.\" Copyright (c) 2018 Mariusz Zaborski <oshogbo@FreeBSD.org>
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.Dd February 26, 2018
.Dt CAP_RANDOM 3
.Os
.Sh NAME
.Nm cap_random_buf
.Nd "library for getting entropy in capability mode"
.Sh LIBRARY
.Lb libcap_random
.Sh SYNOPSIS
.In sys/nv.h
.In libcasper.h
.In casper/cap_random.h
.Ft "int"
.Fn cap_random_buf "cap_channel_t *chan" "void *buf" "size_t nbytes"
.Sh DESCRIPTION
The function
.Fn cap_random_buf
is equivalent to
.Xr arc4random_buf 3
except that the connection to the
.Nm system.random
service needs to be provided.
.Sh EXAMPLES
The following example first opens a capability to casper and then uses this
capability to create the
.Nm system.random
casper service to obtain entropy.
.Bd -literal
cap_channel_t *capcas, *caprandom;
unsigned char buf[16];
int i;

/* Open capability to Casper. */
capcas = cap_init();
if (capcas == NULL)
	err(1, "Unable to contact Casper");

/* Enter capability mode sandbox. */
if (cap_enter() < 0 && errno != ENOSYS)
	err(1, "Unable to enter capability mode");

/* Use Casper capability to create capability to the system.random service. */
caprandom = cap_service_open(capcas, "system.random");
if (caprandom == NULL)
	err(1, "Unable to open system.random service");

/* Close Casper capability, we don't need it anymore. */
cap_close(capcas);

/* Obtain entropy. */
if (cap_random_buf(caprandom, buf, sizeof(buf)) < 0)
	err(1, "Unable to obtain entropy");

for (i = 0; i < sizeof(buf); i++)
	printf("%.2x ", buf[i]);
printf("\\n");
.Ed
.Sh SEE ALSO
.Xr cap_enter 2 ,
.Xr arc4random_buf 3 ,
.Xr err 3 ,
.Xr capsicum 4 ,
.Xr nv 9
.Sh AUTHORS
The
.Nm cap_random
service was implemented by
.An Pawel Jakub Dawidek Aq Mt pawel@dawidek.net
under sponsorship from the FreeBSD Foundation.
.Pp
This manual page was written by
.An Mariusz Zaborski Aq Mt oshogbo@FreeBSD.org .