How to prove false statements: Practical attacks on Fiat-Shamir

https://www.quantamagazine.org/computer-scientists-figure-out-how-to-prove-lies-20250709/

by nsoonhui • 1 year ago

219 125 1 year ago

Paper: https://eprint.iacr.org/2025/118

tempodox• 1 year ago

Does that mean you can fake Bitcoins or cryptocurrency transactions? What exactly could be affected by these vulnerabilities? Is there a better article anywhere that actually spells it out for the layman?

sheiyei• 1 year ago

Extremely theoretically, and the article is very sensational.

The paper is half a year old, and hasn't made a splash; if this were significant news, I would expect to be able to find more coverage on it.

I did find this more nuanced take here: https://blog.cryptographyengineering.com/2025/02/04/how-to-p...

I haven't seen much of Quanta "Magazine", but I feel all of it has been stuff like this?

yorwba3• 1 year ago

The nuanced take was also discussed here at the time: https://news.ycombinator.com/item?id=42939312

verandaguy• 1 year ago

Quanta is a pretty popular, popular science outlet. It tends to be closer to the theory than (capital P, S) Popular Science magazine, but ultimately much of what they publish is digested to a degree for lay consumption.

They had an article just the other day about a more optimal sphere packing that was up my alley as a technical (programmer) person with a casual interest in broader pure math.

They do sensationalize a bit as a side effect of their process though, no argument there.

pas3• 1 year ago

usually they are very thorough (for a magazine targeting curious well-motivated, but of course still a virtually completely laymen audience), but it seems recently their volume has increased whil quality stayed constant :)

intalentive• 1 year ago

Quanta is “pop science” for smart lay people who might also read, for instance, the New Yorker.

karel-3d2• 1 year ago

From my cursory reading, it doesn't seem related to Bitcoin at all, but it might affect some more complex Ethereum protocols. Doesn't seem related to Ethereum itself, but it seems related to some zero-knowledge proofs.

edit: it seems to be related to something called "GKR protocol" that some cryptocurrencies use (?) - can use (?) - for somehow proving ... something? mining?.. using zero-knowledge proofs.... like here - https://www.polyhedra.network/expander (as usual in cryptocurrency, hard to tell what is actually being done/sold)

what I take from this, as a laic, is that... experimental ZK-proofs are indeed experimental.

lxgr4• 1 year ago

Schnorr signatures, which Bitcoin uses, are based on the Fiat-Shamir transform, but I don't know enough about this attack to be able to tell whether there's any problem with that particular instance of it.

baobun• 1 year ago

So the way Ethereum comes in is that the community at large is moving user activity to "L2s" - separate blockchains (sidechains) usually rolled up in and therefore secured by Ethereum Mainnet. Some of the newer L2s where apparently using this. So it affects Ethereum to the extent that its users could be bridging witg unsane protocols and implementations.

There are usually "bridge contracts" deployed on Mainnet to allow briding assets/tokens between them. This (besides obv exchanges) is where most of the ridiculous hacks and online theft of past few years have happened. The Axie/Ronin hack was a huge facepalm and should have been a lesson to be more wary of handwavy security claims of these more experimental networks.

bluGill4• 1 year ago

Probably - but you are likely to be caught as eventually someone will verify your work with a non-broke program. I'm not clear exactly how likely that is (I'm not interested enough in cryptocurrency to bother to dig into the algorithm, but IIRC several different parties need to agree on a transaction before it is considered real - or something like that, I hope I sound confused), but if you are doing a lot of bitcoin fraud someone will notice.

