ALPHABET = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789abcdefghijkmnpqrstuvwxyz!?#&$%+="
SIZES = {16: 11, 61: 41, 260: 175} # password chars -> bytes including CRC
def crc16(data):
crc = 0xFFFF
for byte in data:
crc ^= byte << 8
for _ in range(8):
crc = (((crc << 1) - 0x1021) if crc & 0x8000 else (crc << 1)) & 0xFFFF
return (~crc) & 0xFFFF
def encode(payload):
crc = crc16(payload)
raw = bytearray(payload + crc.to_bytes(2, "big"))
key = raw[-1]
raw[:-1] = bytes(b ^ key for b in raw[:-1])
bits = "".join(f"{b:08b}" for b in raw)
values = [int(bits[i:i + 6].ljust(6, "0"), 2) for i in range(0, len(bits), 6)]
checked = []
for i in range(0, len(values), 9):
group = values[i:i + 9]
checked.extend(group)
if len(group) == 9:
checked.append(sum(group) & 63)
return "".join(ALPHABET[(v + i) & 63] for i, v in enumerate(checked))
def decode(password):
chars = [ALPHABET.index(c) for c in password if not c.isspace()]
byte_count = SIZES[len(chars)]
values = [(v - i) & 63 for i, v in enumerate(chars)]
data6 = []
for i in range(0, len(values), 10):
group = values[i:i + 10]
body = group[:9]
if len(group) == 10 and group[9] != (sum(body) & 63):
raise ValueError(f"bad group {i // 10 + 1}")
data6.extend(body)
bits = "".join(f"{v:06b}" for v in data6)[:byte_count * 8]
raw = bytearray(int(bits[i:i + 8], 2) for i in range(0, len(bits), 8))
key = raw[-1]
raw[:-1] = bytes(b ^ key for b in raw[:-1])
payload, stored_crc = raw[:-2], int.from_bytes(raw[-2:], "big")
if crc16(payload) != stored_crc:
raise ValueError("bad CRC")
return bytes(payload)
