Fictitious blog

Cryptography -- secret writing -- is the practice of keeping secrets. It involves three parties: a sender, a receiver and an attacker (actually, there can be more attackers, senders and recipients, but let's keep this simple). We usually call these people Alice, Bob and Carol.

Let's say we're in the days of the Caesar, the Gallic War. You need to send messages back and forth to your generals, and you'd prefer that the enemy doesn't get hold of them. You can rely on the idea that anyone who intercepts your message is probably illiterate, but that's a tough bet to stake your empire on. You can put your messages into the hands of reliable messengers who'll chew them up and swallow them if captured -- but that doesn't help you if Brad Pitt and his men in skirts skewer him with an arrow before he knows what's hit him.

Etc.