XML-Signature Syntax and Processing
W3C Working Draft 11-July-2000
This version:
http://www.w3.org/TR/2000/WD-xmldsig-core-20000711/
http://www.ietf.org/internet-drafts/draft-ietf-xmldsig-core-08.txt [W3C-mirror]
Latest version:
http://www.w3.org/TR/xmldsig-core/
Previous version:
http://www.w3.org/TR/2000/WD-xmldsig-core-20000601/
http://www.ietf.org/internet-drafts/draft-ietf-xmldsig-core-07.txt [W3C-mirror]
Editors
Donald Eastlake
<dee3@torque.pothole.com>
Joseph Reagle <reagle@w3.org>
David Solo <dsolo@alum.mit.edu>
Authors
Mark Bartel <mbartel@JetForm.com>
John Boyer <jboyer@PureEdge.com>
Barb Fox <bfox@Exchange.Microsoft.com>
Ed Simon <ed.simon@entrust.com>
Contributors
See Acknowledgements
Copyright ©
2000 W3C® (MIT, INRIA, Keio), All
Rights Reserved. W3C liability, trademark, document use and software licensing rules
apply.
Abstract
This document
specifies XML digital signature processing rules and syntax. XML Signatures
provide integrity, message authentication, and/or signer authentication
services for data of any type, whether located within the XML that includes the
signature or elsewhere.
Status of this document
This
specification of the IETF/W3C XML Signature Working Group follows the XML
Signature Last Call and attempts to address all last call comments sent to the list and
those issues discussed at the April meeting.
Additionally, this specification follows the request that the W3C Director and
IESG consider this specification for advancement on to the standards tracks of
each institution; those concerns included minor process/status issues as well
as the requirement that the Canonical XML specification precede the Signature
specification to Candidate REC (including resolving the last couple
internationalization issues). Additionally, prior to the next draft we hope to:
1. Ensure
that our use of schema namespaces and qualifications
provides a single schema that can be used for enveloped
signatures (signature within content being signed), enveloping
signatures (content is within signature being signed) and detached
signatures (over data external to the signature document).
2. Further
test our employment of Schema, URIs, IDs, and XPath.
3. Confirm
that a compliant Signature application ensures an XML instance is valid XML
for the schema (and DTD) that we have specified.
This version highlights (via
red underlined text) a few of the changes from the previous version.
Please send
comments to the editors and cc: the list <w3c-ietf-xmldsig@w3.org>.
Publication as a Working Draft does not imply endorsement by the W3C membership
or IESG. It is inappropriate to cite W3C Drafts as other than "work in
progress." A list of current W3C working drafts can be found at http://www.w3.org/TR/.
Current IETF drafts can be found at http://www.ietf.org/1id-abstracts.html.
Patent
disclosures relevant to this specification may be found on the Working Group's patent disclosure page and
IETF's Intellectual Property Right Notices.
Table of Contents
1. Introduction
3. Versions, Namespaces and Identifiers
2. Signature Overview and Examples
1. Simple Example (Signature,
SignedInfo, Methods, and References)
2. Extended Example (Object
and SignatureProperty)
3. Extended Example (Object
and Manifest)
1. The CanonicalizationMethod Element
2. The SignatureMethod Element
5. Additional Signature Syntax
2. The SignatureProperties Element
6. Algorithms
1. Algorithm Identifiers and Implementation
Requirements
3. Message Authentication Codes
5. Canonicalization Algorithms
2. Base64
4. Enveloped Signature Transform
7. XML Canonicalization and Syntax
Constraint Considerations
1. XML 1.0, Syntax Constraints, and
Canonicalization
2. DOM/SAX Processing and Canonicalization
1. Transforms
1. Only What is Signed is Secure
2. Only What is "Seen" Should be Signed
3. Algorithms, Key Lengths, Etc.
9. Schema, DTD, Data Model,and Valid Examples
10. Definitions
11. References
12. Authors' Address
1.0 Introduction
This document
specifies XML syntax and processing rules for creating and representing digital
signatures. XML Signatures can be applied to any digital content (data object),
including XML. An XML Signature may be applied to the content of one or more
resources. Enveloped or enveloping
signatures are over data within the same XML document as the signature; detached
signatures are over data external to the signature element.
This
specification also defines other useful types including methods of referencing
collections of resources, algorithms, and keying information and management.
1.1 Editorial
Conventions
For readability,
brevity, and historic reasons this document uses the term "signature"
to generally refer to digital authentication values of all types.Obviously, the
term is also strictly used to refer to authentication values that are based on
public keys and that provide signer authentication. When specifically discussing
authentication values based on symmetric secret key codes we use the terms
authenticators or authentication codes. (See section 8.3:Check the Security Model.)
This
specification uses both XML Schemas [XML-schema] and DTDs [XML]. (Readers unfamiliar with DTD syntax may
wish to refer to Ron Bourret's " Declaring Elements and Attributes in
an XML DTD" [Bourret].) The schema definition is presently
normative.
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 [KEYWORDS]:
"they MUST
only be used where it is actually required for interoperation or to limit behavior
which has potential for causing harm (e.g., limiting retransmissions)"
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 Design Philosophy
The design
philosophy and requirements of this specification are addressed in the
XML-Signature Requirements document [XML-Signature-RD].
1.3 Versions,
Namespaces and Identifiers
No provision is
made for an explicit version number in this syntax. If a future version is
needed, it will use a different namespace The XML namespace [XML-ns] URI that MUST
be used by implementations of this (dated) specification is:
xmlns="http://www.w3.org/2000/07/xmldsig#"
This namespace is
also used as the prefix for algorithm identifiers used by this specification.
While applications MUST support XML and XML-namespaces, the use of internal entities [XML] or our
"dsig" XML namespace prefix and
defaulting/scoping conventions are OPTIONAL; we use these facilities to provide
compact and readable examples.
This specification
uses Uniform Resource Identifiers [URI] to
identify resources, algorithms, and semantics. The URI in the namespace
declaration above is also used as a prefix for URIs under the control of this
specification. For resources not under the control of this specification, we
use the designated Uniform Resource Names [URN] or Uniform Resource Locators [URL] defined by its
normative external specification. If an external specification has not
allocated itself a Uniform Resource Identifier we allocate an identifier under
our own namespace. For instance:
SignatureProperties is identified and defined by this
specification's namespace
http://www.w3.org/2000/07/xmldsig#SignatureProperties
XSLT is
identified and defined by an external namespace
http://www.w3.org/TR/1999/PR-xslt-19991008
SHA1 is
identified via this specification's namespace and defined via a normative
reference
http://www.w3.org/2000/07/xmldsig#sha1
FIPS PUB 180-1. Secure Hash Standard. U.S. Department of
Commerce/National Institute of Standards and Technology.
Finally, in order
to provide for terse namespace declarations we sometimes use XML internal entities [XML] as macros
within URIs. For instance:
<?xml version='1.0'?> <!DOCTYPE Signature SYSTEM "xmldsig-core-schema.dtd" [ <!ENTITY dsig "http://www.w3.org/2000/07/xmldsig#"> ]> <Signature xmlns="&dsig;" Id="MyFirstSignature"> <SignedInfo> ...
1.4 Acknowledgements
The contributions
of the following working group members to this specification are gratefully
acknowledged:
·
Mark Bartel, JetForm Corporation (Author)
·
John Boyer, PureEdge (Author)
·
Mariano P. Consens, University of Waterloo
·
John Cowan, Reuters Health
·
Donald Eastlake 3rd, Motorola (Chair,
Author/Editor)
·
Barb Fox, Microsoft (Author)
·
Christian Geuer-Pollmann, University Siegen
·
Tom Gindin, IBM
·
Phillip Hallam-Baker, VeriSign Inc
·
Richard Himes, US Courts
·
Gregor Karlinger, IAIK TU Graz
·
Brian LaMacchia, Microsoft
·
Peter Lipp, IAIK TU Graz
·
Joseph Reagle, W3C (Chair, Author/Editor)
·
Ed Simon, Entrust Technologies Inc. (Author)
·
David Solo, Citigroup (Author/Editor)
·
Petteri Stenius, DONE Information, Ltd
·
Raghavan Srinivas, Sun
·
Kent Tamura, IBM
·
Winchel Todd Vincent III, GSU
·
Carl Wallace, Corsec Security, Inc.
·
Greg Whitehead, Signio Inc.
As are the last
call comments from the following:
·
Dan Connolly, W3C
·
Paul Biron, Kaiser Permanente, on behalf of the XML Schema WG.
·
Martin J. Duerst, W3C; and Masahiro Sekiguchi,
Fujitsu; on behalf of the Internationalization WG/IG.
·
Jonathan Marsh, Microsoft, on behalf of the Extensible Stylesheet Language WG.
2.0 Signature Overview
and Examples
This section
provides an overview and examples of XML digital signature syntax. The specific
processing is given in section 3: Processing Rules. The
formal syntax is found in section 4: Core Signature Syntax and section 5: Additional Signature Syntax.
In this section,
an informal representation and examples are used to describe the structure
of the XML signature syntax. This representation and examples may omit
attributes, details and potential features that are fully explained later.
XML Signatures
are applied to arbitrary digital content (data objects) via an
indirection. Data objects are digested, the resulting value is placed in an
element (with other information) and that element is then digested and
cryptographically signed. XML digital signatures are represented by the Signature element which
has the following structure (where "?" denotes zero or one
occurrence; "+" denotes one or more occurrences; and "*"
denotes zero or more occurrences):
<Signature> <SignedInfo>(CanonicalizationMethod[GK1])?
(SignatureMethod) <Reference (URI=)? > (Transforms)? (DigestMethod) (DigestValue)(</Reference[GK2]>)+
</SignedInfo> (SignatureValue) (KeyInfo)? (Object)* </Signature>
The content that
is signed was, at the time of signature creation, referred to as an identified resource to
which the specified transforms were applied.
Signatures are
related to data objects via URIs [URI].
Within an XML document, signatures are related to local data objects via
fragment identifiers. Such local data can be included within an enveloping
signature or can enclose an enveloped signature. Detached signatures are
over external network resources or local data objects that resides within the
same XML document as sibling elements; in this case, the signature is neither
enveloping (signature is parent) nor enveloped (signature is child). Since a Signature element (and its
Id attribute
value/name) may co-exist or be combined with other elements (and their IDs)
within a single XML document, care should be taken in choosing names such that
there are no subsequent collisions that violate the ID uniqueness validity constraint [XML].
2.1 Simple Example (Signature,
SignedInfo, Methods, and References)
The following
example is a detached signature of the content of the HTML4 in XML
specification.
