File Encryption and Decryption with python

pyAes.py

#!/usr/bin/python3
# pyAesCrypt script

import argparse
import getpass
from sys import exit
from os.path import isfile
import pyAesCrypt

maxPassLen = 1024  # maximum password length (number of chars)

# encryption/decryption buffer size - 64K
bufferSize = 64 * 1024

# parse command line arguments
parser = argparse.ArgumentParser(description=("Encrypt/decrypt a file "
                                              "using AES256-CBC."))
parser.add_argument("filename", type=str,
                    help="file to encrypt/decrypt")
parser.add_argument("-o", "--out", type=str,
                    default=None, help="specify output file")
parser.add_argument("-p", "--password", type=str,
                    default=None, help="specify the password")

# encrypt OR decrypt....
groupED = parser.add_mutually_exclusive_group(required=True)
groupED.add_argument("-e", "--encrypt",
                     help="encrypt file", action="store_true")
groupED.add_argument("-d", "--decrypt",
                     help="decrypt file", action="store_true")
args = parser.parse_args()


# check for input file existence
if not isfile(args.filename):
    exit("Error: file \"" + args.filename + "\" was not found.")

# check if the user has not supplied a password
if not args.password:
    # prompt the user for password
    passw = str(getpass.getpass("Password:"))
else:
    # warn the user
    print("Warning: passing passwords as plaintext "
          "command-line arguments may be unsafe.")
    # get the password from the argument
    passw = args.password

if args.encrypt:
    # check against max password length
    if len(passw) > maxPassLen:
        exit("Error: password is too long")

    # Check password complexity
    # here assume that a good password is at least 8 chars long
    # and includes at least:
    # 1 lowercase char
    # 1 uppercase char
    # 1 digit
    # 1 symbol
    if not((len(passw) > 7) and any(c.islower() for c in passw)
            and any(c.isupper() for c in passw)
            and any(c.isdigit() for c in passw)
            and any(not(c.isalnum()) for c in passw)):
            print("Warning: your password seems weak.")
            print("A password should be at least 8 chars long and should "
                  "contain at least one lowercase char, one uppercase char, "
                  "one digit and one symbol.")

    # re-prompt the user for password
    # if it was not supplied from the command line
    if not args.password:
        passwConf = str(getpass.getpass("Confirm password:"))
        # check the second pass against the first
        if passw != passwConf:
            exit("Error: passwords you provided do not match")

    # open output file
    if args.out is not None:
        ofname = args.out
    else:
        ofname = args.filename+".aes"

    # call encryption function
    try:
        pyAesCrypt.encryptFile(args.filename, ofname, passw, bufferSize)
    # handle IO errors
    except IOError as ex:
        exit(ex)
    # handle value errors
    except ValueError as ex:
        exit(ex)

elif args.decrypt:
    # open output file
    if args.out is not None:
        ofname = args.out
    elif args.filename.endswith(".aes"):
        ofname = args.filename[:-4]
    else:
        exit("Error: if input file extension is not \".aes\", you should "
             "provide the output file name through \"-o\" option.")

    # call decryption function
    try:
        pyAesCrypt.decryptFile(args.filename, ofname, passw, bufferSize)
    # handle IO errors
    except IOError as ex:
        exit(ex)
    # handle value errors
    except ValueError as ex:
        exit(ex)