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 is Part 2 of the W3C Candidate Recommendation for the XML Key Management Specification (XKMS Version 2.0), made available for public review by the XKMS Working Group. Part 1 of this specification covers the XKMS protocols and services. For background on this work, please see the XKMS Activity Statement.

Note that when first released on 18 April 2003, the XKMS specification consisted of two separate documents: XML Key Management Specification Version 2.0 and XML Key Management Specification Version 2.0 Bindings. In order to show their relationship, the Working Group decided to now refer to them as Part-1 and Part-2 of the XKMS Specification. This is just an Editorial modification and doesn't concern the content of those documents.

This document is based upon the XKMS Bindings Version 2.0 Working Draft of 18 April 2003 and feedback received during the review period (see the Disposition of Comments document). The XKMS Working Group believes that this specification addresses its Requirements and all of the Last Call issues.

The purpose of a W3C Candidate Recommendation is to gather implementation experience. The status of implementations will be tracked on the XKMS Implementation Report. The XKMS CR period will last a minimum of six months (1 October 2004) to ensure that enough time is given for providing implementation feedback. Note that the Conformance Section of this document may be improved following the experience gained during the CR period.

This document has been produced under the 24 January 2002 Current Patent Practice as amended by the W3C Patent Policy Transition Procedure. Patent disclosures relevant to this specification may be found on the Working Group's patent disclosure page. 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.

Comments on this document should be sent to public-xkms-comments@w3.org, a mailing list with a public archive. General discussion of related technology is welcome in www-xkms@w3.org (archive).

Publication as a Candidate 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 Acknowledgments

[12] 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.

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

1.3 Definition of Terms

[14] This document uses the terms defined in section 1.3 of part one of this specification in the manner described therein.

1.4 Structure of this documentt

[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/2002/06/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. Each implementation profile is assigned a unique URI to facilitate negotiation of a specific security profile using some mechanism to be described as a part of the wider Web Services framework.

	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

Security Consideration	Variant	Support	XKMS element
Client Authentication Mechanism	I	None
	II	Payload	Request/Signature
Service Authentication Mechanism		Payload	Response/Signature
Request/Response Correspondence	I	Payload	Request/MessageDigest
	II	Payload	Message/RequestId
Replay Attack Protection		Payload	Message/Nonce
Denial Of Service Protection		Payload	Request/RespondWith=Represent
Non Repudiation		Payload	Message/Signature with digital signature

XKMS element	Required
Message/Service	Required
Request/Signature	Required in profile II
Response/Signature	Required
Message/RequestId	Required
Message/ResponseId	Required
Message/Nonce	Optional, may be used to protect against Denial of Service
Request/RespondWith=Represent	Optional, may be used to protect against Denial of Service
Request/MessageDigest	Required in profile I, Optional in profile II

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."

[74] The SSL/TLS binding has three variants specified by the following identifiers:

Security Consideration	Variant	Support	XKMS element
Client Authentication Mechanism	I	None
	II	TLS	Certificate based client authentication
	II	Payload	Request/Signature
Service Authentication Mechanism		TLS
Request/Response Correspondence		TLS
Replay Attack Protection		TLS
Denial Of Service Protection		None	The TLS service is subject to a denial of service attack [Check this]
Non Repudiation		Payload	Message/Signature with digital signature [if required]

XKMS element	Required
Message/Service	Required but not dependent
Request/Signature	Optional, may be used to support non-repudiation
Response/Signature	Optional, may be used to support non-repudiation
Message/RequestId	Required but not dependent
Message/ResponseId	Required but not dependent
Message/Nonce	Unnecessary
Request/RespondWith=Represent	Unnecessary
Request/MessageHash	Unnecessary

Appendix A References

[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/

[84] [WS-TrustAxiom] P. Hallam-Baker et. al., WS-Trust Axiom, To be published

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

[87] [XML-Enc] Donald Eastlake, Joseph Reagle, Takeshi Imamura, Blair Dillaway, Ed Simon, XML Encryption Syntax and Processing, World Wide Web Consortium, http://www.w3.org/TR/xmlenc-core/