XML Key Management Specification (XKMS 2.0) Bindings

Abstract

[2]This document specifies protocol bindings with security characteristics for the XML Key Management Specification (XKMS).

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document is a Proposed Recommendation (PR) of the W3C. This document has been produced by the XKMS Working Group (WG).

The W3C Membership and other interested parties are invited to review the document and send comments thru 3 June 2005 to www-xkms@w3.org, a mailing list with a public archive. Advisory Committee Representatives should consult their WBS questionnaires. After the review the Director will announce the document's disposition. This announcement should be expected no sooner than 14 days after the end of the review.

This is Part 2 of the W3C Proposed Recommendation for the XML Key Management Specification (XKMS Version 2.0). This document covers different protocol bindings with security characteristics for the XML Key Management Specification. Part 1 of this specification covers the XKMS protocols and services. For background on this work, please see the XKMS Activity Statement.

This document is based on the XKMS Version 2.0 Bindings Candidate Recommendation of 5 April 2004. The Working Group has addressed all comments received, making changes as necessary. Feedback received during that review resulted in clarifications but no major changes. Evidence of interoperation between at least two implementations of this specification are documented in the Implementation Report.

This document has been produced under the 24 January 2002 CPP as amended by the W3C Patent Policy Transition Procedure. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) with respect to this specification should disclose the information in accordance with section 6 of the W3C Patent Policy. Patent disclosures relevant to this specification may be found on the Working Group's patent disclosure page.

Publication as a Proposed Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

1 Introduction

1.1 Editorial and Conformance Conventions

[8]This specification uses XML Schemas [XML-schema] to describe the content model.

[9]The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in [RFC2119]:

[11]Consequently, we use these capitalized keywords to unambiguously specify requirements over protocol and application features and behavior that affect the interoperability and security of implementations. These key words are not used (capitalized) to describe XML grammar; schema definitions unambiguously describe such requirements and we wish to reserve the prominence of these terms for the natural language descriptions of protocols and features. For instance, an XML attribute might be described as being "optional." Compliance with the XML-namespace specification [XML-NS] is described as "REQUIRED."

1.2 Definition of Terms

[14]This document uses the terms defined in Section 1.2 of part one of this specification in the manner described therein.

1.3 Structure of this document

[15]The remainder of this document describes the XML Key Information Service Specification and XML Key Registration Service Specification.

2 Security Requirements

[17]The security enhancements required varies according to the application. In the case of a free or un-metered service the service may not require authentication of the request. A responder that requires an authenticated request must know in that circumstance that the request corresponds to the specified response.

2.1 Confidentiality

[18]Message confidentiality protects protocol messages from disclosure to third parties. Confidentiality MAY be a requirement for an XKMS service. Deployments SHOULD consider the extent to which the content of XKMS messages reveal sensitive information. A confidentiality requirement MAY exist even if a service only provides information from public sources as the contents of a request might disclose information about the client.

[19]The use of transport or payload confidentiality protection is NOT a substitute for the encryption of private keys specified in the XKRSS Registration and Recovery services. A service that supports registration of server generated keys or Key Recovery MUST implement the use of XML Encryption with a strong cipher.

[21]The means by which the client determines that the encryption key of the service is trustworthy is outside the scope of this specification. Possible mechanisms include:

[22]An XKMS service MAY determine the trustworthiness of an encryption key by reference to another XKMS service provided that the chain of references is eventually grounded by a mechanism that establishes direct trust between the client and the service.

2.2 Request Authentication

[23]Request Message Authentication MAY be required. An XKMS Service MAY require request authentication in deployments where the XKMS service is restricted to a specific audience, possibly as a paid service. An XKMS Service MAY require request authentication in contexts where different levels of service are supported according to the identity of the requestor.

[24]In addition various forms of Authentication MAY be required in the XKRSS Registration protocol to confirm the credentials of the party initiating the request and their possession of the private key component of the key pair(s) being registered.

2.3 Response Authentication

[26]Message Response Authentication MAY be required. Message Response Authentication is required in any deployment of a Validate service. A Locate service that provides only data that is self-authenticating such as X.509 or PGP certificates does not require Message Response Authentication.

[27]Note that Message Response Authentication is considered separately from Response Replay Protection.

