Welcome back! This lesson is entirely focused on one problem: encryption.
We’re going to modify the normal lesson flow. We’ll start with the homework problem at the top. If you’d like to just go at on your own, go for it! And, if you’d like a bit of help, we’ll break it down piece-by-piece below.
Let’s get to it!
Encryption is an ancient practice of trying to conceal information by scrambling it. Modern encryption techniques are incredibly strong and mathematically sound. But in the past, simpler and more primitive methods were used.
Let's implement a form of encryption known as a Caesar Cipher,
sometimes also known as Rot-13 encryption.
(Rot for rotation, and 13 for one amount that you might rotate.)
Here is how it works. Given a
String and an amount to rotate, we replace each character in the
a new character determined by rotating the original character through a given rotation
For example, given the rotation
String "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz ",
"ABC" rotated 3 would be "DEF", and rotated -1 would be " AB". (Note the space at the end of the rotation
Declare and implement a function called
encrypt that, given a
String and an
int amount, returns the passed
String "encrypted" by rotating it the given amount.
("Encrypted" is in scare quotes because this is not by any means a strong method of encryption!)
Use the rotation
String provided above.
If the passed
null you should return
Note that rotation may be negative, which will require some additional care.
Let’s break down this problem into smaller pieces, and spend a few moments just orienting ourselves and figuring out what to do. We won’t write test cases yet, and instead save them for the smaller pieces that we’re about to create.
Now that we have a sense of what the different pieces are, let’s look at one of the core challenges: remapping each character. We’ll also write some simple test cases for our helper method.
At this point we’ve identified how to remap individual characters.
Next we need to review how to break the input
String into individual characters.
Now that we have our building blocks, let’s integrate everything together!
If you are enjoying
Strings, rotation, and modular arithmetic, and haven’t had enough yet—here is a practice problem that you might enjoy!
This problem combines
Strings, functions, and arrays. Super fun!
Write a function called
rotateRight that takes a
String as its first argument and a non-negative
its second argument and rotates the
String by the given number of characters.
Here's what we mean by rotate:
CS125rotated right by 1 becomes
CS125rotated right by 2 becomes
CS125rotated right by 3 becomes
CS125rotated right by 4 becomes
CS125rotated right by 5 becomes
CS125rotated right by 8 becomes
And so on.
Notice how characters rotated off the right end of the
String wrap around to the left.
This problem is tricky! Here are a few hints:
Stringto an array of characters before you begin.
charactersback into a
If the passed
String argument is
null, you should return
Good luck and have fun!
She received the award in 2012 “for transformative work that laid the complexity-theoretic foundations for the science of cryptography and in the process pioneered new methods for efficient verification of mathematical proofs in complexity theory.” Her work underlies the foundations of our modern data society, including algorithms that you use every day when you chat, browse, shop, and engage online. Here’s a short (if somewhat poorly done) official video describing her contributions:
Need more practice? Head over to the practice page.