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 specification from the IETF/W3C XML Signature Working Group is a second last call that ends on November 5, 2000. We ask Working Group members and other readers to review our approach to all issues raised by the first last call and, more substantively, consider changes resulting from the recent Canonical XML last call upon which this specification is dependent. Barring substantive comment (we expect little), we will request Candidate Recommendation status as soon as possible (following the Canonical XML request). However, we do wish to ensure that readers are aware of following three changes:
We've changed the Reference Processing Model (section 4.3.3.1). to permit the presentation and acceptance of XML node-sets between Transforms (and resulting from some URI References) when appropriate.
We accomplish this by heavily relying upon the XPath specification but still do NOT require a conformant XPath implementation.
We've revised the treatment of pre-pended
algorithm object identifier within the encoded RSA
SignatureValue by the PKCS1
algorithm (section 6.4.2).
We've revised the X509Data element (section
4.4.4) to clarify the treatment of certificate "bags" and CRLs
within that structure.
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 in conformance with W3C policy, and the IETF Page of Intellectual Property Rights Notices in conformance with IETF policy.
This document specifies XML syntax and processing rules for creating and representing digital signatures. XML Signatures can be applied to any digital content (data object), including XML. An XML Signature may be applied to the content of one or more resources. Enveloped or enveloping signatures are over data within the same XML document as the signature; detached signatures are over data external to the signature element. More specifically, this specification defines an XML signature element type and an XML signature application; conformance requirements for each are specified by way of schema definitions and prose respectively. This specification also includes other useful types that identify methods for referencing collections of resources, algorithms, and keying and management information.
The XML Signature is a method of associating a key with referenced data (octets); it does not normatively specify how keys are associated with persons or institutions, nor the meaning of the data being referenced and signed. Consequently, while this specification is an important component of secure XML applications, it itself is not sufficient to address all application security/trust concerns, particularly with respect to using signed XML (or other data formats) as a basis of human-to-human communication and agreement. Such an application must specify additional key, algorithm, processing and rendering requirements. For further information, please see Security Considerations (section 8).
For readability, brevity, and historic reasons this document uses the term "signature" to generally refer to digital authentication values of all types.Obviously, the term is also strictly used to refer to authentication values that are based on public keys and that provide signer authentication. When specifically discussing authentication values based on symmetric secret key codes we use the terms authenticators or authentication codes. (See Check the Security Model, section 8.3 .)
This specification uses both XML Schemas [XML-schema] and DTDs [XML]. (Readers unfamiliar with DTD syntax may wish to refer to Ron Bourret's " Declaring Elements and Attributes in an XML DTD" [Bourret].) The schema definition is presently normative.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in RFC2119 [KEYWORDS]:
"they MUST only be used where it is actually required for interoperation or to limit behavior which has potential for causing harm (e.g., limiting retransmissions)"
Consequently, we use these capitalized keywords to unambiguously specify requirements over protocol and application features and behavior that affect the interoperability and security of implementations. These key words are not used (capitalized) to describe XML grammar; schema definitions unambiguously describe such requirements and we wish to reserve the prominence of these terms for the natural language descriptions of protocols and features. For instance, an XML attribute might be described as being "optional." Compliance with the XML-namespace specification [XML-ns] is described as "REQUIRED."
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/2000/09/xmldsig#"
This namespace is also used as the prefix for algorithm identifiers used by this specification. While applications MUST support XML and XML-namespaces, the use of internal entities [XML] or our "dsig" XML namespace prefix and defaulting/scoping conventions are OPTIONAL; we use these facilities to provide compact and readable examples.
This specification uses Uniform Resource Identifiers [URI] to identify resources, algorithms, and semantics. The URI in the namespace declaration above is also used as a prefix for URIs under the control of this specification. For resources not under the control of this specification, we use the designated Uniform Resource Names [URN] or Uniform Resource Locators [URL] defined by its normative external specification. If an external specification has not allocated itself a Uniform Resource Identifier we allocate an identifier under our own namespace. For instance:
SignatureProperties is identified and defined by this
specification's namespaceFinally, in order to provide for terse namespace declarations we sometimes use XML internal entities [XML] within URIs. For instance:
<?xml version='1.0'?>
<!DOCTYPE Signature SYSTEM
"xmldsig-core-schema.dtd" [ <!ENTITY dsig
"http://www.w3.org/2000/09/xmldsig#"> ]>
<Signature xmlns="&dsig;" Id="MyFirstSignature">
<SignedInfo>
...
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 Processing Rules (section 3). The formal syntax is found in Core Signature Syntax (section 4) and Additional Signature Syntax (section 5).
In this section, an informal representation and examples are used to describe the structure of the XML signature syntax. This representation and examples may omit attributes, details and potential features that are fully explained later.
XML Signatures are applied to arbitrary digital content (data
objects) via an indirection. Data objects are digested, the
resulting value is placed in an element (with other information)
and that element is then digested and cryptographically signed. XML
digital signatures are represented by the Signature
element which has the following structure (where "?" denotes zero
or one occurrence; "+" denotes one or more occurrences; and "*"
denotes zero or more occurrences):
<Signature>
<SignedInfo>
(CanonicalizationMethod)
(SignatureMethod)
(<Reference (URI=)? >
(Transforms)?
(DigestMethod)
(DigestValue)
</Reference>)+
</SignedInfo>
(SignatureValue)
(KeyInfo)?
(Object)*
</Signature>
Signatures are related to data objects via URIs [URI].
Within an XML document, signatures are related to local data
objects via fragment identifiers. Such local data can be included
within an
enveloping signature or can enclose an enveloped signature.