2.4 Persistent Authentication

[29]In some circumstances requests or responses or to both may require persistent authentication. That is a message sent by A and authenticated by B may be subject to forwarding and authentication by C. In addition some applications may require further measures to ensure that messages meet certain legal standards to prevent repudiation.

[30]An XKMS service MAY support Persistent Authentication by means of a digital signature on the message.

2.5 Message Correlation (Response Replay and Request Substitution)

[31]An XKMS service MUST support a means of ensuring correct message correlation. That is the requestor must be assured that the response returned was made in response to the intended request sent to the service and not a modification of that request (Request Substitution attack) or a response to an earlier request (response replay attack).

[32]In order to prevent response replay and request message substitution attacks the requestor SHOULD ensure that the response corresponds to the request. For correspondence verification to be possible authentication of the response is required. In the TLS binding the correspondence between the request and response is provided by the transport layer. For message layer security mechanisms such as payload security the mechanism required depends on whether or not the request is authenticated as follows:

2.6 Request Replay

[33]An XKMS service may require protection against a Request replay attack. In circumstances where no accounting or other auditing is used to keep track of requests made, protection against a request replay attack may not be required.

2.7 Denial of Service

[35]An XKMS service may require protection against a Denial of Service attack by means of protocol measures. Such measures may not be required in circumstances where an XKMS service is protected against Denial of Service by other means such as the service is managed on an isolated, tightly controlled network and does not provide service outside that network.

[36]Denial of service attacks that originate from a single identified source or set of sources may be addressed by applying velocity controls. In cases where the source of the denial of service is disguised lightweight authentication techniques such as the two-phase protocol described bellow may be used to detect requests from forged addresses.

[37]An XKMS service SHOULD support protection against a Denial of Service attack.

2.8 Security Requirements Summary

[38]The following table summarizes the possible security requirements that an XKMS service deployment may be subject to:

Security Issue	Requirement	Comments
Confidentiality	Some	The information provided by an XKMS service may be confidential, the fact that a party has requested particular information from an XKMS service may allow confidential information to be deduced. Many XKMS applications do not require confidentiality however.
Request Authentication	Some	A service only needs to authenticate a request for information if either the information is confidential or some form of charge is to be made for use of the service.
Response Authentication	Most	An XKMS service that provides only a Location service for self authenticating key information such as Digital Certificates does not require authentication.
Persistent Authentication	Some	Although some XKMS applications have a specific requirement to support Non-Repudiation, the ability to repudiate requests and responses is acceptable in many applications.
Message Correspondence	All	The RequestId correspondence mechanism can only be used if the Request Authentication mechanism can be relied upon. Otherwise the Digest Mechanism should be used.
Request Replay	Some	Request replay attacks are likely to only be a concern if the service charges on a per request basis or as a type of Denial of Service attack.
Denial of Service	Most	Any service made available on a public network is likely to be subject to a Denial of Service attack. The risk of a Denial of Service attack is generally considered to be reduced on closed networks such as internal enterprise networks.

[39]Where the security requirements of the XKRSS protocol differ from those of XKISS they are addressed by the XKRSS protocol directly rather than relying upon the message security binding.

[40]For example the XKRSS registration functions are designed to support use in modes in which a client registration request is accepted by a Local Registration Authority and then forwarded to a Master Registration Authority. In this mode it is essential that the proof of possession of the private key being registered can be verified by both the Local Registration Authority and the Master Registration Authority, even though the authentication for the request sent to the Master Registration Authority is likely to be provided by the Local Registration Authority, rather than the original requestor. Similar considerations affect the distribution of private keys..

Security Issue	Requirement	Comments
Confidentiality of Private Key	If Server Generated Key pairs used	If a Register service supports registration of server generated key pairs or key recovery strong encryption of the private key MUST be supported.
Registration Request Authentication	Some	XKMS Registration services SHOULD support the authentication of registration requests for initial registration of a key binding. Registration requests for secondary registration of previously issued credentials (i.e. a signed key binding or a digital certificate) MAY be permitted without authentication.
Registration Proof Of Possession	Some	XKMS Registration services SHOULD support the verification of Proof Of Possession in the initial registration of client generated keys.
Authentication by Revocation Code	Some	The XKMS Revocation code is self authenticating.

3 SOAP Binding

[41]This section describes a mechanism for communicating the XKMS messages defined in Part 1 of this specification using the SOAP message protocol. XKMS implementers should support the SOAP message protocol for interoperability. When doing do, they MUST use the binding described herein. Bindings for both SOAP 1.2 [SOAP1.2-1][SOAP1.2-2] and SOAP 1.1[SOAP] protocols are specified.