I'm not sure if they can trace the fraud to you.

fract0l• 1 year ago

A security researcher showed me years ago that blockchains were hackable. I don’t remember the proof, but since then have had low interest in crypto or blockchains. I’d like to make money off of it, but it’s insecure.

mckirk• 1 year ago

The major blockchains are basically billion-dollar bug bounty programs. If they were hackable that easily, we'd probably know already.

bobbiechen• 1 year ago

(Take this with a grain of salt as I only learned about the Fiat-Shamir heuristic via this HN thread last week https://news.ycombinator.com/item?id=44458168, and I only have basic experience in theoretical cryptography)

There exists the concept of a zero-knowledge proof: check out the Wikipedia page for some intuitive examples of how these work in an interactive context. Basically, by asking someone who wants to prove something (the prover) a bunch of questions (challenges), you can get probabilistic confidence that they actually know that thing: https://en.wikipedia.org/wiki/Zero-knowledge_proof#Abstract_...

You want it to be interactive because that makes it much harder for the prover to "fake it" on the spot. But it would be more convenient if you didn't need to be online and actively talking to each other - so we want a non-interactive way to do the same thing.

The Fiat-Shamir transform (or heuristic) says that we can transform interactive protocols into non-interactive ones by relying on "random" challenges. If the prover can't control the randomness, then it's about as good as you interactively challenging them (and you can e.g. make them do more challenges to make up for it).

How do we get randomness? In computing we don't really have anything totally random, but cryptographic hash functions are believed to be very difficult to predict the output to. So, in cryptography there's the "random oracle model" where you say, "Well, I don't know if this protocol is safe with these real-life hashes. But if the hash function was a truly random oracle, I can prove it's safe." (The Fiat-Shamir transform is only provably secure if you believe in the random oracle model).

In the past, researchers have constructed new protocols that are safe in the random oracle model, but once you use a real hash function they're breakable because of real-world implementation details. As the abstract of this paper says, "So far, all of these examples have been contrived protocols that were specifically designed to fail." See https://crypto.stackexchange.com/q/879 for some discussion of the mechanics of how it might happen, once you choose a real hash function.

This new paper advances the field by showing an attack that targets a real-world protocol that people actually use, GKR. It shows (and again, take my interpretation with a grain of salt) that when you pick a real hash function, the attacker can construct an input (circuit) that results in whatever output the attacker wants.

---

What's the real-world impact?

There do exist real non-interactive zero-knowledge proof systems, mainly used in blockchains. Instead of publicly exposing all the info to the world and doing computation on the (slow) blockchain, you can protect privacy of transactions and/or bundle a bunch of updates into a cheaper one (ZK-rollups). Theoretically these could be attacked using the methods described in the paper.

It's unclear to me whether those are affected here (though my guess is no, since they could have mentioned it if so).

cypherpunks012• 1 year ago

No, this could not allow for faking Bitcoin or Ethereum TXs. This type of vulnerability mainly concerns "zero-knowledge" proof methods, which do not occur inside the Bitcoin or Ethereum base layers. Some teams are building ZK proofs on top of these and other blockchains though, so those systems could be vulnerable, though they are still largely experimental.

austin-cheney9• 1 year ago

Hashes should never be a source of randomness. Randomness makes assumptions far outside their intended use case.

Hashes should only be a reproducible label that cannot be used to produce the material described by the hash. When used for their intended purposes hashes serve as the strongest point of integrity until value collisions are discovered.

ted_bunny• 1 year ago

I was so frustrated as a noob trying to understand why people were using hashes this way. Even a professional told me "yeah but a collision is really unlikely," and compared it to neutrino interference. How is that supposed to be good enough?

Tyyps3• 1 year ago

Hash functions are used to instantiate a random oracle (which is a theoretical object that can't be instantiated because it would be of infinite size but makes it easy to reason about) because it doesn't seems crazy as an assumption that if finding a collision between 2 hashes is hard it should be hard to predict the output of the so called hash function. However it is well known that there was some contrive counter example for protocols that are secure under the Random Oracle model and unsecure when instanciated with any hash function. The problem with this paper is that the protocol it described isn't so contrive anymore. Cryptography is a matter of assumptions and what you believe in or not. You might want to not use random oracle but you will therefore have to restrict yourself in what you can concretely build.

