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Diffstat (limited to 'docs/hazmat/backends/openssl.rst')
| -rw-r--r-- | docs/hazmat/backends/openssl.rst | 53 |
1 files changed, 52 insertions, 1 deletions
diff --git a/docs/hazmat/backends/openssl.rst b/docs/hazmat/backends/openssl.rst index 4db3972d..f7d6b710 100644 --- a/docs/hazmat/backends/openssl.rst +++ b/docs/hazmat/backends/openssl.rst @@ -16,10 +16,61 @@ The `OpenSSL`_ C library. * :class:`~cryptography.hazmat.backends.interfaces.HMACBackend` * :class:`~cryptography.hazmat.backends.interfaces.PBKDF2HMACBackend` - It has one additional public attribute. + It also exposes the following: .. attribute:: name The string name of this backend: ``"openssl"`` + .. method:: activate_osrandom_engine() + + Activates the OS random engine. This will effectively disable OpenSSL's + default CSPRNG. + + .. method:: deactivate_osrandom_engine() + + Deactivates the OS random engine if it is default. This will restore + the default OpenSSL CSPRNG. If the OS random engine is not the default + engine (e.g. if another engine is set as default) nothing will be + changed. + +OS Random Engine +---------------- + +OpenSSL uses a user-space CSPRNG that is seeded from system random ( +``/dev/urandom`` or ``CryptGenRandom``). This CSPRNG is not reseeded +automatically when a process calls ``fork()``. This can result in situations +where two different processes can return similar or identical keys and +compromise the security of the system. + +The approach this project has chosen to mitigate this vulnerability is to +include an engine that replaces the OpenSSL default CSPRNG with one that sources +its entropy from ``/dev/urandom`` on UNIX-like operating systems and uses +``CryptGenRandom`` on Windows. This method of pulling from the system pool +allows us to avoid potential issues with `initializing the RNG`_ as well as +protecting us from the ``fork()`` weakness. + +This engine is **active** by default when importing the OpenSSL backend. When +active this engine will be used to generate all the random data OpenSSL +requests. + +When importing only the binding it is added to the engine list but +**not activated**. + + +OS Random Sources +----------------- + +On OS X and FreeBSD ``/dev/urandom`` is an alias for ``/dev/random`` and +utilizes the `Yarrow`_ algorithm. + +On Windows ``CryptGenRandom`` is backed by `Fortuna`_. + +Linux uses its own PRNG design. ``/dev/urandom`` is a non-blocking source seeded +from the same pool as ``/dev/random``. + + .. _`OpenSSL`: https://www.openssl.org/ +.. _`initializing the RNG`: http://en.wikipedia.org/wiki/OpenSSL#Vulnerability_in_the_Debian_implementation +.. _`Yarrow`: http://en.wikipedia.org/wiki/Yarrow_algorithm +.. _`Fortuna`: http://en.wikipedia.org/wiki/Fortuna_(PRNG) |