SIPPING K. Ono Internet-Draft S. Tachimoto Expires: August 23, 2005 NTT Corporation February 19, 2005 End-to-middle Security in the Session Initiation Protocol (SIP) draft-ono-sipping-end2middle-security-04 Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668. 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 August 23, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract Some services provided by intermediaries depend on the ability to inspect the message bodies in the Session Initiation Protocol (SIP). When sensitive information is included in these bodies, a SIP User Agent (UA) needs to protect it from intermediaries except those that the UA agreed to disclose it to. This document proposes a mechanism for securing information passed between an end user and Ono & Tachimoto Expires August 23, 2005 [Page 1] Internet-Draft End-to-middle security in SIP February 2005 intermediaries using S/MIME. It also proposes mechanisms for an intermediary to notify the UA about its need to inspect an S/MIME-secured message body, and to notify the need for the message body to be transmitted securely. Conventions used in this document 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 [1]. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Generating S/MIME-secured message body . . . . . . . . . . . 3 2.1 Generating S/MIME CMS EnvelopedData . . . . . . . . . . . . 3 2.2 Generating S/MIME CMS SignedData . . . . . . . . . . . . . . 4 3. Indicating the Target Content . . . . . . . . . . . . . . . 5 4. Notification of the Proxy Server's Policies . . . . . . . . 5 5. Behavior of UAs and Proxy Servers . . . . . . . . . . . . . 6 5.1 UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . 7 5.2 UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . . 7 5.3 Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . 8 6. Proxy-Required-Body Header Field Use . . . . . . . . . . . . 9 7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1 Example of Request for End-to-Middle Confidentiality . . . . 10 7.2 Example of Request for End-to-Middle Integrity . . . . . . . 11 8. Security Considerations . . . . . . . . . . . . . . . . . . 12 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 13 10. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 13 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 13 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 12.1 Normative References . . . . . . . . . . . . . . . . . . . . 14 12.2 Informative References . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15 Intellectual Property and Copyright Statements . . . . . . . 16 Ono & Tachimoto Expires August 23, 2005 [Page 2] Internet-Draft End-to-middle security in SIP February 2005 1. Introduction When a SIP [2] UA requires services provided by intermediaries that depend on the message bodies in request/response messages, end-to-end confidentiality currently has to be disabled. This problem is pointed out in Section 23 of [2]. Since such intermediaries are not always adjacent to the UA, this situation requires security between the UA and the intermediary for the message bodies. We call this "end-to-middle security", where by "end" we mean a UA and by "middle" we mean a specific intermediary, typically a proxy server. This document describes proposed mechanisms for providing data confidentiality and integrity for end-to-middle security to meet the requirements discussed in [3]. Since the major requirement is to have little impact on the standardized end-to-end security mechanisms, the proposed mechanisms are based on S/MIME [4]. The mechanisms consist of generating S/MIME-secured message body and indicating the target message body for a proxy server selected by the UA. In addition, this document includes mechanisms for an intermediary to notify the UA about its need to inspect an S/MIME-secured message body, and to notify that the message body needs to be transmitted securely. 2. Generating S/MIME-secured message body 2.1 Generating S/MIME CMS EnvelopedData For end-to-middle confidentiality, a UA MUST generate S/MIME CMS [5] EnvelopedData. Before generating it, a UA needs to identify the target proxy servers. The UA MUST obtain their credentials, such as their public key certificates or shared secrets. The structure of the S/MIME CMS EnvelopedData contains encrypted data specified in the "encryptedContentInfo" field and its recipient list specified in the "recipientInfos" field. The encrypted data is encrypted with a content-encryption-key (CEK) and the recipient list contains the CEKs encrypted with different key-encryption-keys (KEKs), one for each recipient. The KEKs are the public keys of each recipient or the shared keys between the UA and each recipient. If