python-bchlib

This is a python module for encoding and correcting data using BCH codes.

Requirements

For Windows, python3.5 or greater required.
For Linux and MacOS, python2.7 or python3.4 or greater required.

Installing the latest release:

$ pip install bchlib

Installing from source:

Make sure you have python-dev setup. For Windows, this means you need Visual Studio 2015.

$ pip install .

Module Documentation

bchlib.BCH( polynomial, t[, reverse] ) → bch

Constructor creates a BCH object with given polynomial and t bit strength, reverse is an optional boolean that flips the bit order of data. The Galois field order is automatically determined from the polynomial.

bch.encode( data[, ecc] ) → ecc

Encodes data with an optional starting ecc and returns an ecc.

bch.decode( data, ecc ) → ( bitflips, data, ecc )

Corrects data using ecc and returns a tuple.

bch.decode_inplace( data, ecc ) → bitflips

Corrects data using ecc in place, returning the number of bitflips.

bch.decode_syndromes( data, syndromes ) → ( bitflips, data )

Corrects data using a sequence of syndromes, of t*2 elements, returning a tuple.

bch.compute_even_syndromes( syndromes ) → syndromes

Computes even syndromes from odd ones. Takes and returns a sequence of t*2 elements.

bch.ecc_bytes

A readonly field; the number of bytes an ecc takes up.

bch.ecc_bits

A readonly field; the number of bits an ecc takes up.

bch.m

A readonly field; the Galois field order.

bch.n

A readonly field; the maximum codeword size in bits.

bch.syndromes

A readonly field; a tuple of syndromes after performing a correct operation.

bch.t

A readonly field; the number bit errors that can be corrected.

Usage Example

import bchlib
import hashlib
import os
import random

# create a bch object
BCH_POLYNOMIAL = 8219
BCH_BITS = 16
bch = bchlib.BCH(BCH_POLYNOMIAL, BCH_BITS)

# random data
data = bytearray(os.urandom(512))

# encode and make a "packet"
ecc = bch.encode(data)
packet = data + ecc

# print hash of packet
sha1_initial = hashlib.sha1(packet)
print('sha1: %s' % (sha1_initial.hexdigest(),))

def bitflip(packet):
    byte_num = random.randint(0, len(packet) - 1)
    bit_num = random.randint(0, 7)
    packet[byte_num] ^= (1 << bit_num)

# make BCH_BITS errors
for _ in range(BCH_BITS):
    bitflip(packet)

# print hash of packet
sha1_corrupt = hashlib.sha1(packet)
print('sha1: %s' % (sha1_corrupt.hexdigest(),))

# de-packetize
data, ecc = packet[:-bch.ecc_bytes], packet[-bch.ecc_bytes:]

# correct
bitflips = bch.decode_inplace(data, ecc)
print('bitflips: %d' % (bitflips))

# packetize
packet = data + ecc

# print hash of packet
sha1_corrected = hashlib.sha1(packet)
print('sha1: %s' % (sha1_corrected.hexdigest(),))

if sha1_initial.digest() == sha1_corrected.digest():
    print('Corrected!')
else:
    print('Failed')