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Calculating checksums

The Calculator combines a configuration with a register implementation and exposes two methods: checksum and verify.

Create a calculator

from crc import Calculator, Crc8

calculator = Calculator(Crc8.CCITT)

from crc import Calculator, Configuration

config = Configuration(
    width=8,
    polynomial=0x07,
    init_value=0x00,
    final_xor_value=0x00,
    reverse_input=False,
    reverse_output=False,
)

calculator = Calculator(config)

More about configurations

Calculate a checksum

from crc import Calculator, Crc8

expected = 0xBC
data = bytes([0, 1, 2, 3, 4, 5])
calculator = Calculator(Crc8.CCITT)

assert expected == calculator.checksum(data)

from crc import Calculator, Crc8

expected = 0xBC
data = bytes([0, 1, 2, 3, 4, 5])
calculator = Calculator(Crc8.CCITT, optimized=True)  # (1)!

assert expected == calculator.checksum(data)
  1. Builds a lookup table upfront, which trades a bit of memory and setup time for significantly faster checksum calculation.

Verify a checksum

from crc import Calculator, Crc8

expected = 0xBC
data = bytes([0, 1, 2, 3, 4, 5])
calculator = Calculator(Crc8.CCITT)

assert calculator.verify(data, expected)

from crc import Calculator, Crc8

expected = 0xBC
data = bytes([0, 1, 2, 3, 4, 5])
calculator = Calculator(Crc8.CCITT, optimized=True)

assert calculator.verify(data, expected)

Standard vs. optimized

Standard Optimized
Created with Calculator(config) Calculator(config, optimized=True)
Register used Register TableBasedRegister
Setup cost None Builds a lookup table
Calculation speed Slower Significantly faster
Best for One off checksums Lots of data, repeated use

Create it once, reuse it

Creating an optimized calculator builds a lookup table. Create it once and reuse it, rather than creating a new one for every checksum.

Supported input types

Both checksum and verify accept a wide range of input types, see InputType for the formal definition.

from crc import Calculator, Crc8

expected = 0x20
data = 97
calculator = Calculator(Crc8.CCITT, optimized=True)

assert calculator.checksum(data) == expected

from crc import Calculator, Crc8

expected = 0xF4
data = b"123456789"
calculator = Calculator(Crc8.CCITT, optimized=True)

assert calculator.checksum(data) == expected

from crc import Calculator, Crc8

expected = 0xF4
data = bytearray(b"123456789")
calculator = Calculator(Crc8.CCITT, optimized=True)

assert calculator.checksum(data) == expected

from crc import Calculator, Crc8

expected = 0xF4
calculator = Calculator(Crc8.CCITT, optimized=True)

with open("afile.txt", "rb") as f:  # (1)!
    assert calculator.checksum(f) == expected
  1. Make sure to open the file in binary mode ("rb"), files are read in chunks, so even large files do not need to fit into memory.
import io

from crc import Calculator, Crc8

expected = 0xF4
data = io.BytesIO(b"123456789")
calculator = Calculator(Crc8.CCITT, optimized=True)

assert calculator.checksum(data) == expected

from crc import Calculator, Crc8

expected = 0xF4
calculator = Calculator(Crc8.CCITT, optimized=True)
data = (data for data in [b"12", b"34", b"56", b"78", b"9"])

assert calculator.checksum(data) == expected

from crc import Calculator, Crc8


class ByteConvertible:
    def __init__(self, data):
        self._data = data

    def __bytes__(self):
        return self._data.encode("utf-8")


expected = 0xF4
calculator = Calculator(Crc8.CCITT, optimized=True)
data = ByteConvertible("123456789")

assert calculator.checksum(bytes(data)) == expected

Next steps