Steganography program - converting python 2 to 3, syntax error in: base64.b64decode("".join(chars)) - ascii

Question

I have problem with the syntax in the last part of steg program. I tried to convert python 2 version (of the working code) to python 3, and this is the last part of it:
flag = base64.b64decode("".join(chars)) <- error
print(flag)
The program 1. encrypts the message in the Last Significiant Bits of the image as saves it as a new image. Then 2.decrypts the message, which is stored in "flag", and prints it.
* can the error be caused by the wrong type of input?:
message = input("Your message: ")
BELOW: UNHIDING PROGRAM
#coding: utf-8
import base64
from PIL import Image
image = Image.open("after.png")
extracted = ''
pixels = image.load()
#Iterating in 1st row
for x in range(0,image.width):
r,g,b = pixels[x,0]
# Storing LSB of each color
extracted += bin(r)[-1]
extracted += bin(g)[-1]
extracted += bin(b)[-1]
chars = []
for i in range(len(extracted)/8):
byte = extracted[i*8:(i+1)*8]
chars.append(chr(int(''.join([str(bit) for bit in byte]), 2)))
flag = base64.b64decode(''.join(chars))
print flag
BELOW: HIDING PROGRAM:
import bitarray
import base64
from PIL import Image
with Image.open('before.png') as im:
pixels=im.load()
message = input("Your message: ")
encoded_message = base64.b64encode(message.encode('utf-8'))
#Convert the message into an array of bits
ba = bitarray.bitarray()
ba.frombytes(encoded_message)
bit_array = [int(i) for i in ba]
#Duplicate the original picture
im = Image.open("before.png")
im.save("after.png")
im = Image.open("after.png")
width, height = im.size
pixels = im.load()
#Hide message in the first row
i = 0
for x in range(0,width):
r,g,b = pixels[x,0]
#print("[+] Pixel : [%d,%d]"%(x,0))
#print("[+] \tBefore : (%d,%d,%d)"%(r,g,b))
#Default values in case no bit has to be modified
new_bit_red_pixel = 255
new_bit_green_pixel = 255
new_bit_blue_pixel = 255
if i<len(bit_array):
#Red pixel
r_bit = bin(r)
r_last_bit = int(r_bit[-1])
r_new_last_bit = r_last_bit & bit_array[i]
new_bit_red_pixel = int(r_bit[:-1]+str(r_new_last_bit),2)
i += 1
if i<len(bit_array):
#Green pixel
g_bit = bin(g)
g_last_bit = int(g_bit[-1])
g_new_last_bit = g_last_bit & bit_array[i]
new_bit_green_pixel = int(g_bit[:-1]+str(g_new_last_bit),2)
i += 1
if i<len(bit_array):
#Blue pixel
b_bit = bin(b)
b_last_bit = int(b_bit[-1])
b_new_last_bit = b_last_bit & bit_array[i]
new_bit_blue_pixel = int(b_bit[:-1]+str(b_new_last_bit),2)
i += 1
pixels[x,0] = (new_bit_red_pixel,new_bit_green_pixel,new_bit_blue_pixel)
#print("[+] \tAfter: (%d,%d,%d)"%(new_bit_red_pixel,new_bit_green_pixel,new_bit_blue_pixel))
im.save('after.png')
error
ValueError: string argument should contain only ASCII characters

Answer 1

help for base64.b64decode says:
b64decode(s, altchars=None, validate=False)
Decode the Base64 encoded bytes-like object or ASCII string s.
...
Considering that in Python 2 there were "normal" strs and unicode-strs (u-prefixed), I suggest taking closer look at what produce "".join(chars). Does it contain solely ASCII characters?
I suggest adding:
print("Codes:",[ord(c) for c in chars])
directly before:
flag = base64.b64decode("".join(chars))
If there will be number >127 inside codes, that mean it might not work as it is fit only for pure ASCII strs.