XML-Signature Core Syntax

Abstract

This document specifies the core signature syntax  and processing rules of a XML signature application.

Status of this document

This is the first (and rough) public draft of this specification. This draft covers most of the topics the final specification will cover, however parts of the text and syntax within this specification are subject to change (and may be incorrect or inconsistent.)

Please send comments to the editors and cc: the list <w3c-ietf-xmldsig@w3.org>. Publication as a Working Draft does not imply endorsement by the W3C membership or IESG. 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.

1.0 Introduction

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.

The structure allows for both embedded and detached signatures. An embedded signature can include the signature within the signed object or embed the signed object within the signature. A detached signature allows the signature to be independent of the object. The processing structure allows for switching between embedded and detached signatures without invalidating the signature.

In addition to the basic signature document type, this document also defines other useful types including a methods of referencing multiple resources and key management and algorithm definitions.

1.1 Editorial Conventions

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].

The XML namespace [XML-namespace] URI that MUST be used by experimental implementations of this dated specification is:

xmlns="http://www.w3.org/1999/10/signature-core"

While applications MUST support XML-namespaces, the use of our "dsig" XML namespace prefix and defaulting/scoping conventions are OPTIONAL -- we use these facilities so as to provide compact and readable examples.

The URI in the namespace declaration above is also used as a prefix for URIs which identify resources, algorithms, or semantics under control of this specification. We use MIME types to identify algorithithms, resources, or their characteristics under the control of IANA. Otherwise we define a URN Namespace Identifiers [RFC2141] for other organizations, for example: urn:ietf-org:hmac-sha1

This document includes the following abbreviations for long words. (The acronyms are generated by wrapping the word_length-2 in the first and last letter):

• c14n: canonicalization
• i18n: internationalization

Finally, this document includes a list of open issues which are still being addressed by the working group.

Readers unfamiliar with DTD syntax may wish to refer to Ron Bourret's "Declaring Elements and Attributes in an XML DTD."

1.2 Design Philosophy

The design philosophy and requirements of this specification are addressed in the XML-Signature Requirements document [XML-Signature-RD].

1.3 Overview

This section provides a general top down overview of XML digital signature syntax and processing.  The formal specification is provided in later sections. General familiarity with digital signature concepts and XML syntax is assumed.

1.3.1 The Signature Element

XML digital signatures are very flexible and may be used to apply signatures to any type of resource.  The object(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 verification includes the verification of these digests. The algorithms used in calculating the SignatureValue are also included in the signed information. The signature can not cover itself so the SignatureValue element is outside SignedInfo.

KeyInfo indicates what key was used to create the signature. It is optional because in some applications the key is implied by the circumstances. A wide variety of KeyInfo forms are available including certificates, key names, key agreement algorithms and information, etc. The keying information is outside of the signed information so that it need not be signed. KeyInfo might contain auxiliary information it is not desired to reveal to all signature verifiers. If KeyInfo were signed, it would be necessary to pass all of it to all verifiers. On the other hand, if it is desired to bind the keying information in to the signature, its digest and a pointer to it can easily be included in the signed information.

Object is an optional element for carrying the signed data. A signature can be applied to a mix of external and embedded objects.  The data can be optionally typed and/or encoded. While Object elements can appear inside a signature as show above, they can also appear outside of the Signature element in the same document or in other documents.

While there is no explicit provision for "signature attributes", they can be included as a type of Object and thus can easily be secured or not as appropriate.

1.3.2 The SignedInfo Element

The SignedInfo element has the structure indicated below.

<Signature>
  <SignedInfo>
    (CanonicalizationAlgorithm)
    (SignatureAlgorithm)
    (ObjectReference)+
  </SignedInfo>
  (SignatureValue)
  (KeyInfo)?
  (Object)*
</Signature>

The CanonicalizationAlgorithm is the algorithm which is used to canonicalize the SignedInfo element before it is digested as part of the signature operation.

The SignatureAlgorithm 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 such as RSA-SHA1 or HMAC-SHA1. The algorithm names are signed to resist attacks based on substituting a weaker algorithm.

To promote interoperability, there are mandatory to implement canonicalization and signature algorithms. Additional standard algorithms are specified as Recommended or Optional and user defined algorithms are permitted.

