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Keywords: extensible, markup, language
Network Working Group D. Eastlake
Request for Comments: 3075 Motorola
Category: Standards Track J. Reagle
W3C/MIT
D. Solo
Citigroup
March 2001
XML-Signature Syntax and Processing
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2001 The Internet Society & W3C (MIT, INRIA, Keio), All
Rights Reserved.
Abstract
This document specifies XML (Extensible Markup Language) 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 ................................................ 3
1. Editorial Conventions .................................. 3
2. Design Philosophy ...................................... 4
3. Versions, Namespaces and Identifiers ................... 4
4. Acknowledgements ....................................... 5
2. Signature Overview and Examples ............................. 6
1. Simple Example (Signature, SignedInfo, Methods, and
References) ............................................ 7
1. More on Reference ................................. 9
2. Extended Example (Object and SignatureProperty) ........ 10
3. Extended Example (Object and Manifest) ................. 11
3. Processing Rules ............................................ 13
1. Core Generation .... ................................... 13
1. Reference Generation .............................. 13
2. Signature Generation .............................. 13
Eastlake, et al. Standards Track [Page 1]
RFC 3075 XML-Signature Syntax and Processing March 2001
2. Core Validation ........................................ 13
1. Reference Validation .............................. 14
2. Signature Validation .............................. 14
4. Core Signature Syntax ....................................... 14
1. The Signature element .................................. 15
2. The SignatureValue Element ............................. 16
3. The SignedInfo Element ................................. 16
1. The CanonicalizationMethod Element ................ 17
2. The SignatureMethod Element ....................... 18
3. The Reference Element ............................. 19
1. The URI Attribute ............................ 19
2. The Reference Processing Model ............... 21
3. Same-Document URI-References ................. 23
4. The Transforms Element ....................... 24
5. The DigestMethod Element ..................... 25
6. The DigestValue Element ...................... 26
4. The KeyInfo Element .................................... 26
1. The KeyName Element ............................... 27
2. The KeyValue Element .............................. 28
3. The RetrievalMethod Element ....................... 28
4. The X509Data Element .............................. 29
5. The PGPData Element ............................... 31
6. The SPKIData Element .............................. 32
7. The MgmtData Element .............................. 32
5. The Object Element ..................................... 33
5. Additional Signature Syntax ................................. 34
1. The Manifest Element ................................... 34
2. The SignatureProperties Element ........................ 35
3. Processing Instructions ................................ 36
4. Comments in dsig Elements .............................. 36
6. Algorithms .................................................. 36
1. Algorithm Identifiers and Implementation Requirements .. 36
2. Message Digests ........................................ 38
1. SHA-1 ............................................. 38
3. Message Authentication Codes ........................... 38
1. HMAC .............................................. 38
4. Signature Algorithms ................................... 39
1. DSA ............................................... 39
2. PKCS1 ............................................. 40
5. Canonicalization Algorithms ............................ 42
1. Minimal Canonicalization .......................... 43
2. Canonical XML ..................................... 43
6. Transform Algorithms ................................... 44
1. Canonicalization .................................. 44
2. Base64 ............................................ 44
3. XPath Filtering ................................... 45
4. Enveloped Signature Transform ..................... 48
5. XSLT Transform .................................... 48
Eastlake, et al. Standards Track [Page 2]
RFC 3075 XML-Signature Syntax and Processing March 2001
7. XML Canonicalization and Syntax Constraint Considerations ... 49
1. XML 1.0, Syntax Constraints, and Canonicalization ..... 50
2. DOM/SAX Processing and Canonicalization ................ 51
8. Security Considerations ..................................... 52
1. Transforms ............................................. 52
1. Only What is Signed is Secure ..................... 52
2. Only What is "Seen" Should be Signed .............. 53
3. "See" What is Signed .............................. 53
2. Check the Security Model ............................... 54
3. Algorithms, Key Lengths, Etc. .......................... 54
9. Schema, DTD, Data Model,and Valid Examples .................. 55
10. Definitions ................................................. 56
11. References .................................................. 58
12. Authors' Addresses .......................................... 63
13. Full Copyright Statement .................................... 64
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. More specifically, this specification defines an
XML signature element type and an XML signature application;
conformance requirements for each are specified by way of schema
definitions and prose respectively. This specification also includes
other useful types that identify methods for referencing collections
of resources, algorithms, and keying and management information.
The XML Signature is a method of associating a key with referenced
data (octets); it does not normatively specify how keys are
associated with persons or institutions, nor the meaning of the data
being referenced and signed. Consequently, while this specification
is an important component of secure XML applications, it itself is
not sufficient to address all application security/trust concerns,
particularly with respect to using signed XML (or other data formats)
as a basis of human-to-human communication and agreement. Such an
application must specify additional key, algorithm, processing and
rendering requirements. For further information, please see Security
Considerations (section 8).
1.1 Editorial and Conformance Conventions
For readability, brevity, and historic reasons this document uses the
term "signature" to generally refer to digital authentication values
of all types.Obviously, the term is also strictly used to refer to
Eastlake, et al. Standards Track [Page 3]
RFC 3075 XML-Signature Syntax and Processing March 2001
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 Check the Security
Model, section 8.3.)
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/2000/09/xmldsig#"
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This namespace is also used as the prefix for algorithm identifiers
used by this specification. While applications MUST support XML and
XML-namespaces, the use of internal entities [XML] or our "dsig" XML
namespace prefix and defaulting/scoping conventions are OPTIONAL; we
use these facilities to provide compact and readable examples.
This specification uses Uniform Resource Identifiers [URI] to
identify resources, algorithms, and semantics. The URI in the
namespace declaration above is also used as a prefix for URIs under
the control of this specification. For resources not under the
control of this specification, we use the designated Uniform Resource
Names [URN] or Uniform Resource Locators [URL] defined by its
normative external specification. If an external specification has
not allocated itself a Uniform Resource Identifier we allocate an
identifier under our own namespace. For instance:
SignatureProperties is identified and defined by this specification's
namespace
http://www.w3.org/2000/09/xmldsig#SignatureProperties
XSLT is identified and defined by an external URI
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/2000/09/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] within URIs. For instance:
<?xml version='1.0'?>
<!DOCTYPE Signature SYSTEM
"xmldsig-core-schema.dtd" [ <!ENTITY dsig
"http://www.w3.org/2000/09/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
Eastlake, et al. Standards Track [Page 5]
RFC 3075 XML-Signature Syntax and Processing March 2001
* 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
* Merlin Hughes, Baltimore
* Gregor Karlinger, IAIK TU Graz
* Brian LaMacchia, Microsoft
* Peter Lipp, IAIK TU Graz
* Joseph Reagle, W3C (Chair, Author/Editor)
* Ed Simon, Entrust Technologies Inc. (Author)
* David Solo, Citigroup (Author/Editor)
* Petteri Stenius, DONE Information, Ltd
* Raghavan Srinivas, Sun
* Kent Tamura, IBM
* Winchel Todd Vincent III, GSU
* Carl Wallace, Corsec Security, Inc.
* Greg Whitehead, Signio Inc.
As are the last call comments from the following:
* Dan Connolly, W3C
* Paul Biron, Kaiser Permanente, on behalf of the XML Schema WG.
* Martin J. Duerst, W3C; and Masahiro Sekiguchi, Fujitsu; on
behalf of the Internationalization WG/IG.
* Jonathan Marsh, Microsoft, on behalf of the Extensible
Stylesheet Language WG.
2.0 Signature Overview and Examples
This section provides an overview and examples of XML digital
signature syntax. The specific processing is given in Processing
Rules (section 3). The formal syntax is found in Core Signature
Syntax (section 4) and Additional Signature Syntax (section 5).
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
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RFC 3075 XML-Signature Syntax and Processing March 2001
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>
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/2000/09/xmldsig#">
[s02] <SignedInfo>
[s03] <CanonicalizationMethod
Algorithm="http://www.w3.org/TR/2000/CR-xml-c14n-20001026"/>
[s04] <SignatureMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/>
[s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/">
[s06] <Transforms>
[s07] <Transform Algorithm="http://www.w3.org/TR/2000/
CR-xml-c14n-20001026"/>
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[s08] </Transforms>
[s09] <DigestMethod Algorithm="http://www.w3.org/2000/09/
xmldsig#sha1"/>
[s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue>
[s11] </Reference>
[s12] </SignedInfo>
[s13] <SignatureValue>MC0CFFrVLtRlk=...</SignatureValue>
[s14] <KeyInfo>
[s15a] <KeyValue>
[s15b] <DSAKeyValue>
[s15c] <P>...</P><Q>...</Q><G>...</G><Y>...</Y>
[s15d] </DSAKeyValue>
[s15e] </KeyValue>
[s16] </KeyInfo>
[s17] </Signature>
[s02-12] The required SignedInfo element is the information that is
actually signed. Core validation of SignedInfo consists of two
mandatory processes: validation of the signature over SignedInfo and
validation of each Reference digest within SignedInfo. Note that the
algorithms used in calculating the SignatureValue are also included
in the signed information while the SignatureValue element is outside
SignedInfo.
