XML Digital Signatures Working Group D. Eastlake,
INTERNET-DRAFT IBM
draft-ietf-xmldsig-core-08 J. Reagle,
Expires January 11, 2001 W3C/MIT
D. Solo,
Citigroup
XML-Signature Syntax and Processing
Copyright Notice
Copyright (c) 2000 The Internet Society & W3C (MIT, INRIA, Keio), All
Rights Reserved.
IETF Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
W3C Status of this document
This document is a production of the joint IETF/W3C XML Signature
Working Group.
http://www.w3.org/Signature
The comparable html draft of this version may be found at
http://www.w3.org/TR/2000/WD-xmldsig-core-20000711/
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 requests 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
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Internet Draft XML-Signature Syntax and Processing July 2000
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.
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.
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.
Table of Contents
1. Introduction
1. Editorial Conventions
2. Design Philosophy
3. Versions, Namespaces and Identifiers
4. Acknowledgements
2. Signature Overview and Examples
1. Simple Example (Signature, SignedInfo, Methods, and
References)
1. More on Reference
2. Extended Example (Object and SignatureProperty)
3. Extended Example (Object and Manifest)
3. Processing Rules
1. Signature Generation
2. Signature Validation
4. Core Signature Syntax
1. The Signature element
2. The SignatureValue Element
3. The SignedInfo Element
1. The CanonicalizationMethod Element
2. The SignatureMethod Element
3. The Reference Element
1. The Transforms Element
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2. The DigestMethod Element
3. The DigestValue Element
4. The KeyInfo Element
5. The Object Element
5. Additional Signature Syntax
1. The Manifest Element
2. The SignatureProperties Element
3. Processing Instructions
4. Comments in dsig Elements
6. Algorithms
1. Algorithm Identifiers and Implementation Requirements
2. Message Digests
3. Message Authentication Codes
4. Signature Algorithms
5. Canonicalization Algorithms
6. Transform Algorithms
1. Canonicalization
2. Base64
3. XPath Filtering
4. Enveloped Signature Transform
5. XSLT Transform
7. XML Canonicalization and Syntax Constraint Considerations
1. XML 1.0, Syntax Constraints, and Canonicalization
2. DOM/SAX Processing and Canonicalization
8. Security Considerations
1. Transforms
1. Only What is Signed is Secure
2. Only What is "Seen" Should be Signed
3. "See" What is Signed
2. Check the Security Model
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
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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.
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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)
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* 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)?
(SignatureMethod)
<Reference (URI=)? >
(Transforms)?
(DigestMethod)
(DigestValue)
(</Reference>)+
</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
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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/xmld
sig#">
[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-sh
a1"/>
[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-2000
0710"/>
[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.
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[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.
[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-2000
0710"/>
[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
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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.
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].)
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[ ] ...
[p01] <SignedInfo>
[ ] ...
[p02] <Reference URI="http://www.w3.org/TR/xml-stylesheet/">
[ ] ...
[p03] <Reference URI=" #AMadeUpTimeStamp "
[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=" #MySecondSignatu
re ">
[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.
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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.
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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, 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.
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.
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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:
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<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'/>
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>
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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; >
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<!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>
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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). 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.
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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:
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<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:
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<!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].
