743 lines
25 KiB
C
743 lines
25 KiB
C
/**
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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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/*
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* The following functions are 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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#include "common.h"
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#include "constant_time_internal.h"
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#include "mbedtls/constant_time.h"
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#include "mbedtls/error.h"
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#include "mbedtls/platform_util.h"
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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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#if defined(MBEDTLS_SSL_TLS_C)
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#include "ssl_misc.h"
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#endif
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#if defined(MBEDTLS_RSA_C)
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#include "mbedtls/rsa.h"
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#endif
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#include <string.h>
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int mbedtls_ct_memcmp( const void *a,
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const void *b,
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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( (int)diff );
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}
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unsigned mbedtls_ct_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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#if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
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size_t mbedtls_ct_size_mask( size_t value )
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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( - ( ( 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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#endif /* MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC */
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#if defined(MBEDTLS_BIGNUM_C)
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mbedtls_mpi_uint mbedtls_ct_mpi_uint_mask( mbedtls_mpi_uint 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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#endif /* MBEDTLS_BIGNUM_C */
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#if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
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/** Constant-flow mask generation for "less than" comparison:
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* - if \p x < \p 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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* \param x The first value to analyze.
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* \param y The second value to analyze.
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*
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* \return All-bits-one if \p x is less than \p y, otherwise zero.
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*/
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static size_t mbedtls_ct_size_mask_lt( size_t x,
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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_ct_size_mask( sub1 );
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return( mask );
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}
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size_t mbedtls_ct_size_mask_ge( size_t x,
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size_t y )
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{
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return( ~mbedtls_ct_size_mask_lt( x, y ) );
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}
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#endif /* MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC */
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unsigned mbedtls_ct_size_bool_eq( size_t x,
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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 unsigned diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 );
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return( 1 ^ diff1 );
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}
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#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
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/** Constant-flow "greater than" comparison:
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* return x > y
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*
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* This is equivalent to \p x > \p y, 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 x The first value to analyze.
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* \param y The second value to analyze.
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*
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* \return 1 if \p x greater than \p y, otherwise 0.
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*/
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static unsigned mbedtls_ct_size_gt( size_t x,
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size_t y )
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{
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/* Return the sign bit (1 for negative) of (y - x). */
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return( ( y - x ) >> ( sizeof( size_t ) * 8 - 1 ) );
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}
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#endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */
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#if defined(MBEDTLS_BIGNUM_C)
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unsigned mbedtls_ct_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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unsigned mbedtls_ct_uint_if( unsigned condition,
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unsigned if1,
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unsigned if0 )
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{
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unsigned mask = mbedtls_ct_uint_mask( condition );
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return( ( mask & if1 ) | (~mask & if0 ) );
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}
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#if defined(MBEDTLS_BIGNUM_C)
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/** Select between two sign values without branches.
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*
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* This is functionally equivalent to `condition ? if1 : if0` but uses only bit
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* operations in order to avoid branches.
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*
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* \note if1 and if0 must be either 1 or -1, otherwise the result
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* is undefined.
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*
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* \param condition Condition to test.
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* \param if1 The first sign; must be either +1 or -1.
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* \param if0 The second sign; must be either +1 or -1.
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*
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* \return \c if1 if \p condition is nonzero, otherwise \c if0.
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* */
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static int mbedtls_ct_cond_select_sign( unsigned char condition,
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int if1,
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int if0 )
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{
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/* In order to avoid questions about what we can reasonably 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 if1 and if0 are either +1 or -1. */
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unsigned uif1 = if1 + 1;
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unsigned uif0 = if0 + 1;
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/* condition was 0 or 1, mask is 0 or 2 as are uif1 and uif0 */
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const unsigned mask = condition << 1;
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/* select uif1 or uif0 */
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unsigned ur = ( uif0 & ~mask ) | ( uif1 & 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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void mbedtls_ct_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 condition )
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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 condition is 1, all-bits 0 if condition is 0 */
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const mbedtls_mpi_uint mask = -condition;
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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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#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
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/** Shift some data towards the left inside a buffer.
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*
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* `mbedtls_ct_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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static void mbedtls_ct_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_ct_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_ct_uint_if( no_op, current, next );
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}
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buf[total-1] = mbedtls_ct_uint_if( no_op, buf[total-1], 0 );
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}
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}
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#endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */
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#if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
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void mbedtls_ct_memcpy_if_eq( unsigned char *dest,
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const unsigned char *src,
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size_t len,
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size_t c1,
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size_t c2 )
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{
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/* mask = c1 == c2 ? 0xff : 0x00 */
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const size_t equal = mbedtls_ct_size_bool_eq( c1, c2 );
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const unsigned char mask = (unsigned char) mbedtls_ct_size_mask( equal );
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/* dest[i] = c1 == c2 ? src[i] : dest[i] */
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for( size_t i = 0; i < len; i++ )
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dest[i] = ( src[i] & mask ) | ( dest[i] & ~mask );
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}
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void mbedtls_ct_memcpy_offset( unsigned char *dest,
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const unsigned char *src,
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size_t offset,
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size_t offset_min,
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size_t offset_max,
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size_t len )
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{
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size_t offsetval;
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for( offsetval = offset_min; offsetval <= offset_max; offsetval++ )
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{
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mbedtls_ct_memcpy_if_eq( dest, src + offsetval, len,
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offsetval, offset );
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}
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}
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int mbedtls_ct_hmac( mbedtls_md_context_t *ctx,
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const unsigned char *add_data,
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size_t add_data_len,
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const unsigned char *data,
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size_t data_len_secret,
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size_t min_data_len,
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size_t max_data_len,
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unsigned char *output )
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{
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/*
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* This function breaks the HMAC abstraction and uses the md_clone()
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* extension to the MD API in order to get constant-flow behaviour.
