Script that flips bits at random. You can spare first and last bits from the process.

bitrot.py

from os import path
from random import randint
from sys import argv, stderr


def get_file_path():
	if not path.isfile(argv[1]):
		raise FileNotFoundError

	return argv[1]


def get_bitrot_count():
	bitrot_count = int(argv[2]) if len(argv) >= 3 else 1
	if bitrot_count < 0:
		raise ValueError

	return bitrot_count


def get_bitrot_excluded_bits():
	# The "end" variable isn't the end of the range, but the number of trailing bits to exclude from the range.
	# E.g. if start = 3 and end = 2, the first 3 bits and last 2 bits will be excluded.
	start = int(argv[3]) if len(argv) >= 4 else 0
	end = int(argv[4]) if len(argv) == 5 else 0

	if start < 0 or end < 0:
		raise ValueError

	return start, end


def bitrot(file_path, bitrot_count, start, end):
	# Get the file size in bytes and convert it to bits
	file_size_bits = path.getsize(file_path) * 8

	# Create a set of bit_to_flip values to avoid flipping the same bit multiple times
	bits_flipped = set()

	with open(file_path, 'r+b') as f:
		for i in range(bitrot_count):
			if i >= file_size_bits - (start + end):
				print("No more bits to flip in this file.")
				break

			# Choose a random bit to flip within the allowed range
			bit_to_flip = randint(start, file_size_bits - end - 1)

			# If the bit has already been flipped, choose another bit
			while bit_to_flip in bits_flipped:
				bit_to_flip = randint(start, file_size_bits - end - 1)

			# Add the bit to the set of flipped bits
			bits_flipped.add(bit_to_flip)

			# Determine the byte and bit position within that byte
			byte_index = bit_to_flip // 8
			bit_index = bit_to_flip % 8

			# Read the byte at the byte_index
			f.seek(byte_index)
			byte = ord(f.read(1))

			# Flip the bit at bit_index
			byte ^= (0b10000000 >> bit_index)

			# Write the modified byte back to the file
			f.seek(byte_index)
			f.write(bytes([byte]))

			print(f"Flipped bit {bit_to_flip}.")


def main():
	if len(argv) not in range(2, 6):
		print("Usage: python bitrot.py file_path [bitrot_count [start [end]]]", file=stderr)
		return

	try:
		file_path = get_file_path()
	except FileNotFoundError:
		print(f"File does not exist.", file=stderr)
		return

	try:
		bitrot_count = get_bitrot_count()
		start, end = get_bitrot_excluded_bits()
	except ValueError:
		print("Bitrot count and excluded bits must be non-negative integers.", file=stderr)
		return

	bitrot(file_path, bitrot_count, start, end)


if __name__ == "__main__":
	main()