What is Cryptography?

Symmetric Encryption

Encryption is the practice of scrambling data such that only the people you want to see the original message are able to do so. The first forms of cryptography used symmetric encryption. Let's explain this by starting with our two best friends in cryptography, Alice and Bob. 

Alice and Bob want to exchange secret messages with one another. To hide the contents of the message, they will encrypt the data using a secret, or "key". In modern cryptography, these secrets involve sophisticated mathematics, but an encryption secret could be as simple as the instructions to move each letter of a message forward two letters in the alphabet to encrypt, and the instructions to move backwards two letters in the alphabet to decrypt. Using this algorithm, "hello" becomes "jgnnq". While simple, even this method effectively scrambles a message making it unreadable to those without the secret, or key. 

Alice can use this method to encrypt her private messages to Bob and, if Bob knows the secret, then he can decrypt and read that message. He can also use this same secret to encrypt messages back to Alice. This method is called symmetric encryption, and while it works, it has one big flaw. How do Alice and Bob decide on an encryption algorithm, or secret key, for their communication? And if Alice decides on a secret to use in their encryption, how does she communicate that secret to Bob without someone else seeing it?