Copyright © 2000 W3C® (MIT, INRIA, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.
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.
This is an editors version used to track the latest comments/suggestions. This version highlights (via red underlined text) a few of the changes from the previous version. You shouldn't implement this, and you shouldn't even really read it. This may not even include all the tweaks that are present on a editors' local version.
Please send comments to the editors and cc: the list <w3c-ietf-xmldsig@w3.org>. Publication as a
Working Draft does not imply endorsement by the W3C membership or IESG. It is
inappropriate to cite W3C Drafts as other than "work in progress." A list of
current W3C working drafts can be found at http://www.w3.org/TR/.
Current IETF drafts can be found at http://www.ietf.org/1id-abstracts.html.
Patent disclosures relevant to this specification may be found on the Working
Group's patent disclosure page
and IETF's Intellectual Property Right Notices.
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 Section 8: Security Considerations.
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 section 8.3:Check the Security Model.)
This specification uses both XML Schemas [XML-schema] and DTDs [XML]. (Readers unfamiliar with DTD syntax may wish to refer to Ron Bourret's " Declaring Elements and Attributes in an XML DTD" [Bourret].) The schema definition is presently normative. Consequently, Signature element instances MUST be laxly schema valid [XML-schema].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in RFC2119 [KEYWORDS]:
"they MUST only be used where it is actually required for interoperation or to limit behavior which has potential for causing harm (e.g., limiting retransmissions)"
Consequently, we use these capitalized keywords to unambiguously specify requirements over protocol and application features and behavior that affect the interoperability and security of implementations. These key words are not used (capitalized) to describe XML grammar; schema definitions unambiguously describe such requirements and we wish to reserve the prominence of these terms for the natural language descriptions of protocols and features. For instance, an XML attribute might be described as being "optional." Compliance with the XML-namespace specification [XML-ns] is described as "REQUIRED."
The design philosophy and requirements of this specification are addressed in the XML-Signature Requirements document [XML-Signature-RD].
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/2000/07/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 namespaceFinally, in order to provide for terse namespace declarations we sometimes use XML internal entities [XML] as macros within URIs. For instance:
<?xml version='1.0'?>
<!DOCTYPE Signature SYSTEM
"xmldsig-core-schema.dtd" [ <!ENTITY dsig
"http://www.w3.org/2000/07/xmldsig#"> ]>
<Signature xmlns="&dsig;" Id="MyFirstSignature">
<SignedInfo>
...
The contributions of the following working group members to this specification are gratefully acknowledged:
As are the last call comments from the following:
This section provides an overview and examples of XML digital signature syntax. The specific processing is given in section 3: Processing Rules. The formal syntax is found in section 4: Core Signature Syntax and section 5: Additional Signature Syntax.
In this section, an informal representation and examples are used to describe the structure of the XML signature syntax. This representation and examples may omit attributes, details and potential features that are fully explained later.
XML Signatures are applied to arbitrary digital
content (data objects) via an indirection. Data objects are digested, the resulting
value is placed in an element (with other information) and that element is then digested
and cryptographically signed. XML digital signatures are represented by the Signature
element which has the following structure (where "?" denotes zero or one
occurrence; "+" denotes one or more occurrences; and "*" denotes zero
or more occurrences):
<Signature>
<SignedInfo>
(CanonicalizationMethod)
(SignatureMethod)
(<Reference (URI=)? >
(Transforms)?
(DigestMethod)
(DigestValue)
</Reference>)+
</SignedInfo>
(SignatureValue)
(KeyInfo)?
(Object)*
</Signature>
The content that is signed was, at the time of signature creation, referred to as an identified resource to which the specified transforms were applied.
Signatures are related to data objects via URIs [URI]. Within an
XML document, signatures are related to local data objects via fragment identifiers. Such
local data can be included within an enveloping
signature or can enclose an enveloped
signature. Detached signatures are
over external network resources or local data objects that resides within the same XML
document as sibling elements; in this case, the signature is neither enveloping (signature
is parent) nor enveloped (signature is child). Since a Signature element (and
its Id attribute value/name) may co-exist or be combined with other elements
(and their IDs) within a single XML document, care should be taken in choosing names such
that there are no subsequent collisions that violate the ID uniqueness validity constraint [XML].
Signature,
SignedInfo, Methods, and References)The following example is a detached signature of the content of the HTML4 in XML specification.
[s01] <Signature Id="MyFirstSignature" xmlns="http://www.w3.org/2000/07/xmldsig#"> [s02] <SignedInfo> [s03] <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000710"/> [s04] <SignatureMethod Algorithm="http://www.w3.org/2000/07/xmldsig#dsa-sha1"/> [s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000710"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2000/07/xmldsig#sha1"/> [s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [s11] </Reference> [s12] </SignedInfo> [s13] <SignatureValue>MC0CFFrVLtRlk=...</SignatureValue> [s14] <KeyInfo> [s15a] <KeyValue> [s15b] <DSAKeyValue> [s15c] <P>...</P><Q>...</Q><G>...</G><Y>...</Y> [s15d] </DSAKeyValue> [s15e] </KeyValue> [s16] </KeyInfo> [s17] </Signature>
[s02-12] The required SignedInfo element is the information
that is actually signed. Core validation
of SignedInfo consists of two mandatory processes: validation of the signature over SignedInfo
and validation of each Reference
digest within SignedInfo. Note that the algorithms used in calculating the SignatureValue
are also included in the signed information while the SignatureValue element
is outside SignedInfo.
[s03] The CanonicalizationMethod is the algorithm that is
used to canonicalize the SignedInfo element before it is digested as part of
the signature operation.
[s04] The SignatureMethod is the algorithm that is used to
convert the canonicalized SignedInfo into the SignatureValue. It
is a combination of a digest algorithm and a key dependent algorithm and possibly other
algorithms such as padding, for example RSA-SHA1. The algorithm names are signed to resist
attacks based on substituting a weaker algorithm. To promote application interoperability
we specify a set of signature algorithms that MUST be implemented, though their use is at
the discretion of the signature creator. We specify additional algorithms as RECOMMENDED
or OPTIONAL for implementation and the signature design permits arbitrary user algorithm
specification.
[s05-11] Each Reference element includes the digest method
and resulting digest value calculated over the identified data object. It also may include
transformations that produced the input to the digest operation. A data object is signed
by computing its digest value and a signature over that value. The signature is later
checked via reference and signature validation.
[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.
Reference[s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000710"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2000/07/xmldsig#sha1"/> [s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [s11] </Reference>
[s05] The optional URI attribute of Reference
identifies the data object to be signed. This attribute may be omitted on at most one Reference
in a Signature. (This limitation is imposed in order to ensure that
references and objects may be matched unambiguously.)
[s05-08] This identification, along with the transforms, is a description
provided by the signer on how they obtained the signed data object in the form it was
digested (i.e. the digested content). The verifier may obtain the digested content in
another method so long as the digest verifies. In particular, the verifier may obtain the
content from a different location such as a local store than that specified in the URI.
[s06-08] Transforms is an optional ordered list of processing steps that
were applied to the resource's content before it was digested. Transforms can include
operations such as canonicalization, encoding/decoding (including compression/inflation),
XSLT and XPath. XPath transforms permit the signer to derive an XML document that omits
portions of the source document. Consequently those excluded portions can change without
affecting signature validity. For example, if the resource being signed encloses the
signature itself, such a transform must be used to exclude the signature value from its
own computation. If no Transforms element is present, the resource's content
is digested directly. While we specify mandatory (and 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.
Object and SignatureProperty)This specification does not address mechanisms for making statements or assertions.
