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This Note summarizes scenarios, design decisions, and requirements for the XML Signature and Canonical XML specifications, to guide ongoing W3C work to revise these specifications.
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/.
Changes since the previous publication include reformatting of some material and editorial corrections.
This document was published by the XML Security Working Group as an Editor's Draft. If you wish to make comments regarding this document, please send them to public-xmlsec@w3.org (subscribe, archives). All feedback is welcome.
Publication as an Editor's Draft 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.
This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
This use case and requirements document is intended to summarize use cases and requirements driving revisions to XML Signature 2nd Edition [XMLDSIG-CORE], XML Encryption [XMLENC-CORE], and Canonical XML 1.1 [XML-C14N11]. It is not intended to define all possible use cases for these Recommendations, but rather to provide rationale for decisions leading to XML Signature 1.1, XML Encryption 1.1, XML Signature Properties and XML Security Generic Hybrid Ciphers.
This document outlines general principles and use cases leading to requirements and offers some design options. It elaborates on principles and updates requirements expressed for the original XML Security work including original requirements documents (e.g. [XML-CANONICAL-REQ], and [XMLDSIG-REQUIREMENTS]). This document also reflects material from a W3C workshop on next steps for XML Security [XMLSEC-NEXTSTEPS-2007] and position papers associated with the workshop, including [XMLDSIG-COMPLEXITY], [XMLDSIG-SEMANTICS], and [XMLDSIG-THOMPSON].
Design options were documented early on to provide a starting point with the expectation that specifications developed to meet the requirements could subsequently differ in design choices.Thus the design choices in this document should be viewed as historical information.
The following design principles will be used to guide further development of XML Security, including XML Signature, XML Encryption and Canonical XML. These principles are intended to encourage consistent design decisions, to provide insight into design rationale and to anchor discussions on requirements and design. This list includes items from the original requirements for XML Signature [XMLDSIG-REQUIREMENTS] as well as general principles from EXI [EXI]. Listed in alphabetical order:
Backward compatibility should not be broken unnecessarily. Versioning should be clearly considered. Consideration must be given, for example, for interoperability with the First and Second Editions of XML Signature [XMLDSIG-CORE].
XML Security must be consistent with the Web Architecture [WEBARCH].
XML Security should enable efficient implementations, in order to remove barriers to adoption and use.
One of primary objectives of XML Signature is to support a wide variety of use cases requiring digital signatures, including situations requiring multiple signatures, counter-signatures, and signatures including multiple items to be included in a signature. Extensibility should be possible, but by default options should be constrained when the flexibility is not needed.
To reach the broadest set of applications, reduce the security threat footprint and improve efficiency, simple, elegant approaches are preferred to large, analytical or complex ones.
Recognize pragmatic issues, including recognizing that software might be implemented in layers, with a security layer independent of an application layer.
Existing open standards should be reused where possible, as long as other principles can be met.
XML Security should adhere to security best practices, and minimize the opportunities for threats based on XML Security mechanisms.
XML Security must integrate well with existing XML technologies, be compatible with the XML Information Set [XML-INFOSET], in order to maintain interoperability with existing and prospective XML specifications.
XML Signatures should themselves be self-describing first class XML objects [XMLDSIG-REQUIREMENTS]. This means that XML Signatures can be referenced via URI and used in other operations. For example, an XML Signature may be signed or encrypted, or referred to in a statement (such as an RDF statement).
This section outlines the motivation, requirements and design considerations for XML Security 1.1.
Widgets may require signing for integrity protection and source authentication. This signing of a Widget package may be provided using XML Signature.
Provide the ability to sign and verify a widget package using XML Signature. Enable the use of SHA-256 to support sufficient security. Support the use of properties in a XML Signature, including Profile, Role, and Identifier properties to enable interoperable interpretation of signatures. See the Widget Signature specification for a summary of requirements [WIDGETS-DIGSIG].
Define generic widget properties. See XML Signature Properties [XMLDSIG-PROPERTIES].
