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post FTF syntax draft

From: <david.solo@citicorp.com>
Date: Wed, 1 Sep 1999 13:17:45 -0400
Message-Id: <H0000cc4042604f8@MHS>
TO: w3c-ietf-xmldsig@w3.org
The attached update to the syntax draft represents the consensus reached at the 
FTF on 30/31 Aug.  Tentative decisions (pending further review here) reflected 
in the attached include:

- changing the name of signedobjectdata to signedobjectreference
- promoting objecttype within signedobjectreference
- allowing multiple instances of signedobjectreference and signedobject within 
signature
- eliminating signatureattributes (signedattributes) as an explicit item.  It 
is now expected to be handled as an instance of signedobject
- defining other useful types (signatureattributes, manifest)
- filling in a few more blanks
- decision to maintain the grouping around keyinfodata and attributedata (i.e. 
not promoting those types)
- confirming decision to go with the signedobjectreference model with mandatory 
validation of the digest over signedobject at the top level
- confirming decision to keep keyinfo out of the signedinfo element (but note 
it can be bound using a signedobjectreference if an application desires)

Dave


XML Digital Signature Syntax and Processing 
David Solo
Citigroup
dsolo@alum.mit.edu

31 August 1999

1.0 Introduction

This document describes the proposed syntax and processing rules for the 
XML Digital Signature Standard.  This standard 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 may either 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 manifest for referencing multiple 
resources and key management and algorithm definitions.

2.0 General model

The general structure of an XML signature document is:

<signature>
  <signedinfo/>
  <signedobject/>
  <keyinfo/>
  <signaturevalue/>
</signature>

2.1 signedinfo

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.

2.2     signedobject

signedobject is an optional element which may occur one or more times
and which is used to include the object(s) being signed within the
signature document.  When present this element may contain any item
and specifies the encoding.

2.3     keyinfo

keyinfo is an optional element which enables the recipient(s) to obtain 
the key(s) needed to validate the signature.  keyinfo may contain the 
key, may name the key, may include certificates and other public key 
management information, may include inband key distribution or agreement 
data, or use any other method.

2.4     signaturevalue

signaturevalue is an empty element 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").

3.0 signedinfo

The structure of signedinfo is:

<signedinfo>
  <c14nalg/>
  <signaturealg/>
  <signedobjectreference/>
</signedinfo>

The signedinfo element may contain an optional ID attribute that will 
allow it to be referenced by other signatures and objects.

signedinfo does not include an explicit signatureattributes element.
If an application needs to associate attributes (such as signing time,
signing device, etc.) with the signature, it may add an additional
signedobjectreference and signedobject to the signature (see useful
types). 

3.1 c14nalg

c14nalg is an optional element which specifies the c14n algorithm 
applied to the signedinfo element prior to performing signature 
calculations.  If the default c14n algorithm for signedinfo is used, 
this element may be omitted.  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.  
Possible options may include a null algorithm (no changes), a simple 
identity algorithm (CRLF and charset normalization), and more extensive 
transformations such as the W3C c14n proposal.

3.2 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.

3.3 signedobjectreference

signedobjectreference is an element that may occur one or more times.
The structure of signedobjectreference is:

<signedobjectreference>
  <objectlocation/>
  <objecttype/>
  <transformations/>
  <digestalg/>
  <digestvalue/>
</signedobjectreference>

3.3.1 objectlocation

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

3.3.2 objecttype

objecttype 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), namespace qualified element name, or uri.

3.3.3 transformations

transformations is an optional element that contains one or more 
operations to be performed on the signedobject prior to signature 
calculation.  Examples of transformations include c14n, exclusion 
(omitting certain portions of the object from the signature), encoding, 
etc.  Each element within transformations 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.  
If the transformations element is omitted, the only operation performed 
is the default object c14n algorithm (null or identity).

3.3.4 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.

3.3.5 digestvalue

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

4.0 signedobject

signedobject is an optional element which may occur one or more times
and which contains a signed object identified in a signedobject
reference in signedinfo.  This element is used for embedded signatures
where the object being signed is to be included in the signature
document.  The signedobject element may include optional type, ID, and
encoding attributes and may contain any data.

5.0 keyinfo

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 keyinfodata elements 
providing information for the recipient(s).  Some types are defined 
here, although applications may define any mechanism they choose.

