Considering the following OpenVPN (2.4) option from the manual:

--prng alg [nsl]
(Advanced) For PRNG (Pseudo-random number generator), use digest algorithm alg (default=sha1), and set nsl (default=16) to the size in bytes of the nonce secret length (between 16 and 64).
Set alg=none to disable the PRNG and use the OpenSSL RAND_bytes function instead for all of OpenVPN's pseudo-random number needs.

How much would the entropy of the prng be increased by a more secure hashing algorithm, say SHA512?

Also, what exactly is the nonce secret length and how does it impact the entropy?

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