Several open specifications make use of derived keys, e.g. RSA Laboratories' PKCS #5 v2.0 [PKCS5] and OASIS' WS-SecureConversation Version 1.3 [WS-SECURECONVERSATION13]. These derived keys are used for a variety of purposes including encryption and message authentication, and the purpose of key derivation itself is typically a combination of a desire to expand a given, but limited, set of key material and prudent security practices of limiting use (exposure) of such key material.
Contrary to the situation in the ASN.1-based world (e.g. S/MIME [SMIME]), there is currently a lack of general support in the core XML Security specifications, XML Signature and XML Encryption, for derived keys. Amendment 1 of the aforementioned PKCS #5 v2.0 Amendment 1 [PKCS5] adds support for derived keys only in the context of password-based cryptography. Other XML-based open specifications have similar limitations (see below). This means that an originator of an XML document or message cannot generally make use of key derivation in a standardized manner when performing cryptographic operations on that document.
This section outlines the use of derived keys with Web Services specifications related to Web Services Security [WS-SECURITY11].
This specification [WSS-USERNAME11] describes a key derivation technique for passwords using salt and iteration count (PKCS #5 PBKDF1). It does not allow use of PBKDF2, which is the recommended method to derive keys from passwords in PKCS #5 v2.0. Initial key material cannot be referenced other than with wsu:Id. The key length will always be 160 bits.
Ws-Trust Version 1.3 [WS-TRUST13] describes key derivation through a combination of entropies from both parties. The key is never sent on the wire. The key is never referenced directly (but further key material is derived from it). WS-Trust provides one specific method to derive keys which builds on the P_hash (P_SHA-1) function from TLS.
WS-SecurityPolicy Version 1.2 [WS-SECURITYPOLICY12] really only specifies whether derived keys shall be used or not but may also specify the algorithm to derive keys. The specification also identifies when derived key tokens shall appear in message headers (header layout). WS-SecurityPolicy relies on WS-SecureConversation for the definition of derived keys, key derivation methods and derived key token format.
This specification [WS-SECURECONVERSATION13] defines
the wsc:DerivedKeyTokenType token
type. The derived key token can be used to derive keys
from any other
token that contains keys. The key derivation algorithm specified
builds on the P_hash (P_SHA-1) function from TLS, just as the
algorithm in the Web Service Security UsernameToken Profile
document. Citing from the specification: "The
<wsc:DerivedKeyToken>
element is used to indicate that the key for a specific
reference is
generated from the function. This is so that explicit
security tokens,
secrets, or key material need not be exchanged as
often." (This latter
seems dubious since the DerivedKeyToken still needs to
be exchanged.)
Further: "Basically, a signature or encryption
references a
<wsc:DerivedKeyToken> in the
<wsse:Security> header that, in turn,
references the
<wsc:SecurityContextToken>." The
derived key token does
not support references using key identifiers or key names. All
references must use an ID (to a wsu:Id attribute) or a
URI reference
to the <wsc:Identifier> element in
the Security Context Token.
A derived key type shall be possible to use in those situations where existing specifications make use of ad-hoc derived keys or needs a derived key type
The motivation for this requirement is that any XML Security definition shall be generic enough that there shall be no need to continue with "point" solutions for derived keys; i.e. it shall cover existing and foreseeable uses.
A derived key type shall enable the simple use of derived keys with XML Signature or XML Encryption -using applications, and shall not require import of non-W3C developed specifications with complex security tokens.
The motivation for this is that basic use of XML Signature or XML Encryption should not require use of externally defined security tokens or other security specification elements.
An XML Security derived key type shall allow for existing methods to derive keys; i.e. it shall be possible to use already specified key derivation methods with the new derived key type.
This requirement is based on the assumptions that implementations may want to continue with already chosen key derivation schemes.
A derived key type shall allow for arbitrary derived key lengths.
A derived key type shall allow for referencing using any referencing method in use today for other key types used in XMLDsig or XMLEnc.
A derived key type shall allow for forward referencing with reference lists as recommended by WS-I BSP [WSI-BSP10].
