included in either the SETUP or the PLAY message). The server may provide re-keying facilities by sending a new key management message in a SET_PARAMETER or ANNOUNCE messages. In the latter, the RTSP header is not used if the ANNOUNCE message includes a SDP description where the data can be provided in. The response message is then put in the new RTSP header in the response message from the client to the server. Note that the SET PARAMETER and the ANNOUNCE messages are not mandatory to support for the servers. 3.4. Example scenarios Example 1 (SIP) A SIP call is taking place between Alice and Bob. Alice sends an Invite message consisting of the following offer: v=0 o=alice 2891092738 2891092738 IN IP4 lost.somewhere.com s=Cool stuff email@example.com t=0 0 c=IN IP4 lost.somewhere.com a=keymgmt-prot:MIKEY a=keymgmt-data:uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD... m=audio 49000 RTP/SAVP 98 a=rtpmap:98 AMR/8000 m=video 52230 RTP/SAVP 31 a=rtpmap:31 H261/90000 i.e. Alice proposes to set up one audio stream and one video stream that run over SRTP. To set up the security parameters for SRTP, she uses MIKEY. Note that MIKEY is negotiating the crypto suite for both streams (as it is placed at the session level). Bob accepts the offer and sends an answer back to Alice: v=0 o=bob 2891092897 2891092897 IN IP4 found.somewhere.com s=Cool stuff firstname.lastname@example.org t=0 0 c=IN IP4 found.somewhere.com a=keymgmt-prot:MIKEY a=keymgmt-data:skaoqDeMkdwRW278HjKVB... m=audio 49030 RTP/SAVP 98 a=rtpmap:98 AMR/8000 m=video 52230 RTP/SAVP 31 a=rtpmap:31 H261/90000 Example 2 (SDP) This example shows how Alice would have done in the previous example if she wished to protect only the audio stream. v=0 o=alice 2891092738 2891092738 IN IP4 lost.somewhere.com s=Cool stuff email@example.com t=0 0 c=IN IP4 lost.somewhere.com m=audio 49000 RTP/SAVP 98 a=rtpmap:98 AMR/8000 a=keymgmt-prot:MIKEY a=keymgmt-data:uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD... m=video 52230 RTP/AVP 31 a=rtpmap:31 H261/90000 Example 3 (RTSP) A client wants to set up a streaming session and requests a media description from the streaming server. DESCRIBE rtsp://server.example.com/fizzle/foo RTSP/1.0 CSeq: 312 Accept: application/sdp From: firstname.lastname@example.org The server sends back an OK message including a SDP description. As the server RTSP/1.0 200 OK CSeq: 312 Date: 23 Jan 1997 15:35:06 GMT Content-Type: application/sdp v=0 o=actionmovie 2891092738 2891092738 IN IP4 movie.somewhere.com s=Action Movie email@example.com t=0 0 c=IN IP4 movie.somewhere.com a=keymgmt-prot:MIKEY a=keymgmt-data:uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD... m=audio 0 RTP/SAVP 98 a=rtpmap:98 AMR/8000 control:rtsp://movie.somewhere.com/action/audio m=video 0 RTP/SAVP 31 a=rtpmap:31 H261/90000 control:rtsp://movie.somewhere.com/action/video The client is now ready to setup the sessions. It includes the key management data in the first message going back to the server (i.e. the SETUP message). SETUP rtsp://movie.somewhere.com/action/audio RTSP/1.0 CSeq: 313 Transport: RTP/SAVP/UDP;unicast;client_port=3056-3057 KeyMgmt: Prot=MIKEY;Data="skaoqDeMkdwRW278HjKVB..." The server processes the request including checking the validity of the key management header. RTSP/1.0 200 OK CSeq: 313 Session: 12345678 Transport: RTP/SAVP/UDP;unicast;client_port=3056-3057; server_port=5000-5001 The RTSP is then proceeded as usual (with e.g. a SETUP message for the video followed by a PLAY message). 4. Security ConsiderationsAdding a Key management protocol This framework can not be used with all key management protocols. The nature of this document iskey management protocol MUST comply to allowthe requirements described in Section 3. To be able to use a key management protocol with this framework, the following MUST be specified: * the key management protocol name that should be used in the "keymgmt-prot" attribute (e.g. "MIKEY" for MIKEY). * the information the key management needs from SDP and RTSP to support security(Section 3. gives a guideline of what SDP and RTSP needs from the media sessions. Itkey management). The exact interface is therefore not the intentionimplementation specific, but it SHOULD at least support to exchange the specified information. * if the "keymgmt-auth" attribute is needed by the key management, it should also specify the information needed for this. This also includes a precise list of the SDP lines it covers. The encoding of the data for both "keymgmt-data" and "keymgmt-auth" MUST be specified for each key management protocol and comply with the SDP and RTSP definitions. For most protocols, base64 encoding will be most appropriate. 