Tag Archives: CNF

Our tools for solving, counting and sampling

Development, Research, SAT, Tools, , ,

This post is just a bit of a recap of what we have developed over the years as part of our toolset of SAT solvers, counters, and samplers. Many of these tools depend on each other, and have taken greatly from other tools, papers, and ideas. These dependencies are too long to list here, but the list is long, probably starting somewhere around the Greek period, and goes all the way to recent work such as SharpSAT-td or B+E. My personal work stretches back to the beginning of CryptoMiniSat in 2009, and the last addition to our list is Pepin.

Overview

Firstly when I say “we” I loosely refer to the work of my colleagues and myself, often but not always part of the research group lead by Prof Kuldeep Meel. Secondly, almost all these tools depend on CryptoMiniSat, a SAT solver that I have been writing since around 2009. This is because most of these tools use DIMACS CNF as the input format and/or make use of a SAT solver, and CryptoMiniSat is excellent at reading, transforming , and solving CNFs. Thirdly, many of these tools have python interface, some connected to PySAT. Finally, all these tools are maintained by me personally, and all have a static Linux executable as part of their release, but many have a MacOS binary, and some even a Windows binary. All of them build with open source toolchains using open source libraries, and all of them are either MIT licensed or GPL licensed. There are no stale issues in their respective GitHub repositories, and most of them are fuzzed.

CryptoMiniSat

CryptoMiniSat (research paper) our SAT solver that can solve and pre- and inprocess CNFs. It is currently approx 30k+ lines of code, with a large amount of codebase dedicated to CNF transformations, which are also called “inprocessing” steps. These transformations are accessible to the outside via an API that many of the other tools take advantage of. CryptoMiniSat used to be a state-of-the-art SAT solver, and while it’s not too shabby even now, it hasn’t had the chance to shine at a SAT competition since 2020, when it came 3rd place. It’s hard to keep SAT solver competitive, there are many aspects to such an endeavor, but mostly it’s energy and time, some of which I have lately redirected into other projects, see below. Nevertheless, it’s a cornerstone of many of our tools, and e.g. large portions of ApproxMC and Arjun are in fact implemented in CryptoMiniSat, so that improvement in one tool can benefit all other tools.

Arjun

Arjun (research paper) is our tool to make CNFs easier to count with ApproxMC, our approximate counter. Arjun takes a CNF with or without a projection set, and computes a small projection set for it. What this means is that if say the question was: “How many solutions does this CNF has if we only count solutions to be distinct over variables v4, v5, and v6?”, Arjun can compute that in fact it’s sufficient to e.g. compute the solutions over variables v4 and v5, and that will be the same as the solutions over v4, v5, and v6. This can make a huge difference for large CNFs where e.g. the original projection set can be 100k variables, but Arjun can compute a projection set sometimes as small as a few hundred. Hence, Arjun is used as a preprocessor for our model counters ApproxMC and GANAK.

ApproxMC

ApproxMC (research paper) is our probabilistically approximate model counter for CNFs. This means that when e.g. ApproxMC gives a result, it gives it in a form of “The model count is between 0.9*M and 1.1*M, with a probability of 99%, and with a probability of 1%, it can be any value”. Which is very often enough for most cases of counting, and is much easier to compute than an exact count. It counts by basically halfing the solution space K times and then counts the remaining number of solutions. Then, the count is estimated to be 2^(how many times we halved)*(how many solutions remained). This halfing is done using XOR constraints, which CryptoMiniSat is very efficient at. In fact, no other state-of-the-art SAT solver can currently perform XOR reasoning other than CryptoMiniSat.

UniGen

UniGen (research paper) is an approximate probabilistic uniform sample generator for CNFs. Basically, it generates samples that are probabilistically approximately uniform. This can be hepful for example if you want to generate test cases for a problem, and you need the samples to be almost uniform. It uses ApproxMC to first count and then the same idea as ApproxMC to sample: add as many XORs as needed to half the solution space, and then take K random elements from the remaining (small) set of solutions. These will be the samples returned. Notice that UniGen depends on ApproxMC for counting, Arjun for projection minimization, and CryptoMiniSat for the heavy-lifting of solution/UNSAT finding.

