W3C home > Mailing lists > Public > w3c-ietf-xmldsig@w3.org > April to June 2000

RE: ID Conflict avoidance between multiple signature blocks

From: Mariano P. Consens <mconsens@uwaterloo.ca>
Date: Thu, 11 May 2000 23:48:52 -0400
To: <tgindin@us.ibm.com>, "Joseph M. Reagle Jr." <reagle@w3.org>
Cc: "IETF/W3C XML-DSig WG" <w3c-ietf-xmldsig@w3.org>
Message-ID: <NDBBLNCOECHNGPEDNELJCECCCFAA.mconsens@uwaterloo.ca>

The following discussion highlights the problems caused by signing elements
with ID attributes (whose value is required to be unique by XML in the
entire
document scope), when new documents are created that combine existing
signed fragments.
This issue can be considered a more general version of the problem described
in
Tom Gindin's e-mail.

Consider a fragment A occurring in one valid XML document (making use of
Id's
in an element app:a outside Signature, in a Reference, in KeyInfo, in
SignatureProperties, as well as in URI and Target attributes referencing
those
Id's)

<app:a Id=someContent> ... </app:a>
<Signature>
<SignedInfo>
  <CanonicalizationMethod Algorithm="..."></CanonicalizationMethod>
  <SignatureMethod Algorithm="..."></SignatureMethod>
  <Reference Id=contentRef URI="#someContent">
    <Transforms><Transform Algorithm="..."/></Transform></Transforms>
    <DigestMethod Algorithm="..."></DigestMethod>
    <DigestValue> ... </DigestValue>
  </Reference>
  <Reference URI="#Keys"> ... </Reference>
  <Reference URI="#SigProps"> ... </Reference>
</SignedInfo>
<SignatureValue> ... </SignatureValue>
<KeyInfo Id=Keys> ... </KeyInfo>
<Object><SignatureProperties Id=SigProps>
  <SignatureProperty Target="#contentRef"> ... </SignatureProperty>
</SignatureProperties></Object>
</Signature>

And a similar fragment B (below) in another valid XML document, that has all
the
Id's conflicting with the ones in fragment A (to make it easier to highlight
all the
possible places where conflicts could occur)

<app:b Id=someContent> ... </app:b>
<Signature>
<SignedInfo>
  <CanonicalizationMethod Algorithm="..."></CanonicalizationMethod>
  <SignatureMethod Algorithm="..."></SignatureMethod>
  <Reference Id=contentRef URI="#someContent">
    <Transforms><Transform Algorithm="..."/></Transform></Transforms>
    <DigestMethod Algorithm="..."></DigestMethod>
    <DigestValue> ... </DigestValue>
  </Reference>
  <Reference URI="#Keys"> ... </Reference>
  <Reference URI="#SigProps"> ... </Reference>
</SignedInfo>
<SignatureValue> ... </SignatureValue>
<KeyInfo Id=Keys> ... </KeyInfo>
<Object><SignatureProperties Id=SigProps>
  <SignatureProperty Target="#contentRef"> ... </SignatureProperty>
</SignatureProperties></Object>
</Signature>

Finally, consider a new fragment C that is created by simply concatenating
the
two signed fragments A and B above.
The document that contains fragment C is invalid because it violates the XML
validity constraint ID (for the ID values someContent, contentRef, Keys, and
SigProps).

The goal is to be able to produce a valid XML document that includes a
fragment
similar to C that is the result of combining fragments A and B and where the
original signatures and digests can be verified.
This is a requirement for applications such as FSML eCheck documents,
where fragments A and B could be two checks and fragment C a deposit
containing the two checks (where you want the deposit to be one document
without references to separate documents containing the checks A and B)

Note that:
	- The Id's are all referenced from URI attributes with short form #Id
	- Eliminating the Id values before digesting and signing (to allow for
later
modification of the values to produce a valid version of C) also requires
eliminating the corresponding URI's (which may not be acceptable to the
application's security model)
	- Eliminating the Id and URI values before digesting and signing by using
the existing Transformations facility is not possible for the contents of
SignedInfo (for instance, it works for Id=someContent, but it does not work
for URI=#someContent, nor for Id=contentRef)
	- This does not address eliminating other references to an Id value from
attributes of type uri such as Target, or any other such reference in
arbitrary application dependent attributes that are digested

Dismissing the problem by requiring that globally unique Id values should be
generated for fragments A and B ignores that:
	- The applications creating fragments A and B may not have expected to be
used by a third application creating the fragment C
	- Even if all fragments are created by the same application (or
applications
that may be aware of each other), a globally unique Id may be an
undesirable burden (and could produce unreadable and/or long Id values)

The conclusion is that the usage of Id's within fragments that are going to
be
digested and/or signed precludes combining those fragments later in a valid
document where signatures and digests can be verified.

A proposed solution to this problem will be sent in a follow up message.

Regardless of any specific solution being accepted by the group, I would
suggest
to include in the text of the specification a mention of this issue, to make
the
reader aware of the potential drawback of using ID attributes within XML
Signature applications.

Mariano.

  --------------------------------------------------------------------
  Mariano P. Consens                    Department of Computer Science
  e-mail: mconsens@uwaterloo.ca         University of Waterloo
  phone: (519) 888-4567 x-3023          Waterloo, Ontario, N2L 3G1
  fax: (519) 885-1208                   CANADA
  --------------------------------------------------------------------


> -----Original Message-----
> From: w3c-ietf-xmldsig-request@w3.org
> [mailto:w3c-ietf-xmldsig-request@w3.org]On Behalf Of tgindin@us.ibm.com
> Sent: April 13, 2000 7:04 PM
> To: Joseph M. Reagle Jr.
> Cc: IETF/W3C XML-DSig WG
> Subject: ID Conflict avoidance between multiple signature blocks
>
>
>      There is one potential problem caused by the requirement that
> SignatureProperty blocks have document-wide ID's in order to be
> referenced.
> When multiple signers are creating Signature's, it is possible for them to
> have a collision in the ID between signature properties produced by
> essentially similar code on their separate machines.  This problem can be
> avoided between signatures with no extra data because most elements of
> Signature do not have to have an ID.  However, neither Object nor Manifest
> can be referenced within the signature if they do not have one.  Since the
> various signature blocks probably don't get to see each other until after
> the signatures have been completed, they must assign their identities in
> ignorance of whatever the other may be using.  It may be best to assign a
> root for all these labels either to a random value or by the digest of the
> signer's certificate, and form ID's by concatenating programmer-friendly
> strings to that.  The root could be assigned to Signature or SignedInfo.
>
>           Tom Gindin
>
>
>
Received on Thursday, 11 May 2000 23:47:26 GMT

This archive was generated by hypermail 2.2.0 + w3c-0.29 : Thursday, 13 January 2005 12:10:09 GMT