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' 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) 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
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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 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'>
<any namespace='##other' minOccurs='1' 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
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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
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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 A</X509IssuerName>
<X509SerialNumber>12345678</X509SerialNumber>
</X509IssuerSerial>
<X509SKI>31d97bd7</X509SKI>
</X509Data>
<X509Data>
<X509SubjectName>Subject of Certificate 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'/>
</choice>
</sequence>
<element name='X509Certificate' type='ds:CryptoBinary' minOccurs='1' m
axOccurs='1'/>
<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' minOccurs='1' maxOccurs='
1'/>
<element name='X509SerialNumber' type='integer' minOccurs='1' maxOccur
s='1'/>
</sequence>
</complexType>
</element>
DTD:
<!ELEMENT X509Data ((X509IssuerSerial | X509SKI | X509SubjectName),
X509Certificate*, X509CRL*)>
<!ELEMENT X509IssuerSerial (X509IssuerName, X509SerialNumber) >
<!ELEMENT X509IssuerName (#PCDATA) >
<!ELEMENT X509SubjectName (#PCDATA) >
<!ELEMENT X509SerialNumber (#PCDATA) >
<!ELEMENT X509SKI (#PCDATA) >
<!ELEMENT X509Certificate (#PCDATA) >
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<!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' maxOccurs='unbounded'/>
<element name='PGPKeyID' type='string' minOccurs='1' maxOccurs='1'/>
<element name='PGPKeyPacket' type='ds:CryptoBinary' minOccurs='1' maxO
ccurs='1'/>
</sequence>
</complexType>
</element>
DTD:
<!ELEMENT PGPData (PGPKeyID, PGPKeyPacket?) >
<!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 -->
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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' 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
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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 Element
Identifier
Type="http://www.w3.org/2000/07/xmldsig#SignatureProperty"
(this can be used within a Reference element to identify the
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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.
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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 http://www.w3.org/2000/07/xmldsig#sha1
Encoding
Base64 REQUIRED http://www.w3.org/2000/07/xmldsig#base64
MAC
HMAC-SHA1 REQUIRED http://www.w3.org/2000/07/xmldsig#hmac-sha1
Signature
DSAwithSHA1
(DSS) REQUIRED http://www.w3.org/2000/07/xmldsig#dsa-sha1
RSAwithSHA1 RECOMMENDED http://www.w3.org/2000/07/xmldsig#rsa-sha1
Canonicalization
minimal RECOMMENDED http://www.w3.org/2000/07/xmldsig#minimal
Canonical XML REQUIRED
http://www.w3.org/TR/2000/WD-xml-c14n-20000710
Transform
XSLT OPTIONAL http://www.w3.org/TR/1999/REC-xslt-19991116
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XPath RECOMMENDED http://www.w3.org/TR/1999/REC-xpath-19991116
Enveloped Signature* REQUIRED
http://www.w3.org/2000/07/xmldsig#enveloped-signature
* The Enveloped Signature transform removes the Signature element from
the calculation of the signature when the signature is within the
document 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:
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<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
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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' maxO
ccurs='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
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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
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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-200
00710"/>
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
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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>
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...
<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
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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
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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
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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.
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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
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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
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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
xmldsig-core-schema.xsd
Valid XML schema instance based on the Last Call 20000407
Schema/DTD [XML-Schema].
XML Signature DTD
xmldsig-core-schema.dtd
RDF Data Model
xmldsig-datamodel-20000112.gif
XML Signature Object Example
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signature-example.xml
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
signature-example-rsa.xml
An XML Signature example by Kent Tamura with generated
cryptographic values, uses WD-xml-c14n-20000613, that has been
confirmed by Petteri Stenius. (Note: 'X509Name' should be
'X509SubjectName'.)
XML DSA Signature Valid Example
signature-example-dsa.xml
Similar to above but uses DSA.
10.0 Definitions
Authentication Code
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
Authentication, Message
"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]
Authentication, Signer
"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]
Core
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].
Integrity
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The inability to change a message without also changing the
signature value. See message authentication.
Object
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.
Resource
"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.
Signature
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.
Signature, Detached
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.
Signature, Enveloping
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).
Signature, Enveloped
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
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care not to include their own value in the calculation of the
SignatureValue.
Transform
The processing of a octet stream from source content to derived
content. Typical transforms include XML Canonicalization,
XPath, and XSLT.
Validation, Core
The core processing requirements of this specification
requiring signature validation and SignedInfo reference
validation.
Validation, Reference
The hash value of the identified and transformed content,
specified by Reference, matches its specified DigestValue.
Validation, Signature
The SignatureValue matches the result of processing SignedInfo
with CanonicalizationMethod and SignatureMethod as specified
in section 3.2.