5.1 keyname

keyname contains an identifier for the key which may be useful to the 
recipient.  This may be a name, index, etc.   

5.2 keyvalue

keyvalue contains the actual key(s) used to validate the signature.  If 
the key is sent in protected form, the keymgmtdata element should be 
used.  Specific types must be defined for each algorithm type (see 
algorithms).

5.3 subjectname

subjectname contains one or more names for the sender.  Forms to be 
supported include a simple name string, encoded DN, email address, etc.

5.4 keyretrievalmethod

keyretrievalmethod 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).

5.5 x509data

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.

5.6 pgpdata

5.7 keymgmtdata

keymgmtdata contains in-band key distribution or agreement data.  
Examples may include DH key exchange, RSA key encryption etc.

6.0 signaturevalue

signaturevalue is a required empty element which contains the base64 
encoded value of the signature as defined by the signaturealg value in 
signedinfo.

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 Digest algorithms

SHA-1, MD5, (AESH)

7.2 Signature algorithms

HMAC, DSAwithSHA1, RSAwithSHA1, RSAwithMD5

7.3 c14n algorithms

Null, Minimal, DOM-CANON, W3C-SWG

7.4 exclusion

Xpointer, ??

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

a) apply transformations determined by application to object to be 
signed.
b) calculate digest over object
c) create signedobjectreference element(s) including location of object, 
digest, 
and transformation and digest algorithm elements, if required.
d) create signedinfo element with signaturealg, c14nalg (for 
signedinfo), and signedobjectreference(s).
e) canonicalize and calculate signature over signedinfo based on 
algorithms in step d.
f) construct signature document with signedinfo, signedobject(s) (if 
desired, encoding may be different than that used for signing), keyinfo 
(if required), and signaturevalue.

8.2 Validation 

a) locate and apply transformations (e.g. c14n) to the signed object(s) 
based on all signedobjectreference(s) in the signedinfo element.
b) calculate digest over all transformed signed object(s) based on the 
algorithm in signedobjectreference(s).  If the object is contained within the 
signedobject element, only the object itself is hashed (i.e. the 
<signedobject> and </signedobject> tags are excluded).
c) compare value against digest value in signedinfo (if mismatch, 
validation fails).
d) canonicalize the signedinfo element based on the c14n algorithm ID 
in signedinfo (or based on the default if absent).
e) obtain the validation keying info from keyinfo or externally.
f) validate the signaturevalue based on the signaturealg in the 
signedinfo
element, the key obtained in step c, and the results of step d.
- Digest calculation is performed over the signedInfo element including 
start and end tags.


9.0 DTD

<?xml version="1.0"?>

 <!ELEMENT dsig:signature 
   (dsig:signedinfo, dsig:signedobject*, dsig:keyinfo?, 
dsig:signaturevalue)>
 <!ATTLIST dsig:signature
         id     ID      #IMPLIED>

 <!ELEMENT dsig:signedinfo 
     (dsig:c14nalg?, dsig:signaturealg, dsig:signedobjectreference+ )>    
 <!ATTLIST dsig:signedinfo
         id     ID      #IMPLIED>

 <!ELEMENT dsig:signedobject ANY>
 <!ATTLIST dsig:signedobject
          id    CDATA   #IMPLIED
          type  CDATA   #IMPLIED
          encoding      CDATA   #IMPLIED >

 <!ELEMENT dsig:keyinfo  (dsig:keyinfodata)+>
 <!-- must be at least one keyinfodata or omit the element-->  

 <!ELEMENT dsig:keyinfodata ANY>
 <!ATTLIST dsig:keyinfodata
        type    CDATA   #REQUIRED >

 <!ELEMENT dsig:signaturevalue EMPTY)>
 <!ATTLIST dsig:signaturevalue
          value         CDATA   #REQUIRED >
 <!-- base64 encoded signature value -->

 <!ELEMENT dsig:c14nalg ANY>
 <!ATTLIST dsig:c14nalg
        type    CDATA   #REQUIRED >

 <!ELEMENT dsig:signaturealg ANY>
 <!ATTLIST dsig:signaturealg
        type    CDATA   #REQUIRED >

 <!ELEMENT dsig:digestalg ANY>
 <!ATTLIST dsig:digestalg
        type    CDATA   #REQUIRED >

