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XML-Signature Core Syntax

W3C Working Draft 1999-October-01

This Working Draft version:
http://www.w3.org/Signature/Drafts/xmldsig-core-991001.html
Previous Working Group version:
http://www.w3.org/Signature/Drafts/xmldsig-core-990922.html
Editor(s):
...
Author(s)
...
dsolo@alum.mit.edu
reagle@w3.org (990922)
mbartel@JetForm.com (991006)

Copyright  1999 The Internet Society & W3C (MIT, INRIA, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.

W3C Status of this Document

This document includes updates from the 990922 draft based on the results of 990930 teleconference (and any resulting email discussion). The goal is to get some version of this document out as an ietf-draft and W3C technical report as soon as possible. This draft covers most of the topics the final specification will cover, however the text and syntax within this specification is unstable.

Abstract

This document lists the core signature syntax. 

Table of Contents

...

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.

Editorial Comment: Presently most of this work is unspecified. The working group needs to define things like manifest and algorithm definitions more explicitly.

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/Signature/core-19990922"    

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 Identifier of "dsig:" urn:dsig:signedattributes [RFC2141].

Editorial Comment: This mixed use of identification requires further thought as it is confusing as presently specified and used. The WG has not taken any steps to register the "urn:dsig" namespace.

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):

Finally, this document also includes editorial comments (highlighted in a light tan wit a blue border) that point out open issues the WG has not yet closed. 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]. However, a few of the key philosphy axioms are repeated. This specification endeavors to rely upon the syntax -- and the underlying data model -- to represent the functionality and structure of a signature where ever possible. One way this is manifested is by the use URIs to define the relationships between a signature and its context (its constituent parts, the objects it signs, the algorithms it employs, etc.). This philosophy is best captured by the idea that of "if you make statements about statements, signature is merely one type of statement " This allows us to clearly (and perhaps formally) define the meaning of a signature and how that meaning interacts with the semantics of the content it signs. One way this is manifested is in the choice of the WG to avoid things like:

<attribute type="CDATA" and value="CDATA"/>

Instead, if applications need to associate external semantics for key exchange or trust processing they must introduce a set of qualified namespace [XML-namespace] application elements with clearly defined semantics, or they must make a statement about an existing resource within some data model.

Editorial Comment: The syntax defined in this document is not yet associated with a directed labeled graph data model -- which is a requirement the WG has not yet met. The WG has decided not to use the RDF serialization syntax. Some WG members have advocated using a "striped" XML syntax whereby the structure and nesting of the XML can be read as {resource, predicate, value} triples. Unfortunately, this can result in superfluous syntax. The WG is investigating syntactical alternative and the use of a schema definition to bind a formally specified data model to an XML syntax that is intuitive and compact.

2.0 Signature Structure

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

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

A simple example follows:

<Signature xmlns="http://www.w3.org/Signature/core-19990922">
  <SignedInfo>
    <c14nAlg Algorithm="null"/>
    <SignatureAlg algorithm="dsig:rsaWithSHA-1"/>
    <ObjectReference> ;one might use the manifest or package container here
      <Location HREF="http://www.iet.org"/>
      <Type>text/html; charset="us-ascii"</Type>
      <DigestAlg Algorithm="sha-1"/>
      <DigestValue Value="a23bcd43" />
    </ObjectReference>
  </SignedInfo>
  <SignatureValue encoding="urn:dsig:base64">dd2323dd</SignatureValue>
  <KeyInfo>
     <keyname>Solo</keyname>
  </keyinfo>
</Signature>

3.0 SignatureValue

SignatureValue is an empty-element tag [XML] that contains the actual value of the digital signature. The ability to define a SignatureAlg 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 SignatureValuee
          encoding    CDATA     "urn:dsig:base64">

4.0 SignedInfo

The structure of SignedInfo includes a canonicalizatin (c14n) 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(c14nAlg?, SignatureAlg, ObjectReference+ )>   
<!ATTLIST SignedInfo
          Id     ID       #IMPLIED>

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

4.1 c14nAlg (Canonicalization)

c14nAlg is an optional element which specifies the canonicalization (c14n) 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 simple identity algorithm (CRLF and charset normalization), or more extensive operations such as [XML-C14N].

<!ELEMENT c14nAlg ANY>
<!ATTLIST c14nAlg
          Algorithm    CDATA    "null">
     <!-- Where CDATA conforms to the
          productions specified by [URI] -->

4.2 SignatureAlg

SignatureAlg 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 SignatureAlg ANY>
<!ATTLIST SignatureAlg
          Algorithm    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 URI, the type of the object, specified transformations, a digest algorith 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?, DigestAlg, DigestValue) >

4.3.1 Location

Location identifies where to find the Object . This element may be omitted if the location is implicit in the application.

