Caesar's encryption

For codecademy doing the caecar's encrytion

Caesar's encryption is a simple substitution cipher where each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a shift of 3, A would be replaced by D, B would be replaced by E, and so on.

Here we decode and encode shifting with 10

The script

Simply by changing to + or - in the line shifted_number = (number - offset) % 26 will swtich between encoding and decoding.

# Ask for message and offset
input_message = input('Write message: ')

# Encode/Decode 
def caesar_decode(input_message, offset):
    result = ''
    characters = list(input_message.lower())

    # Convert chars to ASII 
    for character in characters:
        if character in ".,?'! ":
            result += character

        elif character.isalpha():
            
            # Convert to number
            number = ord(character.lower()) - 97

            # Calculate the shift - for encode + for decode
            shifted_number = (number - offset) % 26

            # Convert back to char
            shifted_char = chr(shifted_number + 97)
            result += shifted_char
        
        else:
            result += character

    return result 

# bruteforce the shift
for offset in range(26):
    decoded_message = caesar_decode(input_message, offset)

    # Print result
    print(f"Shift {offset}: {decoded_message}")

The ord() turns the character into ASCI value so 'd' becomes 100. So subtracting 97 gives a value of 3. We can then shift by 10, 3 - 10 which is -7 . We then use modulo to -7 % 26 = 19. Finally we add 19 + 97 to get the ASCI char again.