And the reason behind the problem outlined in the paper isn't a biased randomness problem but the fact that you can represent the hash function compared to a RO.

raxxorraxor• 1 year ago

Every hash function will have collisions as long as the input is larger than the resulting hash.

Some are designed that changing a bit has a massive influence on the resulting hash, others do the opposite.

bawolff2• 1 year ago

Whether hashes are appropriate depend on the details of the usecase.

However, if the negligible chance of a collision is what you are worried about, those also happen with random numbers.

thrance• 1 year ago

You should look into the HyperLogLog algorithm, where fair hash "randomness" is required for the algorithm to work. There are use cases where the pseudo-randomness of hashes is useful, is what I'm trying to say.

austin-cheney9• 1 year ago

This is why you should NEVER trust software developers to make security decisions unless certified to do so. True randomness is challenging to achieve, because computers are inherently predictable. Pseudo-randomness is an intended process to intentionally achieve randomness in spite of this high predictability, often through use of physical or electromagnetic criteria outside the computing machine.

Hash algorithms are none of that. They are not pseudo-randomness merely because a software developer merely wishes them to be so. Hash algorithms are intentionally designed to achieve high reproducibility in that a given set of input should always result in the same hash sequence as output. That intended reproducibility is by definition not random.

kbolino2• 1 year ago

But once you've made a function that "cannot be used to produce the material described by the hash", you've also made a very good pseudo-randomizer. In fact, if a cryptographic hash function cannot be trusted for its ability to produce apparent randomness, then it cannot be trusted for its "intended purposes". You get both properties or neither.

austin-cheney9• 1 year ago

This is broken logic.

There is an untested assumption that hashes achieve randomness because they appear to be a random collection of characters. Hash sequences are completely reproducible given a set of input, and that is by definition not random.

I think you are confusing loss of prediction as randomness. Never in mathematics or logic is that line of thinking correct. This can be described by equivocation, fallacy of composition, inductive fallacy, and more.

baby3• 1 year ago

You realize all signatures in use today basically use hash functions as randomness

austin-cheney9• 1 year ago

That is wrong. Most digital signatures in use today use certificates trusted through a certificate trust chain. The algorithms are different.

kurikuri• 1 year ago

What? You’ve managed to mangle so many terms in so few words… Signatures can refer to two things: integrity checks on a file or authentication checks for a recieved file. In the integrity check situation a hash function (e.g., SHA) is often used. In the authentication check situation, we usually use a public/private keypair for asymmetric encryption; the hash function is only part of the process. The key material used to make this keypair (should) comes from some random number generator…

The ‘hash’ function is a deterministic transform, not a source of randomness.

Ar-Curunir• 1 year ago

I'm sorry, but this comment is very vague and unclear.

Cryptographers know that hashes (even cryptographically strong ones!) are deterministic. Yet, it is possible that in going from an interactive proof to a non-interactive one, one does not actually need randomness. Indeed, for some class of protocols, we know how to design hash functions satisfying a particular property (correlation intractability) so that the resulting non-interactive proof is sound. It's just that (a) these hashes are inefficient, and (b) until now no one had found a non-contrived protocol where using standard hashes leads to an attack.

vlovich1232• 1 year ago

You may want to stay away from all modern CSPRNGs then. Eg Yarrow and Fortuna rely on sources of random input data being mixed in but using a strong hash function (nowadays sha-256) to produce the output at arbitrarily fast rates without consuming entropy.

And to your criticism that this is just programmers who don’t know what they’re doing, these algorithms were developed by Bruce Schneier, Niels Ferguson, and John Kelsey.

karel-3d2• 1 year ago

I find the actual paper more readable and understandable than this summarization.

https://eprint.iacr.org/2025/118

soulofmischief• 1 year ago

I also had to just go to the paper. It was really difficult trying to get through the article. Needlessly hyped language like "proving lies" and hyperfixations on things like leaking boats... felt like the author was either out of their element or inexperienced with math comms. However, the article still provides a bit of context that the paper doesn't.