Detached
signatures are over external network resources or local data
objects that resides within the same XML document as sibling
elements; in this case, the signature is neither enveloping
(signature is parent) nor enveloped (signature is child). Since a
Signature element (and its Id attribute
value/name) may co-exist or be combined with other elements (and
their IDs) within a single XML document, care should be taken in
choosing names such that there are no subsequent collisions that
violate the ID uniqueness
validity constraint [XML].
Signature, SignedInfo,
Methods, and References)The following example is a detached signature of the content of the HTML4 in XML specification.
[s01] <Signature Id="MyFirstSignature" xmlns="http://www.w3.org/2000/09/xmldsig#"> [s02] <SignedInfo> [s03] <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000710"/> [s04] <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/> [s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000710"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> [s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [s11] </Reference> [s12] </SignedInfo> [s13] <SignatureValue>MC0CFFrVLtRlk=...</SignatureValue> [s14] <KeyInfo> [s15a] <KeyValue> [s15b] <DSAKeyValue> [s15c] <P>...</P><Q>...</Q><G>...</G><Y>...</Y> [s15d] </DSAKeyValue> [s15e] </KeyValue> [s16] </KeyInfo> [s17] </Signature>
[s02-12] The required SignedInfo
element is the information that is actually signed. Core validation of
SignedInfo consists of two mandatory processes: validation of the
signature over SignedInfo and validation of each
Reference digest within SignedInfo.
Note that the algorithms used in calculating the
SignatureValue are also included in the signed information
while the SignatureValue element is outside
SignedInfo.
[s03] The CanonicalizationMethod is
the algorithm that is used to canonicalize the
SignedInfo element before it is digested as part of the
signature operation.
[s04] The SignatureMethod is the
algorithm that is used to convert the canonicalized
SignedInfo into the SignatureValue. It is a
combination of a digest algorithm and a key dependent algorithm and
possibly other algorithms such as padding, for example RSA-SHA1.
The algorithm names are signed to resist attacks based on
substituting a weaker algorithm. To promote application
interoperability we specify a set of signature algorithms that MUST
be implemented, though their use is at the discretion of the
signature creator. We specify additional algorithms as RECOMMENDED
or OPTIONAL for implementation and the signature design permits
arbitrary user algorithm specification.
[s05-11] Each Reference element
includes the digest method and resulting digest value calculated
over the identified data object. It also may include
transformations that produced the input to the digest operation. A
data object is signed by computing its digest value and a signature
over that value. The signature is later checked via reference and signature
validation.
[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/2000/09/xmldsig#sha1"/> [s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [s11] </Reference>
[s05] The optional URI attribute of
Reference identifies the data object to be signed.
This attribute may be omitted on at most one Reference
in a Signature. (This limitation is imposed in order
to ensure that references and objects may be matched
unambiguously.)
[s05-08] This identification, along with the
transforms, is a description provided by the signer on how they
obtained the signed data object in the form it was digested (i.e.
the digested content). The verifier may obtain the digested content
in another method so long as the digest verifies. In particular,
the verifier may obtain the content from a different location such
as a local store than that specified in the URI.
[s06-08] Transforms is an optional ordered list of
processing steps that were applied to the resource's content before
it was digested. Transforms can include operations such as
canonicalization, encoding/decoding (including
compression/inflation), XSLT and XPath. XPath transforms permit the
signer to derive an XML document that omits portions of the source
document. Consequently those excluded portions can change without
affecting signature validity. For example, if the resource being
signed encloses the signature itself, such a transform must be used
to exclude the signature value from its own computation. If no
Transforms element is present, the resource's content
is digested directly. While we specify mandatory (and 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].)
[ ] <Signature Id="MySecondSignature" ...> [p01] <SignedInfo> [ ] ... [p02] <Reference URI="http://www.w3.org/TR/xml-stylesheet/"> [ ] ... [p03] <Reference URI="#AMadeUpTimeStamp" [p04] Type="http://www.w3.org/2000/09/xmldsig#SignatureProperty"> [p05] <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> [p06] <DigestValue>k3453rvEPO0vKtMup4NbeVu8nk=</DigestValue> [p07] </Reference> [p08] </SignedInfo> [p09] ... [p10] <Object> [p11] <SignatureProperties> [p12] <SignatureProperty Id="AMadeUpTimeStamp" Target="#MySecondSignature"> [p13] <timestamp xmlns="http://www.ietf.org/rfcXXXX.txt"> [p14] <date>19990908</date> [p15] <time>14:34:34:34</time> [p16] </timestamp> [p17] </SignatureProperty> [p18] </SignatureProperties> [p19] </Object> [p20]</Signature>
[p04] The optional Type attribute of
Reference provides information about the resource
identified by the URI. In particular, it can indicate
that it is an Object, SignatureProperty,
or Manifest element. This can be used by applications
to initiate special processing of some Reference
elements. References to an XML data element within an
Object element SHOULD identify the actual element pointed
to. Where the element content is not XML (perhaps it is binary or
encoded data) the reference should identify the Object
and the Reference Type, if given, SHOULD
indicate Object. Note that Type is
advisory and no action based on it or checking of its correctness
is required by core behavior.
[p10] Object is an optional element
for including data objects within the signature element or
elsewhere. The Object can be optionally typed and/or
encoded.
[p11-18] Signature properties, such as time of
signing, can be optionally signed by identifying them from within a
Reference. (These properties are traditionally called
signature "attributes" although that term has no relationship to
the XML term "attribute".)
