SIP K. Ono Internet-Draft S. Tachimoto Expires: January 10, 2006 NTT Corporation July 9, 2005 End-to-middle Security in the Session Initiation Protocol (SIP) draft-ietf-sip-e2m-sec-00 Status of this Memo 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 becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 January 10, 2006. Copyright Notice Copyright (C) The Internet Society (2005). Abstract Some services provided by intermediaries depend on their ability to inspect a message body in the Session Initiation Protocol (SIP). When sensitive information is included in the message body, a SIP User Agent (UA) needs to protect it from other intermediaries than those that the UA agreed to disclose it to. This document proposes a mechanism for securing information passed between an end user and intermediaries using S/MIME. It also proposes mechanisms for a UA to discover intermediaries which need to inspect an S/MIME-secured Ono & Tachimoto Expires January 10, 2006 [Page 1] Internet-Draft End-to-middle security in SIP July 2005 message body, or to receive the message body with data integrity. 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 S/MIME-secured Message Body for Confidentiality . . . . . 3 2.2 S/MIME-secured Message Body for Data Integrity . . . . . . 4 3. Indicating the Target Content . . . . . . . . . . . . . . . 5 4. Discovering the Security Policies of Proxy Servers . . . . . 5 5. Behavior of UAs and Proxy Servers . . . . . . . . . . . . . 7 5.1 UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . 7 5.2 UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . 8 5.3 Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 8 6. Proxy-Required-Body Header Field Use . . . . . . . . . . . . 9 7. Message Examples . . . . . . . . . . . . . . . . . . . . . . 10 7.1 Message Examples of End-to-Middle Confidentiality . . . . 10 7.2 Message Examples of End-to-Middle Integrity . . . . . . . 14 8. Security Considerations . . . . . . . . . . . . . . . . . . 16 8.1 Impersonating a Proxy Server . . . . . . . . . . . . . . . 16 8.2 Tampering with a Message Body . . . . . . . . . . . . . . 16 8.3 Tampering with the Label of the Target Content . . . . . . 17 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 17 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 17 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 11.1 Normative References . . . . . . . . . . . . . . . . . . 17 11.2 Informative References . . . . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . 20 Ono & Tachimoto Expires January 10, 2006 [Page 2] Internet-Draft End-to-middle security in SIP July 2005 1. Introduction When a UA requires services provided by intermediaries that depend on the message body 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 intermediaries for the message body. We call this "end-to-middle security", where by "end" we mean a UA and by "middle" we mean an intermediary, typically a proxy server. End-to-middle security, as well as end-to-end security, consists of peer authentication, data integrity, and data confidentiality. Peer authentication is required to achieve data integrity and data confidentiality respectively. The mechanisms of end-to-middle peer authentication are established with pre-existing mechanisms such as HTTP Digest authentication [7]. Therefore, this document focuses on mechanisms for providing data confidentiality and integrity for end- to-middle security to meet the requirements discussed in [3]. The proposed mechanisms are based on S/MIME [4], since the major requirement is to have little impact on standardized end-to-end security mechanisms, the way of handling S/MIME-secure messages. 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 describes a mechanism for a UA to discover the intermediary which needs to inspect an S/MIME-secured message body, or to receive the message body with data integrity. 2. Generating S/MIME-secured Message Body 2.1 S/MIME-secured Message Body for Confidentiality For end-to-middle confidentiality, a UA MUST generate S/MIME CMS [5] EnvelopedData. Prior to generating it, a UA needs to identify the target proxy servers and obtain their credentials, such as their public key certificates or shared secrets. One method is shown in Section 4. 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 either the public keys of each recipient or the shared keys between the UA and each recipient. Ono & Tachimoto Expires January 10, 2006 [Page 3] Internet-Draft End-to-middle security in SIP July 2005 If the encrypted data is destined for a proxy server, the recipient list MUST contain only the proxy server. If the same encrypted data is destined for multiple proxy servers, or is shared with the user agent server (UAS) and proxy servers, the recipient list MUST be addressed to the proxy servers, or the UAS and the proxy servers. If there are multiple pieces encrypted data destined for each proxy server, the recipient list in each piece of encrypted data MUST contain the relevant proxy server. If a piece of encrypted data is destined for a proxy server and another piece of encrypted data for the UAS, the recipient of each piece of encrypted data MUST be each entity respectively. In order to concatenate more than one CMS EnvelopedData, the user agent client (UAC) MUST generate a multipart MIME body. For example, a UA uses this mechanism when keying materials, such as keys used for Secure RTP (SRTP), are included in the SDP[8]. Although a proxy server needs to view SDP (i.e., for a firewall traversal service), the UA does not want to show the keying materials to the proxy server. In this case, one CMS EnvelopedData contains the SDP, that includes keying materials of the SRTP stream, encrypted for the UA. The other CMS EnvelopedData contains the SDP, that does not include the keying materials, encrypted for the proxy server. As described in [2], proxy servers are prohibited from deleting any message body. Even if a UAC send a piece of encrypted data only to a proxy server, the UAS receives it and cannot decrypt it. In order to avoid unnecessary error conditions in the UAS, the 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. If the multipart MIME body contains a body with the value "required" and another body with the value "optional", the multipart MIME body MUST have either the value "required" in the handling parameter of the "Content-Disposition" MIME header, or no handling parameter, since the default value is "required" as specified in [2]. The UAS SHOULD NOT try to decrypt encrypted data that has the value "optional". 2.2 S/MIME-secured Message Body for Data Integrity For end-to-middle data integrity, a UA SHOULD generate either S/MIME CMS SignedData. A UA MAY generate a signature in the SIP Identity [9] header, if the UA has its own public and private key. These mechanisms allow any entity to verify the data integrity, if it is able to access the UA's public key. This is why the same mechanisms can be used in both end-to-middle and end-to-end data integrity. Ono & Tachimoto Expires January 10, 2006 [Page 4] Internet-Draft End-to-middle security in SIP July 2005 Note: There are other mechanisms which can provide data integrity, such as S/MIME CMS AuthenticatedData, which requires that a UA obtains the credential of the recipient, that is a proxy server, in advance. However, this is not used in [2] and require a mechanism to securely transmit the credential from the proxy server to the UA. Thus, this document does not describe the use of S/MIME CMS AuthenticatedData. 3. Indicating the Target Content A UA needs a way to indicate content that it expects to be viewed by a proxy server, in order for the proxy server to easily determine whether to process a MIME body and if so, which part. To meet this requirement, the UA SHOULD set a label to indicate the proxy server and its target content using a new SIP header, "Proxy-Required-Body". This header consists of one or more proxy servers' hostnames and one or more "content-id" parameter(s) pointing to the "Content-ID" MIME header placed in 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. If a UA needs to label the encrypted data, it SHOULD set the "Proxy- Required-Body" SIP header that contains the address of the proxy server and "content-id" parameter indicating the target S/MIME CMS EnvelopedData. If a UA needs to label the signed 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 SignedData. Note that the signature for part of a MIME body alone is meaningless in providing data integrity. 4. Discovering the Security Policies of Proxy Servers A discovery mechanism for security policies of proxy servers is needed when a UA does not statically know which proxy servers or domains have such policies. Security policies require disclosure of Ono & Tachimoto Expires January 10, 2006 [Page 5] Internet-Draft End-to-middle security in SIP July 2005 data and/or verification in order to provide some services which needs UA's compliance. There are two ways in which a UA can learn the policies of the proxy servers. One is by receiving an error response. A UAC can learn the policies in this way. However, it is not applicable to the UAS because there is no way to react a response message. Alternatively, a policy server can provide a UAC and the UAS a package mentioning proxy's policy as described in [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. When the proxy server receives a request that can not be accepted due to its condition, the proxy server MUST reject with an error response. If the request contains encrypted data and the proxy server cannot view the message body that has to be viewed in order to proceed, the proxy server MUST reject with a 493 (Undecipherable) error response. The proxy's public key certificate and Content-Type to be viewed SHOULD be contained with the error response. The proxy public key certificate SHOULD be set as an "application/pkix-cert" body. The required Content-Type SHOULD be set in the Warning header with a new warn-code, 380. If a digital signature is not attached to the message body in the request and the proxy server requires the integrity check, the proxy server MUST reject with a 495 (Signature Required) error response. This error response does not contain signature required Content-Type, since the attached signature to the whole body is always required. When a proxy server requires both disclosure and an integrity check of the message body in a request message and the message satisfies neither, the proxy server SHOULD send one error response at a time. When a proxy server cannot decrypt the message body in a request message and does not see if the signature is placed inside, a proxy server SHOULD send an error response only for requesting disclosure. After receiving a request message including encrypted data destined for the proxy