Python (Shift) Substitution Cipher

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#==============MODULES================#
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#==============VARIABLES==============#
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menus = ["(1) Encrypt", "(2) Decrypt", "(3) Exit   "]

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

#defines encryption algorithm
def enc(data, key):
    output = ""
    for char in data:
        nums = ord(char) #convert ascii
        nums = (nums + key) #add ascii value and key value to create constant shift
        output = (output + chr(nums)) #create output string with all 'new' characters
        print "The encrypted message is: ", output

#defines decryption algorithm
def dec(data, key):
    output = ""
    for char in data:
        nums = ord(char)
        nums = (nums - key)
        output = (output + chr(nums))
        print "The decrypted message is: ", output

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#===========MAIN PROGRAM==============#
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def main():
    menu(menus)

    choice = input("Please make a selection(1-3): ")

    #Encryption
    if choice == 1:
        key = input("Please enter numerical key: ")
        data = raw_input("Please enter a message to encrypt: ")
        enc(data, key)

    #Decryption
    elif choice == 2:
        key = input("Please enter numerical key: ")
        data = raw_input("Please enter a message to decrypt: ")
        dec(data, key)

    #Exit
    elif choice == 3:
        print "Exiting Program. Goodbye."

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