XML-Signature Syntax and Processing
XML-Signature Syntax and Processing
W3C Recommendation 12 February 2002
This version:
Latest version:
Previous version:
[corresponds to
CR-xmldsig-core-20001031
Editors
Donald Eastlake <
dee3@torque.pothole.com
Joseph Reagle <
reagle@w3.org
David Solo <
dsolo@alum.mit.edu
Authors
Mark Bartel <
mbartel@accelio.com
John Boyer <
jboyer@PureEdge.com
Barb Fox <
bfox@Exchange.Microsoft.com
Brian LaMacchia <
bal@microsoft.com
Ed Simon <
edsimon@xmlsec.com
Contributors
See
Acknowledgements
The Internet Society
W3C
® (
MIT
INRIA
Keio
), All Rights Reserved. W3C
liability
trademark
docum
ent use
and
software
licensing
rules apply.
Abstract
This document specifies XML digital signature processing rules and syntax. XML
Signatures provide
integrity
message authentication
and/or
signer
authentication
services for data of any type, whether located within the XML
that includes the signature or elsewhere.
Status of this document
This document has been reviewed by W3C Members and other interested parties and
has been endorsed by the Director as a W3C Recommendation. It is a stable
document and may be used as reference material or cited as a normative
reference from another document. W3C's role in making the Recommendation is to
draw attention to the specification and to promote its widespread deployment.
This enhances the functionality and interoperability of the Web.
This specification was produced by the IETF/W3C
XML Signature Working Group
W3C Activity Statement
which believes the specification is sufficient for the creation of independent
interoperable implementations; the
Interoperability
Report
shows at least 10 implementations with at least two interoperable
implementations over every feature.
Patent disclosures relevant to this specification may be found on the Working
Group's
patent
disclosure page
, in conformance with W3C policy, and the
IETF Page of Intellectual Property Rights
Notices
, in conformance with IETF policy.
Please report errors in this document to
w3c-ietf-xmldsig@w3.org
archive
).
The list of known errors in this specification is available at
The English version of this specification is the only normative version.
Information about translations of this document (if any) is available
A list of current W3C Technical Reports can be found at
Table of Contents
Introduction
Editorial Conventions
Design Philosophy
Versions, Namespaces and Identifiers
Acknowledgements
Signature Overview and Examples
Simple Example (
Signature
SignedInfo
Method
s, and
Reference
s)
More on
Reference
Extended Example (
Object
and
SignatureProperty
Extended Example (
Object
and
Manifest
Processing Rules
Signature Generation
Signature Validation
Core Signature Syntax
The
Signature
element
The
SignatureValue
Element
The
SignedInfo
Element
The
CanonicalizationMethod
Element
The
SignatureMethod
Element
The
Reference
Element
The
URI
Attribute
The Reference Processing
Model
Same-Document URI-References
The
Transforms
Element
The
DigestMethod
Element
The
DigestValue
Element
The
KeyInfo
Element
The
KeyName
Element
The
KeyValue
Element
The
DSAKeyValue
Element
The
RSAKeyValue
Element
The
RetrievalMethod
Element
The
X509Data
Element
The
PGPData
Element
The
SPKIData
Element
The
MgmtData
Element
The
Object
Element
Additional Signature Syntax
The
Manifest
Element
The
SignatureProperties
Element
Processing Instructions
Comments in dsig Elements
Algorithms
Algorithm Identifiers and Implementation
Requirements
Message Digests
Message Authentication Codes
Signature Algorithms
Canonicalization Algorithms
Transform Algorithms
Canonicalization
Base64
XPath Filtering
Enveloped Signature Transform
XSLT Transform
XML Canonicalization and Syntax Constraint
Considerations
XML 1.0, Syntax Constraints, and Canonicalization
DOM/SAX Processing and Canonicalization
Namespace Context and Portable
Signatures
Security Considerations
Transforms
Only What is Signed is Secure
Only What is "Seen" Should be Signed
"See" What is Signed
Check the Security Model
Algorithms, Key Lengths, Etc.
Schema, DTD, Data Model, and Valid Examples
Definitions
References
Authors' Address
1.0
Introduction
This document specifies XML syntax and processing rules for creating and
representing digital signatures. XML Signatures can be applied to any
digital content (data object)
including XML. An XML Signature may be applied to the content of one or more
resources.
Enveloped
or
enveloping
signatures are
over data within the same XML document as the signature;
detached
signatures are over
data external to the signature element. 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
SignatureProperties
XSLT is identified and defined by an external
URI
SHA1 is identified via this specification's namespace and defined via a
normative reference
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:

"xmldsig-core-schema.dtd" [ "http://www.w3.org/2000/09/xmldsig#"> ]>


...
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, Capslock
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 the following structure (where "?" denotes zero or one occurrence; "+"
denotes one or more occurrences; and "*" denotes zero or more occurrences):




(
()?


)+


()?
()*

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 (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
Reference
)s
The following example is a detached signature of the content of the HTML4 in XML
specification.
[s01] xmlns="http://www.w3.org/2000/09/xmldsig#">
[s02]
[s03] Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
[s04] Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/>
[s05] URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/">
[s06]
[s07]
[s08]
[s09]
[s10] 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. 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
also may include transformations that produced the input to the digest operation.
A data object is signed by computing its digest value and a signature over that
value. The signature is later checked via
reference
and
signature validation
[s14-16]
KeyInfo
indicates the key to be used to
validate the signature. Possible forms for identification include certificates,
key names, and key agreement algorithms and information -- we define only a few.
KeyInfo
is optional for two reasons. First, the signer may not wish
to reveal key information to all document processing parties. Second, the
information may be known within the application's context and need not be
represented explicitly. Since
KeyInfo
is outside of
SignedInfo
, if the signer wishes to bind the keying information to
the signature, a
Reference
can easily identify and include the
KeyInfo
as part of the signature.
2.1.1 More on
Reference
[s05]
[s06]
[s07]
[s08]
[s09]
[s10] 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, 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]
.)
[ ]
[p01]
[ ] ...
[p02]
[ ] ...
[p03] [p04] Type="http://www.w3.org/2000/09/xmldsig#SignatureProperties">
[p05]
[p06] k3453rvEPO0vKtMup4NbeVu8nk=
[p07]
[p08]
[p09] ...
[p10]
[p11]
[p12]
[p13]
[p14] 19990908
[p15]
[p16]
[p17]
[p18]
[p19]
[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 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
Reference
s); 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] [m02] Type="http://www.w3.org/2000/09/xmldsig#Manifest">
[m03]
[m04] 345x3rvEPO0vKtMup4NbeVu8nk=
[m05]
[ ] ...
[m06]
[m07]
[m08]
[m09] ...
[m10]
[m11]
[m12] ...
[m13]
[m14]
[m15]
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:
Apply the
Transforms
, as determined by the application, to the
data object.
Calculate the digest value over the resulting data object.
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
Create
SignedInfo
element with
SignatureMethod
CanonicalizationMethod
and
Reference
(s).
Canonicalize and then calculate the
SignatureValue
over
SignedInfo
based on algorithms specified in
SignedInfo
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
Canonicalize the
SignedInfo
element based on the
CanonicalizationMethod
in
SignedInfo
For each
Reference
in
SignedInfo
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.)
Digest the resulting data object using the
DigestMethod
specified in its
Reference
specification.
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
Obtain the keying information from
KeyInfo
or from an external source.
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:


PUBLIC "-//W3C//DTD XMLSchema 200102//EN" "http://www.w3.org/2001/XMLSchema.dtd"
xmlns:ds CDATA #FIXED "http://www.w3.org/2000/09/xmldsig#">



]>

xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
targetNamespace="http://www.w3.org/2000/09/xmldsig#"
version="0.1" elementFormDefault="qualified">
DTD:










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:





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:











DTD:


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:









DTD:


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:










DTD:

SignatureMethod, Reference+) >
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
Reference
s 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:









DTD:


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:










DTD:


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:












DTD:


