Request for Comments: 3275 Motorola
Obsoletes: 3075 J. Reagle
Category: Standards Track W3C
D. Solo
Citigroup
March 2002
(Extensible Markup Language) 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) 2002 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
1.1 Editorial and Conformance Conventions......................... 4
1.2 Design Philosophy............................................. 4
1.3 Versions, Namespaces and Identifiers.......................... 4
1.4 Acknowledgements.............................................. 6
1.5 W3C Status.................................................... 6
2. Signature Overview and Examples................................ 7
2.1 Simple Example (Signature, SignedInfo, Methods, and References) 8
2.1.1 More on Reference........................................... 9
2.2 Extended Example (Object and SignatureProperty)............... 10
2.3 Extended Example (Object and Manifest)........................ 12
3.0 Processing Rules.............................................. 13
3.1 Core Generation............................................... 13
3.1.1 Reference Generation........................................ 13
3.1.2 Signature Generation........................................ 13
3.2 Core Validation............................................... 14
3.2.1 Reference Validation........................................ 14
3.2.2 Signature Validation........................................ 15
4.0 Core Signature Syntax......................................... 15
4.0.1 The ds:CryptoBinary Simple Type............................. 17
4.1 The Signature element......................................... 17
4.2 The SignatureValue Element.................................... 18
4.3 The SignedInfo Element........................................ 18
4.3.1 The CanonicalizationMethod Element.......................... 19
4.3.2 The SignatureMethod Element................................. 21
4.3.3 The Reference Element....................................... 21
4.3.3.1 The URI Attribute......................................... 22
4.3.3.2 The Reference Processing Model............................ 23
4.3.3.3 Same-Document URI-References.............................. 25
4.3.3.4 The Transforms Element.................................... 26
4.3.3.5 The DigestMethod Element.................................. 28
4.3.3.6 The DigestValue Element................................... 28
4.4 The KeyInfo Element........................................... 29
4.4.1 The KeyName Element......................................... 31
4.4.2 The KeyValue Element........................................ 31
4.4.2.1 The DSAKeyValue Element................................... 32
4.4.2.2 The RSAKeyValue Element................................... 33
4.4.3 The RetrievalMethod Element................................. 34
4.4.4 The X509Data Element........................................ 35
4.4.5 The PGPData Element......................................... 38
4.4.6 The SPKIData Element........................................ 39
4.4.7 The MgmtData Element........................................ 40
4.5 The Object Element............................................ 40
5.0 Additional Signature Syntax................................... 42
5.1 The Manifest Element.......................................... 42
5.2 The SignatureProperties Element............................... 43
5.3 Processing Instructions in Signature Elements................. 44
5.4 Comments in Signature Elements................................ 44
6.0 Algorithms.................................................... 44
6.1 Algorithm Identifiers and Implementation Requirements......... 44
6.2 Message Digests............................................... 46
6.2.1 SHA-1....................................................... 46
6.3 Message Authentication Codes.................................. 46
6.3.1 HMAC........................................................ 46
6.4 Signature Algorithms.......................................... 47
6.4.1 DSA......................................................... 47
6.4.2 PKCS1 (RSA-SHA1)............................................ 48
6.5 Canonicalization Algorithms................................... 49
6.5.1 Canonical XML............................................... 49
6.6 Transform Algorithms.......................................... 50
6.6.1 Canonicalization............................................ 50
6.6.2 Base64...................................................... 50
6.6.3 XPath Filtering............................................. 51
6.6.4 Enveloped Signature Transform............................... 54
6.6.5 XSLT Transform.............................................. 54
7. XML Canonicalization and Syntax Constraint Considerations...... 55
7.1 XML 1.0, Syntax Constraints, and Canonicalization............. 56
7.2 DOM/SAX Processing and Canonicalization....................... 57
7.3 Namespace Context and Portable Signatures..................... 58
8.0 Security Considerations....................................... 59
8.1 Transforms.................................................... 59
8.1.1 Only What is Signed is Secure............................... 60
8.1.2 Only What is 'Seen' Should be Signed........................ 60
8.1.3 'See' What is Signed........................................ 61
8.2 Check the Security Model...................................... 62
8.3 Algorithms, Key Lengths, Certificates, Etc.................... 62
9. Schema, DTD, Data Model, and Valid Examples.................... 63
10. Definitions................................................... 63
Appendix: Changes from RFC3075................................... 67
References........................................................ 67
Authors' Addresses................................................ 72
Full Copyright Statement.......................................... 73
1. 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
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 provides an XML Schema [XML-schema] and DTD [XML].
