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Python Enhancement Proposals

PEP 524 – Make os.urandom() blocking on Linux

Victor Stinner <vstinner at>
Standards Track

Table of Contents


Modify os.urandom() to block on Linux 3.17 and newer until the OS urandom is initialized to increase the security.

Add also a new os.getrandom() function (for Linux and Solaris) to be able to choose how to handle when os.urandom() is going to block on Linux.

The bug

Original bug

Python 3.5.0 was enhanced to use the new getrandom() syscall introduced in Linux 3.17 and Solaris 11.3. The problem is that users started to complain that Python 3.5 blocks at startup on Linux in virtual machines and embedded devices: see issues #25420 and #26839.

On Linux, getrandom(0) blocks until the kernel initialized urandom with 128 bits of entropy. The issue #25420 describes a Linux build platform blocking at import random. The issue #26839 describes a short Python script used to compute a MD5 hash, systemd-cron, script called very early in the init process. The system initialization blocks on this script which blocks on getrandom(0) to initialize Python.

The Python initialization requires random bytes to implement a counter-measure against the hash denial-of-service (hash DoS), see:

Importing the random module creates an instance of random.Random: random._inst. On Python 3.5, random.Random constructor reads 2500 bytes from os.urandom() to seed a Mersenne Twister RNG (random number generator).

Other platforms may be affected by this bug, but in practice, only Linux systems use Python scripts to initialize the system.

Status in Python 3.5.2

Python 3.5.2 behaves like Python 2.7 and Python 3.4. If the system urandom is not initialized, the startup does not block, but os.urandom() can return low-quality entropy (even it is not easily guessable).

Use Cases

The following use cases are used to help to choose the right compromise between security and practicability.

Use Case 1: init script

Use a Python 3 script to initialize the system, like systemd-cron. If the script blocks, the system initialize is stuck too. The issue #26839 is a good example of this use case.

Use case 1.1: No secret needed

If the init script doesn’t have to generate any secure secret, this use case is already handled correctly in Python 3.5.2: Python startup doesn’t block on system urandom anymore.

Use case 1.2: Secure secret required

If the init script has to generate a secure secret, there is no safe solution.

Falling back to weak entropy is not acceptable, it would reduce the security of the program.

Python cannot produce itself secure entropy, it can only wait until system urandom is initialized. But in this use case, the whole system initialization is blocked by this script, so the system fails to boot.

The real answer is that the system initialization must not be blocked by such script. It is ok to start the script very early at system initialization, but the script may blocked a few seconds until it is able to generate the secret.

Reminder: in some cases, the initialization of the system urandom never occurs and so programs waiting for system urandom blocks forever.

Use Case 2: Web server

Run a Python 3 web server serving web pages using HTTP and HTTPS protocols. The server is started as soon as possible.

The first target of the hash DoS attack was web server: it’s important that the hash secret cannot be easily guessed by an attacker.

If serving a web page needs a secret to create a cookie, create an encryption key, …, the secret must be created with good entropy: again, it must be hard to guess the secret.

A web server requires security. If a choice must be made between security and running the server with weak entropy, security is more important. If there is no good entropy: the server must block or fail with an error.

The question is if it makes sense to start a web server on a host before system urandom is initialized.

The issues #25420 and #26839 are restricted to the Python startup, not to generate a secret before the system urandom is initialized.

Fix system urandom

Load entropy from disk at boot

Collecting entropy can take up to several minutes. To accelerate the system initialization, operating systems store entropy on disk at shutdown, and then reload entropy from disk at the boot.

If a system collects enough entropy at least once, the system urandom will be initialized quickly, as soon as the entropy is reloaded from disk.

Virtual machines

Virtual machines don’t have a direct access to the hardware and so have less sources of entropy than bare metal. A solution is to add a virtio-rng device to pass entropy from the host to the virtual machine.

Embedded devices

A solution for embedded devices is to plug an hardware RNG.

For example, Raspberry Pi have an hardware RNG but it’s not used by default. See: Hardware RNG on Raspberry Pi.

Denial-of-service when reading random

Don’t use /dev/random but /dev/urandom

The /dev/random device should only used for very specific use cases. Reading from /dev/random on Linux is likely to block. Users don’t like when an application blocks longer than 5 seconds to generate a secret. It is only expected for specific cases like generating explicitly an encryption key.

When the system has no available entropy, choosing between blocking until entropy is available or falling back on lower quality entropy is a matter of compromise between security and practicability. The choice depends on the use case.

