openntpd-openbsd/src/lib/libc/string/timingsafe_bcmp.3

.\"	$OpenBSD: timingsafe_bcmp.3,v 1.2 2014/06/21 20:22:15 tedu Exp $
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.Dd $Mdocdate: June 21 2014 $
.Dt TIMINGSAFE_BCMP 3
.Os
.Sh NAME
.Nm timingsafe_bcmp ,
.Nm timingsafe_memcmp
.Nd timing-safe byte sequence comparisons
.Sh SYNOPSIS
.In string.h
.Ft int
.Fn timingsafe_bcmp "const void *b1" "const void *b2" "size_t len"
.Ft int
.Fn timingsafe_memcmp "const void *b1" "const void *b2" "size_t len"
.Sh DESCRIPTION
The
.Fn timingsafe_bcmp
and
.Fn timingsafe_memcmp
functions lexicographically compare the first
.Fa len
bytes (each interpreted as an
.Vt unsigned char )
pointed to by
.Fa b1
and
.Fa b2 .
.Pp
Additionally, their running times are independent of the byte sequences compared,
making them safe to use for comparing secret values such as cryptographic MACs.
In contrast,
.Xr bcmp 3
and
.Xr memcmp 3
may short-circuit after finding the first differing byte.
.Sh RETURN VALUES
The
.Fn timingsafe_bcmp
function returns 0 or not zero if the byte sequence pointed to by
.Fa b1
compares equal to or not equal to (respectively)
the byte sequence pointed to by
.Fa b2 .
.Pp
The
.Fn timingsafe_memcmp
function returns a negative value, 0, or positive value if the byte sequence
pointed to by
.Fa b1
compares less than, equal to, or greater than (respectively)
the byte sequence pointed to by
.Fa b2 .
.Sh SEE ALSO
.Xr bcmp 3 ,
.Xr memcmp 3
.Sh STANDARDS
The
.Fn timingsafe_bcmp
and
.Fn timingsafe_memcmp
functions are
.Ox
extensions.
.Sh HISTORY
The
.Fn timingsafe_bcmp
function first appeared in
.Ox 4.9 .
.Pp
The
.Fn timingsafe_memcmp
function first appeared in
.Ox 5.6 .