Which message authentication code (MAC) is designed for speed and efficiency, particularly on devices without AES hardware acceleration, such as older iPhone and Android devices?

The choice of Poly1305 as the correct answer is based on its design principles focused on speed and efficiency, making it particularly suitable for constrained environments like older mobile devices. Poly1305 is a one-time key authentication code that is optimized for performance and can be extremely fast, which is essential for devices that lack the robust processing power or specialized hardware acceleration for encryption methods such as AES.

This lightweight characteristic allows it to process data quickly without heavily taxing the resources of devices that might not be capable of performing more intensive cryptographic operations efficiently. In contrast, HMAC, while secure and widely used, involves a hashing function that may not be as efficient in resource-constrained environments as Poly1305.

MD5 is an older hashing algorithm that, while fast, is no longer recommended for secure applications due to vulnerabilities. SHA-256 offers stronger security but is computationally heavier compared to Poly1305, which may result in slower performance on less capable devices. Therefore, Poly1305 stands out as the optimal choice for achieving high-speed processing with good security in environments where resources are limited.