Encryption cracking challenge

[dila]

-x is the same as ~x + 1

In this case the code is extracting the most-significant bit (MSB) from a and then creating a 64-bit mask from it.

If MSB(a) == 0, the result is 0x0000000000000000
Otherwise, if MSB(a) == 1, the result is 0xFFFFFFFFFFFFFFFF

[chants]

Some compilers might warn or even generate an error over that. But as said it's just a 2s compliment and the sign but is merely interpreted differently as a cast occurred to signed and back to unsigned. This challenge is interesting need to look for a crypto weakness in the inner function perhaps. Brute force won't work with 2^64...