One document matched: draft-ietf-xmldsig-core-03.txt
Differences from draft-ietf-xmldsig-core-02.txt
XML Digital Signatures Working Group D. Eastlake,
INTERNET-DRAFT IBM
draft-ietf-xmldsig-core-03.txt J. Reagle,
Expires July 04, 2000 W3C/MIT
D. Solo,
Citigroup
XML-Signature Core Syntax
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-20000104/
The latest version of this draft series may be found at:
http://www.w3.org/TR/xmldsig-core
This is a public WG Draft. This draft includes many improvements to
the exposition. Few design changes have been made, the most
significant change is a move from the ObjectReference to the Reference
element type. This version includes the experimental use of XML Schema
and XML entity references and DTD declarations. The XML schema
declarations within the specification may contain errors, though the
Eastlke, Reagle, Solo [Page 1]
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complete WG schema definition does validate to the Schema DTD. We
expect the final draft will include a DTD and schema. We hope to issue
a Working Group last call soon, and an institutuional (IETF/W3C) last
call as soon as possible afterwards.
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. This is a draft
document and may be updated, replaced or obsoleted by other documents
at any time. 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
Patent disclosures relevant to this specification may be found on the
WG's patent disclosure page.
Abstract
This document specifies digital signature processing rules and XML
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. Namespaces and Identifiers
4. Versions
2. Signature Overview
1. The Signature Element
2. The SignedInfo Element
3. The Reference Element
4. The Manifest and Package Elements
5. The SignatureProperties Element
3. Core Signature Syntax
1. The Signature element
2. The SignatureValue Element
3. The SignedInfo Element
4. The KeyInfo Element
5. The Object Element
4. Additional Signature Syntax
1. The Manifest and Package Elements
2. The SignatureProperties Element
3. Processing Instructions
4. Comments in dsig Elements
5. Algorithms
1. Algorithm Identifiers, Parameters, and Implementation
Requirements
2. Message Digests
3. Message Authentication Codes
4. Signature Algorithms
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5. Canonicalization Algorithms
6. Transform Algorithms
6. Processing Rules
1. Generation
2. Signature Validation
7. Security Considerations
1. Only What is Signed is Secure
2. Only What is "Seen" Should be Signed
3. Check the Security Model
4. Algorithms, Key Lengths, Etc.
8. Example Syntax
9. Schema
10. Definitions
11. Other Useful Types (normative)
12. References
13. Acknowledgements (non-normative)
14. Open Issues (non-normative)
_________________________________________________________________
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 or other data.
Furthermore, an XML Signature may be applied to the content of one or
more resources: enveloped or envoloping signatures are over data
within the same XML document as the signature; detached signatures are
over data referenced externally via a URI.
This document also defines other useful types including methods of
referencing collections of resources, and key management and algorithm
definitions.
1.1 Editorial Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document includes a list of open issues which are still being
addressed by the working group and may include editorial comments
within the text.
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 Namespaces and Identifiers
The XML namespace [XML-ns] URI that MUST be used by experimental
implementations of this dated specification is:
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xmlns="http://www.w3.org/2000/01/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 so as to provide compact and readable examples.
This specification uses Uniform Resource Identifiers [URI] to
identify resources, algorithms, and semantics. The URI in the
namespace declaration above is also used as a prefix for URIs under
the control of this specification. For resources not under the control
of this specification, we use the designated Uniform Resource Names
[URN] or Uniform Resource Locators [URL] defined by the 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 specifications
namespace
http://www.w3.org/2000/01/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/2000/01/xmldsig/SHA1
FIPS PUB 180-1. Secure Hash Standard. U.S. Department of
Commerce/National Institute of Standards and Technology.
This specification uses both XML Schemas [XML-schema] and DTDs [XML];
while the DTD is presently more buggy, it will likely be the normative
definition. Readers unfamiliar with DTD syntax may wish to refer to
Ron Bourret's "Declaring Elements and Attributes in an XML DTD."
Finally, in order to provide for terse namespace declarations we use
XML internal entities [XML]as macros within URIs. For instance:
<?xml version="1.0" ?>
<!DOCTYPE Signature SYSTEM "xmldsig.dtd" [
<!ENTITY dsig 'http://www.w3.org/2000/01/xmldsig/'>]>
...
<SignedInfo>
<SignatureMethod Algorithm="&dsig;/dsaWithSHA-1"/>
...
</SignedInfo>
Security Comment: XML processors will automatically expand entity
declarations prior to signature generation. Consequently, this feature
does not permit a substitution attack whereby an attacker replaces the
entity declaration with another so as to change the meaning of the
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signature. However, since this specification presently permits a
CanonicalizationMethod of null over SignedInfo, entity declarations
will not be expanded in those instances (or where the schema is not
present) and we have not completely assessed the security risk.
1.4 Versions
No provision is made for an explicit version number in this syntax. If
a future version is needed, it is expected to use a different
Namespace.
2.0 Signature Overview
This section provides an overview of XML digital signature syntax and
processing; the formal specification is in section-3: Core Signature
Syntax. We assume the reader is familiar with basic digital signature
and XML concepts.
In this section, an informal representation is used to describe the
structure of the XML signature syntax. This representation omits many
attributes and details. The following suffix symbols are used to
represent the number of times elements may occur: "?" denotes zero or
one occurance; "+" denotes one or more occurances; and "*" denotes
zero or more occurances.
2.1 The Signature Element
XML Signatures are very flexible and can sign arbitrary digital
content (data objects). An XML Signature is applied 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. While the data object(s) are
not directly operated on by a cryptographic algorithm, we still refer
to the signature as being over the data object(s). Frequently, content
is obtained by dereferencing an identified resource. Within an XML
document, signatures are related to data objects via IDREFs [XML] and
the data can be included within an envoloping signature or can enclose
an enveloped signature. Signatures are related to external data
objects via URIs [URI] and the signature and data object are said to
be detached.
XML digital signatures are represented by the Signature element which
has the following structure:
<Signature>
(SignedInfo)
(SignatureValue)
(KeyInfo)?
(Object)*
</Signature>
The required SignedInfo element is the information which is actually
signed. SignedInfo includes a list of References to data objects and
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their calculated digest value. The core validation consists of two
mandatory processes: validation of the signature over SignedInfo and
validation of each Reference digest within SignedInfo. The algorithms
used in calculating the SignatureValue are also included in the signed
information while the SignatureValue element is outside SignedInfo.
KeyInfo indicates what key is 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, KeyInfo might contain
information the signer does not wish to reveal to all signature
verifiers. Second, the information may be known within the
application's context and need not be represented explicitly. However,
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.
Object is an optional element for including the signed resources
within the signature document. The Object can be optionally typed
and/or encoded.
Signature properties, such as time of signing, can be optionally
included in a SignatureProperties within Object. (These properties are
traditionally called signature "attributes" although that term in that
context has no relationship to the XML term "attribute".)
SignatureProperties can be included within an Object and signed at the
signer's discretion.