The ObjectReference elements specify the things secured by the signature. As specified in more detail below, they point to the thing, specify any transformations, specify the digest algorithm, and include the digest value itself. It is the signing of this digest value and its verification as part of the signature verification that secures the thing pointed to.

The indirect reference to secured things via the ObjectReference means that it is possible to change a Signature from one where the data in enclosed as an Object within the Signature to one where the Object appears elsewhere or to move a secure item between locations outside a Signature without invalidating the signature provided the secured data can still be located from the same ObjectReference.

1.3.3 The ObjectReference Element

The ObjectReference element has the structure indicated below.

...
<SignedInfo>
   (CanonicalizationAlgorithm)?
   (SignatureAlgorithm)
   <ObjectReference>
     (Location)?
     (Type)?
     (Transformations)?
     (DigestAlgorithm)
     (DigestValue)
   </ObjectReference>+
</SignedInfo>
...

The Location says where the secured thing is.

The optional Type element provides information about the content of the thing at Location. In particular, it can indicate that the thing consists of signature attributes or is a Manifest or Package (see below).

Transformations is an optional ordered list of processing steps that are applied to the thing at Location before it is digested. These transformations can include any number of canonicalizations, encoding and decoding including compression and inflation, and XPath based transforms. XPath transforms permit parts of an XML thing to be omitted. For example, if a thing being secured encloses the signature itself, such a transform must be used to exclude the signature from the data covered. If no Transformations element is present, the data pointed at by Location is digested directly.

To promote interoperability, there are mandatory to implement canonicalization and coding algorithms. Additional standard canonicalization, coding, and XPath based transform algorithms are specified as Recommended or Optional and user defined transformation algorithms are permitted.

DigestAlgorithm is the algorithm which, when applied to the thing at Location after Transformations is applied results in DigestValue. The signing of the DigestValue is what secures the thing pointed to.

1.3.4 The Manifest and Package Elements

There are cases where it is efficient to have one signature covering many items.  One approach is to include multiple object references within SignedInfo.  Since the core verification behavior of this specification 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 item within the SignedInfo element of each Signature is too bulky.

To answer these requirements, additional objects 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 but asserts the equivalence of each of its ObjectReference elements.

2.0 Signature Structure

The general structure of an XML signature includes the following elements:

• SignedInfo is the actual data over which the signature is calculated. It contains control information (algorithm identifiers, pre-processing transformations) and digest(s) over the object(s) being signed.
• SignatureValue contains the actual value of the digital signature.
• KeyInfo is an optional element which enables the recipient(s) to obtain the key(s) needed to validate the signature.
• Object is an optional element wherein applications may place (embed) the content being signed.

<!ELEMENT Signature (SignedInfo, SignatureValue, KeyInfo?, Object*)>
<!ATTLIST SignedInfo
          Id     ID       #IMPLIED>

A simple example follows:

<Signature xmlns="http://www.w3.org/1999/10/signature-core">
  <SignedInfo>
    <CanonicalizationAlgorithm name="null"/>
    <SignatureAlgorithm name="dsig:dsaWithSHA-1"/>
    <ObjectReference>
      <Location HREF="http://www.ietf.org"/>
      <Type>text/html; charset="us-ascii"</Type>
      <DigestAlgorithm name="urn:nist-gov:sha1"/>
      <DigestValue encoding="urn:ietf-org:base64">a23bcd43</DigestValue>
    </ObjectReference>
  </SignedInfo>
  <SignatureValue encoding="urn:ietf-org:base64">dd2323dd</SignatureValue>
  <KeyInfo>
     <keyname>Solo</keyname>
  </KeyInfo>
</Signature>

Note: this example will be revised to ensure hash/signature validate.

3.0 SignatureValue

The SignatureValue element contains the actual value of the digital signature. The ability to define a SignatureAlgorithm and SignatureValue pair which includes multiple distinct signatures is explicitly permitted (e.g. "rsawithsha-1 and ecdsawithsha-1").