Instead, this document defines what it means for something to be signed by an XML
Signature (message authentication, integrity, and/or signer authentication). Applications
that wish to represent other semantics must rely upon other technologies, such as [XML, RDF]. For instance, an application might
use a foo:assuredby attribute within its own markup to reference a Signature
element. Consequently, it's the application that must understand and know how to make
trust decisions given the validity of the signature and the meaning of assurdby
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] <SignedInfo> [ ] ... [p02] <Reference URI="http://www.w3.org/TR/xml-stylesheet/"> [ ] ... [p03] <Reference URI="#AMadeUpTimeStamp" [p04] Type="http://www.w3.org/2000/07/xmldsig#SignatureProperty"> [p05] <DigestMethod Algorithm="http://www.w3.org/2000/07/xmldsig#sha1"/> [p06] <DigestValue>k3453rvEPO0vKtMup4NbeVu8nk=</DigestValue> [p07] </Reference> [p08] </SignedInfo> [p09] ... [p10] <Object> [p11] <SignatureProperties> [p12] <SignatureProperty Id="AMadeUpTimeStamp" Target="#MySecondSignature"> [p13] <timestamp xmlns="http://www.ietf.org/rfcXXXX.txt"> [p14] <date>19990908</date> [p15] <time>14:34:34:34</time> [p16] </timestamp> [p17] </SignatureProperty> [p18] </SignatureProperties> [p19] </Object> [p20]</Signature>
[p04] The optional Type attribute of Reference
provides information about the resource identified by the URI. In particular,
it can indicate that it is an Object, SignatureProperty, or Manifest
element. This can be used by applications to initiate special processing of some Reference
elements. References to an XML data element within an Object element SHOULD
identify the actual element pointed to. Where the element content is not XML (perhaps it
is binary or encoded data) the reference should identify the Object and the Reference
Type, if given, SHOULD indicate Object. Note that Type
is advisory and no action based on it or checking of its correctness is required by core
behavior.
[p10] Object is an optional element for including data
objects within the signature element or elsewhere. The Object can be
optionally typed and/or encoded.
[p11-18] Signature properties, such as time of signing, can be optionally
signed by identifying them from within a Reference. (These properties are
traditionally called signature "attributes" although that term has no
relationship to the XML term "attribute".)
Object and Manifest)The Manifest element is provided to meet additional requirements not
directly addressed by the mandatory parts of this specification. Two requirements and the
way the Manifest satisfies them follows.
First, applications frequently need to efficiently sign multiple data objects even
where the signature operation itself is an expensive public key signature. This
requirement can be met by including multiple Reference elements within SignedInfo
since the inclusion of each digest secures the data digested. However, some applications
may not want the core validation
behavior associated with this approach because it requires every Reference
within SignedInfo to undergo reference validation -- the DigestValue elements are
checked. These applications may wish to reserve reference validation decision logic to
themselves. For example, an application might receive a signature valid SignedInfo element that includes three Reference
elements. If a single Reference fails (the identified data object when
digested does not yield the specified DigestValue) the signature would fail core validation. However, the application
may wish to treat the signature over the two valid Reference elements as
valid or take different actions depending on which fails. To accomplish this, SignedInfo
would reference a Manifest element that contains one or more Reference
elements (with the same structure as those in SignedInfo). Then, reference
validation of the Manifest is under application control.
Second, consider an application where many signatures (using different keys) are
applied to a large number of documents. An inefficient solution is to have a separate
signature (per key) repeatedly applied to a large SignedInfo element (with
many References); this is wasteful and redundant. A more efficient solution
is to include many references in a single Manifest that is then referenced
from multiple Signature elements.
The example below includes a Reference that signs a Manifest
found within the Object element.
[ ] ... [m01] <Reference URI="#MyFirstManifest" [m02] Type="http://www.w3.org/2000/07/xmldsig#Manifest"> [m03] <DigestMethod Algorithm="http://www.w3.org/2000/07/xmldsig#sha1"/> [m04] <DigestValue>345x3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [m05] </Reference> [ ] ... [m06] <Object> [m07] <Manifest Id="MyFirstManifest"> [m08] <Reference> [m09] ... [m10] </Reference> [m11] <Reference> [m12] ... [m13] </Reference> [m14] </Object>
The sections below describe the operations to be performed as part of signature generation and validation.
The REQUIRED steps include the generation of Reference elements and the SignatureValue
over SignedInfo.
For each data object being signed:
Transforms, as determined by the application, to the data object.Reference element, including the (optional) identification of the
data object, any (optional) transform elements, the digest algorithm and the DigestValue.SignedInfo element with SignatureMethod, CanonicalizationMethod
and Reference(s).SignatureValue over SignedInfo
based on algorithms specified in SignedInfo.Signature element that includes SignedInfo, Object(s)
(if desired, encoding may be different than that used for signing), KeyInfo
(if required), and SignatureValue.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.
For each Reference in SignedInfo:
SignedInfo element based on the CanonicalizationMethod
in SignedInfo (so as to ensure the application Sees What
is Signed, which is the canonical form).URI) and Transforms provided by the signer in
the Reference element, or it may obtain the content through other means such
as a local cache.)DigestMethod specified in its Reference
specification.DigestValue in the SignedInfo
Reference; if there is any mismatch, validation fails.SignedInfo element based on the CanonicalizationMethod
in SignedInfo.KeyInfo or from an external source.SignatureMethod to validate the SignatureValue
over the SignedInfo element.The general structure of an XML signature is described in section 2: Signature Overview. This section provides detailed syntax of the core signature features and actual examples. Features described in this section are mandatory to implement unless otherwise indicated. The syntax is defined via DTDs and [XML-Schema] with the following XML preamble, declaration, internal entity, and simpleType:
Schema Definition:
<?xml version='1.0'?>
<!DOCTYPE schema
SYSTEM 'http://www.w3.org/1999/XMLSchema.dtd'
[
<!ENTITY dsig 'http://www.w3.org/2000/07/xmldsig#'>
]>
<schema targetNamespace='&dsig;'
version='0.1'
xmlns='http://www.w3.org/1999/XMLSchema'
xmlns:ds='&dsig;'
elementFormDefault='qualified'>
<!-- Basic Types Defined for Signatures -->
<simpleType name='CryptoBinary' base='binary'>
<encoding value='base64'/>
</simpleType>
DTD:
<!-- These entity declarations permit the flexible parts of Signature
content model to be easily expanded -->
<!ENTITY % Object.ANY '(#PCDATA|Signature|SignatureProperties|Manifest)*'>
<!ENTITY % Method.ANY '(#PCDATA|HMACOutputLength)*'>
<!ENTITY % Transform.ANY '(#PCDATA|XPath|XSLT)*'>
<!ENTITY % Key.ANY '(#PCDATA|KeyName|KeyValue|RetrievalMethod|
X509Data|PGPData|MgmtData|DSAKeyValue|RSAKeyValue)*'>
Signature elementThe Signature element is the root element of an XML Signature. Signature
elements MUST be laxly
schema valid [XML-schema] with respect to the
following schema definition:
Schema Definition:
<element name='Signature'>
<complexType content='elementOnly'>
<sequence minOccurs='1' maxOccurs='1'>
<element ref='ds:SignedInfo' minOccurs='1' maxOccurs='1'/>
<element ref='ds:SignatureValue' minOccurs='1' maxOccurs='1'/>
<element ref='ds:KeyInfo' minOccurs='0' maxOccurs='1'/>
<element ref='ds:Object' minOccurs='0' maxOccurs='unbounded'/>
</sequence>
<attribute name='Id' type='ID' use='optional'/>
</complexType>
</element>
DTD:
<!ELEMENT Signature (SignedInfo, SignatureValue, KeyInfo?, Object*) >
<!ATTLIST Signature
xmlns CDATA #FIXED 'http://www.w3.org/2000/07/xmldsig#'
Id ID #IMPLIED >
SignatureValue
ElementThe SignatureValue element contains the actual value of the digital
signature; it is encoded according to the identifier specified in SignatureMethod.
Base64 [MIME] is the encoding method for all SignatureMethods
specified within this specification. While we specify a mandatory and optional to
implement SignatureMethod algorithms, user specified algorithms are
permitted. Both algorithms specified by this specification and user specified ones MUST
use Base64 [MIME] as their encoding method.
Schema Definition: <element name='SignatureValue' type='ds:CryptoBinary'/>
DTD: <!ELEMENT SignatureValue (#PCDATA) >
SignedInfo
ElementThe 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.
