# crypto.py  27/09/2015  D.J.Whale
#
# Crypto engine for OpenThings, including crc calculation


ran = None


def init(pid, pip):
    """Initialise the crypto engine state variables"""
    global ran
    ran = (((pid&0xFF)<<8) ^ pip) & 0xFFFF # maintain U16


def cryptByte(data):
    """crypt a byte of data and update the crypto engine state variable"""
    global ran
    for i in range(5):
        if (ran&0x01) != 0: # bit0
            # bit0 set
            ran = ((ran>>1) ^ 62965) & 0xFFFF # maintain U16
        else:
            # bit0 clear
            ran = ran >> 1

    return (ran ^ data ^ 90) & 0xFF


def cryptPayload(payload, start, length):
    """Encrypt a range of bytes in place by modifying those payload bytes"""
    for i in range(start, start+length):
        payload[i] = cryptByte(payload[i])

# END