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CONSTTIME_MEMEQUAL(3) FreeBSD Library Functions Manual CONSTTIME_MEMEQUAL(3) NAME consttime_memequal - compare byte strings for equality without timing leaks LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <string.h> int consttime_memequal(void *b1, void *b2, size_t len); DESCRIPTION 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 identical. The time taken by consttime_memequal() depends on len, but not on the data at b1 or b2. Thus, consttime_memequal() is appropriate for comparing 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 lexicographic 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 comparison also require only comparison for equality, not lexicographic ordering. SEE ALSO explicit_memset(3), memcmp(3) HISTORY The consttime_memequal() function appeared in NetBSD 7.0. FreeBSD 14.1-RELEASE-p8 March 23, 2015 FreeBSD 14.1-RELEASE-p8