- Private Key
- Alice and Bob share a key K the adversary does not know
- Alice and Bob agree on cryptosystem and key
- Bob encrypts plaintext using key, sends ciphertext to Alice
- Alice decrypts ciphertext with same key and reads the message
- Advantage: fast
- Disadvantage: keys must be distributed secretly
- Disadvantage: if key is compromised, all is lost
- Disadvantage: number of needed keys is
*n*^{2}