subliminal_messaging.py

import math
import random
import sys
import time

nsymbols = int(sys.argv[1]) if len(sys.argv) == 2 else 26
nbits_per_symbol = math.ceil(math.log(nsymbols, 2))
ntotal_values = 2**nbits_per_symbol
ninvalid_values = ntotal_values - nsymbols

if ninvalid_values == 0:
    print('Cannot send any subliminal bits when # of symbols is a power of two')
    sys.exit(0)


def encode(symbol, extra_bit):
    extra_used = False
    if symbol < ninvalid_values:
        if extra_bit == 1:
            symbol += nsymbols
        extra_used = True
    return symbol, extra_used

def decode(symbol):
    extra_bit = None
    if symbol >= nsymbols:
        symbol -= nsymbols
        extra_bit = 1
    elif symbol < ninvalid_values:
        extra_bit = 0
    return symbol, extra_bit


subliminal = 'Hello world!'
sub_bits = []
for c in subliminal:
    sub_bits.extend((int(b) for b in bin(ord(c))[2:].rjust(8, '0')))

recv_sub_bits = []

sent_sum = recv_sum = 0
sent_count = sent_extra = 0
while sub_bits:
    symbol = random.randrange(nsymbols)

    encoded, extra_used = encode(symbol, sub_bits[0])
    sent_sum += symbol
    sent_count += 1
    if extra_used:
        sub_bits.pop(0)
        sent_extra += 1

    decoded, extra_bit = decode(encoded)
    recv_sum += decoded
    if extra_bit is not None:
        recv_sub_bits.append(extra_bit)

    print('Sent sum:', sent_sum)
    print('Recv sum:', recv_sum)

    if sent_extra > 0:
        print('Subliminal bit per %.3f symbols' % (sent_count / sent_extra))
    else:
        print('No subliminal bits sent yet')

    recv_sub = ''
    for i in range(0, len(recv_sub_bits), 8):
        if i + 7 >= len(recv_sub_bits):
            break
        b = 0
        for j in range(8):
            b = (b << 1) | recv_sub_bits[i + j]
        recv_sub += chr(b) 
    print('Subliminal message:', recv_sub)
    print('\033[4A', end='')

    time.sleep(0.02)


print()
print()
print()
print()