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Wednesday, February 28, 2024

Secured #6 – Writing Strong C – Finest Practices for Discovering and Stopping Vulnerabilities

For EIP-4844, Ethereum shoppers want the flexibility to compute and confirm KZG commitments. Slightly than every consumer rolling their very own crypto, researchers and builders got here collectively to put in writing c-kzg-4844, a comparatively small C library with bindings for higher-level languages. The thought was to create a sturdy and environment friendly cryptographic library that each one shoppers may use. The Protocol Safety Analysis group on the Ethereum Basis had the chance to assessment and enhance this library. This weblog submit will focus on some issues we do to make C initiatives safer.


Fuzzing is a dynamic code testing method that entails offering random inputs to find bugs in a program. LibFuzzer and afl++ are two widespread fuzzing frameworks for C initiatives. They’re each in-process, coverage-guided, evolutionary fuzzing engines. For c-kzg-4844, we used LibFuzzer since we had been already well-integrated with LLVM challenge’s different choices.

This is the fuzzer for verify_kzg_proof, one in all c-kzg-4844’s features:

#embody "../base_fuzz.h"

static const size_t COMMITMENT_OFFSET = 0;

int LLVMFuzzerTestOneInput(const uint8_t* information, size_t measurement) 
    if (measurement == INPUT_SIZE) 
        bool okay;
            (const Bytes48 *)(information + COMMITMENT_OFFSET),
            (const Bytes32 *)(information + Z_OFFSET),
            (const Bytes32 *)(information + Y_OFFSET),
            (const Bytes48 *)(information + PROOF_OFFSET),
    return 0;

When executed, that is what the output seems to be like. If there have been an issue, it could write the enter to disk and cease executing. Ideally, you need to be capable of reproduce the issue.

There’s additionally differential fuzzing, which is a way which fuzzes two or extra implementations of the identical interface and compares the outputs. For a given enter, if the output is completely different, and also you anticipated them to be the identical, you understand one thing is unsuitable. This method may be very widespread in Ethereum as a result of we wish to have a number of implementations of the identical factor. This diversification gives an additional stage of security, understanding that if one implementation had been flawed the others might not have the identical difficulty.

For KZG libraries, we developed kzg-fuzz which differentially fuzzes c-kzg-4844 (by way of its Golang bindings) and go-kzg-4844. Up to now, there have not been any variations.


Subsequent, we used llvm-profdata and llvm-cov to generate a protection report from operating the assessments. It is a nice method to confirm code is executed (“lined”) and examined. See the protection goal in c-kzg-4844’s Makefile for an instance of methods to generate this report.

When this goal is run (i.e., make protection) it produces a desk that serves as a high-level overview of how a lot of every perform is executed. The exported features are on the prime and the non-exported (static) features are on the underside.

There may be loads of inexperienced within the desk above, however there’s some yellow and purple too. To find out what’s and is not being executed, seek advice from the HTML file (protection.html) that was generated. This webpage exhibits your entire supply file and highlights non-executed code in purple. On this challenge’s case, many of the non-executed code offers with hard-to-test error circumstances corresponding to reminiscence allocation failures. For instance, this is some non-executed code:

Originally of this perform, it checks that the trusted setup is sufficiently big to carry out a pairing test. There is not a take a look at case which gives an invalid trusted setup, so this does not get executed. Additionally, as a result of we solely take a look at with the proper trusted setup, the results of is_monomial_form is all the time the identical and would not return the error worth.


We do not advocate this for all initiatives, however since c-kzg-4844 is a efficiency vital library we expect it is necessary to profile its exported features and measure how lengthy they take to execute. This might help determine inefficiencies which may probably DoS nodes. For this, we used gperftools (Google Efficiency Instruments) as a substitute of llvm-xray as a result of we discovered it to be extra feature-rich and simpler to make use of.

The next is an easy instance which profiles my_function. Profiling works by checking which instruction is being executed now and again. If a perform is quick sufficient, it might not be observed by the profiler. To cut back the possibility of this, you might must name your perform a number of occasions. On this instance, we name my_function 1000 occasions.

#embody <gperftools/profiler.h>

int task_a(int n) 
    if (n <= 1) return 1;
    return task_a(n - 1) * n;

int task_b(int n) 
    if (n <= 1) return 1;
    return task_b(n - 2) + n;

void my_function(void) 
    for (int i = 0; i < 500; i++) 
        if (i % 2 == 0) 

int fundamental(void) 
    for (int i = 0; i < 1000; i++) 
    return 0;

Use ProfilerStart(“<filename>”) and ProfilerStop() to mark which components of your program to profile. When re-compiled and executed, it can write a file to disk with profiling information. You may then use pprof to visualise this information.

