Document RAND library.

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+=pod
+
+=head1 NAME
+
+RAND_add, RAND_seed, RAND_screen - Add entropy to the PRNG
+
+=head1 SYNOPSIS
+
+ #include <openssl/rand.h>
+
+ void RAND_seed(const void *buf, int num);
+
+ void RAND_add(const void *buf, int num, int entropy);
+
+ void RAND_screen(void);
+
+=head1 DESCRIPTION
+
+RAND_add() mixes the B<num> bytes at B<buf> into the PRNG state. Thus,
+if the data at B<buf> are unpredictable to an adversary, this
+increases the uncertainty about the state and makes the PRNG output
+less predictable. Suitable input comes from user interaction (random
+key presses, mouse movements) and certain hardware events. The
+B<entropy> argument is (the lower bound of) an estimate of how much
+randomness is contained in B<buf>. Details about sources of randomness
+and how to estimate their entropy can be found in the literature,
+e.g. RFC 1750.
+
+RAND_add() may be called with sensitive data such as user entered
+passwords. The seed values cannot be recovered from the PRNG output.
+
+OpenSSL makes sure that the PRNG state is unique for each thread. On
+systems that provide C</dev/random>, the randomness device is used
+to seed the PRNG transparently. However, on all other systems, the
+application is responsible for seeding the PRNG by calling RAND_add()
+or RAND_load_file(3).
+
+RAND_seed() is equivalent to RAND_add() when B<num == entropy>.
+
+The RAND_screen() function is available for the convenience of Windows
+programmers. It adds the current contents of the screen to the PRNG.
+For applications that can catch Windows events, seeding the PRNG with
+the parameters of B<WM_MOUSEMOVE> events is a significantly better
+source of randomness. It should be noted that both methods cannot be
+used on servers that run without user interaction.
+
+=head1 RETURN VALUES
+
+These functions do not return values.
+
+=head1 SEE ALSO
+
+rand(3), RAND_load_file(3), RAND_cleanup(3)
+
+=head1 HISTORY
+
+RAND_seed() and RAND_screen() are available in all versions of SSLeay
+and OpenSSL. RAND_add() was added in OpenSSL 0.9.5.
+
+=cut