GANAK

GANAK (research paper) is our probabilistic exact model counter. In other words, it returns a solution such as “This CNF has 847365 solutions, with a probability of 99.99%, and with 0.01% probability, any other value”. GANAK is based on SharpSAT and some parts of SharpSAT-td and GPMC. In its currently released form, it is in its infancy, and while usable, it needs e.g. Arjun to be ran on the CNF before, and while competitive, its ease-of-use could be improved. Vast improvements are in the works, though, and hopefully things will be better for the next Model Counting Competition.

CMSGen

CMSGen (research paper) is our fast, weighted, uniform-like sampler, which means it tries to give uniform samples the best it can, but it provides no guarantees for its correctness. While it provides no guarantees, it is surprisingly good at generating uniform samples. While these samples cannot be trusted in scenarios where the samples must be uniform, they are very effective in scenarios where a less-than-uniform sample will only degrade the performance of a system. For example, they are great at refining machine learning models, where the samples are taken uniformly at random from the area of input where the ML model performs poorly, to further train (i.e. refine) the model on inputs where it is performing poorly. Here, if the sample is not uniform, it will only slow down the learning, but not make it incorrect. However, generating provably uniform samples in such scenarios may be prohibitively expensive. CMSGen is derived from CryptoMiniSat, but does not import it as a library.

Bosphorus

Bosphorus (research paper) is our ANF solver, where ANF stands for Algebraic Normal Form. It’s a format used widely in cryptography to describe constraints over a finite field via multivariate polynomials over a the field of GF(2). Essentially, it’s equations such as “a XOR b XOR (b AND c) XOR true = false” where a,b,c are booleans. These allow some problems to be expressed in a very compact way and solving them can often be tantamount to breaking a cryptographic primitive such as a symmetric cipher. Bosphorus takes such a set of polynomials as input and either tries to simplify them via a set of inprocessing steps and SAT solving, and/or tries to solve them via translation to a SAT problem. It can output an equivalent CNF, too, that can e.g. be counted via GANAK, which will give the count of solutions to the original ANF. In this sense, Bosphorus is a bridge from ANF into our set of CNF tools above, allowing cryptographers to make use of the wide array of tools we have developed for solving, counting, and sampling CNFs.

Pepin

Pepin (research paper) is our probabilistically approximate DNF counter. DNF is basically the reverse of CNF — it’s trivial to ascertain if there is a solution, but it’s very hard to know if all solutions are present. However, it is actually extremely fast to probabilistically approximate how many solutions a DNF has. Pepin does exactly that. It’s one of the very few tools we have that doesn’t depend on CryptoMiniSat, as it deals with DNFs, and not CNFs. It basically blows all other such approximate counters out of the water, and of course its speed is basically incomparable to that of exact counters. If you need to count a DNF formula, and you don’t need an exact result, Pepin is a great tool of choice.

Conclusions

My personal philosophy has been that if a tool is not easily accessible (e.g. having to email the authors) and has no support, it essentially doesn’t exist. Hence, I try my best to keep the tools I feel responsible for accessible and well-supported. In fact, this runs so deep, that e.g. CryptoMiniSat uses the symmetry breaking tool BreakID, and so I made that tool into a robust library, which is now being packaged by Fedora, because it’s needed by CryptoMiniSat. In other words, I am pulling other people’s tools into the “maintained and supported” list of projects that I work with, because I want to make use of them (e.g. BreakID now builds on Linux, MacOS, and Windows). I did the same with e.g. the Louvain Community library, which had a few oddities/issues I wanted to fix.

Another oddity of mine is that I try my best to make our tools make sense to the user, work as intended, give meaningful (error) messages, and good help pages. For example, none of the tools I develop call subprocesses that make it hard to stop a computation, and none use a random number seed that can lead to reproducibility issues. While I am aware that working tools are sometimes less respected than a highly cited research paper, and so in some sense I am investing my time in a slightly suboptimal way, I still feel obliged to make sure the tax money spent on my academic salary gives something tangible back to the people who pay for it.

On benchmark randomization

Research, SAT, , ,

As many of you have heard, the SAT Competition for this year has been announced. You can send in your benchmarks between the 12th and the 22nd of April, so get started. I have a bunch of benchmarks I have already submitted about 2 years ago, still waiting for any reply from those organizers — but the organizers are different this year, so fingers crossed.

