Encrypt a Single Letter

This next script will encrypt individual letters with the Caesar cipher.  As-written it encrypts the letter M with a key of 5.

Example script: caesar_encrypt_letter

• Change project’s name from  find_letter_in_alphabet to caesar_encrypt_letter.
• Update it to match the script below
• Save the modified script.
• Click the Send to micro:bit button.

# caesar_encrypt_letter
 
from microbit import *
 
sleep(1000)
 
key = 5
letter = "M"
 
alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
index = alpha.find(letter)
 
print("key =", key)
print("letter =", letter, ", index =", index)
 
index = index + key
index = index % 26
result = alpha[index]
 
print("new index =", index, ", result =", result)
 
print()

• Check the results in the serial monitor, and verify that:
  • The plaintext letter is M and its index is 12
  • The new index is 17 with the resulting ciphertext letter R.

Decrypt with Caesar Cipher

What if a script receives a Caesar encrypted character and a key?  Do you need to write another script?  The answer is no.  To decrypt a ciphertext letter to plaintext, just use the negative of the key that encrypted it.  So, if a ciphertext character was encrypted with 5, it can be decrypted with -5.

Remember how a plaintext letter M encrypted to a ciphertext letter R when the key was 5?  To decrypt a ciphertext letter of R with a key of 5, just run it through the same Caesar cipher with a key of -5.  The result will be the plaintext character M.

• If you don’t already have it open, reopen caesar_encrypt_letter.
• Set the key to -5 and the plaintext letter to “R”.
• Flash the modified script into the micro:bit.
• Verify that the ciphertext letter (which is really the plaintext letter) result is “M”.