[s01] <Signature Id="MyFirstSignature" xmlns="http://www.w3.org/2000/07/xmldsig#"> [s02] <SignedInfo> [s03] <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000710"/> [s04] <SignatureMethod Algorithm="http://www.w3.org/2000/07/xmldsig#dsa-sha1"/> [s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000710"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2000/07/xmldsig#sha1"/> [s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [s11] </Reference> [s12] </SignedInfo> [s13] <SignatureValue>MC0CFFrVLtRlk=...</SignatureValue> [s14] <KeyInfo> [s15a] <KeyValue> [s15b] <DSAKeyValue> [s15c] <P>...</P><Q>...</Q><G>...</G><Y>...</Y>
[s15d] </DSAKeyValue>
[s15e] </KeyValue> [s16] </KeyInfo> [s17] </Signature>
[s02-12] The required SignedInfo element is the
information that is actually signed. Core validation of SignedInfo consists of two
mandatory processes: validation of the signature over SignedInfo and validation of each Reference
digest within SignedInfo. Note
that the algorithms used in calculating the SignatureValue are also included in the signed
information while the SignatureValue
element is outside SignedInfo.
[s03] The CanonicalizationMethod is
the algorithm that is used to canonicalize the SignedInfo element before it is digested as part of
the signature operation. In the
absence of a CanonicalizationMethod element, no canonicalization is done[GK3].
[s04] The SignatureMethod is
the algorithm that is used to convert the canonicalized SignedInfo into the SignatureValue. It is a
combination of a digest algorithm and a key dependent algorithm and possibly
other algorithms such as padding, for example RSA-SHA1. The algorithm names are
signed to resist attacks based on substituting a weaker algorithm. To promote
application interoperability we specify a set of signature algorithms that MUST
be implemented, though their use is at the discretion of the signature creator.
We specify additional algorithms as RECOMMENDED or OPTIONAL for implementation
and the signature design permits arbitrary user algorithm specification.
[s05-11] Each Reference element includes the digest method and
resulting digest value calculated over the identified data object. It also may
include transformations that produced the input to the digest operation. A data
object is signed by computing its digest value and a signature over that value.
The signature is later checked via reference and signature validation.
[s14-16] KeyInfo indicates the key to be used to validate
the signature. Possible forms for identification include certificates, key
names, and key agreement algorithms and information -- we define only a few. KeyInfo is optional for
two reasons. First, the signer may not wish to reveal key information to all
document processing parties. Second, the information may be known within the
application's context and need not be represented explicitly. Since KeyInfo is outside of SignedInfo, if the signer
wishes to bind the keying information to the signature, a Reference can easily
identify and include the KeyInfo as
part of the signature.
2.1.1 More on Reference
[s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000710"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2000/07/xmldsig#sha1"/> [s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [s11] </Reference>
[s05] The optional URI attribute of Reference identifies the data object to be signed.
This attribute may be omitted on at most one Reference in a Signature. (This limitation is imposed in order to
ensure that references and objects may be matched unambiguously.)
[s05-08] This identification, along with the
transforms, is a description provided by the signer on how they obtained the
signed data object in the form it was digested (i.e. the digested content). The
verifier may obtain the digested content in another method so long as the
digest verifies. In particular, the verifier may obtain the content from a
different location such as a local store than that specified in the URI.
[s06-08] Transforms is an optional ordered list of
processing steps that were applied to the resource's content before it was
digested. Transforms can include operations such as canonicalization,
encoding/decoding (including compression/inflation), XSLT and XPath. XPath
transforms permit the signer to derive an XML document that omits portions of
the source document. Consequently those excluded portions can change without
affecting signature validity. For example, if the resource being signed
encloses the signature itself, such a transform must be used to exclude the
signature value from its own computation. If no Transforms element is present, the resource's
content is digested directly. While we specify mandatory (and optional) canonicalization
and decoding algorithms, user specified transforms are permitted.
[s09-10] DigestMethod is the algorithm applied to the
data after Transforms is
applied (if specified) to yield the DigestValue. The signing of the DigestValue is what binds a
resources content to the signer's key.
2.2 Extended Example (Object
and SignatureProperty)
This
specification does not address mechanisms for making statements or assertions.
Instead, this document defines what it means for something to be signed by an
XML Signature (message authentication, integrity, and/or signer
authentication). Applications that wish to represent other semantics must rely
upon other technologies, such as [XML, RDF]. For instance,
an application might use a foo:assuredby
attribute within its own markup to reference a Signature element. Consequently, it's the
application that must understand and know how to make trust decisions given the
validity of the signature and the meaning of assurdby syntax. We also define a SignatureProperties element type for
the inclusion of assertions about the signature itself (e.g., signature
semantics, the time of signing or the serial number of hardware used in
cryptographic processes). Such assertions may be signed by including a Reference for the SignatureProperties in SignedInfo. While the
signing application should be very careful about what it signs (it should
understand what is in the SignatureProperty) a
receiving application has no obligation to understand that semantic (though its
parent trust engine may wish to). Any content about the signature generation
may be located within the SignatureProperty element. The mandatory Target attribute references the Signature element to which the property applies[GK4].
Consider the
preceding example with an additional reference to a local Object that includes a SignatureProperty element. (Such a
signature would not only be detached [p02] but enveloping [p03].)
[ ] ... [p01] <SignedInfo> [ ] ... [p02] <Reference URI="http://www.w3.org/TR/xml-stylesheet/"> [ ] ... [p03] <Reference URI=" #AMadeUpTimeStamp [GK5]"
[p04] Type="http://www.w3.org/2000/07/xmldsig#SignatureProperty"> [p05] <DigestMethod Algorithm="http://www.w3.org/2000/07/xmldsig#sha1"/> [p06] <DigestValue>k3453rvEPO0vKtMup4NbeVu8nk=</DigestValue> [p07] </Reference> [p08] </SignedInfo> [p09] ... [p10] <Object> [p11] <SignatureProperties> [p12] <SignatureProperty Id="AMadeUpTimeStamp" Target=" #MySecondSignature [GK6]">
[p13] <timestamp xmlns="http://www.ietf.org/rfcXXXX.txt"> [p14] <date>19990908</date> [p15] <time>14:34:34:34</time> [p16] </timestamp> [p17] </SignatureProperty> [p18] </SignatureProperties> [p19] </Object> [p20]</Signature>
[p04] The optional Type attribute of Reference provides information about the resource
identified by the URI. In
particular, it can indicate that it is an Object, SignatureProperty, or Manifest element. This can be used by applications
to initiate special processing of some Reference elements. References to an XML data
element within an Object
element SHOULD identify the actual element pointed to. Where the element
content is not XML (perhaps it is binary or encoded data) the reference should
identify the Object and
the Reference Type, if given, SHOULD indicate Object. Note that Type is advisory and no action based on
it or checking of its correctness is required by core behavior.
[p10] Object is an
optional element for including data objects within the signature element or
elsewhere. The Object can
be optionally typed and/or encoded.
[p11-18] Signature properties, such as time of
signing, can be optionally signed by identifying them from within a Reference. (These
properties are traditionally called signature "attributes" although
that term has no relationship to the XML term "attribute".)
2.3 Extended Example (Object
and Manifest)
The Manifest element is
provided to meet additional requirements not directly addressed by the
mandatory parts of this specification. Two requirements and the way the Manifest satisfies them
follows.
First,
applications frequently need to efficiently sign multiple data objects even
where the signature operation itself is an expensive public key signature. This
requirement can be met by including multiple Reference elements within SignedInfo since the
inclusion of each digest secures the data digested. However, some applications
may not want the core validation behavior associated with this
approach because it requires every Reference within SignedInfo to undergo reference validation --
the DigestValue elements are
checked. These applications may wish to reserve reference validation decision
logic to themselves. For example, an application might receive a signature valid SignedInfo element that
includes three Reference
elements. If a single Reference fails
(the identified data object when digested does not yield the specified DigestValue) the signature
would fail core validation. However, the application may wish
to treat the signature over the two valid Reference elements as valid or take different
actions depending on which fails. To accomplish this, SignedInfo would reference
a Manifest element that
contains one or more Reference
elements (with the same structure as those in SignedInfo). Then, reference validation of the Manifest is under
application control.
Second, consider
an application where many signatures (using different keys) are applied to a
large number of documents. An inefficient solution is to have a separate
signature (per key) repeatedly applied to a large SignedInfo element (with
many References); this is
wasteful and redundant. A more efficient solution is to include many references
in a single Manifest that
is then referenced from multiple Signature elements.
The example below
includes a Reference that
signs a Manifest found within the
Object element.
[ ] ... [m01] <Reference URI="#MyFirstManifest" [m02] Type="http://www.w3.org/2000/07/xmldsig#Manifest"> [m03] <DigestMethod Algorithm="http://www.w3.org/2000/07/xmldsig#sha1"/> [m04] <DigestValue>345x3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [m05] </Reference> [ ] ... [m06] <Object>[m07] <Manifest Id="MyFirstManifest">
[m08] <Reference>
[m09] ... [m10] </Reference> [m11] <Reference> [m12] ... [m13] </Reference> [m14] </Object>
3.0 Processing Rules
The sections
below describe the operations to be performed as part of signature generation
and validation.
3.1 Core Generation
The REQUIRED
steps include the generation of Reference
elements and the SignatureValue over SignedInfo.
3.1.1 Reference Generation
For each data
object being signed:
1. Apply
the Transforms, as determined
by the application, to the data object.
2. Calculate
the digest value over the resulting data object.
3. Create
a Reference element,
including the (optional) identification of the data object, any (optional)
transform elements, the digest algorithm and the DigestValue.
3.1.2 Signature Generation
1. Create
SignedInfo element with SignatureMethod, CanonicalizationMethod if required[GK7], and Reference(s).
2. Canonicalize
and then calculate the SignatureValue over SignedInfo based on
algorithms specified in SignedInfo.
3. Construct
the Signature element that
includes SignedInfo, Object(s) (if desired, encoding may be
different than that used for signing), KeyInfo (if required), and SignatureValue.
3.2 Core Validation
The REQUIRED
steps of core validation include (1) reference validation, the
verification of the digest contained in each Reference in SignedInfo, and (2) the cryptographic signature validation of
the signature calculated over SignedInfo.
Note, there may
be valid signatures that some signature applications are unable to validate.
Reasons for this include failure to implement optional parts of this
specification, inability or unwillingness to execute specified algorithms, or
inability or unwillingness to dereference specified URIs (some URI schemes may
cause undesirable side effects), etc.
3.2.1 Reference Validation
For each Reference in SignedInfo:
1.
Canonicalize
the SignedInfo
element based on the CanonicalizationMethod in SignedInfo[GK8].
2. Obtain
the data object to be digested. (The signature application may rely upon the
identification (URI) and Transforms provided by the
signer in the Reference
element, or it may obtain the content through other means such as a local
cache.)
3. Digest
the resulting data object using the DigestMethod specified in its Reference specification.
4. Compare
the generated digest value against DigestValue in the SignedInfo Reference; if there is any mismatch, validation
fails.
3.2.2 Signature Validation
1. Canonicalize
the SignedInfo element based on
the CanonicalizationMethod in SignedInfo.
2. Obtain
the keying information from KeyInfo or
from an external source.
3. Use the
specified SignatureMethod to
validate the SignatureValue over
the (optionally canonicalized) SignedInfo
element.