the data is encrypted for a selected proxy server, the recipient list contains only the selected proxy server. If the data is encrypted and destined for multiple proxy servers, the recipient list contains the multiple proxy servers. If there is encrypted data destined for each proxy server, the recipient list of each piece of encrypted data contains the targeted proxy server. In order to concatenate multiple pieces of encrypted data, the UAC MUST generate a multipart MIME body. Ono & Tachimoto Expires August 23, 2005 [Page 3] Internet-Draft End-to-middle security in SIP February 2005 Since proxy servers are prohibited from deleting any body, the encrypted data for the proxy server is transmitted to the user agent server (UAS) but the UAS is not able to decrypt it. In order to avoid causing unnecessary error conditions in the UAS, the user agent client (UAC) MUST set the value "optional" in the handling parameter of the "Content-Disposition" MIME header for the message body. If the multipart MIME body consists of encrypted MIME bodies with the value "optional", the "Content-Disposition" MIME header of the multipart MIME body MUST also contain the value "optional" in the handling parameter. The UAS SHOULD NOT try to decrypt encrypted data that has the value "optional". If the multipart MIME body contains a body with the value "required" and another body with the value "optional", the multipart MIME body SHOULD have the value "required" in the handling parameter of the "Content-Disposition" MIME header. If the encrypted data is meant to be shared with the UAS and selected proxy servers, the recipient list SHOULD be addressed to the UAS and the selected proxy servers. The UAC SHOULD set the value "required" in the handling parameter of the "Content-Disposition" MIME header for the message body. The UAS MUST try to decrypt the encrypted data that has the value "required" in the handling parameter. If the handling parameter is not set, the default behavior is the same as for setting the value "required", as specified in [2]. If a piece of encrypted data is destined for a selected proxy server and another piece of encrypted data for the UAS, the recipient of each piece of encrypted data is each entity respectively. In this case, the UAC MUST generate a multipart MIME body to concatenate the two. For example, a UA uses this mechanism when keying materials, such as keys used for Secure RTP (SRTP), are included in the SDP[9]. One CMS EnvelopedData contains SDP that includes keying materials of an SRTP stream only for the UA. The other CMS EnvelopedData contains an SDP that does not include the keying materials for a selected proxy server that needs to view SDP (i.e., for a firewall traversal service). 2.2 Generating S/MIME CMS SignedData For end-to-end data integrity, a UA generates S/MIME CMS SignedData that can be verified by any entity that knows the public key of the UA. For end-to-middle data integrity, a UA MUST generate S/MIME CMS SignedData in the same way as end-to-end data integrity. Ono & Tachimoto Expires August 23, 2005 [Page 4] Internet-Draft End-to-middle security in SIP February 2005 3. Indicating the Target Content A UA needs a way to indicate content that they expect to be viewed by a proxy server, in order for the proxy server to easily determine whether to process MIME bodies and if so, which one. To meet this requirement, the UAs SHOULD set a label to indicate a selected proxy server and the target content with a new SIP header, "Proxy-Required-Body". This header contains one or more "content-id" parameter(s) for setting the "Content-ID" MIME header into the target body. Note: There were three other options to label a body: a new SIP parameter to an existing SIP header, a new MIME header, or a new parameter to an existing MIME header. 1) Using a new parameter to Route header. Since a proxy server views this header when forwarding a request message, it seems to be a reasonable option. However, it cannot work with strict routing. 2) Using a new MIME header, "Content-Target", as proposed in a previous version of this draft. Since this option is not necessary as a generic mechanism of MIME, it is not preferred. 3) Using a new MIME parameter to "Content-Disposition". The same reason as above. When a UA labels the encrypted data, it SHOULD set the "Proxy-Required-Body" SIP header that contains the address of the server and "content-id" parameter indicating the S/MIME CMS EnvelopedData. When a UA labels the data with signature, it SHOULD set the "Proxy-Required-Body" SIP header that contains the address of the server and "content-id" parameter indicating the S/MIME CMS SignedData. 