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]
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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
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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]
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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:
]>
...
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
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* 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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the following structure (where "?" denotes zero or one occurrence;
"+" denotes one or more occurrences; and "*" denotes zero or more
occurrences):
(CanonicalizationMethod)
(SignatureMethod)
(
(Transforms)?
(DigestMethod)
(DigestValue)
)+
(SignatureValue)
(KeyInfo)?
(Object)*
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]
[s02]
[s03]
[s06]
[s07]
[s09] j6lwx3rvEPO0vKtMup4NbeVu8nk=
[s11]
[s12]
[s13] MC0CFFrVLtRlk=...
[s14]
[s15a]
[s15b]
[s15c] ...
...
... ...
[s15d]
[s15e]
[s16]
[s17]
[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]
[s06]
[s07]
[s09] j6lwx3rvEPO0vKtMup4NbeVu8nk=
[s11]
[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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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].)
[ ]
[p01]
[ ] ...
[p02]
[ ] ...
[p03]
[p05] k3453rvEPO0vKtMup4NbeVu8nk=
[p07]
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RFC 3075 XML-Signature Syntax and Processing March 2001
[p08]
[p09] ...
[p10]
[p20]
[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]
[m03] 345x3rvEPO0vKtMup4NbeVu8nk=
[m05]
[ ] ...
[m06]
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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:
]>
DTD:
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:
DTD:
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:
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DTD:
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:
DTD:
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:
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DTD:
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:
DTD:
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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RFC 3075 XML-Signature Syntax and Processing March 2001
DTD:
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
DTD:
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:
DTD:
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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RFC 3075 XML-Signature Syntax and Processing March 2001
Schema Definition:
DTD:
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:
DTD:
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,
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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
DTD
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
CN=TAMURA Kent, OU=TRL, O=IBM,
L=Yamato-shi, ST=Kanagawa, C=JP
12345678
31d97bd7
Subject of Certificate B
MIICXTCCA..
MIICPzCCA...
MIICSTCCA...
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
DTD
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:
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RFC 3075 XML-Signature Syntax and Processing March 2001
DTD:
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:
RFC, FYI, BCP