Copyright (c) 1999 The Internet Society & W3C (MIT, INRIA, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.
This document specifies the syntax and processing rules for the encoding of digital signatures using XML. Such signatures can provide integrity, message authentication, and/or signer authentication services for data of any type, whether located within the XML that includes the signature or locatable elsewhere.
This is an internal WG Draft that attempts to clarify the exposition.
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 IETF. 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 describes the proposed syntax and processing rules for the XML Digital Signature specification. This specification provides a mechanism for applying digital signatures to XML documents and other Internet resources and encoding those signatures as XML. XML Signatures may be applied to one or more such resources and those resources may be included within the signature document or be referenced externally.
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].
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].
The XML namespace [XML-namespace] URI that MUST be used by experimental implementations of this dated specification is:
xmlns="http://www.w3.org/1999/12/xmldsig-core"
While applications MUST support XML and XML-namespaces, the use of internal entities 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:
Finally, in order to provide for terse namespace declarations we use XML internal entities as macros within URIs. For instance:
<?xml version="1.0" ?>
<!DOCTYPE Signature SYSTEM "xmldsig.dtd" [
<!ENTITY dsig 'http://www.w3.org/1999/10/signature-core'>]>
...
<SignedInfo>
<SignatureMethod Algorithm="&dsig;/dsaWithSHA-1"/>
...
</SignedInfo>
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. Furthemore, we define this entity as part of the Signature XML Schema such that one does not have to rely upon an internal subset declaration. 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.
No provision is made for an explicit version number in this syntax. If a future version is needed, it is expected to use a different Namespace.
This section provides an overview of XML digital signature syntax and processing. The formal specification is provided in section 3. The editors assume the reader is familiar with basic digital signature and XML concepts.
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 occurance; "+" denotes one or more occurances; and "*" denotes zero or more occurances.
XML digital signatures are very flexible and may be used to apply signatures to any type of resource. The resource(s) being signed may be included within the signature, outside the signature in the same document, or completely outside of the document.
XML digital signatures are represented by the Signature element which has the following structure:
<Signature>
(SignedInfo)
(SignatureValue)
(KeyInfo)?
(Object)*
</Signature>
The required SignedInfo element is the information which is actually signed. SignedInfo includes a digest calculated over each of the data objects being signed. The core signature validation consists of two mandatory to perform parts: validation of the signature over SignedInfo and validation of each digest included in SignedInfo. The algorithms used in calculating the SignatureValue are also included in the signed information while the SignatureValue element is outside SignedInfo.
KeyInfo indicates what key is to be used to validate the signature. Possible forms for identifying the key 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, an ObjectReference can easily identify and include the KeyInfo as part of the signature.
Object is an optional element for including the signed resources within the signature document. The resources can be optionally typed and/or encoded.
Signature properties, such as time of signing, can be included in a SignatureProperties element and optionally carried within 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.
<Signature>
<SignedInfo>
(CanonicalizationMethod)?
(SignatureMethod)
(ObjectReference)+
</SignedInfo>
(SignatureValue)
(KeyInfo)?
(Object)*
</Signature>
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 ObjectReference element includes the digest method and resulting digest value calculated over the signed resource. It also may include an identification of the resource and specifies any transformations to be applied to produce the input to the digest operation. A resource is signed by computing the contents digest value and the signature over that value. The signature is later checked via resource and signature validation.
The ObjectReference element has the structure indicated below.
...
<SignedInfo>
(CanonicalizationMethod)?
(SignatureMethod)
<ObjectReference (URI=|IDREF=)? Type=?>
(Transforms)?
(DigestMethod)
(DigestValue)
</ObjectReference>+
</SignedInfo>
...
The optional URI/IDREF attribute of ObjectReference idenitifies the signed resource. This attribute may be omitted on at most one ObjectReference 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 URI/IDREF.
The optional Type attribute provides information about the content of the resource identified by URI/IDREF. In particular, it can indicate that it contains a SignatureProperties, Manifest, or Package element.
Transforms is an optional ordered list of processing steps that are applied to the resource's content before it is digested. Transforms can include arbitrary specifications such as canonicalization, encoding/decoding (including compression/inflation), XSLT and XPath. XSLT/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 we address 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 identified by the URI/IDREF is digested directly.
Arbritrary 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.