<!ELEMENT SignatureValue CDATA)>
<!-- base64 encoded signature value -->
<!ATTLIST SignatureValue
          encoding    CDATA     "urn:ietf-org:base64">
 

4.0 SignedInfo

The structure of SignedInfo includes a canonicalization algorithm, 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 SignedInfo(CanonicalizationAlgorithm, SignatureAlgorithm, ObjectReference+ )>
<!ATTLIST SignedInfo
          Id     ID       #IMPLIED>

SignedInfo does not include explicit signature attributes. If an application needs to associate attributes (such as signing time, signing device, etc.) with the signature, it may add an additional Object that includes that data and reference that Object via an ObjectReference.

4.1 CanonicalizationAlgorithm

CanonicalizationAlgorithm is a mandatory 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 in which an URI is included as an attribute naming the algorithm and optional contents of the element contain any parameter, value, or other information defined by the algorithm name. Possible options may include a null algorithm (no changes), a minimal algorithm (CRLF and charset normalization), or more extensive operations such as [XML-C14N].  An expected default for this value will be defined once the specification of XML aware canonicalization algorithms are finalized.

<!ELEMENT CanonicalizationAlgorithm ANY>
<!ATTLIST CanonicalizationAlgorithm
          name    CDATA >
     <!-- Where CDATA conforms to the
          productions specified by [URI] -->

Note: the ANY for this and all other Algorithm elements may be replace once a decision is reached on how to represent parameters.

4.2 SignatureAlgorithm

SignatureAlgorithm is a required element which specifies the algorithm used for signature generation and validation. This algorithm ID identifies all cryptographic functions involved in the signature operation (e.g. hashing, public key algorithms, MACs, etc.). This element uses the general structure here for algorithms in which a URI is included as an attribute naming the algorithm and optional contents of the element contain any parameter, value, or other information defined by the algorithm name. While there is a single identifier, that identifier may specify a format containing multiple distinct signature values.

<!ELEMENT SignatureAlgorithm ANY>
<!ATTLIST SignatureAlgorithm
          name    CDATA    #REQUIRED >
     <!-- Where CDATA conforms to the
          productions specified by [URI] -->

4.3 ObjectReference

ObjectReference  is an element that may occur one or more times. It includes a pointer to the object being signed, the type of the object, a list of transformations to be applied prior to digesting, a digest algorithm and digest value. Note, it is the content yielded after the URI is dereferenced, decoded, and transformed that the digest algorithm is applied to.

<!ELEMENT ObjectReference (Location?, Type?, Transformations?, DigestAlgorithm, DigestValue) >

4.3.1 Location

Location identifies where to find the Object using a URI. As the terms are defined in RFC2396 [URI], some URIs are used in conjunction with a fragment identifier by use of a separating hash (#), but the URI does not include the fragment identifier. Location only permits a URI, and fragment identification is covered under Transformations.  If this element is omitted, then the receiving application is expected to be able to determine the object to which the signature applies (for example, this approach might be used in associated a signature with a lightweight protocol data unit).  The location may be omitted only if there is a single object reference.  If there are multiple object references, they each must contain an explicit location.

<!ELEMENT Location CDATA>
<!-- The content conforms to the productions specified by [URI] -->

If the URI indicates an XML document, the document is assumed to be unparsed prior to the application of Transformations. If there are no Transformations, then the indicated resource is passed to the digest algorithm unmodified.

4.3.2 Type

Type is an optional element which contains information about the type of object being signed (e.g. manifest, package, document, SignedInfo, PDF file). This may be represented as a name (e.g. MIME type), or URI.  The type element is intended to be advisory for an application to assist in processing objects.  While the type element in ObjectReference should match the type attribute, if present, in object; such a check is not required.

<!ELEMENT Type CDATA >
    <!--  where PCDATA conforms to the productions specified for the
          content of a Content-Type MIME header [RFC 2045] or is
          a namespace qualified element name or conforms to the
          productions specified by [URI] -->

Type is an optional element which contains information about the type of object being signed (e.g. manifest, package, document, SignedInfo, PDF file). This may be represented as a name (e.g. MIME type), or URI. For example:

<Type>text/plain; charset="us-ascii"</Type>
<Type>http://www.w3.org/1999/10/signature-core/manifest</Type>
<Type>urn:ietf-org:hmac-sha1</Type>

4.3.3 Transformations

Transformations is an optional element that contains one or more operations to be performed on the Object prior to signature calculation. Examples of Transformations include encoding, canonicalization, XPointer, XSLT, filtering, encoding, etc. (These operations are applied to the reference object as contrasted with those specified in the signature which are applied to signedinfo.)  Transformations are applied in the order they appear, from left to right.  In additiona, more than one instance of a particular transformation may appear (e.g. encode, canonicalize, encode).  No transformations are applied other than those explicitly identified (i.e., there are no default transformations).