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 RFC 2732 [
URI-Literal
] are
permitted. Disallowed characters must be escaped as follows:
Each disallowed character is converted to [
UTF-8
] as
one or more octets.
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).
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 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
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.
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 node-sets" 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 an octet stream and the next transform requires a
node-set, the signature application MUST attempt to parse the octets yielding
the required node-set via [
XML
] well-formed processing.
If the data object is a node-set and the next transform requires octets, the
signature application MUST attempt to convert the node-set to an octet stream
using Canonical XML [
XML-C14N
].
Users may specify alternative transforms that override 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 transform that requires XML parsing is
applied. (See
Transforms
(section 4.3.3.1).)
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 any
canonicalization
that preserves comments. (Otherwise
URI="#foo" will automatically remove comments before the canonicalization 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 an 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 transform rather than a URI fragment (barename XPointer
resolution in external resources is not REQUIRED in this specification).
URI=""
Identifies the node-set (minus any comment nodes) of the XML resource
containing the signature
URI="#chapter1"
Identifies a node-set containing the element with ID attribute value 'chapter1'
of the XML resource containing the signature. XML Signature (and its
applications) modify this node-set 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 [
XML-C14N
]. 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:
discard point nodes
replace each range node with all XPath nodes having full or partial content
within the range
replace the root node with its children (if it is in the node-set)
replace any element node
with
plus all
descendants of
(text, comment, PI, element) and all
namespace and attribute nodes of
and its descendant
elements.
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. It's necessary because when [
XML-C14N
] is
passed a node-set, it processes the node-set as is: with or without comments.
Only when it's called with an octet stream does it invoke its own XPath
expressions (default or without comments). Therefore to retain the default
behavior of stripping comments when passed a node-set, they are removed in the
last step if the URI is not a full XPointer. 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).
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
s 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:















DTD:


Algorithm CDATA #REQUIRED >


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 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:








DTD:


Algorithm CDATA #REQUIRED >
4.3.3.6 The
DigestValue
Element
DigestValue is an element that contains the encoded value of the digest. The
digest is always encoded using base64 [
MIME
].
Schema Definition:





DTD:



4.4 The
KeyInfo
Element
KeyInfo
is an optional element that enables the recipient(s) to
obtain the key needed to validate the signature.
KeyInfo
may
contain keys, names, certificates and other public key management information,
such as in-band key distribution or key agreement data. This specification
defines a few simple types but applications may extend those types or all
together replace them with their own key identification and exchange semantics
using the XML namespace facility. [
XML-ns
] However,
questions of trust of such key information (e.g., its authenticity or
strength) are out of scope of this specification and left to the application.
If
KeyInfo
is omitted, the recipient is expected to be able to
identify the key based on application context. Multiple declarations within
KeyInfo
refer to the same key. While applications may define and use
any mechanism they choose through inclusion of elements from a different
namespace, compliant versions MUST implement
KeyValue
(section 4.4.2) and SHOULD
implement
RetrievalMethod
(section 4.4.3).
The schema/DTD specifications of many of
KeyInfo
's children (e.g.,
PGPData
SPKIData
X509Data
) permit their
content to be extended/complemented with elements from another namespace. This
may be done only if it is safe to ignore these extension elements while claiming
support for the types defined in this specification. Otherwise, external
elements, including
alternative
structures to those defined by this
specification, MUST be a child of
KeyInfo
. For example, should a
complete XML-PGP standard be defined, its root element MUST be a child of
KeyInfo
. (Of course, new structures from external namespaces can
incorporate elements from the
&dsig;
namespace via features of
the type definition language. For instance, they can create a DTD that mixes
their own and dsig qualified elements, or a schema that permits, includes,
imports, or derives new types based on
&dsig;
elements.)
The following list summarizes the
KeyInfo
types that are allocated
an identifier in the
&dsig;
namespace; these can be used within
the
RetrievalMethod
Type
attribute to describe a remote
KeyInfo
structure.
In addition to the types above for which we define an XML structure, we specify
one additional type to indicate a
binary (ASN.1 DER) X.509 Certificate
Schema Definition:
















DTD:

X509Data|PGPData|SPKIData|MgmtData %KeyInfo.ANY;)* >
Id ID #IMPLIED >
4.4.1 The
KeyName
Element
The
KeyName
element contains a string value (in which white space is
significant) which may be used by the signer to communicate a key identifier to
the recipient. Typically,
KeyName
contains an identifier related to
the key pair used to sign the message, but it may contain other protocol-related
information that indirectly identifies a key pair. (Common uses of
KeyName
include simple string names for keys, a key index, a
distinguished name (DN), an email address, etc.)
Schema Definition:


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). The
KeyValue
element may include externally defined public keys values
represented as PCDATA or element types from an external namespace.
Schema Definition:









DTD:


4.4.2.1 The
DSAKeyValue
Element
Identifier
Type="
(this can be used within a
RetrievalMethod
or
Reference
element to identify the referent's type)
DSA keys and the DSA signature algorithm are specified in [DSS]. DSA public key
values can have the following fields:
a prime modulus meeting the [DSS] requirements
an integer in the range 2**159 < Q < 2**160 which is a prime divisor of
P-1
an integer with certain properties with respect to P and Q
G**X mod P (where X is part of the private key and not made public)
(P - 1) / Q
seed
a DSA prime generation seed
pgenCounter
a DSA prime generation counter
Parameter J is available for inclusion solely for efficiency as it is
calculatable from P and Q. Parameters seed and pgenCounter are used in the DSA
prime number generation algorithm specified in [DSS]. As such, they are optional
but must either both be present or both be absent. This prime generation
algorithm is designed to provide assurance that a weak prime is not being used
and it yields a P and Q value. Parameters P, Q, and G can be public and common to
a group of users. They might be known from application context. As such, they are
optional but P and Q must either both appear or both be absent. If all of
seed
, and
pgenCounter
are present, implementations are not required to check if they are consistent and
are free to use either
and
or
seed
and
pgenCounter
. All parameters are encoded as base64 [
MIME
] values.
Arbitrary-length integers (e.g. "bignums" such as RSA moduli) are represented in
XML as octet strings as defined by the
ds:CryptoBinary
type
Schema Definition:
















DTD Definition:








4.4.2.2 The
RSAKeyValue
Element
Identifier
Type="
(this can be used within a
RetrievalMethod
or
Reference
element to identify the referent's type)
RSA key values have two fields: Modulus and Exponent.

xA7SEU+e0yQH5rm9kbCDN9o3aPIo7HbP7tX6WOocLZAtNfyxSZDU16ksL6W
jubafOqNEpcwR3RdFsT7bCqnXPBe5ELh5u4VEy19MzxkXRgrMvavzyBpVRgBUwUlV
5foK5hhmbktQhyNdy/6LpQRhDUDsTvK+g9Ucj47es9AQJ3U=

AQAB

Arbitrary-length integers (e.g. "bignums" such as RSA moduli) are represented in
XML as octet strings as defined by the
ds:CryptoBinary
type
Schema Definition:







DTD Definition:



4.4.3 The
RetrievalMethod
Element
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.
Type
is an optional identifier for the type of data to be retrieved.
The result of dereferencing a
RetrievalMethod
Reference
for all
KeyInfo
types defined by this specification
(section 4.4) with a corresponding XML structure is an XML element or document
with that element as the root. The
rawX509Certificate
KeyInfo
(for which there is no XML structure) returns a binary X509
certificate.
Schema Definition









DTD


URI CDATA #REQUIRED
Type CDATA #IMPLIED >
4.4.4 The
X509Data
Element
Identifier
Type="
(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 a
revocation list). The content of
X509Data
is:
At least one element, from the following set of element types; any of these may
appear together or more than once iff (if and only if) each instance describes
or is related to the same certificate:
The
X509IssuerSerial
element, which contains an X.509 issuer
distinguished name/serial number pair that SHOULD be compliant with RFC2253
LDAP-DN
],
The
X509SubjectName
element, which contains an X.509 subject
distinguished name that SHOULD be compliant with RFC2253 [
LDAP-DN
],
The
X509SKI
element, which contains the base64 encoded plain
(i.e. non-DER-encoded) value of a X509 V.3 SubjectKeyIdentifier extension.
The
X509Certificate
element, which contains a base64-encoded
X509v3
] certificate, and
Elements from an external namespace which accompanies/complements any of
the elements above.
The
X509CRL
element, which contains a base64-encoded
certificate revocation list (CRL) [
X509v3
].
Any
X509IssuerSerial
X509SKI
, and
X509SubjectName
elements that appear MUST refer to the certificate
or certificates containing the validation key. All such elements that refer to a
particular individual certificate MUST be grouped inside a single
X509Data
element and if the certificate to which they refer appears,
it MUST also be in that
X509Data
element.
Any
X509IssuerSerial
X509SKI
, and
X509SubjectName
elements that relate to the same key but different
certificates MUST be grouped within a single
KeyInfo
but MAY occur
in multiple
X509Data
elements.
All certificates appearing in an
X509Data
element MUST relate to the
validation key by either containing it or being part of a certification chain
that terminates in a certificate containing the validation key.
No ordering is implied by the above constraints. The comments in the following
instance demonstrate these constraints:




