This document specifies XML digital signature processing rules and syntax. XML Signatures provide integrity, message authentication, and/or signer authentication services for data of any type, whether located within the XML that includes the signature or elsewhere.
This is the post-FTF editors' copy.
This is a Working Draft of the IETF/W3C XML
Signature Working Group . This version serves as a WG last call and was drafted in
preparation for the January face-to-face meeting.
It includes many improvements as a response to comments made to the mailing list but no
substantive design changes have been made. If any design changes are made, they will like
result from discussion about the
Reference element type specification (
Transforms). We hope to issue an institutional (IETF/W3C) within six
weeks. This version includes and XML Schema definition and a DTD; both of which are fairly
mature. The treatment of specifying the parameters for algorithm/transform declaration is
inconsistent but will be resolved at the FTF.
Please send comments to the editors and cc: the list <firstname.lastname@example.org>. Publication as a
Working Draft does not imply endorsement by the W3C membership or IESG. This is a draft
document and may be updated, replaced or obsoleted by other documents at any time. 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
Patent disclosures relevant to this specification may be found on the WG's patent disclosure page.
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 or other data. Furthermore, 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 referenced externally via a URI. Given that a signature can include multiple references it may be one or more of the above types at the same time.
This document also defines other useful types including methods of referencing collections of resources, and key management and algorithm definitions.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document 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 formally and unambiguously describe such requirements and we wish to reserve the prominence of these terms for the natural language descriptions of other features and behaviors . For instance, an XML attribute might be described as being "optional." Compliance with the XML-namespace specification is described as "REQUIRED."
This document includes a list of open issues which are still being addressed by the working group and may include editorial comments within the text.
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 experimental implementations of this dated specification is:
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 so as 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 the external specification. If an external specification has not allocated itself a Uniform Resource Identifier we allocate an identifier under our own namespace. For instance:
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."
Finally, in order to provide for terse namespace declarations we use XML internal entities [XML]as macros within URIs. For instance:
<!DOCTYPE Signature SYSTEM "xmldsig-core-schema.dtd" [
<!ENTITY dsig "http://www.w3.org/2000/01/xmldsig#">
Security Comment: XML processors will automatically expand entity declarations prior to signature generation. Consequently, this feature does not permit a substitution attack whereby an attacker replaces the entity declaration with another so as to change the meaning of the signature. However, since this specification presently permits a CanonicalizationMethod of null over SignedInfo, entity declarations will not be expanded in those instances (or where the schema is not present) and we have not completely assessed the security risk.
This section provides an overview of XML digital signature syntax and processing; the formal and normative specification is found in section-3: Core Signature Syntax, section-4: Additional Signature Syntax and section-6: Processing Rules.
In this section, an informal representation is used to describe the structure of the XML signature syntax. This representation omits many attributes and details. The following suffix symbols are used to represent the number of times elements may occur: "?" denotes zero or one occurrence; "+" denotes one or more occurrences; and "*" denotes zero or more occurrences.
XML Signatures are very flexible and can sign arbitrary digital content (data objects). An XML Signature is applied 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. While the data object(s) are not directly operated on by a cryptographic signature algorithm, we still refer to the signature as being over the data object(s). Somtimes content is obtained by dereferencing an identified resource. Within an XML document, signatures are related to data objects via IDREFs [XML] and the data can be included within an enveloping signature or can enclose an enveloped signature. Signatures are related to external data objects via URIs [URI] and the signature and data object are said to be detached.
XML digital signatures are represented by the Signature element which has the following structure:
The required SignedInfo element is the information which is actually signed. SignedInfo
includes a list of
References to data objects and their calculated digest
value. The core validation consists of two mandatory processes: validation of the
SignedInfo and validation of each
digest within SignedInfo. The algorithms used in calculating the SignatureValue
are also included in the signed information while the SignatureValue element is
KeyInfo indicates what key is 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, KeyInfo might contain information the signer does not wish to reveal to all signature verifiers. Second, the information may be known within the application's context and need not be represented explicitly. However, 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.
Object is an optional element for including data objects within the signature document. The Object can be optionally typed and/or encoded.
Signature properties, such as time of signing, can be optionally included in a SignatureProperties
Object. (These properties are traditionally called signature
"attributes" although that term in that context has no relationship to the XML
term "attribute".) SignatureProperties can be included within an Object
and signed at the signer's discretion.
The SignedInfo element has the structure indicated below.
The CanonicalizationMethod is the algorithm which is used to canonicalize the SignedInfo element before it is digested as part of the signature operation. In the absence of a CanonicalizationMethod element, no canonicalization is done.
The SignatureMethod is the algorithm 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 or HMAC-SHA1. The algorithm names are signed to resist attacks based on substituting a weaker algorithm.