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/2000/09/xmldsig#Manifest"> [m03] <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> [m04] <DigestValue>345x3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [m05] </Reference> [ ] ... [m06] <Object> [m07] <Manifest Id="MyFirstManifest"> [m08] <Reference> [m09] ... [m10] </Reference> [m11] <Reference> [m12] ... [m13] </Reference> [m14] </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 Signature Overview (section 2). This section provides detailed syntax of the core signature features. Features described in this section are mandatory to implement unless otherwise indicated. The syntax is defined via DTDs and [XML-Schema] with the following XML preamble, declaration, internal entity, and simpleType:
Schema Definition:
<?xml version='1.0'?>
<!DOCTYPE schema
SYSTEM 'http://www.w3.org/1999/XMLSchema.dtd'
[
<!ENTITY dsig 'http://www.w3.org/2000/09/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/2000/09/xmldsig#'
Id ID #IMPLIED >
SignatureValue ElementThe SignatureValue element contains the actual
value of the digital signature; it is always encoded using Base64
[MIME].While we specify a mandatory and
optional to implement SignatureMethod algorithms, user
specified algorithms are permitted.
Schema Definition: <element name='SignatureValue' type='ds:CryptoBinary'/>
DTD: <!ELEMENT SignatureValue (#PCDATA) >
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.
SignedInfo does not include explicit signature or
digest properties (such as calculation time, cryptographic device
serial number, etc.). If an application needs to associate
properties with the signature or digest, it may include such
information in a SignatureProperties element within an
Object element.
Schema Definition:
<element name='SignedInfo'>
<complexType 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>
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 Algorithm Identifiers and Implementation
Requirements (section 6.1). Implementations MUST support the
REQUIRED Canonical XML [XML-C14N]
method.
Alternatives to the REQUIRED Canonical XML algorithm (section 6.5.2), such as Canonical XML with Comments (section 6.5.2) and Minimal Canonicalization (the CRLF and charset normalization specified in section 6.5.1), may be explicitly specified but are NOT REQUIRED. Consequently, their use may not interoperate with other applications that do no support the specified algorithm (see XML Canonicalization and Syntax Constraint Considerations, section 7). Security issues may also arise in the treatment of entity processing and comments if minimal or other non-XML aware canonicalization algorithms are not properly constrained (see section 8.2: Only What is "Seen" Should be Signed).
The way in which the SignedInfo element is
presented to the canonicalization method is dependent on that
method. The following applies to the two types of algorithms
specified by this document:
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).We RECOMMEND that resource constrained applications that do not implement the Canonical XML [XML-C14N] algorithm and instead choose minimal canonicalization (or some other form) are implemented to generate Canonical XML as their output serialization so as to easily mitigate some of these interoperability and security concerns. (While a result might not be the canonical form of the original, it can still be in canonical form.) For instance, such an implementation SHOULD (at least) generate standalone XML instances [XML].
Schema Definition:
<element name='CanonicalizationMethod'>
<complexType 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 >
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 >
URI
AttributeThe URI attribute identifies a data object using a
URI-Reference, as specified by RFC2396 [URI]. The set of allowed characters for
URI attributes is the same as for XML, namely [Unicode]. However, some Unicode characters are
disallowed from URI references including all non-ASCII characters
and the excluded characters listed in RFC2396 [URI, section 2.4]. However, the number sign (#),
percent sign (%), and square bracket characters re-allowed in RFC
2732 [URI-Literal] are permitted.
Disallowed characters must be escaped as follows:
XML signature applications MUST be able to parse URI syntax. We RECOMMEND they be able to dereference URIs in the HTTP scheme. Dereferencing a URI in the HTTP scheme MUST comply with the Status Code Definitions of [HTTP] (e.g., 302, 305 and 307 redirects are followed to obtain the entity-body of a 200 status code response). Applications should also be cognizant of the fact that protocol parameter and state information, (such as a HTTP cookies, HTML device profiles or content negotiation), may affect the content yielded by dereferencing a URI.
If a resource is identified by more than one URI, the most specific should be used (e.g. http://www.w3.org/2000/06/interop-pressrelease.html.en instead of http://www.w3.org/2000/06/interop-pressrelease). (See the Reference Validation (section 3.2.1) for a further information on reference processing.)
If the URI attribute is omitted altogether, the
receiving application is expected to know the identity of the
object. For example, a lightweight data protocol might omit this
attribute given the identity of the object is part of the
application context. This attribute may be omitted from at most one
Reference in any particular SignedInfo,
or Manifest.
The optional Type attribute contains information about the type of object being signed. This is represented as a URI. For example:
Type="http://www.w3.org/2000/09/xmldsig#Object"
Type="http://www.w3.org/2000/09/xmldsig#Manifest"
The Type attribute applies to the item being pointed at, not its
contents. For example, a reference that identifies an
Object element containing a SignatureProperties
element is still of type #Object. The type attribute
is advisory. No validation of the type information is required by
this specification.
Note: XPath is RECOMMENDED. Signature applications need not conform to [XPath] specification in order to conform to this specification. However, the XPath data model, definitions (e.g., node-sets) and syntax is used within this document in order to describe functionality for those that want to process XML-as-XML (instead of octets) as part of signature generation. For those that want to use these features, a conformant [XPath] implementation is one way to implement these features, but it is not required. Such applications could use a sufficiently functional replacement to a node-set and implement only those XPath expression behaviors REQUIRED by this specification. However, for simplicity we generally will use XPath terminology without including this qualification on every point. Requirements over "XPath nodesets" can include a node-set functional equivalent. Requirements over XPath processing can include application behaviors that are equivalent to the corresponding XPath behavior.
The data-type of the result of URI dereferencing or subsequent Transforms is either an octet stream or an XPath node-set.
The Transforms specified in this document are defined with respect to the input they require. The following is the default signature application behavior:
Users may specify alternative transforms that over-ride these
defaults in transitions between Transforms that expect different
inputs. The final octet stream contains the data octets being
secured. The digest algorithm specified by
DigestMethod is then applied to these data octets, resulting
in the DigestValue.
