Re: [ltans] I-D ACTION:draft-ietf-ltans-xmlers-07.txt

["Denis Pinkas"<[e-mail]>]

Tobias,

See responses in line.

Denis
----- Message reçu ----- 
De : ltans-bounces 
À : ltans 
Date : 2010-09-13, 19:38:14
Sujet : Re: [ltans] I-D ACTION:draft-ietf-ltans-xmlers-07.txt


Hi Denis, 
yes the response is a bit late, but as we still have IETF LC ahead, not too late.  
Based on Carl's decision we may discuss and resolve this here while the draft proceeds to IESG and IETF LC. 
Answers inline.

Kind regards, Tobias


On 09/13/2010 04:48 PM, Denis Pinkas wrote: 
This is a late response.
Comments on draft-ietf-ltans-xmlers-07  
  
1) The abstract from this draft states:  
  
   In many scenarios, users must be able to demonstrate the (time)   
   existence, integrity and validity of data including signed data for   
   long or undetermined period of time. This document specifies XML   
   syntax and processing rules for creating evidence for long-term non-  
   repudiation of existence of data.   
  
The abstract from RFC 4998, which is very similar, states:  
  
   In many scenarios, users must be able prove the existence and  
   integrity of data, including digitally signed data, in a common and  
   reproducible way over a long and possibly undetermined period of  
   time.  This document specifies the syntax and processing of an  
   Evidence Record, a structure designed to support long-term non-  
   repudiation of existence of data.  
  
Then an additional sentence in the draft states:  
  
ERS-XML incorporates alternative syntax and processing rules to ASN.1 ERS syntax 
by using XML language.   
  
The basic question is why an addendum has not been added to RFC 4998 to include 
an XML syntax, equivalent to the ASN.1.   
  
Some explanations are given on page 7 are not convincing:  
  
   Due to the differences in XML processing rules and other   
   characteristics of XML language, XMLERS does not present a direct   
   transformation of ERS in ASN.1 syntax. The XMLERS syntax is based on   
   different processing rules as defined in [RFC4998] and it does not   
   support for example import of ASN.1 values in XML tags. Creating   
   Evidence Records in XML syntax must follow the steps as defined in   
   this draft. XMLERS is a standalone draft and is based on [RFC4998]   
   conceptually only.   
  
The text omits to highlight what the differences are.   
  
For example, ERS states:  
  
   The data (e.g. certificates, Certificate Revocation Lists (CRLs), or  
   Online Certificate Status Protocol (OCSP) responses) needed to verify  
   the timestamp MUST be preserved, and SHOULD be stored in the  
   timestamp itself unless this causes unnecessary duplication.  
  
While the draft states:  
  
   An ATS contains a Time-Stamp Token and optionally other useful   
   data for Time-Stamp validation, e.g. certificates, CRLs or OCSP   
   responses  
  
In one case CRLs should be part of the TST, while in the other case it should be 
external to the TST.  
  
Section 1.1 should be revised to mention the differences with a rational about them.  


In theory we could have added XMLERS to ERS, but XML puts different processing issues to the document, 
for example the ordering problem of the XML elements (to always produce identical hash values 
for the same data structure). XMLERS required further explanation regarding these processing rules and 
a well-thought XML structure, plus identifiers to be assigned by IANA for certain types. All these items are not 
related to ERS or its ASN.1 structure and would have made the RFC unnecessarily complex to read while 
bringing not much benefit. 

So a lot of reasons to put this in a separate draft, and not much harm from doing so.
[Denis] I am requesting to add further information to section 1.1. so that people may understand better 
why there has been no translation of the ASN.1 into XML.
  2) Page 43 states:  
  
   A generic solution for automatic interpretation of security suitability   
   policies in electronic form is not the subject of this specification.   
  
It is a copy and paste of the same text which existed in RFC 4998.  
  
However, this document omits to take into consideration RFC 5698 which has been issued 
in November 2009 (Data Structure for the Security Suitability of Cryptographic Algorithms 
- DSSC) and which respond to the problem raised.  
  
While it is normal that the concept was not supported in August 2007 when ERS was published, 
it is abnormal that the problems highlighted in RFC 5698 are not covered by this draft. In order to 
demonstrate that an algorithm was not weak, it is useful to include a data structure that lists the 
weak algorithms at a given point of time. This is what RFC 5698 allows.  
  
In any case, the text from section 7 (page 43) needs to be updated.  
  

Thank you, I missed that. I agree. We definitely should refer to RFC5698 as a method that may be used and add it to the informative references. 

[Denis] In fact , I am asking more that simply making a reference to RFC 5698, but to be able to include some structures 
defined in RFC 5698 into XMLERS. RFC 5698 should then be in the normative references. 

3) The most important concern is the following one : there is no rule in the document to explain 
how to really verify a given ERS-XML structure. Which root keys need to be used to verify one structure ? 
Which rules to apply to verify an ERS-XML data structure that is 30 years old ? The notion of multiple 
validation policies should be supported, since validation policies that will exist 30 years from now are still unknown.  
  
Having a data structure is fine, but it is insufficient to allow interoperability.

The data structure should be able to reference the validation policy used at each TST addition.
The current data structure is unable to support  the verification of an ERS-XML data structure that is 30 years old.  
  
