Three mistakes from Dart/Flutter's weak PRNG
zellic.io
zellic.io
DTD is local host or over SSH due to the unecrypted websocket.
Browsers block unsecure WS over HTTPS. The key generated is for tagging multiple instances per IDE not security.
Random is instant and is used for runtime collision prevention and performance especially UI, not uniqueness, PRNG is not truly random anyways.
Random.secure has a large overhead accessing OS entropy and isn't always supported. [1]
Go 'math/rand', python 'Random', C# 'Random' to name a few are also not unique or safe for crypto.
Their secure equivalents: Go 'crypto/rand', Python 'SystemRandom', C# 'RNGCryptoServiceProvider'.
This isn't a Dart or Flutter issue [0]:
"A generator of random bool, int, or double values.
The default implementation supplies a stream of pseudo-random bits that are not suitable for cryptographic purposes.
Use the Random.secure constructor for cryptographic purposes."
[0] https://api.dart.dev/dart-math/Random-class.html [1] https://api.dart.dev/dart-math/Random/Random.secure.html
The author acknowledges early on that `Random.secure` is there to provide randomness where it needs to be secure, but then spends the entire article fretting over how the other source of randomness isn't. But it's not supposed to be!
Applications have plenty of reason for cheap, stable, or otherwise controlled forms of randomness and shouldn't have to pay the penalty for -- or face the reproducibility limitations of -- secure randonmness when that's not what they need. It's both useful and traditional for standard libraries to either include both forms, or to leave the secure form for other libraries to contribute.
If an application uses the wrong supply of randomness for their needs, that's an application error. And if developers writing security-sensitive code don't know to think anticipate this distinction and avoid the error, then that sounds like there was a project management error in assigning them such a sensitive task.
But I think we should, as language developers and users, be well beyond the point of pushing out and accepting insecure defaults with a little documentation disclaimer - especially if even the Dart developers themselves can't catch misuse of their own insecure APIs in widespread tools.
At the end of the day, we don't live in a computing world where true randomness is that resource-intensive anymore, and user-facing applications should basically always be relying on it. Non-randomness should really be the opt-in, when developers can actually justify that the performance penalty is a problem. Otherwise, I don't need library authors prematurely optimising my programs at the cost of security.
(There is a similar related discussion to have about the pervasiveness of floating points in programming languages when most applications would be better served by a more expensive, but more precise, numerical representation - since they rarely do that much calculation.)
1. Was there a security problem that resulted from the use of this insecure generator?
2. Were the developers who encountered this problem completely unreasonable and in the wrong, or were they simply non-(security)-experts who developed software that happened to have a security vulnerability?
3. If the latter, did the language/toolkit strongly encourage developers to use an insecure generator -- e.g., by making an insecure generator the default or "simpler" option (such as Random() instead of Random.secure()).
4. Is there some broad and general understanding that this toolkit is only intended for applications, such as monte-carlo testing or graphics rendering, where performance at the cost of security will always be the correct solution?
Given that the authors are able to offer several examples of real security vulnerabilities caused by developers using an insecure PRG, I'm going to read the answers as (1) Yes, (2) not unreasonable, (3) Yes, (4) I don't think so.
The decision to make insecure RNGs the default creates security vulnerabilities. This is demonstrable and has broad and unpredictable effects, given that developers are not security experts. The decision to make secure RNGs the default (easier path) has exactly one implication, which is that a small number of specialized applications will have slightly worse performance, performance that can easily be improved by making a conscious decision to remove the secure RNG.
On #2: The difference between a PRNG and a cryptographically secure random number generator is foundational knowledge for software engineers, and there's (finally) a strong trade consensus that writing security-sensitive code has a higher bar of preparation and shouldn't be done naively. It's entirely unreasonable that someone writing a `uuid` implementation that's meant to be a cryptographically secure library for use by application developers wouldn't anticipate that their standard library would offer both and would prioritize the PRNG as its basic and most accessible generator.
On #4: It's very rare that an application developer needs direct access to a cryptographically secure random number generator, especially because of that trade consensus that security-sensitive applications deserve expert care. In contrast, application developers are building test data pools, shuffling arrays, adding noise to signals, adding surprise to gameplay, writing and running reproducible tests, etc every day and naive use of a cryptographically secure random number generator in those contexts is easily paralyzing and catastrophic. The overhead can be order of magnitude in measure (which matters quite a lot in typical highly-repetitive/looped usage), and in some implementations it can introduce non-deterministic and potentially indefinite blocks on execution. And because they specifically can't be made deterministic by design, they significantly frustrate debug and test efforts that benefit from reproducibility. They're an essential tool in modern standard libraries, when they're needed in unusual cases, but are not suitable for naive everyday use at all.
