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

[Carl Wallace <[e-mail]>]

Comments inline... 

> From: Denis Pinkas [mailto:[e-mail]] 
> 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
call.

> 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
> 
> and
> 
>    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
draft.
 
> 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 
> ArchiveTimeStampSequence. 

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
suggestions.  
 
> 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.

> 
> CONCLUSION
> 
> 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
-validate.
 
> 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.

> 
> Denis
>