[s03] The CanonicalizationMethod is the algorithm that is used to
canonicalize the SignedInfo element before it is digested as part of
the signature operation.
[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.
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[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/
CR-xml-c14n-20001026"/>
[s08] </Transforms>
[s09] <DigestMethod Algorithm="http://www.w3.org/2000/09/
xmldsig#sha1"/>
[s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue>
[s11] </Reference>
[s05] The optional URI attribute of Reference identifies the data
object to be signed. This attribute may be omitted on at most one
Reference in a Signature. (This limitation is imposed in order to
ensure that references and objects may be matched unambiguously.)
[s05-08] This identification, along with the transforms, is a
description provided by the signer on how they obtained the signed
data object in the form it was digested (i.e., the digested content).
The verifier may obtain the digested content in another method so
long as the digest verifies. In particular, the verifier may obtain
the content from a different location such as a local store than that
specified in the URI.
[s06-08] Transforms is an optional ordered list of processing steps
that were applied to the resource's content before it was digested.
Transforms can include operations such as canonicalization,
encoding/decoding (including compression/inflation), XSLT and XPath.
XPath transforms permit the signer to derive an XML document that
omits portions of the source document. Consequently those excluded
portions can change without affecting signature validity. For
example, if the resource being signed encloses the signature itself,
such a transform must be used to exclude the signature value from its
own computation. If no Transforms element is present, the resource's
content is digested directly. While we specify mandatory (and
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RFC 3075 XML-Signature Syntax and Processing March 2001
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 assuredby 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].)
[ ] <Signature Id="MySecondSignature" ...>
[p01] <SignedInfo>
[ ] ...
[p02] <Reference URI="http://www.w3.org/TR/xml-stylesheet/">
[ ] ...
[p03] <Reference URI="#AMadeUpTimeStamp"
[p04] Type="http://www.w3.org/2000/09/
xmldsig#SignatureProperties">
[p05] <DigestMethod Algorithm="http://www.w3.org/2000/09/
xmldsig#sha1"/>
[p06] <DigestValue>k3453rvEPO0vKtMup4NbeVu8nk=</DigestValue>
[p07] </Reference>
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[p08] </SignedInfo>
[p09] ...
[p10] <Object>
[p11] <SignatureProperties>
[p12] <SignatureProperty Id="AMadeUpTimeStamp"
Target="#MySecondSignature">
[p13] <timestamp xmlns="http://www.ietf.org/rfc3075.txt">
[p14] <date>19990908</date>
[p15] <time>14:34:34:34</time>
[p16] </timestamp>
[p17] </SignatureProperty>
[p18] </SignatureProperties>
[p19] </Object>
[p20]</Signature>
[p04] The optional Type attribute of Reference provides information
about the resource identified by the URI. In particular, it can
indicate that it is an Object, SignatureProperty, or Manifest
element. This can be used by applications to initiate special
processing of some Reference elements. References to an XML data
element within an Object element SHOULD identify the actual element
pointed to. Where the element content is not XML (perhaps it is
binary or encoded data) the reference should identify the Object and
the Reference Type, if given, SHOULD indicate Object. Note that Type
is advisory and no action based on it or checking of its correctness
is required by core behavior.
[p10] Object is an optional element for including data objects within
the signature element or elsewhere. The Object can be optionally
typed and/or encoded.
[p11-18] Signature properties, such as time of signing, can be
optionally signed by identifying them from within a Reference.
(These properties are traditionally called signature "attributes"
although that term has no relationship to the XML term "attribute".)
2.3 Extended Example (Object and Manifest)
The Manifest element is provided to meet additional requirements not
directly addressed by the mandatory parts of this specification. Two
requirements and the way the Manifest satisfies them follows.
First, applications frequently need to efficiently sign multiple data
objects even where the signature operation itself is an expensive
public key signature. This requirement can be met by including
multiple Reference elements within SignedInfo since the inclusion of
each digest secures the data digested. However, some applications
may not want the core validation behavior associated with this
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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/2000/09/xmldsig#Manifest">
[m03] <DigestMethod Algorithm="http://www.w3.org/2000/09/
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] </Manifest>
[m15] </Object>
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3.0 Processing Rules
The sections below describe the operations to be performed as part of
signature generation and validation.
3.1 Core Generation
The REQUIRED steps include the generation of Reference elements and
the SignatureValue over SignedInfo.
3.1.1 Reference Generation
For each data object being signed:
1. Apply the Transforms, as determined by the application, to the
data object.
2. Calculate the digest value over the resulting data object.
3. Create a Reference element, including the (optional)
identification of the data object, any (optional) transform
elements, the digest algorithm and the DigestValue.
3.1.2 Signature Generation
1. Create SignedInfo element with SignatureMethod,
CanonicalizationMethod 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.
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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.
Note, SignedInfo is canonicalized in step 1 to ensure the application
Sees What is Signed, which is the canonical form. For instance, if
the CanonicalizationMethod rewrote the URIs (e.g., absolutizing
relative URIs) the signature processing must be cognizant of this.
3.2.2 Signature Validation
1. Obtain the keying information from KeyInfo or from an external
source.
2. Obtain the canonical form of the SignatureMethod using the
CanonicalizationMethod and use the result (and previously obtained
KeyInfo) to validate the SignatureValue over the SignedInfo
element.
Note, KeyInfo (or some transformed version thereof) may be signed via
a Reference element. Transformation and validation of this reference
(3.2.1) is orthogonal to Signature Validation which uses the KeyInfo
as parsed.
Additionally, the SignatureMethod URI may have been altered by the
canonicalization of SignedInfo (e.g., absolutization of relative
URIs) and it is the canonical form that MUST be used. However, the
required canonicalization [XML-C14N] of this specification does not
change URIs.
4.0 Core Signature Syntax
The general structure of an XML signature is described in Signature
Overview (section 2). This section provides detailed syntax of the
core signature features. 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:
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Schema Definition:
<!DOCTYPE schema
PUBLIC "-//W3C//DTD XMLSCHEMA 200010//EN"
"http://www.w3.org/2000/10/XMLSchema.dtd"
[
<!ATTLIST schema
xmlns:ds CDATA #FIXED "http://www.w3.org/2000/09/xmldsig#">
<!ENTITY dsig 'http://www.w3.org/2000/09/xmldsig#'>
]>
<schema xmlns="http://www.w3.org/2000/10/XMLSchema"
xmlns:ds="&dsig;"
targetNamespace="&dsig;"
version="0.1"
elementFormDefault="qualified">
<!-- Basic Types Defined for Signatures -->
<simpleType name="CryptoBinary">
<restriction base="binary">
<encoding value="base64"/>
</restriction>
</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 % SignatureProperty.ANY '(#PCDATA)*'>
<!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 an XML Signature.
Signature elements MUST be laxly schema valid [XML-schema] with
respect to the following schema definition:
Schema Definition:
<element name="Signature">
<complexType>
<sequence>
<element ref="ds:SignedInfo"/>
Eastlake, et al. Standards Track [Page 15]
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<element ref="ds:SignatureValue"/>
<element ref="ds:KeyInfo" minOccurs="0"/>
<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/2000/09/xmldsig#'
Id ID #IMPLIED >
4.2 The SignatureValue Element
The SignatureValue element contains the actual value of the digital
signature; it is always encoded using base64 [MIME]. While we
specify a mandatory and optional to implement SignatureMethod
algorithms, user specified algorithms 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.
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.
