From 107927ee7a675b3bc10f6249d7c08199a50040d6 Mon Sep 17 00:00:00 2001 From: Nick Coghlan Date: Thu, 17 Sep 2015 00:21:45 +1000 Subject: [PATCH] Major simplification of PEP 504 - drop the submodule idea - call random.ensure_repeatable() to opt in to the PRNG - seed(), getstate(), setstate() all call ensure_repeatable() --- pep-0504.txt | 241 +++++++++++++++++++++++++++++---------------------- 1 file changed, 137 insertions(+), 104 deletions(-) diff --git a/pep-0504.txt b/pep-0504.txt index 11d4c4fe3..cfbb2b9d8 100644 --- a/pep-0504.txt +++ b/pep-0504.txt @@ -24,22 +24,19 @@ 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 if documentation and -developer education were going to solve it, they would have done so by now. +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. +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 compatibility break, calling any of the following module level -functions will count as opting in to using the deterministic random number -generator for all future calls to module level functions in the random -module in the same process: - -* ``random.seed`` -* ``random.getstate`` -* ``random.setstate`` +To minimise the impact on existing code, module level APIs that require +determinism will implicitly switch to the deterministic PRNG. Proposal ======== @@ -48,94 +45,130 @@ 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.seed``, ``random.getstate``, or ``random.setstate`` -are ever called in that process. +applications if ``random.ensure_repeatable()`` is ever called (directly or +indirectly) in that process. -This PEP further proposes to make it easier to explicitly opt in to using -either the system random number generator or Python's deterministic PRNG by -converting the random module to a package that exposes the same top-level API, -and offering two new subpackages: +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. -* ``random.system`` -* ``random.seedable`` +By default, this attribute will be bound to a ``random.SystemRandom`` instance. -The ``random.system`` submodule would provide the following bound methods of a -module global ``random.SystemRandom`` instance as module attributes: -``betavariate``, ``choice``, ``expovariate``, ``gammavariate``, ``gauss``, ``getrandbits``, ``lognormvariate``, ``normalvariate``, ``paretovariate``, -``randint``, ``random``, ``randrange``, ``sample``, ``shuffle``, -``triangular``, ``uniform``, ``vonmisesvariate``, ``weibullvariate`` +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):: -The ``random.seedable`` submodule would provide the same operations, but as -methods of a ``random.Random`` instance. In addition, it would provide the -following additional methods which are only meaningful when using a -deterministic random number generator: ``seed``, ``getstate``, ``setstate``. + def ensure_repeatable(): + """Switch to using random.Random() for the module level APIs""" + if not isinstance(_inst, Random): + _inst = random.Random() -Rather than being bound methods of a ``random.Random`` instance as they are -today, the module level callables in ``random`` itself would change to be -functions that, by default, delegated to the ``random.SystemRandom`` instance -in ``random.system``. +To minimise the impact on existing code, calling any of the following module +level functions will implicitly call ``random.ensure_repeatable()``: -Calling any one of ``random.seed``, ``random.getstate``, or ``random.setstate`` -would change the delegation to instead refer to the ``random.Random`` instance -in ``random.seedable``. +* ``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 seedable PRNG will emit a -deprecation warning. This warning will suggest explicitly opting in to either -the system RNG or the seedable PRNG. Possible wording: +In Python 3.6, implicitly opting in to the use of the deterministic PRNG will +emit a deprecation warning using the following check:: - "DeprecationWarning: Implicitly switching to the seedable PRNG. Consider - importing from random.system or random.seedable as appropriate" + if not isinstance(_inst, Random): + warnings.warn(DeprecationWarning, + "Implicitly ensuring repeatability. " + "See help(random.ensure_repeatable) for details") + ensure_repeatable() -Whatever precise wording is chosen should have an answer added to Stack -Overflow as was done for the custom error message that was added for missing -parentheses in a call to print [#print]_. +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 [#print]_. 