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std/crypto: faster (mod 2^255-19) square root computation

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251 squarings, 250 multiplications -> 251 squarings, 11 multiplications
This commit is contained in:
Frank Denis 2020-10-06 14:02:06 +02:00 committed by Andrew Kelley
parent dd4771a5d2
commit fb63a2cfae
1 changed files with 8 additions and 6 deletions
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14
lib/std/crypto/25519/field.zig
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@ -307,12 +307,14 @@ pub const Fe = struct {
}
pub fn pow2523(a: Fe) Fe {
var c = a;
var i: usize = 0;
while (i < 249) : (i += 1) {
c = c.sq().mul(a);
}
return c.sq().sq().mul(a);
var t0 = a.mul(a.sq());
var t1 = t0.mul(t0.sqn(2)).sq().mul(a);
t0 = t1.sqn(5).mul(t1);
var t2 = t0.sqn(5).mul(t1);
t1 = t2.sqn(15).mul(t2);
t2 = t1.sqn(30).mul(t1);
t1 = t2.sqn(60).mul(t2);
return t1.sqn(120).mul(t1).sqn(10).mul(t0).sqn(2).mul(a);
}
pub fn abs(a: Fe) Fe {