V3 secret keys

[Ben Laurie <[e-mail]>]

OK, I had to resort to reading the PGP 2 source to find out what was
going on here.

In essence its fairly simple, but is _definitely_ no explained by the I-D.

Firstly, v3 CFB does not use the IV in a standard way. Instead, what it
does instead is set the IV to all zeroes and then decrypt the IV and
throw away the result.

Secondly, as I think was correctly explained by someone here (but I
didn't get it, sorry), when "resynchronisation" occurs it means "set the
IV to the last 8 bytes of ciphertext".

Note that for any standard-sized key resynchronisation does _not_ occur,
so people who think they've implemented it from AC are in for a surprise
one day.

Obviously the I-D should be updated to reflect this (and clearly no-one
has ever implemented v3 keys from it).

Cheers,

Ben.

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