CONSTTIME_MEMEQUAL(3) NetBSD Library Functions Manual CONSTTIME_MEMEQUAL(3)
consttime_memequal -- compare byte strings for equality without timing leaks
Standard C Library (libc, -lc)
#include <string.h> int consttime_memequal(void *b1, void *b2, size_t len);
The consttime_memequal() function compares len bytes of memory at b1 and b2 for equality, returning 0 if they are distinct and 1 if they are iden- tical. The time taken by consttime_memequal() depends on len, but not on the data at b1 or b2. Thus, consttime_memequal() is appropriate for compar- ing cryptographic secrets, hashes, message authentication codes, etc., without leaking information about them through a timing side channel. In crypto literature, consttime_memequal() is said to take `constant time', meaning time that does not vary depending on the data it processes. Note that unlike memcmp(3), consttime_memequal() does not return a lexi- cographic ordering on the data at b1 and b2; it tells only whether they are equal. NetBSD does not provide a consttime_memcmp() function, because all known use cases that require `constant time' memory compari- son also require only comparison for equality, not lexicographic order- ing.
The consttime_memequal() function appeared in NetBSD 7.0. NetBSD 9.0 March 23, 2015 NetBSD 9.0
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