A developer needs an authentication coding mechanism that supports older iPhone/iPad and Android devices. Which of the following would work best for the developer?

The best choice for supporting older iPhone/iPad and Android devices in terms of authentication coding mechanisms would be Poly1305. This option is designed to provide high performance and security in environments where computational resources may be limited, which aligns well with older mobile devices.

Poly1305 is an efficient message authentication code that can be used alongside symmetric encryption algorithms. It operates with a lightweight design that is particularly well-suited for environments where performance and battery efficiency are crucial, such as on older mobile devices. Furthermore, it allows for rapid processing, which is beneficial in contexts where older hardware may struggle with more computationally intensive algorithms.

Support for older devices is also an important factor. Developers targeting these platforms need mechanisms that do not impose heavy requirements on processing power or memory usage. Poly1305’s design enables it to function effectively across a range of device capabilities, making it more appropriate than other choices that might require more processing power or memory than older devices can provide.

While other options like HMAC, MD5, and SHA-256 could have some utility, they may not deliver the same balance of performance and compatibility for older devices as Poly1305 does, especially in a mobile context.