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42 lines
2.0 KiB
ReStructuredText
42 lines
2.0 KiB
ReStructuredText
.. SPDX-License-Identifier: CC-BY-SA-4.0
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.. Copyright (C) 2017 embedded brains GmbH & Co. KG
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Entropy Source
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**************
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Each BSP must provide an implementation of the :c:func:`getentropy` system
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call. This system call was introduced by
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`OpenBSD <https://man.openbsd.org/getentropy.2>`_
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and is also available in
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`glibc since version 2.25 <http://man7.org/linux/man-pages/man3/getentropy.3.html>`_.
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This system call is used by the Newlib provided
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`ARC4RANDOM(3) <https://man.openbsd.org/arc4random.3>`_ functions, which in
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turn are used by various cryptographic functions.
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.. warning::
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A good entropy source is critical for (nearly) all cryptographic
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applications. The default implementation based on the CPU counter is not
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suitable for such applications.
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The :c:func:`getentropy` implementation must fill the specified memory region
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of the given size with random numbers and return 0 on success. A non-zero
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return may cause the :c:macro:`INTERNAL_ERROR_ARC4RANDOM_GETENTROPY_FAIL`
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internal error by one of the
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`ARC4RANDOM(3) <https://man.openbsd.org/arc4random.3>`_ functions.
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In general, for embedded systems it is not easy to get some real entropy. Normally,
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that can only be reached with some extra hardware support. Some microcontrollers
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integrate a true random number generator or something similar for cryptographic
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applications. That is the preferred source of entropy for most BSPs. For example
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the
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`atsam BSP uses the TRNG for its entropy source <https://git.rtems.org/rtems/tree/bsps/arm/atsam/start/getentropy-trng.c>`_.
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There is also a quite limited
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`default implementation based on the CPU counter <https://git.rtems.org/rtems/tree/bsps/shared/dev/getentropy/getentropy-cpucounter.c>`_.
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Due to the fact that it is a time based source, the values provided by
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:c:func:`getentropy` are quite predictable. This implementation is not
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appropriate for any cryptographic applications but it is good enough for some
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basic tasks. Use it only if you do not have any strong requirements on the
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entropy and if there is no better source.
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