To promote application interoperability we specify mandatory to implement canonicalization, digest, and signature algorithms. We specify additional algorithms as recommended or optional and the signature design permits arbitrary signer algorithm specification.
Each Reference element includes the digest method and resulting digest value calculated over the identified data object. It also may include transformations that produce 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.
The Reference element has the structure indicated below.
(<Reference (URI=|IDREF=)? Type=?>
The optional URI/IDREF attribute of Reference identifies the data object to be signed. This attribute may be omitted on at most one Reference in a Signature.
This identification, along with the transforms, are a description provided by the
signer on how to obtain the signed resource in the form it was digested (i.e. the digested
content). The verifier (i.e., relying party) 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 (particularly a local store) other than that specified in the
The optional Type attribute provides information about the resource identified
by the URI/IDREF. In particular, it can indicate that it is an Object,
SignatureProperties, or Manifest element. This can be used by
applications to initiate special processing of some Reference elements.
References to XML data within an Object element SHOULD identify the actual
element content. Where the element content is not XML (perhaps it is binary or encoded
data) the reference should identify the Object and the
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 behaviour.
Transforms is an optional ordered list of processing steps that are applied to the resource's content before it is 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 (this is how the Working Group satisfied the requirement of signing portions of a document.) 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.
Arbitrary user specified transforms are permitted. To promote interoperability, we specify mandatory to implement canonicalization and decoding transformation algorithms. Additional canonicalization, coding, XSLT, and XPath based transform algorithms are specified as recommended or optional.
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 bind's a resources content to the signer's key.
The Manifest element is provided to meet requirements not directly addressed by this document. The level of indirection provided by these elements readily meets these requirements. Two examples follow.
First, applications frequently need to efficiently sign multiple data objects even
where the signature operation itself is an expensive public key signature. This
requirement can be achieved by including multiple References 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 References within SignedInfo
to undergoe reference validation
DigestValues are checked. Some 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 References. If a single Reference fails (the
identified data object when digested does not yield the specified
the signature would fail core validation.
However, the application may wish to treat the signature over the two valid References
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 redundant.
To address these requirements, additional element types have been defined which may be referenced by SignedInfo References. First, the Manifest element may contain a collection of References and Objects, but leaves reference validation up to the application. Thus the first case above can be solved by simply putting one reference inside SignedInfo to a Manifest which references the three data objects. Second, multiple signatures over a large number of References need only point to a single Manifest with the many references.
The structure of Manifest, which reuses the Reference and Object elements described above, is as follows:
Manifest may appear as the content of an Object. Note that an application could decide whether to verify a DigestValue based on the Type given in the enclosing Reference.
This specification does not address mechanisms for making statements or assertions. Instead, this whole document simply 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, XML-schema, RDF]. However, we do define a SignatureProperties element type for the inclusion of assertions about how the signature was produced (e.g., the time of signing or the serial number of hardware used in cryptographic processes). Such assertions are included within the signature by be identified by a Reference.
(SignatureProperty Target= )+
The structure of SignatureProperties is shown above. Any content about the signature generation may be located within the SignatureProperty element. The mandatory Target attribute references the element to which the property applies. In particular, target may include a reference to a SignedInfo or Reference element.
These sections describe the operations to be performed as part of signature generation and validation. The description is of a logical behavior and does not specify an order of execution, nor specify discrete steps.
Generation of an XML digital signature involves two mandatory steps and such additional optional steps as the application requires. The mandatory steps are the generation of the Reference element or elements that appear inside SignedInfo and the generation of the Signature element.
Validating an XML signature consists of two mandatory processing steps and additional optional steps as dictated by the application.
The required steps are signature validation, the cryptographic validation of the signature calculated over SignedInfo; and reference validation, the verification of the digest contained in each Reference in SignedInfo. Both steps MUST be performed as part of all XML signature validations.
For a signature to be valid for a particular application, additional optional steps are frequently needed.
The general structure of an XML signature is described in section-2: Signature Overview. This section provides detailed syntax of the core signature features and actual examples. Features described in this section are mandatory to implement unless otherwise indicated. The syntax is defined via [XML-Schema] with the following XML preamble, declaration, and internal entity:
Schema Definition: <?xml version='1.0'?> <!DOCTYPE schema SYSTEM 'http://www.w3.org/TR/1999/WD-xmlschema-1-19991217/structures.dtd' [ <!ENTITY dsig 'http://www.w3.org/2000/01/xmldsig#'> ]> <schema targetNamespace='&dsig;' version='0.1' xmlns='http://www.w3.org/1999/XMLSchema' xmlns:ds='&dsig;'>
The Signature element is the root element of a XML Signature. A simple example of a complete signature follows:
<!DOCTYPE Signature [
<!ENTITY dsig 'http://www.w3.org/2000/01/xmldsig#'>]>
Note: this example will be revised to include generated hash/signature values that validate.
