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RE: Cross review of draft ERS from LTANS WG - RE: WG Last Call: draft-ietf-ltans-ers-09.txt- until Jan 23rd

2007-01-11 06:36:57
Comments inline... 

From: Denis Pinkas [mailto:denis(_dot_)pinkas(_at_)bull(_dot_)net] 
Subject: Re: Cross review of draft ERS from LTANS WG - RE: WG 
Last Call: draft-ietf-ltans-ers-09.txt- until Jan 23rd

Nearly no comments were sent on the LTANS mailing list on 
these documents. 

There is just one document that has been submitted for working group last

It may be questionable why. The fact is that these documents 
are badly written, and thus it takes an enormous amount of 
time to attempt to understand them. 

Even though, most readers will fail, but will not dare to 
report that they did. 
I am unsure that I understood them, but I dare to report it.

This draft and its companion documents namely 
draft-ietf-ltans-ari-00 and draft-ietf-ltans-validate-00 
raise a number of questions to be solved.

These two drafts were prepared quickly in advance of the last working group
meeting.  During the meeting, a call was made for additional co-authors to
help clarify and complete the documents.  One person has joined Tobias in
this effort but no follow-up drafts have been released yet.

Let us look now at the details of ERS.

No ASN.1 compiler has been used to validate the syntax, 
otherwise, it would have been noticed that there is an 
obvious ASN.1 error on EvidenceRecord.

The ASN.1 was changed as a result of comments during the previous working
group last call.  Minor errors can be corrected without much cause for
alarm.  I assume in this case you are referring to the missing 's' at the
end of EncryptionInfo.  Also, the id-EvidenceRecord-Internal and
id-EvidenceRecord-External values are missing OBJECT IDENTIFIER.  

   EvidenceRecord ::= SEQUENCE {
      version                   INTEGER { v1(1) } ,
      digestAlgorithms          SEQUENCE OF AlgorithmIdentifier,
      cryptoInfos               [0] CryptoInfos OPTIONAL,
      encryptionInfo            [1] EncryptionInfo OPTIONAL,
      archiveTimeStampSequence  ArchiveTimeStampSequence,

Later on the text defines ArchiveTimeStampSequence as:

   ArchiveTimeStampSequence ::= SEQUENCE OF
                                ArchiveTimeStampChain and 

   ArchiveTimeStampChain ::= SEQUENCE OF ArchiveTimeStamp


   ArchiveTimeStamp ::= SEQUENCE {
     digestAlgorithm [0] AlgorithmIdentifier OPTIONAL,
     reducedHashtree [1] SEQUENCE OF PartialHashtree OPTIONAL,
     timeStamp       ContentInfo}

   PartialHashtree ::= SEQUENCE OF OCTET STRING

Note that this information is spread all around the document, 
rather than presented in sequence which does not ease its reading.

ContentInfo is supposed to carry a TST. This means that 
timeStamp should be defined as TimeStampToken rather than ContentInfo.

timeStamp was defined as a ContentInfo to allow a type other than
TimeStampToken to be used.  

There is no explanation in section 4.1 (top of page 11) to 
know on which data the digest contained in the TimeStampToken 
is computed. 

This info appears in Section 4 (middle of page 10): "The root hash value,
which represents unambiguously all data objects, is timestamped."

If another kind of time-stamp token would need to be carried, 
the syntax would need to be changed. This comes into 
contradiction with the following sentence :

   Other types of timestamp MAY be used, if they contain time data,
   timestamped data and a cryptographically secure 
confirmation from the
   TSA of these data.

See above.

CryptoInfos from EvidenceRecord should be suppressed, since 
it is not proven that this additional item will ever be 
useful: encryption techniques may be different for each data 
item and thus cannot apply globally at the level of an EvidenceRecord.
EncryptionInfo should be suppressed too. No interoperability 
can be obtained on these two fields using this specification.

