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Fix accumulated index comments in felem_inv for p521

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Comments in felem_inv refer to the logarithm with respect to in of the
element that has just been computed. The last two such annotations are
incorrect. By Fermat's last theorem, we hope to compute in^(p-2) in
GF(2^521-1), as such we expect the final index we reach to be 2^521-3.

CLA: Trivial

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19765)
This commit is contained in:
Rohan McLure 2022-11-25 14:42:12 +11:00 committed by Hugo Landau
parent d5e1fe9c04
commit 055d029610
1 changed files with 2 additions and 2 deletions
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4
crypto/ec/ecp_nistp521.c
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@ -843,9 +843,9 @@ static void felem_inv(felem out, const felem in)
felem_reduce(ftmp3, tmp); /* 2^521 - 2^9 */
}
felem_mul(tmp, ftmp3, ftmp4);
felem_reduce(ftmp3, tmp); /* 2^512 - 2^2 */
felem_reduce(ftmp3, tmp); /* 2^521 - 2^2 */
felem_mul(tmp, ftmp3, in);
felem_reduce(out, tmp); /* 2^512 - 3 */
felem_reduce(out, tmp); /* 2^521 - 3 */
}
/* This is 2^521-1, expressed as an felem */