Unless verification rules are explained in detail, this document will not achieve its goal and will not be practically usable.


Verification rules may be different for different legislation and countries and as you pointed out may even evolve over time. 
A problem true for all signatures (be it certs, timestamps, etc.) and not for XMLERS alone required to determine. 
The draft (as with ERS) is based on the fact that all reasonable validation data is stored and preserved 
(e.g. by also using XMLERS/ERS or integrating into the timestamp structures) and is validated based on the policy 
at the time of the validation in the future. So a different validation policy at each TST addition is not significant for the future validation. 
[Denis] We are in strong disagreement here.

Let us use a example. A TST is renewed every 10 years.

After ten years, I apply what a would call an "algorithm maintenance policy" and I reference the OID/URN 
from that policy before applying new TST, a certification path and CRLs captured at that time which demonstrate 
that the TSU certificate from the earlier TST has not been revoked.
In order to verify the structure, I first look at the OID/URN from the "algorithm maintenance policy" and if I find it acceptable, 
I apply the rules of the policy.
Ten years later, I choose another "algorithm maintenance policy" and I reference the OID/URN 
from that policy before applying new TST, a certification path and CRLs captured at that time which demonstrate 
that the TSU certificate from the earlier TST has not been revoked.
In order to verify the structure, I first look at the OID/URN from inner "algorithm maintenance policy" and if I find it acceptable, 
I apply the rules of the policy. Then I look at the OID/URN from outer "algorithm maintenance policy" and if I find it acceptable, 
I apply the rules of the policy.
There is not a global verification, but a verification that is different at each step.
The implication is to add to the current structure a new field to indicate the OID/URN of the "algorithm maintenance policy" 
that has been used for that step.
Note: if we add this information to XMLERS, it would make sense to do the same for ERS. 
So a different validation policy at each TST addition is not significant for the future validation. Even if it might be interesting, 
the key decision making unit will be the validation at the future point (e.g. in 30 years) and not what policies the system 
might have found appropriate at the time of timestamp renewals. 

On a personal note: I started an individual draft https://datatracker.ietf.org/doc/draft-ietf-ltans-validate/ about validation 
and necessary data. It is still very early stage work in progress (as I wanted to wait for XMLERS to finish first) 
and I find the procedures to be probably quite controversial. So the draft will not be continued in the LTANS WG 
but submitted only as an individual informational ID. (If you don't mind, I'd actually like to come back to you on that 
and ask you for your input on it once I have refined it in a few weeks time)
[Denis] Thank you for the information. Until we solve the previous point, it is likely that we will be in disagreement.
However, this illustrates the fact that without addtional information on how to use and verify the data structure, 
the current XMLERS draft is not usable in practice (i.e. there is no interoperability).
4) The text omits to consider how to deal with the risk of TSU key compromission and what to do if a given 
TSU key has been compromised.
As I understand it, TSU key compromission would lead to revocation of its certificate and loss of validity. 
The draft talks about that, i.e. a Time-stamp renewal.
 
However note, as such timestamp renewal would have to be done timely (and actually before TS looses its validity, 
there can be problems with cases of late discoveries of such TSU compromissions. With this possibility in mind 
it is recommended to use two redundant XMLERS with timestamps from different timestamp authorities. 
(see section 7 Security Considerations) 

As the detailed valuation of such events and legal requirements may vary by country, it seemed prudent to describe things 
up to the current level but not further.
[Denis] As the document does not sufficiently deal with the verification of the data structure, the case of TSU compromission 
is not sufficiently addressed.
In short, if when applying a new TST, the TSU certificate is marked as revoked, this is not a problem if the revocation reason 
is anything else than "compromise".

If the revocation reason is "compromise", then the text should say what to do (or what should have been done earlier).
Once again the data structure is described, but there is unsufficient guidance on how to use it and check it.
Denis
  Denis
----- Message reçu ----- 
De : ltans-bounces 
À : ltans 
Date : 2010-08-23, 21:08:54
Sujet : Re: [ltans] I-D ACTION:draft-ietf-ltans-xmlers-07.txt


Working group last call for this draft will begin today and end on
September 7th. Shortly after this draft completes IESG processing the
working group will close, as noted during the meeting last month.

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> Sent: Monday, August 23, 2010 2:00 PM
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> Subject: [ltans] I-D ACTION:draft-ietf-ltans-xmlers-07.txt
> 
> A New Internet-Draft is available from the on-line Internet-Drafts
> directories.
> This draft is a work item of the Long-Term Archive and Notary Services
> Working Group of the IETF.
> 
> Title : Extensible Markup Language Evidence Record
Syntax
> Author(s) : A. Blazic, S. Saljic, T. Gondrom
> Filename : draft-ietf-ltans-xmlers-07.txt
> Pages : 53
> Date : 2010-8-23
> 
> In many scenarios, users must be able to demonstrate the (time)
> existence, integrity and validity of data including signed data for
> long or undetermined period of time. This document specifies XML
> syntax and processing rules for creating evidence for long-term
non-
> repudiation of existence of data. ERS-XML incorporates alternative
> syntax and processing rules to ASN.1 ERS syntax by using XML
> language.
> 
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