Schema Definition:
<element name="SignedInfo">
<complexType>
<sequence>
<element ref="ds:CanonicalizationMethod"/>
<element ref="ds:SignatureMethod"/>
<element ref="ds:Reference" maxOccurs="unbounded"/>
</sequence>
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<attribute name="Id" type="ID" use="optional"/>
</complexType>
</element>
DTD:
<!ELEMENT SignedInfo (CanonicalizationMethod,
SignatureMethod, Reference+) >
<!ATTLIST SignedInfo
Id ID #IMPLIED>
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 Algorithm Identifiers and
Implementation Requirements (section 6.1). Implementations MUST
support the REQUIRED Canonical XML [XML-C14N] method.
Alternatives to the REQUIRED Canonical XML algorithm (section 6.5.2),
such as Canonical XML with Comments (section 6.5.2) and Minimal
Canonicalization (the CRLF and charset normalization specified in
section 6.5.1), 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 XML Canonicalization
and Syntax Constraint Considerations, section 7). 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).
The way in which the SignedInfo element is presented to the
canonicalization method is dependent on that method. The following
applies to the two types of algorithms specified by this document:
* Canonical XML [XML-C14N] (with or without comments)
implementation MUST be provided with an XPath node-set
originally formed from the document containing the SignedInfo
and currently indicating the SignedInfo, its descendants, and
the attribute and namespace nodes of SignedInfo and its
descendant elements (such that the namespace context and
similar ancestor information of the SignedInfo is preserved).
* Minimal canonicalization implementations MUST be provided with
the octets that represent the well-formed SignedInfo element,
from the first character to the last character of the XML
representation, inclusive. This includes the entire text of
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the start and end tags of the SignedInfo element as well as all
descendant markup and character data (i.e., the text) between
those tags.
We RECOMMEND that resource constrained applications that do not
implement the Canonical XML [XML-C14N] algorithm and instead choose
minimal canonicalization (or some other form) be implemented to
generate Canonical XML as their output serialization so as to easily
mitigate some of these interoperability and security concerns.
(While a result might not be the canonical form of the original, it
can still be in canonical form.) For instance, such an
implementation SHOULD (at least) generate standalone XML instances
[XML].
Schema Definition:
<element name="CanonicalizationMethod">
<complexType>
<sequence>
<any namespace="##any" minOccurs="0" maxOccurs="unbounded"/>
</sequence>
<attribute name="Algorithm" type="uriReference" use="required"/>
</complexType>
</element>
DTD:
<!ELEMENT CanonicalizationMethod %Method.ANY; >
<!ATTLIST CanonicalizationMethod
Algorithm CDATA #REQUIRED >
4.3.2 The SignatureMethod Element
SignatureMethod is a required element that specifies the algorithm
used for signature generation and validation. This algorithm
identifies all cryptographic functions involved in the signature
operation (e.g., hashing, public key algorithms, MACs, padding,
etc.). This element uses the general structure here for algorithms
described in section 6.1: Algorithm Identifiers and Implementation
Requirements. While there is a single identifier, that identifier
may specify a format containing multiple distinct signature values.
Schema Definition:
<element name="SignatureMethod">
<complexType>
<sequence>
<any namespace="##any" minOccurs="0" maxOccurs="unbounded"/>
</sequence>
<attribute name="Algorithm" type="uriReference" use="required"/>
</complexType>
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</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>
<sequence>
<element ref="ds:Transforms" minOccurs="0"/>
<element ref="ds:DigestMethod"/>
<element ref="ds:DigestValue"/>
</sequence>
<attribute name="Id" type="ID" use="optional"/>
<attribute name="URI" type="uriReference" use="optional"/>
<attribute name="Type" type="uriReference" use="optional"/>
</complexType>
</element>
DTD:
<!ELEMENT Reference (Transforms?, DigestMethod, DigestValue) >
<!ATTLIST Reference
Id ID #IMPLIED
URI CDATA #IMPLIED
Type CDATA #IMPLIED >
4.3.3.1 The URI Attribute
The URI attribute identifies a data object using a URI-Reference, as
specified by RFC2396 [URI]. The set of allowed characters for URI
attributes is the same as for XML, namely [Unicode]. However, some
Unicode characters are disallowed from URI references including all
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non-ASCII characters and the excluded characters listed in RFC2396
[URI, section 2.4]. However, the number sign (#), percent sign (%),
and square bracket characters re-allowed in RFC 2732 [URI-Literal]
are permitted. Disallowed characters must be escaped as follows:
1. Each disallowed character is converted to [UTF-8] as one or more
bytes.
2. Any octets corresponding to a disallowed character are escaped
with the URI escaping mechanism (that is, converted to %HH, where
HH is the hexadecimal notation of the byte value).
3. The original character is replaced by the resulting character
sequence.
XML signature applications MUST be able to parse URI syntax. We
RECOMMEND they be able to dereference URIs in the HTTP scheme.
Dereferencing a URI in the HTTP scheme MUST comply with the Status
Code Definitions of [HTTP] (e.g., 302, 305 and 307 redirects are
followed to obtain the entity-body of a 200 status code response).
Applications should also 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.
If a resource is identified by more than one URI, the most specific
should be used (e.g. http://www.w3.org/2000/06/interop-
pressrelease.html.en instead of http://www.w3.org/2000/06/interop-
pressrelease). (See the Reference Validation (section 3.2.1) for a
further information on reference processing.)
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 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/2000/09/xmldsig#Object"
Type="http://www.w3.org/2000/09/xmldsig#Manifest"
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.
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4.3.3.2 The Reference Processing Model
Note: XPath is RECOMMENDED. Signature applications need not conform
to [XPath] specification in order to conform to this specification.
However, the XPath data model, definitions (e.g., node-sets) and
syntax is used within this document in order to describe
functionality for those that want to process XML-as-XML (instead of
octets) as part of signature generation. For those that want to use
these features, a conformant [XPath] implementation is one way to
implement these features, but it is not required. Such applications
could use a sufficiently functional replacement to a node-set and
implement only those XPath expression behaviors REQUIRED by this
specification. However, for simplicity we generally will use XPath
terminology without including this qualification on every point.
Requirements over "XPath nodesets" can include a node-set functional
equivalent. Requirements over XPath processing can include
application behaviors that are equivalent to the corresponding XPath
behavior.
The data-type of the result of URI dereferencing or subsequent
Transforms is either an octet stream or an XPath node-set.
The Transforms specified in this document are defined with respect to
the input they require. The following is the default signature
application behavior:
* If the data object is a an octet stream and the next
transformrequires a node-set, the signature application MUST
attempt to parse the octets.
* If the data object is a node-set and the next transformrequires
octets, the signature application MUST attempt to convert the
node-set to an octet stream using the REQUIRED canonicalization
algorithm [XML-C14N].
Users may specify alternative transforms that over-ride these
defaults in transitions between Transforms that expect different
inputs. The final octet stream contains the data octets being
secured. The digest algorithm specified by DigestMethod is then
applied to these data octets, resulting in the DigestValue.
Unless the URI-Reference is a 'same-document' reference as defined in
[URI, Section 4.2], the result of dereferencing the URI-Reference
MUST be an octet stream. In particular, an XML document identified
by URI is not parsed by the signature application unless the URI is a
same-document reference or unless a transformthat requires XML
parsing is applied (See Transforms (section 4.3.3.1).)
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When a fragment is preceded by an absolute or relative URI in the
URI-Reference, the meaning of the fragment is defined by the
resource's MIME type. Even for XML documents, URI dereferencing
(including the fragment processing) might be done for the signature
application by a proxy. Therefore, reference validation might fail
if fragment processing is not performed in a standard way (as defined
in the following section for same-document references).
Consequently, we RECOMMEND that the URI attribute not include
fragment identifiers and that such processing be specified as an
additional XPath Transform.
When a fragment is not preceded by a URI in the URI-Reference, XML
signature applications MUST support the null URI and barename
XPointer. We RECOMMEND support for the same-document XPointers
'#xpointer(/)' and '#xpointer(id("ID"))' if the application also
intends to support Minimal Canonicalization or Canonical XML with
Comments. (Otherwise URI="#foo" will automatically remove comments
before the Canonical XML with Comments can even be invoked.) All
other support for XPointers is OPTIONAL, especially all support for
barename and other XPointers in external resources since the
application may not have control over how the fragment is generated
(leading to interoperability problems and validation failures).
The following examples demonstrate what the URI attribute identifies
and how it is dereferenced:
URI="http://example.com/bar.xml"
Identifies the octets that represent the external resource
'http//example.com/bar.xml', that is probably XML document
given its file extension.