4.0 Core Signature
Syntax
The general
structure of an XML signature is described in section 2: Signature Overview. This
section provides detailed syntax of the core signature features and actual
examples. Features described in this section are mandatory to implement unless
otherwise indicated. The syntax is defined via DTDs and [XML-Schema] with
the following XML preamble, declaration, internal entity, and simpleType:
Schema Definition:
<?xml version='1.0'?> <!DOCTYPE schema SYSTEM 'http://www.w3.org/1999/XMLSchema.dtd' [
<!ENTITY dsig 'http://www.w3.org/2000/07/xmldsig#'> ]>
<schema targetNamespace='&dsig;' version='0.1' xmlns='http://www.w3.org/1999/XMLSchema'xmlns:ds='&dsig;'
elementFormDefault='qualified'>
<!-- Basic Types Defined for Signatures --> <simpleType name='CryptoBinary' base='binary'> <encoding value='base64'/> </simpleType> DTD: <!-- These entity declarations permit the flexible parts of Signature content model to be easily expanded --> <!ENTITY % Object.ANY '(#PCDATA|Signature|SignatureProperties|Manifest)*'> <!ENTITY % Method.ANY '(#PCDATA|HMACOutputLength)*'> <!ENTITY % Transform.ANY '(#PCDATA|XPath|XSLT)*'> <!ENTITY % Key.ANY '(#PCDATA|KeyName|KeyValue|RetrievalMethod| X509Data|PGPData|MgmtData|DSAKeyValue|RSAKeyValue)*'>
4.1 The Signature element
The Signature element is the
root element of a XML Signature. A simple example of a complete signature
follows:
Schema Definition: <element name='Signature'> <complexType content='elementOnly'> <sequence minOccurs='1' maxOccurs='1'> <element ref='ds:SignedInfo' minOccurs='1' maxOccurs='1'/> <element ref='ds:SignatureValue' minOccurs='1' maxOccurs='1'/>
<element ref='ds:KeyInfo' minOccurs='0' maxOccurs='1'/> <element ref='ds:Object' minOccurs='0' maxOccurs='unbounded'/> </sequence> <attribute name='Id' type='ID' use='optional'/> </complexType> </element> DTD: <!ELEMENT Signature (SignedInfo, SignatureValue, KeyInfo?, Object*) ><!ATTLIST Signature
xmlns CDATA #FIXED 'http://www.w3.org/2000/07/xmldsig#' Id ID #IMPLIED >
4.2 The SignatureValue Element
The SignatureValue element contains
the actual value of the digital signature; it is encoded according to the
identifier specified in SignatureMethod.
Base64 [MIME] is
the encoding method for all SignatureMethods
specified within this specification. While we specify a mandatory and optional
to implement SignatureMethod
algorithms, user specified algorithms (with their own encodings) are permitted.
Schema Definition: <element name='SignatureValue' type='ds:CryptoBinary'[GK9]/>
DTD: <!ELEMENT SignatureValue (#PCDATA) >
4.3 The SignedInfo Element
The structure of SignedInfo includes the
canonicalization algorithm, a signature algorithm, and one or more references.
The SignedInfo element may
contain an optional ID attribute that will allow it to be referenced by other
signatures and objects.
Schema Definition:
<element name='SignedInfo'> <complexType content='elementOnly'>
<sequence minOccurs='1' maxOccurs='1'> <element ref='ds:CanonicalizationMethod' minOccurs='1' maxOccurs='1'/> <element ref='ds:SignatureMethod' minOccurs='1' maxOccurs='1'/> <element ref='ds:Reference' minOccurs='1' maxOccurs='unbounded'/> </sequence>
<attribute name='Id' type='ID' use='optional'/> </complexType> </element> DTD: <!ELEMENT SignedInfo (CanonicalizationMethod, SignatureMethod, Reference+) > <!ATTLIST SignedInfo Id ID #IMPLIED>
SignedInfo does not include explicit signature or
digest properties (such as calculation time, cryptographic device serial
number, etc.). If an application needs to associate properties with the
signature or digest, it may include such information in a SignatureProperties element within
an Object element.
4.3.1 The CanonicalizationMethod Element
CanonicalizationMethod
is a required element
that specifies the canonicalization algorithm applied to the SignedInfo element prior to
performing signature calculations. This element uses the general structure for
algorithms described in section 6.1: Algorithm Identifiers and Implementation Requirements. The
default canonicalization algorithm (applied if this element is omitted) is
Canonical XML [XML-C14N].
Alternatives,
such as the minimal canonicalization algorithm (the CRLF and charset
normalization specified in section 6.5.1: Minimal Canonicalization), may be
explicitly specified but are NOT REQUIRED. Consequently, their use may not
interoperate with other applications that do no support the specified algorithm
(see section 7: XML Canonicalization and Syntax Constraint Considerations).
Security issues may also arise in the treatment of entity processing and comments
if minimal or other non-XML aware canonicalization algorithms are not properly
constrained (see section 8.2: Only What is
"Seen" Should be Signed).
We RECOMMEND that
resource constrained applications that do not implement the Canonical XML
[XML-C14N] transform and instead choose minimal canonicalization (or some other
form) are implemented to generate Canonical XML as their output serialization
to easily mitigate some of these interoperability and security concerns. For instance,
such an implementation SHOULD (at least) generate standalone XML instances [XML].
Schema Definition: <element name='CanonicalizationMethod'> <complexType content='elementOnly'> <any minOccurs='0' maxOccurs='unbounded'/> <attribute name='Algorithm' type='uriReference' use='required'/> </complexType> </element> DTD: <!ELEMENT CanonicalizationMethod %Method.ANY; > <!ATTLIST CanonicalizationMethod Algorithm CDATA #REQUIRED >
4.3.2 The SignatureMethod Element
SignatureMethod is a required element that specifies the
algorithm used for signature generation and validation. This algorithm
identifies all cryptographic functions involved in the signature operation
(e.g. hashing, public key algorithms, MACs, padding, etc.). This element uses
the general structure here for algorithms described in section 6.1: Algorithm Identifiers and Implementation
Requirements. While there is a single identifier, that
identifier may specify a format containing multiple distinct signature values.
Schema Definition: <element name='SignatureMethod'> <complexType content='elementOnly'> <any minOccurs='0' maxOccurs='unbounded'/> <attribute name='Algorithm' type='uriReference' use='required'/> </complexType> </element> DTD: <!ELEMENT SignatureMethod %Method.ANY; > <!ATTLIST SignatureMethod Algorithm CDATA #REQUIRED >
4.3.3 The Reference Element
Reference is an element that may occur one or more
times. It specifies a digest algorithm and digest value, and optionally an
identifier of the object being signed, the type of the object, and/or a list of
transforms to be applied prior to digesting. The identification (URI) and
transforms describe how the digested content (i.e., the input to the digest
method) was created. The Type
attribute facilitates the processing of referenced data. For example, while
this specification makes no requirements over external data, an application may
wish to signal that the referent is a Manifest. An optional ID attribute permits a Reference to be referenced
from elsewhere.
Schema Definition: <element name='Reference'> <complexType content='elementOnly'> <sequence minOccurs='1' maxOccurs='1'> <element ref='ds:Transforms' minOccurs='0' maxOccurs='1'/> <element ref='ds:DigestMethod' minOccurs='1' maxOccurs='1'/>
<element ref='ds:DigestValue' minOccurs='1' maxOccurs='1'/> </sequence>
<attribute name='Id' type='ID' use='optional'/> <attribute name='URI' type='uriReference' use='optional'/> <attribute name='Type' type='uriReference' use='optional'/> </complexType> </element> DTD: <!ELEMENT Reference (Transforms?, DigestMethod, DigestValue) > <!ATTLIST Reference Id ID #IMPLIED URI CDATA #IMPLIED Type CDATA #IMPLIED >
The URI attribute identifies a data object
using a URI-Reference, as specified by RFC2396 [URI]. (Non-ASCII characters in a URI should
be represented in UTF-8 [UTF-8] as
one or more bytes, and then escaping these bytes with the URI escaping
mechanism. [XML])
Note that a null URI (URI="") is
permitted and identifies the XML document that the reference is contained
within (the root element[GK10]). XML Signature applications MUST be able
to parse URI syntax. We RECOMMEND they be able to dereference URIs and null
URIs in the HTTP scheme. (See the section 3.2.1:Reference Validation for a further
comment on URI dereferencing.) Applications should be cognizant of the fact
that protocol parameter and state information, (such as a HTTP cookies, HTML
device profiles or content negotiation), may affect the content yielded by
dereferencing a URI.
[URI] permits
identifiers that specify a fragment identifier via a separating number/pound
symbol '#'. (The meaning of the fragment is defined by the resource's MIME
type). XML Signature applications MUST support the XPointer 'bare name' [Xptr] shortcut after '#' so as to identify IDs
within XML documents. The
results are serialized as specified in section 6.6.3:XPath Filtering. For example,
URI="http://example.com/bar.xml"
Identifies the
external XML resource 'http://example.com/bar.xml'.
URI="http://example.com/bar.xml#chapter1"
Identifies the
element with ID attribute value 'chapter1' of the external XML resource
'http://example.com/bar.xml'.
URI=""
Identifies the
XML resource containing the signature..
URI="#chapter1"
Identifies the
element with ID attribute value 'chapter1' of the XML resource containing the
signature.
Otherwise,
support of other fragment/MIME types (e.g., PDF) or XML addressing mechanisms
(e.g., [XPath, Xptr]) is OPTIONAL,
though we RECOMMEND support of
[XPath]. Regardless, such fragment
identification and addressing SHOULD be given under Transforms (not as part of
the URI) so that they can be fully identified and specified. For instance, one
could reference a fragment of a document that is encoded by using the Reference URI to identify the resource, and one Transform to specify
decoding, and a second to specify an XPath selection.
If the URI attribute is omitted altogether,
the receiving application is expected to know the identity of the object. For
example, a lightweight data protocol might omit this attribute given the
identity of the object is part of the application context. This attribute may
be omitted from at most one Reference in
any particular SignedInfo, or Manifest.
The digest
algorithm is applied to the data octets being secured. Typically that is done
by locating (possibly using the URI if
provided) the data and transforming it. If the data is an XML document, the
document is assumed to be unparsed prior to the application of Transforms. If there are no
Transforms, then the data
is passed to the digest algorithm unmodified.
The optional Type
attribute contains information about the type of object being signed. This is
represented as a URI. For example:
Type="http://www.w3.org/2000/07/xmldsig#Object"
Type="http://www.w3.org/2000/07/xmldsig#Manifest"
Type="http://www.w3.org/2000/07/xmldsig#SignatureProperty"
The Type
attribute applies to the item being pointed at, not its contents. For example,
a reference that identifies an Object element containing a SignatureProperties element is still
of type #Object. The type
attribute is advisory. No validation of the type information is required by
this specification.