Each element within Transformations uses the general structure here for algorithms in which a URI is included as a value specifying the algorithm and optional contents of the element contain any parameter, value, or other information defined by the algorithm name.

Note that when transformations are applied the signer is not signing the native (original) document but the resulting (transformed) document. Where transformation processes are well known and widely implemented an application might include native content and specify transformations by reference. Otherwise, an application may perform transformations on the content itself and use the resulting content within the signature.

4.3.4 DigestAlgorithm

DigestAlgorithm 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 in which a URI is included as an attribute naming the algorithm and optional contents of the element contain any parameter, value, or other information defined by the algorithm name.

<!ELEMENT DigestAlgorithm ANY>
<!ATTLIST DigestAlgorithm
           name     CDATA   #REQUIRED >
     <!-- Where CDATA conforms to the
          productions specified by [URI] -->

4.3.5 digestvalue

digestvalue is an element which contains the base64 encoded value of the digest.

<!ELEMENT DigestValue CDATA>
<!ATTLIST DigestValue
          encoding    CDATA     "urn:ietf-org:base64">
 

5.0 Object

Object is an optional element which may occur one or more times. When present this element may contain any item and specifies the encoding.  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, then a transformation 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.

<!ELEMENT Object ANY>
<!ATTLIST Object
          Id    CDATA    #IMPLIED
          Type  CDATA    #IMPLIED
          Encoding       CDATA   #IMPLIED >
     <!-- Where type and encoding CDATA conforms to the
          productions specified by [URI] -->

The Object's ID is referenced from the ObjectReference in SignedInfo. This element is used for embedded signatures where the object being signed is to be included in the signature document. The Object element may include optional type, ID, and encoding attributes.

6.0 KeyInfo

KeyInfo may contain keys, names, certificates and other public key management information (such as inband key distribution or agreement data or use 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 KeyInfo  (#PCDATA | (KeyName | KeyValue |
          SubjectName | RetrievalMethod | x509Data |
          PGPData | MgmtData)* )>

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 or more KeyInfo data elements providing information for the recipient(s). Applications may define and use any mechanism they choose through inclusion of elements from a different namespace.

• KeyName contains an identifier for the key which may be useful to the recipient. This may be a name, index, etc.
• KeyValue contains the actual key(s) used to validate the signature. If the key is sent in protected form, the MgmtData element should be used. Specific types must be defined for each algorithm type (see algorithms).
• SubjectName contains one or more names for the sender. Forms to be supported include a simple name string, encoded DN, email address, etc.
• RetrievalMethod is a URI which may be used to obtain key and/or certificate information. The URI should contain the complete string for retrieving the key needed for this message (rather than a generic URI).
• X509Data contains an identifier of the key/cert used for validation (either an issuerserial value, a subject name, or a subjectkeyID) and an optional collection of certificates and revocation/status information which may be used by the recipient. issuerserial contains the encoded issuer name (RFCxxxx) along with the serial number.
• PGPData data associated with a PGP key.
• MgmtData contains in-band key distribution or agreement data. Examples may include DH key exchange, RSA key encryption etc.

Note:  This section is preliminary.  A more detailed version will be included in a subsequent version of this specification.

7.0 Algorithms

This sections identifies algorithms used with the XML digital signature standard. Entries contain the identifier to be used in signature documents, a reference to the formal specification, and definitions, where applicable, for the representation of keys and the results of cryptographic operations.

7.1 Algorithm Identifiers and Requirements

The 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.

