Home Ethereum Secured #6 – Writing Strong C – Greatest Practices for Discovering and Stopping Vulnerabilities

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

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


For EIP-4844, Ethereum shoppers want the flexibility to compute and confirm KZG commitments. Moderately 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 every one shoppers might use. The Protocol Safety Analysis staff on the Ethereum Basis had the chance to overview and enhance this library. This weblog put up will focus on some issues we do to make C initiatives safer.


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

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

#embody "../base_fuzz.h"

static const size_t COMMITMENT_OFFSET = 0;

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

When executed, that is what the output appears like. If there have been an issue, it will write the enter to disk and cease executing. Ideally, it’s best to be capable of reproduce the issue.

There’s additionally differential fuzzing, which is a method 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 realize one thing is mistaken. This system could be very fashionable in Ethereum as a result of we prefer to have a number of implementations of the identical factor. This diversification supplies an additional degree of security, figuring out that if one implementation have been flawed the others might not have the identical difficulty.

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


Subsequent, we used llvm-profdata and llvm-cov to generate a protection report from working the assessments. It is a nice strategy to confirm code is executed (“coated”) and examined. See the protection goal in c-kzg-4844’s Makefile for an instance of the right way 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 operate is executed. The exported features are on the prime and the non-exported (static) features are on the underside.

There’s a whole lot of inexperienced within the desk above, however there may be some yellow and pink too. To find out what’s and is not being executed, consult with the HTML file (protection.html) that was generated. This webpage exhibits your complete supply file and highlights non-executed code in pink. On this undertaking’s case, a lot of the non-executed code offers with hard-to-test error instances corresponding to reminiscence allocation failures. For instance, here is some non-executed code:

At the start of this operate, it checks that the trusted setup is sufficiently big to carry out a pairing test. There is not a check case which supplies an invalid trusted setup, so this does not get executed. Additionally, as a result of we solely check with the proper trusted setup, the results of is_monomial_form is all the time the identical and does not return the error worth.


We do not advocate this for all initiatives, however since c-kzg-4844 is a efficiency essential library we expect it is essential to profile its exported features and measure how lengthy they take to execute. This may also help establish inefficiencies which might probably DoS nodes. For this, we used gperftools (Google Efficiency Instruments) as an alternative 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 every now and then. If a operate is quick sufficient, it will not be seen by the profiler. To cut back the possibility of this, it’s possible you’ll have to name your operate a number of instances. On this instance, we name my_function 1000 instances.

#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) {
        } else {

int predominant(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 would write a file to disk with profiling knowledge. You possibly can then use pprof to visualise this knowledge.

Right here is the graph generated from the command above:

This is an even bigger instance from one in every of 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 operate’s time is spent performing Montgomery multiplications. That is anticipated.


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

Whenever you view a decompiled operate, 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 may typically see features are inlined right into a single operate, 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 usually superb. It could assist to construct your binary with DWARF debugging data; most SREs can analyze this part to supply higher outcomes.

For instance, that is what blob_to_kzg_commitment initially appears like in Ghidra:

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

Static Evaluation

Clang comes built-in with the Clang Static Analyzer, which is a superb static evaluation software that may establish 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 quite a bit quicker than “dynamic” evaluation instruments which execute code.

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

#embody <stdlib.h>

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

The unix.Malloc checker will establish 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 seen that the reminiscence hadn’t been freed.

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

Given an sudden enter, it was potential 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 unimaginable. Good job, Clang Static Analyzer!


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

Deal with

AddressSanitizer (ASan) is a quick reminiscence error detector which might establish 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 component in a 5 component array. It is a easy instance of a heap-buffer-overflow:

#embody <stdlib.h>

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

When compiled with -fsanitize=deal with and executed, it would output the next error message. This factors you in a superb path (a 4-byte write in predominant). This binary may very well be considered in a disassembler to determine precisely which instruction (at predominant+0x84) is inflicting the issue.

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

#embody <stdlib.h>

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

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


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

int predominant(void) {
    int knowledge[2];
    return knowledge[0];

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

Undefined Habits

UndefinedBehaviorSanitizer (UBSan) detects undefined conduct, which refers back to the scenario the place a program’s conduct is unpredictable and never specified by the langauge customary. 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 predominant(void) {
    int a = INT_MAX;
    return a + 1;

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


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

#embody <pthread.h>

int counter = 0;

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

int predominant(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 would output the next error message:

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


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

The next picture exhibits the output from working c-kzg-4844’s assessments with Valgrind. Within the pink field is a sound discovering for a “conditional soar or transfer [that] will depend on uninitialized worth(s).”

This recognized an edge case in expand_root_of_unity. If the mistaken root of unity or width have been offered, it was potential that the loop will break earlier than out[width] was initialized. On this scenario, the ultimate test would rely 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 Overview

After growth stabilizes, it has been totally examined, and your staff has manually reviewed the codebase themselves a number of instances, it is time to get a safety overview by a good safety group. This would possibly not be a stamp of approval, nevertheless it exhibits that your undertaking is a minimum of considerably safe. Consider there isn’t 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 overview. They produced this report with 8 findings. It comprises one essential 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 fastened, this might have resulted in a consensus bug (a disagreement between implementations) in Ethereum.

Bug Bounty

If a vulnerability in your undertaking may very well be exploited for positive factors, like it’s for Ethereum, take into account establishing a bug bounty program. This enables safety researchers, or anybody actually, to submit vulnerability stories in alternate for cash. Usually, that is particularly for findings which might show that an exploit is feasible. If the bug bounty payouts are cheap, bug finders will notify you of the bug slightly than exploiting it or promoting it to a different social gathering. We advocate beginning your bug bounty program after the findings from the primary safety overview are resolved; ideally, the safety overview would value lower than the bug bounty payouts.


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



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