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',
provided as an octet stream. Again, for the sake of
interoperability, the element identified as 'chapter1' should
be obtained using an XPath transformrather than a URI fragment
(barename XPointer resolution in external resources is not
REQUIRED in this specification).
URI=""
Identifies the nodeset (minus any comment nodes) of the XML
resource containing the signature
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RFC 3075 XML-Signature Syntax and Processing March 2001
URI="#chapter1"
Identifies a nodeset containing the element with ID attribute
value 'chapter1' of the XML resource containing the signature.
XML Signature (and its applications) modify this nodeset to
include the element plus all descendents including namespaces
and attributes -- but not comments.
4.3.3.3 Same-Document URI-References
Dereferencing a same-document reference MUST result in an XPath
node-set suitable for use by Canonical XML. Specifically,
dereferencing a null URI (URI="") MUST result in an XPath node-set
that includes every non-comment node of the XML document containing
the URI attribute. In a fragment URI, the characters after the
number sign ('#') character conform to the XPointer syntax [Xptr].
When processing an XPointer, the application MUST behave as if the
root node of the XML document containing the URI attribute were used
to initialize the XPointer evaluation context. The application MUST
behave as if the result of XPointer processing were a node-set
derived from the resultant location-set as follows:
1. discard point nodes
2. replace each range node with all XPath nodes having full or
partial content within the range
3. replace the root node with its children (if it is in the node-set)
4. replace any element node E with E plus all descendants of E (text,
comment, PI, element) and all namespace and attribute nodes of E
and its descendant elements.
5. if the URI is not a full XPointer, then delete all comment nodes
The second to last replacement is necessary because XPointer
typically indicates a subtree of an XML document's parse tree using
just the element node at the root of the subtree, whereas Canonical
XML treats a node-set as a set of nodes in which absence of
descendant nodes results in absence of their representative text from
the canonical form.
The last step is performed for null URIs, barename XPointers and
child sequence XPointers. To retain comments while selecting an
element by an identifier ID, use the following full XPointer:
URI='#xpointer(id("ID"))'. To retain comments while selecting the
entire document, use the following full XPointer: URI='#xpointer(/)'.
This XPointer contains a simple XPath expression that includes the
root node, which the second to last step above replaces with all
nodes of the parse tree (all descendants, plus all attributes, plus
all namespaces nodes).
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4.3.3.4 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 serves as input to the
next Transform. The input to the first Transform is the result of
dereferencing the URI attribute of the Reference element. 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
Only What is Signed is Secure (section 8.1).)
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 transform input. (See
Algorithm Identifiers and Implementation Requirements (section 6).)
As described in The Reference Processing Model (section 4.3.3.2),
some transforms take an XPath node-set as input, while others require
an octet stream. If the actual input matches the input needs of the
transform, then the transform operates on the unaltered input. If
the transform input requirement differs from the format of the actual
input, then the input must be converted.
Some Transform may require explicit MIME type, 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
explicit information. Such data characteristics are provided as
parameters to the Transform algorithm and should be described in the
specification for the algorithm.
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. Transform Algorithms
(section 6.6) defines the list of standard transformations.
Schema Definition:
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<element name="Transforms">
<complexType>
<sequence>
<element ref="ds:Transform" maxOccurs="unbounded"/>
</sequence>
</complexType>
</element>
<element name="Transform">
<complexType>
<choice maxOccurs="unbounded">
<any namespace="##other" processContents="lax" minOccurs="0"
maxOccurs="unbounded"/>
<element name="XSLT" type="string"/>
<!-- should be an xsl:stylesheet element -->
<element name="XPath" 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) >
4.3.3.5 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 Algorithm
Identifiers and Implementation Requirements (section 6.1).
If the result of the URI dereference and application of Transforms is
an XPath node-set (or sufficiently functional replacement implemented
by the application) then it must be converted as described in the
Reference Processing Model (section 4.3.3.2). If the result of URI
dereference and application of Transforms is an octet stream, then no
conversion occurs (comments might be present if the Minimal
Canonicalization or Canonical XML with Comments was specified in the
Transforms). The digest algorithm is applied to the data octets of
the resulting octet stream.
Schema Definition:
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RFC 3075 XML-Signature Syntax and Processing March 2001
<element name="DigestMethod">
<complexType>
<sequence>
<any namespace="##any" processContents="lax" minOccurs="0"
maxOccurs="unbounded"/>
</sequence>
<attribute name="Algorithm" type="uriReference" use="required"/>
</complexType>
</element>
DTD:
<!ELEMENT DigestMethod %Method.ANY; >
<!ATTLIST DigestMethod
Algorithm CDATA #REQUIRED >
4.3.3.6 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 digest value -->
4.4 The KeyInfo Element
KeyInfo is an optional element that enables the recipient(s) to
obtain the key needed to validate the signature. 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].
If KeyInfo is omitted, the recipient is expected to be able to
identify the key based on application context information. Multiple
declarations within KeyInfo refer to the same key. While
applications may define and use any mechanism they choose through
inclusion of elements from a different namespace, compliant versions
MUST implement KeyValue (section 4.4.2) and SHOULD implement
RetrievalMethod (section 4.4.3).
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The following list summarizes the KeyInfo types defined by this
specification; these can be used within the RetrievalMethod Type
attribute to describe the remote KeyInfo structure as represented as
an octect stream.
* http://www.w3.org/2000/09/xmldsig#X509Data
* http://www.w3.org/2000/09/xmldsig#PGPData
* http://www.w3.org/2000/09/xmldsig#SPKIData
* http://www.w3.org/2000/09/xmldsig#MgmtData
In addition to the types above for which we define structures, we
specify one additional type to indicate a binary X.509 Certificate
* http://www.w3.org/2000/09/xmldsig#rawX509Certificate
Schema Definition:
<element name="KeyInfo">
<complexType>
<choice maxOccurs="unbounded">
<any processContents="lax" namespace="##other" minOccurs="0"
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 >
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.)
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Schema Definition:
<!-- type declared in KeyInfo -->
DTD:
<!ELEMENT KeyName (#PCDATA) >
4.4.2 The KeyValue Element
The KeyValue element contains a single public key that may be useful
in validating the signature. Structured formats for defining DSA
(REQUIRED) and RSA (RECOMMENDED) public keys are defined in Signature
Algorithms (section 6.4).
Schema Definition:
<element name="KeyValue">
<complexType mixed="true">
<choice>
<any namespace="##other" processContents="lax" minOccurs="0"
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
reference to KeyInfo information that is stored at another location.
For example, several signatures in a document might use a key
verified by an X.509v3 certificate chain appearing once in the
document or remotely outside the document; each signature's KeyInfo
can reference this chain using a single RetrievalMethod element
instead of including the entire chain with a sequence of
X509Certificate elements.
RetrievalMethod uses the same syntax and dereferencing behavior as
Reference's URI (section 4.3.3.1) and The Reference Processing Model
(section 4.3.3.2) except that there is no DigestMethod or DigestValue
child elements and presence of the URI is mandatory. Note, if the
result of dereferencing and transforming the specified URI is a node
set, then it may need to be to be canonicalized. All of the KeyInfo
types defined by this specification (section 4.4) represent octets,
Eastlake, et al. Standards Track [Page 28]
RFC 3075 XML-Signature Syntax and Processing March 2001
consequently the Signature application is expected to attempt to
canonicalize the nodeset via the The Reference Processing Model
(section 4.3.3.2)
Type is an optional identifier for the type of data to be retrieved.
Schema Definition
<element name="RetrievalMethod">
<complexType>
<sequence>
<element ref="ds:Transforms" minOccurs="0"/>
</sequence>
<attribute name="URI" type="uriReference"/>
<attribute name="Type" type="uriReference" use="optional"/>
</complexType>
</element>
DTD
<!ELEMENT RetrievalMethod (Transforms?) >
<!ATTLIST RetrievalMethod
URI CDATA #REQUIRED
Type CDATA #IMPLIED >
4.4.4 The X509Data Element
Identifier
Type="http://www.w3.org/2000/09/xmldsig#X509Data"
(this can be used within a RetrievalMethod or Reference element
to identify the referent's type)
An X509Data element within KeyInfo contains one or more identifiers
of keys or X509 certificates (or certificates' identifiers or
revocation lists). Five types of X509Data are defined
1. The X509IssuerSerial element, which contains an X.509 issuer
distinguished name/serial number pair that SHOULD be compliant
with RFC2253 [LDAP-DN],
2. The X509SubjectName element, which contains an X.509 subject
distinguished name that SHOULD be compliant with RFC2253 [LDAP-
DN],
3. The X509SKI element, which contains an X.509 subject key
identifier value.