4.3.3.1 The Transforms Element
The optional Transforms element contains
an ordered list of Transform
elements; these describe how the signer obtained the data object that was
digested. The output of each Transform
(octets) serves as input to the next Transform. The input to the first Transform is the source
data. The output from the last Transform is
the input for the DigestMethod
algorithm. When transforms are applied the signer is not signing the native
(original) document but the resulting (transformed) document, (see section 8.1:
Only What is Signed is Secure).
Each Transform consists of an Algorithm attribute and
content parameters, if any, appropriate for the given algorithm. The Algorithm attribute value
specifies the name of the algorithm to be performed, and the Transform content provides
additional data to govern the algorithm's processing of the input resource,
(see section 6.1: Algorithm
Identifiers and Implementation Requirements).
Some Transform may require
explicit MimeType, Charset (IANA registered character set), or other such
information concerning the data they are receiving from an earlier Transform or the source
data, although no Transform
algorithm specified in this document needs such information. Such data
characteristics are provided as parameters to the Transform algorithm and
should be described in the specification for the algorithm.
Schema Definition: <element name='Transforms' > <complexType content='elementOnly'> <element ref='ds:Transform' minOccurs='1' maxOccurs='unbounded'/> </complexType> </element> <element name='Transform'> <complexType content='mixed'>
<choice minOccurs='1' maxOccurs='unbounded'>
<any namespace='##other' minOccurs='0' maxOccurs='unbounded'/> <element name='Xpath' type='string'/> <element name='XSLT' type='string'/> </choice> <attribute name='Algorithm' type='uriReference' use='required'/> </complexType> </element> DTD: <!ELEMENT Transforms (Transform+)> <!ELEMENT Transform %Transform.ANY; > <!ATTLIST Transform Algorithm CDATA #REQUIRED > <!ELEMENT XPath (#PCDATA) > <!ELEMENT XSLT (#PCDATA) >
Examples of
transforms include but are not limited to base64 decoding [MIME],
canonicalization [XML-C14N],
XPath filtering [XPath], and
XSLT [XSLT]. The
generic definition of the Transform
element also allows application-specific transform algorithms. For example, the
transform could be a decompression routine given by a Java class appearing as a
base64 encoded parameter to a Java Transform algorithm. However, applications should
refrain from using application-specific transforms if they wish their
signatures to be verifiable outside of their application domain. Section 6.6: Transform Algorithms
defines the list of standard transformations.
4.3.3.2 The DigestMethod Element
DigestMethod is a
required element that identifies the digest algorithm to be applied to the
signed object. This element uses the general structure here for algorithms
specified in section 6.1: Algorithm
Identifiers and Implementation Requirements.
Schema Definition:
<element name='DigestMethod'> <complexType content='elementOnly'>
<any minOccurs='0' maxOccurs='unbounded'/> <attribute name='Algorithm' type='uriReference' use='required'/> </complexType> </element> DTD: <!ELEMENT DigestMethod %Method.ANY; > <!ATTLIST DigestMethod Algorithm CDATA #REQUIRED >
4.3.3.3 The DigestValue Element
DigestValue is an
element that contains the encoded value of the digest. The digest is always encoded using base64 [MIME[GK11]].
Schema Definition: <element name='DigestValue' type='ds:CryptoBinary'/> DTD: <!ELEMENT DigestValue (#PCDATA) ><!-- base64 encoded signature value -->
4.4 The KeyInfo Element
KeyInfo may contain keys, names, certificates and
other public key management information, such as in-band key distribution or
key agreement data. This specification defines a few simple types but
applications may place their own key identification and exchange semantics
within this element type through the XML-namespace facility. [XML-ns]
Schema Definition: <element name='KeyInfo'>
<complexType content='elementOnly'> <choice minOccurs='1' maxOccurs='unbounded'><any namespace='##other' minOccurs='1' [GK12]maxOccurs='unbounded'/>
<element name='KeyName' type='string'/> <element ref='ds:KeyValue'/> <element ref='ds:RetrievalMethod'/> <element ref='ds:X509Data'/> <element ref='ds:PGPData'/> <element ref='ds:SPKIData'/> <element name='MgmtData' type='string' /> </choice> <attribute name='Id' type='ID' use='optional'/> </complexType> </element> DTD: <!ELEMENT KeyInfo %Key.ANY; > <!ATTLIST KeyInfo Id ID #IMPLIED >
KeyInfo is an optional element that enables the
recipient(s) to obtain the key(s[GK13])
needed to validate the signature. If omitted, the recipient is expected to be
able to identify the key based on application context information. Multiple
declarations within KeyInfo refer
to the same key. While applications may define and use any mechanism they
choose through inclusion of elements from a different namespace, compliant
versions MUST implement Section 4.4.2: KeyValue and SHOULD implement Section 4.4.3: RetrievalMethod.
4.4.1 The KeyName Element
The KeyName element contains
a string value which may be used by the signer to communicate a key identifier
to the recipient. Typically, KeyName
contains an identifier related to the key pair used to sign the message, but it
may contain other protocol-related information that indirectly identifies a key
pair. (Common uses of KeyName
include simple string names for keys, a key index, a distinguished name (DN),
an email address, etc.)
Schema Definition: <!-- type declared in KeyInfo --> DTD: <!ELEMENT KeyName (#PCDATA) >
4.4.2 The KeyValue Element
The KeyValue element contains
one or more [GK14]public
keys that may be useful in validating the signature. Structured formats for
defining DSA (REQUIRED) and RSA (RECOMMENDED) public keys are defined in
Section 6.4: Signature
Algorithms.
Schema Definition: <element name='KeyValue'> <complexType content='mixed'><choice minOccurs='1' maxOccurs='1[GK15]'>
<any namespace='##other' minOccurs='1' [GK16]maxOccurs='unbounded'/>
<element ref='ds:DSAKeyValue'/> <element ref='ds:RSAKeyValue'/> </choice> </complexType > </element> DTD: <!ELEMENT KeyValue %Key.ANY; >
4.4.3 The RetrievalMethod Element
A RetrievalMethod
element within KeyInfo is
used to convey a pointer to KeyInfo-like
information that is stored at a remote location. For example, an X.509v3
certificate chain may be published somewhere common to a number of documents;
each document can reference this chain using a single RetrievalMethod element instead
of including the entire chain with a sequence of X509Certificate elements.
Each RetrievalMethod element contains
three children elements: Location, Method and Type. Location
contains a URI identifying the actual object. Method describes the process by which the data
retrieved from the Location URI
should be converted into KeyInfo
sub-elements. The Type
sub-element describes the object type and encoding format of the data stored at
the Location URI.
Schema Definition: <element name='RetrievalMethod'> <complexType content='elementOnly'> <sequence minOccurs='1' maxOccurs='1'> <element name='Location' type='uriReference' minOccurs='1' maxOccurs='1'/> <element name='Method' type='string' minOccurs='1' maxOccurs='1'/> <element ref='ds:Type' minOccurs='1' maxOccurs='1'/>
</sequence>
<attribute name='Encoding' type='uriReference' use='optional'/> </complexType> </element> <element name='Type'> <complexType content='mixed'> <any namespace='##other' minOccurs='1' maxOccurs='unbounded'/> <attribute name='Encoding' type='uriReference' use='optional'/> </complexType> </element> DTD: <!ELEMENT RetrievalMethod (Location, Method, Type) > <!ELEMENT Location %Key.ANY; > <!ELEMENT Method %Key.ANY; > <!ELEMENT Type %Key.ANY; > <!ATTLIST Type Encoding CDATA #IMPLIED>
4.4.4 The X509Data Element
An X509Data element within KeyInfo contains one or
more identifiers of keys/X509 certificates that may be useful for validation.
Five types of X509Data
pointers are defined:
1. The X509IssuerSerial element, which
contains an X.509 issuer distinguished name/serial number pair,
2. The X509SubjectName element, which
contains an X.509 subject distinguished name,
3. The X509SKI element, which
contains an X.509 subject key identifier value.
4. The X509Certificate element, which
contains a Base64-encoded X.509v3 certificate, and
5. The X509CRL element, which
contains a Base64-encoded X.509v2 certificate revocation list (CRL).
Multiple
declarations about a single certificate (e.g., a X509SubjectName and X509IssuerSerial element) MUST be grouped inside a single X509Data element;
multiple declarations about the same key but different certificates (related to
that single key) MUST be grouped within a single KeyInfo element but multiple X509Data elements. For
example, the following block contains two pointers to certificate-A
(issuer/serial number & SKI) and a single reference to certificate-B
(Subject Name):
<X509Data> <X509IssuerSerial> <X509IssuerName>My CA for Certificate [GK17]A</X509IssuerName>
<X509SerialNumber>12345678</X509SerialNumber> </X509IssuerSerial><X509SKI>31d97bd7</X509SKI>
</X509Data> <X509Data><X509SubjectName>Subject of Certificate [GK18]B</X509SubjectName>
</X509Data>
Schema Definition: <element name='X509Data'> <complexType content='elementOnly'> <choice minOccurs='1' maxOccurs='1'>
<sequence minOccurs='1' maxOccurs='unbounded'>
<choice minOccurs='1' maxOccurs='1'> <element ref='ds:X509IssuerSerial'/>
<element name='X509SKI' type='ds:CryptoBinary'/><element name='X509SubjectName' type='string'[GK19]/>
</choice> </sequence><element name='X509Certificate' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1[GK20]'/>
<element name='X509CRL' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/>
</choice> </complexType> </element> <element name='X509IssuerSerial'> <complexType content='elementOnly'>
<sequence minOccurs='1' maxOccurs='1'> <element name='X509IssuerName' type='string' [GK21]minOccurs='1' maxOccurs='1'/>
<element name='X509SerialNumber' type='integer' minOccurs='1' maxOccurs='1'/>
</sequence> </complexType> </element> DTD: <!ELEMENT X509Data ((X509IssuerSerial | X509SKI | X509SubjectName), X509Certificate*, X509CRL*)>[GK22]
<!ELEMENT X509IssuerSerial (X509IssuerName, X509SerialNumber) > <!ELEMENT X509IssuerName (#PCDATA) > <!ELEMENT X509SubjectName (#PCDATA) > <!ELEMENT X509SerialNumber (#PCDATA) > <!ELEMENT X509SKI (#PCDATA) ><!ELEMENT X509Certificate (#PCDATA) >
<!ELEMENT X509CRL (#PCDATA) >
4.4.5 The PGPData element
The PGPData element within KeyInfo is used to
convey information related to PGP public key pairs and signatures on such keys.
The PGPKeyID's value is a
string containing a standard PGP public key identifier as defined in Section
11.2 of [PGP]. The
PGPKeyPacket
contains a base64-encoded Key Material Packet as defined in Section 5.5 of [PGP]. Other sub-types
of the PGPData element may be
defined by the OpenPGP working group.
