curve25519-hacl64: simplify u64_eq_mask

Avoid signed right shift.

Z3 script showing equivalence:

>>> from z3 import *
>>>
>>> x = BitVec("x", 64)
>>> y = BitVec("y", 64)
>>>
>>> # Before
... x_ = ~(x ^ y)
>>> x_ &= x_ << 32
>>> x_ &= x_ << 16
>>> x_ &= x_ <<  8
>>> x_ &= x_ <<  4
>>> x_ &= x_ <<  2
>>> x_ &= x_ <<  1
>>> x_ >>= 63
>>>
>>> # After
... y_ = x ^ y
>>> y_ = y_ | -y_
>>> y_ = LShR(y_, 63) - 1
>>>
>>> prove(x_ == y_)
proved

Signed-off-by: Samuel Neves <sneves@dei.uc.pt>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

0 files changed, 0 insertions, 0 deletions