That said, this is honestly just a bad article that is needlessly sensationalized and fails to impart any real knowledge.

some_furry• 1 year ago

That said, this is honestly just a bad article that is needlessly sensationalized and fails to impart any real knowledge.

There's a joke to be made here, since the issue is with zero-knowledge proof systems.

hinterlands• 1 year ago

(Note: the original title was "Computer Scientists Figure Out How To Prove Lies" before being changed by the admin)

I honestly think that Quanta Magazine just found the perfect formula to farm HN traffic. The titles are clearly carefully engineered for this audience: not the outright clickbait of university press releases, but vague profoundness that lets us upvote without reading the whole thing and then chime in with tangential anecdotes.

I don't think they're bad people, but I honestly think they end up on the front page multiple times a week not on the quality of the articles alone.

tyingq• 1 year ago

They do at least link to that in the 4th paragraph of the article. Many of these summary articles don't do that.

baby3• 1 year ago

I recommend this whiteboard session as well o.o https://blog.zksecurity.xyz/posts/pudding3/

lxgr4• 1 year ago

That's the case with many cryptographic explanations for laypeople, in my experience (as mostly a layperson).

Maybe all these elaborate analogies of Alice walking into a cave and Bob yelling which exit she should come out of, Alice wanting to sell Bob a Hamiltonian cycle trustlessly, Alice and Bob mixing buckets of paint and shipping them via the mail back and forth etc. are working for some people, but it's not me.

catfacts• 1 year ago

I recall someone creating a crypto system and then forgetting to protect the constructor of the initial object so other could change the constructor and do whatever they wanted with that crypto system, but in the end the creators were just web developers with a little training in crypto.

In those circumstances those millions of coins flying in or out are not a tragedy (at least for me) but a very plausible outcome.

lxgr4• 1 year ago

That's a completely different and unrelated type of vulnerability, though.

Implementation mistakes leading to mass coin theft would certainly be cryptocurrency news, but they would not be crypto(graphy) news. Breaking an actual peer-reviewed zero knowledge proof scheme would be.

doodlebugging8• 1 year ago

if a professor has assigned 100 problems for homework but doesn’t want to grade a student’s entire assignment, she can randomly choose 10 problems to grade. In the language of computer scientists, she is making 10 “random challenges” to the student’s homework. If the answers to those 10 problems are correct, the professor can feel confident that most of the other answers are correct too.

Eureka! I found the reason that so many things in society have gone to shit in the last few years. Far too many professors are so overworked or maybe just lazy and are using this type of tool to grade student work and the end result is that we have too many students passing through the system who have demonstrably only been able to score a 10/100.

I'm over 60 now and if I had scored lower than my current age back in the day I would fail and need to repeat the grade/course. Now they just kick the can('ts) on down the road and hope no one ever notices.

Too bad some of these failures end up in positions of influence where their uncharted deficiencies have the power to disrupt or destroy functional systems.

Or maybe I'm joking. I'll know once the caffeine hits.

deadbabe2• 1 year ago

Why would they not simply, assign 10 problems?

pas3• 1 year ago

presumably they glance at the other 90 too, and they then do a thorough verification on that random 10

the analogy is not great, but in cryptography something similar is at play (easy to get/check trivial properties and then hard to achieve/produce/fake the interesting ones)

Winsaucerer• 1 year ago

Presumably more questions can cover a wider variety of skills/techniques/topics. If the student doesn't know in advance which 10 will be selected, they either have to pray they're lucky, or work diligently on all problems.

doodlebugging8• 1 year ago

There is a risk here that the professor who only assigns 10 problems will only check one of them for correctness. If 5/10 of the answers are wrong but the professor verifies a correct answer for one of the 5/10 with a correct solution then their conclusion that the other 9/10 answers are correct due to some likelihood or probability function is invalid and a dimwit makes the grade.

tossandthrow2• 1 year ago

Now calculate the probability that the professor at random selects the exact 10 problems that were solved correctly.