2.2 The SignedInfo Element
The SignedInfo element has the structure indicated below.
<Signature>
<SignedInfo>
(CanonicalizationMethod)?
(SignatureMethod)
(Reference)+
</SignedInfo>
(SignatureValue)
(KeyInfo)?
(Object)*
</Signature>
The CanonicalizationMethod is the algorithm which 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.
The SignatureMethod is the algorithm 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 or HMAC-SHA1. The algorithm
names are signed to resist attacks based on substituting a weaker
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algorithm.
To promote application interoperability we specify mandatory to
implement canonicalization, digest, and signature algorithms. We
specify additional algorithms as recommended or optional and the
signature design permits arbitrary signer algorithm specification.
Each Reference element includes the digest method and resulting digest
value calculated over the identified data object. It also may include
transformations that produce 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.
2.3 The Reference Element
The Reference element has the structure indicated below.
...
<SignedInfo>
(CanonicalizationMethod)?
(SignatureMethod)
(<Reference (URI=|IDREF=)? Type=?>
(Transforms)?
(DigestMethod)
(DigestValue)
</Reference>)+
</SignedInfo>
...
The optional URI/IDREF attribute of Reference idenitifies the data
object to be signed. This attribute may be omitted on at most one
Reference in a Signature.
This identification, along with the transforms, are a description
provided by the signer on how to obtain the signed resource in the
form it was digested (i.e. the digested content). The verifier (i.e.,
relying party) 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 (particularly a local store)
other than that specified in the URI/IDREF.
The optional Type attribute provides information about the content of
the resource identified by URI/IDREF. In particular, it can indicate
that it contains a SignatureProperties, Manifest, or Package element.
This information need not be used nor verified by the receiving
application.
Transforms is an optional ordered list of processing steps that are
applied to the resource's content before it is digested. Transforms
can include arbitrary specifications such as canonicalization,
encoding/decoding (including compression/inflation), XSLT and XPath.
XPath transforms permit the signer to derive an XML document that
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omits portions of the source document. Consequently those excluded
portions can change without affecting signature validity (this is how
the Working Group satisfied the requirement of signing portions of a
document.) 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.
Arbritrary user specified transforms are permitted. To promote
interoperability, we specify mandatory to implement canonicalization
and decoding transformation algorithms. Additional canonicalization,
coding, XSLT, and XPath based transform algorithms are specified as
recommended or optional;
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 bind's a resources content to the signer's key.
2.4 The Manifest and Package Elements
The Manifest and Package elements are included to meet requirements
not directly addressed by this document. The level of indirection
provided by these elements readily meets these requirements. Two
examples follow.
First, applications frequently need to efficiently sign multiple data
objects. This requirement can be achieved by including multiple
References within SignedInfo. However, some applications may not want
the core validation behaviour associated with this approach: each
Reference within SignedInfo undergoes reference validation -- the
DigestValues are checked. Some 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 References. 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 References as valid.
Second, consider an application where many signatures (using different
keys) are applied to thousands of documents. An inefficient solution
is to have a seperate signature (per key) repeatedly applied to a
large SignedInfo element (with thousands of References); this is very
redundant.
To address these requirements, additional element types have been
defined which may be referenced by SignedInfo References. First, the
Manifest element may contain a collection of References and Objects,
but leaves reference validation up to the application. Second,
multiple signatures over the thousands of References need only point
to a single Manifest with the many references.
The structure of Manifest, which reuses the Reference and Object
elements described above, is as follows:
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<Manifest>
(Reference)+
(Object)*
</Manifest>
A Package is syntactically identical to a Manifest, and may appear
anywhere a Manifest may appear, but asserts the identity of each of
its Reference elements after Transforms application.
Manifest and Package may appear as the content of an Object.
2.5 The SignatureProperties Element
This specification does not address mechanisms for making statements
or assertions. Instead, this whole document singly 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, XML-schema, RDF]. However, we do define a
SignatureProperties element type for the inclusion of assertions about
how the signature was produced (e.g., the time of signing or the
serial number of hardware used in cryptographic processes). We do not
define those element types however, they may, or may not be, signed
via a Reference, as desired.
<SignatureProperties>
(SignatureProperty Target= )+
</SignatureProperties>
The structure of SignatureProperties is shown above. Any content about
the signature generation may be located within the SignatureProperty
element. The mandatory Target attribute references the element to
which the property applies. In particular, target may include a
reference to a SignedInfo or Reference element.
3.0 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 [XML-Schema] with the following XML preamble,
declaration, and internal entity:
Schema Definition:
<?xml version='1.0'?>
<!DOCTYPE schema
SYSTEM
'http://www.w3.org/TR/1999/WD-xmlschema-1-19991217/structures.dtd'
[
<!ENTITY dsig 'http://www.w3.org/2000/01/xmldsig/'>
]>
<schema targetNamespace='http://www.w3.org/2000/01/xmldsig/'
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version='0.1'
xmlns='http://www.w3.org/1999/XMLSchema'>
3.1 The Signature element
The Signature element is the root element of a XML Signature. A simple
example of a complete signature follows:
Example:
<!DOCTYPE Signature [
<!ENTITY dsig 'http://www.w3.org/2000/01/xmldsig/'>]>
<Signature xmlns="http://www.w3.org/2000/01/xmldsig/">
<SignedInfo>
<CanonicalizationMethod
Algorithm="http://www.w3.org/1999/07/WD-xml-c14n-19990729"/>
<SignatureMethod Algorithm="&dsig;/dsaWithSHA-1"/>
<Reference Location="http://www.mypage.com">
<DigestMethod Algorithm="&dsig;/sha1"/>
<DigestValue encoding="&dsig;/base64">a23bcd43</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>dd2323dd</SignatureValue>
<KeyInfo>
<KeyName>Solo</KeyName>
</KeyInfo>
</Signature>
Note: this example will be revised to include generated hash/signature
values that validate.
Schema Definition:
<element name='Signature'>
<type content='elementOnly'>
<group order='seq' minOccurs='1' maxOccurs='1'>
<element ref='SignedInfo' minOccurs='1' maxOccurs='1'/>
<element ref='SignatureValue' minOccurs='1' maxOccurs='1'/>
<element ref='KeyInfo' minOccurs='0' maxOccurs='1'/>
<element ref='Object' minOccurs='0' maxOccurs='*'/>
</group>
<attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/>
</type>
</element>
DTD:
<!ELEMENT Signature (SignedInfo, SignatureValue, KeyInfo?, Object*)>
<!ATTLIST SignedInfo
Id ID #IMPLIED>
3.2 The SignatureValue Element
The SignatureValue element contains the actual value of the digital
signature. The encoding of this value is determined by
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SignatureMethod. Base64 [MIME] is the encoding method for all
SignatureMethods specified within this specification. The ability to
define a SignatureMethod and SignatureValue pair which includes
multiple distinct signatures is explicitly permitted (e.g.
"rsawithsha-1 and ecdsawithsha-1").