Schema Definition:
<element name='SignedInfo'>
<complexType content='elementOnly'>
<sequence minOccurs='1' maxOccurs='1'>
<element ref='ds:CanonicalizationMethod' minOccurs='1' maxOccurs='1'/>
<element ref='ds:SignatureMethod' minOccurs='1' maxOccurs='1'/>
<element ref='ds:Reference' minOccurs='1' maxOccurs='unbounded'/>
</sequence>
<attribute name='Id' type='ID' use='optional'/>
</complexType>
</element>
DTD:
<!ELEMENT SignedInfo (CanonicalizationMethod,
SignatureMethod, Reference+) >
<!ATTLIST SignedInfo
Id ID #IMPLIED>
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.
CanonicalizationMethod
ElementCanonicalizationMethod 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 section
6.1: Algorithm Identifiers and Implementation Requirements.
Implementations MUST support the REQUIRED Canonical XML
[XML-C14N] method.
The Canonical XML implementation should behave as if it received an
XPath node-set originally formed from the document
containing the SignedInfo and currently indicating the SignedInfo,
its descendants, and the attribute and namespace nodes of SignedInfo and its
descendant elements (such that the namespace context and similar ancestor information of
the SignedInfo is preserved). The REQUIRED Canonical XML algorithm omits
the comments.
Alternatives to the REQUIRED Canonical XML algorithm, such as Canonical XML with Comments and the minimal canonicalization algorithm (the CRLF and charset normalization specified in section 6.5.1: Minimal Canonicalization), may be explicitly specified but are NOT REQUIRED. Consequently, their use may not interoperate with other applications that do no support the specified algorithm (see section 7: XML Canonicalization and Syntax Constraint Considerations). Security issues may also arise in the treatment of entity processing and comments if minimal or other non-XML aware canonicalization algorithms are not properly constrained (see section 8.2: Only What is "Seen" Should be Signed).
We RECOMMEND that resource constrained applications that do not implement the Canonical
XML [XML-C14N] transform algorithm and
instead choose minimal canonicalization (or some other form) are implemented to generate
Canonical XML as their output serialization to easily mitigate some of these interoperability
and security concerns. For instance, such an implementation SHOULD (at least) generate
standalone XML instances [XML].
Schema Definition:
<element name='CanonicalizationMethod'>
<complexType content='elementOnly'>
<any minOccurs='0' maxOccurs='unbounded'/>
<attribute name='Algorithm' type='uriReference' use='required'/>
</complexType>
</element>
DTD:
<!ELEMENT CanonicalizationMethod %Method.ANY; >
<!ATTLIST CanonicalizationMethod
Algorithm CDATA #REQUIRED >
The algorithm specifier for minimal canonicalization is given in Section 6.5.1. The algorithm specifiers for the REQUIRED Canonical XML algorithm and Canonical XML with Comments are given in Section 6.5.2.
SignatureMethod
ElementSignatureMethod is a required element that specifies the algorithm used
for signature generation and validation. This algorithm identifies all cryptographic
functions involved in the signature operation (e.g. hashing, public key algorithms, MACs,
padding, etc.). This element uses the general structure here for algorithms described in
section 6.1: Algorithm Identifiers and Implementation Requirements.
While there is a single identifier, that identifier may specify a format containing
multiple distinct signature values.
Schema Definition:
<element name='SignatureMethod'>
<complexType content='elementOnly'>
<any minOccurs='0' maxOccurs='unbounded'/>
<attribute name='Algorithm' type='uriReference' use='required'/>
</complexType>
</element>
DTD:
<!ELEMENT SignatureMethod %Method.ANY; >
<!ATTLIST SignatureMethod
Algorithm CDATA #REQUIRED >
Reference
ElementReference is an element that may occur one or more times. It specifies a
digest algorithm and digest value, and optionally an identifier of the object being
signed, the type of the object, and/or a list of transforms to be applied prior to
digesting. The identification (URI) and transforms describe how the digested content
(i.e., the input to the digest method) was created. The Type attribute
facilitates the processing of referenced data. For example, while this specification makes
no requirements over external data, an application may wish to signal that the referent is
a Manifest. An optional ID attribute permits a Reference to be
referenced from elsewhere.
Schema Definition:
<element name='Reference'>
<complexType content='elementOnly'>
<sequence minOccurs='1' maxOccurs='1'>
<element ref='ds:Transforms' minOccurs='0' maxOccurs='1'/>
<element ref='ds:DigestMethod' minOccurs='1' maxOccurs='1'/>
<element ref='ds:DigestValue' minOccurs='1' maxOccurs='1'/>
</sequence>
<attribute name='Id' type='ID' use='optional'/>
<attribute name='URI' type='uriReference' use='optional'/>
<attribute name='Type' type='uriReference' use='optional'/>
</complexType>
</element>
DTD:
<!ELEMENT Reference (Transforms?, DigestMethod, DigestValue) >
<!ATTLIST Reference
Id ID #IMPLIED
URI CDATA #IMPLIED
Type CDATA #IMPLIED >
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 Section 2.4 of RFC2396 [URI]. 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:
Note that a null URI (
URI="") is permitted and identifies the
XML document that the reference is contained within (the root node).
If a resource is identified by more than one URI, the most specific should be used (e.g. http://www.w3.org/2000/06/interop-pressrelease.html.en instead of http://www.w3.org/2000/06/interop-pressrelease).
XML Signature applications MUST be able to parse URI syntax. We RECOMMEND they be able to dereference URIsUnless 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 Section 4.3.3.1).
Applications should also be cognizant of the fact that protocol parameter and
state information, (such as a HTTP cookies, HTML device profiles or content negotiation),
may affect the content yielded by dereferencing a URI.
This is particularly true if the URI dereference includes fragment processing
of an XML document.
If a resource is identified by more than one URI, the most specific should be used (e.g. http://www.w3.org/2000/06/interop-pressrelease.html.en
instead of http://www.w3.org/2000/06/interop-pressrelease).
Note, I moved the last struck-out sentence to above, which is why it is struck from here. The wording wasn't changed.
If the URI is a same-document reference, then the signature
application MUST behave as if the URI dereference has resulted in an XPath node-set suitable
for use by Canonical XML. Specifically, a null URI (URI="") MUST behave
as if it resulted 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 crosshatch ('#') 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:
The third 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 fourth step is performed for null URIs, barename XPointers and child sequence XPointers.
To retain comments while selecting an element by an identifier ID, use the following full
XPointer: URI='#xpointer(id("ID"))'. To retain comments while selecting
the entire document, use the following full XPointer:
URI='#xpointer(//. | //@* | //namespace::*)'. This XPointer contains a simple XPath
expression that includes all nodes of the parse tree (the root and all descendants, plus all
attributes, plus all namespaces nodes).
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 dereference (including the fragment processing) may be done for the signature application by a proxy user agent. Therefore, reference validation may fail if fragment processing is not performed in a standard way (as defined above for same-document references). To prevent this interoperability problem when signing XML document fragments, we RECOMMEND using only the document URI and performing the fragment processing using an XPath Transform.
XML Signature applications MUST support the null URI and barename XPointers. We RECOMMEND support for the same-document XPointers '#xpointer(//. | //@* | //namespace::*)' and '#xpointer(id("ID"))' if the application also intends to support Minimal Canonicalization or Canonical XML with Comments. 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 the aforementioned interoperability problems and validation failures). Some examples of URI attributes are
URI="http://example.com/bar.xml"URI="http://example.com/bar.xml#chapter1"URI=""URI="#chapter1"[URI] permits identifiers that specify a fragment identifier via a separating number/pound symbol '#'. (The meaning of the fragment is defined by the resource's MIME type). XML Signature applications MUST support the XPointer 'bare name' [Xptr] shortcut after '#' so as to identify IDs within XML documents. The results are serialized as specified in section 6.6.3:XPath Filtering. For example,
URI="http://example.com/bar.xml"URI="http://example.com/bar.xml#chapter1"URI=""URI="#chapter1"Otherwise, support of other fragment/MIME types (e.g., PDF) or XML addressing
mechanisms (e.g., [XPath, Xptr]) is
OPTIONAL, though we RECOMMEND support of [XPath]. Regardless,
such fragment identification and addressing SHOULD be given under Transforms
(not as part of the URI) so that they can be fully identified and specified. For instance,
one could reference a fragment of a document that is encoded by using the Reference
URI to identify the resource, and one Transform to specify
decoding, and a second to specify an XPath selection.