Schema Definition: <element name='Signature'> <type content='elementOnly'> <group order='seq' 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='*'/> </group> <attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/> </type> </element>
DTD: <!ELEMENT Signature (SignedInfo, SignatureValue, KeyInfo?, Object*) > <!ATTLIST Signature xmlns CDATA #FIXED 'http://www.w3.org/2000/01/xmldsig#' Id ID #IMPLIED >
The SignatureValue element contains the actual value of the digital signature. The encoding of this value is determined by SignatureMethod. Base64 [MIME] is the encoding method for all SignatureMethods specified within this specification. The ability to define a SignatureMethod and SignatureValue pair which includes multiple distinct signatures is explicitly permitted (e.g. "rsawithsha-1 and ecdsawithsha-1").
Schema Definition: <type name='encoded' source='string'> <annotation> <info>A schema type named encoded provides the following. It is reused in DigestValue as well.</info> </annotation> <attribute name='Encoding' type='uri' default='&dsig;Base64' minOccurs='0' maxOccurs='1'/> </type> <element name='SignatureValue' type='ds:encoded'/>
DTD: <!ELEMENT SignatureValue (#PCDATA) >
The structure of SignedInfo includes the canonicalization algorithm (if any), a signature algorithm, and one or more references. The SignedInfo element may contain an optional ID attribute that will allow it to be referenced by other signatures and objects.
Schema Definition: <element name='SignedInfo'> <type content='elementOnly'> <group order='seq' minOccurs='1' maxOccurs='1'> <element ref='ds:CanonicalizationMethod' minOccurs='0' maxOccurs='1'/> <element ref='ds:SignatureMethod' minOccurs='1' maxOccurs='1'/> <element ref='ds:Reference' minOccurs='1' maxOccurs='*'/> </group> <attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/> </type> </element>
DTD: <!ELEMENT SignedInfo (CanonicalizationMethod?, SignatureMethod, Reference+) > <!ATTLIST SignedInfo Id ID #IMPLIED >
SignedInfo does not include explicit signature or digest properties (such as calculation time, cryptographic device serial number, etc.). If an application needs to associate properties with the signature or digest, it may include such information in a SignatureProperties element found within an Object element.
CanonicalizationMethod is an optional element which specifies the canonicalization algorithm applied to the SignedInfo element prior to performing signature calculations. This element uses the general structure here for algorithms described in section-5.1: Algorithm Identifiers. Possible options may include a minimal algorithm (CRLF and charset normalization), or more extensive operations such as [XML-C14N]. If the CanonicalizationMethod is omitted, no change is made to SignedInfo before digesting. (Note this may lead to interoperability failures as other applications may not serialize it as the creators application did by default.)
Schema Definition: <element name='CanonicalizationMethod'> <type content='elementOnly'> <attribute name='Algorithm' type='uri' minOccurs='1' maxOccurs='1'/> </type> </element>
DTD: <!ELEMENT CanonicalizationMethod ANY > <!ATTLIST CanonicalizationMethod Algorithm CDATA #REQUIRED >
SignatureMethod is a required element which 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 5.1. While there is a single identifier, that identifier may specify a format containing multiple distinct signature values.
Schema Definition: <element name='SignatureMethod'> <type content='elementOnly'> <attribute name='Algorithm' type='uri' minOccurs='1' maxOccurs='1'/> </type> </element>
DTD: <!ELEMENT SignatureMethod ANY > <!ATTLIST SignatureMethod Algorithm CDATA #REQUIRED >
Reference is an element that may occur one or more times. It specifies a digest
algorithm and digest value, and optionally the object being signed, the type of the
object, and/or a list of transforms to be applied prior to digesting. The identification,
and transforms are information provided to inform the verifier how the digested content
(i.e., the input to the digest method) may be created. The
facilitates the processing of referenced data. For example, while this specification makes
no requirements over external data, an application may wish to signal that the referent is
a Manifest. An optional ID attribute permits a
Reference to be
referenced from elsewhere.
Schema Definition: <element name='Reference'> <type content='elementOnly'> <group order='seq' 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'/> </group> <attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/> <attribute name='URI' type='uri' minOccurs='0' maxOccurs='1'/> <attribute name='IDREF' type='IDREF' minOccurs='0' maxOccurs='1'/> <attribute name='Type' type='uri' minOccurs='0' maxOccurs='1'/> </type> </element>
<!ELEMENT Reference (Transforms?, DigestMethod, DigestValue) > <!ATTLIST Reference Id ID #IMPLIED URI CDATA #IMPLIED IDREF IDREF #IMPLIED Type CDATA #IMPLIED>
The URI/IDREF attribute identifies a data object using a URI [URI] or IDREF [XML]. We distinguish between URIs and IDREFs so as to provide expositional clarity and ease signature processing. Note there is some popular confusion about URIs and fragment identifiers. As specified by RFC2396 [URI], URIs can be used in conjunction with a fragment identifier by use of a separating pound symbol '#', but the URI proper does not include the fragment identifier. (The meaning of the fragment is defined by the resource's MIME type). URI/IDREF only permits a 'clean' URI or IDREF; fragment identification is specified under Transforms. This choice permits References to identify a fragment of a document that is encoded: the Reference identifies the resource, the first Transform specifies decoding, the second Transform specifies the fragment.