* Most software developers (let's say, 95-99%+) are smart enough to avoid using insecure PRGs in their code.
* The remaining 1-5% have contributed to a litany of avoidable security disasters, nonetheless.
* The impact of these disasters has been vastly outsized when you compare it to the number of bad developers.
* If every developer understood that they needed security expertise (and indeed, that they were even writing security critical code, the world would be a very different place than it is.)
As Thomas mentioned below, I am something of a collector and obsessive when it comes to incidents of bad random number generation. What's fascinating to me about this Dart/Flutter story is not the result, it's that I haven't seen one of these stories in a long time!
By this I mean: this type of incident used to pop up on HN every month or so, and yet in the past few years they've become incredibly rare. And do you know why that is? It is not because developers got better. It's almost entirely due to the fact that development frameworks (particularly JS in browsers) have made a multi-year and systematic effort to reduce the availability of insecure default PRGs to bad developers. The result is that an entire bug class has gone from a common, monthly occurrence, to a relative rarity.
The idea that we should use insecure default RNGs because they're good for reproducible testing is a new one on me. If you need reproducible random numbers for your application, but you are asking to use an interface that does not explicitly include the fact that it generates non-random, reproducible numbers then you are doing both software development and testing wrong, IMHO.
That's a fair take and worth considering.
I suspect this whole dilemma speaks to a widening divide between those who see security as the top and inviolate priority in modern software and those still seeing software as something with much more varied and heterogenous concerns. And that gap gets most contentious when it comes to what things like what should be considered default, what capabilities should be available altogether, how convenient those features should be made, what emphasis should be made in education/training/mentorship, etc.
Undoubtedly, (often sloppy) network-delivered and network-attached software has come to dominate the industry and with it the consequences of security-practice lapses have become be more severe than they once had been. But at the same time, we can watch as things like correctness, stability, resource-efficiency, performance-efficiency, maintenance/repair-ability, etc tumble away and turn much of the same software into byzantine garbage that barely runs on X-core YYY-ghz hardware with ZZ-gigabytes of RAM and faults have the time when it does.
So I think there's a real tension, but I get where you're coming from. It may in fact be true that making a seeded PRNG less easily available would be a wise favor to the security-first-and-always people. :D
1. The default random() call/library should always produce exactly what it says -- real, secure, unpredictable (pseudo)random numbers.
2. For statistical and non-security applications it's perfectly fine to have a generator of the form random.insecureAndFast(). Or call it whatever you want, the important thing is that the developer make a tiny amount of conscious effort in the process of using it.
3. If you require reproducible and insecure generator, just use approach (2) above and add a seeding function. (Honestly I don't like anything that uses global state, but I don't care as long as it's labeled insecure.)
4. If you require reproducible and secure random numbers, then you have a slightly challenging problem on your hands that is way beyond the scope of this discussion.
For people who don't require secure randomness, the total "cost" of my proposal is something like an extra dozen characters added to your code in a few places -- and if that's too much work for you, then you probably aren't writing reproducible tests or spending a lot of time optimizing for speed. On the flipside, you might save someone a few million dollars when their crappy Bitcoin library uses a dependency that relies on random().
They're acknowledging that cryptographically useful entropy is a limited resource for the system as a whole, and naive exhaustion can either causes starvation, unbounded blocking, or degradation depending on the implementation.
Having a casual array shuffle seize up because there's not enough secure entropy available might just be an absurd nuisance in many cases. But having an session key or uuid generator seize up because somebody burnt all the entropy on casual array shuffles can be a real problem for users. (And much worse so if the implementation just silently degrades in quality instead of blocking or failing, as some have done.)
Every mainstream OS is trivially capable of seeding the system secure RNG by the time userspace applications need to consume it. There is no availability / depletion problem for userspace.
When the supply is treated conscientiously, depletion broadly doesn't matter outside of weird race conditions during system startup. But if everybody naively consumes cryptographically secure randomness for every casual occasion, it becomes a much bigger problem.