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 and CRLs 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.
Also, strings in DNames
X509IssuerSerial
X509SubjectName
, and
KeyName
if approriate) should be encoded as follows:
Consider the string as consisting of Unicode characters.
Escape occurrences of the following special characters by prefixing it with the
"\" character:
a "#" character occurring at the beginning of the string
one of the characters ",", "+", """, "\", "<", ">" or ";"
Escape all occurrences of ASCII control characters (Unicode range \x00 - \x1f)
by replacing them with "\" followed by a two digit hex number showing its
Unicode number.
Escape any trailing white space by replacing "\ " with "\20".
Since a XML document logically consists of characters, not octets, the
resulting Unicode string is finally encoded according to the character encoding
used for producing the physical representation of the XML document.
Schema Definition



















DTD

X509Certificate | X509CRL)+ %X509.ANY;)>







4.4.5 The
PGPData
Element
Identifier
Type="
(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 base64Binary sequence 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]. These children
element types can be complemented/extended by siblings from an external namespace
within
PGPData
, or
PGPData
can be replaced all together
with an alternative PGP XML structure as a child of
KeyInfo
PGPData
must contain one
PGPKeyID
and/or one
PGPKeyPacket
and 0 or more elements from an external namespace.
Schema Definition:







maxOccurs="unbounded"/>



maxOccurs="unbounded"/>



DTD:




4.4.6 The
SPKIData
Element
Identifier
Type="
(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.
SPKISexp
is the base64 encoding of a SPKI canonical S-expression.
SPKIData
must have at least one
SPKISexp
SPKISexp
can be complemented/extended by siblings from an external
namespace within
SPKIData
, or
SPKIData
can be entirely
replaced with an alternative SPKI XML structure as a child of
KeyInfo
Schema Definition:








DTD:



4.4.7 The
MgmtData
Element
Identifier
Type="
(this can be used within a
RetrievalMethod
or
Reference
element to identify the referent's type)
The
MgmtData
element within
KeyInfo
is a string value
used to convey in-band key distribution or agreement data. For example, DH key
exchange, RSA key encryption, etc. Use of this element is NOT RECOMMENDED. It
provides a syntactic hook where in-band key distribution or agreement data can be
placed. However, superior interoperable child elements of
KeyInfo
for the transmission of encrypted keys and for key agreement are being specified
by the W3C XML Encryption Working Group and they should be used instead of
MgmtData
Schema Definition:


DTD:


4.5 The
Object
Element
Identifier
Type=
"http://www.w3.org/2000/09/xmldsig#Object"
(this can be used within a
Reference
element to identify
the referent's type)
Object
is an optional element that may occur one or more times. When
present, this element may contain any data. The
Object
element may
include optional MIME type, ID, and encoding attributes.
The
Object
's
Encoding
attributed may be used to provide
a URI that identifies the method by which the object is encoded (e.g., a binary
file).
The
MimeType
attribute is an optional attribute which describes the
data within the
Object
(independent of its encoding). This is a
string with values defined by [
MIME
]. For example, if the
Object
contains base64 encoded
PNG
, the
Encoding
may be
specified as 'base64' and the
MimeType
as 'image/png'. This
attribute is purely advisory; no validation of the
MimeType
information is required by this specification. Applications which require
normative type and encoding information for signature validation should specify
Transforms
with well defined resulting
types and/or encodings.
The
Object
's
Id
is commonly referenced from a
Reference
in
SignedInfo
, or
Manifest
. This
element is typically used for
enveloping signatures
where the object being signed is to be
included in the signature element. The digest is calculated over the entire
Object
element including start and end tags.
Note, if the application wishes to exclude the

tags
from the digest calculation the
Reference
must identify the actual
data object (easy for XML documents) or a transform must be used to remove the
Object
tags (likely where the data object is non-XML). Exclusion of
the object tags may be desired for cases where one wants the signature to remain
valid if the data object is moved from inside a signature to outside the
signature (or vice versa), or where the content of the
Object
is an
encoding of an original binary document and it is desired to extract and decode
so as to sign the original bitwise representation.
Schema Definition:










DTD:


Id ID #IMPLIED
MimeType CDATA #IMPLIED
Encoding CDATA #IMPLIED >
5.0
Additional Signature
Syntax
This section describes the optional to implement
Manifest
and
SignatureProperties
elements and describes the handling of XML
processing instructions and comments. With respect to the elements
Manifest
and
SignatureProperties
this section specifies
syntax and little behavior -- it is left to the application. These elements can
appear anywhere the parent's content model permits; the
Signature
content model only permits them within
Object
5.1 The
Manifest
Element
Identifier
Type=
"http://www.w3.org/2000/09/xmldsig#Manifest"
(this can be used within a
Reference
element to identify
the referent's type)
The
Manifest
element provides a list of
Reference
s. The
difference from the list in
SignedInfo
is that it is application
defined which, if any, of the digests are actually checked against the objects
referenced and what to do if the object is inaccessible or the digest compare
fails. If a
Manifest
is pointed to from
SignedInfo
, the
digest over the
Manifest
itself will be checked by the core
signature validation behavior. The digests within such a
Manifest
are checked at the application's discretion. If a
Manifest
is
referenced from another
Manifest
, even the overall digest of this
two level deep
Manifest
might not be checked.
Schema Definition:








DTD:


Id ID #IMPLIED >
5.2 The
SignatureProperties
Element
Identifier
Type="
(this can be used within a
Reference
element to identify
the referent's type)
Additional information items concerning the generation of the signature(s) can be
placed in a
SignatureProperty
element (i.e., date/time stamp or the
serial number of cryptographic hardware used in signature generation).
Schema Definition:
















DTD:


Id ID #IMPLIED >


Target CDATA #REQUIRED
Id ID #IMPLIED >
5.3
Processing Instructions
in Signature
Elements
No XML processing instructions (PIs) are used by this specification.
Note that PIs placed inside
SignedInfo
by an application will be
signed unless the
CanonicalizationMethod
algorithm discards them.
(This is true for any signed XML content.) All of the
CanonicalizationMethod
s identified within this specification retain
PIs. When a PI is part of content that is signed (e.g., within
SignedInfo
or referenced XML documents) any change to the PI will
obviously result in a signature failure.
5.4
Comments
in Signature Elements
XML comments are not used by this specification.
Note that unless
CanonicalizationMethod
removes comments within
SignedInfo
or any other referenced XML (which [
XML-C14N
] does), they will be signed. Consequently, if
they are retained, a change to the comment will cause a signature failure.
Similarly, the XML signature over any XML data will be sensitive to comment
changes unless a comment-ignoring canonicalization/transform method, such as the
Canonical XML [
XML-C14N
], is specified.
6.0
Algorithms
This section identifies algorithms used with the XML digital signature
specification. Entries contain the identifier to be used in
Signature
elements, a reference to the formal specification, and
definitions, where applicable, for the representation of keys and the results of
cryptographic operations.
6.1
Algorithm
Identifiers and
Implementation Requirements
Algorithms are identified by URIs that appear as an attribute to the element that
identifies the algorithms' role (
DigestMethod
Transform
SignatureMethod
, or
CanonicalizationMethod
). All algorithms used herein take parameters
but in many cases the parameters are implicit. For example, a
SignatureMethod
is implicitly given two parameters: the keying info
and the output of
CanonicalizationMethod
. Explicit additional
parameters to an algorithm appear as content elements within the algorithm role
element. Such parameter elements have a descriptive element name, which is
frequently algorithm specific, and MUST be in the XML Signature namespace or an
algorithm specific namespace.
This specification defines a set of algorithms, their URIs, and requirements for
implementation. Requirements are specified over implementation, not over
requirements for signature use. Furthermore, the mechanism is extensible;
alternative algorithms may be used by signature applications.
Digest
Required SHA1
Encoding
Required base64
base64
MAC
Required HMAC-SHA1
Signature
Required DSAwithSHA1 (DSS)
dsa-sha1
Recommended RSAwithSHA1
rsa-sha1
Canonicalization
Required Canonical XML (omits comments)
Recommended Canonical XML with Comments
Transform
Optional XSLT
Recommended XPath
Required Enveloped Signature*
* The Enveloped Signature transform removes the
Signature
element
from the calculation of the signature when the signature is within the content
that it is being signed. This MAY be implemented via the RECOMMENDED XPath
specification specified in 6.6.4:
Enveloped
Signature Transform
; it MUST have the same effect as that specified by the
XPath Transform
6.2
Message Digests
Only one digest algorithm is defined herein. However, it is expected that one or
more additional strong digest algorithms will be developed in connection with the
US Advanced Encryption Standard effort. Use of
MD5
MD5
is NOT RECOMMENDED because recent advances in cryptanalysis have cast doubt on
its strength.
6.2.1
SHA-1
Identifier:
The
SHA-1
algorithm [
SHA-1
] takes no explicit parameters. An
example of an SHA-1 DigestAlg element is:

A SHA-1 digest is a 160-bit string. The content of the DigestValue element shall
be the base64 encoding of this bit string viewed as a 20-octet octet stream. For
example, the DigestValue element for the message digest:
A9993E36 4706816A BA3E2571 7850C26C 9CD0D89D
from Appendix A of the SHA-1 standard would be:
qZk+NkcGgWq6PiVxeFDCbJzQ2J0=
6.3
Message Authentication Codes
MAC algorithms take two implicit parameters, their keying material determined
from
KeyInfo
and the octet stream output by
CanonicalizationMethod
. MACs and signature algorithms are
syntactically identical but a MAC implies a shared secret key.
6.3.1
HMAC
Identifier:
The
HMAC
algorithm (RFC2104 [
HMAC
]) takes the truncation length in bits as a parameter;
if the parameter is not specified then all the bits of the hash are output. An
example of an HMAC
SignatureMethod
element:

128

The output of the HMAC algorithm is ultimately the output (possibly truncated) of
the chosen digest algorithm. This value shall be base64 encoded in the same
straightforward fashion as the output of the digest algorithms. Example: the
SignatureValue element for the HMAC-SHA1 digest
9294727A 3638BB1C 13F48EF8 158BFC9D
from the test vectors in [
HMAC
] would be
kpRyejY4uxwT9I74FYv8nQ==
Schema Definition:




DTD:


6.4
Signature Algorithms
Signature algorithms take two implicit parameters, their keying material
determined from
KeyInfo
and the octet stream output by
CanonicalizationMethod
. Signature and MAC algorithms are
syntactically identical but a signature implies public key cryptography.
6.4.1
DSA
Identifier:
The DSA algorithm [
DSS
] takes no explicit parameters. An
example of a DSA
SignatureMethod
element is:

The output of the DSA algorithm consists of a pair of integers usually referred
by the pair (r, s). The signature value consists of the base64 encoding of the
concatenation of two octet-streams that respectively result from the
octet-encoding of the values r and s in that order. Integer to octet-stream
conversion must be done according to the I2OSP operation defined in the
RFC 2437
PKCS1
] specification with a
parameter equal
to 20. For example, the SignatureValue element for a DSA signature
) with values specified in hexadecimal:
r = 8BAC1AB6 6410435C B7181F95 B16AB97C 92B341C0
s = 41E2345F 1F56DF24 58F426D1 55B4BA2D B6DCD8C8
from the example in Appendix 5 of the DSS standard would be

i6watmQQQ1y3GB+VsWq5fJKzQcBB4jRfH1bfJFj0JtFVtLotttzYyA==
6.4.2
PKCS1
(RSA-SHA1)
Identifier:
The expression "RSA algorithm" as used in this draft refers to the
RSASSA-PKCS1-v1_5 algorithm described in
RFC 2437
PKCS1
]. The RSA algorithm takes no explicit parameters. An
example of an RSA SignatureMethod element is:

The
SignatureValue
content for an RSA signature is the base64 [
MIME
] encoding of the octet string computed as per
RFC 2437
PKCS1
, section 8.1.1: Signature generation for the
RSASSA-PKCS1-v1_5 signature scheme]. As specified in the EMSA-PKCS1-V1_5-ENCODE
function
RFC 2437
PKCS1
, section 9.2.1], the value input to the signature
function MUST contain a pre-pended algorithm object identifier for the hash
function, but the availability of an ASN.1 parser and recognition of OIDs is not
required of a signature verifier. The PKCS#1 v1.5 representation appears as:
CRYPT (PAD (ASN.1 (OID, DIGEST (data))))
Note that the padded ASN.1 will be of the following form:
01 | FF* | 00 | prefix | hash
where "|" is concatenation, "01", "FF", and "00" are fixed octets of the
corresponding hexadecimal value, "hash" is the SHA1 digest of the data, and
"prefix" is the ASN.1 BER SHA1 algorithm designator prefix required in PKCS1 [RFC
2437], that is,
hex 30 21 30 09 06 05 2B 0E 03 02 1A 05 00 04 14
This prefix is included to make it easier to use standard cryptographic
libraries. The FF octet MUST be repeated the maximum number of times such that
the value of the quantity being CRYPTed is one octet shorter than the RSA
modulus.
The resulting base64 [
MIME
] string is the value of the
child text node of the SignatureValue element, e.g.

IWijxQjUrcXBYoCei4QxjWo9Kg8D3p9tlWoT4t0/gyTE96639In0FZFY2/rvP+/bMJ01EArmKZsR5VW3rwoPxw=

6.5
Canonicalization Algorithms
If canonicalization is performed over octets, the canonicalization algorithms
take two implicit parameters: the content and its charset. The charset is derived
according to the rules of the transport protocols and media types (e.g, RFC2376
XML-MT
] defines the media types for XML). This
information is necessary to correctly sign and verify documents and often
requires careful server side configuration.
Various canonicalization algorithms require conversion to [
UTF-8
].The two algorithms below understand at least [
UTF-8
] and [
UTF-16
] as input
encodings. We RECOMMEND that externally specified algorithms do the same.
Knowledge of other encodings is OPTIONAL.
Various canonicalization algorithms transcode from a non-Unicode encoding to
Unicode. The two algorithms below perform text normalization during transcoding
NFC
NFC-Corrigendum
].
We RECOMMEND that externally specified canonicalization algorithms do the same.
(Note, there can be ambiguities in converting existing charsets to Unicode, for
an example see the XML Japanese Profile [
XML-Japanese
] Note.)
6.5.1
Canonical
XML
Identifier for REQUIRED Canonical XML (omits comments):
Identifier for Canonical XML with Comments:
An example of an XML canonicalization element is:

