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Python Enhancement Proposals

PEP 504 – Using the System RNG by default

Alyssa Coghlan <ncoghlan at>
Standards Track

Table of Contents


Python currently defaults to using the deterministic Mersenne Twister random number generator for the module level APIs in the random module, requiring users to know that when they’re performing “security sensitive” work, they should instead switch to using the cryptographically secure os.urandom or random.SystemRandom interfaces or a third party library like cryptography.

Unfortunately, this approach has resulted in a situation where developers that aren’t aware that they’re doing security sensitive work use the default module level APIs, and thus expose their users to unnecessary risks.

This isn’t an acute problem, but it is a chronic one, and the often long delays between the introduction of security flaws and their exploitation means that it is difficult for developers to naturally learn from experience.

In order to provide an eventually pervasive solution to the problem, this PEP proposes that Python switch to using the system random number generator by default in Python 3.6, and require developers to opt-in to using the deterministic random number generator process wide either by using a new random.ensure_repeatable() API, or by explicitly creating their own random.Random() instance.

To minimise the impact on existing code, module level APIs that require determinism will implicitly switch to the deterministic PRNG.

PEP Withdrawal

During discussion of this PEP, Steven D’Aprano proposed the simpler alternative of offering a standardised secrets module that provides “one obvious way” to handle security sensitive tasks like generating default passwords and other tokens.

Steven’s proposal has the desired effect of aligning the easy way to generate such tokens and the right way to generate them, without introducing any compatibility risks for the existing random module API, so this PEP has been withdrawn in favour of further work on refining Steven’s proposal as PEP 506.


Currently, it is never correct to use the module level functions in the random module for security sensitive applications. This PEP proposes to change that admonition in Python 3.6+ to instead be that it is not correct to use the module level functions in the random module for security sensitive applications if random.ensure_repeatable() is ever called (directly or indirectly) in that process.

To achieve this, rather than being bound methods of a random.Random instance as they are today, the module level callables in random would change to be functions that delegate to the corresponding method of the existing random._inst module attribute.

By default, this attribute will be bound to a random.SystemRandom instance.

A new random.ensure_repeatable() API will then rebind the random._inst attribute to a system.Random instance, restoring the same module level API behaviour as existed in previous Python versions (aside from the additional level of indirection):

def ensure_repeatable():
    """Switch to using random.Random() for the module level APIs

    This switches the default RNG instance from the cryptographically
    secure random.SystemRandom() to the deterministic random.Random(),
    enabling the seed(), getstate() and setstate() operations. This means
    a particular random scenario can be replayed later by providing the
    same seed value or restoring a previously saved state.

    NOTE: Libraries implementing security sensitive operations should
    always explicitly use random.SystemRandom() or os.urandom in order to
    correctly handle applications that call this function.
    if not isinstance(_inst, Random):
        _inst = random.Random()

To minimise the impact on existing code, calling any of the following module level functions will implicitly call random.ensure_repeatable():

  • random.seed
  • random.getstate
  • random.setstate

There are no changes proposed to the random.Random or random.SystemRandom class APIs - applications that explicitly instantiate their own random number generators will be entirely unaffected by this proposal.

Warning on implicit opt-in

In Python 3.6, implicitly opting in to the use of the deterministic PRNG will emit a deprecation warning using the following check:

if not isinstance(_inst, Random):
                  "Implicitly ensuring repeatability. "
                  "See help(random.ensure_repeatable) for details")

The specific wording of the warning should have a suitable answer added to Stack Overflow as was done for the custom error message that was added for missing parentheses in a call to print [10].

In the first Python 3 release after Python 2.7 switches to security fix only mode, the deprecation warning will be upgraded to a RuntimeWarning so it is visible by default.

This PEP does not propose ever removing the ability to ensure the default RNG used process wide is a deterministic PRNG that will produce the same series of outputs given a specific seed. That capability is widely used in modelling and simulation scenarios, and requiring that ensure_repeatable() be called either directly or indirectly is a sufficient enhancement to address the cases where the module level random API is used for security sensitive tasks in web applications without due consideration for the potential security implications of using a deterministic PRNG.

Performance impact

Due to the large performance difference between random.Random and random.SystemRandom, applications ported to Python 3.6 will encounter a significant performance regression in cases where:

  • the application is using the module level random API
  • cryptographic quality randomness isn’t needed
  • the application doesn’t already implicitly opt back in to the deterministic PRNG by calling random.seed, random.getstate, or random.setstate
  • the application isn’t updated to explicitly call random.ensure_repeatable

This would be noted in the Porting section of the Python 3.6 What’s New guide, with the recommendation to include the following code in the __main__ module of affected applications:

if hasattr(random, "ensure_repeatable"):

Applications that do need cryptographic quality randomness should be using the system random number generator regardless of speed considerations, so in those cases the change proposed in this PEP will fix a previously latent security defect.

Documentation changes

The random module documentation would be updated to move the documentation of the seed, getstate and setstate interfaces later in the module, along with the documentation of the new ensure_repeatable function and the associated security warning.

That section of the module documentation would also gain a discussion of the respective use cases for the deterministic PRNG enabled by ensure_repeatable (games, modelling & simulation, software testing) and the system RNG that is used by default (cryptography, security token generation). This discussion will also recommend the use of third party security libraries for the latter task.


Writing secure software under deadline and budget pressures is a hard problem. This is reflected in regular notifications of data breaches involving personally identifiable information [1], as well as with failures to take security considerations into account when new systems, like motor vehicles [2], are connected to the internet. It’s also the case that a lot of the programming advice readily available on the internet [#search] simply doesn’t take the mathematical arcana of computer security into account. Compounding these issues is the fact that defenders have to cover all of their potential vulnerabilities, as a single mistake can make it possible to subvert other defences [11].