Your Eureka moment seems to be misinformed - I hope you can have it returned for another occasion.

doodlebugging8• 1 year ago

It's just a random decaffeinated thought here this morning. Considering that it must be true that even a blind squirrel gets a nut once in a while it is also likely to be true that a professor at random can select the exact 10 problems that were solved correctly.

There's a difference between something with a probability of being true and another thing that is proven to be true. There are no doubts remaining after the proof whereas the probability always leaves wiggle room even if that wiggle room is a pretty tight space.

pas3• 1 year ago

they were shit all along, but the energy (and productivity) surplus allowed for decades of amazing growth, which created the feeling of prosperity

unfortunately most people doesn't understand (and as a consequence doesn't appreciate) how little wealth we have compared to capacity, in other words how much upkeep we do to maintain that wealth

and now that it's time to scale back the waste a bit people are up in arms

doodlebugging8• 1 year ago

As the caffeine sweeps through my system activating latent stores of energy and powers of analysis, I find that I appreciate this response without needing to understand all of it. It's a bit like assuming that the remaining 90/100 answers are correct after only verifying 10 of them.

I feel like I have accomplished more than I actually have and so I have plenty of incentive now to sweep through all the work of the day hoping that each randomly selected set of results yields similar levels of perfection and that all the inaccurate answers assumed to be correct do not cause the student to make assumptions in later life about things that are not supportable by facts.

t_mann2• 1 year ago

If the questions are randomly chosen, the probability of the true score being 10/100 in that scenario is 10!/(100!/90!) ~ 6e-14

doodlebugging8• 1 year ago

I'll accept your math this morning with the note that without checking the other 90/100 answers you have no way outside of accepting probabilities to know whether your random examples are representative of the quality of the full set. It becomes a "trust me bro" situation.

I processed hundreds of thousands of miles of seismic data in my career. The first thing we needed to do for any processing project was to select a subset of the data for use in defining the parameters that would be used to process the full volume of data. We used brute stacks - a preliminary output in the process - to locate interesting areas with complex attributes to make sure we could handle the volume's complexities. In industry slang these were "carefully selected random examples".

It was inevitable that we would find a situation during the processing for which our parameters were not optimized because we had missed that edge case in selecting the examples used for testing.

In the same way in real life if you only demonstrably know that 10% of the test answers are correct then it is also true that some edge case in the 90% of untested answers could leave all or part of that untested space false or sub-optimum.

If there was a point to my original post it is this. You only know something is true when you have proven it to be true. A maximum likelihood of truth is not a guarantee of truth it is only a guarantee that it is likely to be true. You won't know how sharp the sting will be until you peel the onion.

JW_00000• 1 year ago

This is just an analogy used by the article to explain the Fiat-Shamir transform.

metaesn5• 1 year ago

What is a lie? what is the truth? It is very difficult to find the truth, and the descriptions of the same thing are different for different persons. So the title and its content is a lie.

librasteve• 1 year ago

this is a lie

nicman23• 1 year ago

source?

metaesn5• 1 year ago

Please understand the restrictions of Newton's Laws of Motion, many years ago, some people think they are right in every environments. My point here is it is difficult to get the truth for somethings. the title means proving everythings whether they are lies.

zsoltkacsandi2• 1 year ago

and the descriotions of the same thing are different for different persons

When my son says, "I wasn't there" when the glass broke, it's not just a matter of differing descriptions, it's a clear denial of a fact. There are facts, and when someone deliberately twists or denies them, that's not just a different perspective. That's a lie.

metaesn5• 1 year ago

You are right, for somethings, the truth is clear, but for other things, it is difficult to find the last truth, only reach its truth over time

monkeyelite2• 1 year ago

I think this is a good example of a lie because you and your son both have a shared understanding of the world and he is deliberately saying something he does not believe to be true (that does not match his own internal recall of events).