What I want to talk about today is benchmark randomization. This is a very-very touchy topic. However, I fear that it’s touchy for the wrong reasons, and so I think it’s important to talk about it in detail.

What is benchmark randomization?

Benchmark randomization is when a benchmark that is submitted is shuffled around a bit. There are many ways to shuffle a problem, and I will discuss this in a bit, but the point is that the problem at hand that is described by the benchmark CNF should not be changed, or changed only in a very-very minor way, such that everyone agrees that it doesn’t affect the core problem itself as described by the CNF.

Why do we need shuffling?

We need shuffling because simply put, there aren’t enough good benchmarks and so the benchmarks of yesteryear (and the year before, and before, and…) re-appear often. This would be OK if SAT solvers couldn’t be tuned to solving specific problems faster. Note that I am not suggesting that SAT solvers are intentionally manipulated to solve specific problems faster by unscrupulous researchers. Instead, the following happens.

Unintentional random seed improvements

Researchers test the performance of their SAT solvers on specific instances and then tune their solvers, testing the performance again and again on the same instances to check if they have improved performance. Logically this is the best way to test and improve performance: use the same well-defined test-set all the time for meaningful comparison. Since the researcher wants to use the instances that he/she thinks is the current use-case of SAT solvers, he naturally uses the instances of SAT competitions, since those are representative. I did and still do the same.

So, researchers add their idea to a SAT solver, and test. If the idea is not improving things then some change is made and tested again. Since modern CDCL SAT solvers behave quite randomly, and since any change in the source code changes the behaviour quite significantly, a small change in the source code (tuning of a parameter, for example) will change the behaviour. And since the set of problems tested on is fixed, there is a chance that more problems will be solved. If more are solved, the researcher might correctly interpret this as a general improvement, not specific to the problem set. However, it may very well be generic, it is also specific.

The above suggests that the randomness of the SAT solver is completely unintentionally tuned to specific problems — a subset of which will appear next year in the competition.

Easy fixes

Since there aren’t enough benchmark problems, and in particular some benchmark types are rare, I suggest to fix the unintentional tuning of solvers to specific problems by changing the benchmarks in minor ways. Here is a list, with an explanation why I think it’s OK to perform the manipulation:

  1. Propagate variables. Unitary clauses are often part of benchmarks. Propagating some of these, some recursively, gives quite a bit of problem space variation. Propagation is performed by every CDCL SAT solver, and I think many would be  surprised if it didn’t help SAT solvers that worked differently than  current SAT solvers. Agreeing on performing partial propagation is something that shouldn’t be too difficult.
  2. Renumber variables. For some variable X that is not used (or is fixed to a value that has been propagated), every variable that is higher than X is decremented by one, and the CNF header is fixed to reflect this change.  Such a minor renumbering may be approved by every researcher as something that doesn’t change the problem or its structure. Note that if  partial propagation is performed there should be quite a number of variables that can be removed. Renumbering some, but not others is a way to shuffle the problem. A more radical way of renumbering variables would be to completely shuffle them, however that would change the way the problem is described in quite a radical way, so some would correctly object and it’s not necessary anyway.
  3. Replace equivalent literals. Perform strongly connected component analysis and replace equivalent literals. This has been shown to significantly improve performance and I have never seen a case where it doesn’t. Since equivalent literal replacement can be performed with a lot of freedom, this is quite a bit of shuffling space. For example, if v1=v2=v3, then any of the v1, v2, v3 can be the one that replaces the rest in the CNF. Picking one randomly is a way to shuffle the instance

There are other ways of shuffling, but either they change the instance too much (e.g. blocked clause removal), or can be undone quite easily (e.g. shuffling the order of the clauses). In fact, (3) is already quite a touchy issue I think, but with (1) and (2) all could agree on. Neither requires the order of the literals or the order of the clauses to change — some clauses (e.g. unitary ones) and literals (some of those that are set) would be removed, but that’s all. The problem remains essentially unchanged such that most probably even the original problem author would easily recognize it. However, it would be different from a SAT solver point of view: these changes would change the random seed of the solver, forcing the solver to behave in a way that is less tuned to this specific problem instance.

