A little Python tool that demonstrates how to set up an AES cipher using the PyCrypto library.

pycrypto-aes-demo.py

#!/usr/bin/env python

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"""AES CFB Stream Cipher Tool"""

from Crypto.Cipher import AES
from Crypto import Random
from argparse import ArgumentParser
import base64, sys

def main():
    # Set-up a command-line argument parser
    parser = ArgumentParser(description=__doc__, epilog="""Input is read from
        stdin and output is written to stdout. Use the stream redirection
        features of your shell to pass data through this program. If a key is
        not specified, it is generated and written to stderr.""")
    parser.add_argument('-d', '--decrypt', action='store_true')
    parser.add_argument('-k', '--key', metavar='key')
    args = parser.parse_args()

    # Decode the key if it was supplied
    key = base64.b64decode(args.key) if args.key else None

    # -- DECRYPT --

    if args.decrypt:
        # Check to see if a key was supplied
        if key is None:
            sys.stderr.write('Error: Please supply a decryption key.\n')
            exit(1)

        # Read in the initialization vector
        iv = sys.stdin.read(AES.block_size)

        # Create the cipher object and process the input stream
        cipher = AES.AESCipher(key, AES.MODE_CFB, iv)
        method = cipher.decrypt

        # Stream is processed below...

    # -- ENCRYPT --

    else:
        # Create a file object that produces random data when read from
        random = Random.new()

        # Generate a key if one was not supplied
        if key is None:
            key = random.read(AES.key_size[0])
            sys.stderr.write('Encryption Key: %s\n' % base64.b64encode(key))

        # Create the initialization vector
        iv = random.read(AES.block_size)
        sys.stdout.write(iv)

        # Create the cipher object and process the input stream
        cipher = AES.AESCipher(key, AES.MODE_CFB, iv)
        method = cipher.encrypt

        # Stream is processed below...

    # Process the input stream
    while True:
        data = sys.stdin.read(AES.block_size)
        if data == '': break # Check for EOF
        sys.stdout.write(method(data))

if __name__ == '__main__':
    main()