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.Dd August 28, 2024
.Dt GETENTROPY 3
.Os
.Sh NAME
.Nm getentropy
.Nd generate uniform random seeds from system entropy for cryptography
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn getentropy "void *buf" "size_t buflen"
.In limits.h
.Pp
.Li #define GETENTROPY_MAX  256
.Sh DESCRIPTION
The
.Nm
function fills
.Fa buf
with exactly
.Fa buflen
independent uniform random bytes derived from the system's entropy
pool.
.Pp
The output of
.Nm
is meant to be unpredictable to an adversary and fit for use in
cryptography.
See
.Sx CAVEATS
below.
.Pp
.Nm
is meant for seeding random number generators, not for direct use by
applications; most applications should use
.Xr arc4random 3 .
.Pp
.Fa buflen
must be at most 256.
.Sh RETURN VALUES
.Rv -std getentropy
.Sh ERRORS
.Nm
will succeed unless:
.Bl -tag -width Er
.It Bq Er EFAULT
The
.Fa buf
argument points to an invalid memory address.
.It Bq Er EINVAL
More than 256 bytes were requested.
.El
.Sh CAVEATS
Security can only be guaranteed relative to whatever unpredictable
physical processes or secret seed material are available to the system;
see
.Xr entropy 7 .
.Pp
On systems which have no hardware random number generator and which
have not had secret seed material loaded,
.Nx
makes a reasonable effort to incorporate samples from various physical
processes available to it that might be unpredictable from random
jitter in timing.
.Pp
However, the
.Nm
interface alone can make no security guarantees without a physical
system configuration that includes random number generation hardware or
secret seed material from such hardware on another machine.
.Pp
.Nx
attempts to reseed the system entropy pool when it has detected the
system has been cloned as a guest in a virtual machine, so that
subsequent calls to
.Nm
in the clones yield independent outputs.
However, this relies on the virtual machine host to notify the guest,
e.g. through the
.Xr acpivmgenid 4
device, and even so there is an unavoidable small window of time
between when the virtual machine is actually cloned and when the system
is reseeded during which
.Nm
may yield identical outputs in the clones.
.Sh SEE ALSO
.Xr arc4random 3 ,
.Xr rnd 4 ,
.Xr entropy 7
.Sh STANDARDS
The
.Nm
function conforms to
.St -p1003.1-2024 .
.Sh HISTORY
The
.Nm
function first appeared in
.Ox 5.6 ,
then in
.Fx 12.0 ,
and in
.Nx 10.0 .