Conclusion

SAT solvers are currently tuned too much to specific instances. This is not intentional by the researchers, however it still affects the results. To obtain better, less biased results we should shuffle the problem instances we have. Above, I suggested three ways to shuffle the instances in such a way that most would agree they don’t disturb or change the complexity of the underlying problem described by the instance. I hope that some of these suggestions will be employed, if not this year then for next year’s SAT competition such that we could reach better, more meaningful results.

anf2cnf script released

SAT, , ,

I have finally managed to fix the script that converts ANF problems to CNF format in the Sage math system. The original script was having some problems that I blogged about. The new script has corrected most of the shortcomings of the original script, as well as added some textual help for the user.

For instance, the equations

sage: print two_polynoms
[x0*x1 + 1, x0*x1 + x1]

that last time required 13 clauses and 4 variables in CNF, now look like this:

sage: print anf2cnf.cnf(two_polynoms)
p cnf 3 6
c ------------------------------
c Next definition: x0*x1 + 1
3 0
c ------------------------------
c Next definition: x0*x1 + x1
3 -2 0
-3 2 0
c ------------------------------
c Next definition: monomial x0*x1
1 -3 0
2 -3 0
3 -1 -2 0

which is 1 variable and 7 clauses shorter than the original, not to mention the visually cleaner look and human-parseable output. The new script is available here. Hopefully, some of my enhancements included in the Grain-of-Salt package will be included in this script. The problem is mainly that Grain-of-Salt uses radically different data structures, and is written in a different programming language, so porting is not trivial.

anf2cnf hell in Sage

SAT, ,

There is an ANF (Algebraic Normal Form) to CNF (Conjunctive Normal Form) converter by Martin Albrecht in Sage. Essentially, it performs the ANF to CNF conversion that I have described previously in this blog entry. Me, as unsuspecting as anyone else, have been using this for a couple of days now. It seemed to do its job. However, today, I wanted to backport some of my ideas to this converter. And then it hit me.

Let me illustrate with a short example why I think something is wrong with this converter. We will try to encode that variable 0 and variable 1 cannot both be TRUE. This is as simple as saying x0*x1 = 0 in plain old math. In Sage this is done like this:

sage: B = BooleanPolynomialRing(10,'x')
sage: load anf2cnf.py
sage: anf2cnf = ANFSatSolver(B)
sage: polynom = B.gen(0)*B.gen(1)
sage: print polynom
x0*x1

So far, so good. Let’s try to make a CNF out of this:

sage: print anf2cnf.cnf([polynom])
p cnf 4 6
2 -4 0
3 -4 0
4 -2 -3 0
1 0
4 1 0
-4 -1 0

Oooops. Why do we need 6 clauses to describe this? It can be described with exactly one:

p cnf 2 1
-1 -2

This lonely clause simply bans the solution 1 = TRUE, 2 = TRUE, which was our original aim.

Let me just mention one more thing about this converter: it repeats definitions. For example:

sage: print two_polynoms
[x1*x2 + 1, x1*x2 + x1]
sage:  print anf2cnf.cnf(two_polynoms)
p cnf 4 13
2 -4 0
3 -4 0
4 -2 -3 0
1 0
4 0
2 -4 0
3 -4 0
4 -2 -3 0
1 0
4 2 1 0
-4 -2 1 0
-4 2 -1 0
4 -2 -1 0

Notice that clause 2 -4 0 and the two following it have been repeated twice, as well as the clause setting 1 to TRUE.

I have been trying to get around these problems lately. When ready, the new script will be made available, along with some HOWTO. It will have some minor shortcomings, but already, the number of clauses in problem descriptions have dramatically dropped. For example, originally, the description of an example problem in CNF contained 221’612 clauses. After minor corrections, the same can now be described with only 122’042 clauses. This of course means faster solving, cleaner and even human-readable CNF output, etc. Fingers are crossed for an early release ;)

ANF to CNF conversion

SAT, , ,

Algebraic Normal Form, or simply ANF, is the form used to describe cryptographic functions. It looks like this:
a*b \oplus b*c \oplus b \oplus d = 0\\b*c \oplus c \oplus a = 0\\\ldots
Where a,\ldots d are binary variables, the \oplus sign is binary XOR, and the * sign is binary AND. Every item that is connected with the XOR-s is called a monomial, and its degree is the number of independent variables inside it. So, a*b is a 2-degree monomial in the first equation, and c is a 1-degree monomial in the second equation.