server, it finds out whether the message has a signature attached and SHOULD send an error response for requesting signature when the message lacks it. Note: A 495 (Signature Required) response is not only generated by a proxy server, but also by the UAS. This discovery mechanism requires two more messages' exchange for an error condition per each proxy server in the signaling path in order to establish a session between UAs. Since this causes a delay in session establishment, it is desirable that the UAs learn the security policies of the proxy servers in advance. Ono & Tachimoto Expires January 10, 2006 [Page 6] Internet-Draft End-to-middle security in SIP July 2005 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 signaling path as shown in Figure 1. +-----+ +-----+ +-----+ +-----+ | C |-----| C |-----| [C] |-----| C | +-----+ +-----+ +-----+ +-----+ UA #1 Proxy #1 Proxy #2 UA #2 w/Logging function C : Content that UA #1 allows the entities 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. If UA #1 is unaware of the services provided by Proxy #1 that requires inspecting the message body, UA #1 will MAY get a 493 (Undecipherable) error response and the public key of Proxy #1. After getting the error response, 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 by Proxy #1 in the "Proxy-Required- Body" SIP header. When a UAC sends a request message of which message body needs end- to-middle integrity, it SHOULD use S/MIME CMS SignedData to attach a digital signature. If UA #1 does not know the service of Proxy #1 that requires verifying the message body, UA #1 MAY get a 495 (Signature Required) error response. After getting the error response, UA #1 SHOULD generate the CMS SignedData to attach a signature by computing with its own private key. UA #1 SHOULD specify the hostname of Proxy #1 and Content-ID of the CMS SignedData to be validated by Proxy #1 in the "Proxy-Required-Body" SIP header. When a UAC sends a request and needs both end-to-middle confidentiality and integrity for the message body, it SHOULD first attach a digital signature, and then encrypted the message body. In this example, UA #1 SHOULD generate S/MIME CMS SignedData for the contents, and then generate S/MIME CMS EnvelovedData body to encrypt the CMS SignedData. UA#1 SHOULD specify the hostname of Proxy#1 and Content-IDs of the CMS SignedData and the CMS EnvelopedData destined Ono & Tachimoto Expires January 10, 2006 [Page 7] Internet-Draft End-to-middle security in SIP July 2005 for Proxy #1 in the "Proxy-Required-Body". When a UAC generates S/MIME CMS EnvelopedData, the UAC MAY use the CEK reuse mechanism [12][13]. The CEK reuse mechanism has a benefit that enables UAs to efficiently encrypt/decrypt data in subsequent messages. The UAC MAY 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 the UAS receives a request that uses S/MIME, it first decrypts and/or validates the S/MIME bodies as usual. In particular, when the CMS EnvelopedData body of the request contains the "unprotectedAttrs" attribute specifying reuse of the CEK, the UAS MAY keep the CEK with the identifier specified in the "unprotectedAttrs" attribute. When the UAS responds with a 200 OK, the same type of S/MIME CMS data is RECOMMENDED to be used. For example, if the UAS receives an INVITE request in which the SDP is encrypted by using the CMS EnvelopedData, it is RECOMMENDED to respond with a 200 OK response in which the SDP is encrypted by using the CMS EnvelopedData body. If the UAS receives an INVITE request which is attached a digital signature to the SDP by using the CMS SignedData, it is RECOMMENDED to respond with a 200 OK response which is attached a signature to the SDP by using the CMS SignedData. In the above example, however, a 200 OK response to the MESSAGE request does not need to use the same type of S/MIME CMS data since the response does not contain any MIME body. Even when the UAS receives a request that does not use S/MIME, the UAS sometimes needs end-to-end and end-to-middle confidentiality for the message body and/or headers in a response. In this case, the UAS MUST use CMS EnvelopedData to encrypt it. When the UAS sends a response and needs end-to-end and end-to-middle integrity for the message body and/or headers, it SHOULD use CMS SignedData to attach a digital signature. This is not different from how a UAC operates as described in Section 5.1. 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 body specified by the Content-ID. When the specified body is CMS EnvelopedData, the proxy server MUST inspect it and try Ono & Tachimoto Expires January 10, 2006 [Page 8] Internet-Draft End-to-middle security in SIP July 2005 to decrypt the "recipientInfos" field. If the header does not include the server's own name, nor the header exists, the proxy server MAY view the message body. If there is a piece of encrypted data for the proxy, the proxy server will succeed in decryption using the "recipientInfos" field. If the proxy server fails to decrypt the message body that is required to view, it MUST respond with a 493 (Undecipherable) response if it is a request, otherwise any existing dialog MUST be terminated. If the proxy server succeeds in this decryption, it MAY inspect the "unprotectedAttrs" field of the CMS EnvelopedData body. If the attribute gives the key's identifier, the proxy server MAY keep the CEK with its identifier until the lifetime of the CEK expires. If it receives subsequent messages within the lifetime, it MAY 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. When the proxy server forwards the request, it modifies the routing headers as it normally does, but does not modify the message body. The proxy server MAY delete the "Proxy-Required-Body" header that contains its 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 server 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 servers. 