7.2 Message Digests

7.2.1 SHA-1

The SHA-1 algorithm identifier is urn:nist-gov:sha1. The SHA-1 algorithm takes no parameters. An example of an SHA-1 DigestAlg element is

<DigestAlgorithm name="urn:nist-gov:sha1"/>

An 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 an 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>

7.3 Message Authentication Codes

7.3.1 HMAC

The HMAC algorithm identifiers are urn:ietf-org:hmac-sha1 and urn:ietf-org:hmac-md5. The HMAC algorithm takes the truncation length in bits as a parameter (parameter identifier urn:ietf-org:hmac-outputlength).   An example of an HMAC SignatureAlg element:

<SignatureAlgorithm name="urn:ietf-org:hmac-sha1">
  <Parameter type="urn:ietf-org:hmac-outputlength">
     128
  </Parameter>
</SignatureAlg>

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>

7.4 Signature Algorithms

7.4.1 DSA

The DSA algorithm identifier is urn:nist-gov:dsa. The DSA algorithm takes no parameters. An example of a DSA SignatureAlg element is

<SignatureAlgorithm name="urn:nist-gov: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 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>

7.4.2 RSA

The expression "RSA algorithm" as used in this document refers to the RSASSA-PKCS1-v1_5 algorithm described in RFC 2437.

The RSA algorithm identifiers are urn:rsasdi-com:rsa-sha1 and urn:rsasdi-com:rsa-md5. The RSA algorithm takes no parameters. An example of an RSA SignatureAlg element is

<SignatureAlgorithm name="urn:rsasdi-com: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: <insert example here>

7.4.3 ECDSA

The expression ECDSA as used in this document refers to the signature algorithms specified in ANSI X9.62.  Additional details are to be provided.

7.5 Canonicalization Algorithms

7.5.1 Null Canonicalization

The algorithm identifier for the null canonicalization is http://www.w3.org/1999/10/signature-core/null.  An example of a null canonicalization CanonicalizationAlgorithm element is

<CanonicalizationAlgorithm name="http://www.w3.org/1999/10/signature-core/null"/>

The null canonicalization produces a message byte-for-byte identical with the original resource. No character set, line ending, or white space normalization is done.

This algorithm is appropriate for applications where the resource to be signed is not XML, or where the XML document will be exactly preserved. For many applications, one of the other canonicalization algorithms will be more appropriate.

7.5.2 Minimal Canonicalization

The algorithm identifier for the minimal canonicalization is http://www.w3.org/1999/10/signature-core/minimal. An example of a minimal canonicalization CanonicalizationAlg element is

<CanonicalizationAlgorithm name="http://www.w3.org/1999/10/signature-core/minimal"/>

The minimal canonicalization algorithm:

• converts the character encoding to UTF-8, removing the encoding pseudo-attribute
• normalizes line endings

This algorithm is only applicable to XML resources.

7.5.3 Canonical XML

The algorithm identifier for XML canonicalization is http://www.w3.org/1999/07/WD-xml-c14n-19990729. An example of an XML canonicalization CanonicalizationAlg element is

<CanonicalizationAlgorithm name="http://www.w3.org/1999/07/WD-xml-c14n-19990729"/>

See the Canonical XML specification.

7.6 Transformation Algorithms

Application developers are strongly encouraged to support all transformations listed in this section as RECOMMENDED unless the application environment has severe 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 transformations that can be incorporated into applications without extensive development.

7.6.1 Canonicalization

The Algorithm value for canonicalization are defined above.

The Transformation element content MUST include a Canonicalization element, which specifies the canonicalization algorithm that will be applied to the input of the Transformation element.

7.6.2 Base-64 Decoding

The Algorithm value for the base 64 decoding transformation is urn:ietf-org:base64.

The base-64 decoding algorithm identifier is urn:ietf-org:base64.

The base-64 Transformation element has no content. The input (from the Location or from the previous Transformation) is base-64 decoded. This transformation is useful if an application needs to sign the raw data associated with base-64 encoded content of an element.

7.6.3 XPath Filtering

The Algorithm value for the XPath filtering transformation is "http://www.w3.org/TR/1999/PR-xpath-19991008"

The Transformation 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 characters into a single text node.

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 transformation is a new XML document with the following characteristics:

1. The output document has the XML declaration of the input resource (see rule 23 XMLDecl in XML specification). If the encoding is UTF-16, the output document has the same byte order mark as the input resource.
2. The output document contains the nodes in the node-set identified by the XPath, and excludes the nodes of the input resource that are not not in the node-set identified by the XPath.
3. The nodes in the output document appear in the document order (as defined in XPath) of the input resource.
4. The output document has all of the input resource's entity references expanded, except that characters corresponding to illegal XML are reencoded as character references (XML rule 66) except the ampersand and less than symbol, which are encoded using & and <, respectively.
5. Attribute values are normalized in accordance with the rules for a validating XML processor (even if the implementation did not use a validating XML processor to parse the input resource).

