Example of PyCA's high level AESGCM interface.

pyca-aesgcm-example.py

#!/usr/bin/env python3
"""
pyca-aesgcm-example.py

Author: Jack Sullivan

Example of using PyCA cryptography's high level AESGCM interface
to encrypt and decrypt data. Also demonstrates how modifying ciphertext
data or the auth tag (MAC) invalidates decryption.

The high level AESGCM interface doesn't seem to separate out the ciphertext 
data from the auth tag, or bundle it with the IV.

https://cryptography.io/en/latest/hazmat/primitives/aead/#cryptography.hazmat.primitives.ciphers.aead.AESGCM

"""
import binascii
import os

from cryptography.exceptions import InvalidTag
from cryptography.hazmat.primitives.ciphers.aead import AESGCM


TRIGGER_INVALID_TAG = False


def main():

    # Show the plaintext.
    plaintext_string = "a"
    plaintext = plaintext_string.encode()
    print("\nPlain:  ", plaintext)
    display_data("Plain", plaintext)

    # Existing static key.
    static_key_string = "d1884987a6d4dd93da9c5e24f8cccb5935a65f8e4f078cf02df9cbc4d628369e"
    static_key = hex_string_to_bytes(static_key_string)
    display_data("S Key", static_key)

    # Generate key at runtime.
    keylen = 256
    generated_key = AESGCM.generate_key(bit_length=keylen)
    display_data("G Key", generated_key)

    # High level AES GCM instance.
    aesgcm = AESGCM(static_key)

    # Additional data to authenticate. Can be None if none was passed during encryption.
    # associated_data = b"authenticated but unencrypted data"
    associated_data = None

    # Generate IV. Never reuse an IV for multiple messages.
    iv_nonce = os.urandom(12)
    display_data("IV", iv_nonce)

    # Encrypt the plaintext.
    # 1 byte PT --> 29 byte CT = 12byte IV + 16byte data + 1byte tag
    ciphertext = aesgcm.encrypt(iv_nonce, plaintext, associated_data)
    display_data("Cipher", ciphertext)

    # Decompose ciphertext.
    ct_data = [x for x in ciphertext[0:16]]
    ct_tag = ciphertext[16:]
    display_data("CT data", bytes(ct_data))
    display_data("CT tag", bytes(ct_tag))

    # Decrypt the ciphertext.
    plaintext = aesgcm.decrypt(iv_nonce, ciphertext, associated_data)
    display_data("Plain", plaintext)
    print("Plain:  ", plaintext)


    # Fail at decrypting data with modified auth tag.
    if TRIGGER_INVALID_TAG:
        print("\n")
        bad_ct_tag = [x+1 for x in ct_tag]
        display_data("Bad tag", bytes(bad_ct_tag))
        bad_ciphertext = bytes(ct_data + bad_ct_tag)
        try:
            plaintext = aesgcm.decrypt(iv_nonce, bad_ciphertext, associated_data)
        except InvalidTag as e:
            print("Error: Invalid AES-GCM auth tag")


def bytes_to_hex_string(bytes, spaced=True):
    """Get a pretty string of hex bytes, separated by spaces."""
    sep = " " if spaced else None
    pretty_hex_string = binascii.hexlify(bytes, sep=sep)
    return pretty_hex_string.decode("utf-8")


def hex_string_to_bytes(hex_string):
    """Get bytes from a hex string."""
    bytes = binascii.unhexlify(hex_string.replace(' ', ''))
    return bytes


def display_data(name, data):
    """Display data value and size is standard format"""
    bytelen = len(data)
    bitlen = bytelen * 8
    hex_string = bytes_to_hex_string(data)
    print(f"HEX for {name}\t({bytelen} bytes, {bitlen} bits):\t{hex_string}")


if __name__ == "__main__":
    main()

terminal-output.txt

Plain:   b'a'
HEX for Plain   (1 bytes, 8 bits):      61
HEX for S Key   (32 bytes, 256 bits):   d1 88 49 87 a6 d4 dd 93 da 9c 5e 24 f8 cc cb 59 35 a6 5f 8e 4f 07 8c f0 2d f9 cb c4 d6 28 36 9e
HEX for G Key   (32 bytes, 256 bits):   97 74 eb 46 67 20 b3 9c 4d b9 63 fc f8 92 96 d0 bd fc 3b c0 ff a5 c5 5c 57 bd 62 2f a7 a0 8b c5
HEX for IV      (12 bytes, 96 bits):    e6 f5 d3 46 88 55 c9 56 b0 ee 43 33
HEX for Cipher  (17 bytes, 136 bits):   ce d5 aa 0e 7a f6 3f 32 7c 41 36 c6 f1 e0 0d 8c b9
HEX for CT data (16 bytes, 128 bits):   ce d5 aa 0e 7a f6 3f 32 7c 41 36 c6 f1 e0 0d 8c
HEX for CT tag  (1 bytes, 8 bits):      b9
HEX for Plain   (1 bytes, 8 bits):      61
Plain:   b'a'