[42]XKMS 2.0 implementations MUST support the use of SOAP 1.2. For near term compatibility with existing tools and infrastructure, SOAP 1.1 MAY be used

3.1 XKMS SOAP Message Binding

3.1.1 SOAP 1.2 Binding

[43]XKMS implementers shall use SOAP document style request-response messaging with the XKMS messages defined in Part 1 carried as unencoded Body element content. The SOAP 1.2 RPC representation, and requisite encoding style, are not used. The potential benefits of using the RPC representation do not justify the additional effort required to define a mapping from the Part 1 messages to an appropriate encoding style.

[44]The XKMS binding shall use the SOAP Request-Response Message Exchange Pattern defined in [SOAP1.2-2] and message processing shall conform to that model. SOAP 1.2 messages carrying XKMS content shall use the UTF-8 character encoding to insure interoperability..

[45]SOAP 1.2 messages carrying XKMS content shall conform to the following structure:

[48]The XKMS SOAP message binding does not require use of SOAP Headers. Headers may be used with SOAP messages carrying XKMS content to provide additional services such as communications security or routing. The use of such Headers is beyond the scope of this specification. If used however, they must not alter the message encoding style or SOAP processing model specified herein.

[49]Sample XKMS LocateRequest and LocateResponse message communicated using SOAP 1.2 message transport are shown below:

[52]The structure of conformant SOAP 1.2 messages carrying other XKMS message types should be evident based on this example.

3.1.2 SOAP 1.1 Binding

[53]XKMS implementers using SOAP 1.1 messaging shall use request-response messaging and carry the XKMS messages as unencoded content within the SOAP Body element. The SOAP 1.1 Section 5 encoding model shall not be used. SOAP 1.1 messages carrying XKMS content shall use the UTF-8 character encoding to insure interoperability.

[57]As with the SOAP 1.2 binding, the SOAP 1.1 binding does not require use of SOAP Headers. Headers may be used with SOAP messages carrying XKMS content to provide additional services such as communications security or routing providing they don't impact the encoding style or SOAP processing model specified herein.

[58]SOAP 1.1 messages carrying XKMS content will are identical to those using SOAP 1.2 except for the namespace of the Envelope and Body elements. Hence, the examples shown in Section 3.1.1 are conformant once the SOAP 1.2 namespace is replaced by the SOAP 1.1 namespace (http://schemas.xmlsoap.org/soap/envelope/).

3.2 Namespace inclusions

[59]In using the XKMS SOAP binding, XKMS messages are constructed as defined in Part 1 of this specification including all required namespace declarations. The top-level message element is then inserted as a child of the SOAP Body element. Promotion of XKMS namespace declarations to the parent SOAP Body (or grandparent Envelope) element is not required, but may be done at the discretion of the implementer. Such namespace promotion is generally undesirable if the XKMS message contains a digital signature as it may impact subsequent verification.

[60]Insertion of an XKMS message into the SOAP message structure must not alter namespace prefixes, or use of default namespaces, within the XKMS message. Any change in these encodings will likely break an XML Signature internal to the XKMS messages due to the use of QNames and namespace prefixes. The implementer must insure that prefix values used with the SOAP namespaces http://www.w3.org/2003/05/soap-envelope (SOAP 1.2) and http://schemas.xmlsoap.org/soap/envelope/ (SOAP 1.1) do not conflict with prefixes used in the XKMS message.

3.3 Computation of XML Signature Elements in XKMS Messages

[61]Use of the XKMS SOAP binding does not affect processing of the XML Signature-based elements <KeyBindingAuthentication> and <ProofOfPossession>. These are computed as described in XKMS, sections 7.1.4 and 7.1.6 respectively, and the signature validation processing described in XKMS, section 3.1.2 Element <ds:Signature>." That is, the SOAP defined nodes and namespaces do not contribute to the Signature computation.

3.4 Use of SOAP Faults

[62]SOAP Faults shall be used by an XKMS service to communicate errors that prevent the processing of a received XKMS request message. XKMS clients should never send a SOAP Fault message to an XKMS service.

