CPSC120B - Day 21 Notes

Cool Computer Science Thing of the Day

Encryption & Artificial Intelligence

No Reading Questions

No Quiz

Rot13

Did you have secret codes to pass messages to friends when you were a kid?
I did, this is what I used
Replace each letter with the letter 13 later in the alphabet

Let’s write a function to automate this

def rot13(message):
    shift = 13
    encoded_message = ''
    for i in range(len(message)):
        character = message[i]
        if character in string.ascii_lowercase:
            character = character + shift
        encoded_message += character
    return encoded_message

This won’t work because character is a string and shift is a number

To fix, convert character to a number, add, convert back to character

def rot13(message):
    shift = 13
    encoded_message = ''
    for i in range(len(message)):
        character = message[i]
        if character in string.ascii_lowercase:
            index = string.ascii_lowercase.index(character)
            index = index + shift
            character += string.ascii_lowercase[index]
        encoded_message += character
    return encoded_message

This will crash for any letters after m in the alphabet because the shifted index will be out of bounds

To solve could use an if statement to shift if it is out of range

def rot13(message):
    shift = 13
    encoded_message = ''
    for i in range(len(message)):
        character = message[i]
        if character in string.ascii_lowercase:
            index = string.ascii_lowercase.index(character)
            index = index + shift
            if index >= len(string.ascii_lowercase):
                index = index - len(string.ascii_lowercase)
            character += string.ascii_lowercase[index]
        encoded_message += character
    return encoded_message

Another solution is to use mod

def rot13(message):
    shift = 13
    encoded_message = ''
    for i in range(len(message)):
        character = message[i]
        if character in string.ascii_lowercase:
            index = string.ascii_lowercase.index(character)
            index = (index + shift) % len(string.ascii_lowercase)
            character = string.ascii_lowercase[index]
        encoded_message += character
    return encoded_message

This encryption scheme is called rot13, because it rotates 13 characters
Thirteen because it is half the alphabet, so encoding and decoding are the same function
Using different amounts of shift the encryption scheme is called a Caesar cipher because Julius Caesar used it to encrypt messages to his generals

Lab

lab21