i think the problem is (given your example)
the hash algo is (pw, rounds/iterations?, salt and ???)
plain mode 1710 is pw: salt so the problem starts with iterations, iterations take the first output as input and hash it again, this is done (iterations) 20 times? so you have the result of
take this sha512: salt 20 times
repeat 20 times sha512( pw: salt)
so with mode 1710 you will not get the pw
next thing is what about nil? is it appended, prepended to your pass? so your pass ist nil12345 or 12345nil? or what is nil?
the hash algo is (pw, rounds/iterations?, salt and ???)
plain mode 1710 is pw: salt so the problem starts with iterations, iterations take the first output as input and hash it again, this is done (iterations) 20 times? so you have the result of
take this sha512: salt 20 times
repeat 20 times sha512( pw: salt)
so with mode 1710 you will not get the pw
next thing is what about nil? is it appended, prepended to your pass? so your pass ist nil12345 or 12345nil? or what is nil?