constants.py

UDP_MTU = 65535

# UDP channel

IPV4_LEN = 4
PORT_LEN = 2

MIN_PORT = 50000
MAX_PORT = 60000

CHAN_ID_SIZE = 2

MIN_CHAN_ID = 1
MAX_CHAN_ID = 2**(8 * CHAN_ID_SIZE) - 1

REPLAY_WINDOW_SIZE = 10

CHANNEL_TIMEOUT_SEC = 30

# sym encryption

SYM_KEY_LEN = 16

CTR_PREFIX_LEN = 4
CTR_MODE_PADDING = CTR_PREFIX_LEN

CHANNEL_CTR_START = 0

# link encryption

LINK_CTR_START = 0

NONCE_LEN = 16

MSG_TYPE_FLAG_LEN = 1

DATA_MSG_FLAG = b"\x01"
CHAN_INIT_MSG_FLAG = b"\x02"
CHAN_CONFIRM_MSG_FLAG = b"\x03"

GCM_MAC_LEN = 16

LINK_HEADER_LEN = CHAN_ID_SIZE + CTR_PREFIX_LEN + MSG_TYPE_FLAG_LEN

RESERVED_LEN = 0

assert RESERVED_LEN >= 0

# mix messages

MIX_COUNT = 3

# init message

GROUP_ELEMENT_LEN = 29

INIT_PAYLOAD_LEN = IPV4_LEN + PORT_LEN

INIT_OVERHEAD = GROUP_ELEMENT_LEN + MIX_COUNT * 2 * SYM_KEY_LEN + INIT_PAYLOAD_LEN

# data message

DATA_OVERHEAD = 0

# biggest UDP packet seen in the wild was 1370, which gets divided equally by 274

DATA_FRAG_PAYLOAD_SIZE = 274
INIT_FRAG_PAYLOAD_SIZE = DATA_FRAG_PAYLOAD_SIZE - (INIT_OVERHEAD - DATA_OVERHEAD)

assert INIT_OVERHEAD >= DATA_OVERHEAD