m25_protocol.py

#!/usr/bin/env python3
"""
M25 Protocol Layer - The boring but essential bits.

CRC-16, byte stuffing, frame markers. Nothing fancy, but it works.
"""

HEADER_MARKER = 0xEF          # All packets start with this. How original.
HEADER_SIZE = 3
MAX_FRAME_LENGTH = 293
MIN_PACKET_LENGTH = 37
IV_SIZE = 16
CRC_SIZE = 2

# Default USB Key: "Alber_M25_emotio" - yes, it's literally the product name
# This is used for the USB/wired connection. Bluetooth uses per-device keys.
DEFAULT_USB_KEY = bytes([
    65, 108, 98, 101, 114, 95, 77, 50, 53, 95, 101, 109, 111, 116, 105, 111
])

# CRC-16 Lookup Table
CRC_TABLE = [
    0, 49345, 49537, 320, 49921, 960, 640, 49729, 50689, 1728, 1920, 51009, 1280, 50625, 50305, 1088,
    52225, 3264, 3456, 52545, 3840, 53185, 52865, 3648, 2560, 51905, 52097, 2880, 51457, 2496, 2176, 51265,
    55297, 6336, 6528, 55617, 6912, 56257, 55937, 6720, 7680, 57025, 57217, 8000, 56577, 7616, 7296, 56385,
    5120, 54465, 54657, 5440, 55041, 6080, 5760, 54849, 53761, 4800, 4992, 54081, 4352, 53697, 53377, 4160,
    61441, 12480, 12672, 61761, 13056, 62401, 62081, 12864, 13824, 63169, 63361, 14144, 62721, 13760, 13440, 62529,
    15360, 64705, 64897, 15680, 65281, 16320, 16000, 65089, 64001, 15040, 15232, 64321, 14592, 63937, 63617, 14400,
    10240, 59585, 59777, 10560, 60161, 11200, 10880, 59969, 60929, 11968, 12160, 61249, 11520, 60865, 60545, 11328,
    58369, 9408, 9600, 58689, 9984, 59329, 59009, 9792, 8704, 58049, 58241, 9024, 57601, 8640, 8320, 57409,
    40961, 24768, 24960, 41281, 25344, 41921, 41601, 25152, 26112, 42689, 42881, 26432, 42241, 26048, 25728, 42049,
    27648, 44225, 44417, 27968, 44801, 28608, 28288, 44609, 43521, 27328, 27520, 43841, 26880, 43457, 43137, 26688,
    30720, 47297, 47489, 31040, 47873, 31680, 31360, 47681, 48641, 32448, 32640, 48961, 32000, 48577, 48257, 31808,
    46081, 29888, 30080, 46401, 30464, 47041, 46721, 30272, 29184, 45761, 45953, 29504, 45313, 29120, 28800, 45121,
    20480, 37057, 37249, 20800, 37633, 21440, 21120, 37441, 38401, 22208, 22400, 38721, 21760, 38337, 38017, 21568,
    39937, 23744, 23936, 40257, 24320, 40897, 40577, 24128, 23040, 39617, 39809, 23360, 39169, 22976, 22656, 38977,
    34817, 18624, 18816, 35137, 19200, 35777, 35457, 19008, 19968, 36545, 36737, 20288, 36097, 19904, 19584, 35905,
    17408, 33985, 34177, 17728, 34561, 18368, 18048, 34369, 33281, 17088, 17280, 33601, 16640, 33217, 32897, 16448
]


def calculate_crc(data, length):
    """Calculate CRC-16 checksum."""
    crc = 0xFFFF
    for i in range(length):
        crc = (crc >> 8) ^ CRC_TABLE[(crc ^ data[i]) & 0xFF]
    return crc & 0xFFFF


def add_delimiters(buff):
    """Add byte stuffing (double 0xEF bytes except first)."""
    if len(buff) == 0:
        return buff
    output = bytearray([buff[0]])
    for byte in buff[1:]:
        output.append(byte)
        if byte == HEADER_MARKER:
            output.append(HEADER_MARKER)
    return bytes(output)


def remove_delimiters(buff, prot_max_length=MAX_FRAME_LENGTH):
    """Remove byte stuffing. Returns None on failure."""
    if len(buff) > prot_max_length * 2:
        return None

    output = bytearray(len(buff))
    pos = 0
    flag = True

    for byte in buff:
        if byte == HEADER_MARKER:
            if flag:
                output[pos] = byte
                pos += 1
                flag = False
            else:
                flag = True
        else:
            output[pos] = byte
            pos += 1
            flag = False

    return bytes(output[:pos])