4. Notification of the Proxy Server's Policies A notification mechanism for the proxy server's policies is needed when a UA does not know the policies of the proxy server in a signaling path and the proxy server has its own policies for providing some services which needs the UA's compliance. There are two ways in which a UA can learn the policies of the proxy server. The UA MAY learn them by getting a policy package from a policy server.[10] When a proxy server needs to inspect the message body contained in the response, it needs to learn the policies from a policy server before sending the response. Alternatively, the UA MAY learn them by receiving a response with an error code. When the proxy server receives a request in which it cannot view the message body that has to be read in order to proceed, the proxy server MUST send a response with an error code. If the Ono & Tachimoto Expires August 23, 2005 [Page 5] Internet-Draft End-to-middle security in SIP February 2005 request contains encrypted data, the error code SHOULD be 493 (Undecipherable), accompanied with a proxy's public key certificate and required Content-Type for viewing. The proxy public key certificate is set as an "application/pkix-cert" body. The required Content-Type is set in the Warning header. When the proxy server receives a request that can not be accepted due to its condition, the proxy server MUST also send a response with an error code. If a digital signature is not attached to the request and it required for an integrity check, the error code SHOULD be 495 (Signature Required) accompanied with a required Content-Type for the integrity check. The required Content-Type is set in the Warning header. Note: 495 (Signature Required) response is not only generated by a proxy server, but also by the UAS. Using this error code is more explicit than using an existing error code 403, that was described in the last version of this draft. Open Issues: How should the error message indicate the Content-Type to which a signature needs to be attached? Can these Content-Types be nested such as "Content-Type: multipart/mixed" for "Content-Type: application/sdp" and "Content-Type: message/sipfrag"? When proxy servers require both disclosure and an integrity check, how should it be described? When the UAC receives one of the above error codes, it needs to authenticate the proxy server. Therefore, the error code SHOULD contain the digital signature of the proxy server. In the worst scenario, this notification mechanism requires two messages for each proxy server in the signaling path to establish a session between the UAs. In addition, it requires validation procedures using the digital signatures for all proxy servers. Since this causes an increase in the delay before session establishment, it is recommended that a UA learns in advance the policies of as many proxy servers as they can. 5. Behavior of UAs and Proxy Servers We describe here an example of the behavior of UAs and proxy servers in a model in which a proxy server that provides a logging service for instant messages exists in a message path as shown in Figure 1. Ono & Tachimoto Expires August 23, 2005 [Page 6] Internet-Draft End-to-middle security in SIP February 2005 +-----+ +-----+ +-----+ +-----+ | C |-----| C |-----| [C] |-----| C | +-----+ +-----+ +-----+ +-----+ UA #1 Proxy #1 Proxy #2 UA #2 w/Logging function C : Content that UA #1 allows the entity to inspect [C]: Content that UA #1 prevents the entity from inspecting Figure 1: Deployment example 5.1 UAC Behavior When a UAC sends a MESSAGE [11] request including encrypted message content for end-to-end and end-to-middle confidentiality, it MUST use S/MIME CMS EnvelopedData to encrypt them. In this example, UA #1 is assumed to know the services and the public key of Proxy #1. UA #1 MUST use S/MIME CMS EnvelopedData body for UA #2 and Proxy #1. UA #1 SHOULD specify the hostname of Proxy #1 and Content-ID of the S/MIME CMS EnvelopedData to be decrypted in the "Proxy-Required-Body" SIP header. When the UAC sends a request and needs both end-to-end and end-to-middle integrity for the message body, it MUST use S/MIME CMS SignedData to attach a digital signature. In this example, UA #1 MUST use the CMS SignedData body of the contents. UA #1 SHOULD specify the hostname of Proxy#1 and Content-ID of the CMS SignedData to be validated in the "Proxy-Required-Body" SIP header. When the UAC sends multiple requests to the same UAS, the CEK reuse mechanism is beneficial in letting UAs efficiently encrypt/decrypt data. The CEK reuse mechanism is described in [6][7]. The UAC SHOULD use the "unprotectedAttrs" field to stipulate reuse of the CEK and indicate its identifier. When the UAC reuses the CEK in the previous request as the KEK, it generates CMS EnvelopedData with the type "KEKRecipientInfo" of "RecipientInfo" attribute. 