Modifying the key = and letter = statements to encrypt and re-flashing a script to get a ciphertext character isn’t very efficient.  It would be much easier for your script to pass a function, a key, and a word to a function and let it encrypt or decrypt the whole string.  Getting there can be broken into three steps:

1. Get it to work with user-entered characters in a loop
2. Add a loop to make it perform the cipher on all characters in a word
3. Move the working code from the main loop into a function

Another reason for setting it up this way is that you can replace a Caesar cipher function with a different encryption function, and the main script might only need one line modified to get it to work.

Caesar Cipher on Terminal-Entered Characters

In this example, the terminal will prompt you for a key and a letter, and then display the Caesar cipher result.  Previously, the encryption key was hard-coded as 5.  With this script, you can enter 5, or 13 for ROT-13, or any other value you decide to use.  Next, enter the letter to encrypt, and the script displays the ciphertext result in the terminal.

Example script: caesar_terminal_letters

• Enter caesar_terminal_letters into the micro:bit Python Editor. 
• Set the project's name to caesar_terminal_letters.
• Click Save.
• Click the Send to micro:bit button.

# caesar_terminal_letters
 
from microbit import *
 
sleep(1000)
 
print("Set your keyboard to CAPS LOCK.")
print()
 
while True:
    text = input("Enter key: ")
    key = int(text)
 
    letter = input("Enter a letter: ")
    
    letter = letter.upper()
 
    alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
 
    index = alpha.find(letter)
    index = index + key
    index = index % 26
 
    result = alpha[index]
 
    print("result =", result)
    print()

• Make sure your keyboard is set to CAPS LOCK.  (Or you will have to hold down the SHIFT key as you type letters.)
• If the serial monitor is not already open, click Show serial, then click in the serial monitor.
• Follow the prompts, typing in a key number and a letter to encrypt.  Valid key numbers are from -25 to 25.
• Try encrypting a character, and then decrypting with the negative value of the key you encrypted with. 
  Example: Encrypt A with 13, then decrypt N with -13 to get back to A.

How caesar_terminal_letters Works

The techniques for building this script make use of:

• Terminal with input statements: Input Messages
• Looping with while True: Count and Repeat
• Caesar cipher: Encrypt Letters with a Caesar Cipher Script

Instead of hard coding key = 5 and letter = "M", like in caesar_cipher_letter, this script makes it work during runtime with:

    text = input("Enter key: ")
    key = int(text)

    letter = input("Enter a letter: ")

Before doing the Caesar cipher, it uses letter = letter.upper() which changes any lower-case letter to upper-case.  …just in case you forgot to set your keyboard’s CAPS LOCK.

Did You Know?

These steps from the previous example sketch can be performed in one line:

    index = alpha.find(letter)
    index = index + key
    index = index % 26

Here’s how it would look:

    index = ( alpha.find(letter) + key ) % 26

This more compact format will be used in the next example script.