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 Installation documentation.

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

Return text:

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


New in version 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.


Activates the OS random engine. This will effectively disable OpenSSL’s default CSPRNG.


New in version 1.7.

Returns the implementation of OS random engine.


This will activate the default OpenSSL CSPRNG.

Legacy provider in OpenSSL 3.x

New in version 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).

OS random engine


As of OpenSSL 1.1.1d its CSPRNG is fork-safe by default. cryptography does not compile or load the custom engine on >= 1.1.1d.

By default 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 macOS and FreeBSD /dev/urandom is an alias for /dev/random. The implementation on macOS uses the Yarrow algorithm. FreeBSD uses the Fortuna algorithm.

On Windows the implementation of CryptGenRandom depends on which version of the operation system you are using. See the Microsoft documentation for more details.

Linux uses its own PRNG design. /dev/urandom is a non-blocking source seeded from the same pool as /dev/random.



Linux >= 3.17 with working SYS_getrandom syscall


OpenBSD >= 5.6


BSD family (including macOS 10.12+) with SYS_getentropy in sys/syscall.h



/dev/urandom with cached file descriptor