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.\" ========================================================================
.\"
.IX Title "RAND_egd 3"
.TH RAND_egd 3 "2019-12-20" "1.0.2u" "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_egd, RAND_egd_bytes, RAND_query_egd_bytes \- query entropy gathering daemon
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/rand.h>
\&
\& int RAND_egd(const char *path);
\& int RAND_egd_bytes(const char *path, int bytes);
\&
\& int RAND_query_egd_bytes(const char *path, unsigned char *buf, int bytes);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBRAND_egd()\fR queries the entropy gathering daemon \s-1EGD\s0 on socket \fBpath\fR.
It queries 255 bytes and uses \fBRAND_add\fR\|(3) to seed the
OpenSSL built-in \s-1PRNG.\s0 RAND_egd(path) is a wrapper for
RAND_egd_bytes(path, 255);
.PP
\&\fBRAND_egd_bytes()\fR queries the entropy gathering daemon \s-1EGD\s0 on socket \fBpath\fR.
It queries \fBbytes\fR bytes and uses \fBRAND_add\fR\|(3) to seed the
OpenSSL built-in \s-1PRNG.\s0
This function is more flexible than \fBRAND_egd()\fR.
When only one secret key must
be generated, it is not necessary to request the full amount 255 bytes from
the \s-1EGD\s0 socket. This can be advantageous, since the amount of entropy
that can be retrieved from \s-1EGD\s0 over time is limited.
.PP
\&\fBRAND_query_egd_bytes()\fR performs the actual query of the \s-1EGD\s0 daemon on socket
\&\fBpath\fR. If \fBbuf\fR is given, \fBbytes\fR bytes are queried and written into
\&\fBbuf\fR. If \fBbuf\fR is \s-1NULL,\s0 \fBbytes\fR bytes are queried and used to seed the
OpenSSL built-in \s-1PRNG\s0 using \fBRAND_add\fR\|(3).
.SH "NOTES"
.IX Header "NOTES"
On systems without /dev/*random devices providing entropy from the kernel,
the \s-1EGD\s0 entropy gathering daemon can be used to collect entropy. It provides
a socket interface through which entropy can be gathered in chunks up to
255 bytes. Several chunks can be queried during one connection.
.PP
\&\s-1EGD\s0 is available from http://www.lothar.com/tech/crypto/ (\f(CW\*(C`perl
Makefile.PL; make; make install\*(C'\fR to install). It is run as \fBegd\fR
\&\fIpath\fR, where \fIpath\fR is an absolute path designating a socket. When
\&\fBRAND_egd()\fR is called with that path as an argument, it tries to read
random bytes that \s-1EGD\s0 has collected. \fBRAND_egd()\fR retrieves entropy from the
daemon using the daemon's \*(L"non-blocking read\*(R" command which shall
be answered immediately by the daemon without waiting for additional
entropy to be collected. The write and read socket operations in the
communication are blocking.
.PP
Alternatively, the EGD-interface compatible daemon \s-1PRNGD\s0 can be used. It is
available from
http://prngd.sourceforge.net/ .
\&\s-1PRNGD\s0 does employ an internal \s-1PRNG\s0 itself and can therefore never run
out of entropy.
.PP
OpenSSL automatically queries \s-1EGD\s0 when entropy is requested via \fBRAND_bytes()\fR
or the status is checked via \fBRAND_status()\fR for the first time, if the socket
is located at /var/run/egd\-pool, /dev/egd\-pool or /etc/egd\-pool.
.SH "RETURN VALUE"
.IX Header "RETURN VALUE"
\&\fBRAND_egd()\fR and \fBRAND_egd_bytes()\fR return the number of bytes read from the
daemon on success, and \-1 if the connection failed or the daemon did not
return enough data to fully seed the \s-1PRNG.\s0
.PP
\&\fBRAND_query_egd_bytes()\fR returns the number of bytes read from the daemon on
success, and \-1 if the connection failed. The \s-1PRNG\s0 state is not considered.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBrand\fR\|(3), \fBRAND_add\fR\|(3),
\&\fBRAND_cleanup\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fBRAND_egd()\fR is available since OpenSSL 0.9.5.
.PP
\&\fBRAND_egd_bytes()\fR is available since OpenSSL 0.9.6.
.PP
\&\fBRAND_query_egd_bytes()\fR is available since OpenSSL 0.9.7.
.PP
The automatic query of /var/run/egd\-pool et al was added in OpenSSL 0.9.7.

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✏️ RAND_egd_bytes.3ssl