@classmethod
def double_round(cls, x):
    """Perform two rounds of ChaCha cipher"""
    for a, b, c, d in cls._round_mixup_box:
        xa = x[a]
        xb = x[b]
        xc = x[c]
        xd = x[d]
        xa = xa + xb & 4294967295
        xd = xd ^ xa
        xd = xd << 16 & 4294967295 | xd >> 16
        xc = xc + xd & 4294967295
        xb = xb ^ xc
        xb = xb << 12 & 4294967295 | xb >> 20
        xa = xa + xb & 4294967295
        xd = xd ^ xa
        xd = xd << 8 & 4294967295 | xd >> 24
        xc = xc + xd & 4294967295
        xb = xb ^ xc
        xb = xb << 7 & 4294967295 | xb >> 25
        x[a] = xa
        x[b] = xb
        x[c] = xc
        x[d] = xd
