crc32itu

Package crc32itu implements the ITU I.363.5 CRC-32 algorithm (also known as AAL5 CRC). This algorithm was popularised by BZIP2 and is also used in ATM transmissions.

This package is particularly useful for interoperability with PHP, as it produces the same output as PHP's hash('crc32', ...) function. Go's standard library hash/crc32 uses a different polynomial and bit ordering, making it incompatible with PHP's crc32 hash.

The polynomial used is 0x04C11DB7, with reversed bit shifts compared to the IEEE polynomial.

The algorithm is the same as that in POSIX 1003.2-1992 cksum, except cksum appends the message length to the CRC at the end.

A note on CRC32 variants

"CRC32" is not a single algorithm—there are many variants that produce different results despite sharing the same name. When integrating systems or verifying checksums, it's important to know which variant you need:

Variant	Polynomial	Used by
CRC-32/ISO-HDLC (IEEE)	0xEDB88320	Go's hash/crc32, PNG, ZIP, gzip, Ethernet
CRC-32/BZIP2 (ITU)	0x04C11DB7	This package, PHP's hash('crc32', ...), BZIP2, AAL5
CRC-32C (Castagnoli)	0x1EDC6F41	iSCSI, SCTP, ext4

If you're getting mismatched checksums between systems, you're likely using the wrong CRC32 variant. Common pitfalls:

• Go's hash/crc32 (IEEE) does not match PHP's hash('crc32', ...)
• PHP's hash('crc32b', ...) uses IEEE and does match Go's hash/crc32
• Python's binascii.crc32() and zlib.crc32() use IEEE

Use this package when you need compatibility with PHP's hash('crc32', ...) or other systems using the ITU/BZIP2 variant.

Installation

go get github.com/KarpelesLab/crc32itu

Usage

Simple checksum

package main

import (
	"fmt"
	"github.com/KarpelesLab/crc32itu"
)

func main() {
	data := []byte("Hello, World!")
	checksum := crc32itu.Checksum(data)
	fmt.Printf("CRC-32 ITU: %08x\n", checksum)
}

Incremental updates

package main

import (
	"fmt"
	"github.com/KarpelesLab/crc32itu"
)

func main() {
	crc := crc32itu.Checksum([]byte("Hello, "))
	crc = crc32itu.Update(crc, []byte("World!"))
	fmt.Printf("CRC-32 ITU: %08x\n", crc)
}

Using hash.Hash32 interface

package main

import (
	"fmt"
	"github.com/KarpelesLab/crc32itu"
)

func main() {
	h := crc32itu.New()
	h.Write([]byte("Hello, "))
	h.Write([]byte("World!"))
	fmt.Printf("CRC-32 ITU: %08x\n", h.Sum32())
}

API

• Checksum(data []byte) uint32 - Returns the ITU I.363.5 checksum of data.
• Update(crc uint32, data []byte) uint32 - Returns the result of adding bytes to an existing CRC.
• New() hash.Hash32 - Returns a new hash.Hash32 for computing CRC-32 ITU checksums.
• Size - The size of a CRC-32 checksum in bytes (4).

License

This code is extracted from the Go standard library (compress/bzip2) and is licensed under a BSD-style license. See the LICENSE file for details.