Symmetric-key cryptography


This is the "classical" way to do things
Used for ~4000 years

Alice scrambles her message using some secret technique.
Bob knows the secret, and can unscramble the message.
It's the same secret, so it's called "symmetric-key" crypto
Eve does not know the secret, and cannot read the message.

At first, the secret was an algorithm
Non-standard hieroglyphics is the earliest known example
But what happens when the secret leaks out?