There are cases where it is efficient to have one signature cover many items. One approach is to include multiple ObjectReferences within SignedInfo. Since the core validation behavior includes verifying the digests of objects referenced within SignedInfo, some applications may need an alternative approach which allows pushing the validation decision to the application. This allows more complex processing to be defined on an application specific basis. For example, it may be sufficient if the signature's validity for n out of m of the items can be verified or there may be a large number of items that it is desired to sign with multiple signature algorithms and / or keys where listing all of the items within the SignedInfo element of each Signature is too verbose.
To answer these requirements, additional object types have been defined which may be referenced by SignedInfo. The Manifest element is provided which similarly contains a collection of references and objects (like SignedInfo), but leaves it entirely up to the application which digest or digests it will verify. Multiple signatures over the possibly large number of items in a Manifest need only point to the Manifest from one ObjectReference in each signature's SignedInfo.
The structure of Manifest, which reuses the ObjectReference and Object elements described above, is as follows:
<Manifest>
(ObjectReference)+
(Object)*
</Manifest>
A Package is syntactically identical to a Manifest, and may appear anywhere a Manifest may appear, but asserts the identity of each of its ObjectReference elements after Transforms application.
Manifest and Package may appear as the content of an Object.
Statements or assertions concerning data blocks should be included in those data blocks or in other data blocks signed in parallel with them. Statements about the cryptographic process itself, however, such as time of signing or serial number or hardware used in calculation of the signature or digest, can be included in a SignatureProperties block. Such blocks can be signed, via an ObjectReference, or not, as appropriate.
<SignatureProperties>
(SignatureProperty Target= /)+
</SignatureProperties>
The structure of SignatureProperties is shown above. The mandatory Target attribute references the element to which the property applies. In particular, target may include a reference to a SignedInfo or ObjectReference element.
The general structure of an XML signature is described in section 2 above. 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:
<?xml version='1.0'?>
<!DOCTYPE schema
SYSTEM 'http://www.w3.org/TR/1999/WD-xmlschema-1-19991105/structures.dtd'
[
<!ENTITY dsig 'http://www.w3.org/1999/12/signature-core'>
]>
<schema targetNS='http://www.w3.org/1999/12/signature-core'
version='0.1'
xmlns='http://www.w3.org/1999/XMLSchema'>
<textEntity
name="dsig">http://www.w3.org/1999/12/signature-core</textEntity>
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/1999/12/signature-core'>]>
<Signature xmlns="http://www.w3.org/1999/12/xmldsig-core">
<SignedInfo>
<CanonicalizationMethod
Algorithm="http://www.w3.org/1999/07/WD-xml-c14n-19990729"/>
<SignatureMethod Algorithm="&dsig;/dsaWithSHA-1"/>
<ObjectReference Location="http://www.mypage.com">
<DigestMethod Algorithm="&dsig;/sha1"/>
<DigestValue
encoding="&dsig;/base64">a23bcd43</DigestValue>
</ObjectReference>
</SignedInfo>
<SignatureValue >dd2323dd</SignatureValue>
<KeyInfo>
<KeyName>Solo</KeyName>
</KeyInfo>
</Signature>
Note: this example will be revised to include generated hash/signature values that validate.
<element name='Signature'>
<archetype order='seq' content='elemOnly'>
<element ref='SignedInfo'/>
<element ref='SignatureValue'/>
<element ref='KeyInfo' minOccurs='0' maxOccurs='1' />
<element ref='Object' minOccurs='0' maxOccurs='*'/>
<attribute name='Id' type='ID' />
</archetype>
</element>
The SignatureValue element contains the actual value of the digital signature. The encoding of this value is determined by the SignatureMethod used. For all SignatureMethods specified herein, that encoding is Base64 [RFC2045]. The ability to define a SignatureMethod and SignatureValue pair which includes multiple distinct signatures is explicitly permitted (e.g. "rsawithsha-1 and ecdsawithsha-1").
<element name='SignatureValue' type='string'/>
<!ELEMENT SignatureValue CDATA >
The structure of SignedInfo includes the canonicalization algorithm (if any), a signature algorithm, and one or more references to objects. The SignedInfo element may contain an optional ID attribute that will allow it to be referenced by other signatures and objects.