5.2 UAS Behavior When a UAS sends a response to the request with this mechanism, the use of the same type of S/MIME CMS data is recommended. For example, if the UAS receives an INVITE request in which the SDP is encrypted by using CMS EnvelopedData body, the response is RECOMMENDED to be a "200 OK" containing the encrypted SDP based on the CMS EnvelopedData body. In the above example, however, the response of the MESSAGE request does not need to use the same type of S/MIME CMS data since the response does not contain the message content. Ono & Tachimoto Expires August 23, 2005 [Page 7] Internet-Draft End-to-middle security in SIP February 2005 In particular, when the CMS EnvelopedData body of the request contains the "unprotectedAttrs" attribute specifying reuse of the CEK, the UAS SHOULD keep the CEK with the identifier specified in the "unprotectedAttrs" attribute. When the UAS receives a request that uses S/MIME, it decrypts and/or validates the S/MIME bodies as usual. Even when the UAS receives a request without this mechanism, the UAS MAY need end-to-end and end-to-middle confidentiality of the message bodies and/or headers in the response. In this case, the UAS MUST use CMS EnvelopedData to encrypt them. When the UAS sends a response and needs end-to-end and end-to-middle integrity of the message bodies and/or headers, it MUST use CMS SignedData to attach a digital signature. This is no different from how the UAC normally operates with this mechanism. 5.3 Proxy Behavior When a proxy server supporting this mechanism receives a message, it MUST inspect the "Proxy-Required-Body" header. If the header includes the processing server's own hostname, the proxy server MUST inspect the specified body in the Content-ID. When the specified body is CMS EnvelopedData, the proxy server MUST inspect it and try to decrypt the "recipientInfos" field. Even if there is the header does not include the server's own name, nor the header exists, the proxy server MAY view a message body. If the UA encrypted data for the proxy, the proxy server will succeed in decryption using the "recipientInfos" field. If the proxy server fails to decrypt that, it SHOULD cancel the subsequent procedure and respond with a 493 (Undecipherable) response if it is a request, otherwise any existing dialog MAY be terminated. If the proxy server succeeds in this decryption, it MUST inspect the "unprotectedAttrs" field of the CMS EnvelopedData body. If the attribute gives the key's identifier, the proxy server MUST keep the CEK with its identifier until the lifetime of the CEK has expired. If it receives subsequent messages within the lifetime, it MUST try to decrypt the type "KEKRecipientInfo" of the "RecipientInfo" attribute by using this CEK. When the specified content is CMS SignedData body, the proxy server MUST inspect it and validate the digital signature. If the verification fails, the proxy server SHOULD reject the subsequent procedure and respond with a 495 (Signature Required) response if the message is a request, otherwise any existing dialog MAY be terminated. Ono & Tachimoto Expires August 23, 2005 [Page 8] Internet-Draft End-to-middle security in SIP February 2005 When the proxy server forwards the request, it modifies the routing headers normally, but does not modify the message body. The proxy server MAY delete the "Proxy-Required-Body" header that contains own hostname. When a provider operating the proxy server does not allow any information related to its security policies to be revealed to the proxy serving the recipient UA, the proxy server deletes the "Proxy-Required-Body" header. However, when a request message is transmitted to the proxy server via a proxy server operated by another provider, there is no way to conceal the header from the other proxy server. If a proxy does not support this mechanism and receives a message with the "Proxy-Required-Body" header, the proxy MUST ignore the header and operate as usual. 