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).

7.6.4 XPointer Filtering

The Algorithm value for the XPointer filtering transformation is "http://www.w3.org/1999/07/WD-xptr-19990709".

The Transformation 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 transformation MUST be supported by the application.

7.6.5 XSLT Transformation

The Algorithm value for the XSLT transformation is "http://www.w3.org/TR/1999/PR-xslt-19991008"

The Transformation element content MUST conform to the XSL Transformations (XSLT) language syntax.

The processing rules for the XSLT transformation are stated in the XSLT specification.

7.6.6 Java Transformation

The Algorithm value for the Java transformation is urn:ECMA-org:java.

Details to be determined.

Although the Algorithm attribute of a Transformation can take application-specific values, having a Java transformation seems to be the most reasonable way to allow application-specific transformations that can be processed outside of the application domain.

7.7. Algorithm References

Base64

RFC 2045. Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies. N. Freed & N. Borenstein. DRAFT STANDARD.
http://www.ietf.org/rfc/rfc2045.txt

DSS

FIPS PUB 186-1. Digital Signature Standard (DSS). U.S. Department of Commerce/National Institute of Standards and Technology.
http://www.ietf.org/rfc/rfc2104.txt

HMAC

RFC 2104. HMAC: Keyed-Hashing for Message Authentication. H. Krawczyk, M. Bellare, R. Canetti. INFORMATIONAL.

MD5

RFC 1321. The MD5 Message-Digest Algorithm. R. Rivest. INFORMATIONAL.
http://www.ietf.org/rfc/rfc1321.txt.

RSA

RFC 2437. PKCS #1: RSA Cryptography Specifications Version 2.0. B. Kaliski, J. Staddon. INFORMATIONAL.
http://www.ietf.org/rfc/rfc2432.txt

SHA-1

FIPS PUB 180-1. Secure Hash Standard. U.S. Department of Commerce/National Institute of Standards and Technology.
http://csrc.nist.gov/fips/fip180-1.pdf

URNs

RFC 2141. URN Syntax. R. Moats. PROPOSED STANDARD.
ftp://ftp.isi.edu/in-notes/rfc2141.txt

RFC 2611. URN Namespace Definition Mechanisms. L. Daigle, D. van Gulik, R. Iannella, P. Falstrom. BEST CURRENT PRACTICE.
ftp://ftp.isi.edu/in-notes/rfc2611.txt

XML-Canonicalization

Canonical XML. W3C Working Draft
http://www.w3.org/1999/07/WD-xml-c14n-19990729

XPath

XML Path Language (XPath)Version 1.0. W3C Proposed Recommendation
http://www.w3.org/TR/1999/PR-xpath-19991008

XPointer

XML Pointer Language (XPointer). W3C Working Draft.
http://www.w3.org/1999/07/WD-xptr-19990709

XSL

Extensible Stylesheet Language (XSL) W3C Working Draft
http://www.w3.org/TR/1999/WD-xsl-19990421

XSLT

XSL Transformations (XSLT) Version 1.0. W3C Proposed Recommendation
http://www.w3.org/TR/1999/PR-xslt-19991008

8.0 Processing rules

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.

8.1 Generation

1. apply Transformations determined by application to each object being signed.
2. calculate digest over each transformed object (including start and end tags)
3. create ObjectReference element(s) including location of object, digest, digest algorithm, and transformation elements, if required.
4. create SignedInfo element with SignatureAlgorithm, CanonicalizationAlgorithm, and ObjectReference(s).
5. canonicalize and calculate signature over SignedInfo based on algorithms in step d.
6. construct signature document with SignedInfo, Object (s) (if desired, encoding may be different than that used for signing), KeyInfo (if required), and SignatureValue.