<element name='SignedInfo'>
<archetype order='seq' content='elemOnly'>
<element ref='CanonicalizationMethod' minOccurs='0'
maxOccurs='1'/>
<element ref='SignatureMethod'/>
<element ref='ObjectReference' minOccurs='1' maxOccurs='*'/>
<attribute name='Id' type='ID' />
</archtype>
</element>
SignedInfo does not include explicit signature properties. If an application needs to associate properties (such as signing time, signing device, etc.) with the signature or digest, it may add an additional Object that includes that data and reference that Object via an ObjectReference. See the SignatureProperties element below.
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. 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.
<element name='CanonicalizationMethod'>
<archetype content='elemOnly' >
<element ref='Parameter' >
<attribute name='Algorithm' type='uri' />
</archetype>
</element>
<!ELEMENT CanonicalizationMethod Parameter* >
<!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.
<element name='SignatureMethod'>
<archetype content='elemOnly'>
<element ref='Parameter' >
<attribute name='Algorithm' type='uri' />
</archetype>
</element>
<!ELEMENT SignatureMethod Parameter* >
<!ATTLIST SignatureMethod
Algorithm CDATA #REQUIRED >
ObjectReference 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 type is to make it easier to find data which may require special handling, such as a Manifest where the application wishes to check the digests within the Manifest. There is no requirement for a verifier to validate these items. An optional ID attribute permits an ObjectReference to be easily referenced from elsewhere.
<element name='ObjectReference' >
<archetype order='seq'>
<element ref='Transforms' minOccurs='0' maxOccurs='1'/>
<element ref='DigestMethod'/>
<element ref='DigestValue'/>
<attribute name='Id' type='ID' />
<attribute name='URI' type='uri' />
<attribute name='IDREF' type='IDREF' />
<attribute name='Type' type='uri' />
</archetype>
</element>
The URI/IDREF attribute identifies the 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 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 ObjectReferences to identify a fragment of a document that is encoded: the ObjectReference identifies the resource, the first Transform specifies decoding, the second Transform specifies the fragement.
Note that a null URI (URI="") is permitted and identifies the parent document.
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 ommit this attribute given the identity of the object is part of the application context. This attribute may be omitted from at most one ObjectReference in any particular SignedInfo, Manifest, or Package.
The digest algorithm is applied to the data octets being secured. Typically that is doe 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 (e.g. manifest, package, signature properties, document). This is represented as a URI. For example:
Type="&dsig;/Manifest"
Type="&dsig;/SignatureProperty"
Transforms is an optional element that contains one or more operations to be performed on an indicated resource 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 serves 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.
Each Transform consists of an Algorithm attribute, optional MimeType 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 MimeType 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.
<element name='Transforms' >
<archetype>
<element ref='Transform' minOccurs='1' maxOccurs='*' />
</archetype>
</element>
<element name='Transform' >
<archetype>
<element ref='Parameter' minOccurs='0' maxOccurs='*'/>
<attribute name='Algorithm' type='string' />
<attribute name='Type' type='uri' />
<attribute name='Charset' type='string' />
</archetype>
</element>
Examples of resource transforms include but are not limited to base-64 decoding [RFC2045], 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 the Java Transform algorithm. However, applications should refrain from using application-specific transforms whenever possible since the resulting signature will not necessarily be verifiable outside of the application domain. The section Transform Algorithms defines the list of standard transformations.
Implementation Comment: When transforms are applied the signer is not signing the native (original) document but the resulting (transformed) document.
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.
<element name='DigestMethod'>
<archetype content='elemOnly'>
<element ref='Parameter' minOccurs='0' maxOccurs='*'/>
<attribute name='Algorithm' type='uri' />
</archetype>
</element>
<!ELEMENT DigestMethod Parameter* >
<!ATTLIST DigestMethod
Algorithm CDATA #REQUIRED >
DigestValue is an element which contains the encoded value of the digest. The optional Encoding attribute gives the encoding method which defaults to Base 64 [RFC2045].