Schema Definition:
<element name='SignatureValue' type='string'/>
DTD:
<!ELEMENT SignatureValue CDATA >
3.3 The SignedInfo Element
The structure of SignedInfo includes the canonicalization algorithm
(if any), a signature algorithm, and one or more references to
objects. 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'>
<type content='elementOnly'>
<group order='seq' minOccurs='1' maxOccurs='1'>
<element ref='CanonicalizationMethod' minOccurs='0'
maxOccurs='1'/>
<element ref='SignatureMethod' minOccurs='1' maxOccurs='1'/>
<element ref='Reference' minOccurs='1' maxOccurs='*'/>
</group>
<attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/>
</type>
</element>
DTD:
<!ELEMENT SignedInfo(CanonicalizationMethod, SignatureMethod,
Reference+ ) >
<!ATTLIST SignedInfo
Id ID #IMPLIED >
SignedInfo does not include explicit signature properties (such as
signing time, signing device, etc.). If an application needs to
associate properties with the signature or digest, it may include such
information in an Object element that is referenced by a Reference.
See the SignatureProperties element below.
3.3.1 The CanonicalizationMethod Element
CanonicalizationMethod is an optional element which specifies the
canonicalization algorithm applied to the SignedInfo element prior to
performing signature calculations. This element uses the general
structure here for algorithms described in section-5.1: Algorithm
Identifiers. Possible options may include a minimal algorithm (CRLF
and charset normalization), or more extensive operations such as
[XML-C14N]. If the CanonicalizationMethod is omitted, no change is
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made to SignedInfo.
Schema Definition:
<element name='CanonicalizationMethod'>
<type content='elementOnly'>
<element ref='Parameter' minOccurs='1' maxOccurs='1'/>
<attribute name='Algorithm' type='uri' minOccurs='1'
maxOccurs='1'/>
<type>
</element>
DTD:
<!ELEMENT CanonicalizationMethod Parameter* >
<!ATTLIST CanonicalizationMethod
Algorithm CDATA #REQUIRED >
3.3.2 The SignatureMethod Element
SignatureMethod is a required element which 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 5.1. While there is a single identifier, that identifier
may specify a format containing multiple distinct signature values.
Schema Definition:
<element name='SignatureMethod'>
<type content='elementOnly'>
<element ref='Parameter' minOccurs='0' maxOccurs='*'/>
<attribute name='Algorithm' type='uri' minOccurs='1'
maxOccurs='1'/>
</type>
</element>
DTD:
<!ELEMENT SignatureMethod Parameter* >
<!ATTLIST SignatureMethod
Algorithm CDATA #REQUIRED >
3.3.3 The Reference Element
Reference is an element that may occur one or more times. It specifies
a digest algorithm and digest value, and optionally the object being
signed, the type of the object, and/or a list of transforms to be
applied prior to digesting. The identification, and transforms are
information provided to inform the verifier how the digested content
(i.e., the input to the digest method) may be 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 easily referenced from
elsewhere.
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Schema Definition:
<element name='Reference'>
<type content='elementOnly'>
<group order='seq' minOccurs='1' maxOccurs='1'>
<element ref='Transforms' minOccurs='0' maxOccurs='1'/>
<element ref='DigestMethod' minOccurs='1' maxOccurs='1'/>
<element ref='DigestValue' minOccurs='1' maxOccurs='1'/>
</group>
<attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/>
<attribute name='URI' type='uri' minOccurs='0' maxOccurs='1'/>
<attribute name='IDREF' type='IDREF' minOccurs='0' maxOccurs='1'/>
<attribute name='Type' type='uri' minOccurs='0' maxOccurs='1'/>
</type>
</element>
DTD:
<!ELEMENT ObjectReference ( Transforms?, DigestMethod, DigestValue ) >
<!ATTLIST ObjectReference
Id ID #IMPLIED
URI CDATA #IMPLIED
IDREF CDATA #IMPLIED
Type CDATA #IMPLIED>
<!-- The values of URI and Type conform
to the productions specified by [URI] -->
The URI/IDREF attribute identifies a data object using a URI [URI] or
IDREF [XML]. We distinguish between URIs and IDREFs so as to provide
expositional clarity and ease signature processing. Note there is some
confusion about URIs and fragment identifiers. As specified by RFC2396
[URI], URIs can be used in conjunction with a fragment identifier by
use of a separating pound symbol '#', but the URI proper does not
include the fragment identifier. (The meaning of the fragment is
defined by the resource's MIME type). URI/IDREF only permits a 'clean'
URI or IDREF; fragment identification is specified under Transforms.
This choice permits References to identify a fragment of a document
that is encoded: the Reference identifies the resource, the first
Transform specifies decoding, the second Transform specifies the
fragement.
Note that a null URI (URI="") is permitted and identifies the document
the reference is in (the root element).
If the URI/IDREF attribute is omitted all-together, the receiving
application is expected to know the identity of the object. For
example, a lightweight data protocol might ommit 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, Manifest, or Package.
The digest algorithm is applied to the data octets being secured.
Typically that is done by locating (possibly using the URI/IDREF if
provided) the data and transforming it. If the data is an XML
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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 (e.g. manifest, package, signature properties,
document). This is represented as a URI. For example:
Type="&dsig;/Manifest"
Type="&dsig;/SignatureProperty"
3.3.3.1 The Transforms Element
Transforms is an optional element that contains one or more operations
to be performed on an indicated data object prior to digest
calculation. (These operations are different from the
CanonicalizationMethod specified in the Signature which is applied to
SignedInfo.) If the Transforms element is omitted, no operations are
indicated.
The Transforms element contains an ordered list of Transform elements.
The output of each Transform 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 [sec-7.2: Only What
is "Seen" Should be Signed].
Each Transform consists of an Algorithm attribute, optional Type and
Charset attributes, 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.
The optional Type and Charset (IANA registered character set)
attributes are made available to algorithms which need and are
otherwise unable to deduce that information about the data they are
processing.
Schema Definition:
<element name='Transforms' >
<type content='elementOnly'>
<element ref='Transform' minOccurs='1' maxOccurs='*'/>
</type>
</element>
<element name='Transform'>
<type content='elementOnly'>
<element ref='Parameter' minOccurs='0' maxOccurs='*'/>
<attribute name='Algorithm' type='string' minOccurs='1'
maxOccurs='1'/>
<attribute name='Type' type='uri' minOccurs='0' maxOccurs='1'/>
<attribute name='Charset' type='string' minOccurs='0'
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maxOccurs='1'/>
</type>
</element>
DTD:
<!ELEMENT Transforms Transform+>
<!ELEMENT Transform ANY> <!-- Including dsig:Parameter -->
<!ATTLIST Transform
Algorithm CDATA #REQUIRED
Type CDATA #IMPLIED
Charset CDATA #IMPLIED >
<!-- The Type conforms to the
productions specified by [URI] -->
Examples of transforms include but are not limited to base-64 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
base-64 encoded parameter to the 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 5-6: Transform Algorithms defines the list
of standard transformations.
