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What Is a Cipher?
A cipher is an algorithm for performing encryption or decryption.
Ciphers are simply a series of computational steps that result in the scrambling and de-scrambling of a secret message. All of the encryption we've done so far in this course has involved ciphers.
The Caesar Cipher
A Caesar cipher is an extremely simple substitution cipher where each letter in the plaintext is replaced by a different letter a fixed number of positions away in the alphabet. Caesar ciphers are named after Julius Caesar, so we've known about them for a long time!
Ralphie's Decoder ring in A Christmas Story is a great example of a Caesar cipher:
Example
Using the English alphabet, and a key of left shift 3, we would get the following lookup table:
Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher: XYZABCDEFGHIJKLMNOPQRSTUVW
We could then encrypt some text:
Plaintext: THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
Ciphertext: QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD
Decrypting the message is as simple as reversing the process and shifting the letters back to the right.