Evaluating the existing specifications against the requirements gives the following result:
UsernameToken Profile:
R1: Not met (method specified in UsernameToken profile is ad-hoc for UsernameToken specifically)
R2: Not met (method requires use of UsernameToken profile)
R3: Not met (UsernameToken profile mandates use of specified mechanism)
R4: Not met (Only accept length of 160 bits)
R5: Not met (No referencing with KeyName or
KeyIdentifier and no
<referenceList> element)
WS-Trust:
R1: N/A (WS-Trust does not define a derived key type per se; only a method to derive keys)
R2: N/A
R3: Meets (Through use of URI to identify method and extensibility)
R4: Meets
R5: Meets (Choice of STS on how to identify key)
WS-SecurityPolicy:
R1: N/A (WS-SecurityPolicy does not define a derived key type)
R2: N/A
R3: Meets (Through the use of URIs to identify key derivation methods and schema extensibility)
R4: Meets
R5: N/A
WS-SecureConversation:
R1: Meets
R2: Does not meet.
R3: Meets (may use the <Properties> element to carry parameters for
other key derivation methods.
R4: Meets
R5: Does not meet as referencing can only be done to a
<wsse:SecurityTokenReference>
In this design option, the new
DerivedKeyType is modeled after the
xenc:EncryptedKeyType. A *possible* outline of such a
type could be:
Outline of possible DerivedKeyType schema definition <element name="DerivedKey" type="xmlsec:DerivedKeyType"/> <complexType name="DerivedKeyType"> <sequence> <element name="KeyDerivationMethod" type="xmlsec:KeyDerivationMethodType" minOccurs="0"/> <element ref="xenc:ReferenceList" minOccurs="0"/> <element name="CarriedKeyName" type="string" minOccurs="0"/> </sequence> <attribute name="Id" type="ID" use="optional"/> <attribute name="Type" type="anyURI" use="optional"/> </complexType> <complexType name="KeyDerivationMethodType"> <sequence> <any namespace="##other" minOccurs="0" maxOccurs="unbounded"/> </sequence> <attribute name="Algorithm" type="anyURI" use="required"/> </complexType>
The proposal immediately meets requirements R2, R3 (any key derivation method may be used, including the ones specified, e.g., in WS-SecureConversation), R4 and R5. For R1 we have:
Username Token Profile: As the UsernameToken
Profile requires use of an
existing procedure to derive keys, the proposal
cannot formally meet
requirement R1. However, since the
UsernameTokenType is extensible,
syntactically the requirement can be met since a
<ds:DerivedKey>
element could be placed in lieu of the current
<salt> and
<iteration>
elements.
WS-Trust: Use of derived keys in WS-Trust is
_implicit_, since the
derived key is never sent. The derived keys may
be referenced by any
available means in issued tokens and the
requester is only required to
identify particular key derivation
methods. Since URIs are used for
this (the <wst:ComputedKey>
element), any other key derivation method
with a well-known URI may be used. Specifically,
one can also envision
an STS returning a proof token containing a
<DerivedKey> element when
there already is a shared key between the STS and a token
requester. And so, R1 is met.
WS-SecurityPolicy: Not affected by a new key type. R1 is met.
WS-SecureConversation: Use of derived keys in
WS-SecureConversation is
typically based on the establishment of a
session context, from which
specific keys are derived. The proposed
<xmlsec:DerivedKeyType> type
may be
used in a similar fashion, although the
interactive nature of
WS-SecureConversation (exchange of Nonces,
Labels) may still favor use
of the existing DerivedKeyToken in this
context. But as a counterexample, a
party that wishes to send data authenticated
with a key derived from a
key established in the session, may do so using
the
<xmlsec:DerivedKey> element
in the <ds:KeyInfo> element,
and the
element may refer to a SecurityContextToken that
identifies the base
key. This would, it seems, eliminate an absolute
need for a
<wsc:DerivedKeyToken> (and
should be similar in nature as the "Implied
Derived Key" option in
WS-SecureConversation). Also, the
<wsc:DerivedKeyToken> implies
use of a particular key derivation
algorithm (the <Label> and
<Nonce> elements) although it
does not
require them.