Note that a null URI (URI="") is permitted and identifies the document the reference is in (the root element).
If the URI/IDREF attribute is omitted all-together, 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 digest algorithm is applied to the data octets being secured. Typically that is done by locating (possibly using the URI/IDREF if provided) the data and transforming it. If the data is an XML document, the document is assumed to be unparsed prior to the application of Transforms. If there are no Transforms, then the data is passed to the digest algorithm unmodified.
The optional Type attribute contains information about the type of object being signed. This is represented as a URI. For example:
The type attribute is purely advisory, no validation of the type information is required by this document.
Transforms is an optional element that contains one or more operations to be performed on an indicated data object prior to digest calculation. (These operations are different from the CanonicalizationMethod specified in the Signature which is applied to SignedInfo.) If the Transforms element is omitted, no operations are indicated.
The Transforms element contains an ordered list of Transform elements. The output of each Transform (octects) serve as input to the next Transform. The input to the first Transform is the source data. 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 [sec-7.2: Only What is "Seen" Should be Signed].
Each Transform consists of an Algorithm attribute, optional Type and Charset attributes, 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 input resource.
The optional Type and Charset (IANA registered character set) attributes are made available to algorithms which need and are otherwise unable to deduce that information about the data they are processing.
Schema Definition: <element name='Transforms' > <type content='elementOnly'> <element ref='ds:Transform' minOccurs='1' maxOccurs='*'/> </type> </element> <element name='Transform'> <type content='elementOnly'> <attribute name='Algorithm' type='string' minOccurs='1' maxOccurs='1'/> <attribute name='Type' type='uri' minOccurs='0' maxOccurs='1'/> <attribute name='Charset' type='string' minOccurs='0' maxOccurs='1'/> </type> </element>
<!ELEMENT Transforms (Transform+)> <!ELEMENT Transform ANY> <!ATTLIST Transform Algorithm CDATA #REQUIRED Type CDATA #IMPLIED Charset CDATA #IMPLIED > <!-- The Type conforms to the productions specified by [URI] -->
Examples of transforms include but are not limited to base-64 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 base-64 encoded parameter to a Java Transform algorithm. However, applications should refrain from using application-specific transforms if they wish their signatures to be verifiable outside of their application domain. Section 5-6: Transform Algorithms defines the list of standard transformations.
DigestMethod is a required element which identifies the digest algorithm to be applied to the signed object. This element uses the general structure here for algorithms specified in section-5.1: Algorithm Identifiers.
Schema Definition: <element name='DigestMethod'> <type content='elementOnly'> <attribute name='Algorithm' type='uri' minOccurs='1' maxOccurs='1'/> </type> </element>
DTD: <!ELEMENT DigestMethod ANY > <!ATTLIST DigestMethod Algorithm CDATA #REQUIRED >
DigestValue is an element which contains the encoded value of the digest. The optional Encoding attribute is always encoded using Base 64 [MIME].
Schema Definition: <element name='DigestValue' type='ds:encoded'/>
DTD: <!ELEMENT DigestValue (#PCDATA) >
<!-- base64 encoded signature value -->
KeyInfo may contain keys, names, certificates and other public key management information (such as in-band key distribution or agreement data or data supporting any other method.) This specification defines a few simple types but applications may place their own key identification and exchange semantics within this element through the XML-namespace facility. [XML-namespace]
Schema Definition: <element name='KeyInfo'> <type content='elementOnly'> <group order='choice' minOccurs='1' maxOccurs='*'> <element name='KeyName' type='string'/> <element name='KeyValue' type='string'/> <element name='SubjectName' type='string'/> <element name='RetrievalMethod' type='uri'/> <element ref='ds:X509Data'/> <element ref='ds:PGPData'/> <element name='MgmtData' type='string' minOccurs='0' maxOccurs='1'/> <any/> </group> <attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/> </type> </element>
DTD: <!ELEMENT KeyInfo ((KeyName | KeyValue | SubjectName | RetrievalMethod | X509Data | PGPData | MgmtData)*) > <!ATTLIST KeyInfo Id ID #IMPLIED Type CDATA #IMPLIED> <!ELEMENT KeyName (#PCDATA) > <!ELEMENT KeyValue (#PCDATA) > <!ELEMENT SubjectName (#PCDATA) > <!ELEMENT RetrievalMethod (#PCDATA) >
KeyInfo is an optional element which enables the recipient(s) to obtain the key(s) needed to validate the signature. If omitted, the recipient is expected to be able to identify the key based on application context information. This element contains one KeyInfo data element providing information for the recipient(s). Multiple declarations within KeyInfo refer to the same key. Applications may define and use any mechanism they choose through inclusion of elements from a different namespace.
