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

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

$ pip install bchlib

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

$ pip install .

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.

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')