Symmetric Encryption

The Idea

One key does everything.

The same key that encrypts the message also decrypts it.

How It Works

1. Start with a message
2. Encrypt it with a key → get unreadable ciphertext
3. Decrypt the ciphertext with the same key → get the message back

Why “Symmetric”?

Both sides use the same key. Encrypt and decrypt are mirror images.

Same key in, same key out. Balanced. Symmetric.

The Problem

If both people need the same key, how do you share it safely?

You can’t just send the key over the internet. Anyone watching would see it.

And if they have the key, they can decrypt everything.

This is called the key distribution problem.

Where It’s Used

Despite the key-sharing problem, symmetric encryption is fast.

• AES encrypts your phone, your WiFi, your bank connection
• DES was the old standard (now broken)
• XOR is the simplest form (we already learned this)

For bulk data, symmetric encryption is the go-to choice.