[Libre-soc-bugs] [Bug 965] implement chacha20

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=965

--- Comment #1 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
getting very interesting.  svindex is successful in doing the inner rounds,
svstep for the round inner loop, CTR mode for the outer.

there is however an opportunity to reorder the access to elements such that
the parallelism originally intended for chacha20 by bernstein is possible,
and it involves 3D REMAP.

an attempt to only deploy 2D REMAP was not successful, due to the fact that
the Indices are set up to cover both round-groups (the straight group of
16 followed by the rotated group):

       CHACHA_QUARTER_ROUND(w[0], w[4], w[8], w[12]);
       CHACHA_QUARTER_ROUND(w[1], w[5], w[9], w[13]);
       CHACHA_QUARTER_ROUND(w[2], w[6], w[10], w[14]);
       CHACHA_QUARTER_ROUND(w[3], w[7], w[11], w[15]);

       CHACHA_QUARTER_ROUND(w[0], w[5], w[10], w[15]);
       CHACHA_QUARTER_ROUND(w[1], w[6], w[11], w[12]);
       CHACHA_QUARTER_ROUND(w[2], w[7], w[8], w[13]);
       CHACHA_QUARTER_ROUND(w[3], w[4], w[9], w[14]);

the ordering needed was initially believed to be 2D: cycling through
by row (y) before moving to the next column (x)

unfortunately it is necessary to stop half-way down the rows (y=0-3)
before moving on to the next column (x+=1), then after all columns
are done repeat the process with the 2nd group (y=4-7)

this is perfectly possible but requires a 3D version of svindex
and svshape2, which is too much.

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