Use of OpenSSL
==============

``cryptography`` depends on the `OpenSSL`_ C library for all cryptographic
operation. OpenSSL is the de facto standard for cryptographic libraries and
provides high performance along with various certifications that may be
relevant to developers.

A list of supported versions can be found in our :doc:`/installation`
documentation.

In general the backend should be considered an internal implementation detail
of the project, but there are some public methods available for debugging
purposes.

.. data:: cryptography.hazmat.backends.openssl.backend

    .. method:: openssl_version_text()

        :return text: The friendly string name of the loaded OpenSSL library.
            This is not necessarily the same version as it was compiled against.

    .. method:: openssl_version_number()

        .. versionadded:: 1.8

        :return int: The integer version of the loaded OpenSSL library. This is
            defined in ``opensslv.h`` as ``OPENSSL_VERSION_NUMBER`` and is
            typically shown in hexadecimal (e.g. ``0x1010003f``). This is
            not necessarily the same version as it was compiled against.

.. _legacy-provider:

Legacy provider in OpenSSL 3.x
------------------------------

.. versionadded:: 39.0.0

Users can set ``CRYPTOGRAPHY_OPENSSL_NO_LEGACY`` environment variable to
disable the legacy provider in OpenSSL 3.x. This will disable legacy
cryptographic algorithms, including ``Blowfish``, ``CAST5``, ``SEED``,
``ARC4``, and ``RC2`` (which is used by some encrypted serialization formats).

Additionally, the ``CRYPTOGRAPHY_BUILD_OPENSSL_NO_LEGACY`` environment variable
can be set during the build process to prevent the library from ever attempting
to load the legacy provider.

If loading the legacy provider is not disabled and the legacy provider fails to
load, a warning is emitted.

.. _`OpenSSL`: https://www.openssl.org/