The schema definition is 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 key words 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 Namespaces in XML 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#"
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/REC-xslt-19991116
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, Accelio (Author)
* John Boyer, PureEdge (Author)
* Mariano P. Consens, University of Waterloo
* John Cowan, Reuters Health
* Donald Eastlake 3rd, Motorola (Chair, Author/Editor)
* Barb Fox, Microsoft (Author)
* Christian Geuer-Pollmann, University Siegen
* Tom Gindin, IBM
* Phillip Hallam-Baker, VeriSign Inc
* Richard Himes, US Courts
* Merlin Hughes, Baltimore
* Gregor Karlinger, IAIK TU Graz
* Brian LaMacchia, Microsoft (Author)
* Peter Lipp, IAIK TU Graz
* Joseph Reagle, W3C (Chair, Author/Editor)
* Ed Simon, XMLsec (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.
1.5 W3C Status
The World Wide Web Consortium Recommendation corresponding to
this RFCis at:
http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/
2. 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
the following structure (where "?" denotes zero or one occurrence;
"+" denotes one or more occurrences; and "*" denotes zero or more
occurrences):
<Signature ID?>
<SignedInfo>
<CanonicalizationMethod/>
<SignatureMethod/>
(<Reference URI? >
(<Transforms>)?
<DigestMethod>
<DigestValue>
</Reference>)+
</SignedInfo>
<SignatureValue>
(<KeyInfo>)?
(<Object ID?>)*
</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 reside
within the same XML document as sibling elements; in this case, the
signature is neither enveloping (signature is parent) nor enveloped
attribute (signature is child). Since a Signature element (and its
Id 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/2001/REC-xml-c14n-20010315"/>
[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/2001/REC-xml-c14n-20010315"/>
[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. Note that this example, and all examples in
this specification, are not in canonical form.
[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;
the design also permits arbitrary user specified algorithms.
[s05-11] Each Reference element includes the digest method and
resulting digest value calculated over the identified data object.
It may also include transformations that produced the input to the
digest operation. A data object is signed by computing its digest
value and a signature over that value. The signature is later
checked via reference and signature validation.
[s14-16] KeyInfo indicates the key to be used to validate the
signature. Possible forms for identification include certificates,
key names, and key agreement algorithms and information -- we define
only a few. KeyInfo is optional for two reasons. First, the signer
may not wish to reveal key information to all document processing
parties. Second, the information may be known within the
application's context and need not be represented explicitly. Since
KeyInfo is outside of SignedInfo, if the signer wishes to bind the
keying information to the signature, a Reference can easily identify
and include the KeyInfo as part of the signature.
2.1.1 More on Reference
[s05] <Reference
URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/">
[s06] <Transforms>
[s07] <Transform
Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
[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, as
opposed to 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, XPath, XML
schema validation, or XInclude. 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 the Working Group has specified mandatory (and
optional) canonicalization and decoding algorithms, user specified
transforms are permitted.
[s09-10] DigestMethod is the algorithm applied to the data after
Transforms is applied (if specified) to yield the DigestValue. The
signing of the DigestValue is what binds a resources content to the
signer's key.
2.2 Extended Example (Object and SignatureProperty)
This specification does not address mechanisms for making statements
or assertions. Instead, this document defines what it means for
something to be signed by an XML Signature (integrity, message
authentication, 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>
[p08] </SignedInfo>
[p09] ...
[p10] <Object>
[p11] <SignatureProperties>
[p12] <SignatureProperty Id="AMadeUpTimeStamp"
Target="#MySecondSignature">
[p13] <timestamp xmlns="http://www.ietf.org/rfcXXXX.txt">
[p14] <date>19990908</date>
[p15] <time>14:34:34:34</time>
[p16] </timestamp>
[p17] </SignatureProperty>
[p18] </SignatureProperties>
[p19] </Object>
[p20]</Signature>
[p04] The optional Type attribute of Reference provides information
about the resource identified by the URI. In particular, it can
indicate that it is an Object, SignatureProperty, or Manifest
element. This can be used by applications to initiate special
processing of some Reference elements. References to an XML data
element within an Object element SHOULD identify the actual element
pointed to. Where the element content is not XML (perhaps it is
binary or encoded data) the reference should identify the Object and
the Reference Type, if given, SHOULD indicate Object. Note that Type
is advisory and no action based on it or checking of its correctness
is required by core behavior.
[p10] Object is an optional element for including data objects within
the signature element or elsewhere. The Object can be optionally
typed and/or encoded.
[p11-18] Signature properties, such as time of signing, can be
optionally signed by identifying them from within a Reference.
(These properties are traditionally called signature "attributes"
although that term has no relationship to the XML term "attribute".)
2.3 Extended Example (Object and Manifest)
The Manifest element is provided to meet additional requirements not
directly addressed by the mandatory parts of this specification. Two
requirements and the way the Manifest satisfies them follow.