3.3.3.2 The DigestMethod Element
DigestMethod is a required element which identifies the digest
algorithm to be applied to the signed object. This element uses the
general structure here for algorithms specified in section-5.1:
Algorithm Identifiers.
Schema Definition:
<element name='DigestMethod'>
<type content='elementOnly'>
<element ref='Parameter' minOccurs='0' maxOccurs='*'/>
<attribute name='Algorithm' type='uri' minOccurs='1'
maxOccurs='1'/>
</type>
</element>
DTD:
<!ELEMENT DigestMethod Parameter* >
<!ATTLIST DigestMethod
Algorithm CDATA #REQUIRED >
<!-- Where CDATA conforms to the
productions specified by [URI] -->
3.3.3.3 The DigestValue Element
DigestValue is an element which contains the encoded value of the
digest. The optional Encoding attribute gives the encoding method
which defaults to Base 64 [MIME].
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Schema Definition:
<element name='DigestValue'>
<type source='string'>
<attribute name='Encoding' type='uri' default='&dsig;/Base64'
minOccurs='0' maxOccurs='1'/>
</type>
</element>
DTD:
<!ELEMENT DigestValue CDATA>
<!-- base64 encoded signature value -->
3.4 The KeyInfo Element
KeyInfo may contain keys, names, certificates and other public key
management information (such as inband key distribution or agreement
data or data supporting any other method.) This specification defines
a few simple types but applications may place their own key
identification and exchange semantics within this element through the
XML-namespace facility. [XML-namespace]
Schema Definition:
<element name='KeyInfo'>
<type content='elementOnly'>
<group order='choice' minOccurs='1' maxOccurs='1'>
<element name='KeyName' type='string'/>
<element name='KeyValue' type='string'/>
<element name='SubjectName' type='string'/>
<element name='RetrievalMethod' type='string'/>
<element ref='X509Data'/>
<element ref='PGPData'/>
<element name='MgmtData' type='string' minOccurs='0'
maxOccurs='1'/>
<any/>
</group>
<attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/>
</type>
</element>
DTD:
<!ELEMENT KeyInfo (KeyName | KeyValue | SubjectName
| RetrievalMethod | x509Data | PGPData
| MgmtData)* )>
<!ATTLIST KeyInfo
Id ID #IMPLIED>
<!ELEMENT KeyName (#PCDATA)>
<!ELEMENT KeyValue (#PCDATA)>
<!ELEMENT SubjectName (#PCDATA) >
<!ELEMENT RetrievalMethod (#PCDATA) >
KeyInfo is an optional element which enables the recipient(s) to
obtain the key(s) needed to validate the signature. If omitted, the
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recipient is expected to be able to identify the key based on
application context information. This element contains one KeyInfo
data element providing information for the recipient(s). Applications
may define and use any mechanism they choose through inclusion of
elements from a different namespace.
Compliant versions implementing KeyInfo MUST implement KeyValue, and
SHOULD implement RetrievalMethod.
* KeyName contains an identifier for the key which may be useful to
the recipient. This may be a name, index, etc.
* KeyValue contains the actual key(s) used to validate the
signature. If the key is sent in protected form, the MgmtData
element should be used. Specific types must be defined for each
algorithm type (see algorithms).
* SubjectName contains one or more names for the sender. Forms to be
supported include a simple name string, encoded DN, email address,
etc.
* RetrievalMethod is a URI which may be used to obtain key and/or
certificate information. The URI should contain the complete
string for retrieving the key needed for this message (rather than
a generic URI).
* X509Data contains an identifier of the key/cert used for
validation (either an IssuerSerial value, a subject name, or a
subjectkeyID) and an optional collection of certificates and
revocation/status information which may be used by the recipient.
IssuerSerial contains the encoded issuer name (RFC 2253) along
with the serial number.
* PGPData data associated with a PGP key.
* MgmtData contains in-band key distribution or agreement data.
Examples may include DH key exchange, RSA key encryption etc.
Schema Definition:
<element name='X509Data'>
<type content='elementOnly'>
<group order='seq' minOccurs='1' maxOccurs='1'>
<group order='choice' minOccurs='1' maxOccurs='1'>
<element ref='X509IssuerSerial'/>
<element name='X509SKI' type='string'/>
<element name='X509Name' type='string'/>
</group>
<element name='X509Certificate' type='string' minOccurs='0'
maxOccurs='1'/>
<element name='X509CRL' type='string' minOccurs='0'
maxOccurs='1'/>
<group>
</type>
</element>
<element name='X509IssuerSerial'>
<type content='elementOnly'>
<group order='seq' minOccurs='1' maxOccurs='1'>
<element name='X509Name' type='string' minOccurs='1'
maxOccurs='1'/>
<element name='X509SerialNumber' type='string' minOccurs='1'
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maxOccurs='1'/>
</group>
</type>
</element>
<element name='PGPData'>
<type content='elementOnly'>
<group order='seq' minOccurs='1' maxOccurs='1'>
<element name='PGPKeyID' type='string' minOccurs='1'
maxOccurs='1'/>
<element name='PGPKeyPacket' type='string' minOccurs='1'
maxOccurs='1'/>
</group>
</type>
</element>
DTD:
<!ELEMENT X509Data ((X509IssuerSerial | X509SKI | X509Name),
(X509Certificate | X509CRL)* ) >
<!ELEMENT X509IssuerSerial (X509Name, X509SerialNumber)>
<!ELEMENT X509Name (#PCDATA)>
<!-- Where the name is encoded accroding to RFC 2253 -->
<!ELEMENT X509SerialNumber (#PCDATA)>
<!-- Where the data is the serial number encoded as a decimal integer
-->
<!ELEMENT X509SKI (#PCDATA)>
<!-- Where the data consists of the SKI base64 encoded -->
<!ELEMENT X509Certificate (#PCDATA)>
<!-- Where the data conists of the base64 encoded certificate -->
<!ELEMENT X509CRL (#PCDATA)>
<!-- Where the data consists of the base64 encoded CRL -->
<!ELEMENT PGPData (PGPKeyID, PGPKeyPacket?)>
<!ELEMENT PGPKeyID (#PCDATA)>
<!-- Where the data conists of the hex encoding of the key ID. -->
<!ELEMENT PGPKeyPacket (#PCDATA)>
<!-- Where the data consists of the base64 encoded key packet --->
<!ELEMENT MgmtData (#PCDATA)>
3.5 The Object Element
Object is an optional element which may occur one or more times. When
present, this element may contain any data. The Object element may
include optional type, ID, and encoding attributes.
