CRCmod

1.7 · active · verified Thu Apr 09

CRCmod is a Python module for calculating Cyclic Redundancy Checks (CRCs). It provides functionality to generate custom CRC algorithms and access a wide range of predefined CRC algorithms, such as CRC-CCITT and CRC-32. The current version is 1.7. The library has a slow release cadence, with updates primarily focused on bug fixes and stability.

Warnings

gotcha Input data to CRC functions (e.g., `update`, `mkCrcFun`) MUST be bytes (`bytes`), not strings (`str`). Passing a Python string will result in a `TypeError`.
Fix: Always encode strings to bytes before passing them to crcmod functions, e.g., `my_string.encode('utf-8')`.
gotcha There are different ways to use crcmod: `crcmod.predefined.Crc` (object-oriented for standard CRCs), `crcmod.mkCrcFun` (functional for custom CRCs), and `crcmod.predefined.mkCrcFun` (functional for standard CRCs). Confusing these can lead to errors.
Fix: Use `crcmod.predefined.Crc('crc-name')` for an object to update incrementally with a standard CRC. Use `crcmod.mkCrcFun(...)` or `crcmod.predefined.mkCrcFun('crc-name')` if you need a simple function that calculates the CRC of an entire byte string in one call.

Install

pip install crcmod Install stable version

Imports

crcmod
```
import crcmod
```
Crc
```
from crcmod.predefined import Crc
```
While `crcmod.Crc` exists, `crcmod.predefined.Crc` is commonly used for standard CRC algorithms.
mkCrcFun
```
from crcmod import mkCrcFun
```
`mkCrcFun` is a function that *returns* a CRC calculation function, it's not a method to be called directly on the module.

Quickstart

This quickstart demonstrates how to calculate CRCs using both predefined algorithms and custom parameters. It shows both the object-oriented approach (`Crc` objects) and the functional approach (`mkCrcFun` functions) for flexibility.

import crcmod
import crcmod.predefined

# 1. Using a predefined CRC algorithm (object-oriented approach)
# Get a CRC-16-CCITT-FALSE calculator
crc16_ccitt = crcmod.predefined.Crc('crc-ccitt-false')
data_bytes = b'Hello, world!'
crc16_ccitt.update(data_bytes)
print(f"CRC-16-CCITT-FALSE for '{data_bytes.decode()}': {hex(crc16_ccitt.crcValue)}")

# You can reset and reuse the object
crc16_ccitt.reset()
crc16_ccitt.update(b'Another test')
print(f"CRC-16-CCITT-FALSE for 'Another test': {hex(crc16_ccitt.crcValue)}")

# 2. Creating a custom CRC function (functional approach)
# Example: CRC-32 (Ethernet) - polynomial 0x104C11DB7, initial 0xFFFFFFFF, xorOut 0xFFFFFFFF, revIn/Out True
crc32_func = crcmod.mkCrcFun(0x104C11DB7, initCrc=0xFFFFFFFF, xorOut=0xFFFFFFFF, rev=True)
data_bytes_32 = b'123456789'
result_32 = crc32_func(data_bytes_32)
print(f"CRC-32 for '{data_bytes_32.decode()}': {hex(result_32)}")

# 3. Using a predefined CRC algorithm (functional approach)
crc32_predef_func = crcmod.predefined.mkCrcFun('crc-32')
result_predef_32 = crc32_predef_func(b'123456789')
print(f"CRC-32 (predefined func) for '123456789': {hex(result_predef_32)}")

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