1=pod
2
3=head1 NAME
4
5RAND_add, RAND_seed, RAND_status, RAND_event, RAND_screen - add
6entropy to the PRNG
7
8=head1 SYNOPSIS
9
10 #include <openssl/rand.h>
11
12 void RAND_seed(const void *buf, int num);
13
14 void RAND_add(const void *buf, int num, double entropy);
15
16 int  RAND_status(void);
17
18 int  RAND_event(UINT iMsg, WPARAM wParam, LPARAM lParam);
19 void RAND_screen(void);
20
21=head1 DESCRIPTION
22
23RAND_add() mixes the B<num> bytes at B<buf> into the PRNG state. Thus,
24if the data at B<buf> are unpredictable to an adversary, this
25increases the uncertainty about the state and makes the PRNG output
26less predictable. Suitable input comes from user interaction (random
27key presses, mouse movements) and certain hardware events. The
28B<entropy> argument is (the lower bound of) an estimate of how much
29randomness is contained in B<buf>, measured in bytes. Details about
30sources of randomness and how to estimate their entropy can be found
31in the literature, e.g. RFC 1750.
32
33RAND_add() may be called with sensitive data such as user entered
34passwords. The seed values cannot be recovered from the PRNG output.
35
36OpenSSL makes sure that the PRNG state is unique for each thread. On
37systems that provide C</dev/urandom>, the randomness device is used
38to seed the PRNG transparently. However, on all other systems, the
39application is responsible for seeding the PRNG by calling RAND_add(),
40L<RAND_egd(3)|RAND_egd(3)>
41or L<RAND_load_file(3)|RAND_load_file(3)>.
42
43RAND_seed() is equivalent to RAND_add() when B<num == entropy>.
44
45RAND_event() collects the entropy from Windows events such as mouse
46movements and other user interaction. It should be called with the
47B<iMsg>, B<wParam> and B<lParam> arguments of I<all> messages sent to
48the window procedure. It will estimate the entropy contained in the
49event message (if any), and add it to the PRNG. The program can then
50process the messages as usual.
51
52The RAND_screen() function is available for the convenience of Windows
53programmers. It adds the current contents of the screen to the PRNG.
54For applications that can catch Windows events, seeding the PRNG by
55calling RAND_event() is a significantly better source of
56randomness. It should be noted that both methods cannot be used on
57servers that run without user interaction.
58
59=head1 RETURN VALUES
60
61RAND_status() and RAND_event() return 1 if the PRNG has been seeded
62with enough data, 0 otherwise.
63
64The other functions do not return values.
65
66=head1 SEE ALSO
67
68L<rand(3)|rand(3)>, L<RAND_egd(3)|RAND_egd(3)>,
69L<RAND_load_file(3)|RAND_load_file(3)>, L<RAND_cleanup(3)|RAND_cleanup(3)>
70
71=head1 HISTORY
72
73RAND_seed() and RAND_screen() are available in all versions of SSLeay
74and OpenSSL. RAND_add() and RAND_status() have been added in OpenSSL
750.9.5, RAND_event() in OpenSSL 0.9.5a.
76
77=cut
78