On Linux, /dev/urandom is secure, it should be used instead of /dev/random. See Myths about /dev/urandom by Thomas Hühn: “Fact: /dev/urandom is the preferred source of cryptographic randomness on UNIX-like systems”

getrandom(size, 0) can block forever on Linux

The origin of the Python issue #26839 is the Debian bug report #822431: in fact, getrandom(size, 0) blocks forever on the virtual machine. The system succeeded to boot because systemd killed the blocked process after 90 seconds.

Solutions like Load entropy from disk at boot reduces the risk of this bug.


On Linux, reading the /dev/urandom can return “weak” entropy before urandom is fully initialized, before the kernel collected 128 bits of entropy. Linux 3.17 adds a new getrandom() syscall which allows to block until urandom is initialized.

On Python 3.5.2, os.urandom() uses the getrandom(size, GRND_NONBLOCK), but falls back on reading the non-blocking /dev/urandom if getrandom(size, GRND_NONBLOCK) fails with EAGAIN.

Security experts promotes os.urandom() to generate cryptographic keys because it is implemented with a Cryptographically secure pseudo-random number generator (CSPRNG). By the way, os.urandom() is preferred over ssl.RAND_bytes() for different reasons.

This PEP proposes to modify os.urandom() to use getrandom() in blocking mode to not return weak entropy, but also ensure that Python will not block at startup.


Make os.urandom() blocking on Linux

All changes described in this section are specific to the Linux platform.


  • Modify os.urandom() to block until system urandom is initialized: os.urandom() (C function _PyOS_URandom()) is modified to always call getrandom(size, 0) (blocking mode) on Linux and Solaris.
  • Add a new private _PyOS_URandom_Nonblocking() function: try to call getrandom(size, GRND_NONBLOCK) on Linux and Solaris, but falls back on reading /dev/urandom if it fails with EAGAIN.
  • Initialize hash secret from non-blocking system urandom: _PyRandom_Init() is modified to call _PyOS_URandom_Nonblocking().
  • random.Random constructor now uses non-blocking system urandom: it is modified to use internally the new _PyOS_URandom_Nonblocking() function to seed the RNG.

Add a new os.getrandom() function

A new os.getrandom(size, flags=0) function is added: use getrandom() syscall on Linux and getrandom() C function on Solaris.

The function comes with 2 new flags:

  • os.GRND_RANDOM: read bytes from /dev/random rather than reading /dev/urandom
  • os.GRND_NONBLOCK: raise a BlockingIOError if os.getrandom() would block

The os.getrandom() is a thin wrapper on the getrandom() syscall/C function and so inherit of its behaviour. For example, on Linux, it can return less bytes than requested if the syscall is interrupted by a signal.

Examples using os.getrandom()

Best-effort RNG

Example of a portable non-blocking RNG function: try to get random bytes from the OS urandom, or fallback on the random module.

def best_effort_rng(size):
    # getrandom() is only available on Linux and Solaris
    if not hasattr(os, 'getrandom'):
        return os.urandom(size)

    result = bytearray()
        # need a loop because getrandom() can return less bytes than
        # requested for different reasons
        while size:
            data = os.getrandom(size, os.GRND_NONBLOCK)
            result += data
            size -= len(data)
    except BlockingIOError:
        # OS urandom is not initialized yet:
        # fallback on the Python random module
        data = bytes(random.randrange(256) for byte in range(size))
        result += data
    return bytes(result)

This function can block in theory on a platform where os.getrandom() is not available but os.urandom() can block.


Example of function waiting timeout seconds until the OS urandom is initialized on Linux or Solaris:

def wait_for_system_rng(timeout, interval=1.0):
    if not hasattr(os, 'getrandom'):

    deadline = time.monotonic() + timeout
    while True:
            os.getrandom(1, os.GRND_NONBLOCK)
        except BlockingIOError:

        if time.monotonic() > deadline:
            raise Exception('OS urandom not initialized after %s seconds'
                            % timeout)


This function is not portable. For example, os.urandom() can block on FreeBSD in theory, at the early stage of the system initialization.

Create a best-effort RNG

Simpler example to create a non-blocking RNG on Linux: choose between Random.SystemRandom and Random.Random depending if getrandom(size) would block.

def create_nonblocking_random():
    if not hasattr(os, 'getrandom'):
        return random.Random()

        os.getrandom(1, os.GRND_NONBLOCK)
    except BlockingIOError:
        return random.Random()
        return random.SystemRandom()

This function is not portable. For example, random.SystemRandom can block on FreeBSD in theory, at the early stage of the system initialization.


