>> > It allows hardware sources to be used via the same API. >> >> I rather doubt this. For example, my smartcard (accessible via PKCS#11) is a hardware source, which I >> occasionally use. How do you see it used with the same API? > > We have a similar situation, on a small hardware device with little own entropy > but with a smartcard reader.
Yes, but in most cases you cannot count on the smartcard (or smartcard-like device) being in the reader. Which is why in my opinion this is an ideal case for “additional input” source, but rather poor as the “main” entropy source. > We implemented a get_entropy() call which fetches the entropy via PKCS#11, and > modified the rand_method such that RAND_DRBG_generate() is always called with > prediction_resistance=1. So every generate() triggers a reseed(), the entropy is fetched > from the smartcard and it is immediately postprocessed by the AES-CTR DRBG. > The /dev/urandom device was only used as additional input. > So we didn't feel the need for an extra API call. I would do exactly the opposite. “Normal” entropy is fetched from the default sources (/dev/urandom). But when a sensitive (aka long-term) keys are generated, a (portable :) hardware RNG is plugged in and used with RAND_add() equivalent. Reason – in my setup reliable trusted hardware RNG is not always on. If it were, I’d use it as the main entropy source and be done with it.
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