A cipher is an algorithm that transforms readable information, called plaintext, into an unreadable form, called ciphertext, and back again, using a secret value known as a key. Historical ciphers swapped or shifted letters by hand; modern ciphers operate on bits and are designed so that, without the key, recovering the plaintext is computationally infeasible. Symmetric ciphers such as the Advanced Encryption Standard (AES) use the same key to encrypt and decrypt and protect stored data and card transactions. Asymmetric schemes such as RSA use a public key and a private key, and underpin secure web connections and many digital signatures. Ciphers are the core tool of cryptography. Also see Data Encryption and Decryption.