2021-07-12 14:31:22 +00:00
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/**
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* Constant-time functions
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*
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* Copyright The Mbed TLS Contributors
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* SPDX-License-Identifier: Apache-2.0
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*
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* Licensed under the Apache License, Version 2.0 (the "License"); you may
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* not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include "common.h"
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2021-09-27 09:28:54 +00:00
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#include "constant_time.h"
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2021-09-27 10:55:33 +00:00
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#if defined(MBEDTLS_BIGNUM_C)
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#include "mbedtls/bignum.h"
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#endif
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2021-09-27 09:28:54 +00:00
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/* constant-time buffer comparison */
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int mbedtls_ssl_safer_memcmp( const void *a, const void *b, size_t n )
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{
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size_t i;
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volatile const unsigned char *A = (volatile const unsigned char *) a;
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volatile const unsigned char *B = (volatile const unsigned char *) b;
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volatile unsigned char diff = 0;
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for( i = 0; i < n; i++ )
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{
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/* Read volatile data in order before computing diff.
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* This avoids IAR compiler warning:
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* 'the order of volatile accesses is undefined ..' */
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unsigned char x = A[i], y = B[i];
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diff |= x ^ y;
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}
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return( diff );
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}
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/* Compare the contents of two buffers in constant time.
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* Returns 0 if the contents are bitwise identical, otherwise returns
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* a non-zero value.
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* This is currently only used by GCM and ChaCha20+Poly1305.
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*/
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int mbedtls_constant_time_memcmp( const void *v1, const void *v2,
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size_t len )
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{
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const unsigned char *p1 = (const unsigned char*) v1;
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const unsigned char *p2 = (const unsigned char*) v2;
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size_t i;
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unsigned char diff;
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for( diff = 0, i = 0; i < len; i++ )
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diff |= p1[i] ^ p2[i];
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return( (int)diff );
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}
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/* constant-time buffer comparison */
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unsigned char mbedtls_nist_kw_safer_memcmp( const void *a, const void *b, size_t n )
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{
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size_t i;
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volatile const unsigned char *A = (volatile const unsigned char *) a;
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volatile const unsigned char *B = (volatile const unsigned char *) b;
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volatile unsigned char diff = 0;
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for( i = 0; i < n; i++ )
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{
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/* Read volatile data in order before computing diff.
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* This avoids IAR compiler warning:
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* 'the order of volatile accesses is undefined ..' */
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unsigned char x = A[i], y = B[i];
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diff |= x ^ y;
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}
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return( diff );
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}
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/* constant-time buffer comparison */
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int mbedtls_safer_memcmp( const void *a, const void *b, size_t n )
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{
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size_t i;
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const unsigned char *A = (const unsigned char *) a;
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const unsigned char *B = (const unsigned char *) b;
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unsigned char diff = 0;
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for( i = 0; i < n; i++ )
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diff |= A[i] ^ B[i];
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return( diff );
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}
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2021-09-27 09:40:03 +00:00
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/** Turn zero-or-nonzero into zero-or-all-bits-one, without branches.
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*
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* \param value The value to analyze.
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* \return Zero if \p value is zero, otherwise all-bits-one.
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*/
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unsigned mbedtls_cf_uint_mask( unsigned value )
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{
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/* MSVC has a warning about unary minus on unsigned, but this is
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* well-defined and precisely what we want to do here */
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#if defined(_MSC_VER)
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#pragma warning( push )
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#pragma warning( disable : 4146 )
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#endif
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return( - ( ( value | - value ) >> ( sizeof( value ) * 8 - 1 ) ) );
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#if defined(_MSC_VER)
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#pragma warning( pop )
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#endif
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}
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2021-09-27 09:49:42 +00:00
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/*
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* Turn a bit into a mask:
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* - if bit == 1, return the all-bits 1 mask, aka (size_t) -1
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* - if bit == 0, return the all-bits 0 mask, aka 0
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*
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* This function can be used to write constant-time code by replacing branches
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* with bit operations using masks.
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*
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* This function is implemented without using comparison operators, as those
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* might be translated to branches by some compilers on some platforms.
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*/
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size_t mbedtls_cf_size_mask( size_t bit )
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{
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/* MSVC has a warning about unary minus on unsigned integer types,
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* but this is well-defined and precisely what we want to do here. */
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#if defined(_MSC_VER)
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#pragma warning( push )
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#pragma warning( disable : 4146 )
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#endif
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return -bit;
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#if defined(_MSC_VER)
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#pragma warning( pop )
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#endif
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}
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2021-09-27 09:53:54 +00:00
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/*
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* Constant-flow mask generation for "less than" comparison:
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* - if x < y, return all bits 1, that is (size_t) -1
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* - otherwise, return all bits 0, that is 0
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*
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* This function can be used to write constant-time code by replacing branches
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* with bit operations using masks.