The normative specification of Canonical XML is [
XML-C14N
]. The algorithm is capable of taking as input
either an octet stream or an XPath node-set (or sufficiently functional
alternative). The algorithm produces an octet stream as output. Canonical XML is
easily parameterized (via an additional URI) to omit or retain comments.
6.6
Transform
Algorithms
Transform
algorithm has a single implicit parameter: an octet
stream from the
Reference
or the output of an earlier
Transform
Application developers are strongly encouraged to support all transforms listed
in this section as RECOMMENDED unless the application environment has resource
constraints that would make such support impractical. Compliance with this
recommendation will maximize application interoperability and libraries should be
available to enable support of these transforms in applications without extensive
development.
6.6.1
Canonicalization
Any canonicalization algorithm that can be used for
CanonicalizationMethod
(such as those in
Canonicalization Algorithms
(section 6.5)) can be used as
Transform
6.6.2
Base64
Identifiers:
The normative specification for base64 decoding transforms is [
MIME
]. The base64
Transform
element has no
content. The input is decoded by the algorithms. This transform is useful if an
application needs to sign the raw data associated with the encoded content of an
element.
This transform requires an octet stream for input. If an XPath node-set (or
sufficiently functional alternative) is given as input, then it is converted to
an octet stream by performing operations logically equivalent to 1) applying an
XPath transform with expression
self::text()
, then 2) taking the
string-value of the node-set. Thus, if an XML element is identified by a barename
XPointer in the
Reference
URI, and its content consists solely of
base64 encoded character data, then this transform automatically strips away the
start and end tags of the identified element and any of its descendant elements
as well as any descendant comments and processing instructions. The output of
this transform is an octet stream.
6.6.3
XPath
Filtering
Identifier:
The normative specification for XPath expression evaluation is [
XPath
]. The XPath expression to be evaluated appears as the
character content of a transform parameter child element named
XPath
The input required by this transform is an XPath node-set. Note that if the
actual input is an XPath node-set resulting from a null URI or barename XPointer
dereference, then comment nodes will have been omitted. If the actual input is an
octet stream, then the application MUST convert the octet stream to an XPath
node-set suitable for use by Canonical XML with Comments. (A subsequent
application of the REQUIRED Canonical XML algorithm would strip away these
comments.) In other words, the input node-set should be equivalent to the one
that would be created by the following process:
Initialize an XPath evaluation context by setting the initial node equal to the
input XML document's root node, and set the context position and size to 1.
Evaluate the XPath expression
(//. | //@* | //namespace::*)
The evaluation of this expression includes all of the document's nodes (including
comments) in the node-set representing the octet stream.
The transform output is also an XPath node-set. The XPath expression appearing in
the
XPath
parameter is evaluated once for each node in the input
node-set. The result is converted to a boolean. If the boolean is true, then the
node is included in the output node-set. If the boolean is false, then the node
is omitted from the output node-set.
Note:
Even if the input node-set has had comments removed, the
comment nodes still exist in the underlying parse tree and can separate text
nodes. For example, the markup
Hello, world!
contains two text nodes. Therefore, the expression
self::text()[string()="Hello, world!"]
would fail. Should this
problem arise in the application, it can be solved by either canonicalizing the
document before the XPath transform to physically remove the comments or by
matching the node based on the parent element's string value (e.g. by using the
expression
self::text()[string(parent::e)="Hello, world!"]
).
The primary purpose of this transform is to ensure that only specifically defined
changes to the input XML document are permitted after the signature is affixed.
This is done by omitting precisely those nodes that are allowed to change once
the signature is affixed, and including all other input nodes in the output. It
is the responsibility of the XPath expression author to include all nodes whose
change could affect the interpretation of the transform output in the application
context.
An important scenario would be a document requiring two enveloped signatures.
Each signature must omit itself from its own digest calculations, but it is also
necessary to exclude the second signature element from the digest calculations of
the first signature so that adding the second signature does not break the first
signature.
The XPath transform establishes the following evaluation context for each node of
the input node-set:
context node
equal to a node of the input node-set.
context position
, initialized to 1.
context size
, initialized to 1.
library of functions
equal to the function set defined in
XPath]
plus a function named
here
A set of variable bindings. No means for initializing these is defined. Thus,
the set of variable bindings used when evaluating the XPath expression is
empty, and use of a variable reference in the XPath expression results in an
error.
The set of namespace declarations in scope for the XPath expression.
As a result of the context node setting, the XPath expressions appearing in this
transform will be quite similar to those used in used in [
XSLT
], except that the size and position are always 1 to
reflect the fact that the transform is automatically visiting every node (in
XSLT, one recursively calls the command
apply-templates
to visit the
nodes of the input tree).
The function
here()
is defined as follows:
Function:
node-set
here
()
The
here
function returns a
node-set containing the attribute or processing instruction node or the parent
element of the text node that directly bears the XPath expression.  This
expression results in an error if the containing XPath expression does not appear
in the same XML document against which the XPath expression is being evaluated.
As an example, consider creating an enveloped signature (a
Signature
element that is a descendant of an element being signed). Although the signed
content should not be changed after signing, the elements within the
Signature
element are changing (e.g. the digest value must be put
inside the
DigestValue
and the
SignatureValue
must be
subsequently calculated). One way to prevent these changes from invalidating the
digest value in
DigestValue
is to add an XPath
Transform
that omits all
Signature
elements and their
descendants. For example,

...


...




not(ancestor-or-self::dsig:Signature)









...

Due to the null
Reference
URI in this example, the XPath transform
input node-set contains all nodes in the entire parse tree starting at the root
node (except the comment nodes). For each node in this node-set, the node is
included in the output node-set except if the node or one of its ancestors has a
tag of
Signature
that is in the namespace given by the replacement
text for the entity
&dsig;
A more elegant solution uses the
here
function to omit only the
Signature
containing the XPath Transform, thus allowing enveloped
signatures to sign other signatures. In the example above, use the
XPath
element:

count(ancestor-or-self::dsig:Signature |
here()/ancestor::dsig:Signature[1]) >
count(ancestor-or-self::dsig:Signature)
Since the XPath equality operator converts node sets to string values before
comparison, we must instead use the XPath union operator (|). For each node of
the document, the predicate expression is true if and only if the node-set
containing the node and its
Signature
element ancestors does not
include the enveloped
Signature
element containing the XPath
expression (the union does not produce a larger set if the enveloped
Signature
element is in the node-set given by
ancestor-or-self::Signature
).
6.6.4
Enveloped
Signature
Transform
Identifier:
An enveloped signature transform
removes the whole
Signature
element containing
from the
digest calculation of the
Reference
element containing
. The entire string of characters used by an XML
processor to match the
Signature
with the XML production
element
is removed. The output of the transform is equivalent to the
output that would result from replacing
with an XPath
transform containing the following
XPath
parameter element:

count(ancestor-or-self::dsig:Signature |
here()/ancestor::dsig:Signature[1]) >
count(ancestor-or-self::dsig:Signature)
The input and output requirements of this transform are identical to those of the
XPath transform, but may only be applied to a node-set from its parent XML
document. Note that it is not necessary to use an XPath expression evaluator to
create this transform. However, this transform MUST produce output in exactly the
same manner as the XPath transform parameterized by the XPath expression above.
6.6.5
XSLT
Transform
Identifier:
The normative specification for XSL Transformations is [
XSLT
]. Specification of a namespace-qualified stylesheet
element, which MUST be the sole child of the
Transform
element,
indicates that the specified style sheet should be used. Whether this
instantiates in-line processing of local XSLT declarations within the resource is
determined by the XSLT processing model; the ordered application of multiple
stylesheet may require multiple
Transforms
. No special provision is
made for the identification of a remote stylesheet at a given URI because it can
be communicated via an
xsl:include
or
xsl:import
within the
stylesheet
child of the
Transform
This transform requires an octet stream as input. If the actual input is an XPath
node-set, then the signature application should attempt to convert it to octets
(apply
Canonical XML
]) as described in
the Reference Processing Model
(section
4.3.3.2).
The output of this transform is an octet stream. The processing rules for the XSL
style sheet or transform element are stated in the XSLT specification [
XSLT
]. We RECOMMEND that XSLT transform authors use an
output method of
xml
for XML and HTML. As XSLT implementations do
not produce consistent serializations of their output, we further RECOMMEND
inserting a transform after the XSLT transform to canonicalize the output. These
steps will help to ensure interoperability of the resulting signatures among
applications that support the XSLT transform. Note that if the output is actually
HTML, then the result of these steps is logically equivalent [
XHTML
].
7.0
XML
Canonicalization
and Syntax Constraint Considerations
Digital signatures only work if the verification calculations are performed on
exactly the same bits as the signing calculations. If the surface representation
of the signed data can change between signing and verification, then some way to
standardize the changeable aspect must be used before signing and verification.
For example, even for simple ASCII text there are at least three widely used line
ending sequences. If it is possible for signed text to be modified from one line
ending convention to another between the time of signing and signature
verification, then the line endings need to be canonicalized to a standard form
before signing and verification or the signatures will break.
XML is subject to surface representation changes and to processing which discards
some surface information. For this reason, XML digital signatures have a
provision for indicating canonicalization methods in the signature so that a
verifier can use the same canonicalization as the signer.
Throughout this specification we distinguish between the canonicalization of a
Signature
element and other signed XML data objects. It is possible
for an isolated XML document to be treated as if it were binary data so that no
changes can occur. In that case, the digest of the document will not change and
it need not be canonicalized if it is signed and verified as such. However, XML
that is read and processed using standard XML parsing and processing techniques
is frequently changed such that some of its surface representation information is
lost or modified. In particular, this will occur in many cases for the
Signature
and enclosed
SignedInfo
elements since they,
and possibly an encompassing XML document, will be processed as XML.
Similarly, these considerations apply to
Manifest
Object
, and
SignatureProperties
elements if those
elements have been digested, their
DigestValue
is to be checked, and
they are being processed as XML.
The kinds of changes in XML that may need to be canonicalized can be divided into
four categories. There are those related to the basic [
XML
], as described in 7.1 below. There are those related to
DOM
], [
SAX
], or similar processing
as described in 7.2 below. Third, there is the possibility of coded character set
conversion, such as between UTF-8 and UTF-16, both of which all  [
XML
] compliant processors are required to support, which is
described in the paragraph immediately below. And, fourth, there are changes that
related to namespace declaration and XML namespace attribute context as described
in 7.3 below.
Any canonicalization algorithm should yield output in a specific fixed coded
character set. All canonicalization
algorithms
identified in this document use UTF-8 (without a byte order mark (BOM)) and do
not provide character normalization. We RECOMMEND that signature applications
create XML content (
Signature
elements and their
descendents/content) in Normalization Form C [
NFC
NFC-Corrigendum
] and check that any XML being
consumed is in that form as well; (if not, signatures may consequently fail to
validate). Additionally, none of these algorithms provide data type
normalization. Applications that normalize data types in varying formats (e.g.,
(true, false) or (1,0)) may not be able to validate each other's signatures.
7.1
XML 1.0
, Syntax Constraints, and
Canonicalization
XML 1.0 [
XML
] defines an interface where a conformant
application reading XML is given certain information from that XML and not other
information. In particular,
line endings are normalized to the single character #xA by dropping #xD
characters if they are immediately followed by a #xA and replacing them with
#xA in all other cases,
missing attributes declared to have default values are provided to the
application as if present with the default value,
character references are replaced with the corresponding character,
entity references are replaced with the corresponding declared entity,
attribute values are normalized by
replacing character and entity references as above,
replacing occurrences of #x9, #xA, and #xD with #x20 (space) except that
the sequence #xD#xA is replaced by a single space, and
if the attribute is not declared to be CDATA, stripping all leading and
trailing spaces and replacing all interior runs of spaces with a single
space.
Note that items (2), (4), and (5.3) depend on the presence of a schema, DTD or
similar declarations. The
Signature
element type is
laxly
schema valid
XML-schema
], consequently
external XML or even XML within the same document as the signature may be (only)
well-formed or from another namespace (where permitted by the signature schema);
the noted items may not be present. Thus, a signature with such content will only
be verifiable by other signature applications if the following syntax constraints
are observed when generating any signed material including the
SignedInfo
element:
attributes having default values be explicitly present,
all entity references (except "amp", "lt", "gt", "apos", "quot", and other
character entities not representable in the encoding chosen) be expanded,
attribute value white space be normalized
7.2
DOM/SAX
Processing and
Canonicalization
In addition to the canonicalization and syntax constraints discussed above, many
XML applications use the Document Object Model [
DOM
] or
the Simple API for XML  [
SAX
]. DOM maps XML into a
tree structure of nodes and typically assumes it will be used on an entire
document with subsequent processing being done on this tree. SAX converts XML
into a series of events such as a start tag, content, etc. In either case, many
surface characteristics such as the ordering of attributes and insignificant
white space within start/end tags is lost. In addition, namespace declarations
are mapped over the nodes to which they apply, losing the namespace prefixes in
the source text and, in most cases, losing where namespace declarations appeared
in the original instance.
If an XML Signature is to be produced or verified on a system using the DOM or
SAX processing, a canonical method is needed to serialize the relevant part of a
DOM tree or sequence of SAX events. XML canonicalization specifications, such as
XML-C14N
], are based only on information which is
preserved by DOM and SAX. For an XML Signature to be verifiable by an
implementation using DOM or SAX, not only must the
XML 1.0
syntax constraints given in the previous section
be followed but an
appropriate XML canonicalization MUST be specified so that the verifier can
re-serialize DOM/SAX mediated input into the same octet stream that was signed.
7.3
Namespace
Context
and Portable Signatures
In [
XPath
] and consequently the Canonical XML data model
an element has namespace nodes that correspond to those declarations within the
element and its ancestors:
Note:
An element
has namespace
nodes that represent its namespace declarations
as well as
any
namespace declarations made by its ancestors that have not been overridden in
's declarations, the default namespace if it is
non-empty, and the declaration of the prefix
xml
." [
XML-C14N
When serializing a
Signature
element or signed XML data that's the
child of other elements using these data models, that
Signature
element and its children, may contain namespace declarations from its ancestor
context. In addition, the Canonical XML and Canonical XML with Comments
algorithms import all xml namespace attributes (such as
xml:lang
from the nearest ancestor in which they are declared to the apex node of
canonicalized XML unless they are already declared at that node. This may
frustrate the intent of the signer to create a signature in one context which
remains valid in another. For example, given a signature which is a child of
and a grandchild of


...
...




when either the element
or the signed element
is
moved into a [
SOAP
] envelope for transport:

...



...