Leave os.urandom() unchanged, add os.getrandom()

os.urandom() remains unchanged: never block, but it can return weak entropy if system urandom is not initialized yet.

Only add the new os.getrandom() function (wrapper to the getrandom() syscall/C function).

The secrets.token_bytes() function should be used to write portable code.

The problem with this change is that it expects that users understand well security and know well each platforms. Python has the tradition of hiding “implementation details”. For example, os.urandom() is not a thin wrapper to the /dev/urandom device: it uses CryptGenRandom() on Windows, it uses getentropy() on OpenBSD, it tries getrandom() on Linux and Solaris or falls back on reading /dev/urandom. Python already uses the best available system RNG depending on the platform.

This PEP does not change the API:

  • os.urandom(), random.SystemRandom and secrets for security
  • random module (except random.SystemRandom) for all other usages

Raise BlockingIOError in os.urandom()


PEP 522: Allow BlockingIOError in security sensitive APIs on Linux.

Python should not decide for the developer how to handle The bug: raising immediately a BlockingIOError if os.urandom() is going to block allows developers to choose how to handle this case:

  • catch the exception and falls back to a non-secure entropy source: read /dev/urandom on Linux, use the Python random module (which is not secure at all), use time, use process identifier, etc.
  • don’t catch the error, the whole program fails with this fatal exception

More generally, the exception helps to notify when sometimes goes wrong. The application can emit a warning when it starts to wait for os.urandom().


For the use case 2 (web server), falling back on non-secure entropy is not acceptable. The application must handle BlockingIOError: poll os.urandom() until it completes. Example:

def secret(n=16):
        return os.urandom(n)
    except BlockingIOError:

    print("Wait for system urandom initialization: move your "
          "mouse, use your keyboard, use your disk, ...")
    while 1:
        # Avoid busy-loop: sleep 1 ms
            return os.urandom(n)
        except BlockingIOError:

For correctness, all applications which must generate a secure secret must be modified to handle BlockingIOError even if The bug is unlikely.

The case of applications using os.urandom() but don’t really require security is not well defined. Maybe these applications should not use os.urandom() at the first place, but always the non-blocking random module. If os.urandom() is used for security, we are back to the use case 2 described above: Use Case 2: Web server. If a developer doesn’t want to drop os.urandom(), the code should be modified. Example:

def almost_secret(n=16):
        return os.urandom(n)
    except BlockingIOError:
        return bytes(random.randrange(256) for byte in range(n))

The question is if The bug is common enough to require that so many applications have to be modified.

Another simpler choice is to refuse to start before the system urandom is initialized:

def secret(n=16):
        return os.urandom(n)
    except BlockingIOError:
        print("Fatal error: the system urandom is not initialized")
        print("Wait a bit, and rerun the program later.")

Compared to Python 2.7, Python 3.4 and Python 3.5.2 where os.urandom() never blocks nor raise an exception on Linux, such behaviour change can be seen as a major regression.

Add an optional block parameter to os.urandom()

See the issue #27250: Add os.urandom_block().

Add an optional block parameter to os.urandom(). The default value may be True (block by default) or False (non-blocking).

The first technical issue is to implement os.urandom(block=False) on all platforms. Only Linux 3.17 (and newer) and Solaris 11.3 (and newer) have a well defined non-blocking API (getrandom(size, GRND_NONBLOCK)).

As Raise BlockingIOError in os.urandom(), it doesn’t seem worth it to make the API more complex for a theoretical (or at least very rare) use case.

As Leave os.urandom() unchanged, add os.getrandom(), the problem is that it makes the API more complex and so more error-prone.


The PEP was accepted on 2016-08-08 by Guido van Rossum.


Operating system random functions

os.urandom() uses the following functions:

On Linux, commands to get the status of /dev/random (results are number of bytes):

$ cat /proc/sys/kernel/random/entropy_avail
$ cat /proc/sys/kernel/random/poolsize

Why using os.urandom()?

Since os.urandom() is implemented in the kernel, it doesn’t have issues of user-space RNG. For example, it is much harder to get its state. It is usually built on a CSPRNG, so even if its state is “stolen”, it is hard to compute previously generated numbers. The kernel has a good knowledge of entropy sources and feed regularly the entropy pool.

That’s also why os.urandom() is preferred over ssl.RAND_bytes().


Last modified: 2023-09-09 17:39:29 GMT