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On Mar 28, 2013, at 1:21 PM, ianG <[email protected]> wrote:
>
> Correct me if I'm wrong, but the parity bits in DES guard the key, which
> doesn't need correcting? And the block which does need correcting has no
> space for parity bits?
"Guard" is perhaps a bit strong. They're just parity bits.
In those days, people bought parity memory, and it was worth it. As Steve says,
hardware errors that would just happen were pretty common.
Now, there is a little more to it than that -- remember that when Lucifer
became DES, it was knocked down from a 64-bit key to a 56-bit key. When they
did that, they chose to knock one bit off of each octet (note that I'm saying
octet, not byte, because also in those days it was not presumed that "bytes"
had eight bits) rather than have 56 packed bits.
If you do it that way, using the orphaned bits as parity is a pretty reasonable
use for them.
>
> Layering was the "big idea" of the ISO 7 layer model. From memory this first
> started appearing in standards committees around 1984 or so? So likely it
> was developed as a concept in the decade before then -- late 1970s to early
> 1980s.
Earlier than that. But arguably, the full seven layers are still aspirational,
but the word "conceptual" was used for a long, long time. The bottom four
layers are pretty easy to know what goes where. But what makes a protocol be in
5, 6, or 7 is subject to debate.
Jon
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