8.2 Signature Validation

1. locate object and apply Transformations  to the specified resource based on each ObjectReference(s) in the SignedInfo element.  Each transformation is applied in order from left to right to the object with the output of each transformation being the input to the next.
2. calculate digest over each transformed signed object(s) (including start and end tags) based on the algorithm in ObjectReference(s).
3. compare value against DigestValue in SignedInfo for each reference (if any mismatch, validation fails).
4. canonicalize the SignedInfo element based on the CanonicalizationAlgorithm in SignedInfo.
5. obtain the validation keying info from KeyInfo or externally.
6. validate the SignatureValue based on the SignatureAlgorithm in the SignedInfo element, the key obtained in step e, and the results of step d. - Digest calculation is performed over the SignedInfo element including start and end tags.

Any processing beyond cryptographic validation (e.g. certificate validation, applicability decisions, time related processing) is outside the scope of this specification.

9 DTD

[TBD: Combined DTD]

10.0 Example syntax

11.0 Open Issues

1. Additional review is required on use of the dsig namespace; other use of namespaces; specification of types; etc.
2. Need to review Default CanonicalizationAlgorithm algorithms for SignedInfo and for objects. Other defaults. Mandatory to implement cryptographic algorithms.
3. Much more detail for KeyInfo types.
4. How to represent optional parameters for all algorithms.  Candidate choices include (EMPTY | Parameter+) where Parameter has a name attribute and is of type ANY; or allowing multiple elements such as <keySize>128</keySize>.
5. Make sure we are consistent with respect to types, algorithm IDs, URIs, etc.
6. The signature data structures specified in this document are not yet associated with a data model.

12.0 Security Considerations

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.

Only What is Signed is Secure

The flexible Transformations 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.

Only What is "Seen" Should be Signed

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 possible 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.

Check the Security Model

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.

Algorithms, Key Lengths, Etc.

The strength of a particular signature depends on all links in the security chain.  This includes the signature and digest algorithms used, the strength of the key generation [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.

13.0 References

Other references can be found in section7.7 .

DOMHASH

Internet Draft. Digest Values for DOM (DOMHASH)
http://search.ietf.org/internet-drafts/draft-hiroshi-dom-hash-01.txt .

RDF

RDF Schema

http://www.w3.org/TR/1999/PR-rdf-schema-19990303

RDF Model and Syntax
http://www.w3.org/TR/1999/REC-rdf-syntax-19990222

[RFC2119]

RFC2119 -- Key words for use in RFCs to Indicate Requirement Levels.
http://www.ietf.org/rfc/rfc2119.txt

URI

Uniform Resource Identifiers (URI): Generic Syntax

http://www.ietf.org/rfc/rfc2396.txt

XLink

XML Linking Language
http://www.w3.org/1999/07/WD-xlink-19990726

XML

Extensible Markup Language (XML) Recommendation.
http://www.w3.org/TR/1998/REC-xml-19980210

XML-namespace

Namespaces in XML
http://www.w3.org/TR/1999/REC-xml-names-19990114

XML-schema

XML Schema Part 1: Structures
http://www.w3.org/1999/05/06-xmlschema-1/
XML Schema Part 2: Datatypes
http://www.w3.org/1999/05/06-xmlschema-2/

XML-Signature-RD

XML-Signature Requirements
http://www.w3.org/1999/08/WD-xmldsig-requirements-990820

WebData

Web Architecture: Describing and Exchanging Data.
http://www.w3.org/1999/04/WebData

14.0 Acknowledgements

• Milton Anderson, FSTC
• Mark Bartel, JetForm Corporation
• John Boyer, UWI.com
• Richard Brown, Globeset
• Donald Eastlake 3rd, IBM
• Barb Fox, Microsoft
• Phillip Hallam-Baker, VeriSign Inc
• Joseph Reagle, W3C
• Ed Simon , Entrust Technologies Inc.
• Chris Smithies, PenOp
• David Solo, Citigroup
• Winchel Todd Vincent III, GSU
• Greg Whitehead, Signio Inc.

15.0 Other Useful Types

We define the following types for use in identifying XML resources that include Signture semantics.

http://www.w3.org/1999/10/signature-core/signatureattributes

designates that the referenced resource is a statement about the referring signature.

http://www.w3.org/1999/10/signature-core/manifest

designates that the referenced resource is a collection of other resources.

http://www.w3.org/1999/10/signature-core/package

designates that the referenced resources is a collection of other resources and the creator of that collection asserts that the specified resources, when transformed as specified, yield the same exact content.