Unless the URI-Reference is a 'same-document' reference as defined in [URI, Section 4.2], the result of dereferencing the URI-Reference MUST be an octet stream. In particular, an XML document identified by URI is not parsed by the signature application unless the URI is a same-document reference or unless a Transform that requires XML parsing is applied (See Transforms (section 4.3.3.1).)
When a fragment is preceded by an absolute or relative URI in
the URI-Reference, the meaning of the fragment is defined by the
resource's MIME type. Even for XML documents, URI dereferencing
(including the fragment processing) might be done for the signature
application by a proxy. Therefore, reference validation might fail
if fragment processing is not performed in a standard way (as
defined in the following section for same-document references).
Consequently, we RECOMMEND that the URI
attribute not include fragment identifiers and that such processing
be specified as an additional XPath
Transform.
When a fragment is not preceded by a URI in the URI-Reference, XML signature applications MUST support the null URI and barename XPointer. We RECOMMEND support for the same-document XPointers '#xpointer(/)' and '#xpointer(id("ID"))' if the application also intends to support Minimal Canonicalization or Canonical XML with Comments. (Otherwise URI="#foo" will automatically remove comments before the Canonical XML with Comments can even be invoked.) All other support for XPointers is OPTIONAL, especially all support for barename and other XPointers in external resources since the application may not have control over how the fragment is generated (leading to interoperability problems and validation failures).
The following examples demonstrate what the URI attribute identifies and how it is dereferenced:
URI="http://example.com/bar.xml"URI="http://example.com/bar.xml#chapter1"URI=""URI="#chapter1"Dereferencing a same-document reference MUST result in an XPath
node-set suitable for use by Canonical XML. Specifically,
dereferencing a null URI (URI="") MUST result in an
XPath node-set that includes every non-comment node of the XML
document containing the URI attribute. In a fragment
URI, the characters after the number sign ('#') character conform
to the XPointer syntax [Xptr]. When
processing an XPointer, the application MUST behave as if the root
node of the XML document containing the URI attribute
were used to initialize the XPointer evaluation context. The
application MUST behave as if the result of XPointer processing
were a node-set derived from the resultant location-set as
follows:
The second to last replacement is necessary because XPointer typically indicates a subtree of an XML document's parse tree using just the element node at the root of the subtree, whereas Canonical XML treats a node-set as a set of nodes in which absence of descendant nodes results in absence of their representative text from the canonical form.
The last step is performed for null URIs, barename XPointers and
child sequence XPointers. To retain comments while selecting an
element by an identifier ID, use the following full
XPointer: URI='#xpointer(id("ID"))'. To retain
comments while selecting the entire document, use the following
full XPointer: URI='#xpointer(/)'. This XPointer
contains a simple XPath expression that includes the root node,
which the second to last step above replaces with all nodes of the
parse tree (all descendants, plus all attributes, plus all
namespaces nodes).
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 serves as input to the next
Transform. The input to the first Transform is
the result of dereferencing the URI attribute of the
Reference element. The output from the last
Transform is the input for the DigestMethod
algorithm. When transforms are applied the signer is not signing
the native (original) document but the resulting (transformed)
document. (See Only What is Signed is
Secure (section 8.1).)
Each Transform consists of an
Algorithm attribute and content parameters, if any,
appropriate for the given algorithm. The Algorithm
attribute value specifies the name of the algorithm to be
performed, and the Transform content provides
additional data to govern the algorithm's processing of the
transform input. (See Algorithm Identifiers
and Implementation Requirements (section 6).)
As described in The Reference Processing Model (section 4.3.3.2), some transforms take an XPath node-set as input, while others require an octet stream. If the actual input matches the input needs of the transform, then the transform operates on the unaltered input. If the transform input requirement differs from the format of the actual input, then the input must be converted.
Some Transform may require explicit 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.
Examples of transforms include but are not limited to base64
decoding [MIME], canonicalization [XML-C14N], XPath filtering [XPath], and XSLT [XSLT].
The generic definition of the Transform element also
allows application-specific transform algorithms. For example, the
transform could be a decompression routine given by a Java class
appearing as a base64 encoded parameter to a Java
Transform algorithm. However, applications should refrain
from using application-specific transforms if they wish their
signatures to be verifiable outside of their application domain.Transform Algorithms (section 6.6)
defines the list of standard transformations.
Schema Definition:
<element name='Transforms' >
<complexType content='elementOnly'>
<element ref='ds:Transform' minOccurs='1' maxOccurs='unbounded'/>
</complexType>
</element>
<element name='Transform'>
<complexType content='elementOnly'>
<choice minOccurs='1' maxOccurs='unbounded'>
<any namespace='##other' processContents='lax' minOccurs='0'
maxOccurs='unbounded'/>
<element name='XSLT' type='string'/>
<!-- must be an xsl:stylesheet element -->
<element name='XPath' type='string'/>
</choice>
<attribute name='Algorithm' type='uriReference' use='required'/>
</complexType>
</element>
DTD:
<!ELEMENT Transforms (Transform+)>
<!ELEMENT Transform %Transform.ANY; >
<!ATTLIST Transform
Algorithm CDATA #REQUIRED >
<!ELEMENT XPath (#PCDATA) >
<!ELEMENT XSLT (#PCDATA) >
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
Algorithm Identifiers and Implementation
Requirements (section 6.1).
If the result of the URI dereference and application of Transforms is an XPath node-set (or sufficiently functional replacement implemented by the application) then it must be converted as described in the Reference Processing Model (section 4.3.3.2). If the result of URI dereference and application of Transforms is an octet stream, then no conversion occurs (comments might be present if the Minimal Canonicalization or Canonical XML with Comments was specified in the Transforms). The digest algorithm is applied to the data octets of the resulting octet stream.