<!ELEMENT Location EMPTY >
<!ATTLIST Location
          HREF           CDATA   #IMPLIED >
     <!-- Where CDATA conforms to the
          productions specified by [URI] -->

Editorial Comment: WG's members feel that an optional Location is necessary for closed model signature applications that implicitly know what the referant is based on its processing context. However, these applications abuse the notion of the open Web model and their signature will be of no utility outside their own closed domain.

4.3.2 Type

;do week keep the type in the reference and object? Or only place it in one place. -JR
; still under specified. Also, mixing the MIME type and URI's seems awkard. Maybe we need <Type owner="IANA"> or <Type owner="W3C"> etc.

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.

<!ELEMENT Type (ANY | #PCDATA) >
    <!-- 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 -->

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/Signature/1999/07/cert</Type>
<Type>urn:dsig:signedattributes</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, c14n, XPointer, XSL, filtering, encoding, etc. (These operations are different from those specified in the signature; those are are applied over signedinfo.)

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. If the Transformations element is omitted, the only operation performed is the default object c14n algorithm (null or identity).

<!ELEMENT Transformations ((Generic | c14nAlg? | Encoding? | XSLT?
            Stylesheet? | XPointer?)*) >

<!ELEMENT Generic EMPTY >
<!ATTLIST Generic
          Algorithm    CDATA    #REQUIRED >

<!-- While not necessary because of the Generic, we
define a few specific transformation types.

<!ELEMENT Encoding EMPTY >
<!ATTLIST Encoding
          Algorithm    CDATA    #REQUIRED >

<!ELEMENT c14nAlg EMPTY >
<!ATTLIST c14nAlg
          Algorithm    CDATA    #REQUIRED >

<!ELEMENT XSLT EMPTY >
<!ATTLIST XSLT
          Algorithm    CDATA    #REQUIRED >

<!ELEMENT Stylesheet EMPTY >
<!ATTLIST Stylesheet
          Algorithm    CDATA    #REQUIRED >

<!ELEMENT XPointer EMPTY >
<!ATTLIST XPointer
          Algorithm    CDATA    #REQUIRED >

     <!-- Where CDATA conforms to the
          productions specified by [URI] -->

Implementation Comment: When transformations are applied the signer is not signing the native (original) document but the resulting (transformed) document that is not captured explicitly in the signature syntax. 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.

Security Comment: Applications are recommended to ensure signers understand the actual resulting content that is being signed after transformations are applied. Users should not be tricked into signing a native content that is transformed into something that the user would not have signed otherwise. This recommendation applied to transformations specified in the signature block, as well as transformations found within the document itself.

4.3.4 DigestAlg

DigestAlg 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 DigestAlg ANY>
<!ATTLIST DigestAlg
           Algorithm     CDATA   #REQUIRED >
     <!-- Where CDATA conforms to the
          productions specified by [URI] -->

4.3.5 digestvalue

digestvalue is a required empty-element tag which contains the base64 encoded value of the digest.

<!ELEMENT DigestValue EMPTY>
<!-- base64 encoded digest value -->
<!ATTLIST DigestValue
          value          CDATA   #REQUIRED >

5.0 Object

. When present this element may contain any item and specifies the encoding. It is important to note that the content of the Object itself is not part of the hash algorithm over the signature.

<!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] -->

; Define what is meant by type, define the types. Also, do we allow duplication in reference and object? Does the reference type refer to the type of the Object tag, which will always be XML. Don should tweak. -JR

Object is an optional element which may occur one or more times.  The purpose of Object element is to wrap elements defined by any XML application as introduced through the namespace [XML-namespace] facility. For instance, the following Object references a timestamp document that is defined by some other specification.

  <Object id="timestamp1" Type="signatureattributes" > ;who defined this type?
    <timestamp xmlsn="http://www.ietf.org/rfc/1234">
      <date>19990908</date>
      <time>14:34:34:34</time>
    </timestamp>
  </Object>

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. Note there is nothing that prevents these other XML applications from referencing and making statements about signatures.

<!ELEMENT Object ANY>
<!ATTLIST Object
           id    CDATA    #IMPLIED
           type  CDATA    #IMPLIED
           encoding       CDATA   #IMPLIED >

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  (ANY | (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.

<!ELEMENT KeyName (#PCDATA)>
<!ELEMENT KeyValue (#PCDATA)>
<!ELEMENT SubjectName (#PCDATA)>
<!ELEMENT RetrievalMethod (#PCDATA)>
<!ELEMENT X509Data (#PCDATA)>
<!ELEMENT MgmtData (#PCDATA)>

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 the following URNs that are associated with algorithms we employ. Requirements are specified over implementation, not requirements for signature use.

