Asymmetric Encryption

The Idea

Two keys instead of one.

A public key and a private key.

They’re mathematically linked, but you can’t figure out one from the other.

How It Works

Public key: Share it with everyone. Anyone can use it to encrypt messages to you.

Private key: Keep it secret. Only you can decrypt those messages.

Solving the Key Distribution Problem

Remember the problem with symmetric encryption? Alice couldn’t safely send Bob a key.

With asymmetric encryption:

1. Bob creates a key pair
2. Bob publishes his public key (anyone can see it)
3. Alice encrypts her message with Bob’s public key
4. Only Bob can decrypt it (he has the private key)

Eve can see the public key. It doesn’t help her. She can’t decrypt anything.

The Tradeoff

Asymmetric encryption is slow. Much slower than symmetric.

So in practice, we use both:

1. Use asymmetric encryption to securely share a symmetric key
2. Use symmetric encryption for the actual data

Best of both worlds. Secure key exchange, fast encryption.

Examples

• RSA - the classic, based on prime factorization
• Diffie-Hellman - key exchange protocol
• ECC - elliptic curve cryptography, smaller keys, same security