Schema Definition:
<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 >
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 is an optional element that enables the
recipient(s) to obtain the key needed to validate the signature.
KeyInfo may contain keys, names, certificates and
other public key management information, such as in-band key
distribution or key agreement data. This specification defines a
few simple types but applications may place their own key
identification and exchange semantics within this element type
through the XML-namespace facility. [XML-ns]
If KeyInfo is omitted, the recipient is expected to
be able to identify the key based on application context
information. Multiple declarations within KeyInfo
refer to the same key. While applications may define and use any
mechanism they choose through inclusion of elements from a
different namespace, compliant versions MUST implement KeyValue (section 4.4.2) and
SHOULD implement
RetrievalMethod (section 4.4.3).
The following list summarizes the KeyInfo types
defined by this specification; these can used within the
RetrievalMethod Type attribute to describe the
remote KeyInfo structure.
In addition to the types above for which we define structures, we specify one additional type to indicate a binary X.509 Certificate
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 >
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 Signature Algorithms
(section 6.4).
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 reference to
KeyInfo information that is stored at another location. For
example, several sigantures in a document might use a key verified
by an X.509v3 certificate chain appearing once in the document or
remotely outside the document; each signature's
KeyInfo can reference this chain using a single
RetrievalMethod element instead of including the entire
chain with a sequence of X509Certificate elements.
RetrievalMethod uses the same syntax as
Reference except that there is no DigestMethod
or DigestValue child elements and presence of the
URI is mandatory; Type is an optional
identifier for the type of data to be retrieved.
Schema Definition
<element name='RetrievalMethod'>
<complexType content='elementOnly'>
<element ref='dsTransforms' minOccurs='0' maxOccurs='1'/>
<attribute name='URI' type='uriReference'/>
<attribute name='Type' type='uriReference'/>
</complexType>
</element>
DTD
<!ELEMENT RetrievalMethod (Transforms?) >
<!ATTLIST Type
URI CDATA #REQUIRED
Type CDATA #REQUIRED >
X509Data ElementType="http://www.w3.org/2000/09/xmldsig#X509Data"RetrievalMethod or
Reference element to identify the referent's type)An X509Data element within KeyInfo
contains one or more identifiers of keys or X509 certificates (or
certificates' identifiers or revocation lists). Five types of
X509Data are defined
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, andX509CRL 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 MAY occur in multiple
X509Data elements. For example, the following block contains
two pointers to certificate-A (issuer/serial number and SKI) and a
single reference to certificate-B (SubjectName) and also shows use
of certificate elements
<KeyInfo>
<X509Data> <!-- two pointers to certificate-A -->
<X509IssuerSerial>
<X509IssuerName>CN=TAMURA Kent, OU=TRL, O=IBM,
L=Yamato-shi, ST=Kanagawa, C=JP</X509IssuerName>
<X509SerialNumber>12345678</X509SerialNumber>
</X509IssuerSerial>
<X509SKI>31d97bd7</X509SKI>
</X509Data>
<X509Data> <!-- single pointer to certificate-B -->
<X509SubjectName>Subject of Certificate B</X509SubjectName>
</X509Data> <!-- certificate chain -->
<!--Signer cert, issuer C=US,O=IBM,OU=FVT,CN=arbolCA serial 4-->
<X509Certificate>MIICXTCCA..</X509Certificate>
<!-- Intermediate cert subject C=US,O=IBM,OU=FVT,CN=arbolCA issuer,C=US,O=Bridgepoint,OU=FVT,CN=tootiseCA -->
<X509Certificate>MIICPzCCA...</X509Certificate>
<!-- Root cert subject C=US,O=Bridgepoint,OU=FVT,CN=tootiseCA -->
<X509Certificate>MIICSTCCA...</X509Certificate>
</X509Data>
</KeyInfo>
Note, there is no direct provision for a PKCS#7 encoded "bag" of
certificates or CRLs. However, a set of certificates or a CRL can
occur within an X509Data element and multiple
X509Data elements can occur in a KeyInfo.
Whenever multiple certificates occur in an X509Data
element, at least one such certificate must contain the public key
which verifies the signature.
Schema Definition
<element name='X509Data'>
<complexType content='elementOnly'>
<choice minOccurs='1' maxOccurs='1'>
<sequence minOccurs='1' maxOccurs='unbounded'>
<choice minOccurs='1' maxOccurs='1'>
<element ref='dsX509IssuerSerial'/>
<element name='X509SKI' type='dsCryptoBinary'/>
<element name='X509SubjectName' type='string'/>
<element name='X509Certificate' type='dsCryptoBinary'/>
</choice>
</sequence>
<element name='X509CRL' type='dsCryptoBinary'/>
</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 elementType="http://www.w3.org/2000/09/xmldsig#PGPData"RetrievalMethod or
Reference element to identify the referent's type)The PGPData element within KeyInfo is
used to convey information related to PGP public key pairs and
signatures on such keys. The PGPKeyID's value is a
string containing a standard PGP public key identifier as defined
in [PGP, section 11.2]. The
PGPKeyPacket contains a base64-encoded Key Material Packet
as defined in [PGP, section 5.5]. Other
sub-types of the PGPData element may be defined by the
OpenPGP working group.
Schema Definition:
<element name='PGPData'>
<complexType 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 elementType="http://www.w3.org/2000/09/xmldsig#SPKIData"RetrievalMethod or
Reference element to identify the referent's type)The SPKIData element within KeyInfo is
used to convey information related to SPKI public key pairs,
certificates and other SPKI data. The content of this element type
is 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 elementType="http://www.w3.org/2000/09/xmldsig#MgmtData"RetrievalMethod or
Reference element to identify the referent's type)The MgmtData element within KeyInfo is
a string value used to convey in-band key distribution or agreement
data. For example, DH key exchange, RSA key encryption, etc.