There is a physical component to this lie but it seems to me that the social part dominates.

blueprint2• 1 year ago

Your question is very simple to solve for someone who has done enough real philosophy lol :) a lie is when you tell something differently than it is. the truth means "what exists as it is".

If you say that "it is difficult to find the truth", aside from the blatant subjectivity of your claim, then , if we believe your statement, then that itself must be a "truth". And yet you invalidate your own claim immediately by saying "different for different persons". Well, if that statement is true, then it invalidates itself as well.

You cannot disprove the existence of truth by using a system that relies on truth and falsehood.

metaesn5• 1 year ago

You seem to have some misunderstandings about truth. Many truths are our understanding of real existence, not existence itself. Obviously, our understanding of complex things may be different, but complex things do exist.

For example, the movement of things exists, but Newton's understanding of this existence is different from Einstein's understanding of the same existence (or the movement of more things).

sesm2• 1 year ago

There is a famous "4 theories of truth" classification: Correspondence, Coherence, Consensus and Pragmatic.

In this case they are talking about mathematical truth, which is a case of Coherence truth.

metaesn5• 1 year ago

For mathematical truth, there is an example, please search "The Limitations of Euclidean Geometry"

JanisErdmanis• 1 year ago

Can you recommend a book that expands on this classification?

bawolff2• 1 year ago

In context, these terms have specific definitions where this is not an issue.

PretzelPirate2• 1 year ago

When he shared his thoughts with Ethereum’s cryptographers, he was startled to learn that they were unfamiliar with this work

It would be nice if the article included timelines. Ethereum researchers have been talking about GKR since 2020,so it's hard to imagine the lack of familiarity.

yorwba3• 1 year ago

The time is given as "last October," and the work they were unfamiliar with is presumably "contrived proof protocols that are vulnerable to attack, no matter which hash function you use" as stated in the immediately preceding sentence.

PretzelPirate2• 1 year ago

That's confusing to me because back in 2020 when they were looking into GKR inside of a Snark, they were worried about these attacks. Following up in 2022, Ethereum researchers were talking about attacking GKR by forging proofs and not having sufficient randomness/collision resistant.

It's hard to align what's being researched on Ethresar.ch and this statement.

shkkmo• 1 year ago

I don't believe the "this work" that the article is talking about here is GKR, but work that is referenced earlier in the article:

In the early 2000s, computer scientists showed how to do just that, contriving interactive proof protocols that were specifically designed to fail when they underwent Fiat-Shamir

Indeed, the artcile points out that targeting GKR was the idea of the Ethereum Foundation researcher.

Soukhanov had the idea to target a Fiat-Shamir proof system based on something called the GKR protocol

bluGill4• 1 year ago

My initial prediction is they just divided by zero someplace - just like every other undergrad in any degree program that requires some advanced math has. I'm happy to see they didn't make this obvious mistake.

baby3• 1 year ago

Here's a whiteboard session going over that but https://blog.zksecurity.xyz/posts/pudding3/

GTP6• 1 year ago

The article is lacking a lot of details, so maybe I'll check the paper if I have the time in the coming days. But, my understanding from the article is that this attack works by breaking a premise of the considered protocols that doesn't have much to do with the random oracle model. They basically say that, if you agree on a program to use and you hash it as part of your commitment, then you can use the Fiat-Shamir transform to prove claims regarding the program's output. But it seems natural to me that, if you are tricked into accepting the use of a malicious program, then the protocol breaks. After all, the hashing of the program at the beginning is meant to ensure that you're using a specific binary you agreed upon, but it does nothing to show that such a binary works as intended. This has to be verified outside of such protocol.