Compliant versions implementing KeyInfo MUST implement KeyValue, and SHOULD implement RetrievalMethod.
Schema Definition <element name='X509Data'> <type content='elementOnly'> <group order='seq' minOccurs='1' maxOccurs='1'> <group order='choice' minOccurs='1' maxOccurs='1'> <element ref='ds:X509IssuerSerial'/> <element name='X509SKI' type='string'/> <element name='X509SubjectName' type='string'/> </group> <element name='X509Certificate' type='string' minOccurs='0' maxOccurs='*'/> <element name='X509CRL' type='string' minOccurs='0' maxOccurs='*'/> </group> </type> </element> <element name='X509IssuerSerial'> <type content='elementOnly'> <group order='seq' minOccurs='1' maxOccurs='1'> <element name='X509IssuerName' type='string' minOccurs='1' maxOccurs='1'/> <element name='X509SerialNumber' type='string' minOccurs='1' maxOccurs='1'/> </group> </type> </element> <element name='PGPData'> <type content='elementOnly'> <group order='seq' minOccurs='1' maxOccurs='1'> <element name='PGPKeyID' type='string' minOccurs='1' maxOccurs='1'/> <element name='PGPKeyPacket' type='string' minOccurs='1' maxOccurs='1'/> </group> </type> </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) > <!ELEMENT PGPData (PGPKeyID, PGPKeyPacket?) > <!ELEMENT PGPKeyPacket (#PCDATA) > <!ELEMENT PGPKeyID (#PCDATA) > <!ELEMENT MgmtData (#PCDATA)>
Object is an optional element which may occur one or more times. When present, this element may contain any data. The Object element may include optional type, ID, and encoding attributes.
ID is commonly referenced from an 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 document. The digest is calculated
over the entire Object element including start and end tags.
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' > <type content='mixed'> <any namespace='##targetNamespace'/> <attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/> <attribute name='Type' type='uri' minOccurs='0' maxOccurs='1'/> <attribute name='Encoding' type='uri' minOccurs='0' maxOccurs='1'/> </type> </element>
DTD: <!ELEMENT Object (#PCDATA) > <!ATTLIST Object Id ID #IMPLIED Type 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
model only permits them within
The Manifest element provides a list of References. The difference from the list in SignedInfo is that it is application defined which, if any, of the digests are actually checked against the objects referenced and what to do if the object is inaccessible or the digest compare fails. If a Manifest is pointed to from SignedInfo, the digest over the Manifest itself will be checked by the core signature validation behavior. The digests within such a Manifest are checked at application discretion. If a Manifest is referenced from another Manifest, even the overall digest of this two level deep Manifest might not be checked.
Schema Definition: <element name='Manifest'> <type content='elementOnly'> <group order='seq' minOccurs='1' maxOccurs='1'> <element ref='ds:Reference' minOccurs='1' maxOccurs='*'/> <element ref='ds:Object' minOccurs='0' maxOccurs='*'/> </group> <attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/> </type> </element>
DTD: <!ELEMENT Manifest ((Reference | Object)+) > <!ATTLIST Manifest Id ID #IMPLIED >
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'> <type content='elementOnly'> <element ref='ds:SignatureProperty' minOccurs='1' maxOccurs='*'/> <attribute name='Id' type='ID' minOccurs='0' maxOccurs='1'/> </type> </element> <element name='SignatureProperty'> <type content='mixed'> <any namespace='##other'/> <attribute name='Target' type='IDREF' minOccurs='1' maxOccurs='1'/> </type> </element>
DTD: <!ELEMENT SignatureProperties (SignatureProperty+) > <!ATTLIST SignatureProperties Id ID #IMPLIED > <!ELEMENT SignatureProperty (#PCDATA) > <!ATTLIST SignatureProperty Target IDREF #REQUIRED >
We define the following URIs for use in identifying XML resources that include non-core but signature related semantics.
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 document retain PIs. When a PI is part of content that is signed (e.g., within SignedInfo or referenced XML documents) any change to the PI will obviously result in a signature failure.
XML comments are not used by this specification.
Note that unless CanonicalizationMethod removes comments within SignedInfo or any other referenced XML, they will be signed. Consequently, a change to the comment will cause a signature failure. Similarly, the XML signature over any XML data will be sensitive to comment changes unless a comment-ignoring canonicalization/transform method, such as the Canonical XML [XML-canonicalization], is specified.