Schema Definition: <!-- type declared in KeyInfo -->
DTD: <!ELEMENT MgmtData (#PCDATA)>
Object ElementType="http://www.w3.org/2000/09/xmldsig#Object"
(this can be used within a Reference element
to identify the referent's type)Object is an optional element that may occur one or
more times. When present, this element may contain any data. The
Object element may include optional MIME type, ID, and
encoding attributes.
The MimeType attribute is an optional attribute
which describes the data within the Object. This is a
string with values defined by [MIME]. For
example, if the Object contains XML, the
MimeType could be text/xml. This attribute is purely
advisory; no validation of the MimeType information is
required by this specification.
The Object's Id is commonly referenced
from a Reference in SignedInfo, or
Manifest. This element is typically used for enveloping
signatures where the object being signed is to be included in
the signature element. The digest is calculated over the entire
Object element including start and end tags.
The Object's Encoding attributed may
be used to provide a URI that identifies the method by which the
object is encoded (e.g., a binary file).
Note, if the application wishes to exclude the
<Object> tags from the digest calculation the
Reference must identify the actual data object (easy for XML
documents) or a transform must be used to remove the
Object tags (likely where the data object is non-XML).
Exclusion of the object tags may be desired for cases where one
wants the signature to remain valid if the data object is moved
from inside a signature to outside the signature (or vice-versa),
or where the content of the Object is an encoding of
an original binary document and it is desired to extract and decode
so as to sign the original bitwise representation.
Schema Definition:
<element name='Object' >
<complexType 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/2000/09/xmldsig#Manifest"
(this can be used within a Reference element
to identify the referent's type)The Manifest element provides a list of
References. The difference from the list in
SignedInfo is that it is application defined which, if any,
of the digests are actually checked against the objects referenced
and what to do if the object is inaccessible or the digest compare
fails. If a Manifest is pointed to from
SignedInfo, the digest over the Manifest itself
will be checked by the core signature validation behavior. The
digests within such a Manifest are checked at the
application's discretion. If a Manifest is referenced
from another Manifest, even the overall digest of this
two level deep Manifest might not be checked.
Schema Definition:
<element name='Manifest'>
<complexType 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/2000/09/xmldsig#SignatureProperties"
(this can be used within a Reference element
to identify the referent's type)Additional information items concerning the generation of the
signature(s) can be placed in a SignatureProperty
element (i.e., date/time stamp or the serial number of
cryptographic hardware used in signature generation).
Schema Definition:
<element name='SignatureProperties'>
<complexType 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='0' 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 (#PCDATA) >
<!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
(which [XML-C14N] does), they will be
signed. Consequently, if they are retained, a change to the comment
will cause a signature failure. Similarly, the XML signature over
any XML data will be sensitive to comment changes unless a
comment-ignoring canonicalization/transform method, such as the
Canonical XML [XML-C14N], is
specified.
This section identifies algorithms used with the XML digital
signature specification. Entries contain the identifier to be used
in Signature elements, a reference to the formal
specification, and definitions, where applicable, for the
representation of keys and the results of cryptographic
operations.
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 sometimes abbreviated in XML syntax (e.g., "&dsig;base64").)
| Algorithm Type | Algorithm | Requirements | Algorithm URI |
| Digest | |||
| SHA1 | REQUIRED | http://www.w3.org/2000/09/xmldsig#sha1 | |
| Encoding | |||
| Base64 | REQUIRED | http://www.w3.org/2000/09/xmldsig#base64 | |
| MAC | |||
| HMAC-SHA1 | REQUIRED | http://www.w3.org/2000/09/xmldsig#hmac-sha1 | |
| Signature | |||
| DSAwithSHA1 (DSS) |
REQUIRED | http://www.w3.org/2000/09/xmldsig#dsa-sha1 | |
| RSAwithSHA1 | RECOMMENDED | http://www.w3.org/2000/09/xmldsig#rsa-sha1 | |
| Canonicalization | |||
| minimal | RECOMMENDED | http://www.w3.org/2000/09/xmldsig#minimal | |
| Canonical XML with Comments | RECOMMENDED | http://www.w3.org/TR/2000/WD-xml-c14n-20000907#WithComments | |
| Canonical XML (omits comments) | REQUIRED | http://www.w3.org/TR/2000/WD-xml-c14n-20000907 | |
| 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/2000/09/xmldsig#enveloped-signature |
* The Enveloped Signature transform removes the
Signature element from the calculation of the signature when
the signature is within the content that it is being signed. This
MAY be implemented via the RECOMMENDED XPath specification
specified in 6.6.4: Enveloped
Signature Transform; it MUST have the same effect as that
specified by the XPath Transform.
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]. The RSA algorithm takes no explicit parameters. An example of an RSA SignatureMethod element is:
<SignatureMethod Algorithm="&dsig;rsa-sha1"/>
The SignatureValue content for an RSA signature is
the base64 encoding of the octet string computed as per RFC 2437 [PKCS1, section 8.1.1: Signature generation for the
RSASSA-PKCS1-v1_5 signature scheme]. As specified in the
EMSA-PKCS1-V1_5-ENCODE function RFC 2437 [PKCS1, section 9.2.1], the value input to the
signaute function MUST contain a pre-pended algorithm object
identifier for the hash function, but the availability of an ASN.1
parser and recognition of OIDs is not required of a signature
verifier. The PKCS#1 v1.5 representation appears as:
CRYPT (PAD (ASN.1 (OID, DIGEST (data))))
Note that the padded ASN.1 will be of the following form:
01 | FF* | 00 | prefix | hash
where "|" is concatentation, "01", "FF", and "00" are fixed octets of the corresponding hexadecimal value, "hash" is the SHA1 digest of the data, and "prefix" is the ASN.1 BER SHA1 algorithm designator prefix required in PKCS1 [RFC 2437], that is,
hex 30 21 30 1F 06 05 2B 0E 03 02 1A 05 00 04 14
This prefix is included to make it easier to use standard cryptographic libraries. The FF octet MUST be repeated the maximum number of times such that the value of the quantity being CRYPTed is one octet shorter than the RSA modulus.
