Re: [ltans] I-D Action:draft-ietf-ltans-dssc-12.txt

<ErnstG.Giessmann@t-systems.com> Wed, 30 September 2009 14:09 UTC

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Subject: Re: [ltans] I-D Action:draft-ietf-ltans-dssc-12.txt
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Hi all,
there are some clashes in the ASN.1 code of the draft that prevents to use it with e.g. RFC3280/RFC5280.

1.
The Draft defines
-- Extension ::= SEQUENCE {
--   extensionType           OBJECT IDENTIFIER,
--   extension               ANY DEFINED BY extensionType
--   }

which has a (presumably better defined) twin in RFC5280:
Extension  ::=  SEQUENCE  {
     extnID      OBJECT IDENTIFIER,
     critical    BOOLEAN DEFAULT FALSE,
     extnValue   OCTET STRING
                 -- contains the DER encoding of an ASN.1 value
                 -- corresponding to the extension type identified
                 -- by extnID
     }

I suggest to import the type Extension from PKIX1Explicit88. Please take into account that already since 1997 the ASN.1 types ANY/ANY DEFINED BY are deprecated (http://www.itu.int/ITU-T/studygroups/com07/changing-ASN.html).


2. 
The Draft defines
-- Validity ::= SEQUENCE {
--   start  [0] GeneralizedTime OPTIONAL,
--   end    [1] GeneralizedTime OPTIONAL
--   }

which clashes also with a twin in RFC3280/RFC5280:

Validity ::= SEQUENCE {
     notBefore      Time,
     notAfter       Time  }

It seems to me that this is based on a misunderstanding already in section 1.1 (Motivation)

   Cryptographic algorithms that are used in signatures shall be
   selected to resist such attacks during their period of use.  For
   signature keys included in public key certificates, it is the
   validity period of the certificate.  Cryptographic algorithms that
   are used for encryption shall resist during the time during which it
   is planned to keep the information confidential.

The validity period of a certificate is primarily *not* a period where resistance against attacks is assured but "is the time interval during which the CA warrants that it will maintain information about the status of the certificate". If the signatures are made in a authentication context, then the validity is almost the same as the usage time. But if the signatures are related to a non-repudiation context then the documented in the certificate validity time may be longer then predicted at time of certificate generation. If the algorithm becomes weaker then the certificate can be easily revoked, which reduces the "usage time" immediately, leaving the "warranty time" unchanged. If on the other side it turned out that the algorithm is harder than predicted, it is not necessary to revoke the certificate based on a still secure algorithm.

Just another thought: For a certificate the dates notBefore and notAfter make sense. But how we understand the notBefore date for algorithm resistance? A certificate may become valid later, but how a weak algorithm may become hard after a while?

I would suggest to remove the Validity type and replace it by a single notAfter date.

Regards,
Ernst.
--
	Ernst G Giessmann
	T-Systems Enterprise Services GmbH
	ICT Operations
	Security and Smart Card Solutions
	Goslarer Ufer 35, D-10589 Berlin
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> -----Ursprüngliche Nachricht-----
> Von: ltans-bounces@ietf.org [mailto:ltans-bounces@ietf.org] 
> Im Auftrag von Internet-Drafts@ietf.org
> Gesendet: Mittwoch, 30. September 2009 10:00
> An: i-d-announce@ietf.org
> Cc: ltans@ietf.org
> Betreff: [ltans] I-D Action:draft-ietf-ltans-dssc-12.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           : Data Structure for the Security 
> Suitability of Cryptographic Algorithms (DSSC)
> 	Author(s)       : T. Kunz, et al.
> 	Filename        : draft-ietf-ltans-dssc-12.txt
> 	Pages           : 48
> 	Date            : 2009-09-30
> 
> Since cryptographic algorithms can become weak over the 
> years, it is necessary to evaluate their security 
> suitability.  When signing or verifying data, or when 
> encrypting or decrypting data, these evaluations must be 
> considered.  This document specifies a data structure that 
> enables an automated analysis of the security suitability of 
> a given cryptographic algorithm at a given point of time 
> which may be in the past, at the present time or in the 
> future.Conventions used in this document

...
> A URL for this Internet-Draft is:
> http://www.ietf.org/internet-drafts/draft-ietf-ltans-dssc-12.txt
...