Here is an example of how to use the Pycrypto library to encrypt and decrypt a file:

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import os

from Crypto.Cipher import AES

def encrypt_file(key, in_filename, out_filename=None, chunksize=64*1024):

    """ Encrypts a file using AES (CBC mode) with the

        given key.

        key:

            The encryption key - a string that must be

            either 16, 24 or 32 bytes long. Longer keys

            are more secure.

        in_filename:

            Name of the input file

        out_filename:

            If None, '<in_filename>.enc' will be used.

        chunksize:

            Sets the size of the chunk which the function

            uses to read and encrypt the file. Larger chunk

            sizes can be faster for some files and machines.

            chunksize must be divisible by 16.

    """

    if not out_filename:

        out_filename = in_filename + '.enc'

    iv = os.urandom(16)

    encryptor = AES.new(key, AES.MODE_CBC, iv)

    filesize = os.path.getsize(in_filename)

    with open(in_filename, 'rb') as infile:

        with open(out_filename, 'wb') as outfile:

            outfile.write(struct.pack('<Q', filesize))

            outfile.write(iv)

            while True:

                chunk = infile.read(chunksize)

                if len(chunk) == 0:

                    break

                elif len(chunk) % 16 != 0:

                    chunk += b' ' * (16 - len(chunk) % 16)

                outfile.write(encryptor.encrypt(chunk))

def decrypt_file(key, in_filename, out_filename=None, chunksize=24*1024):

    """ Decrypts a file using AES (CBC mode) with the

        given key. Parameters are similar to encrypt_file,

        with one difference: out_filename, if not supplied

        will be in_filename without its last extension

        (i.e. '.enc')

    """

    if not out_filename:

        out_filename = os.path.splitext(in_filename)[0]

    with open(in_filename, 'rb') as infile:

        origsize = struct.unpack('<Q', infile.read(struct.calcsize('Q')))[0]

        iv = infile.read(16)

        decryptor = AES.new(key, AES.MODE_CBC, iv)

        with open(out_filename, 'wb') as outfile:

            while True:

                chunk = infile.read(chunksize)

                if len(chunk) == 0:

                    break

                outfile.write(decryptor.decrypt(chunk))

            outfile.truncate(origsize)

You can use these functions as follows:

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key = b'Sixteen byte key'

encrypt_file(key, 'inputfile.txt', 'outputfile.txt.enc')

decrypt_file(key, 'outputfile.txt.enc', 'decrypted_inputfile.txt')