The Object's ID is commonly referenced from an Reference in
SignedInfo, Manifest or Package. This element is typically used for
envoloping signatures where the object being signed is to be included
in the signature document. 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 acual 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
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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' >
<type content='mixed'>
<attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/>
<attribute name='Type' type='uri' minOccurs='0' maxOccurs='1'/>
<attribute name='Encoding' type='uri' minOccurs='0'
maxOccurs='1'/>
</type>
</element>
DTD:
<!ELEMENT Object ANY>
<!ATTLIST Object
Id ID #IMPLIED
Type CDATA #IMPLIED
Encoding CDATA #IMPLIED >
<!-- Where type and encoding CDATA conforms to the
productions specified by [URI] -->
3.6 The Parameter Element
Algorithms are provided with parameters and input data, when
necessary, by having Parameter elements in the content of the
algorithm element. Algorithms also have implicit input, such as the
canonicalized SignedInfo for SignatureMethod and the transformed data
for DigestMethod.
Where more than one Parameter appears, they are passed to the
algorithm as an ordered vector corresponding to the order they appear
in the algorithm element content.
Schema Definition:
<element name='Parameter'>
<type content='mixed'>
<attribute name='Encoding' type='uri' minOccurs='0'
maxOccurs='1'/>
</type>
</element>
DTD:
<!ELEMENT Parameter #PCDATA>
<!ATTLIST Parameter
Encoding CDATA #IMPLIED >
<!-- Encoding CDATA conforms to the productions
specified by [URI] -->
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4.0 Additional Signature Syntax
This section describes the optional to implement Manifest,
SignatureProperties, and Package elements and describes the handling
of XML Processing Instructions and Comments. With respect to the
elements Manifest, SignatureProperties, and Package, this section
specifies syntax and little behaviour -- 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.
4.1 The Manifest and Package Elements
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.
A Package is syntactically identical to a Manifest, and may appear
anywhere a Manifest may appear, but asserts the identity of each of
its Reference elements after Transforms application. The testing of
this relationship and consequent action is at the discretion of the
applicaiton.
Schema Definition:
<element name='Manifest'>
<type content='elementOnly'>
<group order='seq' minOccurs='1' maxOccurs='1'>
<element ref='Reference' minOccurs='1' maxOccurs='*'/>
<element ref='Object' minOccurs='0' maxOccurs='*'/>
</group>
<attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/>
</type>
</element>
<element name='Package'>
<type content='elementOnly'>
<group order='seq' minOccurs='1' maxOccurs='1'>
<element ref='Reference' minOccurs='1' maxOccurs='*'/>
<element ref='Object' minOccurs='0' maxOccurs='*'/>
</group>
<attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/>
</type>
</element>
DTD:
<!ELEMENT Manifest ( (Reference | Object )+ ) >
<!ATTLIST Manifest
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Id ID #IMPLIED >
<!ELEMENT Package ( (Reference | Object )+ ) >
<!ATTLIST Package
Id ID #IMPLIED >
4.2 The SignatureProperties Element
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'>
<type content='elementOnly'>
<element ref='SignatureProperty' minOccurs='1' maxOccurs='*'/>
<attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/>
</type>
</element>
<element name='SignatureProperty'>
<type content='mixed'>
<attribute name='Target' type='IDREF' minOccurs='1'
maxOccurs='1'/>
</type>
</element>
DTD:
<!ELEMENT SignatureProperties SignatureProperty >
<!ATTLIST SignatureProperties
Id ID #IMPLIED>
<!ELEMENT SignatureProperty ANY >
<!ATTLIST SignatureProperty
Target IDREF #REQUIRED >
4.3 Processing Instructions
TDB - will specify the use, if any, of XML processing instructions by
this specification and the handling of PIs appearing within elements
specified in this document.
4.4 Comments in dsig Elements
TDB - will specify the use, if any, and handling of XML comments
appearing within elements specified in this document.
5.0 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.
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5.1 Algorithm Identifiers, Parameters, 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 (or SignedInfo
directly if there is no 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
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.
Algorithm Type Algorithm Requirements Algorithm URI
Digest
SHA1 REQUIRED http://www.w3.org/2000/01/xmldsig/sha1
Encoding
Base64 REQUIRED http://www.w3.org/2000/01/xmldsig/base64
QuotedPrintable RECOMMENDED http://www.w3.org/2000/01/xmldsig/qp
MAC
HMAC-SHA1 REQUIRED http://www.w3.org/2000/01/xmldsig/hmac-sha1
Signature
DSAwithSHA1 (DSS) REQUIRED http://www.w3.org/2000/01/xmldsig/dsa
RSAwithSHA1 RECOMMENDED http://www.w3.org/2000/01/xmldsig/rsa-sha1
ECDSAwithSHA1 OPTIONAL http://www.w3.org/2000/01/xmldsig/ecdsa
Canonicalization
minimal REQUIRED http://www.w3.org/2000/01/xmldsig/minimal
XML-Canonicalization RECOMMENDED
http://www.w3.org/TR/1999/WD-xml-c14n-19991115
Transform
XSLT RECOMMENDED http://www.w3.org/TR/1999/REC-xslt-19991116
XPath RECOMMENDED http://www.w3.org/TR/1999/REC-xpath-19991116
XPointer RECOMMENDED http://www.w3.org/TR/1999/WD-xptr-19991206
Java OPTIONAL urn:ECMA-org:java
Note that the normative identifier is the complete URIs in the table
though they are frequently abbreviated in XML syntax as
"&dsig;/hmac".
5.2 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.
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Digest algorithms take as an implicit parameter a byte string to be
digested.
5.2.1 SHA-1
Identifier:
http://www.w3.org/2000/01/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. 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>
5.3 Message Authentication Codes
MAC algorithms take two implicit parameters, their keying material
determined from KeyInfo and the byte stream output by
CanonicalizationMethod or SignedInfo directly if there is no
CanonicalizationMethod. MACs and signature algorithms are
syntactically identical but a MAC implies a shared secret key.
5.3.1 HMAC
Identifier:
http://www.w3.org/2000/01/xmldsig/hmac-sha1
The HMAC algorithm [RFC2104:HMAC] takes the truncation length in bits
as a parameter. An example of an HMAC SignatureMethod element:
<SignatureMethod Algorithm="&dsig;/hmac-sha1">
<hmac-outputlength xmlns="&dsig;/hmac-sha1">
128
</hmac-outputlength>
</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-MD5 digest
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9294727A 3638BB1C 13F48EF8 158BFC9D
from the test vectors in [HMAC] would be
<SignatureValue>kpRyejY4uxwT9I74FYv8nQ==</SignatureValue>
5.4 Signature Algorithms
Signature algorithms take two implicit parameters, their keying
material determined from KeyInfo and the byte stream output by
CanonicalizationMethod or SignedInfo directly if there is no
CanonicalizationMethod. Signature and MAC algorithms are syntactically
identical but a signature implies public key cryptography.
Note: the schema and DTD declarations within this section are not yet
part of sec-9: schemas.
5.4.1 DSA
Identifier:
http://www.w3.org/2000/01/xmldsig/dsa
The DSA algorithm [DSA] 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 shall consist 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 shall be done according to the
I2OSP operation defined in the RFC 2437 [RSA] specification with a k
parameter equal to 20. 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==</SignatureVa
lue>
DSA key values have the following set of fields: P, Q, G and Y are
mandatory when appearing as a key value, J, seed and pgenCounter are
optional but SHOULD be present. (The seed and pgenCounter fields MUST
both either appear or be absent). All parameters are encoded as base64
values.