6. Proxy-Required-Body Header Field Use The following syntax specification uses the augmented Backus-Naur Form (BNF) as described in RFC-2234 [8]. The new header "Proxy-Required-Body" is defined as a SIP header. Proxy-Required-Body = "Proxy-Required-Body" HCOLON required-proxy SEMI target-body required-proxy = host target-body = cid-param *(COMMA cid-param) cid-param = "cid" EQUAL content-id content-id = LDQUOT dot-atom "@" (dot-atom / host) RDQUOT dot-atom = atom *( "." atom ) atom = 1*( alphanum / "-" / "!" / "%" / "*" / "_" / "+" / "'" / "`" / "~" ) Information about the use of headers in relation to SIP methods and proxy processing is summarized in Table 1. Header field where proxy ACK BYE CAN INV OPT REG -------------------------------------------------------------- Proxy-Required-Body R adr - o - o o o Proxy-Required-Body 200-699 adr - o - o o o Proxy-Required-Body 1xx adr - - - o - - Header field where proxy SUB NOT PRK IFO UPD MSG -------------------------------------------------------------- Proxy-Required-Body R adr o o - o o o Proxy-Required-Body 200-699 adr o o - o o o Table 1: Summary of header field use Ono & Tachimoto Expires August 23, 2005 [Page 9] Internet-Draft End-to-middle security in SIP February 2005 The "where" column gives the request and response types in which the header field can be used. The values in the "where" column are as follows: * R: The header field may appear in requests * 200-699: A numeral range indicates response codes with which the header field can be used. The "proxy" column gives the operations a proxy may perform on the header field: * a: A proxy can add or concatenate the header field if it is not present. * r: A proxy must be able to read the header field, so it cannot be encrypted. * d: A proxy can delete a header field value. The next six columns relate to the presence of a header field in a method: * o: The header field is optional. * -: The header field is not applicable. 7. Examples The following examples illustrate the use of the mechanism defined in the previous section. 7.1 Example of Request for End-to-Middle Confidentiality In the following example, a UA needs the message content in a MESSAGE request to be confidential and it allows a selected proxy server to view the message content. It also needs to protect the label of the target content. In addition, it needs to reuse the CEK in the subsequent request messages. In the example encrypted message below, the text with the box of asterisks ("*") is encrypted: MESSAGE alice@atlanta.example.com --> ss1.atlanta.example.com MESSAGE sip:bob@biloxi.example.com SIP/2.0 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9 Max-Forwards: 70 Route: From: Alice ;tag=9fxced76sl To: Bob Call-ID: 3848276298220188511@atlanta.example.com CSeq: 1 MESSAGE Date: Fri, 20 June 2003 13:02:03 GMT Proxy-Required-Body: ss1.atlanta.example.com; cid=1234@atlanta.example.com Content-Type: multipart/signed;protocol="application/pkcs7-signature"; micalg=sha1;boundary=boundary1 Ono & Tachimoto Expires August 23, 2005 [Page 10] Internet-Draft End-to-middle security in SIP February 2005 Content-Length: ... --boundary1 Content-Type: application/pkcs7-mime;smime-type=enveloped-data; name=smime.p7m Content-Transfer-Encoding: binary Content-ID: 1234@atlanta.example.com Content-Disposition: attachment;filename=smime.p7m;handling=required Content-Length: ... ****************************************************************** * (encryptedContentInfo) * * Content-Type: text/plain * * Content-Length: ... * * * * Hello. * * This is confidential. * * * * (recipientInfos) * * RecipientInfo[0] for ss1.atlanta.example.com public key * * RecipientInfo[1] for Bob's public key * * * * (unprotectedAttrs) * * CEKReference * ****************************************************************** --boundary1-- Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: binary Content-Disposition: attachment; filename=smime.p7s;handling=required [binary data] --boundary1-- 7.2 Example of Request for End-to-Middle Integrity In the following example, a UA needs the integrity of the message content in a MESSAGE request to be validated by a selected proxy server before it views the message content. It also needs to protect the label of the target content. Ono & Tachimoto Expires August 23, 2005 [Page 11] Internet-Draft End-to-middle security in SIP February 2005 MESSAGE alice@atlanta.example.com --> ss1.atlanta.example.com MESSAGE sip:bob@biloxi.example.com SIP/2.0 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9 Max-Forwards: 70 Route: From: Alice ;tag=9fxced76sl To: Bob Call-ID: 3848276298220188511@atlanta.example.com CSeq: 1 MESSAGE Date: Fri, 20 June 2003 13:02:03 GMT Proxy-Required-Body: ss1.atlanta.example.com; cid=1234@atlanta.example.com Content-Type: multipart/signed;protocol="application/pkcs7-signature"; micalg=sha1;boundary=boundary1 Content-Length: ... --boundary1 Content-Type: text/plain Content-Length: ... Hello. This is protected with the signature. --boundary1-- Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: binary Content-ID:1234@atlanta.example.com Content-Disposition: attachment; filename=smime.p7s;handling=required [binary data] --boundary1-- 8. Security Considerations This document describes a mechanism to encrypt data for multiple recipients, such as multiple proxy servers, or the recipient UA and