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 result of resolving the URI is passed to the Transforms, if any,
for further processing. The result of the Transforms, or of the URI dereference if
there are no transforms, is either an octet stream or an XPath node-set (or a sufficiently
functional replacement). If the result is not an octet stream, the node-set is converted
to an octet stream using the REQUIRED Canonical XML algorithm (which omits comments).
The resulting 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.
The digest algorithm is then applied to the data octets being secured. Typically that is
done by locating (possibly using the URI if provided) the data and
transforming it. If the data is an XML document, the document is assumed to be unparsed
prior to the application of Transforms. If there are no Transforms,
then the data is passed to the digest algorithm unmodified.
The optional Type attribute contains information about the type of object being signed. This is represented as a URI. For example:
Type="http://www.w3.org/2000/07/xmldsig#Object"
Type="http://www.w3.org/2000/07/xmldsig#Manifest"
Type="http://www.w3.org/2000/07/xmldsig#SignatureProperty"
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.
Transforms
ElementThe 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 (octets) serves as input to the
next Transform.
The input to the first Transform is the source data
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 section 8.1: Only What is
Signed is Secure).
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 resource,
(see section 6.1: Algorithm Identifiers and Implementation Requirements).
Some transforms take an XPath node-set (or sufficiently functional alternative) 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. Each transform defines how this conversion is performed.
Some Transform may require explicit MimeType, 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.
Schema Definition:
<element name='Transforms' >
<complexType content='elementOnly'>
<element ref='ds:Transform' minOccurs='1' maxOccurs='unbounded'/>
</complexType>
</element>
<element name='Transform'>
<complexType content='mixed'>
<choice minOccurs='1' maxOccurs='unbounded'>
<any namespace='##other' processContents='lax' minOccurs='0' maxOccurs='unbounded'/>
<!-- Including well formed XSLT elements -->
<element name='XSLT' type='string'/> <!-- Must be well formed XSL stylesheet or transform element -->
<element name='XPath' type='string'/>
</choice>
<attribute name='Algorithm' type='uriReference' use='required'/>
</complexType>
</element>
DTD:
<!ELEMENT Transforms (Transform+)>
<!ELEMENT Transform %Transform.ANY; >
<!ATTLIST Transform
Algorithm CDATA #REQUIRED >
<!ELEMENT XPath (#PCDATA) >
<!ELEMENT XSLT (#PCDATA) >
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.
Section 6.6: Transform Algorithms defines the list of
standard transformations.
DigestMethod
ElementDigestMethod 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 section 6.1: Algorithm Identifiers and Implementation Requirements.
Schema Definition:
<element name='DigestMethod'>
<complexType content='elementOnly'>
<any processContents='lax' minOccurs='0' maxOccurs='unbounded'/>
<attribute name='Algorithm' type='uriReference' use='required'/>
</complexType>
</element>
DTD:
<!ELEMENT DigestMethod %Method.ANY; >
<!ATTLIST DigestMethod
Algorithm CDATA #REQUIRED >
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 the application MUST convert the node-set to an octet stream using the REQUIRED Canonical XML algorithm (or perform a logically equivalent operation, omitting comments). If the result of URI dereference and application of Transforms is an octet stream, then no conversion occurs (comments may be included if the Minimal Canonicalization or Canonical XML with Comments was specified in the Transforms). The digest algorithm is applied to the data octets of the resulting octet stream.
DigestValue
ElementDigestValue is an element that contains the encoded value of the digest. The digest is always encoded using base64 [MIME].
Schema Definition: <element name='DigestValue' type='ds:CryptoBinary'/>
DTD:
<!ELEMENT DigestValue (#PCDATA) >
<!-- base64 encoded digest value -->
KeyInfo ElementKeyInfo may contain keys, names, certificates and other public key
management information, such as in-band key distribution or key agreement data. This
specification defines a few simple types but applications may place their own key
identification and exchange semantics within this element type through the XML-namespace
facility. [XML-ns]
Schema Definition:
<element name='KeyInfo'>
<complexType content='elementOnly'>
<choice minOccurs='1' maxOccurs='unbounded'>
<any namespace='##other' processContents='lax' minOccurs='0'
maxOccurs='unbounded'/>
<element name='KeyName' type='string'/>
<element ref='ds:KeyValue'/>
<element ref='ds:RetrievalMethod'/>
<element ref='ds:X509Data'/>
<element ref='ds:PGPData'/>
<element ref='ds:SPKIData'/>
<element name='MgmtData' type='string' />
</choice>
<attribute name='Id' type='ID' use='optional'/>
</complexType>
</element>
DTD:
<!ELEMENT KeyInfo %Key.ANY; >
<!ATTLIST KeyInfo
Id ID #IMPLIED >
KeyInfo is an optional element that enables the recipient(s) to obtain the
key needed to validate the signature. If omitted, the recipient is expected to be able to
identify the key based on application context information. Multiple declarations within KeyInfo
refer to the same key. While applications may define and use any mechanism they choose
through inclusion of elements from a different namespace, compliant versions MUST
implement Section 4.4.2: KeyValue and SHOULD
implement Section 4.4.3: RetrievalMethod.
KeyName ElementThe KeyName element contains a string value which may be used by the
signer to communicate a key identifier to the recipient. Typically, KeyName
contains an identifier related to the key pair used to sign the message, but it may
contain other protocol-related information that indirectly identifies a key pair. (Common
uses of KeyName include simple string names for keys, a key index, a
distinguished name (DN), an email address, etc.)
Schema Definition: <!-- type declared in KeyInfo -->
DTD: <!ELEMENT KeyName (#PCDATA) >
KeyValue ElementThe 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 Section 6.4: Signature
Algorithms.
Schema Definition:
<element name='KeyValue'>
<complexType content='mixed'>
<choice minOccurs='1' maxOccurs='1'>
<any namespace='##other' processContents='lax' minOccurs='0' maxOccurs='unbounded'/>
<element ref='ds:DSAKeyValue'/>
<element ref='ds:RSAKeyValue'/>
</choice>
</complexType >
</element>
DTD: <!ELEMENT KeyValue %Key.ANY; >
RetrievalMethod
ElementA RetrievalMethod element within KeyInfo is used to convey a
pointer to KeyInfo-like information that is stored at a remote location. For
example, an X.509v3 certificate chain may be published somewhere common to a number of
documents; each document can reference this chain using a single RetrievalMethod
element instead of including the entire chain with a sequence of X509Certificate elements.
Each RetrievalMethod element contains three children elements: Location,
Method and Type. Location contains a URI
identifying the actual object. Method describes the process by which the data
retrieved from the Location URI should be converted into KeyInfo
sub-elements. The Type sub-element describes the object type and encoding
format of the data stored at the Location URI.
