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up vote 3 down vote favorite

As the standard is not completely clear in this matter, I'm trying to found out if the XAdES standard (ETSI TS 101 903 v1.4.2) describes distributed case in which the SigAndRefsTimeStamp element or ArchiveTimeStamp elements can secure multiple SignedProperties elements.

Basically what I want is something like this:

      file-1.xml        other-X-L.xml                 old-Archival.xml
      +--------+       +----------------+            /
      |+------+|\      | XAdES-X-L      |          ,'
      ||+----+|| |     |  Time Stamp    |         /
      |||XML ||| \     +--------------+-+       ,'
      |||Doc.|||  \                    \       /
      |||    |||   \                    `.    ;
      ||+----+||    \                     \   |  archival-20121004.xml
      ||XAdES ||     | daily-20121001.xml  \  | +--------------------+
      || BES  ||     \ +----------------+   \ : |+------------------+|
      |+------+|      \|+--------------+|\   \ `++old-Archival.xml  ||
      |XAdES-T |       +|file-1.xml ref|| \   \ ||SHA-256 Digest    ||
      +--------+       ||SHA-1 Digest  ||  \   \|+------------------+|
                       |+--------------+|   `.  +|other-X-L.xml ref ||
      file-2.xml   __..+|file-2.xml ref||     \ ||SHA-256 Digest    ||
      +--------+-''    ||SHA-1 Digest  ||      \|+------------------+|
      |+------+|       |+--------------+|       +|daily-20121004.xml||
      ||+----+||       |XAdES-X-L       |       ||SHA-256 Digest    ||
      |||XML |||       | Time Stamp     |       |+------------------+|
      |||Doc.|||       +----------------+       |XAdES-A             |
      |||    |||                                | Time Stamp         |
      ||+----+||                                +--------------------+
      ||XAdES ||
      || BES  ||
      |+------+|
      |XAdES-T |
      +--------+

Is this possible?

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1+ for the ascii map – Digital Fire Oct 8 '12 at 18:56

2 Answers

up vote 3 down vote accepted

Yes, this is possible.

The XAdES standard uses regular expressions to describe the various forms. This is how the SignedInfo field of the XAdES-X forms is described:

< ds:SignedInfo >                 

  < ds:CanonicalizationMethod/ > 

  < ds:SignatureMethod/ >         

  (< ds:Reference URI? >         

    (< ds:Transforms >)?          

    < ds:DigestMethod >           

    < ds:DigestValue >            

  < /ds:Reference >)+

< /ds:SignedInfo >

The '+' after the reference field means that this field may appear once or more times.

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But wouldn't that require actually signing this information every time I create a new form or archive version (daily-20121001.xml, archival-20121004.xml, old-Archival.xml, etc.)? Then verifying it wouldn't be exactly straight-forward, wouldn't it? – Hubert Kario Oct 8 '12 at 21:40
Yes, every new form needs to be signed. It's not clear to me why this would make verification more difficult. – David Wachtfogel Oct 9 '12 at 10:18
Because if it was done using Include elements in ArchiveTimeStamp then verifier wouldn't have to be modified (it's standard usage). But with Reference elements it has to preserve current verification context (time and verified certificates) to verify the XML document in Reference. I still have to think about it a bit, but it looks like I'll use your idea. It will abuse the standard but won't create non conformant implementation. – Hubert Kario Oct 9 '12 at 10:27
up vote 1 down vote

This side-steps your question, but you might want to use the Evidence Record Syntax to do this kind of job, instead of XAdES-A. ERS includes the use of hash trees to allow sharing a given time stamp between multiple documents; and, thanks to "reduced hash trees", verification can be performed without knowing the whole set of documents.

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That would probably be a good solution if we didn't have to use XML. – Hubert Kario Oct 9 '12 at 10:19

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