pycryptodome : MAC Check Failed (using decrypt_and_verify) - python-3.x

Question

I am working on an encryption program with Pycryptodome in Python 3.6 I am trying to encrypt a file and then decrypt it and verify the MAC tag. When I get to verify it, an error is thrown
import os
from Crypto.Cipher import AES
bib Cryptodome imported
aad = b'any thing'
nonce = b'\xde\xe2G\xca\xe8Lq9\xeb\x8b\x84\xe7'
key = b'\xde\xe9\xad\xe9\x14_\x07\x1aq7C\\\xd7\x9c\xae\xfcJ\x1c%\xec\xe6C\xbe\xf0eO\xfaJ1\x08\x0c\xae'
I set the nonce and key as constant just to start things first. then i will use nonce = get_random_bytes(12) unique for every file.
def encrypt(filename):
chunksize = 64 * 1024
outputFile = "(encrypted)" + filename
filesize = str(os.path.getsize(filename))
cipher = AES.new(key, AES.MODE_GCM, nonce)
cipher.update(aad)
with open(filename, 'rb') as infile:
with open(outputFile, 'wb') as outfile:
outfile.write(filesize.encode('utf-8'))
while True:
chunk = infile.read(chunksize)
if len(chunk) == 0:
break
elif len(chunk) % 16 != 0:
chunk += ' '.encode('utf-8') * (16 - (len(chunk) % 16))
ciphertext,sender_tag = cipher.encrypt_and_digest(chunk)
print (sender_tag)
outfile.write(ciphertext)
the decryption part using decrypt_and_verify so no need to worry about which come first the decryption or verification
def decrypt(filename,received_tag):
chunksize = 64 * 1024
outputFile = "(clear)"+ filename
cipher = AES.new(key, AES.MODE_GCM, nonce)
cipher.update(aad)
with open(filename, 'rb') as infile:
with open(outputFile, 'wb') as outfile:
while True:
chunk = infile.read(chunksize)
if len(chunk) == 0:
break
clearmessage = cipher.decrypt_and_verify(chunk, b'received_tag')
outfile.write(clearmessage)
outfile.truncate()
def Main():
choice = input("Would you like to (E)ncrypt or (D)ecrypt?: ")
if choice == 'E':
filename = input("File to encrypt: ")
#password = input("Password: ")
encrypt(filename)
print ("Done.")
elif choice == 'D':
filename = input("File to decrypt: ")
#password = input("Password: ")
received_tag = input("enter received tag:")
decrypt(filename,received_tag)
print ("Done.")
else:
print ("No Option selected, closing...")
if __name__ == '__main__':
Main()
And this is the error:
raise ValueError("MAC check failed")
ValueError: MAC check failed
I don't know where i messed up . by the way i get a tag similar to b'\x1c\xd1\xd8\x1a6\x07\xf3G\x8c_s\x94"*(b'
Update :
i did correct a mistake in the code sender_tag,ciphertext = cipher.encrypt_and_digest(chunk) insted of ciphertext,sender_tag = cipher.encrypt_and_digest(chunk) but still the problem persists

Answer 1

Quite a few of things look dodgy:
You write the file size, but then you don't read it back. Actually, I don't think you should write it at all.
You pad the chunk to the 16 bytes boundary before encrypting it, but GCM does not require that.
None should not be fixed, but random per encryption.
Nonce and MAC tag are typically stored along with the ciphertext (which therefore becomes longer than the plaintext).
By writing chunks independently one from the other you are allowing an attacker to remove, duplicate and reorder chunks in transit.