<element name='DigestValue' type='string'>
<archetype>
<attribute name='Encoding' type='uri'
default="&dsig;/Base64"/ />
</archetype>
</element>
KeyInfo may contain keys, names, certificates and other public key management information (such as inband key distribution or agreement data or data supporting any other method.) This specification defines a few simple types but applications may place (embed) their own key identification and exchange semantics within this element through the XML-namespace facility. [XML-namespace]
<element name='KeyInfo' minOccurs='0' maxOccurs='1'>
<archetype order='seq' content='any'>
<element name='KeyName' type='string' />
<element name='KeyValue' type='string' />
<element name='SubjectName' type='string' />
<element name='RetrievalMethod' type='string' />
<element ref='X509Data' type='string'>
<element ref='PGPData' type='string'>
<element ref='MgmtData' type='string' minOccurs='0'
maxOccurs='1'/>
<attribute name='Id' type='ID' />
</archetype>
</element>
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). 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.
<element name='X509Data' type='string'>
<archetype order='seq' content='any'>
<element name='X509IssuerSerial' type='string'>
<archetype order='seq' content='elemOnly'>
<element name='X509Name' type='string' />
<element name='X509SerialNumber'
type='string' />
</archetype>
</element>
<element name='X509SKI' type='string' />
<element name='X509Name' type='string' />
<element name='X509Certificate' type='string' />
<element name='X509CRL' type='string' />
</archetype>
</element>
<element name='PGPData' type='string'>
<archetype order='seq' content='elemOnly'>
<element name='PGPKeyID' type='string' />
<element name='PGPKeyPacket' type='string' />
</archetype>
</element>
<element name='MgmtData' type='string' minOccurs='0' maxOccurs='1'/>
</archetype>
</element>
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.
The Object's ID is commonly referenced from an ObjectReference in SignedInfo or a Manifest or Package. This element is typically used for embedded 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. If the application wishes to exclude the <Object> tags from the digest calculation a transform must be used. (Exclusion of the object tags may be desired for cases where the signature is intended to survive a change between embedded and detached objects 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.)
<element name='Object' >
<archetype content='any'>
<attribute name='Id' type='ID' />
<attribute name='Type' type='uri' />
<attribute name='Encoding' type='uri' />
</archetype>
</element>
Algorithms are provided with parameters and input data, when necessary, by having Parameter elements in the content of the algorithm element. Algorithms also have implicit input, such as the canonicalized SignedInfo for SignatureMethod and the transformed data for DigestMethod.
Where more than one Parameter appears, they are passed to the algorithm as an ordered vector corresponding to the order they appear in the algorithm element content.
<element name='Parameter' >
<archetype content='any'>
<attribute name='Encoding' type='uri' />
</archetype>
</element>
This section describes the optional to implement Manifest, SignatureProperties, and Package elements and describes the handling of XML Processing Instructions and Comments.
The Manifest element provides a list of ObjectReferences. 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.
A Package has the same syntax as a Manifest, and can appear wherever Manifest may appear, but also asserts the equality of each of its referenced objects, after any transforms. The testing of this equality and action if it fails is also entirely at the discretion of the applicaiton.
<element name='Manifest'>
<archetype>
<element ref='ObjectReference' minOccurs='1' maxOccurs='*'/>
<element ref='Object' minOccurs='0' maxOccurs='*'/>
<attribute name='Id' type='id' />
</archetype>
</element>
<element name='Package'>
<archetype>
<element ref='ObjectReference' minOccurs='1' maxOccurs='*'/>
<element ref='Object' minOccurs='0' maxOccurs='*'/>
<attribute name='Id' type='id' />
</archetype>
</element>
Additional information items concerning the signature or particular ObjectReferences can be placed in SignatureProperty elements within a SignatureProperties element within an Object. This should be such information as signing time or the serial number of crypto hardware used. An additional information concerning data being signed should be with that data.
<element name='SignatureProperties' >
<archetype content='elemOnly' >
<element ref='SignatureProperty' minOccurs='1' maxOccurs='*'>
<attribute name='Id' type='id' />
</archetype>
</element>
<element name='SignatureProperty' >
<archetype content='any' >
<attribute name='Target' type='idref' />
</archetype>
</element>
TDB - will specify the use, if any, of XML processing instructions by this specification and the handling of PIs appearing within elements specified in this document.
TDB - will specify the use, if any, and handling of XML comments appearing within elements specified in this document.
