[rtems-docs commit] bsp-howto: Add getentropy.

[Sebastian Huber]

Module:    rtems-docs
Branch:    master
Commit:    d0b961ac3492533c70ba886470c2b5b09bbeaf39
Changeset: http://git.rtems.org/rtems-docs/commit/?id=d0b961ac3492533c70ba886470c2b5b09bbeaf39

Author:    Christian Mauderer <Christian.Mauderer at embedded-brains.de>
Date:      Thu Nov 16 16:01:16 2017 +0100

bsp-howto: Add getentropy.

Update #3239.

---

 bsp-howto/getentropy.rst | 39 +++++++++++++++++++++++++++++++++++++++
 bsp-howto/index.rst      |  1 +
 2 files changed, 40 insertions(+)

diff --git a/bsp-howto/getentropy.rst b/bsp-howto/getentropy.rst
new file mode 100644
index 0000000..b460450
--- /dev/null
+++ b/bsp-howto/getentropy.rst
@@ -0,0 +1,39 @@
+.. comment SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. COMMENT: COPYRIGHT (c) 2017 embedded brains GmbH <rtems at embedded-brains.de>
+.. COMMENT: All rights reserved.
+
+Entropy Source
+**************
+
+Each BSP must provide an implementation of the :c:func:`getentropy` system
+call.  This system call was introduced by
+`OpenBSD <https://man.openbsd.org/getentropy.2>`_
+and is also available in
+`glibc since version 2.25 <http://man7.org/linux/man-pages/man3/getentropy.3.html>`_.
+This system call is used by the Newlib provided
+`ARC4RANDOM(3) <https://man.openbsd.org/arc4random.3>`_ functions, which in
+turn are used by various cryptographic functions.
+
+.. warning::
+    A good entropy source is critical for (nearly) all cryptographic
+    applications. The default implementation based on the CPU counter is not
+    suitable for such applications.
+
+The :c:func:`getentropy` implementation must fill the specified memory region
+of the given size with random numbers and return 0 on success.
+
+In general, for embedded systems it is not easy to get some real entropy. Normally,
+that can only be reached with some extra hardware support. Some microcontrollers
+integrate a true random number generator or something similar for cryptographic
+applications. That is the preferred source of entropy for most BSPs. For example
+the
+`atsam BSP uses the TRNG for its entropy source <https://git.rtems.org/rtems/tree/c/src/lib/libbsp/arm/atsam/startup/getentropy-trng.c>`_.
+
+There is also a quite limited
+`default implementation based on the CPU counter <https://git.rtems.org/rtems/tree/c/src/lib/libbsp/shared/getentropy-cpucounter.c>`_.
+Due to the fact that it is a time based source, the values provided by
+:c:func:`getentropy` are quite predictable. This implementation is not
+appropriate for any cryptographic applications but it is good enough for some
+basic tasks. Use it only if you do not have any strong requirements on the
+entropy and if there is no better source.
diff --git a/bsp-howto/index.rst b/bsp-howto/index.rst
index 67e31d2..de93be0 100644
--- a/bsp-howto/index.rst
+++ b/bsp-howto/index.rst
@@ -49,6 +49,7 @@ to the Community Project hosted at http://www.rtems.org/.
 	initilization_code
 	console
 	clock
+	getentropy
 	i2c
 	spi
 	real_time_clock