Editing Timing attack (section)

=== Algorithm ===

The following [[C (programming language)|C]] code demonstrates a typical insecure string comparison which stops testing as soon as a character doesn't match. For example, when comparing "ABCDE" with "ABxDE" it will return after 3 loop iterations:

<syntaxhighlight lang="c">
bool insecureStringCompare(const void *a, const void *b, size_t length) {
  const char *ca = a, *cb = b;
  for (size_t i = 0; i < length; i++)
    if (ca[i] != cb[i])
      return false;
  return true;
}
</syntaxhighlight>

By comparison, the following version runs in constant-time by testing all characters and using a [[bitwise operation]] to accumulate the result:
<syntaxhighlight lang="c">
bool constantTimeStringCompare(const void *a, const void *b, size_t length) {
  const char *ca = a, *cb = b;
  bool result = true;
  for (size_t i = 0; i < length; i++)
    result &= ca[i] == cb[i];
  return result;
}
</syntaxhighlight>

In the world of C library functions, the first function is analogous to {{code|memcmp()}}, while the latter is analogous to NetBSD's {{code|consttime_memequal()}} or<ref>{{Cite web|url=https://www.freebsd.org/cgi/man.cgi?query=consttime_memequal&manpath=NetBSD+7.0|title = Consttime_memequal}}</ref> OpenBSD's {{code|timingsafe_bcmp()}} and {{code|timingsafe_memcmp}}. On other systems, the comparison function from cryptographic libraries like [[OpenSSL]] and [[libsodium]] can be used.<!-- There should be a big flashy "don't roll your own crypto" paragraph here, but I have no idea how to phrase it. Whatever. -->