Private-Key Encryption

Private- or symmetric-key encryption schemes  are intuitive ciphers your students may have heard about. These ciphers depend upon a shared secret key. For example, if Alice and Bob are using an encryption system to communicate, they both have to know the secret key. That way, Alice can encipher a message with the key, and Bob can decipher it with the same key. Then Bob can encipher a message with the key, and Alice can decipher it. And so on. But how do Alice and Bob share the key in the first place? What if Alice lives in California and Bob lives in New York? There must be a secure way in which they can share the secret key before being able to use it. Ask your students how they think Alice and Bob should share their secret key and discuss difficulties that might come up.

The following are some examples of symmetric-key encryption systems. Although most of them are considered insecure and are not used for security purposes today, the ideas behind them are fundamental to modern private-key encryption.

Caesar Ciphers
Substitution Ciphers
Vigenère Cipher
One-time Pads
The Enigma Machine
Current private-key encryption systems
Stream and Block Ciphers