This section identifies algorithms used with the XML digital signature standard. Entries contain the identifier to be used in Signature elements, a reference to the formal specification, and definitions, where applicable, for the representation of keys and the results of cryptographic operations.
Algorithms are identified by URIs that appear as an attribute to the element that identifies the algorithms' role (DigestMethod, Transform, SignatureMethod, or CanonicalizationMethod). All algorithms used herein take parameters but in many cases the parameters are implicit. For example, a SignatureMethod is implicitly given two parameters: the keying info and the output of CanonicalizationMethod (or SignedInfo directly if there is no 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.
|Algorithm Type||Algorithm||Requirements||Algorithm URI|
Note that the normative identifier is the complete URIs in the table though they are frequently abbreviated in XML syntax as "&dsig;hmac".
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.
Digest algorithms take as an implicit parameter a byte string to be digested.
The SHA-1 algorithm [SHA-1] takes no explicit parameters. An example of an SHA-1 DigestAlg element is:
A SHA-1 digest is a 160-bit string. The content of the DigestValue element shall be the base64 encoding of this bit string viewed as a 20-octet octet stream. Example, the DigestValue element for the message digest:
A9993E36 4706816A BA3E2571 7850C26C 9CD0D89D
from Appendix A of the SHA-1 standard would be:
MAC algorithms take two implicit parameters, their keying material determined from KeyInfo and the byte stream output by CanonicalizationMethod or SignedInfo directly if there is no 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. An example of an HMAC SignatureMethod element:
<hmac-outputlength xmlns="&dsig;hmac-sha1"> <!-- need to have a DTD for this -->
</hmac-outputlength> <!-- define length -->
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
Signature algorithms take two implicit parameters, their keying material determined from KeyInfo and the byte stream output by CanonicalizationMethod or SignedInfo directly if there is no CanonicalizationMethod. Signature and MAC algorithms are syntactically identical but a signature implies public key cryptography.
Note: the schema and DTD declarations within this section are not yet part of sec-9: schemas.
The DSA algorithm [DSA] takes no explicit parameters. An example of a DSA SignatureMethod element is:
The output of the DSA algorithm consists of a pair of integers usually referred by the pair (r, s). The signature value shall consist 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 shall be done according to the I2OSP operation defined in the RFC 2437 [RSA] specification with a k parameter equal to 20. Example: the SignatureValue element for a DSA signature (r, s) with values specified in hexadecimal
r = 8BAC1AB6 6410435C B7181F95 B16AB97C 92B341C0
s = 41E2345F 1F56DF24 58F426D1 55B4BA2D B6DCD8C8
from the example in Appendix 5 of the DSS standard would be
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 both either appear or be absent). All parameters are encoded as base64 values.
Schema: <element name='DSAKeyValue'> <type content='elementOnly'> <group order='seq' minOccurs='1' maxOccurs='1'> <element name='ds:P' type='string' minOccurs='1' maxOccurs='1'/> <element name='ds:Q' type='string' minOccurs='1' maxOccurs='1'/> <element name='ds:G' type='string' minOccurs='1' maxOccurs='1'/> <element name='ds:Y' type='string' minOccurs='1' maxOccurs='1'/> <element name='ds:J' type='string' minOccurs='0' maxOccurs='1'/> </group> <group order='seq' minOccurs='0' maxOccurs='1'> <element name='ds:Seed' type='string' minOccurs='1' maxOccurs='1'/> <element name='ds:PgenCounterQ' type='string' minOccurs='1' maxOccurs='1'/> </group> </type> </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) > <!-- Each of these fields consists a PCDATA where the data is base64 encoded -->
The expression "RSA algorithm" as used in this document refers to the RSASSA-PKCS1-v1_5 algorithm described in RFC 2437 [RSA]. The RSA algorithm takes no parameters. An example of an RSA SignatureMethod element is:
The output of the RSA algorithm is an octet string. The SignatureValue content for an RSA signature shall be the base64 encoding of this octet string. Example: TBD
RSA key values have two fields: Modulus and Exponent.
Schema: <element name='RSAKeyValue'> <type content='elementOnly'> <element name='ds:Modulus' type='string' minOccurs='1' maxOccurs='1'/> <element name='ds:Exponent' type='string' minOccurs='1' maxOccurs='1'/> </type> </element>
DTD: <!ELEMENT RSAKeyValue (Modulus, Exponent) > <!ELEMENT Modulus (#PCDATA) > <!ELEMENT Exponent (#PCDATA) > <!-- Each field contains a CDATA which is the value for that item base64 encoded -->
The algorithm identifier for the minimal canonicalization is &dsig;minimal. An example of a minimal canonicalization element is:
The minimal canonicalization algorithm:
An example of an XML canonicalization element is:
The normative specification of Canonical XML is [XML-c14n].
A Transform algorithm has three implicit parameters. The first is a byte stream from the Reference or as the output of an earlier Transform. The second and third are the optional MimeType and Charset attributes that can be specified on the Transform element.