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 [6]. The new header "Proxy- Required-Body" is defined as a SIP header. Ono & Tachimoto Expires January 10, 2006 [Page 9] Internet-Draft End-to-middle security in SIP July 2005 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 dr - o - o o o Proxy-Required-Body 100-699 dr - o - o o o Header field where proxy SUB NOT PRK IFO UPD MSG -------------------------------------------------------------- Proxy-Required-Body R dr o o - o o o Proxy-Required-Body 100-699 dr o o - o o o Table 1: Summary of header field use 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 * 100-699: 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: * d: A proxy can delete a header field value. * r: A proxy must be able to read the header field, so it cannot be encrypted. The next 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. Message Examples The following examples illustrate the use of the mechanism defined in the previous sections. 7.1 Message Examples of End-to-Middle Confidentiality In the following example, a UAC needs message content in a MESSAGE Ono & Tachimoto Expires January 10, 2006 [Page 10] Internet-Draft End-to-middle security in SIP July 2005 request to be confidential and it allows a proxy server to view the message body. It also needs to reuse the CEK in the subsequent request messages. Even though the Content-Length has no digit, the appropriate length is to be set. In the example 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: 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 * ****************************************************************** If the proxy server successfully views the message body, the UAC receives a 200 OK from the UAS normally. However, if a proxy server fails to view the message body, the UAC receives a 493 Ono & Tachimoto Expires January 10, 2006 [Page 11] Internet-Draft End-to-middle security in SIP July 2005 (Undecipherable) error response from the proxy server, as follows: 493 Undecipherable alice@atlanta.example.com <-- ss1.atlanta.example.com SIP/2.0 493 Undeciperable Warning: 380 ss1.atlanta.example.com "Required to view 'text/plain'" Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9 ;received=192.0.2.101 From: Alice ;tag=9fxced76sl To: Bob ;tag=8321234356 Call-ID: 3848276298220188511@atlanta.example.com CSeq: 1 MESSAGE Content-Type: application/pkix-cert Content-Length: ... In the following example, a UA needs the SDP in an INVITE request to be confidential and it allows a proxy server to view the SDP. It also needs to reuse the CEK of the encrypted data in the subsequent request messages. Ono & Tachimoto Expires January 10, 2006 [Page 12] Internet-Draft End-to-middle security in SIP July 2005 INVITE alice@atlanta.example.com --> ss1.atlanta.example.com INVITE sip:bob@biloxi.example.com SIP/2.0 Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9 Max-Forwards: 70 From: Alice ;tag=9fxced76sl To: Bob Call-ID: 3848276298220188511@atlanta.example.com CSeq: 1 INVITE Date: Fri, 20 June 2003 13:02:03 GMT Contact: Proxy-Required-Body: ss1.atlanta.example.com; cid=1234@atlanta.example.com 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: application/sdp * * Content-Length: 151 * * * * v=0 * * o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com* * s=- * * c=IN IP4 192.0.2.101 * * t=0 0 * * m=audio 49172 RTP/AVP 0 * * a=rtpmap:0 PCMU/8000 * * * * (recipientInfos) * * RecipientInfo[0] for ss1.atlanta.example.com public key * * RecipientInfo[1] for Bob's public key * * * * (unprotectedAttrs) * * CEKReference * ****************************************************************** When the proxy server successfully views the SDP, and the UAS responds with a 200 OK. The 200 OK is to be encrypted as follows: Ono & Tachimoto Expires January 10, 2006 [Page 13] Internet-Draft End-to-middle security in SIP July 2005 200 OK alice@atlanta.example.com <-- ss1.atlanta.example.com SIP/2.0 200 OK Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9 ;received=192.0.2.101 From: Alice ;tag=9fxced76sl To: Bob ;tag=8321234356 Call-ID: 3848276298220188511@atlanta.example.com CSeq: 1 INVITE Contact: Content-Type: application/pkcs7-mime;smime-type=enveloped-data; name=smime.p7m Content-Transfer-Encoding: binary Content-ID: 1234@atlanta.example.com ****************************************************************** * (encryptedContentInfo) * * Content-Type: application/sdp * * Content-Length: 147 * * * * v=0 * * o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com* * s=- * * c=IN IP4 192.0.2.201 * * t=0 0 * * m=audio 3456 RTP/AVP 0 * * a=rtpmap:0 PCMU/8000 * * * * (recipientInfos) * * RecipientInfo[0] for Alice's public key * ****************************************************************** 7.2 Message Examples of End-to-Middle Integrity In the following example, a UA needs the integrity of message content in a MESSAGE request to be validated by a proxy server before it views message content. Even