ietf-smime
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Re: Comment on ESS and Privacy Marks

1998-02-16 14:12:04
Hi there,

Type UTF8String has UNIVERSAL 12 as its tag. X.680 states 
that "In the value notation all BMPString values are valid 
UniversalString and UTF8String values", and notes that the 
notation for defining individual character values is the 
same for these types. 

X.680 also states that "UTF8String is synonymous with 
UniversalString at the abstract level and can be used 
wherever UniversalString is used (subject to rules requiring 
distinct tags) but has a different tag and is a distinct type.

NOTE - The encoding is different from that of UniversalString, 
and for most text will be less verbose.

The UniversalString and UTF8String types carries carry any 
character from ISO/IEC 10646-1. The set of characters in ISO/IEC 
10646-1 is generally too large for meaningful conformance to be 
required, and should normally be subsetted to a combination of 
the standard collections of characters in Annex A of ISO/IEC 10646-1.

Hope this helps. There are other places in X.680 where it is
recommended that a hole be left in the abstract syntax of a 
specification which allows an implementation to specify the
set of characters in this type that are supported in that
implementation. This technique allows the protocol to be
completely specified, but allows each implementation to 
avoid having to be able to receive and properly process the
entire 10646 set of characters, which is huge.

This technique, however, involves use of the ABSTRACT-SYNTAX
class, which I've never personally used. However, in a recent
lecture by the ASN.1 Rapporteur, it was explained in some detail.
But, I don't believe that such an approach would be well understood
enough by enough implementors to be of use to SMIME. And I doubt
seriously that there are more than a handful of tools out there
that properly handle this class definition. This is an ISO 
technique, which involves pics and performa statements and the
infrastructure to manage these things, so I doubt this would be 
of any use for our purposes. 

Phil

Blake Ramsdell wrote:

On Friday, February 13, 1998 3:30 PM, jsp(_at_)jgvandyke(_dot_)com
[SMTP:jsp(_at_)jgvandyke(_dot_)com] wrote:
   utf8String   OCTET STRING -- MUST contain UTF-8-encoded characters

My understanding is that 1997 ASN.1 is going to include UTF-8.  Is there
a universal tag that we can use from any draft 1997 ASN.1 documents?  We
can employ the same hack we are currently using for BMP STRING --
defining a universal tag in an ASN.1 module.  I am concerned about the
backwards compatibility of saying to future clients "ESSPrivacyMark
SHOULD accept utf8String encoded as an OCTET STRING as well as a UTF8
STRING (or whatever the universal tag ends up being)".

Blake
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