 <!ELEMENT dsig:encoding ANY>
 <!ATTLIST dsig:encoding
        type    CDATA   #REQUIRED >

 <!ELEMENT dsig:exclusion ANY>
 <!ATTLIST dsig:exclusion
        type    CDATA   #REQUIRED >

 <!ELEMENT dsig:signedobjectreference 
    (dsig:objectlocation?, dsig:objecttype?, dsig:transformations?,
dsig:digestalg, dsig:digestvalue) >

 <!ELEMENT dsig:objectlocation EMPTY >
 <!ATTLIST dsig:objectlocation
          href          CDATA   #REQUIRED >

 <!ELEMENT dsig:objecttype EMPTY >
 <!ATTLIST dsig:objecttype
          type          CDATA   #REQUIRED >

 <!ELEMENT dsig:transformations
  (dsig:c14nalg?, dsig:encoding?, dsig:exclusion?) >

 <!ELEMENT dsig:digestvalue EMPTY>
 <!-- base64 encoded digest value -->
 <!ATTLIST dsig:digestvalue
          value         CDATA   #REQUIRED >

----

 <!ELEMENT dsig:signatureattributes 
    (dsig:attributedata+, objectlocation?>

 <!ELEMENT dsig:attributedata ANY>
 <!ATTLIST dsig:attributedata
        type    CDATA   #REQUIRED >

 <!ELEMENT dsig:manifest
    (dsig:signedobjectreference+, signedobject*)
 <!ATTLIST dsig:manifest
        id      ID      #IMPLIED >

10.0 Example syntax:

<dsig:signature>
  <dsig:signedinfo id="...">
    <dsig:c14nalg type="null"/>
    <dsig:signaturealg type="rsaWithSHA-1"/>
    <dsig:signedobjectreference>
      <dsig:objectlocation href="..."/> 
        <!-- pointer to external signedobject  -->
      <dsig:objecttype type="http://..." />
      <dsig:transformations>
         <dsig:c14nalg type="http://..."/>
         <dsig:encoding type="http://..."/>
      </dsig:transformations>
      <dsig:digestalg type="sha-1"/>
      <dsig:digestvalue value="a23bcd43" />
    </dsig:signedobjectreference>
    <dsig:signedobjectreference>
      <dsig:objectlocation href="..."/> 
        <!-- pointer to signedobject below -->
      <dsig:objecttype type="dsig:signatureattributes" />
      <dsig:transformations>
         <dsig:c14nalg type="http://..."/>
      </dsig:transformations>
      <dsig:digestalg type="sha-1"/>
      <dsig:digestvalue value="a53uud43" />
    </dsig:signedobjectreference>
  </dsig:signedinfo>
  <dsig:signedobject id="..." type="dsig:signatureattributes" >
    <dsig:signatureattributes>
      <dsig:attributedata type="http://...">   19990824132700Z  
      </dsig:attributedata>
      <objectlocation href="...">
        <!-- pointer to signedinfo above -->
    </dsig:signatureattributes>
  </dsig:signedobject>
  <dsig:keyinfo>
     <dsig:keyinfodata type="keyname"> 3 </dsig:keyinfodata>
  </dsig:keyinfo>
  <dsig:signaturevalue value="dd2323dd"/>
</dsig:signature>

11.0 Open Issues

1) Default c14n algorithms for signedinfo and for objects.  Other 
defaults.  Mandatory to implement cryptographic algorithms and keyinfo 
types.

2) Identifying signed object.  How do we handle the pointer from 
signedinfo to the signed object, and what rules need to be defined for 
the ID attributes in signedinfo and signedobject?

12.0 Security Considerations

13.0 References

14.0 Acknowledgements

15.0 Other Useful Types

15.1  signatureattributes

signatureattributes is an optional element which contains one or more
attributedata elements.  Each attributedata element contains a type
attribute naming the attribute type with a uri or qualifed element
name and has a value defined by the type.  signatureattributes contains
information associated with the signature itself.  signatureattributes
also contains an optional reference to the signedinfo element with
which it is associated.

15.2 manifest

manifest is an element type which is used to collect a number of
instances of signedobjectreference and optionally signedobject.
manifest is one instance of a type that may be used as a signedobject
to create a signature over multiple items.


Received on Wednesday, 1 September 1999 13:19:03 GMT

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