though the Content-Length has no digit, the appropriate length is to be set. Ono & Tachimoto Expires January 10, 2006 [Page 14] Internet-Draft End-to-middle security in SIP July 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-- If the proxy server successfully validates the integrity of the message body, the UAC normally receives a 200 OK from the UAS. However, if a proxy server does not receive a signature for the whole message body, the UAC receives a 495 (Signature Required) error response from the proxy server, as follows: Ono & Tachimoto Expires January 10, 2006 [Page 15] Internet-Draft End-to-middle security in SIP July 2005 495 Signature Required alice@atlanta.example.com <-- ss1.atlanta.example.com SIP/2.0 495 Signature Required Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9 ;received=192.0.2.101 From: Alice ;tag=9fxced76sl To: Bob ;tag=8321234356 Call-ID: 3848276298220188511@atlanta.example.com CSeq: 1 MESSAGE Content-Length: 0 8. Security Considerations 8.1 Impersonating a Proxy Server In the discovery mechanism in Section 4, a UA receives a 493 (Undecipherable) error response with the public key certificate of the proxy server requesting the disclosure of the message body. The public key certificate in the error response is vulnerable to be forged by a malicious user. To make sure that the response is sent by a proper proxy server, a UA needs to authenticate the response. Since the UA is not always adjacent to the proxy server, the UA cannot directly authenticate the proxy server by security mechanisms of the transport layer or the below. A UA SHOULD verify the chains to a trusted certificate authority of the public key certificate. 8.2 Tampering with a Message Body This document describes a mechanism to encrypt data for multiple recipients, such as multiple proxy servers, or a recipient UA and proxy servers. A piece of encrypted data is decipherable and vulnerable to tampering by proxy servers at the previous hops. In order to prevent such tampering, 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 to see if the encrypted data has been modified after decryption by an entity listed in the "recipientInfos" field. Ono & Tachimoto Expires January 10, 2006 [Page 16] Internet-Draft End-to-middle security in SIP July 2005 8.3 Tampering with the Label of the Target Content 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 is not inspected by the specified proxy server, and some services requiring its content can not be provided. Or a proxy server will conduct an unnecessary processing on message bodies such as unpacking MIME structure, and/or signature verification. This is a possible cause for a Denial-of-Services attack to a proxy server. To prevent such attacks, data integrity for the label is needed. UAs and proxy servers SHOULD use TLS mechanism to communicate with each other. Since a proxy server trusted to provide SIP routing is basically trusted to process SIP headers other than those related to routing, hop-by-hop security is reasonable to protect the label. In order to further protect the integrity of the label, UAs MAY generate a "message/sipfrag" body and attach a digital signature for the whole body. 9. IANA Considerations This document defines a new SIP header, "Proxy-Required-Body", of which the syntax is shown in Section 6. This document also defines a new SIP response-code, 495 "Signature Required", and a new Warn-code, 380 "Required to view Content-Type", as described in Section 4. 10. 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, and Russ Housely. 11. References 11.1 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)", draft-ietf-sipping-e2m-sec-reqs-06 (work in progress), March 2005. Ono & Tachimoto Expires January 10, 2006 [Page 17] Internet-Draft End-to-middle security in SIP July 2005 [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] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. 11.2 Informative References [7] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication: Basic and Digest Access Authentication", RFC 2617, June 1999. [8] Andreasen, F., Baugher, M., and D. Wing, "Session Description Protocol Security Descriptions for Media Streams", draft-ietf-mmusic-sdescriptions-11 (work in progress), June 2005. [9] Peterson, J. and C. Jennings, "Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP)", draft-ietf-sip-identity-05 (work in progress), May 2005. [10] Hilt, V., Camarillo, G., and J. Rosenberg, "Session Initiation Protocol (SIP) Session Policies - Document Format and Session- Independent Delivery Mechanism", draft-ietf-sipping-session-indep-policy-02 (work in progress), February 2005. [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] Farrell, S. and S. Turner, "Reuse of CMS Content Encryption Keys", RFC 3185, October 2001. [13] Ono, K. and S. Tachimoto, "Key reuse in S/MIME for SIP", draft-ono-sipping-keyreuse-smime-00 (work in progress), February 2004. [14] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420, November 2002. Ono & Tachimoto Expires January 10, 2006 [Page 18] Internet-Draft End-to-middle security in SIP July 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 January 10, 2006 [Page 19] Internet-Draft End-to-middle security in SIP July 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. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. 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 January 10, 2006 [Page 20]