The resulting Base64 string is the value of the child text node of the SignatureValue element, e.g.
<SignatureValue>IWijxQjUrcXBYoCei4QxjWo9Kg8D3p9tlWoT4
t0/gyTE96639In0FZFY2/rvP+/bMJ01EArmKZsR5VW3rwoPxw=
</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) >
If canonicalization is performed over octets, the Canonicalization algorithms take two implicit parameter: the content and its charset. The charset is derived according to the rules of the transport protocols and media types (e.g, RFC2376 [XML-MT] defines the media types for XML). This information is necessary to correctly sign and verify documents and often requires careful server side configuration.
Various canonicalization algorithms require conversion to [UTF-8].The two algorithms below understand at least [UTF-8] and [UTF-16] as input encodings. We RECOMMEND that externally specified algorithms do the same. Knowledge of other encodings is OPTIONAL.
Various canonicalization algorithms transcode from a non-Unicode encoding to Unicode. The two algorithms below perform text normalization during transcoding [NFC]. We 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; the algorithm outputs an octet stream.
When used to canonicalize SignedInfo the algorithm
MUST be provided with the octets that represent the well-formed
SignedInfo element (and its children and content) as
described in The
CanonicalizationMethod Element (section 4.3.1).
If the signature application has a node set, then the signature application must convert it into octets as described in The Reference Processing Model (section 4.3.3.2). However, Minimal Canonicalization is NOT RECOMMENDED for processing XPath node-sets, the results of same-document URI references, and the output of other types of XML based transforms. It is only RECOMMENDED for simple character normalization of well formed XML that has no namespace or external entity complications.
An example of an XML canonicalization element is:
<CanonicalizationMethod Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000907"/>
The normative specification of Canonical XML is [XML-C14N]. The algorithm is capable of taking as input either an octet stream or an XPath node-set (or sufficiently functional alternative). The algorithm produces an octet stream as output. Canonical XML is easily parameterized (via an additional URI) to omit or retain comments.
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 (such as those in Canonicalization Algorithms (section 6.5)) 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 performing
operations logically equivalent to 1) applying an XPath transform
with expression self::text(), then 2) taking the
string-value of the node-set. Thus, if an XML element is identified
by a barename XPointer in the Reference URI, and its
content consists solely of base-64 encoded character data, then
this transform automatically strips away the start and end tags of
the identified elements and any of its
descendant elements as well as any descendant comments and
processing instructions. 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 child element named
XPath.
The input required by this transform is an XPath node-set. Note that if the actual input is an XPath node-set resulting from a null URI or barename XPointer dereference, then comment nodes will have been omitted. If the actual input is an octet stream, then the application MUST convert the octet stream to an XPath node-set suitable for use by Canonical XML with Comments (a subsequent application of the REQUIRED Canonical XML algorithm would strip away these comments). In other words, the input node-set should be equivalent to the one that would be created by the following process:
(//. | //@* |
//namespace::*)The evaluation of this expression includes all of the document's nodes (including comments) in the node-set representing the octet stream.
The transform output is also an XPath node-set. The XPath
expression appearing in the XPath parameter is
evaluated once for each node in the input node-set. The result is
converted to a boolean. If the boolean is true, then the node is
included in the output node-set. If the boolean is false, then the
node is omitted from the output node-set.
Note: Even if the input node-set has had comments
removed, the comment nodes still exist in the underlying parse tree
and can separate text nodes. For example, the markup
<e>Hello, <!-- comment --> world!</e>
contains two text nodes. Therefore, the expression
self::text()[string()="Hello, world!"] would fail. Should
this problem arise in the application, it can be solved by either
canonicalizing the document before the XPath transform to
physically remove the comments or by matching the node based on the
parent element's string value (e.g. by using the expression
self::text()[string(parent::e)="Hello, world!"]).
The primary purpose of this transform is to ensure that only specifically defined changes to the input XML document are permitted after the signature is affixed. This is done by omitting precisely those nodes that are allowed to change once the signature is affixed, and including all other input nodes in the output. It is the responsibility of the XPath expression author to include all nodes whose change could affect the interpretation of the transform output in the application context.
An important scenario would be a document requiring two enveloped signatures. Each signature must omit itself from its own digest calculations, but it is also necessary to exclude the second signature element from the digest calculations of the first signature so that adding the second signature does not break the first signature.
The XPath transform establishes the following evaluation context for each node of the input node-set:
As a result of the context node setting, the XPath expressions
appearing in this transform will be quite similar to those used in
used in [XSLT], except that
the size and position are always 1 to reflect the fact that the
transform is automatically visiting every node (in XSLT, one
recursively calls the command apply-templates to visit
the nodes of the input tree).
The function here() is defined as
follows:
The here function returns a node-set containing the attribute or processing instruction node or the parent element of the text node that directly bears the XPath expression. This expression results in an error if the containing XPath expression does not appear in the same XML document against which the XPath expression is being evaluated.
Note: The function definition for
here() is intended to be consistent with its definition in
XPointer. However, some minor differences are presently being
discussed between the Working Groups.