3.4.1 SOAP 1.2 Faults

[63]The following SOAP Fault messages are defined for use with the XKMS SOAP 1.2 binding. Consistent with the SOAP 1.2 specification, these Fault messages shall contain the mandatory Code and Reason element information items. Inclusion of other elements is at the discretion of the implementer.

[64]In response to an XKMS request message, the receiver shall respond with one of the following SOAP Faults if it is unable to process the message. If it is able to process the message, then the response should conform to a valid XKMS response message as described in Part 1.

[65]A sample SOAP 1.2 fault message that would be returned when the received XKMS request message isn't supported by the service is shown below:

3.4.2 SOAP 1.1 Faults

[66]The following SOAP Fault messages are defined for use with the XKMS SOAP 1.1 binding. Consistent with the SOAP 1.1 specification, these Fault messages shall contain the faultcode and faultstring elements. Inclusion of other elements is at the discretion of the implementer.

[67]In response to an XKMS request message, the receiver shall respond with one of the following SOAP Faults if it is unable to process the message. If it is able to process the message, then the response should conform to a valid XKMS response message as described in [XKMS1].

[68]A sample SOAP 1.1 fault message that would be returned when the received XKMS request message isn't supported by the service is shown below:

3.5 SOAP over HTTP binding

[69]When the XKMS binding to SOAP 1.2 is implemented, the SOAP messages should be sent using HTTP POST consistent with the recommendations of Section 6.4.2 in [SOAP1.2-2]. Processing shall be consistent with Section 7, SOAP HTTP Binding in that specification.

[70]When the XKMS binding to SOAP 1.1 is implemented, the SOAP messages should be sent using HTTP POST consistent with the of Section 6.1 in [SOAP].

4 Security Bindings

[71]This specification describes two principal security bindings each of which specifies two or more specific implementation profiles.

Feature	Payload Security	Transaction Layer Security
Dependencies	Authentication defined by XKMS specification, client does not need to implement a comprehensive framework	Authentication mechanism defined by TLS which clients must implement
Use of XML Signature	Uses XML Signature in Enveloped Mode requiring slightly more complex processing	Not required
Support for Routing	Specification describes bi-lateral authentication only, multi-hop message routing and multi-party transactions are not supported	None
Support for Confidentiality	None, although applications may employ TLS to establish a secure channel	Supported
Non-Repudiation	Supported	Requires additional payload security
Unspecified Party Authentication	Supported	Requires additional payload security
Client Authentication	Supported	Supported through certificate client authentication or through use of payload security

4.1 Payload Authentication Binding

[71b]In the following table, Request/Signature means that a XKMS Request element includes a <dsig:Signature> element in enveloped mode. This signature is calculated using a digital signature method. Request/MAC has similar meaning, except that the signature is calculated using a Message Authentication Code (MAC).

Security Consideration	Client Authentication Mode	Support	Comment
Client Authentication Mechanism	No authentication mechanism is used.	None
Authentication using payload security	Payload	Request/Signature
Service Authentication Mechanism	Not Applicable	Payload	Response/Signature
Request/Response Correspondence	Authentication using payload security	Payload	Message/RequestId
Replay Attack Protection	Any	Payload	Message/Nonce in Two-phase protocol
Denial Of Service Protection	Any	Payload	Request/RespondWith=Represent
Non Repudiation	Any	Payload	Message/Signature with digital signature

XKMS element.attribute name	Required
MessageAbstractType.Service	Required
MessageAbstractType.Signature	Required in profile where client is authenticated using payload security for both Request and Response Messages
ResultType.RequestId	Required
PendingRequestType.ResponseId	Required
MessageAbstractType.Nonce	Optional, may be used to protect against Denial of Service
MessageAbstractType.RespondWith=Represent	Optional, may be used to protect against Denial of Service
Request.Signature with MAC	Required in profile where no mechanism is used to authenticate the client, Optional in profile where client is authenticated using payload security

4.2 Secure Socket Layer and Transaction Layer (SSL/TLS)Security Binding

[73]When TLS is to be used in XKMS, XKMS responders MUST support server authenticated TLS. Note that this means that an XKMS client need only support anonymous TLS, since to require otherwise would mean that all XKMS clients would have to be able to store root certificates for TLS usage.

All XKMS clients and responders which support TLS MUST support the TLS_RSA_WITH_3DES-EDE_CBC_SHA ciphersuite.
Other ciphersuites MAY be supported, but weak ciphersuites intended to meet export restrictions ("export grade") are NOT RECOMMENDED to be supported."

