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

RC4 operates as a stream cipher, encrypting data one byte at a time as it flows, which produces minimal latency and processing overhead compared to block ciphers. This design is well-suited for real-time streaming protocols such as VoIP and video conferencing where delay and jitter must be minimized. RC4 was used in protocols like WEP and early SSL/TLS for this reason.

RC5 is a block cipher that processes fixed-size data blocks, introducing buffering and padding overhead that adds latency unsuitable for real-time bidirectional streaming.

MD4 is a cryptographic hash function used for data integrity verification, not encryption, so it cannot provide confidentiality for voice or video streams.

MD5 is also a cryptographic hash function used for integrity checking and is not an encryption algorithm, making it entirely incapable of encrypting communications.