4. The X509Certificate element, which contains a base64-encoded
[X509v3] certificate, and
5. The X509CRL element, which contains a base64-encoded certificate
revocation list (CRL) [X509v3].
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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 MAY occur in multiple
X509Data elements. For example, the following block contains two
pointers to certificate-A (issuer/serial number and SKI) and a single
reference to certificate-B (SubjectName) and also shows use of
certificate elements
<KeyInfo>
<X509Data> <!-- two pointers to certificate-A -->
<X509IssuerSerial>
<X509IssuerName>CN=TAMURA Kent, OU=TRL, O=IBM,
L=Yamato-shi, ST=Kanagawa, C=JP</X509IssuerName>
<X509SerialNumber>12345678</X509SerialNumber>
</X509IssuerSerial>
<X509SKI>31d97bd7</X509SKI>
</X509Data>
<X509Data> <!-- single pointer to certificate-B -->
<X509SubjectName>Subject of Certificate B</X509SubjectName>
</X509Data> <!-- certificate chain -->
<!--Signer cert, issuer CN=arbolCA,OU=FVT,O=IBM,C=US, serial 4-->
<X509Certificate>MIICXTCCA..</X509Certificate>
<!-- Intermediate cert subject CN=arbolCA,OU=FVTO=IBM,C=US
issuer,CN=tootiseCA,OU=FVT,O=Bridgepoint,C=US -->
<X509Certificate>MIICPzCCA...</X509Certificate>
<!-- Root cert subject CN=tootiseCA,OU=FVT,O=Bridgepoint,C=US -->
<X509Certificate>MIICSTCCA...</X509Certificate>
</X509Data>
</KeyInfo>
Note, there is no direct provision for a PKCS#7 encoded "bag" of
certificates or CRLs. However, a set of certificates or a CRL can
occur within an X509Data element and multiple X509Data elements can
occur in a KeyInfo. Whenever multiple certificates occur in an
X509Data element, at least one such certificate must contain the
public key which verifies the signature.
Schema Definition
<element name="X509Data">
<complexType>
<choice>
<sequence maxOccurs="unbounded">
<choice>
<element ref="ds:X509IssuerSerial"/>
<element name="X509SKI" type="ds:CryptoBinary"/>
<element name="X509SubjectName" type="string"/>
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<element name="X509Certificate" type="ds:CryptoBinary"/>
</choice>
</sequence>
<element name="X509CRL" type="ds:CryptoBinary"/>
</choice>
</complexType>
</element>
<element name="X509IssuerSerial">
<complexType>
<sequence>
<element name="X509IssuerName" type="string"/>
<element name="X509SerialNumber" type="integer"/>
</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) >
<!ELEMENT X509CRL (#PCDATA) >
4.4.5 The PGPData element
Identifier
Type="http://www.w3.org/2000/09/xmldsig#PGPData"
(this can be used within a RetrievalMethod or Reference element
to identify the referent's type)
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 [PGP, section 11.2]. The PGPKeyPacket
contains a base64-encoded Key Material Packet as defined in [PGP,
section 5.5]. Other sub-types of the PGPData element may be defined
by the OpenPGP working group.
Schema Definition:
<element name="PGPData">
<complexType>
<choice>
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<any namespace="##other" processContents="lax" minOccurs="0"
maxOccurs="unbounded"/>
<sequence>
<element name="PGPKeyID" type="string"/>
<element name="PGPKeyPacket" type="ds:CryptoBinary"/>
</sequence>
</choice>
</complexType>
</element>
DTD:
<!ELEMENT PGPData (PGPKeyID, PGPKeyPacket) >
<!ELEMENT PGPKeyPacket (#PCDATA) >
<!ELEMENT PGPKeyID (#PCDATA) >
4.4.6 The SPKIData element
Identifier
Type="http://www.w3.org/2000/09/xmldsig#SPKIData"
(this can be used within a RetrievalMethod or Reference element
to identify the referent's type)
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 expected to be a Canonical S-
expression.
Schema Definition:
<element name="SPKIData" type="string"/>
DTD:
<!ELEMENT SPKIData (#PCDATA) >
4.4.7 The MgmtData element
Identifier
Type="http://www.w3.org/2000/09/xmldsig#MgmtData"
(this can be used within a RetrievalMethod or Reference element
to identify the referent's type)
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:
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<!-- type declared in KeyInfo -->
DTD:
<!ELEMENT MgmtData (#PCDATA)>
4.5 The Object Element
Identifier
Type="http://www.w3.org/2000/09/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.
The Object's Encoding attributed may be used to provide a URI that
identifies the method by which the object is encoded (e.g., a binary
file).
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 mixed="true">
<sequence maxOccurs="unbounded">
<any namespace="##any" processContents="lax"/>
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</sequence>
<attribute name="Id" type="ID" use="optional"/>
<attribute name="MimeType" type="string" use="optional"/>
<!-- add a grep facet -->
<attribute name="Encoding" type="uriReference" use="optional"/>
</complexType>
</element>
DTD:
<!ELEMENT Object %Object.ANY; >
<!ATTLIST Object
Id ID #IMPLIED
MimeType CDATA #IMPLIED
Encoding CDATA #IMPLIED >
5.0 Additional Signature Syntax
This section describes the optional to implement Manifest and
SignatureProperties elements and describes the handling of XML
processing instructions and comments. With respect to the elements
Manifest and SignatureProperties this section specifies syntax and
little behavior -- it is left to the application. These elements can
appear anywhere the parent's content model permits; the Signature
content model only permits them within Object.
5.1 The Manifest Element
Identifier
Type="http://www.w3.org/2000/09/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 the application's 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>
<sequence>
<element ref="ds:Reference" maxOccurs="unbounded"/>
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</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/2000/09/xmldsig#SignatureProperties"
(this can be used within a Reference element to identify the
referent's type)
Additional information items concerning the generation of the
signature(s) can be placed in a SignatureProperty element (i.e.,
date/time stamp or the serial number of cryptographic hardware used
in signature generation).
Schema Definition:
<element name="SignatureProperties">
<complexType>
<sequence>
<element ref="ds:SignatureProperty" maxOccurs="unbounded"/>
</sequence>
<attribute name="Id" type="ID" use="optional"/>
</complexType>
</element>
<element name="SignatureProperty">
<complexType mixed="true">
<choice minOccurs="0" maxOccurs="unbounded">
<any namespace="##other" processContents="lax" minOccurs="0"
maxOccurs="unbounded"/>
</choice>
<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 >
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<!ELEMENT SignatureProperty %SignatureProperty.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 (which [XML-C14N] does), they
will be signed. Consequently, if they are retained, a change to the
comment will cause a signature failure. Similarly, the XML signature
over any XML data will be sensitive to comment changes unless a
comment-ignoring canonicalization/transform method, such as the
Canonical XML [XML-C14N], is specified.
6.0 Algorithms
This section identifies algorithms used with the XML digital
signature specification. 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
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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 sometimes abbreviated in XML syntax (e.g.,
"&dsig;base64").)
Algorithm Type
Algorithm - Requirements - Algorithm URI
Digest
SHA1 - REQUIRED - &dsig;sha1
Encoding
base64 - REQUIRED - &dsig;base64
MAC
HMAC-SHA1 - REQUIRED - &dsig;hmac-sha1
Signature
DSAwithSHA1(DSS) - REQUIRED - &dsig;dsa-sha1
RSAwithSHA1 - RECOMMENDED - &dsig;rsa-sha1
Canonicalization
minimal - RECOMMENDED - &dsig;minimal
Canonical XML with Comments - RECOMMENDED -
http://www.w3.org/TR/2000/CR-xml-c14n-20001026#WithComments
Canonical XML (omits comments) - REQUIRED -
http://www.w3.org/TR/2000/CR-xml-c14n-20001026
Transform
XSLT - OPTIONAL - http://www.w3.org/TR/1999/REC-xslt-19991116
XPath - RECOMMENDED -
http://www.w3.org/TR/1999/REC-xpath-19991116
Enveloped Signature* - REQUIRED - &dsig;enveloped-signature
* The Enveloped Signature transform removes the Signature element
from the calculation of the signature when the signature is within
the content 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 Transform.