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. The Working Group goal is to maximize application interoperability on XML signatures, and the working group expects ubiquitous availability of software to support these transforms that can be incorporated into applications without extensive development.
Any canonicalization algorithm that can be used for CanonicalizationMethod can be used as a Transform.
The normative specification for base 64 and quoted-printable decoding transforms is [MIME]. The base-64 Transform element has no content. The input is base-64 decoded by this algorithm. This transform is useful if an application needs to sign the raw data associated with base-64 encoded content of an element.
Transform element content MUST conform to the XML Path Language [XPath] syntax. XPath is a language for addressing parts of an XML
document. Hence, an XPath expression MUST be applied to an entire well-formed XML
Note: The current output of a
Reference's IDREF cannot be
used as input to an XPath transform. The XPath transform could be defined to provide an
XML declaration when one is found not to exist since the encoding attribute could be set
equal to the XPath transform's Charset attribute. However, there is currently no way to
communicate the correct byte order mark to the transform. For security reasons, a default
cannot be selected.
The XPath transform applies the W3C XML canonicalization [XML-C14N] to the input resource. This ensures all entity reference substitutions and attribute normalizations are performed in a manner consistent with a validating XML processor. Linefeeds are normalized, and CDATA sections are eliminated. The types of quotes around attributes are normalized, and the order of attributes is defined. Namespace attributes are created in descendant elements that use namespace definitions. All of these modifications are necessary to achieve a consistent interpretation of the XPath expression and a consistent output of the XPath transform.
Finally, the XPath expression is evaluated assuming that the entity references created by canonicalization have been replaced by the corresponding entity values and that each block of consecutive text characters has been replaced by a single text node.
The result of the XPath is a string, boolean, number, or node-set. If the result of the XPath expression is a string, then the string is the output of the XPath transform. If the XPath result is a boolean or number, then the result is converted to a string using the XPath string() function. If the result of the XPath expression is a node-set, then the output of the transform is a string containing the text rendering of the nodes in the node-set. The nodes are selected for rendering based on the document order (as defined in [XPath]) of the canonicalized input resource. The text rendering is performed in accordance with [XML-C14N].
It is RECOMMENDED that the XPath be constructed such that the result of this operation is a well-formed XML document. This should be the case if root element of the input resource is included by the XPath (even if a number of its descendant elements and attributes are omitted by the XPath).
The Transform element content MUST conform to the XSL Transforms [XSLT] language syntax. The processing rules for the XSLT transform are stated in the XSLT specification [XSLT].
Digital signatures only work if the verification calculations are performed on exactly the same bits as the signing calculations. If the surface representation of the signed data can change between signing and verification, then some way to standardize the changeable aspect must be used before signing and verification. For example, even for simple ASCII text there are at least three widely used line ending sequences. If it is possible for signed text to be modified from one line ending convention to another between the time of signing and signature verification, then the line endings need to be canonicalized to a standard form before signing and verification or the signatures will break.
XML is subject to surface representation changes and to processing which discards some surface information. For this reason, XML digital signatures have a provision for indicating canonicalization methods in the signature so that a verifier can use the same canonicalization as the signer.
Throughout this document we distinguish between the canonicalization of a Signature data object 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 character set conversion, such as between UTF-8 and UTF-16, both of which all XML standards compliant processors are required to support. Any canonicalization algorithm should yield output in a specific fixed character set. For both the minimal canonicalization defined in this document and the W3C Canonical XML [XML-c14n], that character set is UTF-8.
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), (5C), and (6) depend on specific Schema, DTD, or similar declarations. In the general case, such declarations will not be available to or used by the signature verifier. Thus, to interoperate between different XML implementations, the following syntax contraints MUST be observed when generating any signed material to be processed as XML, 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 the where namespace declarations appeared in the original instance.
If an XML Signature is to be produced or verified on a system using the DOM or SAX processing, a canonical method is needed to serialize the relevant part of a DOM tree or sequence of SAX events. XML canonicalization specifications, such as [XML-c14n], are based only on information which is preserved by DOM and SAX. For an XML Signature to be verifiable by an implementation using DOM or SAX, not only must the syntax constraints given in section-7.1 be followed but an appropriate XML canonicalization MUST be specified so that the verifier can re-serialize DOM/SAX mediated input into the same byte sequence 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." [3.1.3 XML-Signature-RD]
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 the form itself might use XPath [XPath]
to exclude those portions the user needs to change.
may be arbitrarily specified and may include canonicalization instructions or even XSLT
transformations. Of course, signatures over such a dervied document do not secure any
information discarded by the
Furthermore, 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 application might require that content be freshly dereferenced and transformed.