Schema Definition:
<element name='PGPData'> <complexType content='elementOnly'>
<sequence minOccurs='1' maxOccurs='1'><any namespace='##other' minOccurs='1[GK23]' maxOccurs='unbounded'/>
<element name='PGPKeyID' type='string' minOccurs='1' maxOccurs='1'/> <element name='PGPKeyPacket' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> </sequence> </complexType> </element> DTD: <!ELEMENT PGPData (PGPKeyID, PGPKeyPacket[GK24]?) >
<!ELEMENT PGPKeyPacket (#PCDATA) > <!ELEMENT PGPKeyID (#PCDATA) >
4.4.6 The SPKIData element
The SPKIData element within KeyInfo is used to
convey information related to SPKI public key pairs, certificates and other
SPKI data. The content of this element type is open and can be defined
elsewhere.
Schema Definition: <element name='SPKIData'> <complexType content='elementOnly'> <any namespace='##other' minOccurs='1' maxOccurs='unbounded'/> </complexType> </element> DTD: <!ELEMENT SPKIData (#PCDATA) >
4.4.6 The MgmtData element
The MgmtData element within KeyInfo is a string
value used to convey in-band key distribution or agreement data. For example,
DH key exchange, RSA key encryption, etc.
Schema Definition: <!-- type declared in KeyInfo --> DTD: <!ELEMENT MgmtData (#PCDATA)>
4.5 The Object Element
Identifier
Type="http://www.w3.org/2000/07/xmldsig#Object"
(this can be used within a Reference element to identify the referent's type)
Object is an optional element that may occur one
or more times. When present, this element may contain any data. The Object element may include optional MIME
type, ID, and encoding attributes.
The MimeType attribute is an
optional attribute which describes the data within the Object. This is a string with values
defined by [MIME]. For
example, if the Object
contains XML, the MimeType could
be text/xml. This attribute is purely advisory; no validation of the MimeType information is
required by this specification.
The Object's Id is commonly referenced from a Reference in SignedInfo, or Manifest. This element is
typically used for enveloping signatures where
the object being signed is to be included in the signature element. The digest
is calculated over the entire Object
element including start and end tags.
Note, if the
application wishes to exclude the <Object> tags from the digest calculation the Reference must identify
the actual data object (easy for XML documents) or a transform must be used to
remove the Object tags
(likely where the data object is non-XML). Exclusion of the object tags may be
desired for cases where one wants the signature to remain valid if the data
object is moved from inside a signature to outside the signature (or
vice-versa), or where the content of the Object is an encoding of an original binary document
and it is desired to extract and decode so as to sign the original bitwise
representation.
Schema Definition: <element name='Object' > <complexType content='mixed'> <element ref='ds:Manifest' minOccurs='1' maxOccurs='unbounded'/><any namespace='##any' minOccurs='1'[GK25] maxOccurs='unbounded'/>
<attribute name='Id' type='ID' use='optional'/> <attribute name='MimeType' type='string' use='optional'/> <!-- add a grep facet --> <attribute name='Encoding' type='uriReference' use='optional'/> </complexType> </element> DTD: <!ELEMENT Object %Object.ANY; > <!ATTLIST Object Id ID #IMPLIED MimeType CDATA #IMPLIED Encoding CDATA #IMPLIED >
5.0 Additional Signature
Syntax
This section
describes the optional to implement Manifest and SignatureProperties elements and describes the handling
of XML processing instructions and comments. With respect to the elements Manifest and SignatureProperties this section specifies
syntax and little behavior -- it is left to the application. These elements can
appear anywhere the parent's content model permits; the Signature content model
only permits them within Object.
5.1 The Manifest Element
Identifier
Type="http://www.w3.org/2000/07/xmldsig#Manifest"
(this can be used within a Reference element to identify the referent's type)
The Manifest element provides
a list of References. The
difference from the list in SignedInfo is
that it is application defined which, if any, of the digests are actually
checked against the objects referenced and what to do if the object is
inaccessible or the digest compare fails. If a Manifest is pointed to from SignedInfo, the digest over
the Manifest itself will be
checked by the core signature validation behavior. The digests within such a Manifest are checked at
application discretion. If a Manifest is
referenced from another Manifest, even
the overall digest of this two level deep Manifest might not be checked.
Schema Definition:
<element name='Manifest'> <complexType content='elementOnly'>
<sequence minOccurs='1' maxOccurs='1'><element ref='ds:Reference' minOccurs='1' maxOccurs='unbounded'/>
</sequence> <attribute name='Id' type='ID' use='optional'/> </complexType>
</element> DTD: <!ELEMENT Manifest (Reference+) > <!ATTLIST Manifest Id ID #IMPLIED >
5.2 The SignatureProperties [GK26]Element
Identifier
Type="http://www.w3.org/2000/07/xmldsig#SignatureProperty"
(this can be used within a Reference element to identify the referent's type)
Additional
information items concerning the generation of the signature(s) can be placed
in a SignatureProperty
element (i.e., date/time stamp or the serial number of cryptographic hardware
used in signature generation).
Schema Definition: <element name='SignatureProperties'> <complexType content='elementOnly'> <element ref='ds:SignatureProperty' minOccurs='1' maxOccurs='unbounded'/> <attribute name='Id' type='ID' use='optional'/> </complexType> </element> <element name='SignatureProperty'> <complexType content='mixed'> <any namespace='##other' minOccurs='1' maxOccurs='unbounded'/> <attribute name='Target' type='uriReference' use='required'/> <attribute name='Id' type='ID' use='optional'/> </complexType> </element> DTD: <!ELEMENT SignatureProperties (SignatureProperty+) > <!ATTLIST SignatureProperties Id ID #IMPLIED > <!ELEMENT SignatureProperty %Object.ANY; > <!ATTLIST SignatureProperty Target CDATA #REQUIRED Id ID #IMPLIED >
5.3 Processing Instructions
in Signature Elements
No XML processing
instructions (PIs) are used by this specification.
Note that PIs
placed inside SignedInfo by an
application will be signed unless the CanonicalizationMethod algorithm discards them. (This is
true for any signed XML content.) All of the CanonicalizationMethods specified within this
specification retain PIs. When a PI is part of content that is signed (e.g.,
within SignedInfo or referenced
XML documents) any change to the PI will obviously result in a signature
failure.
5.4 Comments in
Signature Elements
XML comments are
not used by this specification.
Note that unless CanonicalizationMethod removes comments
within SignedInfo or any other
referenced XML, they will be signed. Consequently, a change to the comment will
cause a signature failure. Similarly, the XML signature over any XML data will
be sensitive to comment changes unless a comment-ignoring
canonicalization/transform method, such as the Canonical XML [XML-C14N], is specified.
6.0 Algorithms
This section
identifies algorithms used with the XML digital signature standard. Entries
contain the identifier to be used in Signature elements, a reference to the formal
specification, and definitions, where applicable, for the representation of
keys and the results of cryptographic operations.
6.1 Algorithm Identifiers
and Implementation Requirements
Algorithms are
identified by URIs that appear as an attribute to the element that identifies
the algorithms' role (DigestMethod, Transform, SignatureMethod, or CanonicalizationMethod). All algorithms
used herein take parameters but in many cases the parameters are implicit. For
example, a SignatureMethod is
implicitly given two parameters: the keying info and the output of CanonicalizationMethod. Explicit
additional parameters to an algorithm appear as content elements within the
algorithm role element. Such parameter elements have a descriptive element
name, which is frequently algorithm specific, and MUST be in the XML Signature
namespace or an algorithm specific namespace.
This
specification defines a set of algorithms, their URIs, and requirements for
implementation. Requirements are specified over implementation, not over
requirements for signature use. Furthermore, the mechanism is extensible,
alternative algorithms may be used by signature applications.
(Note that the
normative identifier is the complete URI in the table though they are
frequently abbreviated in XML syntax (e.g., "&dsig;base64").)
|
Algorithm
Type |
Algorithm |
Requirements |
Algorithm URI |
|
Digest |
|
|
|
|
|
SHA1 |
REQUIRED |
|
|
Encoding |
|
|
|
|
|
Base64 |
REQUIRED |
|
|
MAC |
|
|
|
|
|
HMAC-SHA1 |
REQUIRED |
|
|
Signature |
|
|
|
|
|
DSAwithSHA1 |
REQUIRED |
|
|
|
RSAwithSHA1 |
RECOMMENDED |
|
|
Canonicalization |
|
|
|
|
|
minimal |
RECOMMENDED |
|
|
|
Canonical XML |
REQUIRED |
|
|
Transform |
|
|
|
|
|
XSLT |
OPTIONAL |
|
|
|
XPath |
RECOMMENDED |
|
|
|
Enveloped
Signature* |
REQUIRED |
* The Enveloped
Signature transform removes the Signature element
from the calculation of the signature when the signature is within the document [GK27]that it is being signed. This MAY be
implemented via the RECOMMENDED XPath specification specified in 6.6.4: Enveloped Signature Transform; it
MUST have the same effect as that specified by the XPath specification.
6.2 Message Digests
Only one digest
algorithm is defined herein. However, it is expected that one or more
additional strong digest algorithms will be developed in connection with the US
Advanced Encryption Standard effort. Use of MD5 [MD5] is NOT
RECOMMENDED because recent advances in cryptography have cast doubt on its
strength.
6.2.1 SHA-1
Identifier:
http://www.w3.org/2000/07/xmldsig#sha1
The SHA-1 algorithm [SHA-1] takes no
explicit parameters. An example of an SHA-1 DigestAlg element is:
<DigestMethod Algorithm="&dsig;sha1"/>
A SHA-1 digest is
a 160-bit string. The content of the DigestValue element shall be the base64
encoding of this bit string viewed as a 20-octet octet stream. For example, the
DigestValue element for the message digest:
A9993E36 4706816A BA3E2571 7850C26C 9CD0D89D
from Appendix A
of the SHA-1 standard would be:
<DigestValue>qZk+NkcGgWq6PiVxeFDCbJzQ2J0=</DigestValue>
6.3 Message Authentication
Codes
MAC algorithms
take two implicit parameters, their keying material determined from KeyInfo and the octet
stream output by CanonicalizationMethod. MACs
and signature algorithms are syntactically identical but a MAC implies a shared
secret key.
6.3.1 HMAC
Identifier:
http://www.w3.org/2000/07/xmldsig#hmac-sha1
The HMAC algorithm
(RFC2104 [HMAC])
takes the truncation length in bits as a parameter; if the parameter is not
specified then all the bits of the hash are output. An example of an HMAC SignatureMethod element:
<SignatureMethod Algorithm="&dsig;hmac-sha1"> <HMACOutputLength>128</HMACOutputLength> </SignatureMethod>
The output of the
HMAC algorithm is ultimately the output (possibly truncated) of the chosen
digest algorithm. This value shall be base64 encoded in the same
straightforward fashion as the output of the digest algorithms. Example: the
SignatureValue element for the HMAC-SHA1 digest
9294727A 3638BB1C 13F48EF8 158BFC9D
from the test
vectors in [HMAC]
would be
<SignatureValue>kpRyejY4uxwT9I74FYv8nQ==</SignatureValue> Schema Definition: <element name='HMACOutputLength' type='integer' minOccurs='0' maxOccurs='1'/> DTD: <!ELEMENT HMACOutputLength (#PCDATA)>
6.4 Signature
Algorithms
Signature
algorithms take two implicit parameters, their keying material determined from KeyInfo and the octet
stream output by CanonicalizationMethod.