The parent comment is correct: entropy depletion is a weird meme. Unfortunately, Linux encoded that meme into its legacy KRNG ("/dev/random"), so lots of people believe it to be real. It is not, and modern userland programs don't use "/dev/random" anyways.
It's not the only such system, nor has that "legacy KPRN" been eradicated from real-world use.
Moreso, I can't understand how someone who seemingly has as much seasoning and experience as yourself can fathom making generalizing claims like "modern userland programs don't X".
Application software and the platforms that it runs are insanely more diverse than that, especially now that we live in a world where people just paste in verbatim docker stacks that they read about in some blogspam tutorial written 12 years ago, and countless others are building on frameworks and dependencies that are themselves broken in ways they oughtn't be (like Flutter, in the article).
While there exist contexts where cryptographically secure randomness may be treated as inexhaustible, we're decades away from being able to take that for granted across "modern userland programs" and its much safer to have people anticipate the dangers of exhaustion than it is to have them presume its universally inexhaustible.
"LRNG" => (idiosyncratic) shorthand for "Linux's KRNG".
The LRNG never had the problem you're alluding to. It's not as if the literature eventually found "inexhaustible" designs. They were never exhaustible. There is no such danger. You can get to this axiomatically by looking at how a CSPRNG works (again: it's effectively a stream cipher construction), or empirically by reading about it. Either way: no, you're wrong here.
As recently as node 16 or 18 or so, which is still in real-world use despite its age, reaning on the real-word docker images that these projects use, it happens. And when it does, you can rightly guess that all the client's javascript developers were utterly perplexed specifically because they'd never heard of such a concern. But you point to the stack trace for when their call to uuid never resolves and provide all the documentation from familiar bloggers to help them wrap their heads around it, and voila -- the mysterious and catastrophic issue that's been plaguing them is suddenly remedied.
As I said, I appreciate that exhaustion doesn't need to happen in anything with an appropriate design. That's genuinely great. But even if it's just a certain and inappropriate way of using the linux RNG in a certain window of Linux versions (and I'm not strictly convinced its even so narrow a scenario), that's a actually a pretty common real thing that real developers will continue to encounter in the wild for longer than any us might want, and not some baseless myth or meme.
Honestly it's kind of refreshing to see a thread like this. This was a live issue on HN like 10 years ago, when cryptographers started yelling at Ted T'so about how dumb "/dev/random" was, and generalist engineers on HN argued about it. But it has since become a settled matter --- here, I mean. Among cryptography engineers: never a doubt.
There are KRNG security issues! You can come up with cases, especially with embedded software or older hypervisors, where code runs with an unseeded RNG. That's very bad. But those issues aren't relevant to ordinary userland code; if your system can't run with the assumption that "/dev/urandom" is seeded, you're fucked for other reasons, so distributions (and hypervisors) go out of their way to ensure that can't happen.
At any rate: there is no case where "exhaustion" comes into play, and there never has been, but there was an archaic kernel interface in Linux that thought there could be such a thing, so there's a whole generation of Linux developers who believe it's a big deal. It is not.
A language designer or library developer, especially, generally can't assert that their code will only run on the most robust, modern, popular systems currently available. They need to work defensively, anticipating the issues that they may plausibly encounter -- especially where those issues will not be apparent to less expert application developers.
And those less expert application developers are in an even worse position, because many barely even know what they're doing in the first place, making all kinds of foolish decisions while they paste together their docker script and relying on inexpert devops/sre people (or end users!) who make their own choices about what things are running on.
It's encouraging to learn that there are now established, widespread, inexhaustibly secure RNG's out there. But it remains irrelevant to a discussion of what developers need to keep themselves mindful of because there remain many implementations in the wild that don't behave like that and most developers aren't in a position to guarantee which their code will encounter.
(Yes, I know, you have shared a workaround for a long time prior: https://sockpuppet.org/blog/2014/02/25/safely-generate-rando... . But that sort of presumes a clued-in user.)
That has the advantage of avoiding problems in a generation's time: if most platforms moved to random() being secure, it would be excusable if young programmers started assuming that would be the case on older platform too.
IMO, it's not worth it: It makes working with random numbers very problematic when working on cryptographic use cases.
I don't think rng seeding has anything to do with the algorithm you choose? Seeding from the os rng is usually what you want even for a prng. If you want to use use the current time there is a `seed_from_u64` if you want.