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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/2000/09/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/2000/09/xmldsig#hmac-sha1
The HMAC algorithm (RFC2104 [HMAC]) takes the truncation length in
bits as a parameter; if the parameter is not specified then all the
bits of the hash are output. An example of an HMAC SignatureMethod
element:
<SignatureMethod Algorithm="&dsig;hmac-sha1">
<HMACOutputLength>128</HMACOutputLength>
</SignatureMethod>
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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"/>
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/2000/09/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
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from the example in Appendix 5 of the DSS standard would be
<SignatureValue>
i6watmQQQ1y3GB+VsWq5fJKzQcBB4jRfH1bfJFj0JtFVtLotttzYyA==</SignatureValue>
DSA key values have the following set of fields: P, Q, G and Y are
mandatory when appearing as a key value, J, seed and pgenCounter are
optional but should be present. (The seed and pgenCounter fields
must appear together or be absent). All parameters are encoded as
base64 [MIME] values.
Schema:
<element name="DSAKeyValue">
<complexType>
<sequence>
<sequence>
<element name="P" type="ds:CryptoBinary"/>
<element name="Q" type="ds:CryptoBinary"/>
<element name="G" type="ds:CryptoBinary"/>
<element name="Y" type="ds:CryptoBinary"/>
<element name="J" type="ds:CryptoBinary" minOccurs="0"/>
</sequence>
<sequence minOccurs="0">
<element name="Seed" type="ds:CryptoBinary"/>
<element name="PgenCounter" type="ds:CryptoBinary"/>
</sequence>
</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/2000/09/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
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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 [MIME] encoded. (The conversion from integer to octet string
is equivalent to IEEE 1363's I2OSP [1363] with minimal length).
The expression "RSA algorithm" as used in this document refers to the
RSASSA-PKCS1-v1_5 algorithm described in RFC 2437 [PKCS1]. 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 is the base64 [MIME]
encoding of the octet string computed as per RFC 2437 [PKCS1, section
8.1.1: Signature generation for the RSASSA-PKCS1-v1_5 signature
scheme]. As specified in the EMSA-PKCS1-V1_5-ENCODE function RFC
2437 [PKCS1, section 9.2.1], the value input to the signature
function MUST contain a pre-pended algorithm object identifier for
the hash function, but the availability of an ASN.1 parser and
recognition of OIDs is not required of a signature verifier. The
PKCS#1 v1.5 representation appears as:
CRYPT (PAD (ASN.1 (OID, DIGEST (data))))
Note that the padded ASN.1 will be of the following form:
01 | FF* | 00 | prefix | hash
where "|" is concatentation, "01", "FF", and "00" are fixed octets of
the corresponding hexadecimal value, "hash" is the SHA1 digest of the
data, and "prefix" is the ASN.1 BER SHA1 algorithm designator prefix
required in PKCS1 [RFC 2437], that is,
hex 30 21 30 09 06 05 2B 0E 03 02 1A 05 00 04 14
This prefix is included to make it easier to use standard
cryptographic libraries. The FF octet MUST be repeated the maximum
number of times such that the value of the quantity being CRYPTed is
one octet shorter than the RSA modulus.
The resulting base64 [MIME] string is the value of the child text
node of the SignatureValue element, e.g.
<SignatureValue>IWijxQjUrcXBYoCei4QxjWo9Kg8D3p9tlWoT4
t0/gyTE96639In0FZFY2/rvP+/bMJ01EArmKZsR5VW3rwoPxw=
</SignatureValue>
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RSA key values have two fields Modulus and Exponent
<RSAKeyValue>
<Modulus>xA7SEU+e0yQH5rm9kbCDN9o3aPIo7HbP7tX6WOocLZAtNfyxSZDU16ksL6W
jubafOqNEpcwR3RdFsT7bCqnXPBe5ELh5u4VEy19MzxkXRgrMvavzyBpVRgBUwUlV
5foK5hhmbktQhyNdy/6LpQRhDUDsTvK+g9Ucj47es9AQJ3U=
</Modulus>
<Exponent>AQAB</Exponent>
</RSAKeyValue>
Schema:
<element name="RSAKeyValue">
<complexType>
<sequence>
<element name="Modulus" type="ds:CryptoBinary"/>
<element name="Exponent" type="ds:CryptoBinary"/>
</sequence>
</complexType>
</element>
DTD:
<!ELEMENT RSAKeyValue (Modulus, Exponent) >
<!ELEMENT Modulus (#PCDATA) >
<!ELEMENT Exponent (#PCDATA) >
6.5 Canonicalization Algorithms
If canonicalization is performed over octets, the canonicalization
algorithms take two implicit parameter: the content and its charset.
The charset is derived according to the rules of the transport
protocols and media types (e.g., RFC2376 [XML-MT] defines the media
types for XML). This information is necessary to correctly sign and
verify documents and often requires careful server side
configuration.
Various canonicalization algorithms require conversion to [UTF-8].The
two algorithms below understand at least [UTF-8] and [UTF-16] as
input encodings. We RECOMMEND that externally specified algorithms
do the same. Knowledge of other encodings is OPTIONAL.
Various canonicalization algorithms transcode from a non-Unicode
encoding to Unicode. The two algorithms below perform text
normalization during transcoding [NFC]. We RECOMMEND that externally
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specified canonicalization algorithms do the same. (Note, there can
be ambiguities in converting existing charsets to Unicode, for an
example see the XML Japanese Profile [XML-Japanese] NOTE.)
6.5.1 Minimal Canonicalization
Identifier:
http://www.w3.org/2000/09/xmldsig#minimal
An example of a minimal canonicalization element is:
<CanonicalizationMethod Algorithm="&dsig;minimal"/>
The minimal canonicalization algorithm:
* converts the character encoding to UTF-8 (without any byte
order mark (BOM)). If an encoding is given in the XML
declaration, it must be removed. Implementations MUST
understand at least [UTF-8] and [UTF-16] as input encodings.
Non-Unicode to Unicode transcoding MUST perform text
normalization [NFC].
* normalizes line endings as provided by [XML]. (See XML and
Canonicalization and Syntactical Considerations (section 7).)
This algorithm requires as input the octet stream of the resource to
be processed; the algorithm outputs an octet stream. When used to
canonicalize SignedInfo the algorithm MUST be provided with the
octets that represent the well-formed SignedInfo element (and its
children and content) as described in The CanonicalizationMethod
Element (section 4.3.1).
If the signature application has a node set, then the signature
application must convert it into octets as described in The Reference
Processing Model (section 4.3.3.2). However, Minimal
Canonicalization is NOT RECOMMENDED for processing XPath node-sets,
the results of same-document URI references, and the output of other
types of XML based transforms. It is only RECOMMENDED for simple
character normalization of well formed XML that has no namespace or
external entity complications.
6.5.2 Canonical XML
Identifier for REQUIRED Canonical XML (omits comments):
http://www.w3.org/TR/2000/CR-xml-c14n-20001026
Identifier for Canonical XML with Comments:
http://www.w3.org/TR/2000/CR-xml-c14n-20001026#WithComments
An example of an XML canonicalization element is:
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<CanonicalizationMethod Algorithm="http://www.w3.org/TR/2000/CR-xml-
c14n-20001026"/>
The normative specification of Canonical XML is [XML-C14N]. The
algorithm is capable of taking as input either an octet stream or an
XPath node-set (or sufficiently functional alternative). The
algorithm produces an octet stream as output. Canonical XML is
easily parameterized (via an additional URI) to omit or retain
comments.
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 (such as those in Canonicalization Algorithms
(section 6.5)) can be used as a Transform.
6.6.2 Base64
Identifiers:
http://www.w3.org/2000/09/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.
This transform requires an octet stream for input. If an XPath
node-set (or sufficiently functional alternative) is given as input,
then it is converted to an octet stream by performing operations
logically equivalent to 1) applying an XPath transform with
expression self::text(), then 2) taking the string-value of the
node-set. Thus, if an XML element is identified by a barename
XPointer in the Reference URI, and its content consists solely of
base64 encoded character data, then this transform automatically
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strips away the start and end tags of the identified element and any
of its descendant elements as well as any descendant comments and
processing instructions. The output of this transform is an octet
stream.
6.6.3 XPath Filtering
Identifier:
http://www.w3.org/TR/1999/REC-xpath-19991116
The normative specification for XPath expression evaluation is
[XPath]. The XPath expression to be evaluated appears as the
character content of a transform parameter child element named XPath.