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 removing the ability to seed the default RNG used -process wide - it's not a good idea relative to the alternative of explicitly -importing from the appropriate submodule (hence the eventually -visible-by-default warning), but it's also a concern that can be more -readily addressed on a project-by-project basis. +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"): + 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 associated security warning. +along with the documentation of the new ``ensure_repeatable`` function and the +associated security warning. -The docs would gain a discussion of the respective use cases for the seedable -PRNG (games, modelling & simulation, software testing) and the system RNG -(cryptography, security token generation). +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. Rationale ========= Writing secure software under deadline and budget pressures is a hard problem. -This is reflected in ongoing problems with data breaches involving personally +This is reflected in regular notifications of data breaches involving personally identifiable information [#breaches]_, as well as with failures to take security considerations into account when new systems, like motor vehicles -[#uconnect]_, are connected to the internet. Compounding the issue is the fact -that a lot of the programming advice readily available on the internet [#search] -simply doesn't take the mathemetical arcana of computer security into account, -and the fact that defenders have to cover *all* of their potential -vulnerabilites, as a single mistake can make it possible to subvert other -defences [#bcrypt]_. +[#uconnect]_, 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 mathemetical arcana of computer security into account. +Compounding these issues is the fact that defenders have to cover *all* of +their potential vulnerabilites, as a single mistake can make it possible to +subvert other defences [#bcrypt]_. 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 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). +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 [#owasptopten]_) feel that security engineers @@ -151,8 +184,8 @@ threats, and less time fighting with default language behaviours. Discussion ========== -Why "seedable" over "deterministic"? ------------------------------------- +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. @@ -163,16 +196,17 @@ 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 aren't familiar with the technical meaning. +folks that are familiar with the conventional meaning, but aren't familiar with +the additional qualifiers on the technical meaning. -The other problem with "deterministic" as a description for the traditional RNG -is that it doesn't tell you what you can *do* with the traditional RNG that you -can't do with the system one. +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. -"seedable" aims to address both those problems, as it doesn't have a misleading -common meaning, and it's a word form that means "you can seed this", which then -leads naturally into an exploration of what it means to "seed" a random number -generator. +"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+ ----------------------------------------- @@ -184,9 +218,9 @@ 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 the additional effort and -stability risks involved in making such an intrusive change in a maintenance -release. +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 ---------------------------------- @@ -198,18 +232,24 @@ 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. -Implicitly opting in to the deterministic RNG ---------------------------------------------- +Warning when implicitly opting in to the deterministic RNG +---------------------------------------------------------- -Python is widely used for modelling and simulation purposes, and in many cases, -these software models won't have a dedicated maintenance team tasked with -ensuing they keep working on the latest versions of Python. +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, preserves -compatibility with this existing software, while still nudging future users -that need a deterministic generator towards importing ``random.seedable`` -explicitly. +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 explicitily calling ``random.ensure_repeatable()``. Avoiding the introduction of a userspace CSPRNG ----------------------------------------------- @@ -224,23 +264,9 @@ 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. -What about the performance impact? ----------------------------------- - -Rather than introducing a userspace CSPRNG, this PEP instead proposes that we -accept the performance regression in cases where: - -* an 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 import from ``random.seedable`` - rather than ``random`` - -Applications that need cryptographic quality randomness should be using the -system random number generator regardless of speed considerations, while other -applications where speed is a more important consideration are better off with -the current PRNG implementation than they would be with a new CSPRNG. +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"? --------------------------------------------- @@ -252,6 +278,11 @@ studies of PHP's random number generator [#php]_ 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 ---------------------------------------- @@ -264,9 +295,9 @@ 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 backwards -compatibility improvements in this proposal relative to the initial draft -concept posted to python-ideas [#draft]_. +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 [#draft]_. Acknowledgements ================ @@ -284,6 +315,8 @@ Acknowledgements 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 References ==========