Beginning AES with Python3

Encryption is a vast field and one post can never do it justice. But I’ll try to provide code examples on how to use the PyCrypto library to work with AES.

Disclaimer: My programming skills might not be up to par when it comes to encryption. Try to learn from my mistakes (when I make them).

Install the library in Fedora:

yum install python3 python3-crypto

Let’s just dive into the code. I have tried to make it simple and clear.

#!/usr/bin/env python3
from Crypto.Cipher import AES
from Crypto import Random
from base64 import b64encode, b64decode
from Crypto.Util import Counter
from binascii import hexlify

print('AES block size: {0}'.format(AES.block_size))
original_key = 'This is my k\u00eay!! The extra stuff will be truncated before using it.'
key = original_key.encode('utf-8')[0:32]
print('Original Key: {0}'.format(original_key))
print('Usable Key: {0}'.format(key))
print('Base64 Encoded key: {0}'.format(b64encode(key).decode('utf-8')))
message = '0123456789'.encode('utf-8')
print('Original Message: {0}'.format(message))


print('```MODE CFB```')
cfb_iv =
print('Base64 Encoded IV: {0}'.format(b64encode(cfb_iv).decode('utf-8')))

cfb_cipher_encrypt =, AES.MODE_CFB, cfb_iv)
cfb_msg_encrypt = b64encode(cfb_cipher_encrypt.encrypt(message))
print ('Mode CFB, Base64 Encoded, Encrypted message: {0}'.format( cfb_msg_encrypt.decode('utf-8')))

cfb_cipher_decrypt =, AES.MODE_CFB, cfb_iv)
cfb_msg_decrypt = cfb_cipher_decrypt.decrypt(b64decode(cfb_msg_encrypt)).decode('utf-8')
print('Mode CFB, Decrypted message: {0}'.format(cfb_msg_decrypt))


print('```MODE CTR```')
def ctr_pad_message(in_message):
    # We use PKCS7 padding
    length = 16 - (len(in_message) % 16)
    return (in_message + bytes([length])*length)
def ctr_unpad_message(in_message):
    return in_message[:-in_message[-1]]

ctr_iv = int(hexlify(, 16)
print('CTR IV (int): {0}'.format(ctr_iv))
ctr_encrypt_counter =, initial_value=ctr_iv)
ctr_decrypt_counter =, initial_value=ctr_iv)

ctr_padded_message = ctr_pad_message(message)
print('Mode CTR, Padded message: {0}'.format(ctr_padded_message))
ctr_cipher_encrypt =, AES.MODE_CTR, counter=ctr_encrypt_counter)
ctr_msg_encrypt = b64encode(ctr_cipher_encrypt.encrypt(ctr_padded_message))
print('Mode CTR, Base64 Encoded, Encrypted message: {0}'.format( ctr_msg_encrypt.decode('utf-8')))

ctr_cipher_decrypt =, AES.MODE_CTR, counter=ctr_decrypt_counter)
ctr_msg_decrypt = ctr_cipher_decrypt.decrypt(b64decode(ctr_msg_encrypt))
ctr_unpadded_message = ctr_unpad_message(ctr_msg_decrypt)
print('Mode CTR, Decrypted message: {0}'.format(ctr_msg_decrypt))
print('Mode CTR, Unpadded, Decrypted message: {0}'.format(ctr_unpadded_message))

Here I have provided examples for two modes: CFB and CTR. Although both of them should not use fixed blocks for some reason CTR expects you to use fixed blocks in this library.

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