First, applications frequently need to efficiently sign multiple data
objects even where the signature operation itself is an expensive
public key signature. This requirement can be met by including
multiple Reference elements within SignedInfo since the inclusion of
each digest secures the data digested. However, some applications
may not want the core validation behavior associated with this
approach because it requires every Reference within SignedInfo to
undergo reference validation -- the DigestValue elements are checked.
These applications may wish to reserve reference validation decision
logic to themselves. For example, an application might receive a
signature valid SignedInfo element that includes three Reference
elements. If a single Reference fails (the identified data object
when digested does not yield the specified DigestValue) the signature
would fail core validation. However, the application may wish to
treat the signature over the two valid Reference elements as valid or
take different actions depending on which fails. To accomplish this,
SignedInfo would reference a Manifest element that contains one or
more Reference elements (with the same structure as those in
SignedInfo). Then, reference validation of the Manifest is under
application control.
Second, consider an application where many signatures (using
different keys) are applied to a large number of documents. An
inefficient solution is to have a separate signature (per key)
repeatedly applied to a large SignedInfo element (with many
References); this is wasteful and redundant. A more efficient
solution is to include many references in a single Manifest that is
then referenced from multiple Signature elements.
The example below includes a Reference that signs a Manifest found
within the Object element.
[ ] ...
[m01] <Reference URI="#MyFirstManifest"
[m02] Type="http://www.w3.org/2000/09/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>
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. (Note, it is
the canonical form of these references that are signed in 3.1.2
and validated in 3.2.1.)
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.
Note, if the Signature includes same-document references, [XML] or
[XML-schema] validation of the document might introduce changes that
break the signature. Consequently, applications should be careful to
consistently process the document or refrain from using external
contributions (e.g., defaults and entities).
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.
Comparison of values in reference and signature validation are over
the numeric (e.g., integer) or decoded octet sequence of the value.
Different implementations may produce different encoded digest and
signature values when processing the same resources because of
variances in their encoding, such as accidental white space. But if
one uses numeric or octet comparison (choose one) on both the stated
and computed values these problems are eliminated.
3.2.1 Reference Validation
1. Canonicalize the SignedInfo element based on the
CanonicalizationMethod in SignedInfo.
2. For each Reference in SignedInfo:
2.1 Obtain the data object to be digested. (For example, the
signature application may dereference the URI and execute
Transforms provided by the signer in the Reference element, or
it may obtain the content through other means such as a local
cache.)
2.2 Digest the resulting data object using the DigestMethod
specified in its Reference specification.
2.3 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. The application must
ensure that the CanonicalizationMethod has no dangerous side affects,
such as rewriting URIs, (see CanonicalizationMethod (section 4.3))
and that it Sees What is Signed, which is the canonical form.
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 confirm 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, and internal entity.
Schema Definition:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE schema
PUBLIC "-//W3C//DTD XMLSchema 200102//EN"
"http://www.w3.org/2001/XMLSchema.dtd"
[
<!ATTLIST schema
xmlns:ds CDATA #FIXED "http://www.w3.org/2000/09/xmldsig#">
<!ENTITY dsig 'http://www.w3.org/2000/09/xmldsig#'>
<!ENTITY % p ''>
<!ENTITY % s ''>
]>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
targetNamespace="http://www.w3.org/2000/09/xmldsig#"
version="0.1" elementFormDefault="qualified">
DTD:
<!--
The following entity declarations enable external/flexible content
in the Signature content model.
#PCDATA emulates schema:string; when combined with element types
it emulates schema mixed="true".
%foo.ANY permits the user to include their own element types from
other namespaces, for example:
<!ENTITY % KeyValue.ANY '| ecds:ECDSAKeyValue'>
...
<!ELEMENT ecds:ECDSAKeyValue (#PCDATA) >
-->
<!ENTITY % Object.ANY ''>
<!ENTITY % Method.ANY ''>
<!ENTITY % Transform.ANY ''>
<!ENTITY % SignatureProperty.ANY ''>
<!ENTITY % KeyInfo.ANY ''>
<!ENTITY % KeyValue.ANY ''>
<!ENTITY % PGPData.ANY ''>
<!ENTITY % X509Data.ANY ''>
<!ENTITY % SPKIData.ANY ''>
4.0.1 The ds:CryptoBinary Simple Type
This specification defines the ds:CryptoBinary simple type for
representing arbitrary-length integers (e.g., "bignums") in XML as
octet strings. The integer value is first converted to a "big
endian" bitstring. The bitstring is then padded with leading zero
bits so that the total number of bits == 0 mod 8 (so that there are
an integral number of octets). If the bitstring contains entire
leading octets that are zero, these are removed (so the high-order
octet 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).