Signature and MAC algorithms are syntactically identical but a signature
implies public key cryptography.
6.4.1 DSA
Identifier:
http://www.w3.org/2000/07/xmldsig#dsa-sha1
The DSA algorithm
[DSS]
takes no explicit parameters. An example of a DSA SignatureMethod element is:
<SignatureMethod Algorithm="&dsig;dsa"/>
The output of the
DSA algorithm consists of a pair of integers usually referred by the pair (r,
s). The signature value consists of the base64 encoding of the concatenation of
two octet-streams that respectively result from the octet-encoding of the
values r and s. Integer to
octet-stream conversion must be done according to the I2OSP operation defined
in the RFC 2437 [PKCS1]
specification with a k parameter equal to 20. For example, the
SignatureValue element for a DSA signature (r, s) with values specified in
hexadecimal:
r = 8BAC1AB6 6410435C B7181F95 B16AB97C 92B341C0
s = 41E2345F 1F56DF24 58F426D1 55B4BA2D B6DCD8C8
from the example
in Appendix 5 of the DSS standard would be
<SignatureValue>
i6watmQQQ1y3GB+VsWq5fJKzQcBB4jRfH1bfJFj0JtFVtLotttzYyA==</SignatureValue>
DSA key values
have the following set of fields: P, Q, G and Y are mandatory when appearing as
a key value, J, seed and
pgenCounter are optional but SHOULD be present. (The seed and
pgenCounter fields MUST appear together or be absent). All parameters are
encoded as base64 values.
Schema:
<element name='DSAKeyValue'> <complexType content='elementOnly'> <sequence minOccurs='1' maxOccurs='1'>
<element name='P' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> <element name='Q' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> <element name='G' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> <element name='Y' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> <element name='J' type='ds:CryptoBinary' minOccurs='0' maxOccurs='1'/> </sequence> <sequence minOccurs='0' maxOccurs='1'> <element name='Seed' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> <element name='PgenCounterQ' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> </sequence> </complexType> </element> DTD:
<!ELEMENT DSAKeyValue (P, Q, G, Y, J?, (Seed, PgenCounter)?) >
<!ELEMENT P (#PCDATA) ><!ELEMENT Q (#PCDATA) >
<!ELEMENT G (#PCDATA) >
<!ELEMENT Y (#PCDATA) >
<!ELEMENT J (#PCDATA) >
<!ELEMENT Seed (#PCDATA) > <!ELEMENT PgenCounter (#PCDATA) >
6.4.2 PKCS1
Identifier:
http://www.w3.org/2000/07/xmldsig#rsa-sha1
Arbitrary-length
integers (e.g. "bignums" such as RSA modulii) are represented in XML
as octet strings. The integer value is first converted to a "big
endian" bitstring. The bitstring is then padded with leading zero bits so
that the total number of bits == 0 mod 8 (so that there are an even number of
bytes). If the bitstring contains entire leading bytes that are zero, these are
removed (so the high-order byte is always non-zero). This octet string is then
Base64 encoded. (The
conversion from integer to octet string is equivalent to IEEE P1363's I2OSP [P1363] with minimal length).
The expression
"RSA algorithm" as used in this draft refers to the RSASSA-PKCS1-v1_5
algorithm described in RFC 2437 [PKCS1]. (Note that
support for PKCS1 Version 2 is planned as soon as that standard is finalized).
The RSA algorithm takes no explicit parameters. An example of an RSA
SignatureMethod element is:
<SignatureMethod Algorithm="&dsig;rsa-sha1"/>
The SignatureValue content for an
RSA signature shall be the base64 encoding of the octet string. Signatures are
interpreted as unsigned integers. A signature MAY contain a pre-pended algorithm object identifier, but the
availability of an ASN.1 parser and recognition of OIDs is not required of a
signature verifier.
<SignatureValue>F8aupsHjmbIApjAH4AVYjcsmQkXChyjGYleVJe1KLAmmXWww 3PqkDPUMojithbwbVWVJJ0UhdT407nl0fBrohvkunDq8gzfGkjvO+zDJws1HkRtZ vl1IIBLVWf/qgcLJOgid/2A66niC20GwKcJgIp3o1L+6l7LlSKiZ/CkgDO4= </SignatureValue>
RSA key values
have two fields: Modulus and Exponent
<RSAKeyValue> <Exponent>AQAB</Exponent> <Modulus>xA7SEU+e0yQH5rm9kbCDN9o3aPIo7HbP7tX6WOocLZAtNfyxSZDU16ksL6W jubafOqNEpcwR3RdFsT7bCqnXPBe5ELh5u4VEy19MzxkXRgrMvavzyBpVRgBUwUlV 5foK5hhmbktQhyNdy/6LpQRhDUDsTvK+g9Ucj47es9AQJ3U= </Modulus> </RSAKeyValue> Schema:
<element name='RSAKeyValue'> <complexType content='elementOnly'> <element name='Modulus' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> <element name='Exponent' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/> </complexType>
</element> DTD:
<!ELEMENT RSAKeyValue (Modulus, Exponent) > <!ELEMENT Modulus (#PCDATA) ><!ELEMENT Exponent (#PCDATA) >
6.5 Canonicalization
Algorithms
Canonicalization
algorithms take one implicit parameter when they appear as a CanonicalizationMethod within the SignedInfo element.
6.5.1 Minimal
Canonicalization
Identifier:
http://www.w3.org/2000/07/xmldsig#minimal
An example of a
minimal canonicalization element is:
<CanonicalizationMethod Algorithm="&dsig;minimal"/>
The minimal
canonicalization algorithm:
·
converts the character encoding to UTF-8 (without any
byte order mark (BOM)).
·
normalizes line endings as provided by [XML]. (See section
7: XML and Canonicalization and Syntactical Considerations.)
6.5.2 Canonical
XML
Identifier:
http://www.w3.org/TR/2000/WD-xml-c14n-20000710
An example of an
XML canonicalization element is:
<CanonicalizationMethod Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000710"/>
The normative
specification of Canonical XML is [XML-C14N].
6.6 Transform Algorithms
A Transform algorithm has a
single implicit parameters: an octet stream from the Reference or the output of
an earlier Transform.
Application
developers are strongly encouraged to support all transforms listed in this
section as RECOMMENDED unless the application environment has resource constraints
that would make such support impractical. Compliance with this recommendation
will maximize application interoperability and libraries should be available to
enable support of these transforms in applications without extensive
development.
6.6.1 Canonicalization
Any
canonicalization algorithm that can be used for CanonicalizationMethod can be used as a Transform.
6.6.2 Base64
Identifiers:
http://www.w3.org/2000/07/xmldsig#base64
The normative
specification for base 64 decoding transforms is [MIME]. The base64 Transform element has no
content. The input is decoded by the algorithms. This transform is useful if an
application needs to sign the raw data associated with the encoded content of
an element.
6.6.3 XPath
Filtering
Identifier:
http://www.w3.org/TR/1999/REC-xpath-19991116
The XPath transform output
is the result of applying the XML canonicalization algorithm [XML-C14N], parameterized
by a given XPath expression, to
the XML document received as the transform input. The XPath expression
appears as the character content of a transform parameter subelement named XPath.
The primary
purpose of this transform is to ensure that only specifically defined changes
to the input XML document are permitted after the signature is affixed. The
XPath expression can be created such that it includes all elements except those
meeting specific criteria. It is the responsibility of the XPath expression
author to ensure that all necessary information has been included in the output
such that modification of the excluded information does not affect the
interpretation of the transform output in the application context.
The XPath
transform establishes the following evaluation context for the XML
canonicalization algorithm:
·
A context node, initialized to the input XML
document's root node.
·
A context position, initialized to 1.
·
A context size, initialized to 1.
·
A library of functions equal to the function
set defined in XPath plus a function named here.
·
A set of variable bindings. No means for initializing
these is defined. Thus, the set of variable bindings used when evaluating the
XPath expression is empty, and use of a variable reference in the XPath
expression results in an error.
·
The set of
namespace declarations in scope for the XPath expression.
·
The XPath expression appearing as the character
content of the XPath
parameter element.
The function
definition for here() is
consistent with its definition in XPointer. It is defined as follows:
Function: node-set here()
The here
function returns a node-set containing the single node that directly bears the
XPath expression. The node could be of any type capable of directly bearing
text, especially text and attribute. This expression results in an error if the
containing XPath expression does not appear in an XML document.
As an example,
consider creating an enveloped signature (a Signature element that is a descendant of an
element being signed). Although the signed content should not be changed after
signing, the elements within the Signature element are changing (e.g. the digest
value must be put inside the DigestValue and
the SignatureValue must
be subsequently calculated). One way to prevent these changes from invalidating
the digest value in DigestValue is to
add an XPath Transform that
omits all Signature
elements and their descendants. For example,
<Document>
...
<Signature xmlns="&dsig;">
<SignedInfo>
...
<Reference URI="">
<Transforms>
<Transform
Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
<XPath
xmlns:dsig="&dsig;">
(//. | //@* | //namespace::*)[not(ancestor-or-self::dsig:Signature)]
</XPath>
</Transform>
</Transforms>
<DigestMethod
Algorithm="http://www.w3.org/2000/07/xmldsig#sha1"/>
<DigestValue></DigestValue>
</Reference>
</SignedInfo>
<SignatureValue></SignatureValue>
</Signature>
...
</Document>
The subexpression
(//. | //@* | //namespace::*) means
that all nodes in the entire parse tree starting at the root node are
candidates for the result node-set. For each node candidate, the node is
included in the resultant node-set if and only if the node test (the boolean
expression in the square brackets) evaluates to "true" for that node.
The node test returns true for all nodes except nodes that either have or have
an ancestor with a tag of Signature.
A more elegant
solution uses the here
function to omit only the Signature
containing the XPath Transform, thus allowing enveloped signatures to sign
other signatures. In the example above, use the XPath element:
<XPath xmlns:dsig="&dsig;">(//. | //@* | //namespace::*)
[count(ancestor-or-self::dsig:Signature |
here()/ancestor::dsig:Signature[1]) >
count(ancestor-or-self::dsig:Signature)]</XPath>
Since the XPath
equality operator converts node sets to string values before comparison, we must
instead use the XPath union operator (|). For each node of the document, the
predicate expression is true if and only if the node-set containing the node
and its Signature element
ancestors does not include the enveloped Signature element containing the XPath expression
(the union does not produce a larger set if the enveloped Signature element is in
the node-set given by ancestor-or-self::Signature).
It is RECOMMENDED that the XPath be constructed such
that the result of this operation is a well-formed XML document. This
should be the case if the root element of the input resource is included by the
XPath (even if a number of its descendant nodes are omitted by the XPath
expression). It is also RECOMMENDED that nodes should not be omitted from the
input if they affect the interpretation of the output nodes in the application
context. The XPath expression author is responsible for this since the XPath
expression author knows the application context.