An important problem in SAT is to translate an ANF into the input of SAT solvers, Conjunctive Normal Form, or simply CNF. A CNF formula looks like this:
a \vee b \vee c = 1\\a \vee \neg d = 1\\\ldots
Where again a,\ldots d are binary variables, the \vee sign is the binary OR, and the \neg sign is the binary NOT (i.e. inverse).

The scientific reference paper

The most quoted article about ANF-to-CNF translation is that by Courtois and Bard, which advocates for the following translation process:

  1. Introduce internal variables for every monomial of degree larger than 1
  2. Describe the equations as large XORs using the recently introduced variables

The example problem in CNF

According to the original method, the equations presented above are first translated to the following form:
v1 = a*b\\v2 = b*c\\v1 \oplus v2 \oplus b \oplus d = 0\\v2 \oplus c \oplus a = 0

Where v1, v2 are fresh binary variables. Then, each of the above equations are translated to CNF. The internal variables are translated as such:

  1. Translation of v1 = a*b:
    v1 \vee \neg a \vee \neg b = 1\\\neg v1 \vee a = 1\\\neg 1 \vee b = 1
  2. Translation of v2 = b*c
    v2 \vee \neg b \vee \neg c = 1\\\neg v2 \vee b = 1\\\neg v2 \vee c = 1
  3. Translation of v1 + v2 + b + d = 0:
    \neg v1 \vee v2 \vee b \vee d = 1\\v1 \vee \neg v2 \vee b \vee d = 1\\v1 \vee v2 \vee \neg b \vee d = 1\\v1 \vee v2 \vee b \vee -d = 1\\\neg v1 \vee \neg v2 \vee \neg b \vee d = 1\\\neg v1 \vee \neg v2 \vee b \vee \neg d = 1\\\neg v1 \vee v2 \vee \neg b \vee \neg d = 1\\v1 \vee \neg v2 \vee \neg b \vee \neg d = 1
  4. Translation of v2 + c + a = 0 :
    v2 \vee c \vee \neg a = 1\\v2 \vee \neg c \vee a = 1\\\neg v2 \vee c \vee a = 1\\\neg v2 \vee \neg c \vee \neg a = 1

We are now done. The final CNF file is this. This final CNF  has a small header, and some  fluffs have been removed: variables are not named, but referred to with a number, and the = true-s have been replaced with a line-ending 0.

As you can imagine, there are many ways to enhance this process. I have written a set of them down in this paper. The set of improvements in a nutshell are the following:

  1. If a variable’s value is given, (e.g. a = true), first substitute this value in the ANF, and transcribe the resulting ANF to CNF.
  2. If there are two monomials, such as: a*b + b in an equation, make a non-standard monomial (-a)*b from the two, and transcribe this to CNF. Since the CNF can use negations, this reduces the size of the resulting CNF
  3. If the ANF can be described using Karnaugh maps shorter than with the method presented above, use that translation method.

An automated way

I just got a nice script to perform step (1) from Martin Albrecht, one of the developers of Sage:

sage: B = BooleanPolynomialRing(4500,'x')
sage: L = [B.random_element(degree=2,terms=10) 
      for _ in range(4000)]
sage: s = [B.gen(i) + GF(2).random_element() 
      for i in range(1000)]
sage: %time F = 
      mq.MPolynomialSystem(L+s).
      eliminate_linear_variables(maxlength=1)
CPU time: 1.03 s,  Wall time: 1.11 s

In this code, Martin generates a boolean system of equations with 4500 variables, with 4000 random equations each with randomly selected monomials of degree 2 and of XOR size 10. Then, he sets 1000 random variables to a random value (true or false), and finally, he substitutes the assigned values, and simplifies the system. If you think this is a trivial issue, alas, it is not. Both Maple and Sage take hours to perform it if using the standard eval function. The function above uses a variation of the ElimLin algorithm from the Courtois-Bard paper to do this efficiently.