## Symmetric Ciphers

Symmetric ciphers use the same cryptographic keys for both encryption of plaintext and decryption of ciphertext. They are faster than asymmetric ciphers and allow encrypting large sets of data. However, they require sophisticated mechanisms to securely distribute the secret keys to both parties.

Definition
A symmetric cipher defined over (K, M, C), where:

- K - a set of all possible keys,
- M - a set of all possible messages,
- C - a set of all possible ciphertexts

- E: K × M -> C
- D: K × C -> M

- D(k, E(k, m)) = m (the consistency rule)

- Function E is often randomized
- Function D is always deterministic