CONSTTIME_MEMEQUAL(3)

CONSTTIME_MEMEQUAL(3) | 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.

# SEE ALSO

explicit_memset(3), memcmp(3)

# HISTORY

The consttime_memequal() function appeared in NetBSD 7.0.

November 14, 2013 | NetBSD 7.1