The canonical form of the signature in this context will contain new namespace
declarations from the
SOAP:Envelope
context, invalidating the
signature. Also, the canonical form will lack namespace declarations it may have
originally had from element
's context, also invalidating the
signature. To avoid these problems, the application may:
Rely upon the enveloping application to properly divorce its body (the
signature payload) from the context (the envelope) before the signature is
validated. Or,
Use a canonicalization method that "repels/excludes" instead of "attracts"
ancestor context. [
XML-C14N
] purposefully attracts
such context.
8.0
Security Considerations
The XML Signature specification provides a very flexible digital signature
mechanism. Implementors must give consideration to their application threat
models and to the following factors.
8.1
Transforms
A requirement of this specification is to permit signatures to "apply to a
part or totality of a XML document." (See [
XML-Signature-RD
, section 3.1.3].) The
Transforms
mechanism meets this requirement by permitting one to
sign data derived from processing the content of the identified resource. For
instance, applications that wish to sign a form, but permit users to enter
limited field data without invalidating a previous signature on the form might
use [
XPath
] to exclude those portions the user needs to
change.
Transforms
may be arbitrarily specified and may include
encoding transforms, canonicalization instructions or even XSLT transformations.
Three cautions are raised with respect to this feature in the following sections.
Note,
core validation
behavior
does not confirm that the signed data was obtained by applying each step of the
indicated transforms. (Though it does check that the digest of the resulting
content matches that specified in the signature.)  For example, some
applications may be satisfied with verifying an XML signature over a cached copy
of already transformed data. Other applications might require that content be
freshly dereferenced and transformed.
8.1.1
Only What is Signed is
Secure
First, obviously, signatures over a transformed document do not secure any
information discarded by transforms: only what is signed is secure.
Note that the use of Canonical  XML [
XML-C14N
ensures that all internal entities and XML namespaces are expanded within the
content being signed. All entities are replaced with their definitions and the
canonical form explicitly represents the namespace that an element would
otherwise inherit. Applications that do not canonicalize XML content (especially
the
SignedInfo
element) SHOULD NOT use internal entities and SHOULD
represent the namespace explicitly within the content being signed since they can
not rely upon canonicalization to do this for them. Also, users concerned with
the integrity of the element type definitions associated with the XML instance
being signed may wish to sign those definitions as well (i.e., the schema, DTD,
or natural language description associated with the namespace/identifier).
Second, an envelope containing signed information is not secured by the
signature. For instance, when an encrypted envelope contains a signature, the
signature does not protect the authenticity or integrity of unsigned envelope
headers nor its ciphertext form, it only secures the plaintext actually signed.
8.1.2
Only What is "Seen" Should be Signed
Additionally, the signature secures any information introduced by the transform:
only what is "seen" (that which is represented to the user via visual, auditory
or other media) should be signed. If signing is intended to convey the judgment
or consent of a user (an automated mechanism or person), then it is normally
necessary to secure as exactly as practical the information that was presented to
that user. Note that this can be accomplished by literally signing what was
presented, such as the screen images shown a user. However, this may result in
data which is difficult for subsequent software to manipulate. Instead, one can
sign the data along with whatever filters, style sheets, client profile or other
information that affects its presentation.
8.1.3
"See" What is Signed
Just as a user should only sign what he or she "sees," persons and automated
mechanism that trust the validity of a transformed document on the basis of a
valid signature should operate over the data that was transformed (including
canonicalization) and signed, not the original pre-transformed data. This
recommendation applies to transforms specified within the signature as well as
those included as part of the document itself. For instance, if an XML document
includes an
embedded
style sheet
XSLT
] it is the transformed document
that should be represented to the user and signed. To meet this recommendation
where a document references an external style sheet, the content of that external
resource should also be signed as via a signature
Reference
otherwise the content of that external content might change which alters the
resulting document without invalidating the signature.
Some applications might operate over the original or intermediary data but should
be extremely careful about potential weaknesses introduced between the original
and transformed data. This is a trust decision about the character and meaning of
the transforms that an application needs to make with caution. Consider a
canonicalization algorithm that normalizes character case (lower to upper) or
character composition ('e and accent' to 'accented-e'). An adversary could
introduce changes that are normalized and consequently inconsequential to
signature validity but material to a DOM processor. For instance, by changing the
case of a character one might influence the result of an XPath selection. A
serious risk is introduced if that change is normalized for signature validation
but the processor operates over the original data and returns a different result
than intended.
As a result:
All documents operated upon and generated by signature applications MUST be in
NFC
NFC-Corrigendum
] (otherwise intermediate
processors might unintentionally break the signature)
Encoding normalizations SHOULD NOT be done as part of a signature transform, or
(to state it another way) if normalization does occur, the application SHOULD
always "see" (operate over) the normalized form.
8.2
Check the Security Model
This specification uses public key signatures and keyed hash authentication
codes. These have substantially different security models. Furthermore, it
permits user specified algorithms which may have other models.
With public key signatures, any number of parties can hold the public key and
verify signatures while only the parties with the private key can create
signatures. The number of holders of the private key should be minimized and
preferably be one. Confidence by verifiers in the public key they are using and
its binding to the entity or capabilities represented by the corresponding
private key is an important issue, usually addressed by certificate or online
authority systems.
Keyed hash authentication codes, based on secret keys, are typically much more
efficient in terms of the computational effort required but have the
characteristic that all verifiers need to have possession of the same key as the
signer. Thus any verifier can forge signatures.
This specification permits user provided signature algorithms and keying
information designators. Such user provided algorithms may have different
security models. For example, methods involving biometrics usually depend on a
physical characteristic of the authorized user that can not be changed the way
public or secret keys can be and may have other security model differences.
8.3 Algorithms,
Key Lengths
Certificates, Etc.
The strength of a particular signature depends on all links in the security
chain. This includes the signature and digest algorithms used, the strength of
the key generation [
RANDOM
] and the size of the key,
the security of key and certificate authentication and distribution mechanisms,
certificate chain validation policy, protection of cryptographic processing from
hostile observation and tampering, etc.
Care must be exercised by applications in executing the various algorithms that
may be specified in an XML signature and in the processing of any "executable
content" that might be provided to such algorithms as parameters, such as XSLT
transforms. The algorithms specified in this document will usually be implemented
via a trusted library but even there perverse parameters might cause unacceptable
processing or memory demand. Even more care may be warranted with application
defined algorithms.
The security of an overall system will also depend on the security and integrity
of its operating procedures, its personnel, and on the administrative enforcement
of those procedures. All the factors listed in this section are important to the
overall security of a system; however, most are beyond the scope of this
specification.
9.0
Schema
, DTD, Data Model, and Valid
Examples
XML Signature Schema Instance
xmldsig-core-schema.xsd
Valid XML schema instance based on the 20001024 Schema/DTD [
XML-Schema
].
XML Signature DTD
xmldsig-core-schema.dtd
RDF Data Model
xmldsig-datamodel-20000112.gif
XML Signature Object Example
signature-example.xml
A cryptographical fabricated XML example that includes foreign content and
validates under the schema, it also uses
schemaLocation
to aid
automated schema fetching and validation.
RSA XML Signature Example
signature-example-rsa.xml
An XML Signature example with generated cryptographic values by Merlin Hughes
and validated by Gregor Karlinger.
DSA XML Signature Example
signature-example-dsa.xml
Similar to above but uses DSA.
10.0
Definitions
Authentication
Code
Protected
Checksum
A value generated from the application of a shared key to a message via a
cryptographic algorithm such that it has the properties of
message authentication
(and
integrity
) but not
signer authentication
Equivalent to
protected checksum
, "A checksum that is computed for a
data object by means that protect against active attacks that would attempt to
change the checksum to make it match changes made to the data object."
SEC
Authentication, Message
The property, given an
authentication code
protected checksum
, that tampering with both the data and
checksum, so as to introduce changes while seemingly preserving
integrity
, are still detected. "A
signature should identify what is signed, making it impracticable to falsify or
alter either the signed matter or the signature without detection." [