Algorithm Type Algorithm Requirements Algorithm URI URN Derivation
Digest        
  SHA1 REQUIRED urn:nist-gov:sha1 IOTP
  MD5 RECOMMENDED urn:rsasdi-com:md5 extrapolated from IOTP
  AESH (forthcoming) urn:nist-gov:aesh IOTP
MAC        
  HMAC-SHA1 REQUIRED urn:ietf-org:hmac-sha1 extrapolated from IOTP
  HMAC-MD5 OPTIONAL urn:ietf-org:hmac-md5 extrapolated from IOTP
Signature        
  DSAwithSHA1 (DSS) REQUIRED urn:nist-gov:dsa IOTP
  RSAwithSHA1 RECOMMENDED urn:rsasdi-com:rsa-sha1 extrapolated from IOTP
  RSAwithMD5 OPTIONAL urn:rsasdi-com:rsa-md5 extrapolated from IOTP
  ECDSA (forthcoming) urn:nist-gov:ecdsa extrapolated from IOTP
Canonicalization        
  :null REQUIRED http://www.w3.org/Signature/core-19990922/null suggested W3C
  :minimal REQUIRED http://www.w3.org/Signature/core-19990922/minimal suggested W3C
  XML-c14n RECOMMENDED http://www.w3.org/1999/07/WD-xml-c14n-19990729 W3C
  DOM-c14n RECOMMENDED urn:ietf-org:dom-xml-canonicalization suggested
Transformation        
  XPointer REQUIRED http://www.w3.org/1999/07/WD-xptr-19990709 W3C

; I have attempted to match the URN scheme of IOTP. Some URNs were pulled directly from IOTP. The urn:rsasdi-com:*, urn:ietf-org:hmac-*, and urn:nist-gov:ecdsa URNs were extrapolated from urn:rsasdi-com:rsa-encryption, urn:ietf-org:hmac, and urn:nist-gov:dsa, respectively, in IOTP. For the other URNs, I alternated ietf-org and w3-org (presumably we would pick one). -MB

; URN namespace definition is discussed in RFC 2611. As far as I can tell, there are no registered namespaces. Therefore, rather than attempting the scheme above which has many urn namespaces, I would suggest we either prompt the ieft or w3c to define a namespace and then hang everything off of that, or just define a namespace for this standard. The first case would result in URNs like urn:ietf-org:sha1, the second would result in urn:w3c-xml-dsig:sha1. I think the first is preferable. -MB

; Question, mostly to the chairs: are there URN namespace applications in process for the ietf or for the w3c? If so, where do we find the processes (and criteria) to add our own URNs to those namespaces? If not, how do we make it happen if we want to go that route? I presume we shouldn't unilaterally apply for urn:ietf-org or urn:w3c-org. -MB

; Mark: The W3C would not likely use any URNs for anything in their space, we prefer namespace URI's that look more like dated URLs, so I've made the approriate tweaks that also fit with section 1.1 .

To be done: add examples of the algorithm identification elements (ie <DigestAlg/>) to the algorithms below; most are obvious. -MB

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 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>

7.2.2 MD5

The MD5 algorithm identifier is urn:rsasdi-com:md5. The MD5 algorithm takes no parameters.

The MD5 digest algorithm produces a sequence of bytes. The content of the DigestValue element shall be the base64 encoding of this sequence. Example: the DigestValue element for the message digest

F96B697D 7CB7938D 525A2F31 AAF161D0

from the test suite of the sample implemention of in RFC 1321 would be

<DigestValue>+WtpfXy3k41SWi8xqvFh0A==</DigestValue>

7.3 Message Authentication Codes

7.3.1 HMAC

The HMAC algorithm identifiers are urn:dsig:hmac-sha1 and urn:dsig:hmac-md5. The HMAC algorithm takes the truncation length in bits as a parameter.

; This is different than what the Brown draft had, which appears to have come from IOTP. I can see some added flexibility to the IOTP approach but I'd rather stick to the RFC 2104 specification. HOWEVER: I am speaking from ignorance; I don't now why IOTP did it that way. -MB

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.

; This description was lifted almost word-for-word directly from IOTP. Does it matter? Should it be reworded? -MB

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.

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>

7.5 Canonicalization Algorithms

7.5.1 Null Canonicalization

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.

The null canonicalization 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 minimal canonicalization algorithm:

This algorithm is only applicable to XML resources.

7.5.3 XML Canonicalization

See the XML Canonicalization specification.

