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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 .
