[UniSoft]

#define EXP7(x)  ((x) * (x) * (x) * (x) * (x) * (x) * (x))

uint64_t res = 0x90de757572b51cd3ULL;
uint64_t key = 0;
uint64_t mask = 1ULL;

while (mask)
{
    if ((EXP7(key) ^ res) & mask)
    {
        key |= mask;
        if ((EXP7(key) ^ res) & mask)
            break;
    }
    mask <<= 1;
}

if (mask)
    printf("not found");
else
    printf("found: 0x%llX", key);