Public-key cryptography / asymmetric cryptography, shared secret agreement. Alice combines her own private key with Bob's public key to create a shared secret. Likewise Bob combines his own private key with Alice's public key and gets the same shared secret. This secret is now only known by Alice and Bob. The shared secret can be used as a symmetric key for encrypted communication between Alice and Bob and for many other purposes such as MACing and to create multiple message keys etc. Diffie–Hellman key exchange is a method of this type.

Original illustration by David Göthberg, Sweden.