Ciphers refer to algorithms, which are used to encrypt and decrypt data so its security is maintained. The original data or plaintext is encrypted to ciphertext before it is sent. Though ciphertext contains the original data, it is not readable. When the ciphertext is received by the intended user, it is decrypted back to plaintext.
The commonly used ciphers for encryption include block ciphers and stream ciphers. According to Stallings, a block cipher chunks the message together into a fixed-length bit stream and then processes the blocks. A stream cipher, on the other hand, processes bits one element at a time.