Which of the following is optimized for confidential communications, such as bidirectional voice and video?

RC4 is a symmetric stream cipher that encrypts data one byte at a time, making it highly efficient for continuous, real-time bidirectional data streams such as voice over IP and video conferencing where low latency and minimal processing overhead are critical.

RC5 is a symmetric block cipher that processes fixed-size blocks, introducing latency and overhead that makes it less suited for low-latency real-time streams than a stream cipher.

MD4 is a cryptographic hash function used for producing message digests, not for encrypting confidential communications.

MD5 is a cryptographic hash function used for integrity verification, not for encrypting or securing bidirectional communications streams.