Schema Definition:
<element name='RetrievalMethod'>
<complexType content='elementOnly'>
<sequence minOccurs='1' maxOccurs='1'>
<element name='Location' type='uriReference' minOccurs='1' maxOccurs='1'/>
<element name='Method' type='string' minOccurs='1' maxOccurs='1'/>
<element ref='ds:Type' minOccurs='1' maxOccurs='1'/>
</sequence>
</complexType>
</element>
<element name='Type'>
<complexType content='mixed'>
<any namespace='##other' minOccurs='1' maxOccurs='unbounded'/>
<attribute name='Encoding' type='uriReference' use='optional'/>
</complexType>
</element>
DTD:
<!ELEMENT RetrievalMethod (Location, Method, Type) >
<!ELEMENT Location %Key.ANY; >
<!ELEMENT Method %Key.ANY; >
<!ELEMENT Type %Key.ANY; >
<!ATTLIST Type
Encoding CDATA #IMPLIED>
X509Data ElementAn X509Data element within KeyInfo contains one or more
identifiers of keys/X509 certificates that may be useful for validation. Five types of X509Data
pointers are defined:
X509IssuerSerial element, which contains an X.509 issuer distinguished
name/serial number pair that SHOULD be compliant with RFC2253 [LDAP-DN],
X509SubjectName element, which contains an X.509 subject distinguished
name that SHOULD be compliant with RFC2253 [LDAP-DN], X509SKI element, which contains an X.509 subject key identifier value. X509Certificate element, which contains a Base64-encoded X.509v3
certificate, and X509CRL element, which contains a Base64-encoded X.509v2 certificate
revocation list (CRL). Multiple declarations about a single certificate (e.g., a X509SubjectName
and X509IssuerSerial element) MUST be grouped inside a single X509Data
element; multiple declarations about the same key but different certificates (related to
that single key) MUST be grouped within a single KeyInfo element but multiple
X509Data elements. For example, the following block contains two pointers to
certificate-A (issuer/serial number and SKI) and a single reference to certificate-B
(SubjectName):
<X509Data> <!-- two pointers to certificate-A -->
<X509IssuerSerial>
<X509IssuerName>CN=TAMURA Kent, OU=TRL, O=IBM,
L=Yamato-shi, ST=Kanagawa, C=JP</X509IssuerName>
<X509SerialNumber>12345678</X509SerialNumber>
</X509IssuerSerial>
<X509SKI>31d97bd7</X509SKI>
</X509Data>
<X509Data> <!-- single pointer to certificate-B -->
<X509SubjectName>Subject of Certificate B</X509SubjectName>
</X509Data>
Schema Definition:
<element name='X509Data'>
<complexType content='elementOnly'>
<choice minOccurs='1' maxOccurs='1'>
<sequence minOccurs='1' maxOccurs='unbounded'>
<choice minOccurs='1' maxOccurs='1'>
<element ref='ds:X509IssuerSerial'/>
<element name='X509SKI' type='ds:CryptoBinary'/>
<element name='X509SubjectName' type='string'/>
</choice>
</sequence>
<element name='X509Certificate' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/>
<element name='X509CRL' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/>
</choice>
</complexType>
</element>
<element name='X509IssuerSerial'>
<complexType content='elementOnly'>
<sequence minOccurs='1' maxOccurs='1'>
<element name='X509IssuerName' type='string' minOccurs='1' maxOccurs='1'/>
<element name='X509SerialNumber' type='integer' minOccurs='1' maxOccurs='1'/>
</sequence>
</complexType>
</element>
DTD:
<!ELEMENT X509Data ((X509IssuerSerial | X509SKI | X509SubjectName)+ |
X509Certificate | X509CRL)>
<!ELEMENT X509IssuerSerial (X509IssuerName, X509SerialNumber) >
<!ELEMENT X509IssuerName (#PCDATA) >
<!ELEMENT X509SubjectName (#PCDATA) >
<!ELEMENT X509SerialNumber (#PCDATA) >
<!ELEMENT X509SKI (#PCDATA) >
<!ELEMENT X509Certificate (#PCDATA) >
<!ELEMENT X509CRL (#PCDATA) >
PGPData elementThe PGPData element within KeyInfo is used to convey
information related to PGP public key pairs and signatures on such keys. The PGPKeyID's
value is a string containing a standard PGP public key identifier as defined in Section
11.2 of [PGP]. The PGPKeyPacket contains a
base64-encoded Key Material Packet as defined in Section 5.5 of [PGP].
Other sub-types of the PGPData element may be defined by the OpenPGP working
group.
Schema Definition:
<element name='PGPData'>
<complexType content='elementOnly'>
<choice minOccurs='1' maxOccurs='1'>
<any namespace='##other' processContents='lax' minOccurs='0' maxOccurs='unbounded'/>
<sequence minOccurs='1' maxOccurs='1'>
<element name='PGPKeyID' type='string' minOccurs='1' maxOccurs='1'/>
<element name='PGPKeyPacket' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/>
</sequence>
</choice>
</complexType>
</element>
DTD: <!ELEMENT PGPData (PGPKeyID, PGPKeyPacket) > <!ELEMENT PGPKeyPacket (#PCDATA) > <!ELEMENT PGPKeyID (#PCDATA) >
SPKIData elementThe SPKIData element within KeyInfo is used to convey
information related to SPKI public key pairs, certificates and other SPKI data. The
content of this element type is open and can be defined elsewhere.
Schema Definition:
<element name='SPKIData'>
<complexType content='elementOnly'>
<any namespace='##other' processContents='lax' minOccurs='1' maxOccurs='unbounded'/>
</complexType>
</element>
DTD: <!ELEMENT SPKIData (#PCDATA) >
MgmtData elementThe MgmtData element within KeyInfo is a string value used to
convey in-band key distribution or agreement data. For example, DH key exchange, RSA key
encryption, etc.
Schema Definition: <!-- type declared in KeyInfo -->
DTD: <!ELEMENT MgmtData (#PCDATA)>
Object ElementType="http://www.w3.org/2000/07/xmldsig#Object"
(this can be used within a Reference element to identify the
referent's type)Object is an optional element that may occur one or more times. When
present, this element may contain any data. The Object element may include
optional MIME type, ID, and encoding attributes.
The MimeType attribute is an optional attribute which describes the data
within the Object. This is a string with values defined by [MIME]. For example, if the Object contains XML, the MimeType
could be text/xml. This attribute is purely advisory; no validation of the MimeType
information is required by this specification.
The Object's Id is commonly referenced from a Reference
in SignedInfo, or Manifest. This element is typically used for enveloping signatures where the
object being signed is to be included in the signature element. The digest is calculated
over the entire Object element including start and end tags.
The Object's Encoding attributed may be used to provide a
URI that identifies the method by which the object is encoded (e.g., a binary file).
Note, if the application wishes to exclude the <Object> tags from
the digest calculation the Reference must identify the actual data object
(easy for XML documents) or a transform must be used to remove the Object
tags (likely where the data object is non-XML). Exclusion of the object tags may be
desired for cases where one wants the signature to remain valid if the data object is
moved from inside a signature to outside the signature (or vice-versa), or where the
content of the Object is an encoding of an original binary document and it is
desired to extract and decode so as to sign the original bitwise representation.
Schema Definition:
<element name='Object' >
<complexType content='mixed'>
<choice minOccurs='1' maxOccurs='1'>
<element ref='ds:Manifest' minOccurs='0' maxOccurs='unbounded'/>
<any namespace='##any' processContents='lax' minOccurs='0' maxOccurs='unbounded'/>
</choice>
<attribute name='Id' type='ID' use='optional'/>
<attribute name='MimeType' type='string' use='optional'/> <!-- add a grep facet -->
<attribute name='Encoding' type='uriReference' use='optional'/>
</complexType>
</element>
DTD:
<!ELEMENT Object %Object.ANY; >
<!ATTLIST Object
Id ID #IMPLIED
MimeType CDATA #IMPLIED
Encoding CDATA #IMPLIED >
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.
Manifest ElementType="http://www.w3.org/2000/07/xmldsig#Manifest"
(this can be used within a Reference element to identify the
referent's type)The Manifest element provides a list of References. The
difference from the list in SignedInfo is that it is application defined
which, if any, of the digests are actually checked against the objects referenced and what
to do if the object is inaccessible or the digest compare fails. If a Manifest
is pointed to from SignedInfo, the digest over the Manifest
itself will be checked by the core signature validation behavior. The digests within such
a Manifest are checked at application discretion. If a Manifest
is referenced from another Manifest, even the overall digest of this two
level deep Manifest might not be checked.
Schema Definition:
<element name='Manifest'>
<complexType content='elementOnly'>
<sequence minOccurs='1' maxOccurs='1'>
<element ref='ds:Reference' minOccurs='1' maxOccurs='unbounded'/>
</sequence>
<attribute name='Id' type='ID' use='optional'/>
</complexType>
</element>
DTD:
<!ELEMENT Manifest (Reference+) >
<!ATTLIST Manifest
Id ID #IMPLIED >
SignatureProperties
ElementType="http://www.w3.org/2000/07/xmldsig#SignatureProperty"
(this can be used within a Reference element to identify the
referent's type)Additional information items concerning the generation of the signature(s) can be
placed in a SignatureProperty element (i.e., date/time stamp or the serial
number of cryptographic hardware used in signature generation).
Schema Definition:
<element name='SignatureProperties'>
<complexType content='elementOnly'>
<element ref='ds:SignatureProperty' minOccurs='1' maxOccurs='unbounded'/>
<attribute name='Id' type='ID' use='optional'/>
</complexType>
</element>
<element name='SignatureProperty'>
<complexType content='mixed'>
<any namespace='##other' processContents='lax' minOccurs='1' maxOccurs='unbounded'/>
<attribute name='Target' type='uriReference' use='required'/>
<attribute name='Id' type='ID' use='optional'/>
</complexType>
</element>
DTD:
<!ELEMENT SignatureProperties (SignatureProperty+) >
<!ATTLIST SignatureProperties
Id ID #IMPLIED >
<!ELEMENT SignatureProperty %Object.ANY; >
<!ATTLIST SignatureProperty
Target CDATA #REQUIRED
Id ID #IMPLIED >
No XML processing instructions (PIs) are used by this specification.
Note that PIs placed inside SignedInfo by an application will be signed
unless the CanonicalizationMethod algorithm discards them. (This is true for
any signed XML content.) All of the CanonicalizationMethods specified within
this specification retain PIs. When a PI is part of content that is signed (e.g., within SignedInfo
or referenced XML documents) any change to the PI will obviously result in a signature
failure.
XML comments are not used by this specification.
Note that unless CanonicalizationMethod removes comments within SignedInfo
or any other referenced XML, they will be signed. Consequently, 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.
This section identifies algorithms used with the XML digital signature standard.
Entries contain the identifier to be used in Signature elements, a reference
to the formal specification, and definitions, where applicable, for the representation of
keys and the results of cryptographic operations.
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.
(Note that the normative identifier is the complete URI in the table though they are frequently abbreviated in XML syntax (e.g., "&dsig;base64").)
| Algorithm Type | Algorithm | Requirements | Algorithm URI |
| Digest | |||
| SHA1 | REQUIRED | http://www.w3.org/2000/07/xmldsig#sha1 | |
| Encoding | |||
| Base64 | REQUIRED | http://www.w3.org/2000/07/xmldsig#base64 | |
| MAC | |||
| HMAC-SHA1 | REQUIRED | http://www.w3.org/2000/07/xmldsig#hmac-sha1 | |
| Signature | |||
| DSAwithSHA1 (DSS) |
REQUIRED | http://www.w3.org/2000/07/xmldsig#dsa-sha1 | |
| RSAwithSHA1 | RECOMMENDED | http://www.w3.org/2000/07/xmldsig#rsa-sha1 | |
| Canonicalization | |||
| minimal | RECOMMENDED | http://www.w3.org/2000/07/xmldsig#minimal | |
| Canonical XML with Comments | RECOMMENDED | http://www.w3.org/TR/2000/WD-xml-c14n-20000710#WithComments | |
| Canonical XML (omits comments) | REQUIRED | http://www.w3.org/TR/2000/WD-xml-c14n-20000710 | |
| Transform | |||
| XSLT | OPTIONAL | http://www.w3.org/TR/1999/REC-xslt-19991116 | |
| XPath | RECOMMENDED | http://www.w3.org/TR/1999/REC-xpath-19991116 | |
| Enveloped Signature* | REQUIRED | http://www.w3.org/2000/07/xmldsig#enveloped-signature |
* The Enveloped Signature transform removes the Signature element from the
calculation of the signature when the signature is within the
element 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
XPath Transform specification.
Only one digest algorithm is defined herein. However, it is expected that one or more additional strong digest algorithms will be developed in connection with the US Advanced Encryption Standard effort. Use of MD5 [MD5] is NOT RECOMMENDED because recent advances in cryptography have cast doubt on its strength.
The SHA-1 algorithm [SHA-1] takes no explicit parameters. An example of an SHA-1 DigestAlg element is:
<DigestMethod Algorithm="&dsig;sha1"/>
A SHA-1 digest is a 160-bit string. The content of the DigestValue element shall be the base64 encoding of this bit string viewed as a 20-octet octet stream. For example, the DigestValue element for the message digest:
A9993E36 4706816A BA3E2571 7850C26C 9CD0D89D
from Appendix A of the SHA-1 standard would be:
<DigestValue>qZk+NkcGgWq6PiVxeFDCbJzQ2J0=</DigestValue>
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.
The HMAC algorithm (RFC2104 [HMAC]) takes the truncation length in bits as a parameter; if the
parameter is not specified then all the bits of the hash are output. An example of an HMAC
SignatureMethod element:
<SignatureMethod Algorithm="&dsig;hmac-sha1">
<HMACOutputLength>128</HMACOutputLength>
</SignatureMethod>
The output of the HMAC algorithm is ultimately the output (possibly truncated) of the chosen digest algorithm. This value shall be base64 encoded in the same straightforward fashion as the output of the digest algorithms. Example: the SignatureValue element for the HMAC-SHA1 digest
9294727A 3638BB1C 13F48EF8 158BFC9D
from the test vectors in [HMAC] would be
<SignatureValue>kpRyejY4uxwT9I74FYv8nQ==</SignatureValue>
Schema Definition: <element name='HMACOutputLength' type='integer' minOccurs='0' maxOccurs='1'/>
DTD: <!ELEMENT HMACOutputLength (#PCDATA)>
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.
The DSA algorithm [DSS] takes no explicit parameters. An example
of a DSA SignatureMethod element is:
<SignatureMethod Algorithm="&dsig;dsa"/>
The output of the DSA algorithm consists of a pair of integers usually referred by the pair (r, s). The signature value consists of the base64 encoding of the concatenation of two octet-streams that respectively result from the octet-encoding of the values r and s. Integer to octet-stream conversion must be done according to the I2OSP operation defined in the RFC 2437 [PKCS1] specification with a k parameter equal to 20. For example, the SignatureValue element for a DSA signature (r, s) with values specified in hexadecimal:
r = 8BAC1AB6 6410435C B7181F95 B16AB97C 92B341C0s = 41E2345F 1F56DF24 58F426D1 55B4BA2D B6DCD8C8
from the example in Appendix 5 of the DSS standard would be
<SignatureValue>i6watmQQQ1y3GB+VsWq5fJKzQcBB4jRfH1bfJFj0JtFVtLotttzYyA==</SignatureValue>
DSA key values have the following set of fields: P, Q, G and Y are mandatory when appearing as a key value, J, seed and pgenCounter are optional but SHOULD be present. (The seed and pgenCounter fields MUST appear together or be absent). All parameters are encoded as base64 values.
Schema:
<element name='DSAKeyValue'>
<complexType content='elementOnly'>
<sequence minOccurs='1' maxOccurs='1'>
<element name='P' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/>
<element name='Q' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/>
<element name='G' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/>
<element name='Y' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/>
<element name='J' type='ds:CryptoBinary' minOccurs='0' maxOccurs='1'/>
</sequence>
<sequence minOccurs='0' maxOccurs='1'>
<element name='Seed' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/>
<element name='PgenCounterQ' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/>
</sequence>
</complexType>
</element>
DTD:
<!ELEMENT DSAKeyValue (P, Q, G, Y, J?, (Seed, PgenCounter)?) >
<!ELEMENT P (#PCDATA) >
<!ELEMENT Q (#PCDATA) >
<!ELEMENT G (#PCDATA) >
<!ELEMENT Y (#PCDATA) >
<!ELEMENT J (#PCDATA) >
<!ELEMENT Seed (#PCDATA) >
<!ELEMENT PgenCounter (#PCDATA) >
Arbitrary-length integers (e.g. "bignums" such as RSA modulii) are represented in XML as octet strings. The integer value is first converted to a "big endian" bitstring. The bitstring is then padded with leading zero bits so that the total number of bits == 0 mod 8 (so that there are an even number of bytes). If the bitstring contains entire leading bytes that are zero, these are removed (so the high-order byte is always non-zero). This octet string is then Base64 encoded. (The conversion from integer to octet string is equivalent to IEEE P1363's I2OSP [P1363] with minimal length).