Seeding from an entropy source is critical for encryption, but that can take time depending on how it works.
Since dart/flutter is multi platform, using Random.secure for animation has it's own performance issues with interfacing host entropy RNG.
The majority of Dart/Flutter users are creating UI apps.
Few browsers with security policies and OS combination does not allow access to the entropy with Flutter Web in which Random.secure will fail, this isn't exclusive to Dart/Flutter. [1]
NaCL [0] offloads these concerns for developers, especially indie/startup.
[0] https://en.wikipedia.org/wiki/NaCl_(software)
[1] https://api.dart.dev/dart-math/Random/Random.secure.html
I'm more a fan of "make the defaults as secure as you can reasonably expect to get away with for each step of the way". It'll never be as secure as everyone wants but if you but up against "it's as secure as people would want to put up with by default" then things are at least at a good starting point for others to build from. The hard part is finding out what people are willing to put up with and which tradeoffs are worth it. That default random number generators "only" go at GB/s on most PCs because they produce really good random numbers is probably an easy tradeoff though.
- randomWeak() - randomSecure()
[0] In my experience, most of them -- who reads documentation these days...? Or programmers using editors like vim (no slight on vim by the way, don't shoot me!) which (generally) won't alert the programmer to the library options, unless it's been set up with a suggestion/completion/intellisense plugin?
[1] randomSecure() vs randomInsecure(), which can look similar at 3:00am on three hours sleep. Naming the functions explicitly differently makes it patently clear which function is cryptographically safe. Also helps when the interpreter/compiler won't understand the context the function is being called in, therefore can't throw warnings or errors to alert the overtired/overworked programmer that they're using insecure random number generation for crypto.
Big brain idea: Perhaps cryptographically-weak/insecure random number generators should be type incompatible by default, requiring an explicit cast to integer/float? Probably overkill, but definitely wards off mistakes and/or misuses by forcing the developer to type cast out, otherwise the compiler simply refuses, then explains why by fatal error at compile time, or by exception thrown.
But then why not make secure random number generators type incompatible by default? Because we're trying to encourage secure-by-default programming, making it slightly harder (but not onerous) to use the equivalent insecure function over secure one. Create a speed bump to prompt the programmer to think about their code, not a road block.
Instead we take sources that are still believed to be statistically random and influenced by underlying quantum randomness. E.g. RF interference, clock phasing, or an avalanche amplifier that are measures of entropy that is considered to be random to the same extent as say the ideal gas laws. But even those measures can still be imperfect; a probe might have a bias or manufacturing error that shows up later than QC.
To cope with this, we have PRNG designs that can use mixed sources in ways that will provably debias any such errors from a single source, and remain secure. All modern RNGs use these kind of PRNGs or DRBG/NRBG designs as a last stage. Far from being insecure, they are what give us more confidence in the overall system.
I was under the impression that's how entropy generators for servers work?
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-3991...
December 11, 2024 - Dart SDK 3.6.0 has been released. December 12, 2024 - Flutter SDK 3.27.0 have been released.
Alternately develop something that at least passes all the Dieharder tests.
There are many good options for a variety of requirements.
Maybe the following perspective is missing some key context here (?) but when I skimmed the code for this, I was stunned that such a low quality algorithm was used by Zellic (some kind of high end cyber genius cabal, it seems) and in cryptocurrency purposes. I'm rarely harsh on projects here but bloody hell that's utterly stupid. Especially when there are so many good options.
// TODO: Make this actually random
static int _initialSeed() => 0xCAFEBABEDEADBEEF;
It's simply incorrect for them to have naively used a PRNG there, regardless of whether it has a static seed or not. They should have used Random.secure all along, and anyone working in a cryptographically sensitive module, like `uuid` should be familiar enough with the distinction to make the right choice.
There's nothing especially harmful about having that static seed and there are even some diagnostic and debugging advantages offered by it. Shuffling the seed on init is a generous convenience for people who want to make sure there's more per-run variety in their randomness without setting a seed themselves, but (when using in a non-secure context) it quickly doesn't matter since anything but toy programs tend to have enough complexity, branching, and data-dependency that the sequence of invocations isn't consistent anyway.
Good to know Google doesn't care about my security. Long time Flutter user, but Google expresses disdain for it at every opportunity...
Might see if the C# mobile frameworks have caught up...