This type is used by "bignum" values such as RSAKeyValue and
DSAKeyValue. If a value can be of type base64Binary or
ds:CryptoBinary they are defined as base64Binary. For example, if
the signature algorithm is RSA or DSA then SignatureValue represents
a bignum and could be ds:CryptoBinary. However, if HMAC-SHA1 is the
signature algorithm then SignatureValue could have leading zero
octets that must be preserved. Thus SignatureValue is generically
defined as of type base64Binary.
Schema Definition:
<simpleType name="CryptoBinary">
<restriction base="base64Binary">
</restriction>
</simpleType>
4.1 The Signature element
The Signature element is the root element of an XML Signature.
Implementation MUST generate laxly schema valid [XML-schema]
Signature elements as specified by the following schema:
Schema Definition:
<element name="Signature" type="ds:SignatureType"/>
<complexType name="SignatureType">
<sequence>
<element ref="ds:SignedInfo"/>
<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>
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
identify two SignatureMethod algorithms, one mandatory and one
optional to implement, user specified algorithms may be used as well.
Schema Definition:
<element name="SignatureValue" type="ds:SignatureValueType"/>
<complexType name="SignatureValueType">
<simpleContent>
<extension base="base64Binary">
<attribute name="Id" type="ID" use="optional"/>
</extension>
</simpleContent>
</complexType>
DTD:
<!ELEMENT SignatureValue (#PCDATA) >
<!ATTLIST SignatureValue
Id ID #IMPLIED>
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" type="ds:SignedInfoType"/>
<complexType name="SignedInfoType">
<sequence>
<element ref="ds:CanonicalizationMethod"/>
<element ref="ds:SignatureMethod"/>
<element ref="ds:Reference" maxOccurs="unbounded"/>
</sequence>
<attribute name="Id" type="ID" use="optional"/>
</complexType>
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 canonicalization algorithms.
Alternatives to the REQUIRED canonicalization algorithms (section
6.5), such as Canonical XML with Comments (section 6.5.1) or a
minimal canonicalization (such as CRLF and charset normalization),
may be explicitly specified but are NOT REQUIRED. Consequently,
their use may not interoperate with other applications that do not
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 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 algorithms which process XML as nodes or characters:
* XML based canonicalization implementations MUST be provided
with a [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.
* Text based canonicalization algorithms (such as CRLF and
charset normalization) should be provided with the UTF-8 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
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. Use of text based canonicalization of SignedInfo
is NOT RECOMMENDED.
We recommend applications that implement a text-based instead of
XML-based canonicalization -- such as resource constrained apps --
generate canonicalized XML as their output serialization so as to
mitigate interoperability and security concerns. For instance, such
an implementation SHOULD (at least) generate standalone XML instances
[XML].
NOTE: The signature application must exercise great care in accepting
and executing an arbitrary CanonicalizationMethod. For example, the
canonicalization method could rewrite the URIs of the References
being validated. Or, the method could massively transform SignedInfo
so that validation would always succeed (i.e., converting it to a
trivial signature with a known key over trivial data). Since
CanonicalizationMethod is inside SignedInfo, in the resulting
canonical form it could erase itself from SignedInfo or modify the
SignedInfo element so that it appears that a different
canonicalization function was used! Thus a Signature which appears to
authenticate the desired data with the desired key, DigestMethod, and
SignatureMethod, can be meaningless if a capricious
CanonicalizationMethod is used.
Schema Definition:
<element name="CanonicalizationMethod"
type="ds:CanonicalizationMethodType"/>
<complexType name="CanonicalizationMethodType" mixed="true">
<sequence>
<any namespace="##any" minOccurs="0" maxOccurs="unbounded"/>
<!-- (0,unbounded) elements from (1,1) namespace -->
</sequence>
<attribute name="Algorithm" type="anyURI" use="required"/>
</complexType>
DTD:
<!ELEMENT CanonicalizationMethod (#PCDATA %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" type="ds:SignatureMethodType"/>
<complexType name="SignatureMethodType" mixed="true">
<sequence>
<element name="HMACOutputLength" minOccurs="0"
type="ds:HMACOutputLengthType"/>
<any namespace="##other" minOccurs="0" maxOccurs="unbounded"/>
<!-- (0,unbounded) elements from (1,1) external namespace -->
</sequence>
<attribute name="Algorithm" type="anyURI" use="required"/>
</complexType>
DTD:
<!ELEMENT SignatureMethod
(#PCDATA|HMACOutputLength %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" type="ds:ReferenceType"/>
<complexType name="ReferenceType">
<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="anyURI" use="optional"/>
<attribute name="Type" type="anyURI" use="optional"/>
</complexType>
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
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 RFC2732 [URI-Literal]
are permitted. Disallowed characters must be escaped as follows:
1. Each disallowed character is converted to [UTF-8] as one or more
octets.
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 octet value).