The input required by this transform is an XPath node-set. Note that
if the actual input is an XPath node-set resulting from a null URI or
barename XPointer dereference, then comment nodes will have been
omitted. If the actual input is an octet stream, then the
application MUST convert the octet stream to an XPath node-set
suitable for use by Canonical XML with Comments (a subsequent
application of the REQUIRED Canonical XML algorithm would strip away
these comments). In other words, the input node-set should be
equivalent to the one that would be created by the following process:
1. Initialize an XPath evaluation context by setting the initial node
equal to the input XML document's root node, and set the context
position and size to 1.
2. Evaluate the XPath expression (//. | //@* | //namespace::*)
The evaluation of this expression includes all of the document's
nodes (including comments) in the node-set representing the octet
stream.
The transform output is also an XPath node-set. The XPath expression
appearing in the XPath parameter is evaluated once for each node in
the input node-set. The result is converted to a boolean. If the
boolean is true, then the node is included in the output node-set.
If the boolean is false, then the node is omitted from the output
node-set.
Note: Even if the input node-set has had comments removed, the
comment nodes still exist in the underlying parse tree and can
separate text nodes. For example, the markup <e>Hello, <!-- comment
--> world!</e> contains two text nodes. Therefore, the expression
self::text()[string()="Hello, world!"] would fail. Should this
problem arise in the application, it can be solved by either
canonicalizing the document before the XPath transform to physically
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remove the comments or by matching the node based on the parent
element's string value (e.g., by using the expression
self::text()[string(parent::e)="Hello, world!"]).
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. This is done by omitting precisely
those nodes that are allowed to change once the signature is affixed,
and including all other input nodes in the output. It is the
responsibility of the XPath expression author to include all nodes
whose change could affect the interpretation of the transform output
in the application context.
An important scenario would be a document requiring two enveloped
signatures. Each signature must omit itself from its own digest
calculations, but it is also necessary to exclude the second
signature element from the digest calculations of the first signature
so that adding the second signature does not break the first
signature.
The XPath transform establishes the following evaluation context for
each node of the input node-set:
* A context node equal to a node of the input node-set.
* 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.
As a result of the context node setting, the XPath expressions
appearing in this transform will be quite similar to those used in
used in [XSLT], except that the size and position are always 1 to
reflect the fact that the transform is automatically visiting every
node (in XSLT, one recursively calls the command apply-templates to
visit the nodes of the input tree).
The function here() is defined as follows:
Function: node-set here()
The here function returns a node-set containing the attribute or
processing instruction node or the parent element of the text node
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that directly bears the XPath expression. This expression results in
an error if the containing XPath expression does not appear in the
same XML document against which the XPath expression is being
evaluated.
Note: The function definition for here() is intended to be consistent
with its definition in XPointer. However, some minor differences are
presently being discussed between the Working Groups.
As an example, consider creating an enveloped signature (a Signature
element that is a descendant of an element being signed). Although
the signed content should not be changed after signing, the elements
within the Signature element are changing (e.g., the digest value
must be put inside the DigestValue and the SignatureValue must be
subsequently calculated). One way to prevent these changes from
invalidating the digest value in DigestValue is to add an XPath
Transform that omits all Signature elements and their descendants.
For example,
<Document>
<Signature xmlns="&dsig;">
<SignedInfo>
...
<Reference URI="">
<Transforms>
<Transform
Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
<XPath xmlns:dsig="&dsig;">
not(ancestor-or-self::dsig:Signature)
</XPath>
</Transform>
</Transforms>
<DigestMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
<DigestValue></DigestValue>
</Reference>
</SignedInfo>
<SignatureValue></SignatureValue>
</Signature>
...
</Document>
Due to the null Reference URI in this example, the XPath transform
input node-set contains all nodes in the entire parse tree starting
at the root node (except the comment nodes). For each node in this
node-set, the node is included in the output node-set except if the
node or one of its ancestors has a tag of Signature that is in the
namespace given by the replacement text for the entity &dsig;.
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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;">
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).
6.6.4 Enveloped Signature Transform
Identifier:
http://www.w3.org/2000/09/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;">
count(ancestor-or-self::dsig:Signature |
here()/ancestor::dsig:Signature[1]) >
count(ancestor-or-self::dsig:Signature)</XPath>
The input and output requirements of this transform are identical to
those of the XPath transform. 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
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The normative specification for XSL Transformations is [XSLT]. The
XSL style sheet or transform to be evaluated appears as the character
content of a transform parameter child element named XSLT. The root
element of a XSLT style sheet SHOULD be <xsl:stylesheet>.
This transform requires an octet stream as input. If the actual
input is an XPath node-set, then the signature application should
attempt to covert it to octets (apply Canonical XML]) as described in
the Reference Processing Model (section 4.3.3.2).
The output of this transform is an octet stream. The processing
rules for the XSL style sheet or transform element are stated in the
XSLT specification [XSLT]. We RECOMMEND that XSLT transformauthors
use an output method of xml for XML and HTML. As XSLT
implementations do not produce consistent serializations of their
output, we further RECOMMEND inserting a transformafter the XSLT
transformto perform canonicalize the output. These steps will help
to ensure interoperability of the resulting signatures among
applications that support the XSLT transform. Note that if the
output is actually HTML, then the result of these steps is logically
equivalent [XHTML].
7.0 XML Canonicalization and Syntax Constraint Considerations
Digital signatures only work if the verification calculations are
performed on exactly the same bits as the signing calculations. If
the surface representation of the signed data can change between
signing and verification, then some way to standardize the changeable
aspect must be used before signing and verification. For example,
even for simple ASCII text there are at least three widely used line
ending sequences. If it is possible for signed text to be modified
from one line ending convention to another between the time of
signing and signature verification, then the line endings need to be
canonicalized to a standard form before signing and verification or
the signatures will break.
XML is subject to surface representation changes and to processing
which discards some surface information. For this reason, XML
digital signatures have a provision for indicating canonicalization
methods in the signature so that a verifier can use the same
canonicalization as the signer.
Throughout this specification we distinguish between the
canonicalization of a Signature element 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
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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 Canonical XML [XML-C14N] that coded
character set is UTF-8 (without a byte order mark (BOM)).Neither the
minimal canonicalization nor the Canonical XML [XML-C14N] algorithms
provide character normalization. We RECOMMEND that signature
applications create XML content (Signature elements and their
descendents/content) in Normalization Form C [NFC] and check that any
XML being consumed is in that form as well (if not, signatures may
consequently fail to validate). Additionally, 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.
7.1 XML 1.0, Syntax Constraints, and Canonicalization
XML 1.0 [XML] defines an interface where a conformant application
reading XML is given certain information from that XML and not other
information. In particular,
1. line endings are normalized to the single character #xA by
dropping #xD characters if they are immediately followed by a #xA
and replacing them with #xA in all other cases,
2. missing attributes declared to have default values are provided to
the application as if present with the default value,
3. character references are replaced with the corresponding
character,
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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 the presence of a
schema, DTD or similar declarations. The Signature element type is
laxly schema valid [XML-schema], consequently external XML or even
XML within the same document as the signature may be (only) well
formed or from another namespace (where permitted by the signature
schema); the noted items may not be present. Thus, a signature with
such content will only be verifiable by other signature applications
if the following syntax constraints are observed when generating any
signed material 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
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Signature to be verifiable by an implementation using DOM or SAX, not
only must the XML1.0 syntax constraints given in the previous section
be followed but an appropriate XML canonicalization MUST be specified
so that the verifier can re-serialize DOM/SAX mediated input into the
same octect stream that was signed.
8.0 Security Considerations
The XML Signature specification provides a very flexible digital
signature mechanism. Implementors must give consideration to their
application threat models and to the following factors.
8.1 Transforms
A requirement of this specification is to permit signatures to "apply
to a part or totality of a XML document." (See [XML-Signature-RD,
section 3.1.3].) 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] to
exclude those portions the user needs to change. Transforms may be
arbitrarily specified and may include encoding transforms,
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
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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" (that which is represented to the user
via visual, auditory or other media) should be signed. If signing is
intended to convey the judgment or consent of a user (an automated
mechanism or person), then it is normally necessary to secure as
exactly as practical the information that was presented to that user.
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
Just as a user 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. This recommendation applies to transforms
specified within the signature as well as those included as part of
the document itself. For instance, if an XML document includes an
embedded style sheet [XSLT] it is the transformed document that that
should be represented to the user and signed. To meet this
recommendation where a document references an external style sheet,
the content of that external resource should also be signed as via a
signature Reference -- otherwise the content of that external content
might change which alters the resulting document without invalidating
the signature.
