HAP_rand or HAPPlatformRandomNumberFill? about homekitadk HOT 5 CLOSED

HAP_rand allows Darwin to use either OpenSSL and MbedTLS as crypto backends.

With that said, this really was just a convenient shortcut. If Darwin didn't rely on the crypto backend to provide the RNG, HAP_rand wouldn't need to exist. Could probably make Darwin behave the same as the POSIX PAL and remove HAP_rand.

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@rojer You are right, we could potentially replace HAP_rand with HAPPlatformRandomNumberFill

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Watch out, HAP_rand is referenced in the documentation here : https://github.com/apple/HomeKitADK/blob/master/Documentation/PAL.md. Maybe it is used on purpose ?

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HAPPlatformRandomNumberFill is the proper PAL API to use to generate a random number.

HAP_rand is probably better thought of as an internal API used by some platforms.
Currently HAP_rand is only used by Darwin's HAPPlatformRandomNumberFill implementation to make use of the supported crypto libraries's (ex: OpenSSL and MbedTLS) built-in random number generator, and by the MbedTLS wrapper using it's own rng function rather than going back to the PAL's HAPPlatformRandomNumberFill.

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but in turn, mbedtls implementation uses arc4random_buf, which is not part of mbedtls.
imo HAP_rand should be eliminated. if arc4random_buf is good enough on darwin, then HAPPlatformRandomNumberFill should just use that.
i ran into this problem when porting to a platform that has mbedtls but not arc4random

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