Digital Signature
Guidelines
ABA
].
Authentication, Signer
The property that the identity of the signer is as claimed. "A signature should
indicate who signed a document, message or record, and should be difficult for
another person to produce without authorization." [
Digital Signature
Guidelines
ABA
] Note, signer authentication is an
application decision (e.g., does the signing key actually correspond to a
specific identity) that is supported by, but out of scope, of this
specification.
Checksum
"A value that (a) is computed by a function that is dependent on the contents
of a data object and (b) is stored or transmitted together with the object, for
the purpose of detecting changes in the data."  [
SEC
Core
The syntax and processing defined by this specification, including
core validation
. We use this
term to distinguish other markup, processing, and applications semantics from
our own.
Data Object
(Content/Document)
The actual binary/octet data being operated on (transformed, digested, or
signed) by an application -- frequently an
HTTP
entity
HTTP
]. Note that the proper noun
Object
designates a specific XML element. Occasionally we refer to
a data object as a
document
or as a
resource
's content
. The term
element content
is used to describe the data between XML start and end tags [
XML
]. The term
XML document
is used to describe
data objects which conform to the XML specification [
XML
].
Integrity
"The property that data has not been changed, destroyed, or lost in an
unauthorized or accidental manner." [
SEC
] A simple
checksum
can provide integrity from
incidental changes in the data;
message authentication
is similar but also protects
against an active attack to alter the data whereby a change in the checksum is
introduced so as to match the change in the data.
Object
An XML Signature element wherein arbitrary (non-
core
) data may be placed. An
Object
element
is merely one type of digital data (or document) that can be signed via a
Reference
Resource
"A resource can be anything that has identity. Familiar examples include an
electronic document, an image, a service (e.g., 'today's weather report for Los
Angeles'), and a collection of other resources.... The resource is the
conceptual mapping to an entity or set of entities, not necessarily the entity
which corresponds to that mapping at any particular instance in time. Thus, a
resource can remain constant even when its content---the entities to which it
currently corresponds---changes over time, provided that the conceptual mapping
is not changed in the process." [
URI
] In order to avoid
a collision of the term
entity
within the URI and XML specifications,
we use the term
data object
content
or
document
to
refer to the actual bits/octets being operated upon.
Signature
Formally speaking, a value generated from the application of a private key to a
message via a cryptographic algorithm such that it has the properties of
integrity
message authentication
and/or
signer
authentication
. (However, we sometimes use the term signature generically
such that it encompasses
Authentication Code
values as well, but we are careful to
make the distinction when the property of
signer authentication
is relevant to the exposition.) A
signature may be (non-exclusively) described as
detached
enveloping
, or
enveloped
Signature,
Application
An application that implements the MANDATORY (REQUIRED/MUST) portions of this
specification; these conformance requirements are over application behavior,
the structure of the
Signature
element type and its children
(including
SignatureValue
) and the specified algorithms.
Signature,
Detached
The signature is over content external to the
Signature
element,
and can be identified via a
URI
or transform. Consequently, the
signature is "detached" from the content it signs. This definition typically
applies to separate data objects, but it also includes the instance where the
Signature
and data object reside within the same XML document but
are sibling elements.
Signature,
Enveloping
The signature is over content found within an
Object
element of
the signature itself. The
Object
(or its content) is identified
via a
Reference
(via a
URI
fragment identifier or
transform).
Signature,
Enveloped
The signature is over the XML content that contains the signature as an
element. The content provides the root XML document element. Obviously,
enveloped signatures must take care not to include their own value in the
calculation of the
SignatureValue
Transform
The processing of a data from its source to its derived form. Typical
transforms include XML Canonicalization, XPath, and XSLT.
Validation, Core
The core processing requirements of this specification requiring
signature validation
and
SignedInfo
reference validation
Validation,
Reference
The hash value of the identified and transformed content, specified by
Reference
, matches its specified
DigestValue
Validation,
Signature
The
SignatureValue
matches the result of processing
SignedInfo
with
CanonicalizationMethod
and
SignatureMethod
as specified in
Core
Validation
(section 3.2).
Validation, Trust/Application
The application determines that the semantics associated with a signature are
valid. For example, an application may validate the time stamps or the
integrity of the signer key -- though this behavior is external to this
core
specification.
11.0
References
ABA
Digital Signature
Guidelines.
DOM
Document Object
Model (DOM) Level 1 Specification.
W3C Recommendation. V. Apparao, S.
Byrne, M. Champion, S. Isaacs, I. Jacobs, A. Le Hors, G. Nicol, J. Robie, R.
Sutor, C. Wilson, L. Wood. October 1998.
DSS
FIPS
PUB 186-2
Digital Signature Standard (DSS).
U.S. Department of
Commerce/National Institute of Standards and Technology.
HMAC
RFC 2104
HMAC:
Keyed-Hashing for Message Authentication.
H. Krawczyk, M. Bellare, R.
Canetti. February 1997.
HTTP
RFC 2616
Hypertext Transfer Protocol -- HTTP/1.1
. J. Gettys, J. Mogul, H.
Frystyk, L. Masinter, P. Leach, T. Berners-Lee. June 1999.
KEYWORDS
RFC 2119.
Key words for
use in RFCs to Indicate Requirement Levels.
S. Bradner. March 1997.
LDAP-DN
RFC 2253
Lightweight
Directory Access Protocol (v3): UTF-8 String Representation of Distinguished
Names.
M. Wahl, S. Kille, T. Howes. December 1997.
MD5
RFC 1321
The MD5
Message-Digest Algorithm.
R. Rivest. April 1992.
MIME
RFC 2045
Multipurpose
Internet Mail Extensions (MIME) Part One: Format of Internet Message
Bodies
. N. Freed & N. Borenstein. November 1996.
NFC
TR15, Unicode Normalization Forms.
M. Davis, M. Dürst. Revision
18: November 1999.
NFC-Corrigendum
Normalization Corrigendum
. The Unicode Consortium.
PGP
RFC 2440
OpenPGP Message
Format.
J. Callas, L. Donnerhacke, H. Finney, R. Thayer. November
1998.
RANDOM
RFC 1750
Randomness
Recommendations for Security.
D. Eastlake, S. Crocker, J. Schiller.
December 1994.
RDF
Resource
Description Framework (RDF) Schema Specification 1.0.
W3C Candidate
Recommendation. D. Brickley, R.V. Guha. March 2000.
Resource
Description Framework (RDF) Model and Syntax Specification
. W3C
Recommendation. O. Lassila, R. Swick. February 1999.
1363
IEEE 1363: Standard Specifications for Public Key Cryptography. August 2000.
PKCS1
RFC 2437
PKCS #1: RSA
Cryptography Specifications Version 2.0.
B. Kaliski, J. Staddon. October
1998.
SAX
SAX: The Simple API for
XML
. D. Megginson, et al. May 1998.
SEC
RFC 2828
Internet
Security Glossary.
R. Shirey. May 2000.
SHA-1
FIPS
PUB 180-1
Secure Hash Standard.
U.S. Department of
Commerce/National Institute of Standards and Technology.
SOAP
Simple Object Access
Protocol (SOAP) Version 1.1
. W3C Note. D. Box, D. Ehnebuske, G. Kakivaya,
A. Layman, N. Mendelsohn, H. Frystyk Nielsen, S. Thatte, D. Winer. May 2001.
Unicode
The Unicode Consortium.
The Unicode Standard.
UTF-16
RFC 2781
UTF-16, an
encoding of ISO 10646.
P. Hoffman , F. Yergeau. February 2000.
UTF-8
RFC 2279
UTF-8, a
transformation format of ISO 10646
. F. Yergeau. January 1998.
URI
RFC 2396
Uniform
Resource Identifiers (URI): Generic Syntax.
T. Berners-Lee, R. Fielding,
L. Masinter. August 1998.
URI-Literal
RFC 2732
Format for
Literal IPv6 Addresses in URL's
. R. Hinden, B. Carpenter, L. Masinter.
December 1999.
URL
RFC 1738.
Uniform
Resource Locators (URL).
T. Berners-Lee, L. Masinter, and M. McCahill.
December 1994.
URN
RFC 2141
URN
Syntax.
R. Moats. May 1997.
RFC 2611
URN Namespace
Definition Mechanisms.
L. Daigle, D. van Gulik, R. Iannella, P. Falstrom.
June 1999.
X509v3
ITU-T Recommendation X.509 version 3 (1997). "Information Technology - Open
Systems Interconnection - The Directory Authentication Framework"  ISO/IEC
9594-8:1997.
XHTML 1.0
XHTML(tm) 1.0: The
Extensible Hypertext Markup Language
. W3C Recommendation. S. Pemberton, D.
Raggett, et al. January 2000.
XLink
XML Linking
Language.
W3C Recommendation. S. DeRose, E. Maler, D. Orchard. June 2001.
XML
Extensible Markup Language
(XML) 1.0 (Second Edition).
W3C Recommendation. T. Bray, E. Maler, J.
Paoli, C. M. Sperberg-McQueen. October 2000.
XML-C14N
Canonical XML.
W3C Recommendation. J. Boyer. March 2001.
XML-Japanese
XML Japanese
Profile
. W3C Note.
M.
Murata
April 2000
XML-MT
RFC 2376
XML Media
Types
. E. Whitehead, M. Murata. July 1998.
XML-ns
Namespaces in
XML
. W3C Recommendation. T. Bray, D. Hollander, A. Layman. January 1999.
XML-schema
XML Schema Part
1: Structures
. W3C Recommendation. D. Beech, M. Maloney, N. Mendelsohn, H.
Thompson. May 2001.
XML Schema Part
2: Datatypes
W3C Recommendation. P. Biron, A. Malhotra. May 2001.
XML-Signature-RD
RFC 2807
XML Signature
Requirements.
W3C Working Draft. J. Reagle, April 2000.
XPath
XML Path Language
(XPath) Version 1.0
. W3C Recommendation. J. Clark, S. DeRose. October
1999.
XPointer
XML Pointer Language
(XPointer)
. W3C Candidate Recommendation. S. DeRose, R. Daniel, E. Maler.
January 2001.
XSL
Extensible Stylesheet
Language (XSL)
. W3C Recommendation. S. Adler, A. Berglund, J. Caruso, S.
Deach, T. Graham, P. Grosso, E. Gutentag, A. Milowski, S. Parnell, J. Richman,
S. Zilles. October 2001.
XSLT
XSL Transforms
(XSLT) Version 1.0
. W3C Recommendation. J. Clark. November 1999.
12.
Authors'
Address
Donald E. Eastlake 3rd
Motorola, 20 Forbes Boulevard
Mansfield, MA 02048 USA
Email:
Donald.Eastlake@motorola.com
Joseph M. Reagle Jr.,
W3C
Massachusetts Institute of Technology
Laboratory for Computer Science
NE43-350, 545 Technology Square
Cambridge, MA 02139
Phone: + 1.617.258.7621
Email:
reagle@w3.org
David Solo
Citigroup
909 Third Ave, 16th Floor
NY, NY 10043 USA
Phone +1-212-559-2900
Email:
dsolo@alum.mit.edu

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