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.\" ========================================================================
.\"
.IX Title "RAND_add 3"
.TH RAND_add 3 "2009-07-19" "1.0.1n" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
RAND_add, RAND_seed, RAND_status, RAND_event, RAND_screen \- add
entropy to the PRNG
.SH "LIBRARY"
libcrypto, -lcrypto
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/rand.h>
\&
\& void RAND_seed(const void *buf, int num);
\&
\& void RAND_add(const void *buf, int num, double entropy);
\&
\& int  RAND_status(void);
\&
\& int  RAND_event(UINT iMsg, WPARAM wParam, LPARAM lParam);
\& void RAND_screen(void);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fIRAND_add()\fR mixes the \fBnum\fR bytes at \fBbuf\fR into the \s-1PRNG\s0 state. Thus,
if the data at \fBbuf\fR are unpredictable to an adversary, this
increases the uncertainty about the state and makes the \s-1PRNG\s0 output
less predictable. Suitable input comes from user interaction (random
key presses, mouse movements) and certain hardware events. The
\&\fBentropy\fR argument is (the lower bound of) an estimate of how much
randomness is contained in \fBbuf\fR, measured in bytes. Details about
sources of randomness and how to estimate their entropy can be found
in the literature, e.g. \s-1RFC 1750.\s0
.PP
\&\fIRAND_add()\fR may be called with sensitive data such as user entered
passwords. The seed values cannot be recovered from the \s-1PRNG\s0 output.
.PP
OpenSSL makes sure that the \s-1PRNG\s0 state is unique for each thread. On
systems that provide \f(CW\*(C`/dev/urandom\*(C'\fR, the randomness device is used
to seed the \s-1PRNG\s0 transparently. However, on all other systems, the
application is responsible for seeding the \s-1PRNG\s0 by calling \fIRAND_add()\fR,
\&\fIRAND_egd\fR\|(3)
or \fIRAND_load_file\fR\|(3).
.PP
\&\fIRAND_seed()\fR is equivalent to \fIRAND_add()\fR when \fBnum == entropy\fR.
.PP
\&\fIRAND_event()\fR collects the entropy from Windows events such as mouse
movements and other user interaction. It should be called with the
\&\fBiMsg\fR, \fBwParam\fR and \fBlParam\fR arguments of \fIall\fR messages sent to
the window procedure. It will estimate the entropy contained in the
event message (if any), and add it to the \s-1PRNG.\s0 The program can then
process the messages as usual.
.PP
The \fIRAND_screen()\fR function is available for the convenience of Windows
programmers. It adds the current contents of the screen to the \s-1PRNG.\s0
For applications that can catch Windows events, seeding the \s-1PRNG\s0 by
calling \fIRAND_event()\fR is a significantly better source of
randomness. It should be noted that both methods cannot be used on
servers that run without user interaction.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fIRAND_status()\fR and \fIRAND_event()\fR return 1 if the \s-1PRNG\s0 has been seeded
with enough data, 0 otherwise.
.PP
The other functions do not return values.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fIopenssl_rand\fR\|(3), \fIRAND_egd\fR\|(3),
\&\fIRAND_load_file\fR\|(3), \fIRAND_cleanup\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fIRAND_seed()\fR and \fIRAND_screen()\fR are available in all versions of SSLeay
and OpenSSL. \fIRAND_add()\fR and \fIRAND_status()\fR have been added in OpenSSL
0.9.5, \fIRAND_event()\fR in OpenSSL 0.9.5a.