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Re: Trivial checksums for BASE-64

1991-11-15 02:07:50
Your message dated: Thu, 14 Nov 91 17:54:12 EST

I'm unclear as to why a checksum is so problematic across ASCII-EBCDIC
gateways.  You just have to know that the checksum is to be computed
using ASCII, not EBCDIC, codes.  This is what Kermit implementations on
IBM mainframes do all the time.  Works fine.

It is important that the ASCII-EBCDIC translate tables be correct.
With something like BASE64, where the character set is restricted to
the invariant subset, it is not a problem.  I don't recall offhand if
quoted-printable restricts itself to the invariant subset.  If not,
that could be a problem when more than one EBCDIC host is involved.  In
such a case a checksum would tell you when something did not translate
properly.  This could be important for some kinds of text, such as
source code.

There is no such thing as a 'correct ASCII-EBCDIC table'. Even in Finland there
are several (atleast four, propably more) different tables in use.  Also many
tables are not reversible (ASCII->EBCDIC->ASCII change the text file). The 
behind selecting the characters to base64 coding is that it contains the 
subset that is reversible in ASCII<>EBCDIC tables.

This problem with converions makes checksumming anything that isn't encoded
not very useable. 

Also checksum computed in ASCII isn't very much use if your user-agent
is using EBCDIC (or any other charcater set than ASCII).

Pekka Kyt|laakso
netmgr(_at_)venus(_dot_)csc(_dot_)fi      Centre for Scientific Computing
netmgr(_at_)csc(_dot_)fi            PL 40   SF-02101 Espoo FINLAND
Phone: +358 0 4571       Telefax: + 358 0 4572302

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