What type of cipher is RC4?

RC4 is classified as a symmetric-key stream cipher. This type of cipher encrypts data one bit or byte at a time, providing a continuous stream of keys that can be used to encrypt plaintext messages into ciphertext. The "symmetric-key" aspect means that the same key is used for both encryption and decryption, allowing both parties in communication to securely exchange information. Stream ciphers like RC4 are particularly useful for applications that require the encryption of data streams of variable sizes, such as in SSL/TLS communications.

In contrast to block ciphers, which encrypt fixed-size blocks of data, stream ciphers process data in a stream, making them more efficient for scenarios where data is continuously flowing, such as video or audio streaming. This is why the choice of "symmetric-key stream cipher" accurately describes RC4’s function within cryptographic protocols.