Schema:
<element name='DSSKeyValue'>
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<type content='elementOnly'>
<group order='seq' minOccurs='1' maxOccurs='1'>
<element name='P' type='string' minOccurs='1' maxOccurs='1'/>
<element name='Q' type='string' minOccurs='1' maxOccurs='1'/>
<element name='G' type='string' minOccurs='1' maxOccurs='1'/>
<element name='Y' type='string' minOccurs='1' maxOccurs='1'/>
<element name='J' type='string' minOccurs='0' maxOccurs='1'/>
</group>
<group order='seq' minOccurs='0' maxOccurs='1'>
<element name='seed' type='string' minOccurs='1' maxOccurs='1'/>
<element name='pgenCounterQ' type='string' minOccurs='1'
maxOccurs='1'/>
</group>
</type>
</element>
DTD:
<!ELEMENT DssKeyValue (P, Q, G, Y, J?, (seed, pgenCounter)?) >
<!-- Each of these fields consists a CDATA
where the data is base64 encoded -->
5.4.2 RSA
Identifier:
http://www.w3.org/2000/01/xmldsig/rsa-sha1
The expression "RSA algorithm" as used in this document refers to the
RSASSA-PKCS1-v1_5 algorithm described in RFC 2437 [RSA]. The RSA
algorithm takes no parameters. An example of an RSA SignatureMethod
element is:
<SignatureMethod Algorithm="&dsig;/rsa-sha1"/>
The output of the RSA algorithm is an octet string. The SignatureValue
content for an RSA signature shall be the base64 encoding of this
octet string. Example: TBD
RSA key values have two fields: Modulus and Exponent.
Schema:
<element name='RSAKeyValue'>
<type content='elementOnly'>
<element name='Modulus' type='string' minOccurs='1'
maxOccurs='1'/>
<element name='Exponent' type='string' minOccurs='1'
maxOccurs='1'/>
</type>
</element>
DTD:
<!ELEMENT RSAKeyValue ( Modulus, Exponent ) >
<!-- Each field contains a CDATA which is the
value for that item base64 encoded -->
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5.4.3 ECDSA
The expression ECDSA [ECDSA] as used in this document refers to the
signature algorithms specified in ANSI X9.62. Additional details are
to be provided.
5.5 Canonicalization Algorithms
5.5.1 Null Canonicalization
Null canonicalization, i.e., no modification whatsoever, can be
achieved for digested data by simply not putting any canonicalization
in the Transforms element (omitting it entirely if no other tranforms
are needed) for a data object or omitting CanonicalizationMethod for
SignedInfo.
5.5.2 Minimal Canonicalization
Identifier:
http://www.w3.org/2000/01/xmldsig/minimal
The algorithm identifier for the minimal canonicalization is
&dsig;/minimal. An example of a minimal canonicalization element is:
<CanonicalizationMethod Algorithm="&dsig;/minimal"/>
The minimal canonicalization algorithm:
* converts the character encoding to UTF-8, removing the encoding
pseudo-attribute
* normalizes line endings
5.5.3 Canonical XML
Identifier:
http://www.w3.org/TR/1999/WD-xml-c14n-19991115
An example of an XML canonicalization element is:
<CanonicalizationMethod
Algorithm="http://www.w3.org/TR/1999/WD-xml-c14n-19991115"/>
The normative specificatin of Canonical XML is [XML-c14n].
5.6 Transform Algorithms
A Transform algorithm has three implicit parameters. The first is a
byte stream from the Reference or as the output of an earlier
Transform. The second and third are the optional MimeType and Charset
attributes that can be specified on the Transform element.
Application developers are strongly encouraged to support all
transforms listed in this section as RECOMMENDED unless the
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application environment has resource constraints that would make such
support impractical. The working group goal is to maximize application
interoperability on XML signatures, and the working group expects
ubiquitous availability of software to support these transforms that
can be incorporated into applications without extensive development.
5.6.1 Canonicalization
Any canonicalization algorithm that can be used for
CanonicalizationMethod can be used as a Transform.
5.6.2 Base-64 and Quoted-Printable Decoding
Identifiers:
http://www.w3.org/2000/01/xmldsig/base-64
http://www.w3.org/2000/01/xmldsig/qp
The normative specification for base 64 and quoted-printable decoding
transforms is [MIME]. The base-64 Transform element has no content.
The input is base-64 decoded by this algorithm. This transform is
useful if an application needs to sign the raw data associated with
base-64 encoded content of an element.
5.6.3 XPath Filtering
Identifier:
http://www.w3.org/TR/1999/REC-xpath-19991116
The Transform element content MUST conform to the XML Path Language
[XPath] syntax. XPath is a language for addressing parts of an XML
document. Hence, an XPath expression MUST be applied to an entire
well-formed XML document.
Note: The current output of a Reference's IDREF cannot be used as
input to an XPath transform. The XPath transform could be defined to
provide an XML declaration when one is found not to exist since the
encoding attribute could be set equal to the XPath transform's Charset
attribute. However, there is currently no way to communicate the
correct byte order mark to the transform. For security reasons, a
default cannot be selected.
The XPath transform applies the W3C XML canonicalization [XML-C14N] to
the input resource. This ensures all entity reference substitutions
and attribute normalizations are performed in a manner consistent with
a validating XML processor. Linefeeds are normalized, and CDATA
sections are eliminated. The types of quotes around attributes are
normalized, and the order of attributes is defined. Namespace
attributes are created in descendant elements that use namespace
definitions. All of these modifications are necessary to achieve a
consistent interpretation of the XPath expression and a consistent
output of the XPath transform.
Finally, the XPath expression is evaluated assuming that the entity
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references created by canonicalization have been replaced by the
corresponding entity values and that each block of consecutive text
characters has been replaced by a single text node.
The result of the XPath is a string, boolean, number, or node-set. If
the result of the XPath expression is a string, then the string is the
output of the XPath transform. If the XPath result is a boolean or
number, then the result is converted to a string using the XPath
string() function. If the result of the XPath expression is a
node-set, then the output of the transform is a string containing the
text rendering of the nodes in the node-set. The nodes are selected
for rendering based on the document order (as defined in [XPath]) of
the canonicalized input resource. The text rendering is performed in
accordance with [XML-C14N].
It is RECOMMENDED that the XPath be constructed such that the result
of this operation is a well-formed XML document. This should be the
case if root element of the input resource is included by the XPath
(even if a number of its descendant elements and attributes are
omitted by the XPath).
5.6.4 XPointer Filtering
Identifier:
http://www.w3.org/TR/1999/WD-xptr-19991206
The Transform element content MUST conform to the XML Pointer Language
[XPointer] syntax.
The processing rules for XPointer filtering are identical to those for
XPath filtering (stated above), except that the additional
functionality offered by XPointer can be utilized in constructing the
output.
5.6.5 XSLT Transform
Identifier:
http://www.w3.org/TR/1999/REC-xslt-19991116
The Transform element content MUST conform to the XSL Transforms
[XSLT] language syntax. The processing rules for the XSLT transform
are stated in the XSLT specification [XSLT].
5.6.6 Java Transform
The Algorithm value for the Java transform is urn:ECMA-org:java.
Details to be determined.
Although the Algorithm attribute of a Transform can take
application-specific values, having a Java transform seems to be the
most reasonable way to allow application-specific transforms that can
be processed outside of the application domain.
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6.0 Processing rules
These sections describe the operations to be performed as part of
signature generation and validation. The description is of a logical
behavior and does not specify an order of execution, nor specify
discrete steps.
6.1 Generation
1. apply Transforms determined by application to each object being
signed.
2. calculate digest over each transformed object
3. create Reference element(s) including location of object, digest,
digest algorithm, and transform elements, if required.
4. create SignedInfo element with SignatureMethod,
CanonicalizationMethod if required, and Reference(s).
5. canonicalize and calculate signature over SignedInfo based on
algorithms in step 4.
6. construct signature document with SignedInfo, Object (s) (if
desired, encoding may be different than that used for signing),
KeyInfo (if required), and SignatureValue.
6.2 Core Validation
Validating an XML signature consists of two mandatory processing
steps. These are signature validation, the cryptographic validation of
the signature calculated over SignedInfo; and reference validation,
the verification of the digest contained in each Reference in
SignedInfo. Both steps MUST be performed as part of all XML signature
validations.
6.2.1 Signature Validation
1. canonicalize the SignedInfo element based on the
CanonicalizationMethod, if any, in SignedInfo.
2. obtain the validation keying info from KeyInfo or externally.
3. validate the SignatureValue based on the SignatureMethod in the
SignedInfo element, the key obtained in step 2, and the results of
step 1. Digest calculation is performed over the SignedInfo
element including start and end tags.
6.2.2 Reference Validation
1. For each object reference in SignedInfo, obtain digested content
(this may be obtained by locating object and applying Transforms
to the specified resource based on each Reference(s) in the
SignedInfo element. Each transform is applied in order from left
to right to the object with the output of each transform being the
input to the next.).
2. calculate digest over each transformed signed object(s) based on
the algorithm in Reference(s).
3. compare value against DigestValue in SignedInfo for each reference
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(if any mismatch, validation fails).
Any processing beyond cryptographic validation (e.g. certificate
validation, applicability decisions, time related processing) is
outside the scope of this specification.
7.0 Security Considerations
The XML Signature specification provides a very flexible digital
signature mechanism. Implementors must give consideration to the
application threat models and to the following factors.
7.1 Only What is Signed is Secure
A requirement of this specification is to permit signatures to "apply
to a part or totality of a XML document." [3.1.3 XML-Signature-RD] The
Transforms mechanism meets this requirement by permitting one to sign
a document derived from processing the identified source document. For
instance, applications that wish to sign a form, but permit users to
enter field data without invalidating the form itself might use XPath
[XPath] to select only those portions the user does not change.
However, Transforms may be arbitrarly specified and may include
canonicalization instructions or even XSLT transformations. We stress
that the signature is placed over the derived document, and those
portions that were excluded by transformation can be arbitrarily
modified and the signature will still validate! This is a feature,
though one that is used at the application's risk. (Some applications
may not be willing to trust such signatures all-together.)
Furthermore, core validation behaviour does not confirm that the
signed resource was obtained by applying transforms to the specified
source document. This behaviour is left to the application as core
validation only checks the digest values of the source document and
the signature over SignedInfo. If this fact is important, then
additional information (such as by including References to both the
original and transformed documents) is needed.
7.2 Only What is "Seen" Should be Signed
If signing is intended to convey the judgment or consent of an
automated mechanism or person concerning some information, then it is
normally necessary to secure as exactly as practical the information
that was presented to that mechanism or person. Note that this can be
accomplished by literally signing what was presented, for example the
screen images shown a user. However, this may result in data which it
is difficult for subsequent software to manipulate. It can be
effective instead to secure the full data along with whatever filters,
style sheets, or the like were used to control the part of the
information that was presented.
7.3 Check the Security Model
This standard specifies public key signatures and secret key keyed
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hash authentication codes. These have substantially different security
models. Furthermore, it permits user specified additions 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 secret key
can create signatures. The number of holders of the secret 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 secret key is an
important issue, usually addressed by certificate or online authority
systems.
Keyed hash authentication codes, based on secret keys, are typically
much more efficient in terms of the computational effort required but
have the characteristic that all verifiers need to have possession of
the same key as the signer. Thus any verifier can forge signatures.
This standard permits user provided signature algorithms and keying
information designators. Such user provided algorithms may have
further different security models. For example, methods involving
biometrics usually depend on a "key" which is a physical
characteristic of the user and thus can not be changed the way public
or secret keys can be and may have other security model differences.
7.4 Key Lengths, Algorithms, 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 [RFC1750] and the size of the
key, the security of key and certificate authentication and
distribution mechanisms, protection of all cryptographic processing
from hostile observation and tampering, etc. The security of an
overall system would also depend on the security and integrity of its
operating procedures, its personnel, and on the administrative
enforcement of those procedures. The factors listed in this paragraph,
while critical to the overall security of a system, are mostly beyond
the scope of this document.
8.0 Example syntax
<Signature xmlns="http://www.w3.org/1999/11/xmldsig-core">
<SignedInfo ID="5">
<CanonicalizationMethod
Algorithm="http://www.w3.org/1999/07/WD-xml-c14n-19990729"/>
<SignatureMethod Algorithm="&dsig;/dsa"/>
<Reference URI="http://www.mypage.com">
<Transforms>
<Transform Algorithm="&dsig;/null">
<Encoding Algorithm="&dsig;/base64"/>
</Transforms>
<DigestMethod Algorithm="&dsig;/sha1"/>
<DigestValue>a23bcd43</DigestValue>
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</Reference>
<Reference IDREF="timestamp"
Type="&dsig;/signatureattributes">
<Transforms>
<CanonicalizationMethod name="http://..."/>
</Transforms>
<DigestMethod Algorithm="&dsig;/sha1"/>
<DigestValue>a53uud43</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>dd2323dd</SignatureValue>
<Object ID="timestamp"
type="&dsig;/SignatureProperties" >
<SignatureProperties>
<SignatureProperty>
<timestamp about="5"
xmlns="http://www.ietf.org/rfcXXXX.txt">
<date>19990908</date>
<time>14:34:34:34</time>
</timestamp>
</SignatureProperty>
</SignatureProperties>
</Object>
<KeyInfo>
<keyname>Solo</keyname>
</KeyInfo>
</Signature>
9.0 Schema
http://www.w3.org/TR/2000/WD-xmldsig-core-20000104/xmldsig-core-schema
.xml
10 Definitions
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] See
non-repudiation.