In summary, WS-Trust and WS-SecurityPolicy are not directly affected by this proposal. UsernameToken profile could use the proposal if the (artificial) requirement to only use the key derivation method specified in the UsernameToken Profile document was relaxed. WS-SecureConversation comes close in establishing an alternative but the specification defines a token primarily for use in interactive sessions based on a security context and which is designed for a particular key derivation method. It also seems strange to require use of such a token in more basic XMLDsig or XMLEnc situations. Finally, the proposal seems to be able to replace the DerivedKeyToken currently used in WS-SecureConversation.
XML Signature specifies algorithm identifiers and implementation requirements for algorithms related to various aspects of signature processing, including digest and signature algorithms. The algorithms listed in XML Signature, Second Edition date from the original XML Signature Recommendation, published in 2002. Since that time there have been new algorithms introduced to address security risks associated with earlier algorithms (e.g. SHA-256 versus SHA-1), changes in patent status related to algorithms (e.g. RSA signing no longer has licensing requirements), and additional algorithms introduced to meet additional requirements (Suite B algorithms [SUITEB], [ECC-ALGS]).
In order to meet the principle of "Secure" and "Pragmatic", new algorithm requirements should be met.
In order to address concerns related to potential risks associated with SHA-1 [SHA-1-Collisions], the following algorithm requirements that update the SHA algorithm should be met in XML Signature 1.1 and XML Encryption 1.1:
Digest:
SHA256 be required.
SHA384 and SHA512 optional.
Mac:
HMAC-SHA256 recommended.
HMAC-SHA384 and HMAC-SHA512 optional. (Note these are Recommended in XML Signature 1.1.)
Signature:
RSAwithSHA256 required.
RSAwithSHA384, RSAwithSHA512 optional.
In order to discourage the use of DSAwithSHA1 but to continue to enable interoperability, the following algorithm changes are requirements;
Signature:
Continue to require DSAwithSHA1 for signature verification, but change DSAwithSHA1 to optional (from required) for signature generation.
In order to:
enable long term security for digital signatures (including in commercial contexts),
ensure that the XML Signature standard is cryptographically secure and makes use of the best current practices for digital signature algorithms, and
enable use of XML Signature technology in a wide variety of commercial and government applications, including those that require Suite B
The additional algorithm requirements are as follows:
Signature:
Require ECDSAwithSHA256.
ECDSAwithSHA1, ECDSAwithSHA384, ECDSAwithSHA512 optional. (Note ECDSAwithSHA1 is Discouraged in XML Signature 1.1 due to concerns with SHA-1.)
Define ECKeyValue element to enable interoperable exchange of EC public key values in XML Signature context.
Provide profile guidance for use of RFC 4050 [RFC4050] when it continues to be used in XML Signature context but indicate preference for mechanism defined in XML Signature.
The last two requirements are discussed in more detail in the following design section.
RFC 4050 is an informational RFC that defines a method of representing ECDSA public keys and ECC curve parameters for use with XML Signature, but it has some issues related to XML Signature:
The RFC 4050 definition of an ECDSAKeyValue is larger than necessary.
An ECDSAKeyValue is defined by the type ECPointType, which has subelements X and Y. X and Y are defined as FieldParamsType which is an abstract type. Separate derived types are defined for prime fields, trinomial base fields, pentanomial base fields, and odd characteristic extension fields. In order to validate against the 4050 schema, one must include the type attribute from the XML schema instance namespace. This is not a significant problem but it does make the public key larger than necessary.
ECPointType definition is inconsistent with ANSI X9.62 and RFC 3279.
ECPointType is reused in the definition of the ExplicitParamsType to describe the base point of a curve. The field parameters are already included in the FieldParams element. The use of the FieldParamsType in the ECPointType definition appears to be a mistake in 4050. If you look at the ASN.1 definition for ECC public keys in RFC 3279 [RFC3279] , ECPoint simply references the Point to Octet String conversion function in ANSI X9.62 (section A.5.6 in the 2005 version, section 4.3.6 in the 1998 version). The conversion functions in X9.62 are not ASN.1 specific and it appears they would be implemented as part of any ECC crypto library. It appears that RFC 4050 tried to avoid using any of the conversion functions in X9.62 but somehow mixed up the definitions between a field type and a field element.