Am I missing something? Or maybe the point is that, under the random oracle model, it should be hard to write a program that contains its own hash? But then again, would the trick of reading the hash from an external configuration file that isn't considered as part of the hashing be fair game?

quantumgarbage4• 1 year ago

Yes, what you are missing is that attacks on Fiat Shamir were very contrived up to now.

However the paper shows that there in fact exists a pretty simple way to break the Fiat Shamir heuristic, for a protocol operating in the RO model. And such kind of efficient attacks are rather concerning in cryptography land.

So this isn't about the attack per se, rather it's about the existence of such an easy one.

ShroudedNight• 1 year ago

What does "easy" mean in this context? From my [ignorant] reading, it sounds like it requires being able compute a fixed point for the hash function in order to be able to integrate it into the program and respond differently under test. I thought that was one of the things cryptographically secure hash functions explicitly made difficult?

GTP6• 1 year ago

I understand that this is the claim being made, but I think I'm still missing what is the attack's heart. From the article, it seems to boil down to "if you use a malicious program, then Fiat-Shamir is broken". But to me it seems more like that Fiat-Shamir would still give a correct proof of the program's output, it's just that the output itself was wrong from the start (I'm referring to the point in the article where they say such a malicious program behaves differently than intended when it detects its own hash being used). Is this attack actually letting you generate a valid proof for an output that the program doesn't generate under the given input?

_alternator_2• 1 year ago

The “choosing the program” part is vague, but in many practical cases the user gets to choose the program by choosing input data.

This goes back to the rather fuzzy distinction between “data” and “program” you may remember from your early CS days. More precisely, from a theoretical CS perspective, there is no solid difference between data and program.

Almost all practical ZK schemes require the user to choose some input (eg the root of the merkle tree representing the “current state” of the blockchain and secrets like keys and the amount of a transaction).

From some perspective, you get a different program for each different input; sometimes people call this “currying” or “partial evaluation”.

So yeah, it’s more serious than it seems at first blush.

Groxx• 1 year ago

It seems to be pretty explicit that the "program" being run contains the full hashing algorithm used (to output the correct answers most of the time) plus additional logic to allow cheating.

That rather clearly goes wildly beyond what most ZK schemes use. That's arbitrary code execution of your choice, either as input or as part of selecting the program. Which seems like it puts this somewhere near "if you allow `eval` on user input in your script, it could do anything", doesn't it?

Plus like. They fixed it. That seems to imply it's more of an implementation flaw than a fundamental, even if it may be a surprisingly achievable one.

GTP6• 1 year ago

So, in the end, what is the core concept of the attack? Were they able to generate a program (maybe exloit the fuzzyness of the distinction between data and program to generate a program) that contains the hash of itself? I doubt that this is it, as if this were the case, then it would be likely to be an issue with a specific hash function and not a general issue. Unless they are using some trick like the one I presented above, but then it seems to me that the problem wouldn't be with Fiat-Shamir itself.

Sniffnoy• 1 year ago

I think the thing that you're missing here is that the "malicicous program" being discussed here isn't malicious from the usual point of view of what "malicious" means. It's truly functionally identical to the original program -- it returns the same outputs on the same inputs, there isn't some secret input that makes it do something wrong.

Despite that, it's nonetheless "malicious" in that, with the modifications made to it, FS can be made to "prove" false things about it. So you can "prove" that M(x)=y, even though when you actually run M(x), you find that you don't get y.

EGreg• 1 year ago

The key to why this even works (and didn’t work before) is here: https://community.intercoin.app/t/paper-shows-relying-on-has...

Simply put, a reliable random oracle in an adversarial environment should be based on sources of randomness from multiple sources and participants, usually the sources are the participants’ meaningful actions to prevent collusion.

It has been known for quite a while that if the space of inputs being hashed is small, the hashing is relatively useless for most benefits of a true one-way function (eg hashing a phone number in USA).

How to prove false statements: Practical attacks on Fiat-Shamir

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