This comment was made during a previous last call and resulted in Tobias'
posting to the list a few instances of structures that could be conveyed in
these fields.  These could be defined in a peer document or defined in this
ArchiveTimeStampSequence is a sequence of 
ArchiveTimeStampChain which is itself a sequence of 
ArchiveTimeStamp. However, there is no way to make a 
difference between an ArchiveTimeStampChain and an 

The two are not used interchangably.  Why is this a problem?
If within an EvidenceRecord, one element within of an 
archiveTimeStampSequence is suppressed (e.g. a 
ArchiveTimeStamp) this is not detectable. What is the real 
value of the EvidenceRecord structure ? One would expect that 
an EvidenceRecord includes a time stamp. However, it does not not.

Why would one element be suppressed?  What would you want to have happen in
this case?  As-is, verification would fail.  I have commented previously
that it might be better if the chain supported shallow verification, which
would enable you to suppress a contiguous set of elements from the beginning
of the chain up to the point from which verification was desired.  There was
no support for making a change along these lines, and there seemed to be no
real need to have this feature.  

What do you mean the EvidenceRecord does not include a timestamp?  It
includes ArchiveTimeStampSequence, which may have one or more timestamps.
The current draft does not allow to apply the processing 
recursively. Having constructed one EvidenceRecord (that 
would include a time stamp), it is currently not possible to 
construct another one using a previous one. 

An EvidenceRecord is a wrapper around an ArchiveTimeStampSequence.  The
draft describes how to add additional ArchiveTimeStamps to the last
ArchiveTimeStampChain in an ArchiveTimeStampSequence as well as how to add a
new ArchiveTimeStampChain to the ArchiveTimeStampSequence.  I don't disagree
that trying to follow discussion of these three similarly-named structures
is difficult, but have failed to come up with clearer alternative
This document is supposed to apply to a long term archive 
service. However, there is no signature from an archive 
service. The TSA should not be confused with a Archive 
Authority. With the current structure the Archive service may 
not be held responsible of the storage of the data.

This document defines a syntax for representing a chain of cryptographic
evidence.  This syntax may be used by an archive service.  However, no
archive service is required to produce an evidence record.  Archive service
signatures are addressed in the context of a protocol for interacting with
an archive service.
The draft claims to protect against weak algorithms or keys. 
However, it does not make a clear and clean separation 
between the cases of :

-     the key of a Time-Stamping Unit has been compromised,
-     an asymmetric algorithm has been broken for a given key size,
-     a hash algorithm exhibits collisions.

With "Timestamp renewal" the text omits to say that it is 
necessary using "out of bands means" that a given key from a 
Time Stamping Unit has been compromised or a that given 
asymmetric algorithm has been broken for a given key size.

This should be clarified.

"Hash-Tree renewal" would apply when hash algorithm exhibits 
collisions. However a complete reconstruction is needed and 
it is not clearly explained which data from the previous 
structure may re-used.

Appendix A contains an annex and it is not said whether it is 
informative or normative.
Nevertheless, the benefits of the inclusion of an 
EvidenceRecord as an unsigned attribute are not explained. 
The advantages and drawbacks of each case is not explained either.

Good point.


In my opinion, none of these documents is ready to be sent to 
the IESG and it does not make sense to send ERS alone. The 
usefulness of the whole work is very questionable. These 
documents are not mature and contain numerous typos.

We should not cloud the review of ERS by referring to typos in -ari and
The primary question is : " Is this work really needed ?". SC 
27 has issued ISO 18014-3: 
"Time-stamp services. Mechanisms producing linked tokens" 
that is very close.

It would be interesting that the authors position their 
documents towards the ISO standard. Duplication of work 
should be avoided. If there is no duplication, then this 
should be explained in an informative annex. If no acceptable 
explanations may be given, then this work should be stopped.

The ISO drafts have been discussed on the list and are referenced by this
document as a source for alternative timestamp formats.

P.S. I spent several hours to read the documents and to write 
this message, 
        but I do not have the time available to sustain long 
discussions on that topic.

Thanks for the review.


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