6.6.4 Enveloped Signature Transform
Identifier:
http://www.w3.org/2000/07/xmldsig#enveloped-signature
An enveloped
signature transform T removes the whole Signature element
containing T from the digest calculation of the Reference element
containing T. The entire string of characters used by an XML
processor to match the Signature with
the XML production element is
removed. The output of the transform is equivalent to the output that would
result from replacing T with an XPath transform containing the
following XPath
parameter element:
<XPath xmlns:dsig="&dsig;">(//. | //@* | //namespace::*)
[count(ancestor-or-self::dsig:Signature |
here()/ancestor::dsig:Signature[1]) >
count(ancestor-or-self::dsig:Signature)]</XPath>
Note that it is
not necessary to use an XPath expression evaluator to create this transform.
However, this transform MUST produce output in exactly the same manner as the
XPath transform parameterized by the XPath expression above.
6.6.5 XSLT
Transform
Identifier:
http://www.w3.org/TR/1999/REC-xslt-19991116
The Transform element contains
a single parameter child element called XSLT, whose content MUST conform to the XSL Transforms [XSLT] language
syntax. The processing rules for the XSLT transform are stated in the XSLT
specification [XSLT].
7.0 XML
Canonicalization and Syntax Constraint Considerations
Digital
signatures only work if the verification calculations are performed on exactly
the same bits as the signing calculations. If the surface representation of the
signed data can change between signing and verification, then some way to
standardize the changeable aspect must be used before signing and verification.
For example, even for simple ASCII text there are at least three widely used
line ending sequences. If it is possible for signed text to be modified from
one line ending convention to another between the time of signing and signature
verification, then the line endings need to be canonicalized to a standard form
before signing and verification or the signatures will break.
XML is subject to
surface representation changes and to processing which discards some surface
information. For this reason, XML digital signatures have a provision for
indicating canonicalization methods in the signature so that a verifier can use
the same canonicalization as the signer.
Throughout this specification
we distinguish between the canonicalization of a Signature data object and other signed XML data
objects. It is possible for an isolated XML document to be treated as if it
were binary data so that no changes can occur. In that case, the digest of the
document will not change and it need not be canonicalized if it is signed and
verified as such. However, XML that is read and processed using standard XML
parsing and processing techniques is frequently changed such that some of its
surface representation information is lost or modified. In particular, this
will occur in many cases for the Signature and enclosed SignedInfo elements since
they, and possibly an encompassing XML document, will be processed as XML.
Similarly, these
considerations apply to Manifest, Object, and SignatureProperties elements if those elements have
been digested, their DigestValue is to
be checked, and they are being processed as XML.
The kinds of
changes in XML that may need to be canonicalized can be divided into three categories.
There are those related to the basic [XML], as
described in 7.1 below. There are those related to [DOM], [SAX], or similar
processing as described in 7.2 below. And, third, there is the possibility of
coded character set conversion, such as between UTF-8 and UTF-16, both of which
all [XML]
compliant processors are required to support.
Any
canonicalization algorithm should yield output in a specific fixed coded
character set. For both the minimal canonicalization defined in this
specification and the W3C Canonical XML [XML-C14N] that coded character set is UTF-8
(without a byte order mark (BOM)). Additinally, none of these algorithms
provide data type normalization. Applications that normalize data types in
varying formats (e.g., (true, false) or (1,0)) may not be able to validate each
other's signatures. Neither the minimal canonicalization nor the Canonical XML
[XML-C14N]
algorithms provide character normalization. We RECOMMEND that signature applications produce XML
content in Normalized Form C [NFC] and
check that any XML being consumed is in that form as well (if not, signatures
may consequently fail to validate).
7.1 XML 1.0, Syntax
Constraints, and Canonicalization
XML 1.0 [XML] defines an
interface where a conformant application reading XML is given certain
information from that XML and not other information. In particular,
1. line
endings are normalized to the single character #xA by dropping #xD characters
if they are immediately followed by a #xA and replacing them with #xA in all
other cases,
2. missing
attributes declared to have default values are provided to the application as
if present with the default value,
3. character
references are replaced with the corresponding character,
4. entity
references are replaced with the corresponding declared entity,
5. attribute
values are normalized by
A. replacing
character and entity references as above,
B. replacing
occurrences of #x9, #xA, and #xD with #x20 (space) except that the sequence
#xD#xA is replaced by a single space, and
C. if the
attribute is not declared to be CDATA, stripping all leading and trailing
spaces and replacing all interior runs of spaces with a single space.
Note that items
(2), (4), and (5C) depend on specific schema, DTD, or similar declarations. In
the general case, such declarations will not be available to or used by the
signature verifier. Thus, to interoperate between different XML
implementations, the following syntax contraints MUST be observed when
generating any signed material to be processed as XML, including the SignedInfo element:
1. attributes
having default values be explicitly present,
2. all
entity references (except "amp", "lt", "gt",
"apos", "quot", and other character entities not
representable in the encoding chosen) be expanded,
3. attribute
value white space be normalized
7.2 DOM/SAX Processing
and Canonicalization
In addition to
the canonicalization and syntax constraints discussed above, many XML
applications use the Document Object Model [DOM] or The Simple API for XML [SAX]. DOM maps XML
into a tree structure of nodes and typically assumes it will be used on an
entire document with subsequent processing being done on this tree. SAX
converts XML into a series of events such as a start tag, content, etc. In
either case, many surface characteristics such as the ordering of attributes
and insignificant white space within start/end tags is lost. In addition,
namespace declarations are mapped over the nodes to which they apply, losing
the namespace prefixes in the source text and, in most cases, losing where
namespace declarations appeared in the original instance.
If an XML
Signature is to be produced or verified on a system using the DOM or SAX
processing, a canonical method is needed to serialize the relevant part of a
DOM tree or sequence of SAX events. XML canonicalization specifications, such
as [XML-C14N], are
based only on information which is preserved by DOM and SAX. For an XML
Signature to be verifiable by an implementation using DOM or SAX, not only must
the syntax constraints given in section 7.1 be followed but an appropriate XML
canonicalization MUST be specified so that the verifier can re-serialize
DOM/SAX mediated input into the same octect stream that was signed.
8.0 Security
Considerations
The XML Signature
specification provides a very flexible digital signature mechanism.
Implementors must give consideration to their application threat models and to
the following factors.
8.1 Transforms
A requirement of
this specification is to permit signatures to "apply to a part or
totality of a XML document." (See section 3.1.3 of [XML-Signature-RD].)
The Transforms mechanism meets
this requirement by permitting one to sign data derived from processing the
content of the identified resource. For instance, applications that wish to
sign a form, but permit users to enter limited field data without invalidating
a previous signature on the form might use XPath [XPath] to exclude
those portions the user needs to change. Transforms may be arbitrarily specified and may
include encoding tranforms, canonicalization instructions or even XSLT
transformations. Three cautions are raised with respect to this feature in the
following sections.
Note, core validation
behavior does not confirm that the signed data was obtained by applying each
step of the indicated transforms. (Though it does check that the digest of the
resulting content matches that specified in the signature.) For example,
some application may be satisfied with verifying an XML signature over a cached
copy of already transformed data. Other applications might require that content
be freshly dereferenced and transformed.
8.1.1 Only
What is Signed is Secure
First, obviously,
signatures over a transformed document do not secure any information discarded
by transforms: only what is signed is secure.
Note that the use
of Canonical XML [XML-C14N]
ensures that all internal entities and XML namespaces are expanded within the
content being signed. All entities are replaced with their definitions and the
canonical form explicitly represents the namespace that an element would
otherwise inherit. Applications that do not canonicalize XML content
(especially the SignedInfo
element) SHOULD NOT use internal entities and SHOULD represent the namespace
explicitly within the content being signed since they can not rely upon
canonicalization to do this for them.
8.1.2 Only What is
"Seen" Should be Signed
Additionally, the
signature secures any information introduced by the transform: only what is
"seen" should be signed. If signing is intended to convey the
judgment or consent of an automated mechanism or person, then it is normally
necessary to secure as exactly as practical the information that was presented
to that mechanism or person. Note that this can be accomplished by literally
signing what was presented, such as the screen images shown a user. However,
this may result in data which is difficult for subsequent software to
manipulate. Instead, one can sign the data along with whatever filters, style
sheets, client profile or other information that affects its presentation.
8.1.3 "See"
What is Signed
Note:
This new recommendation is actually a combination/inverse of the earlier
recommendations and is still under discussion.
Just as a person
or automatable mechanism should only sign what it "sees," persons and
automated mechanisms that trust the validity of a transformed document on the
basis of a valid signature SHOULD operate over the data that was transformed
(including canonicalization) and signed, not the original pre-transformed data.
Some applications might operate over the original or intermediary data but
SHOULD be extremely careful about potential weaknesses introduced between the
original and transformed data. This is a trust decision about the character and
meaning of the transforms that an application needs to make with caution.
Consider a canonicalization algorithm that normalizes character case (lower to
upper) or character composition ('e and accent' to 'accented-e'). An adversary
could introduce changes that are normalized and consequently inconsequential to
signature validity but material to a DOM processor. For instance, by changing
the case of a character one might influence the result of an XPath selection. A
serious risk is introduced if that change is normalized for signature
validation but the processor operates over the original data and returns a
different result than intended.
Consequently,
while we RECOMMEND all documents operated upon and generated by signature
applications be in [NFC]
(otherwise intermediate processors might unintentionally break the signature)
encoding normalizations SHOULD NOT be done as part of a signature transform, or
(to state it another way) if normalization does occur, the application SHOULD
always "see" (operate over) the normalized form.
8.2 Check the Security Model
This standard
specifies public key signatures and keyed hash authentication codes. These have
substantially different security models. Furthermore, it permits user specified
algorithms which may have other models.
With public key
signatures, any number of parties can hold the public key and verify signatures
while only the parties with the private key can create signatures. The number
of holders of the private key should be minimized and preferably be one.
Confidence by verifiers in the public key they are using and its binding to the
entity or capabilities represented by the corresponding private key is an
important issue, usually addressed by certificate or online authority systems.
Keyed hash
authentication codes, based on secret keys, are typically much more efficient
in terms of the computational effort required but have the characteristic that
all verifiers need to have possession of the same key as the signer. Thus any
verifier can forge signatures.
This standard
permits user provided signature algorithms and keying information designators.
Such user provided algorithms may have different security models. For example,
methods involving biometrics usually depend on a physical characteristic of the
authorized user that can not be changed the way public or secret keys can be
and may have other security model differences.
8.3 Algorithms, Key
Lengths, Certificates, Etc.
The strength of a
particular signature depends on all links in the security chain. This includes
the signature and digest algorithms used, the strength of the key generation [RANDOM] and the size of
the key, the security of key and certificate authentication and distribution
mechanisms, certificate chain validation policy, protection of cryptographic
processing from hostile observation and tampering, etc.