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
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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 specification uses public key signatures and keyed hash
authentication codes. These have substantially different security
models. Furthermore, it permits user specified algorithms which may
have other models.
With public key signatures, any number of parties can hold the public
key and verify signatures while only the parties with the private key
can create signatures. The number of holders of the private key
should be minimized and preferably be one. Confidence by verifiers
in the public key they are using and its binding to the entity or
capabilities represented by the corresponding private key is an
important issue, usually addressed by certificate or online authority
systems.
Keyed hash authentication codes, based on secret keys, are typically
much more efficient in terms of the computational effort required but
have the characteristic that all verifiers need to have possession of
the same key as the signer. Thus any verifier can forge signatures.
This specification 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.
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Care must be exercised by applications 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
http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/xmldsig-
core-schema.xsd Valid XML schema instance based on the
20000922 Schema/DTD [XML-Schema].
XML Signature DTD
http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/xmldsig-
core-schema.dtd
RDF Data Model
http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/xmldsig-
datamodel-20000112.gif
XML Signature Object Example
http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/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).
RSA XML Signature Example
http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/signature-
example-rsa.xml
An XML Signature example with generated cryptographic values by
Merlin Hughes and validated by Gregor Karlinger.
DSA XML Signature Example
http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/signature-
example-dsa.xml Similar to above but uses DSA.
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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
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
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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, Application
An application that implements the MANDATORY (REQUIRED/MUST)
portions of this specification; these conformance requirements
are over the structure of the Signature element type and its
children (including SignatureValue) and mandatory to support
algorithms.
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 identifier or transform).
Signature, Enveloped
The signature is over the XML content that contains the
signature as an element. The content provides the root XML
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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 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 Core Validation (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/xmldtd.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/
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DSS FIPS PUB 186-1. Digital Signature Standard (DSS).
U.S. Department of Commerce/National Institute of
Standards and Technology.
http://csrc.nist.gov/fips/fips1861.pdf
HMAC Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC
2104, February 1997.
http://www.ietf.org/rfc/rfc2104.txt
HTTP Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P. and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1", RFC
2616, June 1999.
http://www.ietf.org/rfc/rfc2616.txt
KEYWORDS Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
http://www.ietf.org/rfc/rfc2119.txt
LDAP-DN Wahl, M., Kille, S. and T. Howes, "Lightweight
Directory Access Protocol (v3): UTF-8 String
Representation of Distinguished Names", RFC 2253,
December 1997. http://www.ietf.org/rfc/rfc2253.txt
MD5 Rivest, R., "The MD5 Message-Digest Algorithm", RFC
1321, April 1992.
http://www.ietf.org/rfc/rfc1321.txt
MIME Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.
http://www.ietf.org/rfc/rfc2045.txt
NFC TR15. Unicode Normalization Forms. M. Davis, M.
Drst. Revision 18: November 1999.
PGP Callas, J., Donnerhacke, L., Finney, H. and R.
Thayer, "OpenPGP Message Format", November 1998.
http://www.ietf.org/rfc/rfc2440.txt
RANDOM Eastlake, D., Crocker, S. and J. Schiller,
"Randomness Recommendations for Security", RFC
1750, December 1994.
http://www.ietf.org/rfc/rfc1750.txt
Eastlake, et al. Standards Track [Page 59]
RFC 3075 XML-Signature Syntax and Processing March 2001
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/
1363 IEEE 1363: Standard Specifications for Public Key
Cryptography. August 2000.
PKCS1 Kaliski, B. and J. Staddon, "PKCS #1: RSA
Cryptography Specifications Version 2.0", RFC 2437,
October 1998. http://www.ietf.org/rfc/rfc2437.txt
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
Unicode The Unicode Consortium. The Unicode Standard.
http://www.unicode.org/unicode/standard/standard.html
UTF-16 Hoffman, P. and F. Yergeau, "UTF-16, an encoding of
ISO 10646", RFC 2781, February 2000.
http://www.ietf.org/rfc/rfc2781.txt
UTF-8 Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
http://www.ietf.org/rfc/rfc2279.txt
URI Berners-Lee, T., Fielding, R. and L. Masinter,
"Uniform Resource Identifiers (URI): Generic
Syntax", RFC 2396, August 1998.
http://www.ietf.org/rfc/rfc2396.txt
URI-Literal Hinden, R., Carpenter, B. and L. Masinter, "Format
for Literal IPv6 Addresses in URL's", RFC 2732,
December 1999. http://www.ietf.org/rfc/rfc2732.txt
URL Berners-Lee, T., Masinter, L. and M. McCahill,
"Uniform Resource Locators (URL)", RFC 1738,
December 1994. http://www.ietf.org/rfc/rfc1738.txt
Eastlake, et al. Standards Track [Page 60]
RFC 3075 XML-Signature Syntax and Processing March 2001
URN Moats, R., "URN Syntax" RFC 2141, May 1997.
http://www.ietf.org/rfc/rfc2141.txt
Daigle, L., van Gulik, D., Iannella, R. and P.
Faltstrom, "URN Namespace Definition Mechanisms",
RFC 2611, June 1999.
http://www.ietf.org/rfc/rfc2611.txt
X509v3 ITU-T Recommendation X.509 version 3 (1997).
"Information Technology - Open Systems
Interconnection - The Directory Authentication
Framework" ISO/IEC 9594-8:1997.
XHTML 1.0 XHTML(tm) 1.0: The Extensible Hypertext Markup
Language Recommendation. S. Pemberton, D. Raggett,
et. al. January 2000.
http://www.w3.org/TR/2000/REC-xhtml1-20000126/
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 J. Boyer, "Canonical XML Version 1.0", RFC 3076,
September 2000. http://www.w3.org/TR/2000/CR-xml-
c14n-20001026
http://www.ietf.org/rfc/rfc3076.txt
XML-Japanese XML Japanese Profile. W3C NOTE. M. MURATA April
2000 http://www.w3.org/TR/2000/NOTE-japanese-xml-
20000414/
XML-MT Whitehead, E. and M. Murata, "XML Media Types",
July 1998. http://www.ietf.org/rfc/rfc2376.txt
XML-ns Namespaces in XML Recommendation. T. Bray, D.
Hollander, A. Layman. Janury 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. September 2000.
http://www.w3.org/TR/2000/WD-xmlschema-1-20000922/
Eastlake, et al. Standards Track [Page 61]
RFC 3075 XML-Signature Syntax and Processing March 2001
XML Schema Part 2: Datatypes Working Draft. P.
Biron, A. Malhotra. September 2000.
http://www.w3.org/TR/2000/WD-xmlschema-2-20000922/
XML-Signature-RD Reagle, J., "XML Signature Requirements", RFC 2907,
April 2000. http://www.w3.org/TR/1999/WD-xmldsig-
requirements-19991014
http://www.ietf.org/rfc/rfc2807.txt
XPath XML Path Language (XPath)Version 1.0.
Recommendation. J. Clark, S. DeRose. October 1999.
http://www.w3.org/TR/1999/REC-xpath-19991116
XPointer XML Pointer Language (XPointer). Candidate
Recommendation. S. DeRose, R. Daniel, E. Maler.
http://www.w3.org/TR/2000/CR-xptr-20000607
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
Eastlake, et al. Standards Track [Page 62]
RFC 3075 XML-Signature Syntax and Processing March 2001
12. Authors' Addresses
Donald E. Eastlake 3rd
Motorola, Mail Stop: M2-450
20 Forbes Boulevard
Mansfield, MA 02048 USA
Phone: 1-508-261-5434
EMail: Donald.Eastlake@motorola.com
Joseph M. Reagle Jr., W3C
Massachusetts Institute of Technology
Laboratory for Computer Science
NE43-350, 545 Technology Square
Cambridge, MA 02139
Phone: 1.617.258.7621
EMail: reagle@w3.org
David Solo
Citigroup
909 Third Ave, 16th Floor
NY, NY 10043 USA
Phone: +1-212-559-2900
EMail: dsolo@alum.mit.edu
Eastlake, et al. Standards Track [Page 63]
RFC 3075 XML-Signature Syntax and Processing March 2001
13. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
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English.
The limited permissions granted above are perpetual and will not be
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Eastlake, et al. Standards Track [Page 64]