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*
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* This function is implemented without using comparison operators, as those
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* might be translated to branches by some compilers on some platforms.
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*/
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size_t mbedtls_cf_size_mask_lt( size_t x, size_t y )
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{
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/* This has the most significant bit set if and only if x < y */
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const size_t sub = x - y;
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/* sub1 = (x < y) ? 1 : 0 */
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const size_t sub1 = sub >> ( sizeof( sub ) * 8 - 1 );
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/* mask = (x < y) ? 0xff... : 0x00... */
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const size_t mask = mbedtls_cf_size_mask( sub1 );
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return( mask );
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}
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2021-09-27 09:58:31 +00:00
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/*
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* Constant-flow mask generation for "greater or equal" comparison:
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* - if x >= y, return all bits 1, that is (size_t) -1
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* - otherwise, return all bits 0, that is 0
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*
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* This function can be used to write constant-time code by replacing branches
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* with bit operations using masks.
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*
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* This function is implemented without using comparison operators, as those
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* might be translated to branches by some compilers on some platforms.
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*/
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size_t mbedtls_cf_size_mask_ge( size_t x, size_t y )
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{
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return( ~mbedtls_cf_size_mask_lt( x, y ) );
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}
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2021-09-27 10:15:19 +00:00
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/*
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* Constant-flow boolean "equal" comparison:
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* return x == y
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*
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* This function can be used to write constant-time code by replacing branches
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* with bit operations - it can be used in conjunction with
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* mbedtls_ssl_cf_mask_from_bit().
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*
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* This function is implemented without using comparison operators, as those
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* might be translated to branches by some compilers on some platforms.
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*/
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size_t mbedtls_cf_size_bool_eq( size_t x, size_t y )
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{
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/* diff = 0 if x == y, non-zero otherwise */
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const size_t diff = x ^ y;
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/* MSVC has a warning about unary minus on unsigned integer types,
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* but this is well-defined and precisely what we want to do here. */
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#if defined(_MSC_VER)
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#pragma warning( push )
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#pragma warning( disable : 4146 )
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#endif
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/* diff_msb's most significant bit is equal to x != y */
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const size_t diff_msb = ( diff | (size_t) -diff );
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#if defined(_MSC_VER)
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#pragma warning( pop )
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#endif
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/* diff1 = (x != y) ? 1 : 0 */
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const size_t diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 );
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return( 1 ^ diff1 );
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}
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2021-09-27 10:25:07 +00:00
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/** Check whether a size is out of bounds, without branches.
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*
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* This is equivalent to `size > max`, but is likely to be compiled to
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* to code using bitwise operation rather than a branch.
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*
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* \param size Size to check.
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* \param max Maximum desired value for \p size.
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* \return \c 0 if `size <= max`.
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* \return \c 1 if `size > max`.
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*/
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unsigned mbedtls_cf_size_gt( size_t size, size_t max )
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{
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/* Return the sign bit (1 for negative) of (max - size). */
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return( ( max - size ) >> ( sizeof( size_t ) * 8 - 1 ) );
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}
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2021-09-27 10:55:33 +00:00
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#if defined(MBEDTLS_BIGNUM_C)
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/** Decide if an integer is less than the other, without branches.
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*
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* \param x First integer.
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* \param y Second integer.
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*
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* \return 1 if \p x is less than \p y, 0 otherwise
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*/
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unsigned mbedtls_cf_mpi_uint_lt( const mbedtls_mpi_uint x,
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const mbedtls_mpi_uint y )
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{
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mbedtls_mpi_uint ret;
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mbedtls_mpi_uint cond;
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/*
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* Check if the most significant bits (MSB) of the operands are different.
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*/
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cond = ( x ^ y );
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/*
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* If the MSB are the same then the difference x-y will be negative (and
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* have its MSB set to 1 during conversion to unsigned) if and only if x<y.
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*/
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ret = ( x - y ) & ~cond;
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/*
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* If the MSB are different, then the operand with the MSB of 1 is the
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* bigger. (That is if y has MSB of 1, then x<y is true and it is false if
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* the MSB of y is 0.)
