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/* $OpenBSD: constant_time.h,v 1.3 2023/04/06 18:43:47 tb Exp $ */
/*-
* Utilities for constant-time cryptography.
*
* Author: Emilia Kasper (emilia@openssl.org)
* Based on previous work by Bodo Moeller, Emilia Kasper, Adam Langley
* (Google).
* ====================================================================
* Copyright (c) 2014 The OpenSSL Project. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. All advertising materials mentioning features or use of this software
* must display the following acknowledgement:
* "This product includes cryptographic software written by
* Eric Young (eay@cryptsoft.com)"
* The word 'cryptographic' can be left out if the rouines from the library
* being used are not cryptographic related :-).
* 4. If you include any Windows specific code (or a derivative thereof) from
* the apps directory (application code) you must include an acknowledgement:
* "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
*
* THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*
* The licence and distribution terms for any publically available version or
* derivative of this code cannot be changed. i.e. this code cannot simply be
* copied and put under another distribution licence
* [including the GNU Public Licence.]
*/
#ifndef HEADER_CONSTANT_TIME_H
#define HEADER_CONSTANT_TIME_H
__BEGIN_HIDDEN_DECLS
/*-
* The boolean methods return a bitmask of all ones (0xff...f) for true
* and 0 for false. This is useful for choosing a value based on the result
* of a conditional in constant time. For example,
*
* if (a < b) {
* c = a;
* } else {
* c = b;
* }
*
* can be written as
*
* unsigned int lt = constant_time_lt(a, b);
* c = constant_time_select(lt, a, b);
*/
/*
* Returns the given value with the MSB copied to all the other
* bits. Uses the fact that arithmetic shift shifts-in the sign bit.
* However, this is not ensured by the C standard so you may need to
* replace this with something else on odd CPUs.
*/
static inline unsigned int constant_time_msb(unsigned int a);
/*
* Returns 0xff..f if a < b and 0 otherwise.
*/
static inline unsigned int constant_time_lt(unsigned int a, unsigned int b);
/* Convenience method for getting an 8-bit mask. */
static inline unsigned char constant_time_lt_8(unsigned int a,
unsigned int b);
/*
* Returns 0xff..f if a >= b and 0 otherwise.
*/
static inline unsigned int constant_time_ge(unsigned int a, unsigned int b);
/* Convenience method for getting an 8-bit mask. */
static inline unsigned char constant_time_ge_8(unsigned int a,
unsigned int b);
/*
* Returns 0xff..f if a == 0 and 0 otherwise.
*/
static inline unsigned int constant_time_is_zero(unsigned int a);
/* Convenience method for getting an 8-bit mask. */
static inline unsigned char constant_time_is_zero_8(unsigned int a);
/*
* Returns 0xff..f if a == b and 0 otherwise.
*/
static inline unsigned int constant_time_eq(unsigned int a, unsigned int b);
/* Convenience method for getting an 8-bit mask. */
static inline unsigned char constant_time_eq_8(unsigned int a,
unsigned int b);
/* Signed integers. */
static inline unsigned int constant_time_eq_int(int a, int b);
/* Convenience method for getting an 8-bit mask. */
static inline unsigned char constant_time_eq_int_8(int a, int b);
/*-
* Returns (mask & a) | (~mask & b).
*
* When |mask| is all 1s or all 0s (as returned by the methods above),
* the select methods return either |a| (if |mask| is nonzero) or |b|
* (if |mask| is zero).
*/
static inline unsigned int constant_time_select(unsigned int mask,
unsigned int a,
unsigned int b);
/* Convenience method for unsigned chars. */
static inline unsigned char constant_time_select_8(unsigned char mask,
unsigned char a,
unsigned char b);
/* Convenience method for signed integers. */
static inline int constant_time_select_int(unsigned int mask, int a, int b);
static inline unsigned int constant_time_msb(unsigned int a)
{
return 0 - (a >> (sizeof(a) * 8 - 1));
}
static inline unsigned int constant_time_lt(unsigned int a, unsigned int b)
{
return constant_time_msb(a ^ ((a ^ b) | ((a - b) ^ b)));
}
static inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b)
{
return (unsigned char)(constant_time_lt(a, b));
}
static inline unsigned int constant_time_ge(unsigned int a, unsigned int b)
{
return ~constant_time_lt(a, b);
}
static inline unsigned char constant_time_ge_8(unsigned int a, unsigned int b)
{
return (unsigned char)(constant_time_ge(a, b));
}
static inline unsigned int constant_time_is_zero(unsigned int a)
{
return constant_time_msb(~a & (a - 1));
}
static inline unsigned char constant_time_is_zero_8(unsigned int a)
{
return (unsigned char)(constant_time_is_zero(a));
}
static inline unsigned int constant_time_eq(unsigned int a, unsigned int b)
{
return constant_time_is_zero(a ^ b);
}
static inline unsigned char constant_time_eq_8(unsigned int a, unsigned int b)
{
return (unsigned char)(constant_time_eq(a, b));
}
static inline unsigned int constant_time_eq_int(int a, int b)
{
return constant_time_eq((unsigned)(a), (unsigned)(b));
}
static inline unsigned char constant_time_eq_int_8(int a, int b)
{
return constant_time_eq_8((unsigned)(a), (unsigned)(b));
}
static inline unsigned int constant_time_select(unsigned int mask,
unsigned int a,
unsigned int b)
{
return (mask & a) | (~mask & b);
}
static inline unsigned char constant_time_select_8(unsigned char mask,
unsigned char a,
unsigned char b)
{
return (unsigned char)(constant_time_select(mask, a, b));
}
static inline int constant_time_select_int(unsigned int mask, int a, int b)
{
return (int)(constant_time_select(mask, (unsigned)(a), (unsigned)(b)));
}
void err_clear_last_constant_time(int clear);
__END_HIDDEN_DECLS
#endif /* !HEADER_CONSTANT_TIME_H */
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