Right here is the graph generated from the command above:

This is a much bigger instance from one in all c-kzg-4844’s features. The next picture is the profiling graph for compute_blob_kzg_proof. As you possibly can see, 80% of this perform’s time is spent performing Montgomery multiplications. That is anticipated.


Subsequent, view your binary in a software program reverse engineering (SRE) device corresponding to Ghidra or IDA. These instruments might help you perceive how high-level constructs are translated into low-level machine code. We expect it helps to assessment your code this manner; like how studying a paper in a distinct font will drive your mind to interpret sentences in a different way. It is also helpful to see what sort of optimizations your compiler makes. It is uncommon, however typically the compiler will optimize out one thing which it deemed pointless. Hold an eye fixed out for this, one thing like this truly occurred in c-kzg-4844, among the assessments had been being optimized out.

While you view a decompiled perform, it is not going to have variable names, complicated varieties, or feedback. When compiled, this data is not included within the binary. Will probably be as much as you to reverse engineer this. You will typically see features are inlined right into a single perform, a number of variables declared in code are optimized right into a single buffer, and the order of checks are completely different. These are simply compiler optimizations and are typically fantastic. It might assist to construct your binary with DWARF debugging data; most SREs can analyze this part to offer higher outcomes.

For instance, that is what blob_to_kzg_commitment initially seems to be like in Ghidra:

With a little bit work, you possibly can rename variables and add feedback to make it simpler to learn. This is what it may appear like after a couple of minutes:

Static Evaluation

Clang comes built-in with the Clang Static Analyzer, which is a wonderful static evaluation device that may determine many issues that the compiler will miss. Because the title “static” suggests, it examines code with out executing it. That is slower than the compiler, however rather a lot sooner than “dynamic” evaluation instruments which execute code.

This is a easy instance which forgets to free arr (and has one other downside however we are going to speak extra about that later). The compiler is not going to determine this, even with all warnings enabled as a result of technically that is fully legitimate code.

#embody <stdlib.h>

int fundamental(void) 
    int* arr = malloc(5 * sizeof(int));
    arr[5] = 42;
    return 0;

The unix.Malloc checker will determine that arr wasn’t freed. The road within the warning message is a bit deceptive, nevertheless it is sensible if you consider it; the analyzer reached the return assertion and observed that the reminiscence hadn’t been freed.

Not the entire findings are that easy although. This is a discovering that Clang Static Analyzer present in c-kzg-4844 when initially launched to the challenge:

Given an surprising enter, it was doable to shift this worth by 32 bits which is undefined conduct. The answer was to limit the enter with CHECK(log2_pow2(n) != 0) in order that this was not possible. Good job, Clang Static Analyzer!


Santizers are dynamic evaluation instruments which instrument (add directions) to applications which may level out points throughout execution. These are notably helpful at discovering widespread errors related to reminiscence dealing with. Clang comes built-in with a number of sanitizers; listed here are the 4 we discover most helpful and straightforward to make use of.

Deal with

AddressSanitizer (ASan) is a quick reminiscence error detector which may determine out-of-bounds accesses, use-after-free, use-after-return, use-after-scope, double-free, and reminiscence leaks.

Right here is identical instance from earlier. It forgets to free arr and it’ll set the sixth ingredient in a 5 ingredient array. It is a easy instance of a heap-buffer-overflow:

#embody <stdlib.h>

int fundamental(void) 
    int* arr = malloc(5 * sizeof(int));
    arr[5] = 42;
    return 0;

When compiled with -fsanitize=deal with and executed, it can output the next error message. This factors you in route (a 4-byte write in fundamental). This binary could possibly be seen in a disassembler to determine precisely which instruction (at fundamental+0x84) is inflicting the issue.

Equally, this is an instance the place it finds a heap-use-after-free:

#embody <stdlib.h>

int fundamental(void) 
    int *arr = malloc(5 * sizeof(int));
    return arr[2];

It tells you that there is a 4-byte learn of freed reminiscence at fundamental+0x8c.