7.5.4 DOM Canonicalization

To be determined.

7.6 Transformation Algorithms

To be determined.

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.
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-C14N
XML Canonicalization Requirements. W3C Working Draft
http://www.w3.org/1999/07/WD-xml-c14n-19990729
XPointer
XML Pointer Language (XPointer) W3C Working Draft.
http://www.w3.org/1999/07/WD-xptr-19990709

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 object being signed.
  2. calculate digest over object
  3. create Object reference element(s) including location of object, digest, and transformation and digest algorithm elements, if required.
  4. create SignedInfoelement with signaturealg, c14nAlg (for SignedInfo), and Object reference(s).
  5. canonicalize and calculate signature over SignedInfobased on algorithms in step d. f) 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 and apply Transformations (e.g. c14n) to the signed object(s) based on all Object reference(s) in the SignedInfoelement.
  2. calculate digest over all transformed signed object(s) based on the algorithm in Object reference(s). If the object is contained within the Object element, only the object itself is hashed (i.e. the <Object > and </Object > tags are excluded).
  3. compare value against digest value in SignedInfo(if mismatch, validation fails).
  4. canonicalize the SignedInfoelement based on the c14n algorithm ID in SignedInfo(or based on the default if absent).
  5. obtain the validation keying info from KeyInfo or externally.
  6. validate the SignatureValue based on the SignatureAlg in the SignedInfoelement, the key obtained in step c, and the results of step d. - Digest calculation is performed over the SignedInfoelement including start and end tags.

Note that while a signature may validate (a key was properly applied to content to yield the stated SignatureValue) this does not mean applications will necessarily trust or accept the signature. For instance, perhaps the assertions within the content that was signed could not be confirmed or validated themselves.

9 DTD

Editorial Comment: Declarations are now in the body of the text.

10.0 Example syntax

<Signature xmlns="http://www.w3.org/Signature/core-19990922">
  <SignedInfo Id="5">
    <c14nAlg Algorithm="null"/>
    <SignatureAlg algorithm="dsig:rsaWithSHA-1"/> ; not a namespace but an http urn scheme.
    <ObjectReference> ;one might use the manifest or package container here
      <Location HREF="..."/> 
        <!-- pointer to external signedobject  -->
      <Type>text/plain; charset="us-ascii"</Type> ;don't really understand
      <Transformations>
         <C14NAlg Algorithm="http://..."/>
         <Encoding Algorithm="http://..."/>
      </Transformations>
      <DigestAlg Algorithm="sha-1"/>
      <DigestValue Value="a23bcd43" />
    </ObjectReference>
    <ObjectReference>
      <Location HREF="#timestamp"/>
           <!-- points to Object below -->
      <Type type="signatureattributes" /> ;who defines this type? awkward
      <Transformations>
         <c14nAlg Algorithm="http://..."/>
      </Transformations>
      <DigestAlg Algorithm="sha-1"/>
      <DigestValue Value="a53uud43" />
    </ObjectReference>
  </SignedInfo>
  <SignatureValue encoding="urn:dsig:base64">dd2323dd</SignatureValue>
  <Object id="timestamp1" type="signatureattributes" > ;who defined this type?
    <timestamp about="#5" xmlsn="http://www.ietf.org/rfc/1234">
      <date>19990908</date>
      <time>14:34:34:34</time>
    </timestamp>
  </Object>
  <KeyInfo>
     <keyname>Solo</keyname> ;changed to be elements
  </keyinfo>
</Signature>

11.0 Open Issues

  1. Default c14n algorithms for SignedInfoand for objects. Other defaults. Mandatory to implement cryptographic algorithms and KeyInfo types.
  2. Identifying signed object. How do we handle the pointer from SignedInfoto the signed object, and what rules need to be defined for the ID attributes in SignedInfoand Object ?

12.0 Security Considerations

...

13.0 References

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
[RFC2141]
RFC 2141 -- URN Syntax
http://www.ietf.org/rfc/rfc2141.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-C14N
XML Canonicalization Requirements.
http://www.w3.org/TR/1999/NOTE-xml-canonical-req-19990605
XML-Fragment
XML-Fragment Interchange
http://www.w3.org/1999/06/WD-xml-fragment-19990630.html
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/
XPointer
XML Pointer Language (XPointer)
http://www.w3.org/1999/07/WD-xptr-19990709
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

Members of the WG

15.0 Other Useful Types

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

http://www.w3.org/Signature/core-19990922/signatureattributes
designates that the referenced resource is a statement about the referring signature.
http://www.w3.org/Signature/core-19990922/manifest
designates that the referenced resource is a collection of other resources.
http://www.w3.org/Signature/core-19990922/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.