Limitation of the decimal type in XSD
RFC 4050 defines X and Y (at least for prime and odd characteristic extension fields) as xs:nonNegativeInteger which derives from the xs:decimal primitive type. However, XSD requires implementations to support only a maximum of 18 digits (see section 3.2.3 in [XMLSCHEMA11-2] ). It is possible to create an example requiring 77 and 78 digits for X and Y respectively. This means that there is no guarantee that an RFC 4050 compliant ECDSAKeyValue element will actually validate against the RFC 4050 schema.
Collision between the RFC 4050 DTD and the XMLDSIG DTD
Merging the RFC 4050 DTD into the XMLDSIG DTD is a problem due to conflicting DTD definitions. In ECDSAKeyValue, Y is defined as follows:
Definition of Y in ECDSAKeyValue <!ELEMENT Y EMPTY> <!ATTLIST Y Value CDATA #REQUIRED>
However, DSAKeyValue defines Y as follows:
Definition of Y in DSAKeyValue <!ELEMENT Y (#PCDATA) >
ECDSAKeyValue also contains identical definition for elements SEED and P as DSAKeyValue.
It does not seem possible to scope the definition of Y under a specific element in DTD.
Because of these issues, a possible proposed solution is for XML Signature 1.1 to define a new ECPublicKey element in the ds namespace rather than attempt to reuse the RFC 4050 ECDSAPublicKey elements. This new element will be based on the ASN.1 definition ANSI X9.62 and RFC 3279. Changing the name of the element to ECPublicKey means it can be also used in XML Encryption to support ECDH. (Note, XML Signature 1.1 defined ECKeyValue instead).
To maximize interoperability with existing RFC 4050 implementations, we should also put a note in 1.1 to recommend implementations to support a profile of RFC 4050. The profile will support only named prime curves.
This section summarizes the motivation for new features designed to address known issues. (This section of the requirements document was written after the XML Signature 1.1 specification was updated in order to record the rationale for the changes.)
X509IssueSerial
The X509IssuerSerialNumber child element of
the X509IssuerSerialType
XML Schema type
was defined to be an integer
holding an X.509 certificate serial number.
XML
Schema validators may not support integer types with decimal
data exceeding 18 decimal digits [XMLSCHEMA-2]
and this
maximum length has proven
insufficient as many Certificate Authorities issue
certificates with large random serial numbers that
exceed this
limit. A
new element is defined in XML Signature 1.1 with a
different type definition,
the sig11:X509Digest element, and a warning
that
deployments that make use of
the X509IssuerSerial element
should take care if schema validation is involved.
ds:KeyInfo
The RetrievalMethod is ambiguous about whether the result
is an element within KeyInfo or the KeyInfo
element itself. It also supports the use of ds:Transform
adding complexity. The new KeyInfoReference element
removes the ambiguity by always referencing the KeyInfo
element itself. It also is simpler in that it does not allow
any ds:Transform children.
KeyValue type interoperability
XML Signature 1.1 defines XML formats to
convey key information in the KeyValue
element. There are scenarios
where at least one of signer and/or verifier are not able to
serialize keys in those XML formats.
The DEREncodedKeyValue element has been
added to XML Signature 1.1 to support use
of other binary encodings.
It is sometimes useful to provide an OCSP
response along
with an X.509 certificate. The OCSPResponse
element was
added to X509Data to support this use case.
Contributions received from the members of the XML Security Working Group: Scott Cantor, Juan Carlos Cruellas, Pratik Datta, Gerald Edgar, Ken Graf, Phillip Hallam-Baker, Brad Hill, Frederick Hirsch, Brian LaMacchia, Konrad Lanz, Hal Lockhart, Cynthia Martin, Rob Miller, Sean Mullan, Shivaram Mysore, Magnus Nyström, Bruce Rich, Thomas Roessler, Ed Simon, Chris Solc, John Wray, Kelvin Yiu.
Dated references below are to the latest known or appropriate edition of the referenced work. The referenced works may be subject to revision, and conformant implementations may follow, and are encouraged to investigate the appropriateness of following, some or all more recent editions or replacements of the works cited. It is in each case implementation-defined which editions are supported.