getentropy(3) - NetBSD Manual Pages

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GETENTROPY(3)           NetBSD Library Functions Manual          GETENTROPY(3)


NAME

     getentropy -- generate uniform random seeds from system entropy for cryp-
     tography


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <unistd.h>

     int
     getentropy(void *buf, size_t buflen);

     #include <limits.h>

     #define GETENTROPY_MAX 256


DESCRIPTION

     The getentropy function fills buf with exactly buflen independent uniform
     random bytes derived from the system's entropy pool.

     The output of getentropy is meant to be unpredictable to an adversary and
     fit for use in cryptography.  See CAVEATS below.

     getentropy is meant for seeding random number generators, not for direct
     use by applications; most applications should use arc4random(3).

     buflen must be at most 256.


RETURN VALUES

     The getentropy() function returns the value 0 if successful; otherwise
     the value -1 is returned and the global variable errno is set to indicate
     the error.


ERRORS

     getentropy will succeed unless:

     [EFAULT]           The buf argument points to an invalid memory address.

     [EINVAL]           More than 256 bytes were requested.


CAVEATS

     Security can only be guaranteed relative to whatever unpredictable physi-
     cal processes or secret seed material are available to the system; see
     entropy(7).

     On systems which have no hardware random number generator and which have
     not had secret seed material loaded, NetBSD makes a reasonable effort to
     incorporate samples from various physical processes available to it that
     might be unpredictable from random jitter in timing.

     However, the getentropy interface alone can make no security guarantees
     without a physical system configuration that includes random number gen-
     eration hardware or secret seed material from such hardware on another
     machine.

     NetBSD 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 sub-
     sequent calls to getentropy in the clones yield independent outputs.
     However, this relies on the virtual machine host to notify the guest,
     e.g. through the acpivmgenid(4) device, and even so there is an unavoid-
     able small window of time between when the virtual machine is actually
     cloned and when the system is reseeded during which getentropy may yield
     identical outputs in the clones.


SEE ALSO

     arc4random(3), rnd(4), entropy(7)


STANDARDS

     The getentropy function conforms to IEEE Std 1003.1-2024 (``POSIX.1'').


HISTORY

     The getentropy function first appeared in OpenBSD 5.6, then in
     FreeBSD 12.0, and in NetBSD 10.0.

NetBSD 10.1                     August 28, 2024                    NetBSD 10.1