Security Consideration	Client Authentication Mode	Support	Comment
Client Authentication Mechanism	No authentication mechanism is used	None
TLS certificate based client authentication is used.	TLS	Certificate based client authentication
Authentication using Payload security	Payload	Request/Signature
Service Authentication Mechanism	Not applicable	TLS
Request/Response Correspondence	Any	TLS	Message/RequestId
Replay Attack Protection	Any	TLS	Message/Nonce in Two-phase protocol
Denial Of Service Protection	Any	None	TLS has no specific countermeasures against denial of service attacks
Non Repudiation	Any	Payload	Message/Signature with digital signature [if required]

XKMS element.attribute name	Required
MessageAbstractType.Service	Required, but not dependent
MessageAbstractType.Signature	Optional, may be used to support non-repudiation for both Request and Response messages
ResultType.RequestId	Required, but not dependent
PendingRequestType.ResponseId	Required, but not dependent
MessageAbstractType.Nonce	Unnecessary
MessageAbstractType.RespondWith=Represent	Unnecessary

Appendix A References (Non-Normative)

[79] [RFC3280] R. Housley et. al., Public Key Infrastructure (X.509) (PKIX) Certificate and Certificate Revocation List (CRL) Profile, IETF RFC 3280, April 2002 ,http://www.ietf.org/rfc/rfc3280.txt

[80] [SOAP] D. Box, D Ehnebuske, G. Kakivaya, A. Layman, N. Mendelsohn, H. Frystyk Nielsen, S Thatte, D. Winer. Simple Object Access Protocol (SOAP) 1.1, W3C Note 08 May 2000. http://www.w3.org/TR/2000/NOTE-SOAP-20000508/

[85] [XML-SIG] D. Eastlake, J. R., D. Solo, M. Bartel, J. Boyer , B. Fox , E. Simon. XML-Signature Syntax and Processing, W3C Recommendation. http://www.w3.org/TR/xmldsig-core/

[89] [XML-Schema1] H. S. Thompson, D. Beech, M. Maloney, N. Mendelsohn. XML Schema Part 1: Structures Second Edition, W3C Recommendation 28 October 2004. http://www.w3.org/TR/xmlschema-1/

Appendix B Acknowledgments (Non-Normative)

[91]This specification is the work of the W3C XML Key Management Working Group. The contributions of the following Working Group members to this specification are gratefully acknowledged in accordance with the contributor policies and the active WG roster.

[92]Participants in the Working Group are (at the time of writing, and by alphabetical order): Guillermo Alvaro Rey (Trinity College Dublin), Stephen Farrell (Trinity College Dublin, Co-Chair), José Kahan (W3C, staff contact), Berin Lautenbach (Apache Software Foundation), Tommy Lindberg (Markup Security), Roland Lockhart (Entrust, Inc.), Vamsi Motukuru (Oracle Corp.), Shivaram Mysore (Co-Chair; Editor since 13 Apr 2004), Rich Salz (DataPower Technology, Inc.), Yunhao Zhang (SQLData Systems).

[93]Previous participants were (by alphabetical order): Daniel Ash (Identrus), Blair Dillaway (Microsoft), Donald Eastlake 3rd (Motorola), Yassir Elley (Sun Microsystems), Jeremy Epstein (webMethods), Slava Galperin (Sun Microsystems), Phillip Hallam-Baker (VeriSign Inc, Editor until 13 Apr 2004), Loren Hart (VeriSign Inc.), Mack Hicks (Bank of America), Merlin Hughes (Baltimore), Frederick Hirsch (Nokia Mobile Phones), Mike Just (Treasury Board of Canada Secretariat), Brian LaMacchia (Microsoft), Pradeep Lamsal, Joseph Reagle (W3C, previous staff contact), Dave Remy (GeoTrust, Inc.), Peter Rostin (RSA Security Inc.), Ed Simon (XMLsec Inc.)

[94]The authors also acknowledge the extensive assistance provided in the design stage of this specification by David Solo (CitiGroup) and Barbara Fox (Microsoft), and the contributions of (by alphabetical order) Dr. Paul Boisen (NSA), Alex Deacon, Dan Guinan, Marc Hayes, Jeremy Epstein (webMethods), Andrew Layman (Microsoft), Mingliang Pei (VeriSign).