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.