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.Dd March 23, 2015 .Dt CONSTTIME_MEMEQUAL 3 .Os .Sh NAME .Nm consttime_memequal .Nd compare byte strings for equality without timing leaks .Sh LIBRARY .Lb libc .Sh SYNOPSIS n string.h .Ft int .Fn consttime_memequal "void *b1" "void *b2" "size_t len" .Sh DESCRIPTION The .Fn consttime_memequal function compares .Fa len bytes of memory at .Fa b1 and .Fa b2 for equality, returning 0 if they are distinct and 1 if they are identical.
p The time taken by .Fn consttime_memequal depends on .Fa len , but not on the data at .Fa b1 or .Fa b2 . Thus, .Fn 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, .Fn consttime_memequal is said to take .Sq constant time , meaning time that does not vary depending on the data it processes.
p Note that unlike .Xr memcmp 3 , .Fn consttime_memequal does not return a lexicographic ordering on the data at .Fa b1 and .Fa b2 ; it tells only whether they are equal. .Nx does not provide a .Fn consttime_memcmp function, because all known use cases that require .Sq constant time memory comparison also require only comparison for equality, not lexicographic ordering. .Sh SEE ALSO .Xr explicit_memset 3 , .Xr memcmp 3 .Sh HISTORY The .Fn consttime_memequal function appeared in .Nx 7.0 .