proxy servers. This means that the encrypted data is decipherable at a previous hop and it may be modified by an entity at the previous hop. Therefore, the UA SHOULD protect the data integrity before encryption, when the encrypted data is meant to be shared with multiple proxy servers, or to be shared with the UAS and selected proxy servers. The UA SHOULD generate S/MIME CMS SignedData and then SHOULD generate the EnvelopedData to encrypt attached data with a digital signature. The recipient entity SHOULD verify the signature Ono & Tachimoto Expires August 23, 2005 [Page 12] Internet-Draft End-to-middle security in SIP February 2005 to see if the encrypted data has been modified after decryption at another entity on the recipient list. This document also describes a new SIP header for labeling a message body for a proxy server. If a malicious user or proxy server modified/added/deleted the label, the specified message body will not be inspected by the specified proxy server, and then some service using its content can not be served. Or a proxy server will do unnecessary processing on message bodies such as unpacking MIME structure, and/or signature verification. This causes a Denial-of-Services attack to a proxy server. To prevent such attacks, protection of the label integrity is needed. UAs and proxy servers SHOULD use TLS mechanism to communicate each other. A proxy server trusted to provide SIP routing is basically trusted to process SIP headers other than those related to routing. Therefore, hop-by-hop security is reasonable for the protection. Of course, UAs MAY generate a "message/sipfrag" body and attach a digital signature for the whole body in order to protect the label integrity. 9. IANA Considerations This document requires a new "Proxy-Required-Body" SIP header. 10. Open Issues o How should the error message indicate the Content-Type to which a signature needs to be attached? Can these Content-Types be nested such as "Content-Type: multipart/mixed" for "Content-Type:application/sdp" and "Content-Type: message/sipfrag"? o When proxy servers require both disclosure and an integrity check, how should it be described? o If the label of the target body has a wrong value, what should we do? 403 (Forbidden) response? o Do we need to add text about CMS AuthenticationData for the integrity protection? o Do we need to add text about the reason we don't have to define a new option-tag? 11. Acknowledgments Thanks to Rohan Mahy and Cullen Jennings for their initial support of this concept and to many people for useful comments, especially Jon Peterson, Jonathan Rosenberg, Eric Burger. Ono & Tachimoto Expires August 23, 2005 [Page 13] Internet-Draft End-to-middle security in SIP February 2005 12. References 12.1 Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, BCP 14, March 1997. [2] 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. [3] Ono, K. and S. Tachimoto, "Requirements for end-to-middle security in the Session Initiation Protocol (SIP)", Internet-Draft draft-ietf-sipping-e2m-sec-reqs-04, October 2004. [4] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Certificate Handling", RFC 3850, July 2004. [5] Housley, R., "Cryptographic Message Syntax", RFC 2630, June 1999. [6] Farrell, S. and S. Turner, "Reuse of CMS Content Encryption Keys", RFC 3185, October 2001. [7] Ono, K. and S. Tachimoto, "Key reuse in S/MIME for SIP", Internet-Draft draft-ono-sipping-keyreuse-smime-00, February 2004. [8] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. 12.2 Informative References [9] Andreasen, F., Baugher, M. and D. Wing, "Session Description Protocol Security Descriptions for Media Streams", Internet-Draft draft-ietf-mmusic-sdescriptions-09, February 2005. [10] Hilt, V., Camarillo, G. and J. Rosenberg, "Profile Data for Session Initiation Protocol (SIP) Policies", September 2003. [11] Campbell, Ed., B., Rosenberg, J., Schulzrinne, H., Huitema, C. and D. Gurle, "Session Initiation Protocol (SIP) Extension for Instant Messaging", RFC 3428, December 2002. [12] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420, November 2002. Ono & Tachimoto Expires August 23, 2005 [Page 14] Internet-Draft End-to-middle security in SIP February 2005 Authors' Addresses Kumiko Ono Network Service Systems Laboratories, NTT Corporation Musashino-shi, Tokyo 180-8585 Japan Email: ono.kumiko@lab.ntt.co.jp Shinya Tachimoto Network Service Systems Laboratories, NTT Corporation Musashino-shi, Tokyo 180-8585 Japan Email: tachimoto.shinya@lab.ntt.co.jp Ono & Tachimoto Expires August 23, 2005 [Page 15] Internet-Draft End-to-middle security in SIP February 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Ono & Tachimoto Expires August 23, 2005 [Page 16]