For secret equality checks in C, use a cryptographic library function documented to compare inputs in constant time for a fixed length—not ordinary memcmp. To clear a sensitive buffer, use an explicit-erasure function documented for your platform; a final ordinary memset can be removed when the compiler sees that the cleared object is never used again. Neither technique hides a secret-dependent length or guarantees that every copy of a secret has been erased.
Why ordinary memcmp is risky for secret comparisons
memcmp compares byte sequences and may stop at the first difference. When the compared data is secret—such as an authentication tag or key—that behavior can make execution time depend on how many initial bytes match. Repeated observations may reveal information about the value.
For secret equality checks, use an API whose documentation promises content-independent timing for the relevant input length. Libsodium calls this essential for comparisons involving secret data in its Helpers documentation.
Choose a comparison API that matches your need
The documented guarantees below concern timing dependence on input contents for a given length. They do not promise identical wall-clock time across machines, operating conditions, compilers, or application contexts.
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| API | Documented timing behavior | Result and intended use |
|---|---|---|
sodium_memcmp |
Libsodium documents constant-time equality comparison for inputs of the same length. | Returns 0 when equal and -1 otherwise. Equality test only; not lexicographic ordering. Libsodium Helpers |
CRYPTO_memcmp |
OpenSSL documents runtime as dependent on length but independent of the contents of the memory regions. | Returns 0 when equal and nonzero otherwise. Unequal results have no meaningful ordering contract. OpenSSL CRYPTO_memcmp |
timingsafe_bcmp |
OpenBSD documents content-independent running time. | Equality/inequality result; an OpenBSD extension. |
timingsafe_memcmp |
OpenBSD documents content-independent running time. | Lexicographic comparison result; an OpenBSD extension. |
Check length and return semantics
These APIs are not interchangeable in every use. In particular, sodium_memcmp and CRYPTO_memcmp are equality tests, not substitutes for memcmp when code needs lexicographic ordering. Check each library’s headers and documentation for declarations and platform availability.
Also ensure the length is not itself secret-dependent if revealing it would leak information. A content-independent comparison for a fixed length does not make the length or surrounding control flow constant-time.
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Why a final memset can disappear
Consider clearing a key buffer with memset just before the buffer goes out of scope or is otherwise never read again. Under the C program’s abstract behavior, those writes have no observable effect if no later code can access the object. Dead-store elimination can therefore remove the call. GCC compiler developer Zack Weinberg described this issue in a 2015 GCC mailing-list post.
Ordinary memset is useful for initializing or clearing data when the resulting memory contents remain relevant to program behavior. It is not, by itself, a reliable secure-erasure primitive when the compiler can prove the cleared data is never used again.
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Use an explicit-erasure API supported by the target
Choose an erasure function documented by the C library or operating-system platform you actually target. GNU libc documents explicit_bzero and memset_explicit; it specifies that their designated writes are retained even when the compiler can determine that no correct program path will read them. See GNU libc’s Erasing Sensitive Data documentation.
Availability and names vary. Depending on the platform, related facilities may include explicit_memset, C11 Annex K’s memset_s, or Windows SecureZeroMemory. Confirm that the function is declared and supported by the target headers and documentation. Do not silently fall back to ordinary memset or assume an unverified hand-written volatile loop provides the required guarantee.
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What explicit erasure does—and does not—guarantee
An explicit-erasure API prevents the designated writes from being discarded as unnecessary under the function’s documented contract. That is a narrower promise than removing every copy of a secret from a running process.
- Earlier copies in other buffers are not automatically cleared.
- Compiler-created temporaries, stack scratch space, or values held in registers may remain.
- The function’s guarantee does not establish that all traces of a secret have disappeared.
GNU libc warns that explicit erasure disables only removal of unnecessary writes; other optimizations may still occur. Libsodium likewise notes that stack clearing cannot clear values held in registers. Treat erasure as one measure in a broader design, limiting unnecessary copies and their lifetimes.
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Practical implementation checklist
- For secret equality: identify a documented constant-time equality API in the cryptographic library available on the target.
- For the comparison: provide the intended fixed length and use the documented return convention; do not interpret equality-only results as an ordering.
- For length and control flow: check that length selection and surrounding branches do not reveal information the comparison is meant to protect.
- For buffer clearing: use the target platform’s documented explicit-erasure function and verify its declaration and availability.
- For assurance: if the security requirement depends on the generated implementation, include generated-code inspection in the project’s validation process. Documentation alone is not a code-generation test for your build.
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