As an example, consider creating an enveloped signature (a
Signature element that is a descendant of an element
being signed). Although the signed content should not be changed
after signing, the elements within the Signature
element are changing (e.g. the digest value must be put inside the
DigestValue and the SignatureValue must
be subsequently calculated). One way to prevent these changes from
invalidating the digest value in DigestValue is to add
an XPath Transform that omits all
Signature elements and their descendants. For example,
<Document>
...
<Signature xmlns="&dsig;">
<SignedInfo>
...
<Reference URI="">
<Transforms>
<Transform
Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
<XPath
xmlns:dsig="&dsig;">
not(ancestor-or-self::dsig:Signature)
</XPath>
</Transform>
</Transforms>
<DigestMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
<DigestValue></DigestValue>
</Reference>
</SignedInfo>
<SignatureValue></SignatureValue>
</Signature>
...
</Document>
Due to the null Reference URI in this example, the
XPath transform input node-set contains all nodes in the entire
parse tree starting at the root node (except the comment nodes).
For each node in this node-set, the node is included in the output
node-set except if the node or one of its ancestors has a tag of
Signature that is in the namespace given by the
replacement text for the entity &dsig;.
A more elegant solution uses the
here function to omit only the Signature
containing the XPath Transform, thus allowing enveloped signatures
to sign other signatures. In the example above, use the
XPath element:
<XPath
xmlns:dsig="&dsig;">
count(ancestor-or-self::dsig:Signature |
here()/ancestor::dsig:Signature[1]) >
count(ancestor-or-self::dsig:Signature)</XPath>
Since the XPath equality operator converts node sets to string
values before comparison, we must instead use the XPath union
operator (|). For each node of the document, the predicate
expression is true if and only if the node-set containing the node
and its Signature element ancestors does not include
the enveloped Signature element containing the XPath
expression (the union does not produce a larger set if the
enveloped Signature element is in the node-set given
by ancestor-or-self::Signature).
An enveloped signature transform T removes the
whole Signature element containing T
from the digest calculation of the Reference element
containing T. The entire string of characters used by
an XML processor to match the Signature with the XML
production element is removed. The output of the
transform is equivalent to the output that would result from
replacing T with an XPath transform containing the
following XPath parameter element:
<XPath
xmlns:dsig="&dsig;">
count(ancestor-or-self::dsig:Signature |
here()/ancestor::dsig:Signature[1]) >
count(ancestor-or-self::dsig:Signature)</XPath>
The input and output requirements of this transform are identical to those of the XPath transform. Note that it is not necessary to use an XPath expression evaluator to create this transform. However, this transform MUST produce output in exactly the same manner as the XPath transform parameterized by the XPath expression above.
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 child element named
XSLT.
This transform requires an octet stream as input. If the actual input is an XPath node-set, then the signature application should attempt to covert it to octets (apply Canonical XML]) as described in the Reference Processing Model (section 4.3.3.2).
The output of this transform is an octet stream. The processing
rules for the XSL 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. As XSLT implementations do not
produce consistent serializations of their output, we further
RECOMMEND inserting a Transform after the XSLT Transform to perform
canonicalize the output. These steps will help to ensure
interoperability of the resulting signatures among applications
that support the XSLT transform. Note that if the output is
actually HTML, then the result of these steps is logically
equivalent [XHTML].
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 XML1.0 sytnax constraints given in the previous section be followed but an appropriate XML canonicalization MUST be specified so that the verifier can re-serialize DOM/SAX mediated input into the same octect stream that was signed.
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 [XML-Signature-RD, section 3.1.3].)
The Transforms mechanism meets this requirement by
permitting one to sign data derived from processing the content of
the identified resource. For instance, applications that wish to
sign a form, but permit users to enter limited field data without
invalidating a previous signature on the form might use [XPath] to exclude those portions the user needs to
change. Transforms may be arbitrarily specified and
may include encoding 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.
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 specification uses public key signatures and keyed hash authentication codes. These have substantially different security models. Furthermore, it permits user specified algorithms which may have other models.
With public key signatures, any number of parties can hold the public key and verify signatures while only the parties with the private key can create signatures. The number of holders of the private key should be minimized and preferably be one. Confidence by verifiers in the public key they are using and its binding to the entity or capabilities represented by the corresponding private key is an important issue, usually addressed by certificate or online authority systems.
Keyed hash authentication codes, based on secret keys, are typically much more efficient in terms of the computational effort required but have the characteristic that all verifiers need to have possession of the same key as the signer. Thus any verifier can forge signatures.
This specification permits user provided signature algorithms and keying information designators. Such user provided algorithms may have different security models. For example, methods involving biometrics usually depend on a physical characteristic of the authorized user that can not be changed the way public or secret keys can be and may have other security model differences.
The strength of a particular signature depends on all links in the security chain. This includes the signature and digest algorithms used, the strength of the key generation [RANDOM] and the size of the key, the security of key and certificate authentication and distribution mechanisms, certificate chain validation policy, protection of cryptographic processing from hostile observation and tampering, etc.
Care must be exercised by applications in executing the various algorithms that may be specified in an XML signature and in the processing of any "executable content" that might be provided to such algorithms as parameters, such as XSLT transforms. The algorithms specified in this document will usually be implemented via a trusted library but even there perverse parameters might cause unacceptable processing or memory demand. Even more care may be warranted with application defined algorithms.
The security of an overall system will also depend on the security and integrity of its operating procedures, its personnel, and on the administrative enforcement of those procedures. All the factors listed in this section are important to the overall security of a system; however, most are beyond the scope of this specification.
Note, the examples below are based on the http://www.w3.org/2000/07/xmldsig# namespace.
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 children (including 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
Core Validation (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