glibc/manual/crypt.texi

@node Cryptographic Functions, Debugging Support, System Configuration, Top
@chapter Cryptographic Functions
@c %MENU% A few functions to support cryptographic applications

@Theglibc{} includes only one type of special-purpose cryptographic
functions; these allow use of a source of cryptographically strong
pseudorandom numbers, if such a source is provided by the operating
system.  Programs that need general-purpose cryptography should use
a dedicated cryptography library, such as
@uref{https://www.gnu.org/software/libgcrypt/,,libgcrypt}.

@menu
* Unpredictable Bytes::         Randomness for cryptographic purposes.
@end menu

@node Unpredictable Bytes
@section Generating Unpredictable Bytes
@cindex randomness source
@cindex random numbers, cryptographic
@cindex pseudo-random numbers, cryptographic
@cindex cryptographic random number generator
@cindex deterministic random bit generator
@cindex CRNG
@cindex CSPRNG
@cindex DRBG

Cryptographic applications often need random data that will be as
difficult as possible for a hostile eavesdropper to guess.
The pseudo-random number generators provided by @theglibc{}
(@pxref{Pseudo-Random Numbers}) are not suitable for this purpose.
They produce output that is @emph{statistically} random, but fails to
be @emph{unpredictable}.  Cryptographic applications require a
@dfn{cryptographic random number generator} (CRNG), also known as a
@dfn{cryptographically strong pseudo-random number generator} (CSPRNG)
or a @dfn{deterministic random bit generator} (DRBG).

Currently, @theglibc{} does not provide a cryptographic random number
generator, but it does provide functions that read cryptographically
strong random data from a @dfn{randomness source} supplied by the
operating system.  This randomness source is a CRNG at heart, but it
also continually ``re-seeds'' itself from physical sources of
randomness, such as electronic noise and clock jitter.  This means
applications do not need to do anything to ensure that the random
numbers it produces are different on each run.

The catch, however, is that these functions will only produce
relatively short random strings in any one call.  Often this is not a
problem, but applications that need more than a few kilobytes of
cryptographically strong random data should call these functions once
and use their output to seed a CRNG.

Most applications should use @code{getentropy}.  The @code{getrandom}
function is intended for low-level applications which need additional
control over blocking behavior.

@deftypefun int getentropy (void *@var{buffer}, size_t @var{length})
@standards{GNU, sys/random.h}
@safety{@mtsafe{}@assafe{}@acsafe{}}

This function writes exactly @var{length} bytes of random data to the
array starting at @var{buffer}.  @var{length} can be no more than 256.
On success, it returns zero.  On failure, it returns @math{-1}, and
@code{errno} is set to indicate the problem.  Some of the possible
errors are listed below.

@table @code
@item ENOSYS
The operating system does not implement a randomness source, or does
not support this way of accessing it.  (For instance, the system call
used by this function was added to the Linux kernel in version 3.17.)

@item EFAULT
The combination of @var{buffer} and @var{length} arguments specifies
an invalid memory range.

@item EIO
@var{length} is larger than 256, or the kernel entropy pool has
suffered a catastrophic failure.
@end table

A call to @code{getentropy} can only block when the system has just
booted and the randomness source has not yet been initialized.
However, if it does block, it cannot be interrupted by signals or
thread cancellation.  Programs intended to run in very early stages of
the boot process may need to use @code{getrandom} in non-blocking mode
instead, and be prepared to cope with random data not being available
at all.

The @code{getentropy} function is declared in the header file
@file{sys/random.h}.  It is derived from OpenBSD.
@end deftypefun

@deftypefun ssize_t getrandom (void *@var{buffer}, size_t @var{length}, unsigned int @var{flags})
@standards{GNU, sys/random.h}
@safety{@mtsafe{}@assafe{}@acsafe{}}

This function writes up to @var{length} bytes of random data to the
array starting at @var{buffer}.  The @var{flags} argument should be
either zero, or the bitwise OR of some of the following flags:

@table @code
@item GRND_RANDOM
Use the @file{/dev/random} (blocking) source instead of the
@file{/dev/urandom} (non-blocking) source to obtain randomness.

If this flag is specified, the call may block, potentially for quite
some time, even after the randomness source has been initialized.  If it
is not specified, the call can only block when the system has just
booted and the randomness source has not yet been initialized.

@item GRND_NONBLOCK
Instead of blocking, return to the caller immediately if no data is
available.

@item GRND_INSECURE
Write random data that may not be cryptographically secure.
@end table

Unlike @code{getentropy}, the @code{getrandom} function is a
cancellation point, and if it blocks, it can be interrupted by
signals.

On success, @code{getrandom} returns the number of bytes which have
been written to the buffer, which may be less than @var{length}.  On
error, it returns @math{-1}, and @code{errno} is set to indicate the
problem.  Some of the possible errors are:

@table @code
@item ENOSYS
The operating system does not implement a randomness source, or does
not support this way of accessing it.  (For instance, the system call
used by this function was added to the Linux kernel in version 3.17.)

@item EAGAIN
No random data was available and @code{GRND_NONBLOCK} was specified in
@var{flags}.

@item EFAULT
The combination of @var{buffer} and @var{length} arguments specifies
an invalid memory range.

@item EINTR
The system call was interrupted.  During the system boot process, before
the kernel randomness pool is initialized, this can happen even if
@var{flags} is zero.

@item EINVAL
The @var{flags} argument contains an invalid combination of flags.
@end table

The @code{getrandom} function is declared in the header file
@file{sys/random.h}.  It is a GNU extension.

@end deftypefun