Validation, Trust/Application
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
ABA
Digital Signature Guidelines.
http://www.abanet.org/scitech/ec/isc/dsgfree.html
Bourret
Declaring Elements and Attributes in an XML DTD. Ron Bourret.
http://www.informatik.tu-darmstadt.de/DVS1/staff/bourret/xml/xm
ldtd.html
DOM
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/
DOMHASH
Will be RFC 2803. Digest Values for DOM (DOMHASH). H. Maruyama,
K. Tamura, N. Uramoto. April 2000
DSS
FIPS PUB 186-1. Digital Signature Standard (DSS). U.S.
Department of Commerce/National Institute of Standards and
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Technology.
http://csrc.nist.gov/fips/fips1861.pdf
HMAC
RFC 2104. HMAC: Keyed-Hashing for Message Authentication. H.
Krawczyk, M. Bellare, R. Canetti. February 1997.
HTTP
RFC 2616. Hypertext Transfer Protocol -- HTTP/1.1. J. Gettys,
J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee.
June 1999.
KEYWORDS
RFC2119 Key words for use in RFCs to Indicate Requirement
Levels. S. Bradner. March 1997.
MD5
RFC 1321. The MD5 Message-Digest Algorithm. R. Rivest. April
1992.
MIME
RFC 2045. Multipurpose Internet Mail Extensions (MIME) Part
One: Format of Internet Message Bodies. N. Freed & N.
Borenstein. November 1996.
NFC
TR15. Unicode Normalization Forms. M. Davis, M. Dürst. Revision
18: November 1999.
PGP
RFC 2440 OpenPGP Message Format. J. Callas, L. Donnerhacke, H.
Finney, R. Thayer. November 1998.
RANDOM
RFC1750 Randomness Recommendations for Security. D. Eastlake,
S. Crocker, J. Schiller. December 1994.
RDF
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/
P1363
IEEE P1363: Standard Specifications for Public Key
Cryptography.
PKCS1
RFC 2437. PKCS #1: RSA Cryptography Specifications Version 2.0.
B. Kaliski, J. Staddon. October 1998.
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SAX
SAX: The Simple API for XML David Megginson et. al. May 1998.
http://www.megginson.com/SAX/index.html
SHA-1
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.
UTF-8
RFC2279. UTF-8, a transformation format of ISO 10646. F.
Yergeau. Janaury 1998.
URI
RFC2396. Uniform Resource Identifiers (URI): Generic Syntax. T.
Berners-Lee, R. Fielding, L. Masinter. August 1998
URL
RFC1738. Uniform Resource Locators (URL). Berners-Lee, T.,
Masinter, L., and M. McCahill. December 1994.
URN
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.
XLink
XML Linking Language.Working Draft. S. DeRose, D. Orchard, B.
Trafford. July 1999.
http://www.w3.org/1999/07/WD-xlink-19990726
XML
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
XML-C14N
Canonical XML. Working Draft. J. Boyer. July 2000.
http://www.w3.org/TR/2000/WD-xml-c14n-20000710
XML-Japanese
XML Japanese Profile. W3C NOTE. M. MURATA April 2000
http://www.w3.org/TR/2000/NOTE-japanese-xml-20000414/
XML-MT
RFC 2376. XML Media Types. E. Whitehead, M. Murata. July 1998.
XML-ns
Namespaces in XML Recommendation. T. Bray, D. Hollander, A.
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Layman. Janaury 1999.
http://www.w3.org/TR/1999/REC-xml-names-19990114
XML-schema
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/
XML-Signature-RD
Will be RFC 2807. XML Signature Requirements. J. Reagle, April
2000.
http://www.w3.org/TR/xmldsig-requirements
XPath
XML Path Language (XPath)Version 1.0. Proposed Recommendation.
J. Clark, S. DeRose. October 1999.
http://www.w3.org/TR/1999/PR-xpath-19991008
XPointer
XML Pointer Language (XPointer). Working Draft. S. DeRose, R.
Daniel.
http://www.w3.org/1999/07/WD-xptr-19990709
XSL
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
XSLT
XSL Transforms (XSLT) Version 1.0. Recommendation. J. Clark.
November 1999.
http://www.w3.org/TR/1999/REC-xslt-19991116.html
WebData
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
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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
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