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

RC4 is a symmetric stream cipher designed for speed and simplicity, processing data sequentially byte by byte without the block-alignment overhead of block ciphers. This characteristic made it well-suited for real-time, bidirectional communications such as voice and video streams where low computational overhead and continuous data flow are required. It was widely used in protocols like SSL/TLS and WEP specifically because of this streaming efficiency.

RC5 is a symmetric block cipher that operates on fixed-size data blocks, introducing padding and block-alignment requirements that are not optimal for continuous streaming media.

MD4 is a cryptographic hash function used for producing message digests, not for encrypting or securing bidirectional data streams.

MD5 is a cryptographic hash function used for data integrity verification, not for encrypting confidential communications of any kind.