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 they 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 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 |
| Digest | |||
| SHA1 | REQUIRED | &dsig;/sha1 | |
| Encoding | |||
| Base64 | REQUIRED | &dsig;/base64 | |
| QuotedPrintable | RECOMMENDED | &dsig;/qp | |
| MAC | |||
| HMAC-SHA1 | REQUIRED | &dsig;/hmac-sha1 | |
| Signature | |||
| DSAwithSHA1 (DSS) | REQUIRED | &dsig;/dsa | |
| RSAwithSHA1 | RECOMMENDED | &dsig;/rsa-sha1 | |
| ECDSAwithSHA1 | OPTIONAL | &dsig;/ecdsa | |
| Canonicalization | |||
| minimal | REQUIRED | &dsig;/minimal | |
| XML-Canonicalization | RECOMMENDED | http://www.w3.org/1999/07/WD-xml-c14n-19990729 | |
| Transform | |||
| XSLT | RECOMMENDED | http://www.w3.org/TR/1999/PR-xslt-19991008 | |
| XPath | RECOMMENDED | http://www.w3.org/TR/1999/PR-xpath-19991008 | |
| XPointer | RECOMMENDED | http://www.w3.org/1999/07/WD-xptr-19990709 | |
| Java | OPTIONAL | urn:ECMA-org:java |
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 [RFC 1321] 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] identifier is &dsig;/sha1. The SHA-1 algorithm 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. 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 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 [HMAC] identifiers are &dsig;/hmac-sha1. The HMAC algorithm takes the truncation length in bits as a parameter. An example of an HMAC SignatureMethod element:
<SignatureMethod
Algorithm="&dsig;/hmac-sha1">
<hmac-outputlength xmlns="&dsig;/hmac-sha1">
128
</hmac-outputlength>
</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-MD5 digest
9294727A 3638BB1C 13F48EF8 158BFC9D
from the test vectors in [RFC 2104] would be
<SignatureValue>kpRyejY4uxwT9I74FYv8nQ==</SignatureValue>
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.
The DSA algorithm [DSA] identifier is &dsig;/dsa. The DSA algorithm 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 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 PKCS #1 [RFC 2437] 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
<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 both either appear or be absent). All parameters are encoded as base64 values.
<!ELEMENT DssKeyValue (P, Q, G, Y, J?, (seed, pgenCounter)?)
>
<!-- Each of these fields consists a CDATA 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 identifier is &dsig;/rsa-sha1. The RSA algorithm takes no parameters. An example of an RSA SignatureMethod element is
<SignatureMethod Algorithm="&dsig;/rsa-sha1"/>
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.
<!ELEMENT RSAKeyValue ( Modulus, Exponent ) > <!-- Each field contains a CDATA which is the value for that item base64 encoded -->
The expression ECDSA [ECDSA] as used in this document refers to the signature algorithms specified in ANSI X9.62. Additional details are to be provided.
Null canonicalization, i.e., no modification whatsoever, can be achieved for digested data by simply not putting any canonicalization in the Transforms element (omitting it entirely if no other tranforms are needed) for a data object or omitting CanonicalizationMethod for SignedInfo.
The algorithm identifier for the minimal canonicalization is &dsig;/minimal. An example of a minimal canonicalization element is
<CanonicalizationMethod Algorithm="&dsig;/minimal"/>
The minimal canonicalization algorithm:
The algorithm identifier for XML canonicalization is http://www.w3.org/1999/07/WD-xml-c14n-19990729. An example of an XML canonicalization element is
<CanonicalizationMethod Algorithm="http://www.w3.org/1999/07/WD-xml-c14n-19990729"/>
See the Canonical XML specification.
A Transform algorithm has three implicit parameters. The first is a byte stream from the ObjectReference 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 Algorithm values for the base 64 and quoted-printable decoding transforms [RFC2045] are &dsig;/base64 and &dsig;/qp.
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.
The Algorithm value for the XPath filtering transform is "http://www.w3.org/TR/1999/PR-xpath-19991008"
The Transform element content MUST conform to the XML Path Language (XPath) syntax.
XPath assumes that an XML processor has processed the input resource. So, for example, entity reference expansion, normalization of linefeeds and attribute values are normalized, and CDATA section replacement are expected. As well, XPath joins all consecutive text characters into a single text nodes.
The input resource MUST be a well-formed XML document. The result of applying the XPath to the input resource MUST be a node-set (as defined in XPath). The output of this transform is a new XML document with the following characteristics:
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 Algorithm value for the XPointer filtering transform is "http://www.w3.org/1999/07/WD-xptr-19990709".
