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From: nick@stca77.stc.oz (Nick Lochrin)
Newsgroups: comp.protocols.iso.dev-environ
Subject: Re: The innards of the FTAM implementation
Keywords: ftam modification, functions, data structures
Message-ID: <1276@stca77.stc.oz>
Date: 3 Mar 91 23:56:46 GMT
References: <47905.27b2d259@ccvax.ucd.ie> <1991Feb11.113524.222@solar.card.inpu.oz.au>
Organization: Alcatel-STC, Alexandria, AUSTRALIA
Lines: 34
Disclaimer: The views expressed herein are those of the author only

In article <1991Feb11.113524.222@solar.card.inpu.oz.au> brett@solar.card.inpu.oz.au (Brett Sealey) writes:
>clissman@ccvax.ucd.ie writes:
>
>>I want to get at this data and encrypt it before it arrives in level 6, and
>>then 
>>decrypt it as it goes from level 6 to level 7. Has anyone got clever ideas
>>on this one ? 
>>My gratitude for any help received,
>
>I would have thought that level 6, being the Presentation Layer, would
>be a logical place _within_ which do perform encryption & decryption?

I would agree that layer 6 is a reasonable place for this. I think this fits
into the concept of a presentation context,

where:
	presentation context = abstract syntax + transfer syntax

X.208 describes ASN.1 (abstract syntax notation number one) and
X.209 describes BER (basic encoding rules) transfer syntax.

also of use here is the concept of a Defined Context Set (DCS) which refers
to the set of currently active presentation contexts. Before any information
exchange can occur, both end systems must reach an agreement on which abstract
syntaxes are to be used. This happens via the P-CONNECT request primitive.

I think you could define a new transfer syntax which say used an encrpted
version of BER. IMHO this could also be applied to compression/decompression
of application data.
-- 
Nick Lochrin                 nick@stca77.stc.oz
Alcatel STC Australia        ...!uunet!stca77.stc.oz!nick
252-280 Botany Road,         nick%stca77.stc.oz@uunet.UU.NET
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