Python CAST String Encryption & Decryption

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#==============MODULES================#
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from Crypto.Cipher import *
from Crypto import Random
import base64

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#==============VARIABLES==============#
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key_size = 16
modes = ["(1) ECB    ", "(2) CBC    ", "(3) CFB    "]
mtypes = ["(1) Encrypt", "(2) Decrypt", "(3) Exit   "]
castbt = ["(1) 128    ", "(2) 256    "]


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#==============FUNCTIONS==============#
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def menu(parameter):
    print "     +===========+"
    print "     |    MENU   |"
    print "     |-----------|"
    for word in parameter:
        print "     |%s|" % word
        print "     |----------|"


#defines encryption algorithm
def enc(key_size, block_size, iv_size, mode):

    #get key and pad it if necessary
    key = raw_input("Please enter a key: ")
    key += (key_size - len(key) % key_size) * chr(key_size - len(key) % key_size)

    #get data and pad if necessary
    data = raw_input("Please enter a string to encrypt: ")
    pad = lambda data: data + (block_size - len(data) % block_size) * chr(block_size - len(data) % block_size)
    data = pad(data)

    #generate random IV
    iv = Random.new().read(iv_size)

    #define what is needed for CAST
    cipher = CAST.new(key, mode, iv)

    #encode using base64 and encrypt data and add IV
    output = base64.b64encode(iv + cipher.encrypt(data))

    print "The encrypted message is: ", output

#defines the decryption algorithm
def dec(block_size, key_size, iv_size, mode):
    #get key and pad it if necessary
    key = raw_input("Please enter a key: ")
    key += (key_size - len(key) % key_size) * chr(key_size - len(key) % key_size)

    #get data and decode base64
    data = raw_input("Please enter a string to encrypt: ")
    data = base64.b64decode(data)

    #get IV
    iv = data[:iv_size]

    #strip padding
    strippad = lambda data: data[0:-ord(data[-1])]

    #define what is needed for CAST
    cipher = CAST.new(key, mode, iv)

    #strip off padding and decrypt data
    output = strippad(cipher.decrypt(data[iv_size:]))

    print "The decrypted message is: ", output

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#===========MAIN PROGRAM==============#
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def main():
    menu(mtypes)
    mtypes_choice = input("Please select to encrypt, decrypt, or exit(1-3): ")

    #Encryption
    if mtypes_choice == 1:
        menu(castbt)
        castbt_choice = input("Please select to use 128 or 256 bits(1-2): ")

        #128 Bit
        if castbt_choice == 1:
            block_size = 8
            iv_size = 8
            menu(modes)
            modes_choice = input("Please select a mode of encryption(1-3): ")

            #MODES
            if modes_choice == 1:
                mode = CAST.MODE_ECB
                enc(key_size, block_size, iv_size, mode)
            elif mode_choice == 2:
                mode = CAST.MODE_CBC
                enc(key_size, block_size, iv_size, mode)
            elif mode_choice == 3:
                mode = CAST.MODE_CFB
                enc(key_size, block_size, iv_size, mode)

        #256 Bit
        elif castbt_choice == 2:
            block_size = 16
            iv_size = 8
            menu(modes)
            modes_choice = input("Please select a mode of encryption(1-3): ")

            #MODES
            if modes_choice == 1:
                mode = CAST.MODE_ECB
                enc(key_size, block_size, iv_size, mode)
            elif mode_choice == 2:
                mode = CAST.MODE_CBC
                enc(key_size, block_size, iv_size, mode)
            elif mode_choice == 3:
                mode = CAST.MODE_CFB
                enc(key_size, block_size, iv_size, mode)

    #Decryption
    elif mtypes_choice == 2:

        #128 Bit
        if castbt_choice == 1:
            block_size = 8
            iv_size = 8
            menu(modes)
            modes_choice = input("Please select a mode of encryption(1-3): ")

            #MODES
            if modes_choice == 1:
                mode = CAST.MODE_ECB
                dec(key_size, block_size, iv_size, mode)
            elif mode_choice == 2:
                mode = CAST.MODE_CBC
                dec(key_size, block_size, iv_size, mode)
            elif mode_choice == 3:
                mode = CAST.MODE_CFB
                dec(key_size, block_size, iv_size, mode)

        #256 Bit
        elif castbt_choice == 2:
            block_size = 16
            iv_size = 8
            menu(modes)
            modes_choice = input("Please select a mode of encryption(1-3): ")

            #MODES
            if modes_choice == 1:
                mode = CAST.MODE_ECB
                dec(key_size, block_size, iv_size, mode)
            elif mode_choice == 2:
                mode = CAST.MODE_CBC
                dec(key_size, block_size, iv_size, mode)
            elif mode_choice == 3:
                mode = CAST.MODE_CFB
                dec(key_size, block_size, iv_size, mode)

    #Exit
    elif mtypes_choice == 3:
        print "Exiting Program. Goodbye!"
        print
        return

    #Fault
    else:
        print "ERROR. Invalid Selection."
        print
        return main()