Caesar Cipher: ci = (pi + k) % 26 wherein % 26 here means “remainder when dividing by 26.”
Let’s write a program called caesar that enables you to encrypt messages using Caesar’s cipher. At the time the user executes the program, they should decide, by providing a command-line argument, on what the key should be in the secret message they’ll provide at runtime. We shouldn’t necessarily assume that the user’s key is going to be a number; though you may assume that, if it is a number, it will be a positive integer.
Here are a few examples of how the program might work. For example, if the user inputs a key of 1 and a plaintext of HELLO:
$ ./caesar 1
plaintext: HELLO
ciphertext: IFMMP
Here’s how the program might work if the user provides a key of 13 and a plaintext of hello, world:
$ ./caesar 13
plaintext: hello, world
ciphertext: uryyb, jbeyq
How about one more? Here’s how the program might work if the user provides a key of 13 again, with a more complex plaintext:
$ ./caesar 13
plaintext: be sure to drink your Ovaltine
ciphertext: or fher gb qevax lbhe Binygvar
And what if a user doesn’t cooperate?
$ ./caesar HELLO
Usage: ./caesar key
$ ./caesar
Usage: ./caesar key
$ ./caesar 1 2 3
Usage: ./caesar key
Full instructions available here