divisions in the HAETAE reference code

[D. J. Bernstein]

The function

int32_t fix_round(int32_t num) {
num += (num >> 31) & (-LN + 1);
num +=
LN /
2; // total \floor(LN / 2) if positive, -\floor(LN / 2) + 1 if negative
return num / LN;
}

in the HAETAE reference code is called on secret inputs and can end up
using division instructions (see, e.g., https://godbolt.org/z/rEEcn89x6
saying "idiv"), presumably allowing timing attacks on some platforms.
For the HAETAE integration into SUPERCOP, I'm changing this to

int32_t fix_round(int32_t num) {
return (num + LNHALF) >> LNBITS;
}

with

#define LNHALF 4096
#define LNBITS 13

in params.h. The C standard doesn't define the behavior of right shifts
on negative integers, but SUPERCOP uses the -fwrapv option for gcc and
clang, and this option guarantees twos-complement right shifts.

---D. J. Bernstein

[Georg Land]

Thanks Dan, 

we have fixed this internally. Soon, there will be an updated code release that fixes several issues that have been brought up in the past months and brings some additional improvements.

Best,

Georg