One of the factors that contributes to making this last aspect particularly difficult is APIs where using them inappropriately creates a silent security failure - one where the only way to find out that what you’re doing is incorrect is for someone reviewing your code to say “that’s a potential security problem”, or for a system you’re responsible for to be compromised through such an oversight (and you’re not only still responsible for that system when it is compromised, but your intrusion detection and auditing mechanisms are good enough for you to be able to figure out after the event how the compromise took place).

This kind of situation is a significant contributor to “security fatigue”, where developers (often rightly [9]) feel that security engineers spend all their time saying “don’t do that the easy way, it creates a security vulnerability”.

As the designers of one of the world’s most popular languages [8], we can help reduce that problem by making the easy way the right way (or at least the “not wrong” way) in more circumstances, so developers and security engineers can spend more time worrying about mitigating actually interesting threats, and less time fighting with default language behaviours.


Why “ensure_repeatable” over “ensure_deterministic”?

This is a case where the meaning of a word as specialist jargon conflicts with the typical meaning of the word, even though it’s technically the same.

From a technical perspective, a “deterministic RNG” means that given knowledge of the algorithm and the current state, you can reliably compute arbitrary future states.

The problem is that “deterministic” on its own doesn’t convey those qualifiers, so it’s likely to instead be interpreted as “predictable” or “not random” by folks that are familiar with the conventional meaning, but aren’t familiar with the additional qualifiers on the technical meaning.

A second problem with “deterministic” as a description for the traditional RNG is that it doesn’t really tell you what you can do with the traditional RNG that you can’t do with the system one.

“ensure_repeatable” aims to address both of those problems, as its common meaning accurately describes the main reason for preferring the deterministic PRNG over the system RNG: ensuring you can repeat the same series of outputs by providing the same seed value, or by restoring a previously saved PRNG state.

Only changing the default for Python 3.6+

Some other recent security changes, such as upgrading the capabilities of the ssl module and switching to properly verifying HTTPS certificates by default, have been considered critical enough to justify backporting the change to all currently supported versions of Python.

The difference in this case is one of degree - the additional benefits from rolling out this particular change a couple of years earlier than will otherwise be the case aren’t sufficient to justify either the additional effort or the stability risks involved in making such an intrusive change in a maintenance release.

Keeping the module level functions

In additional to general backwards compatibility considerations, Python is widely used for educational purposes, and we specifically don’t want to invalidate the wide array of educational material that assumes the availability of the current random module API. Accordingly, this proposal ensures that most of the public API can continue to be used not only without modification, but without generating any new warnings.

Warning when implicitly opting in to the deterministic RNG

It’s necessary to implicitly opt in to the deterministic PRNG as Python is widely used for modelling and simulation purposes where this is the right thing to do, and in many cases, these software models won’t have a dedicated maintenance team tasked with ensuring they keep working on the latest versions of Python.

Unfortunately, explicitly calling random.seed with data from os.urandom is also a mistake that appears in a number of the flawed “how to generate a security token in Python” guides readily available online.

Using first DeprecationWarning, and then eventually a RuntimeWarning, to advise against implicitly switching to the deterministic PRNG aims to nudge future users that need a cryptographically secure RNG away from calling random.seed() and those that genuinely need a deterministic generator towards explicitly calling random.ensure_repeatable().

Avoiding the introduction of a userspace CSPRNG

The original discussion of this proposal on python-ideas[#csprng]_ suggested introducing a cryptographically secure pseudo-random number generator and using that by default, rather than defaulting to the relatively slow system random number generator.

The problem [7] with this approach is that it introduces an additional point of failure in security sensitive situations, for the sake of applications where the random number generation may not even be on a critical performance path.

Applications that do need cryptographic quality randomness should be using the system random number generator regardless of speed considerations, so in those cases.

Isn’t the deterministic PRNG “secure enough”?

In a word, “No” - that’s why there’s a warning in the module documentation that says not to use it for security sensitive purposes. While we’re not currently aware of any studies of Python’s random number generator specifically, studies of PHP’s random number generator [3] have demonstrated the ability to use weaknesses in that subsystem to facilitate a practical attack on password recovery tokens in popular PHP web applications.

However, one of the rules of secure software development is that “attacks only get better, never worse”, so it may be that by the time Python 3.6 is released we will actually see a practical attack on Python’s deterministic PRNG publicly documented.

Security fatigue in the Python ecosystem

Over the past few years, the computing industry as a whole has been making a concerted effort to upgrade the shared network infrastructure we all depend on to a “secure by default” stance. As one of the most widely used programming languages for network service development (including the OpenStack Infrastructure-as-a-Service platform) and for systems administration on Linux systems in general, a fair share of that burden has fallen on the Python ecosystem, which is understandably frustrating for Pythonistas using Python in other contexts where these issues aren’t of as great a concern.

This consideration is one of the primary factors driving the substantial backwards compatibility improvements in this proposal relative to the initial draft concept posted to python-ideas [6].


  • Theo de Raadt, for making the suggestion to Guido van Rossum that we seriously consider defaulting to a cryptographically secure random number generator
  • Serhiy Storchaka, Terry Reedy, Petr Viktorin, and anyone else in the python-ideas threads that suggested the approach of transparently switching to the random.Random implementation when any of the functions that only make sense for a deterministic RNG are called
  • Nathaniel Smith for providing the reference on practical attacks against PHP’s random number generator when used to generate password reset tokens
  • Donald Stufft for pursuing additional discussions with network security experts that suggested the introduction of a userspace CSPRNG would mean additional complexity for insufficient gain relative to just using the system RNG directly
  • Paul Moore for eloquently making the case for the current level of security fatigue in the Python ecosystem



Last modified: 2023-10-11 12:05:51 GMT