If signing is intended to convey the judgment or consent of an automated mechanism or person concerning some information, 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, for example the screen images shown a user. However, this may result in data which it is difficult for subsequent software to manipulate. It can be effective instead to secure the full data along with whatever filters, style sheets, or the like were used to control the part of the information that was presented.
This standard specifies public key signatures and keyed hash authentication codes. These have substantially different security models. Furthermore, it permits user specified additions which may have other models.
With public key signatures, any number of parties can hold the public key and verify signatures while only the parties with the private key can create signatures. The number of holders of the private key should be minimized and preferably be one. Confidence by verifiers in the public key they are using and its binding to the entity or capabilities represented by the corresponding private key is an important issue, usually addressed by certificate or online authority systems.
Keyed hash authentication codes, based on secret keys, are typically much more efficient in terms of the computational effort required but have the characteristic that all verifiers need to have possession of the same key as the signer. Thus any verifier can forge signatures.
This standard permits user provided signature algorithms and keying information designators. Such user provided algorithms may have further different security models. For example, methods involving biometrics usually depend on a physical characteristic of the authorized user which 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 valiation policy, protection of cryptographic processing from hostile observation and tampering, etc. The security of an overall system would also depend on the security and integrity of its operating procedures, its personnel, and on the administrative enforcement of those procedures. The factors listed in this paragraph, while critical to the overall security of a system, are mostly beyond the scope of this document.
<Signature xmlns="http://www.w3.org/2000/01/xmldsig"> <SignedInfo Id="mypage"> <CanonicalizationMethod Algorithm="http://www.w3.org/1999/07/WD-xml-c14n-19990729"> </CanonicalizationMethod> <SignatureMethod Algorithm="http://www.w3.org/2000/01/xmldsig/dsa"> </SignatureMethod> <Reference URI="http://www.w3.org/TR/xml-stylesheet/"> <Transforms> <Transform Algorithm="http://www.w3.org/2000/01/xmldsig/base64"/> <Transform Algorithm="http://www.w3.org/2000/01/xmldsig/null"/> </Transforms> <DigestMethod Algorithm="http://www.w3.org/2000/01/xmldsig/sha1"> </DigestMethod> <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue> </Reference> <Reference URI="http://www.w3.org/TR/REC-xml-names/"> <Transforms> <Transform Algorithm="http://www.w3.org/2000/01/xmldsig/base64"/> </Transforms> <DigestMethod Algorithm="http://www.w3.org/2000/01/xmldsig/sha1"> </DigestMethod> <DigestValue>UrXLDLBIta6skoV5/A8Q38GEw44=</DigestValue> </Reference> </SignedInfo> <SignatureValue>MC0CFFrVLtRlkMc3Daon4BqqnkhCOlEaAhUAk8pH1iRNK+q1I +sisDTz2TFEALE=</SignatureValue> <KeyInfo> <KeyValue xmlns:java="http://xsl.lotus.com/java" xmlns:dsig="http://www.w3.org/2000/01/xmldsig"> MIIBtzCCASwGByqGSM44BAEwggEfAoGBAP1/U4EddRIpUt9KnC7s5Of2E bdSPO9EAMMeP4C2USZpRV1AIlH7WT2NWPq/xfW6MPbLm1Vs14E7gB00b/ JmYLdrmVClpJ+f6AR7ECLCT7up1/63xhv4O1fnxqimFQ8E+4P208UewwI1 VBNaFpEy9nXzrith1yrv8iIDGZ3RSAHHAhUAl2BQjxUjC8yykrmCouuEC/ BYHPUCgYEA9+GghdabPd7LvKtcNrhXuXmUr7v6OuqC+VdMCz0HgmdRWVeO utRZT+ZxBxCBgLRJFnEj6EwoFhO3zwkyjMim4TwWeotUfI0o4KOuHiuzpn WRbqN/C/ohNWLx+2J6ASQ7zKTxvqhRkImog9/hWuWfBpKLZl6Ae1UlZAFM O/7PSSoDgYQAAoGAQFL0+RhXZbDxdt17o05PlMzQGqDnAq2NM1eun+ie21 4okrmIp4r0CGKvHM1HbFgwXMlBpkXyStYg64RTMnL9dtShw5rCkEv145TV 0EYVoxBQ5X0gmrQ2NftRHH8imBhx9glz//y6NE4JhfIVPu3o+55VYUwdFP 0cbBvWkKOngo0= </KeyValue> </KeyInfo> </Signature>
Objectdesignates 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].
Objectelement is merely one type of digital data (or document) that can be signed via a
Signatureelement, and can be identified via a
IDREF, 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
Signatureand data object reside within the same XML document but are sibling elements.
Objectelement of the signature itself. The
Object(or its content) is identified via a
IDREFor transform). The enveloping
Signatureelement is used to provide the root document element.
Reference, matches its specified
SignatureValuematches the result of processing
SignatureMethodas specified in section 6.2.