Crypto · C

How to fuzz LibTomCrypt

A portable crypto toolkit common in embedded firmware — wide deployment, niche test coverage.

LibTomCrypt's portability focus means it avoids OS-specific mitigations, making overflows more exploitable. Its RSA and ECC key import paths parse DER-encoded structures with hand-rolled length arithmetic that rarely receives the same fuzzing attention as OpenSSL-equivalent paths, yet it appears in many embedded and IoT firmware images.

Common bug classes

•Heap buffer overflow in DER SEQUENCE length decode
•Integer overflow in RSA key size to buffer allocation
•Out-of-bounds write in ECC point decompression
•Null dereference in PKCS#1 v1.5 padding check on empty input
•Use-after-free in error path of multi-step key import

Recommended setup

Fuzzers

→ AFL++
→ libFuzzer

Sanitizers

→ ASan
→ UBSan

Harness scaffold

#include <stdint.h>
#include <stddef.h>
#include <tomcrypt.h>

int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
  rsa_key key;
  /* Attempt DER import of RSA key */
  rsa_import(data, (unsigned long)size, &key);
  rsa_free(&key);
  /* Attempt ECC import */
  ecc_key ekey;
  ecc_import(data, (unsigned long)size, &ekey);
  ecc_free(&ekey);
  return 0;
}

Save this as fuzz_target.cc, build with your compiler + sanitizer flags, and you have a working starting point.

Start fuzzing LibTomCrypt on Fuzze.rs →

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