Stream ciphers

Stream ciphers are encryption algorithms that apply encryption algorithms on a bit-by-bit basis (one bit at a time) to plaintext using a keystream. There are two types of stream ciphers: synchronous stream ciphers and asynchronous stream ciphers:

• Synchronous stream ciphers are those where the keystream is dependent only on the key
• Asynchronous stream ciphers have a keystream that is also dependent on the encrypted data

In stream ciphers, encryption and decryption are the same function because they are simple modulo-2 additions or XOR operations. The fundamental requirement in stream ciphers is the security and randomness of keystreams. Various techniques ranging from pseudorandom number generators to true random number generators implemented in hardware have been developed to generate random numbers, and it is vital that all key generators be cryptographically secure: