Quick wins for a faster PC:
Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →For secret equality checks in C, use a cryptographic library function documented for constant-time comparison at the required length—not ordinary memcmp. To erase a password or key, use an explicit-erasure API documented by your target platform; a final ordinary memset can be optimized away when the cleared object is never used again.
Compare secret values with an equality API
memcmp is designed to compare byte sequences and can stop at the first differing byte. When the input contains a secret—such as an authentication tag or key—that behavior can make execution time depend on how much of the input matches. Choose a library function whose documentation promises content-independent timing for the comparison you need.
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These guarantees are about the compared contents for a given length, not identical wall-clock timing under every scheduler, processor, cache state, or application context. The length and surrounding control flow can also disclose information if they depend on secret data. Keep the length independent of the secret where possible, and avoid adding secret-dependent branches around the comparison.
| Function | Documented timing and purpose | Result and availability |
|---|---|---|
sodium_memcmp |
Libsodium documents constant-time equality comparison for inputs of the same length. See Libsodium Helpers. | Returns 0 if equal and -1 otherwise. It does not provide lexicographic ordering. |
CRYPTO_memcmp |
OpenSSL documents runtime that depends on len but not on the contents of the memory regions. See OpenSSL documentation. |
Returns 0 if equal and nonzero otherwise. Unequal inputs have no meaningful ordering contract. |
timingsafe_bcmp |
OpenBSD documents content-independent running time for equality testing. | OpenBSD extension; use only where the target environment provides and documents it. |
timingsafe_memcmp |
OpenBSD documents content-independent running time and lexicographic comparison. | OpenBSD extension. Its ordering behavior makes it distinct from equality-only APIs. |
Pick the API your target library supports and follow its declared length and return-value contract. In particular, do not treat an equality-only constant-time function as a drop-in replacement when your program needs lexicographic ordering. Libsodium specifically advises using a constant-time comparison for secret data such as keys and authentication tags; see its helper documentation.
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Why an ordinary memset may disappear
A call such as memset(secret, 0, size) looks like it clears memory, but the compiler can remove it if the object is dead afterward—meaning no later operation in the program can observe the cleared bytes. In that case, the write has no effect on the program’s abstract behavior. A 2015 GCC mailing-list explanation describes the compiler’s rationale: it is entitled to delete a final memset when the object is dead after the call. See the GCC discussion.
Use an explicit-erasure function provided by the platform
Check the official documentation and headers for the C library or operating system you actually target. GNU libc documents explicit_bzero and memset_explicit; their writes are retained even when the compiler can determine that no correct program path will read them. Other environments may offer APIs such as memset_s or Windows SecureZeroMemory. Names, declarations, and availability vary, so verify support rather than assuming an API is portable.
The guarantee is deliberately narrow. GNU libc says of explicit_bzero: “The only optimization that explicit_bzero disables is removal of ‘unnecessary’ writes to memory.” See GNU libc’s erasure documentation. The function preserves its designated writes against dead-store removal; it does not disable all optimization or establish that every copy of the secret has been removed.
The Tool Desk
Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →- Use the target platform’s documented explicit-erasure API, with the correct buffer and length.
- Do not silently fall back to ordinary
memsetif the explicit API is unavailable. - Do not assume a hand-written
volatileloop is a universally reliable substitute; CERT secure-coding guidance cautions against that assumption. - If the security requirement depends on the exact generated implementation, inspect generated code as part of your own project’s validation.
What explicit erasure cannot promise
An explicit wipe of one buffer does not necessarily clear other copies. A secret may also have been placed in a register, a compiler scratch area, a stack temporary, or another buffer. GNU libc notes that register-held values are beyond what stack clearing can reach. Treat explicit erasure as protection against one specific failure—the compiler discarding the requested writes—not proof that all traces of a secret are gone.
For robust handling, minimize copies and the time secrets remain in memory, use established cryptographic libraries, and consider the whole program and platform rather than relying on one wipe call. The appropriate measures depend on how the secret is created, copied, and used.
Quick Recap
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Practical decision checklist
- For equality: identify the cryptographic library available on the target, then use its documented constant-time equality function for a fixed, appropriate length.
- For ordering: use an API that explicitly documents lexicographic behavior; equality-only functions do not provide it.
- For erasure: check the target’s official headers and documentation for an explicit-erasure function, then use that function rather than relying on a final ordinary
memset. - For validation: if correctness depends on generated code, inspect what the project’s compiler emits under its actual build settings and revalidate when the compiler or target changes.
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