The Transform element content MUST conform to the XML Pointer Language (XPointer) syntax.
The processing rules for XPointer filtering are identical to those for XPath filtering (stated above), except that the additional functionality offered by XPointer can be utilized in constructing the output node-set.
The XPointer filter is particularly important if the input resource is processed by a validating XML processor since the XPointer barename shortcut could then be used to implement the well-known fragment identification by ID attribute.
NOTE: In application environments with severe resource limitations, applications MAY constrain XPointer support to barename processing and also to determination of the ID attribute by means other than a validating XML processor. In fact, the use of an XML processor for barename resolution is OPTIONAL. However, the output expectations of this transform MUST be supported by the application.
The Algorithm value for the XSLT transform is "http://www.w3.org/TR/1999/PR-xslt-19991008"
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.
The Algorithm value for the Java transform is urn:ECMA-org:java.
Details to be determined.
Although the Algorithm attribute of a Transform can take application-specific values, having a Java transform seems to be the most reasonable way to allow application-specific transforms that can be processed outside of the application domain.
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.
Validating an XML signature consists of two mandatory processing steps. These are signature validation, the cryptographic validation of the signature calculated over SignedInfo; and reference validation, the verification of the digest contained in each ObjectReference in SignedInfo. Both steps MUST be performed as part of all XML signature validations.
Any processing beyond cryptographic validation (e.g. certificate validation, applicability decisions, time related processing) is outside the scope of this specification.
The XML digital signature standard provides a very flexible mechanism. In designing a system to make use of it, due consideration should be given to the threat model being defended against and to the factors covered in the subsections below.
The flexible Transforms mechanism, including canonicalization and explicit filtering and extraction, permit securing only a subset of data in an object. This is good for many applications where a limited portion of an object must change after the signature or different signatures secure different parts or the application modifies aspects of the object that are not significant and can be omitted from signature coverage or the like. Keep in mind that whenever this is done, those aspects that are not signed can be arbitrarily modified and the signature will still validate.
Further, the fact that the signed resource was obtained by applying transforms to a different document is not verified by the signature validation process. If this fact is important, then additional information (such as by including ObjectReferences to both the original and transformed documents) is needed.
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 secret key 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 secret key can create signatures. The number of holders of the secret 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 secret key is an important issue, usually addressed by certificate or on line 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 "key" which is a physical characteristic of the user and thus 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 [RFC 1750] and the size of the key, the security of key and certificate authentication and distribution mechanisms, protection of all 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/1999/11/xmldsig-core">
<SignedInfo ID="5">
<CanonicalizationMethod
Algorithm="http://www.w3.org/1999/07/WD-xml-c14n-19990729"/>
<SignatureMethod Algorithm="&dsig;/dsa"/>
<ObjectReference URI="http://www.mypage.com">
<Transforms>
<Transform
Algorithm="&dsig;/null">
<Encoding
Algorithm="&dsig;/base64"/>
</Transforms>
<DigestMethod Algorithm="&dsig;/sha1"/>
<DigestValue>a23bcd43</DigestValue>
</ObjectReference>
<ObjectReference IDREF="timestamp"
Type="&dsig;/signatureattributes">
<Transforms>
<CanonicalizationMethod
name="http://..."/>
</Transforms>
<DigestMethod Algorithm="&dsig;/sha1"/>
<DigestValue>a53uud43</DigestValue>
</ObjectReference>
</SignedInfo>
<SignatureValue>dd2323dd</SignatureValue>
<Object ID="timestamp"
type="&dsig;/SignatureAttributes" >
<timestamp about="5"
xmlns="http://www.ietf.org/rfcXXXX.txt">
<date>19990908</date>
<time>14:34:34:34</time>
</timestamp>
</Object>
<KeyInfo>
<keyname>Solo</keyname>
</KeyInfo>
</Signature>
[TBD: http://www.w3.org/TR/1999/WD-xmldsig-core-19991119/xmldsig-core-schema.xml]
SignedInfo reference validation.ObjectReference,
matches its specified DigetsValue.SignatureValue matches the result of processing SignedInfo
with CanonicalizationMethod and SignatureMethod as
specified in 6.2.We define the following types for use in identifying XML resources that include Signture semantics.
Note: This section is preliminary. A more detailed version will be included in a subsequent version of this specification.