The expression "RSA algorithm" as used in this draft refers to the RSASSA-PKCS1-v1_5 algorithm described in RFC 2437 [PKCS1]. (Note that support for PKCS1 Version 2 is planned as soon as that standard is finalized). The RSA algorithm takes no explicit parameters. An example of an RSA SignatureMethod element is:
<SignatureMethod Algorithm="&dsig;rsa-sha1"/>
The SignatureValue content for an RSA signature shall be the base64
encoding of the octet string. Signatures are interpreted as unsigned integers. A signature
MAY contain a pre-pended algorithm object identifier, but the availability of an ASN.1
parser and recognition of OIDs is not required of a signature verifier.
<SignatureValue>F8aupsHjmbIApjAH4AVYjcsmQkXChyjGYleVJe1KLAmmXWww 3PqkDPUMojithbwbVWVJJ0UhdT407nl0fBrohvkunDq8gzfGkjvO+zDJws1HkRtZ vl1IIBLVWf/qgcLJOgid/2A66niC20GwKcJgIp3o1L+6l7LlSKiZ/CkgDO4= </SignatureValue>
RSA key values have two fields: Modulus and Exponent
<RSAKeyValue>
<Exponent>AQAB</Exponent>
<Modulus>xA7SEU+e0yQH5rm9kbCDN9o3aPIo7HbP7tX6WOocLZAtNfyxSZDU16ksL6W
jubafOqNEpcwR3RdFsT7bCqnXPBe5ELh5u4VEy19MzxkXRgrMvavzyBpVRgBUwUlV
5foK5hhmbktQhyNdy/6LpQRhDUDsTvK+g9Ucj47es9AQJ3U=
</Modulus>
</RSAKeyValue>
Schema:
<element name='RSAKeyValue'>
<complexType content='elementOnly'>
<element name='Modulus' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/>
<element name='Exponent' type='ds:CryptoBinary' minOccurs='1' maxOccurs='1'/>
</complexType>
</element>
DTD:
<!ELEMENT RSAKeyValue (Modulus, Exponent) >
<!ELEMENT Modulus (#PCDATA) >
<!ELEMENT Exponent (#PCDATA) >
Canonicalization algorithms take two implicit parameter when they appear as a CanonicalizationMethod
within the SignedInfo element: the content and its charset. The charset is
derived according to the rules of the transport protocols and media types (e.g, RFC2376 [XML-MT] defines the media types for XML). This information is
necessary to correctly sign and verify documents and often requires careful server side
configuration.
Various canonicalization algorithms require conversion to [UTF-8].The two algorithms below understand at least [UTF-8] and [UTF-16] as input encodings. We RECOMMEND that externally specified algorithms do the same. Knowledge of other encodings is OPTIONAL.
Various canonicalization algorithms transcode from a non-Unicode encoding to Unicode. The two algorithms below perform text normalization during transcoding [NFC]. We RECOMMENDED 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.)
An example of a minimal canonicalization element is:
<CanonicalizationMethod Algorithm="&dsig;minimal"/>
The minimal canonicalization algorithm:
This algorithm requires as input the octet stream of the resource to be processed.
However, the actual input to this algorithm may be an XPath node-set (or a sufficiently
functional replacement implemented by the application). This is true when canonicalizing
SignedInfo, the result of a same-document URI dereference, or the output
of certain types of transforms. In such cases, it is assumed that the application has access to
the necessary portions of the original octet stream such that the algorithm's conversions
can be done without XML processing. The XPath node-set (or sufficiently functional replacement)
exists solely to indicate the portions of the original octet stream required to form the output.
The algorithm outputs an octet stream.
Since this algorithm will typically be implemented without the formal use of XPath
node-sets, note that comments are omitted from same-document URI references created by
the null URI or barename XPointers. We RECOMMEND using the full XPointer equivalents
of these URIs that retain comments (see Section 4.3.3) due to
the difficulty of removing comments from a surface string representation of XML.
Note that the comment start token (<!--) can appear as data (i.e. not as a
begin comment mark) inside attribute values, CDATA sections,
processing instructions, and certain places in the document type declaration
(specifically, in entity and notation declarations where the ExternalID permits
a SystemLiteral).
An example of an XML canonicalization element is:
<CanonicalizationMethod Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000710"/>
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 to omit or retain comments.
Transform
AlgorithmsA Transform algorithm has a single implicit parameters: an octet stream
from the Reference or the output of an earlier Transform.
Application developers are strongly encouraged to support all transforms listed in this section as RECOMMENDED unless the application environment has resource constraints that would make such support impractical. Compliance with this recommendation will maximize application interoperability and libraries should be available to enable support of these transforms in applications without extensive development.
Any canonicalization algorithm that can be used for CanonicalizationMethod
can be used as a Transform.
The normative specification for base 64 decoding transforms is [MIME].
The base64 Transform element has no content. The input is decoded by the
algorithms. This transform is useful if an application needs to sign the raw data
associated with the encoded content of an element.
This transform requires an octet stream for input. If an XPath node-set (or sufficiently
functional alternative) is given as input, then it is converted to an octet stream by
taking the string-value of the node-set (or logically equivalent operation). Thus, if
an XML element is identified by a barename XPointer in the Reference URI, and
its content consists solely of base-64 encoded character data, then this transform automatically
strips away the start and end tags of the identified elements. The output of this transform
is an octet stream.
The normative specification for XPath expression evaluation is [XPath]. The XPath expression
to be evaluated appears as the character content of a transform parameter subelement
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 there may not be comment nodes. 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:
(//. | //@* | //namespace::*)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.
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:
As a result of the context node setting, the XPath expressions appearing in this
transform will be quite similar to those used in XSLT template matching, 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 XPath transform output
is the result of applying the XML canonicalization algorithm [XML-C14N],
parameterized by a given XPath expression, to the XML document received as the transform
input. The XPath expression appears as the character content of a transform parameter
subelement named XPath.
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. The XPath expression can be created such that it includes all elements except those meeting specific criteria. It is the responsibility of the XPath expression author to ensure that all necessary information has been included in the output such that modification of the excluded information does not affect the interpretation of the transform output in the application context.
The XPath transform establishes the following evaluation context for the XML canonicalization algorithm:
XPath
parameter element.The function definition for here() is consistent with its definition in
XPointer. It is defined as follows:
The here function returns a node-set containing the
single node that directly bears the XPath expression. The node could be of any type
capable of directly bearing text, especially text and attribute. This expression results
in an error if the containing XPath expression does not appear in
an XML documentthe 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,
<Document>
...
<Signature xmlns="&dsig;">
<SignedInfo>
...
<Reference URI="">
<Transforms>
<Transform
Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
<XPath
xmlns:dsig="&dsig;">
(//. | //@* | //namespace::*)[not(ancestor-or-self::dsig:Signature)]
not(ancestor-or-self::dsig:Signature)
</XPath>
</Transform>
</Transforms>
<DigestMethod
Algorithm="http://www.w3.org/2000/07/xmldsig#sha1"/>
<DigestValue></DigestValue>
</Reference>
</SignedInfo>
<SignatureValue></SignatureValue>
</Signature>
...
</Document>
The subexpression (//. | //@* | //namespace::*) means that all nodes in
the entire parse tree starting at the root node are candidates for the result node-set.
