[PATCH] g10: Fix ECDH secret compressed/uncompressed format

[Werner Koch]

On Tue, 25 Oct 2016 09:14, arnaud.fontaine at ssi.gouv.fr said:

> My comment "Compressed format" is actually imprecise, it is the raw x coordinate
> (without any prefix), as it is the expected output shared secret of ECDH.

I see.  For EdDSA we extended the meaning of the flag octet like this:

     Flag  Description
     ----  -----------
     0x04  Standard flag for uncompressed format
     0x40  Native point format of the curve follows
     0x41  Only X coordinate follows.
     0x42  Only Y coordinate follows.

We only use 0x40 for EdDSA but the the new flag 0x41 is what I suggest
for your use.  We can prefix the x-coordinate in scdaemon with 0x41 so
that we do not run in ambiguities in other parts of the code.


Shalom-Salam,

   Werner

-- 
Die Gedanken sind frei.  Ausnahmen regelt ein Bundesgesetz.
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