Care must be
exercised by validaters in executing the various algorithms that may be
specified in an XML signature and in the processing of any "executable
content" that might be provided to such algorithms as parameters, such as
XSLT transforms. The algorithms specified in this document will usually be
implemented via a trusted library but even there perverse parameters might
cause unacceptable processing or memory demand. Even more care may be warranted
with application defined algorithms.
The security of
an overall system will also depend on the security and integrity of its
operating procedures, its personnel, and on the administrative enforcement of
those procedures. All the factors listed in this section are important to the
overall security of a system; however, most are beyond the scope of this
specification.
9.0 Schema, DTD, Data
Model, and Valid Examples
XML Signature
Schema Instance
Valid XML schema
instance based on the Last Call 20000407 Schema/DTD [XML-Schema].
XML Signature DTD
RDF Data Model
xmldsig-datamodel-20000112.gif
XML Signature
Object Example
A cryptographical
invalid XML example that includes foreign content and validates under the
schema. (It validates under the DTD when the foreign content is removed or the
DTD is modified accordingly).
XML RSA Signature
Valid Example
An XML Signature
example by Kent Tamura with generated cryptographic values, uses WD-xml-c14n-20000613, that
has been confirmed by Petteri Stenius.
XML DSA Signature
Valid Example
Similar to above
but uses DSA.
10.0 Definitions
A value generated
from the application of a shared key to a message via a cryptographic algorithm
such that it has the properties of message authentication (integrity) but
not signer authentication
"A signature
should identify what is signed, making it impracticable to falsify or alter
either the signed matter or the signature without detection." [Digital Signature Guidelines, ABA]
"A signature
should indicate who signed a document, message or record, and should be
difficult for another person to produce without authorization." [Digital Signature Guidelines, ABA]
The syntax and
processing defined by this specification, including core validation. We
use this term to distinguish other markup, processing, and applications
semantics from our own.
Data Object
(Content/Document)
The actual
binary/octet data being operated on (transformed, digested, or signed) by an
application -- frequently an HTTP entity [HTTP]. Note that the
proper noun Object designates
a specific XML element. Occasionally we refer to a data object as a document
or as a resource's
content. The term element content is used to
describe the data between XML start and end tags [XML]. The term XML
document is used to describe data objects which conform to the XML
specification [XML].
The inability to
change a message without also changing the signature value. See message authentication.
An XML Signature
element wherein arbitrary (non-core) data may be
placed. An Object
element is merely one type of digital data (or document) that can be signed via
a Reference.
"A resource
can be anything that has identity. Familiar examples include an electronic
document, an image, a service (e.g., 'today's weather report for Los Angeles'),
and a collection of other resources.... The resource is the conceptual mapping
to an entity or set of entities, not necessarily the entity which corresponds
to that mapping at any particular instance in time. Thus, a resource can remain
constant even when its content---the entities to which it currently
corresponds---changes over time, provided that the conceptual mapping is not
changed in the process." [URI] In
order to avoid a collision of the term entity within the URI and XML
specifications, we use the term data object, content or document
to refer to the actual bits being operated upon.
Formally
speaking, a value generated from the application of a private key to a message
via a cryptographic algorithm such that it has the properties of signer authentication
and message authentication (integrity).
(However, we sometimes use the term signature generically such that it
encompasses Authentication Code
values as well, but we are careful to make the distinction when the property of
signer authentication is
relevant to the exposition.) A signature may be (non-exclusively) described as detached, enveloping, or enveloped.
The signature is
over content external to the Signature
element, and can be identified via a URI or transform. Consequently, the signature is
"detached" from the content it signs. This definition typically
applies to separate data objects, but it also includes the instance where the Signature and data object
reside within the same XML document but are sibling elements.
The signature is
over content found within an Object
element of the signature itself. The Object(or its content) is identified via a Reference (via a URI fragment idenitifier or transform).
The signature is
over the XML content that contains the signature as an element. The content
provides the root XML document element. Obviously, enveloped signatures must
take care not to include their own value in the calculation of the SignatureValue.
The processing of
a octet stream from source content to derived content. Typical transforms
include XML Canonicalization, XPath, and XSLT.
The core
processing requirements of this specification requiring signature validation and SignedInfo reference validation.
The hash value of
the identified and transformed content, specified by Reference, matches its
specified DigestValue.
The SignatureValue matches the
result of processing SignedInfo
with CanonicalizationMethod and SignatureMethod as specified in
section 3.2.
The application determines
that the semantics associated with a signature are valid. For example, an
application may validate the time stamps or the integrity of the signer key --
though this behavior is external to this core specification.
11.0 References
Digital Signature Guidelines.
http://www.abanet.org/scitech/ec/isc/dsgfree.html
Declaring Elements and Attributes in
an XML DTD. Ron Bourret.
http://www.informatik.tu-darmstadt.de/DVS1/staff/bourret/xml/xmldtd.html
Document Object Model (DOM) Level 1
Specification. W3C Recommendation. V. Apparao, S. Byrne,
M. Champion, S. Isaacs, I. Jacobs, A. Le Hors, G. Nicol, J. Robie, R. Sutor, C.
Wilson, L. Wood. October 1998.
http://www.w3.org/TR/1998/REC-DOM-Level-1-19981001/
Will be RFC 2803. Digest Values for DOM (DOMHASH). H. Maruyama,
K. Tamura, N. Uramoto. April 2000
FIPS PUB 186-1. Digital
Signature Standard (DSS). U.S. Department of Commerce/National Institute of
Standards and Technology.
http://csrc.nist.gov/fips/fips1861.pdf
RFC 2104. HMAC:
Keyed-Hashing for Message Authentication. H. Krawczyk, M.
Bellare, R. Canetti. February 1997.
RFC 2616. Hypertext Transfer Protocol -- HTTP/1.1. J. Gettys, J.
Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee. June 1999.
RFC2119 Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. March 1997.
RFC 1321. The
MD5 Message-Digest Algorithm. R. Rivest. April 1992.
RFC 2045. Multipurpose
Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies.
N. Freed & N. Borenstein. November 1996.
TR15. Unicode
Normalization Forms. M. Davis, M. Dürst. Revision 18: November 1999.
RFC 2440 OpenPGP Message Format. J. Callas, L.
Donnerhacke, H. Finney, R. Thayer. November 1998.
RFC1750 Randomness Recommendations for Security.
D. Eastlake, S. Crocker, J. Schiller. December 1994.
RDF Schema W3C
Candidate Recommendation. D. Brickley, R.V. Guha. March 2000.
http://www.w3.org/TR/2000/CR-rdf-schema-20000327/
RDF Model and Syntax W3C
Recommendation. O. Lassila, R. Swick. February 1999.
http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/
IEEE P1363:
Standard Specifications for Public Key Cryptography.
RFC 2437. PKCS #1: RSA
Cryptography Specifications Version 2.0. B. Kaliski, J. Staddon. October
1998.
SAX: The Simple API for XML David
Megginson et. al. May 1998.
http://www.megginson.com/SAX/index.html
FIPS PUB 180-1. Secure
Hash Standard. U.S. Department of Commerce/National Institute of Standards
and Technology.
http://csrc.nist.gov/fips/fip180-1.pdf
[UTF-16]
RFC2781. UTF-16, an
encoding of ISO 10646. P.
Hoffman , F. Yergeau. February 2000.
RFC2279. UTF-8, a
transformation format of ISO 10646. F. Yergeau. Janaury 1998.
RFC2396. Uniform
Resource Identifiers (URI): Generic Syntax. T. Berners-Lee, R. Fielding, L.
Masinter. August 1998
RFC1738. Uniform Resource Locators (URL).
Berners-Lee, T., Masinter, L., and M. McCahill. December 1994.
RFC 2141. URN Syntax. R. Moats. May
1997.
RFC 2611. URN
Namespace Definition Mechanisms. L. Daigle, D. van Gulik, R. Iannella,
P. Falstrom. June 1999.
XML Linking Language.Working
Draft. S. DeRose, D. Orchard, B. Trafford. July 1999.
http://www.w3.org/1999/07/WD-xlink-19990726
Extensible Markup Language (XML) 1.0
Recommendation. T. Bray, J. Paoli, C. M. Sperberg-McQueen. February 1998.
http://www.w3.org/TR/1998/REC-xml-19980210
Canonical XML. Working
Draft. J. Boyer. July 2000.
http://www.w3.org/TR/2000/WD-xml-c14n-20000710
XML Japanese Profile. W3C
NOTE. MURATA April 2000 http://www.w3.org/TR/2000/NOTE-japanese-xml-20000414/
RFC 2376. XML Media
Types. E. Whitehead, M. Murata. July 1998.
Namespaces in XML
Recommendation. T. Bray, D. Hollander, A. Layman. Janaury 1999.
http://www.w3.org/TR/1999/REC-xml-names-19990114
XML Schema Part 1: Structures
Working Draft. D. Beech, M. Maloney, N. Mendelshohn. April 2000.
http://www.w3.org/TR/2000/WD-xmlschema-1-20000407/
XML Schema Part 2: Datatypes
Working Draft. P. Biron, A. Malhotra. April 2000.
http://www.w3.org/TR/2000/WD-xmlschema-2-20000407/
Will be RFC 2807. XML Signature Requirements. J.
Reagle, April 2000.
http://www.w3.org/TR/xmldsig-requirements
XML Path Language (XPath)Version 1.0.
Proposed Recommendation. J. Clark, S. DeRose. October 1999.
http://www.w3.org/TR/1999/PR-xpath-19991008
XML Pointer Language (XPointer). Working
Draft. S. DeRose, R. Daniel.
http://www.w3.org/1999/07/WD-xptr-19990709
Extensible Stylesheet Language (XSL)
Working Draft. S. Adler, A.
Berglund, J. Caruso, S. Deach, P. Grosso, E. Gutentag, A. Milowski, S. Parnell,
J. Richman, S. Zilles. March 2000.
http://www.w3.org/TR/2000/WD-xsl-20000327/xslspec.html
XSL Transforms (XSLT) Version 1.0.
Recommendation. J. Clark. November 1999.
http://www.w3.org/TR/1999/REC-xslt-19991116.html
Web Architecture: Describing and Exchanging
Data. W3C Note. T. Berners-Lee, D. Connolly, R. Swick. June
1999.
http://www.w3.org/1999/04/WebData
12. Authors' Address
Donald E.
Eastlake 3rd
Motorola, Mail Stop: M4-10
20 Forbes Boulevard
Mansfield, MA 02048 USA
Phone: 1-508-261-5434
Email: Donald.Eastlake@motorola.com
Joseph M. Reagle
Jr., W3C
Massachusetts Institute of Technology
Laboratory for Computer Science
NE43-350, 545 Technology Square
Cambridge, MA 02139
Phone: + 1.617.258.7621
Email: reagle@w3.org
David Solo
Citigroup
666 Fifth Ave, 3rd Floor
NY, NY 10103 USA
Phone: +1-212-830-8118
Email: dsolo@alum.mit.edu