MemorySanitizer (MSan) is a detector of uninitialized reads. This is a easy instance which reads (and returns) an uninitialized worth:

int fundamental(void) 
    int information[2];
    return information[0];

When compiled with -fsanitize=reminiscence and executed, it can output the next error message:

Undefined Conduct

UndefinedBehaviorSanitizer (UBSan) detects undefined conduct, which refers back to the state of affairs the place a program’s conduct is unpredictable and never specified by the langauge normal. Some widespread examples of this are accessing out-of-bounds reminiscence, dereferencing an invalid pointer, studying uninitialized variables, and overflow of a signed integer. For instance, right here we increment INT_MAX which is undefined conduct.

#embody <limits.h>

int fundamental(void) 
    int a = INT_MAX;
    return a + 1;

When compiled with -fsanitize=undefined and executed, it can output the next error message which tells us precisely the place the issue is and what the circumstances are:


ThreadSanitizer (TSan) detects information races, which may happen in multi-threaded applications when two or extra threads entry a shared reminiscence location on the identical time. This case introduces unpredictability and may result in undefined conduct. This is an instance through which two threads increment a worldwide counter variable. There are no locks or semaphores, so it is totally doable that these two threads will increment the variable on the identical time.

#embody <pthread.h>

int counter = 0;

void *increment(void *arg) 
    for (int i = 0; i < 1000000; i++)
    return NULL;

int fundamental(void) 
    pthread_t thread1, thread2;
    pthread_create(&thread1, NULL, increment, NULL);
    pthread_create(&thread2, NULL, increment, NULL);
    pthread_join(thread1, NULL);
    pthread_join(thread2, NULL);
    return 0;

When compiled with -fsanitize=thread and executed, it can output the next error message:

This error message tells us that there is a information race. In two threads, the increment perform is writing to the identical 4 bytes on the identical time. It even tells us that the reminiscence is counter.


Valgrind is a robust instrumentation framework for constructing dynamic evaluation instruments, however its greatest recognized for figuring out reminiscence errors and leaks with its built-in Memcheck device.

The next picture exhibits the output from operating c-kzg-4844’s assessments with Valgrind. Within the purple field is a sound discovering for a “conditional bounce or transfer [that] is dependent upon uninitialized worth(s).”

This recognized an edge case in expand_root_of_unity. If the unsuitable root of unity or width had been supplied, it was doable that the loop will break earlier than out[width] was initialized. On this state of affairs, the ultimate test would depend upon an uninitialized worth.

static C_KZG_RET expand_root_of_unity(
    fr_t *out, const fr_t *root, uint64_t width
    out[0] = FR_ONE;
    out[1] = *root;

    for (uint64_t i = 2; !fr_is_one(&out[i - 1]); i++) 
        CHECK(i <= width);
        blst_fr_mul(&out[i], &out[i - 1], root);

    return C_KZG_OK;

Safety Evaluation

After improvement stabilizes, it has been completely examined, and your group has manually reviewed the codebase themselves a number of occasions, it is time to get a safety assessment by a good safety group. This would possibly not be a stamp of approval, nevertheless it exhibits that your challenge is a minimum of considerably safe. Consider there is no such thing as a such factor as good safety. There’ll all the time be the chance of vulnerabilities.

For c-kzg-4844 and go-kzg-4844, the Ethereum Basis contracted Sigma Prime to conduct a safety assessment. They produced this report with 8 findings. It accommodates one vital vulnerability in go-kzg-4844 that was a very good discover. The BLS12-381 library that go-kzg-4844 makes use of, gnark-crypto, had a bug which allowed invalid G1 and G2 factors to be sucessfully decoded. Had this not been mounted, this might have resulted in a consensus bug (a disagreement between implementations) in Ethereum.

Bug Bounty

If a vulnerability in your challenge could possibly be exploited for features, like it’s for Ethereum, contemplate organising a bug bounty program. This permits safety researchers, or anybody actually, to submit vulnerability stories in trade for cash. Typically, that is particularly for findings which may show that an exploit is feasible. If the bug bounty payouts are cheap, bug finders will notify you of the bug somewhat than exploiting it or promoting it to a different occasion. We advocate beginning your bug bounty program after the findings from the primary safety assessment are resolved; ideally, the safety assessment would price lower than the bug bounty payouts.


The event of strong C initiatives, particularly within the vital area of blockchain and cryptocurrencies, requires a multi-faceted strategy. Given the inherent vulnerabilities related to the C language, a mix of greatest practices and instruments is important for producing resilient software program. We hope our experiences and findings from our work with c-kzg-4844 present useful insights and greatest practices for others embarking on related initiatives.

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