The Assassin's Curse (The Blackthorn Key, #3)

by Kevin Sands

That’s where Vigenère comes in. At its heart, Vigenère is a letter shift, like I showed you. What makes it unbreakable, however, is that the amount you shift each letter changes every time.”

Code breaking

Because what defines that shift is what we call a key.

take the original message, and above it, write out our key, repeating it as many times as we need to get to the end of the message.”