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*/
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ret |= y & cond;
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ret = ret >> ( sizeof( mbedtls_mpi_uint ) * 8 - 1 );
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return (unsigned) ret;
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}
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#endif /* MBEDTLS_BIGNUM_C */
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2021-09-27 10:59:30 +00:00
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/** Choose between two integer values, without branches.
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*
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* This is equivalent to `cond ? if1 : if0`, but is likely to be compiled
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* to code using bitwise operation rather than a branch.
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*
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* \param cond Condition to test.
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* \param if1 Value to use if \p cond is nonzero.
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* \param if0 Value to use if \p cond is zero.
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* \return \c if1 if \p cond is nonzero, otherwise \c if0.
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*/
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unsigned mbedtls_cf_uint_if( unsigned cond, unsigned if1, unsigned if0 )
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{
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unsigned mask = mbedtls_cf_uint_mask( cond );
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return( ( mask & if1 ) | (~mask & if0 ) );
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}
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2021-09-27 11:03:57 +00:00
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/**
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* Select between two sign values in constant-time.
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*
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* This is functionally equivalent to second ? a : b but uses only bit
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* operations in order to avoid branches.
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*
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* \param[in] a The first sign; must be either +1 or -1.
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* \param[in] b The second sign; must be either +1 or -1.
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* \param[in] second Must be either 1 (return b) or 0 (return a).
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*
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* \return The selected sign value.
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*/
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int mbedtls_cf_cond_select_sign( int a, int b, unsigned char second )
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{
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/* In order to avoid questions about what we can reasonnably assume about
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* the representations of signed integers, move everything to unsigned
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* by taking advantage of the fact that a and b are either +1 or -1. */
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unsigned ua = a + 1;
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unsigned ub = b + 1;
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/* second was 0 or 1, mask is 0 or 2 as are ua and ub */
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const unsigned mask = second << 1;
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/* select ua or ub */
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unsigned ur = ( ua & ~mask ) | ( ub & mask );
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/* ur is now 0 or 2, convert back to -1 or +1 */
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return( (int) ur - 1 );
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}
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2021-09-27 11:17:15 +00:00
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#if defined(MBEDTLS_BIGNUM_C)
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/*
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* Conditionally assign dest = src, without leaking information
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* about whether the assignment was made or not.
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* dest and src must be arrays of limbs of size n.
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* assign must be 0 or 1.
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*/
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void mbedtls_cf_mpi_uint_cond_assign( size_t n,
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mbedtls_mpi_uint *dest,
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const mbedtls_mpi_uint *src,
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unsigned char assign )
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{
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size_t i;
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/* MSVC has a warning about unary minus on unsigned integer types,
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* but this is well-defined and precisely what we want to do here. */
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#if defined(_MSC_VER)
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#pragma warning( push )
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#pragma warning( disable : 4146 )
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#endif
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/* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
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const mbedtls_mpi_uint mask = -assign;
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#if defined(_MSC_VER)
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#pragma warning( pop )
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#endif
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for( i = 0; i < n; i++ )
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dest[i] = ( src[i] & mask ) | ( dest[i] & ~mask );
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}
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#endif /* MBEDTLS_BIGNUM_C */
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2021-09-27 11:31:06 +00:00
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/** Shift some data towards the left inside a buffer without leaking
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* the length of the data through side channels.
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*
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* `mbedtls_cf_mem_move_to_left(start, total, offset)` is functionally
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* equivalent to
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* ```
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* memmove(start, start + offset, total - offset);
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* memset(start + offset, 0, total - offset);
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* ```
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* but it strives to use a memory access pattern (and thus total timing)
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* that does not depend on \p offset. This timing independence comes at
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* the expense of performance.
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*
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* \param start Pointer to the start of the buffer.
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* \param total Total size of the buffer.
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* \param offset Offset from which to copy \p total - \p offset bytes.
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*/
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void mbedtls_cf_mem_move_to_left( void *start,
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size_t total,
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size_t offset )
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{
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volatile unsigned char *buf = start;
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size_t i, n;
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if( total == 0 )
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return;
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for( i = 0; i < total; i++ )
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{
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unsigned no_op = mbedtls_cf_size_gt( total - offset, i );
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/* The first `total - offset` passes are a no-op. The last
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* `offset` passes shift the data one byte to the left and
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* zero out the last byte. */
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for( n = 0; n < total - 1; n++ )
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{
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unsigned char current = buf[n];
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unsigned char next = buf[n+1];
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buf[n] = mbedtls_cf_uint_if( no_op, current, next );
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}
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buf[total-1] = mbedtls_cf_uint_if( no_op, buf[total-1], 0 );
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}
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}
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