def reverseDwords(data):
    assert len(data) % 4 == 0, "Data length must be multiple of 4 but is %s" % len(data)
    l = []
    i = iter(data)
    for x in range(len(data) // 4):
        l1 = [i.next(), i.next(), i.next(), i.next()]
        l1.reverse() 
        l += l1
    return "".join(l)

def foo(s):
        # assert ...
	result = ''
	for step in xrange(len(s)/4):
		result = ''.join((result, s[4*step:4*(step+1)][::-1]))
	return result

def reverse_with_struct(data):
    assert len(data) % 4 == 0, "Data length must be multiple of 4 but is %s" % len(data)
    ndwords = len(data)//4
    return struct.pack(">%dI" % ndwords, *struct.unpack("<%dI" % ndwords, data))

def reverse(data):
    assert len(data) % 4 == 0, "Data length must be multiple of 4 but is %s" % len(data)
    b = bytearray(data)
    for x in xrange(0, len(data), 4):
        b[0+x], b[1+x], b[2+x], b[3+x] = b[3+x], b[2+x], b[1+x], b[0+x]
    return b

reverse(decrypt(reverse))