Encrypt a big file that does not fit in RAM with AES-GCM

Question

This code works for a file myfile which fits in RAM:

import Crypto.Random, Crypto.Cipher.AES   # pip install pycryptodome

nonce = Crypto.Random.new().read(16)
key = Crypto.Random.new().read(16)  # in reality, use a key derivation function, etc. ouf of topic here
cipher = Crypto.Cipher.AES.new(key, Crypto.Cipher.AES.MODE_GCM, nonce=nonce)

out = io.BytesIO()
with open('myfile', 'rb') as g:
    s = g.read()
ciphertext, tag = cipher.encrypt_and_digest(s)
out.write(nonce)
out.write(ciphertext)
out.write(tag)

But how to encrypt a 64 GB file using this technique?

Obviously, the g.read(...) should use a smaller buffer-size, e.g. 128 MB.

But then, how does it work for the crypto part? Should we keep a (ciphertext, tag) for each 128-MB chunk?

Or is it possible to have only one tag for the whole file?

Answer 1

As mentioned in @PresidentJamesK.Polk's comment, this seems to be the solution:

out.write(nonce)
while True:
    block = g.read(65536)
    if not block:
        break
    out.write(cipher.encrypt(block))
out.write(cipher.digest())  # 16-byte tag at the end of the file

The only problem is that, when reading back this file for decryption, stopping at the end minus 16 bytes is a bit annoying.

Or maybe one should do this:

out.write(nonce)
out.seek(16, 1)  # go forward of 16 bytes, placeholder for tag
while True:
   ...
   ...
out.seek(16)
out.write(cipher.digest())  # write the tag at offset #16 of the output file

?