For each node candidate, the node is included in the resultant node-set if and only if the
node test (the boolean expression in the square brackets) evaluates to "true"
for that node. The node test returns true for all nodes except nodes that either have or
have an ancestor with a tag of 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:
<XPath
xmlns:dsig="&dsig;">(//. | //@* | //namespace::*)
[count(ancestor-or-self::dsig:Signature | here()/ancestor::dsig:Signature[1])
>
count(ancestor-or-self::dsig:Signature)]</XPath>
<XPath
xmlns:dsig="&dsig;">
count(ancestor-or-self::dsig:Signature | here()/ancestor::dsig:Signature[1]) >
count(ancestor-or-self::dsig:Signature)</XPath>
Since the XPath equality operator converts node sets to string values before
comparison, we must instead use the XPath union operator (|). For each node of the
document, the predicate expression is true if and only if the node-set containing the node
and its Signature element ancestors does not include the enveloped Signature
element containing the XPath expression (the union does not produce a larger set if the
enveloped Signature element is in the node-set given by ancestor-or-self::Signature).
It is RECOMMENDED that the XPath be constructed such that the result of this operation is a well-formed XML document. This should be the case if the root element of the input resource is included by the XPath (even if a number of its descendant nodes are omitted by the XPath expression). It is also RECOMMENDED that nodes should not be omitted from the input if they affect the interpretation of the output nodes in the application context. The XPath expression author is responsible for this since the XPath expression author knows the application context.
An enveloped signature transform T removes the whole Signature
element containing T from the digest calculation of the Reference
element containing T. The entire string of characters used by an XML
processor to match the Signature with the XML production element
is removed. The output of the transform is equivalent to the output that would result from
replacing T with an XPath transform containing the following XPath
parameter element:
<XPath
xmlns:dsig="&dsig;">(//. | //@* | //namespace::*)
[count(ancestor-or-self::dsig:Signature | here()/ancestor::dsig:Signature[1])
>
count(ancestor-or-self::dsig:Signature)]</XPath>
<XPath
xmlns:dsig="&dsig;">
count(ancestor-or-self::dsig:Signature | here()/ancestor::dsig:Signature[1])
>
count(ancestor-or-self::dsig:Signature)</XPath>
The input and output requirements of this transform are identical to those of the XPath transform. Note that it is not necessary to use an XPath expression evaluator to create this transform. However, this transform MUST produce output in exactly the same manner as the XPath transform parameterized by the XPath expression above.
The normative specification for XSL Transformations is [XSLT]. The XSL stylesheet or
transform to be evaluated appears as the character content of a transform parameter subelement
named XSLT. This transform requires an octet stream as input. If the
actual input is an XPath node-set, then it should be converted to an octet stream
using the required Canonical XML (omit comments) [XML-C14N].
The output of this transform is an octet stream. The processing rules for the XSL stylesheet
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. We RECOMMEND inserting a Transform after the
XSLT Transform to perform the required Canonical XML (omit comments). 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].
The Transform element contains a single parameter child element called XSLT,
whose content MUST conform to the XSL Transforms [XSLT] language syntax. The
processing rules for the XSLT transform are stated in the XSLT specification [XSLT].
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 three categories. There are those related to the basic [XML], as described in 7.1 below. There are those related to [DOM], [SAX], or similar processing as described in 7.2 below. And, third, there is the possibility of coded character set conversion, such as between UTF-8 and UTF-16, both of which all [XML] compliant processors are required to support.
Any canonicalization algorithm should yield output in a specific fixed coded character
set. For both the minimal canonicalization defined in this specification and Canonical XML
[XML-C14N] that coded character set is UTF-8 (without a byte
order mark (BOM)).Neither the minimal canonicalization nor the Canonical XML [XML-C14N] algorithms provide character normalization. We
RECOMMEND that signature applications create XML content (Signature
elements and their descendents/content) in Normalized Form C [NFC]
and check that any XML being consumed is in that form as well (if not, signatures may
consequently fail to validate). Additionally, none of these algorithms provide data
type normalization. Applications that normalize data types in varying formats (e.g.,
(true, false) or (1,0)) may not be able to validate each other's signatures.
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,
Note that items (2), (4), and (5C) depend on the presence of a schema, DTD or
similar declarations. The Signature element type is laxly schema valid
[XML-schema], consequently external XML or even XML within
the same document as the signature may be (only) well formed or from another namespace
(where permitted by the signature schema); the noted items may not be present. Thus, a
signature with such content will only be verifiable by other signature applications if the
following syntax contraints are observed when generating any signed material including the
SignedInfo element:
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 syntax constraints given in section 7.1 be followed but an appropriate XML canonicalization MUST be specified so that the verifier can re-serialize DOM/SAX mediated input into the same octect stream that was signed.
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.
A requirement of this specification is to permit signatures to "apply to a
part or totality of a XML document." (See section 3.1.3 of [XML-Signature-RD].) 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 [XPath] to exclude those portions the user
needs to change. Transforms may be arbitrarily specified and may include
encoding tranforms, canonicalization instructions or even XSLT transformations. Three
cautions are raised with respect to this feature in the following sections.
Note, core validation behavior does not confirm that the signed data was obtained by applying each step of the indicated transforms. (Though it does check that the digest of the resulting content matches that specified in the signature.) For example, some application may be satisfied with verifying an XML signature over a cached copy of already transformed data. Other applications might require that content be freshly dereferenced and transformed.
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.
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 an automated mechanism or person, then it is normally necessary to secure as exactly as practical the information that was presented to that mechanism or person. Note that this can be accomplished by literally signing what was presented, 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.
Note: This new recommendation is actually a combination/inverse of the earlier recommendations and is still under discussion.
Just as a person or automatable mechanism should only sign what it "sees,"
persons and automated mechanisms that trust the validity of a transformed document on the
basis of a valid signature SHOULD operate over the data that was transformed (including
canonicalization) and signed, not the original pre-transformed data. This recommendation
applies to transforms specified within the signature as well as those included as part of
the document itself. For instance, if an XML document includes an embedded
stylesheet [XSLT] it is the transformed document that that
SHOULD be represented to the user and signed. To meet this recommendation where a document
references an external style sheet, the content of that external resource SHOULD also be
signed as via a signature Reference -- otherwise the content of that external
content might change which alters the resulting document without invalidating the
signature.
Some applications might operate over the original or intermediary data but SHOULD be extremely careful about potential weaknesses introduced between the original and transformed data. This is a trust decision about the character and meaning of the transforms that an application needs to make with caution. Consider a canonicalization algorithm that normalizes character case (lower to upper) or character composition ('e and accent' to 'accented-e'). An adversary could introduce changes that are normalized and consequently inconsequential to signature validity but material to a DOM processor. For instance, by changing the case of a character one might influence the result of an XPath selection. A serious risk is introduced if that change is normalized for signature validation but the processor operates over the original data and returns a different result than intended. Consequently, while we RECOMMEND all documents operated upon and generated by signature applications be in [NFC] (otherwise intermediate processors might unintentionally break the signature) encoding normalizations SHOULD NOT be done as part of a signature transform, or (to state it another way) if normalization does occur, the application SHOULD always "see" (operate over) the normalized form.
This standard specifies 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 standard 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.
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 validaters 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.
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].Object element is merely one type of digital data (or
document) that can be signed via a Reference.Signature
element type and its child's content SignatureValue and mandatory to support
algorithms.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.Object element of the
signature itself. The Object(or its content) is identified via a Reference
(via a URI fragment idenitifier or transform).SignatureValue.SignedInfo
reference validation.Reference,
matches its specified DigestValue.SignatureValue matches the result of processing SignedInfo
with CanonicalizationMethod and SignatureMethod as
specified in section 3.2.Donald E. Eastlake 3rd
Motorola, Mail Stop: M4-10
20 Forbes Boulevard
Mansfield, MA 02048 USA
Phone: 1-508-261-5434
Email: Donald.Eastlake@motorola.com
Joseph M. Reagle Jr., W3C
Massachusetts Institute of Technology
Laboratory for Computer Science
NE43-350, 545 Technology Square
Cambridge, MA 02139
Phone: + 1.617.258.7621
Email: reagle@w3.org
David Solo
Citigroup
666 Fifth Ave, 3rd Floor
NY, NY 10103 USA
Phone: +1-212-830-8118
Email: dsolo@alum.mit.edu