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.
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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 Objectdesignates 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
The inability to change a message without also changing the
signature value. See message authentication.
Non-repudiation
The inability of a key holder to assert that their key was not
associated with a message given a strong signature algorithm.
(This definition speaks nothing of the number of key holders,
the key length, whether the key is comprised, whether the
signature was coerced, etc.) See signer 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
A value generated from the application of a key to a message
via a cryptographic algorithm such that it has the properties
of signer authentication, integrity, and non-repudiation.
Signature, Detached
The signature is over external content identified via a URI.
Cosequently, the signature is "detached" from the content it
signs.
Signature, Enveloping
The signature is over content found within the signature itself
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via an IDREF to an Object element. The Signature provides the
root XML document element.
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
care not to include their own value in the calculation of the
SignatureValue.
Transform
The processing of a byte 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 DigetsValue.
Validation, Signature
The SignatureValue matches the result of processing SignedInfo
with CanonicalizationMethod and SignatureMethod as specified
in 6.2.
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 behvaiour is external to this core specification.
11.0 Other Useful Types (normative)
We define the following URIs for use in identifying XML resources that
include non-core but signature related semantics.
http://www.w3.org/1999/11/xmldsig-core/SignatureProperties
designates that the referenced resource is a statement about
the referring signature.
http://www.w3.org/1999/11/xmldsig-core/Manifest
designates that the referenced resource is a collection of
other resources.
http://www.w3.org/1999/11/xmldsig-core/Package
designates that the referenced resources is a collection of
other resources and the creator of that collection asserts that
the specified resources, when transformed as specified, yield
the same exact content.
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12.0 References
ABA
Digital Signature Guidelines.
http://www.abanet.org/scitech/ec/isc/dsgfree.html
DOMHASH
Internet Draft. Digest Values for DOM (DOMHASH)
http://search.ietf.org/internet-drafts/draft-hiroshi-dom-hash-0
1.txt .
DSS
FIPS PUB 186-1. Digital Signature Standard (DSS). U.S.
Department of Commerce/National Institute of Standards and
Technology.
http://www.ietf.org/rfc/rfc2104.txt
ECSDA
?ANSI X9.62
HMAC
RFC 2104. HMAC: Keyed-Hashing for Message Authentication. H.
Krawczyk, M. Bellare, R. Canetti. INFORMATIONAL.
HTTP
RFC 2616.Hypertext Transfer Protocol -- HTTP/1.1. J. Gettys, J.
Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee.
http://www.ietf.org/rfc/rfc2616.txt
MD5
RFC 1321. The MD5 Message-Digest Algorithm. R. Rivest.
INFORMATIONAL.
http://www.ietf.org/rfc/rfc1321.txt
MIME
RFC 2045. Multipurpose Internet Mail Extensions (MIME) Part
One: Format of Internet Message Bodies. N. Freed & N.
Borenstein. DRAFT STANDARD.
http://www.ietf.org/rfc/rfc2045.txt
RDF
RDF Schema
http://www.w3.org/TR/1999/PR-rdf-schema-19990303
RDF Model and Syntax
http://www.w3.org/TR/1999/REC-rdf-syntax-19990222
RFC1750
RFC1750 -- Randomness Recommendations for Security.
http://www.ietf.org/rfc/rfc1750.txt
RFC2119
RFC2119 -- Key words for use in RFCs to Indicate Requirement
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Levels.
http://www.ietf.org/rfc/rfc2119.txt
RSA
RFC 2437. PKCS #1: RSA Cryptography Specifications Version 2.0.
B. Kaliski, J. Staddon. INFORMATIONAL.
http://www.ietf.org/rfc/rfc2432.txt
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
URI
RFC2396 - Uniform Resource Identifiers (URI): Generic Syntax
http://www.ietf.org/rfc/rfc2396.txt
URL
RFC1738. Uniform Resource Locators (URL). Berners-Lee, T.,
Masinter, L., and M. McCahill . December 1994.
http://www.ietf.org/rfc/rfc1738.txt
URN
RFC 2141. URN Syntax. R. Moats. PROPOSED STANDARD.
ftp://ftp.isi.edu/in-notes/rfc2141.txt
RFC 2611. URN Namespace Definition Mechanisms. L. Daigle, D.
van Gulik, R. Iannella, P. Falstrom. BEST CURRENT PRACTICE.
ftp://ftp.isi.edu/in-notes/rfc2611.txt
XLink
XML Linking Language
http://www.w3.org/1999/07/WD-xlink-19990726
XML
Extensible Markup Language (XML) Recommendation.
http://www.w3.org/TR/1998/REC-xml-19980210
XML-c14n
Canonical XML. W3C Working Draft
http://www.w3.org/TR/1999/WD-xml-c14n-19991115
XML-ns
Namespaces in XML
http://www.w3.org/TR/1999/REC-xml-names-19990114
XPath
XML Path Language (XPath)Version 1.0. W3C Proposed
Recommendation
http://www.w3.org/TR/1999/PR-xpath-19991008
XPointer
XML Pointer Language (XPointer). W3C Working Draft.
http://www.w3.org/1999/07/WD-xptr-19990709
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XML-schema
XML Schema Part 1: Structures
http://www.w3.org/TR/1999/WD-xmlschema-1-19991217/
XML Schema Part 2: Datatypes
http://www.w3.org/TR/1999/WD-xmlschema-2-19991217/
XML-Signature-RD
XML-Signature Requirements
http://www.w3.org/1999/08/WD-xmldsig-requirements-990820
XSL
Extensible Stylesheet Language (XSL) W3C Working Draft
http://www.w3.org/TR/1999/WD-xsl-19990421
XSLT
XSL Transforms (XSLT) Version 1.0. W3C Proposed Recommendation
http://www.w3.org/TR/1999/PR-xslt-19991008
WebData
Web Architecture: Describing and Exchanging Data.
http://www.w3.org/1999/04/WebData
13.0 Acknowledgements (non-normative)
* Milton Anderson, FSTC
* Mark Bartel, JetForm Corporation (Author)
* John Boyer, UWI.com (Author)
* Richard Brown, Globeset
* Donald Eastlake 3rd, IBM (Chair, Editor)
* Barb Fox, Microsoft (Author)
* Phillip Hallam-Baker, VeriSign Inc
* Richard Himes, US Courts
* Joseph Reagle, W3C (Chair, Editor)
* Ed Simon , Entrust Technologies Inc.
* Chris Smithies, PenOp
* David Solo, Citigroup (Editor)
* Winchel Todd Vincent III, GSU
* Greg Whitehead, Signio Inc.
14.0 Open Issues (non-normative)
1. More detail for KeyInfo types, based on IETF'46, we need proposals
for the actual XML'ized algorithm parameters.
2. Make sure we are consistent with respect to types, algorithm IDs,
URIs, etc.
3. The signature data structures specified in this document are not
yet associated with a data model.
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