5. Security Considerations The nature of this document is to allow SDP and RTSP to support security of the media sessions. It is therefore not the intention of this document to describe possible security solution or to define possible security problems. The defined SDP and RTSP extensions are not believed to introduce any new security risks to SDP and RTSP. The "keymgmt-auth" attribute may be (optionally) used to guarantee an authenticated binding between the session(s) and the security parameters, e.g. authenticating both the key management lines and (parts of) the surrounding SDP description. Each key management specifies the coverage of such "keymgmt-auth" attribute. A denial-of- service attack may be open if such authenticated binding is not provided. Note however, the "keymgmt-auth" cannot work when NATs are present.Note that the purpose of the key management fields is to secure the media streams themselves. Under the assumption that the key management schemes are secure, the SDP payloads can be passed along unprotected, and the media streams will still be secure even if some attackers gained knowledge of the SDP contents. There may however, be other reasons to protect the SDP payloads such as preventing attackers from gaining any information regarding the session or the used equipment. 5.If the SDP messages are not sent authenticated between the parties, it is possible for an attacker to change attributes without being detected. The "keymgmt-data" is protected by itself, but as it relies on the session information from SDP, an attack on SDP may give indirect consequences on the key management. The result of the attacks is not that severe, as it mainly will result in re-direction of the streams, or other DoS attacks. To circumvent the problem, the "keymgmt-auth" attribute may (optionally) be used to guarantee an authenticated binding between the session(s) and the security parameters, e.g., authenticating both the key management lines and (parts of) the surrounding SDP description. Each key management specifies the exact coverage of such "keymgmt-auth" attribute, i.e., a precise list of which SDP lines are covered. Note however, the "keymgmt-auth" cannot work when NATs are present (as this may require that fields in the SDP are changed). 6. IANA Considerations Three new attributes fields for SDP (see Section 2.1) and one new RTSP header are registered (see Section 2.2). 6.7. Conclusions A security solution for real-time applications needs a key management infrastructure. Integrating the key management scheme with the session establishment protocol could be done efficiently in most of the scenarios. This draft proposes a framework that integrates a key management protocol (e.g.(e.g., MIKEY) into SIP and RTSP, and which can be accompanied by different key management protocols. A set of new attributes and headers has been defined in SDP and RTSP to support this. 7.8. Acknowledgments Thanks to: Rolf Blom, Magnus Westerlund, and the rest involved in the MMUSIC WG and the MSEC WG. A special thanks to Joerg Ott and Colin Perkins. 8.9. Author's Addresses Jari Arkko Ericsson 02420 Jorvas Phone: +358 40 5079256 Finland Email: firstname.lastname@example.org Elisabetta Carrara Ericsson Research SE-16480 Stockholm Phone: +46 8 50877040 Sweden EMail: email@example.com Fredrik Lindholm Ericsson Research SE-16480 Stockholm Phone: +46 8 58531705 Sweden EMail: firstname.lastname@example.org Mats Naslund Ericsson Research SE-16480 Stockholm Phone: +46 8 58533739 Sweden EMail: email@example.com Karl Norrman Ericsson Research SE-16480 Stockholm Phone: +46 8 4044502 Sweden EMail: firstname.lastname@example.org 9.10. References [KERB] Kohl, J., Neuman, C., "The Kerberos Network Authentication Service (V5)", IETF, RFC 1510. [MIKEY] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and Norrman, K., "MIKEY: Multimedia Internet KEYing", Internet Draft, IETF, Work in progress (MSEC). [OAM] Rosenberg, J. and Schulzrinne, H., "An Offer/Answer Model with SDP", Internet Draft, IETF, Work in progress (MMUSIC). [RTSP] Schulzrinne, H., Rao, A., and Lanphier, R., "Real Time Streaming Protocol (RTSP)", IETF, RFC 2326, April 1998. [SDP]2326. [SDPnew] Handley, M., andJacobson, V., "Sessionand Perkins, C., "SDP: Session Description Protocol (SDP)",Protocol", Internet Draft, IETF, RFC2327Work in progress (MMUSIC). [SIP] Handley, M., Schulzrinne, H., Schooler, E., and Rosenberg, J., "SIP: Session Initiation Protocol", IETF, RFC2543.RFC 2543. [SRTP] Baugher, M., Blom, R., Carrara, E., McGrew, D., Naslund, M, Norrman, K., and Oran, D., "The Secure Real Time Transport Protocol", Internet Draft, IETF, Work in Progress (AVT). This Internet-Draft expires in August 2002.