From d0b961ac3492533c70ba886470c2b5b09bbeaf39 Mon Sep 17 00:00:00 2001 From: Christian Mauderer Date: Thu, 16 Nov 2017 16:01:16 +0100 Subject: bsp-howto: Add getentropy. Update #3239. --- bsp-howto/getentropy.rst | 39 +++++++++++++++++++++++++++++++++++++++ bsp-howto/index.rst | 1 + 2 files changed, 40 insertions(+) create mode 100644 bsp-howto/getentropy.rst (limited to 'bsp-howto') 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 +.. 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 `_ +and is also available in +`glibc since version 2.25 `_. +This system call is used by the Newlib provided +`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 `_. + +There is also a quite limited +`default implementation based on the CPU counter `_. +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 -- cgit v1.2.3