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*
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* HMAC(msg) is defined as HASH(okey + HASH(ikey + msg)) where + means
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* concatenation, and okey/ikey are the XOR of the key with some fixed bit
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* patterns (see RFC 2104, sec. 2), which are stored in ctx->hmac_ctx.
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*
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* We'll first compute inner_hash = HASH(ikey + msg) by hashing up to
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* minlen, then cloning the context, and for each byte up to maxlen
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* finishing up the hash computation, keeping only the correct result.
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*
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* Then we only need to compute HASH(okey + inner_hash) and we're done.
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*/
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const mbedtls_md_type_t md_alg = mbedtls_md_get_type( ctx->md_info );
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/* TLS 1.2 only supports SHA-384, SHA-256, SHA-1, MD-5,
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* all of which have the same block size except SHA-384. */
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const size_t block_size = md_alg == MBEDTLS_MD_SHA384 ? 128 : 64;
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const unsigned char * const ikey = ctx->hmac_ctx;
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const unsigned char * const okey = ikey + block_size;
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const size_t hash_size = mbedtls_md_get_size( ctx->md_info );
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unsigned char aux_out[MBEDTLS_MD_MAX_SIZE];
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mbedtls_md_context_t aux;
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size_t offset;
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int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
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mbedtls_md_init( &aux );
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#define MD_CHK( func_call ) \
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do { \
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ret = (func_call); \
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if( ret != 0 ) \
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goto cleanup; \
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} while( 0 )
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MD_CHK( mbedtls_md_setup( &aux, ctx->md_info, 0 ) );
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/* After hmac_start() of hmac_reset(), ikey has already been hashed,
|
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* so we can start directly with the message */
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MD_CHK( mbedtls_md_update( ctx, add_data, add_data_len ) );
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MD_CHK( mbedtls_md_update( ctx, data, min_data_len ) );
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|
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/* For each possible length, compute the hash up to that point */
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for( offset = min_data_len; offset <= max_data_len; offset++ )
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{
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MD_CHK( mbedtls_md_clone( &aux, ctx ) );
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MD_CHK( mbedtls_md_finish( &aux, aux_out ) );
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/* Keep only the correct inner_hash in the output buffer */
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mbedtls_ct_memcpy_if_eq( output, aux_out, hash_size,
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offset, data_len_secret );
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|
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if( offset < max_data_len )
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MD_CHK( mbedtls_md_update( ctx, data + offset, 1 ) );
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}
|
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|
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/* The context needs to finish() before it starts() again */
|
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MD_CHK( mbedtls_md_finish( ctx, aux_out ) );
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|
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/* Now compute HASH(okey + inner_hash) */
|
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MD_CHK( mbedtls_md_starts( ctx ) );
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MD_CHK( mbedtls_md_update( ctx, okey, block_size ) );
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MD_CHK( mbedtls_md_update( ctx, output, hash_size ) );
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MD_CHK( mbedtls_md_finish( ctx, output ) );
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|
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/* Done, get ready for next time */
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MD_CHK( mbedtls_md_hmac_reset( ctx ) );
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|
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#undef MD_CHK
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cleanup:
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mbedtls_md_free( &aux );
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return( ret );
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}
|
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|
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#endif /* MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC */
|
|
|
|
#if defined(MBEDTLS_BIGNUM_C)
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|
|
#define MPI_VALIDATE_RET( cond ) \
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MBEDTLS_INTERNAL_VALIDATE_RET( cond, MBEDTLS_ERR_MPI_BAD_INPUT_DATA )
|
|
|
|
/*
|
|
* Conditionally assign X = Y, without leaking information
|
|
* about whether the assignment was made or not.
|
|
* (Leaking information about the respective sizes of X and Y is ok however.)
|
|
*/
|
|
int mbedtls_mpi_safe_cond_assign( mbedtls_mpi *X,
|
|
const mbedtls_mpi *Y,
|
|
unsigned char assign )
|
|
{
|
|
int ret = 0;
|
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size_t i;
|
|
mbedtls_mpi_uint limb_mask;
|
|
MPI_VALIDATE_RET( X != NULL );
|
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MPI_VALIDATE_RET( Y != NULL );
|
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|
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/* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
|
|
limb_mask = mbedtls_ct_mpi_uint_mask( assign );;
|
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|
|
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, Y->n ) );
|
|
|
|
X->s = mbedtls_ct_cond_select_sign( assign, Y->s, X->s );
|
|
|
|
mbedtls_ct_mpi_uint_cond_assign( Y->n, X->p, Y->p, assign );
|
|
|
|
for( i = Y->n; i < X->n; i++ )
|
|
X->p[i] &= ~limb_mask;
|
|
|
|
cleanup:
|
|
return( ret );
|
|
}
|
|
|
|
/*
|
|
* Conditionally swap X and Y, without leaking information
|
|
* about whether the swap was made or not.
|
|
* Here it is not ok to simply swap the pointers, which whould lead to
|
|
* different memory access patterns when X and Y are used afterwards.
|
|
*/
|
|
int mbedtls_mpi_safe_cond_swap( mbedtls_mpi *X,
|
|
mbedtls_mpi *Y,
|
|
unsigned char swap )
|
|
{
|
|
int ret, s;
|
|
size_t i;
|
|
mbedtls_mpi_uint limb_mask;
|
|
mbedtls_mpi_uint tmp;
|
|
MPI_VALIDATE_RET( X != NULL );
|
|
MPI_VALIDATE_RET( Y != NULL );
|
|
|
|
if( X == Y )
|
|
return( 0 );
|
|
|
|
/* all-bits 1 if swap is 1, all-bits 0 if swap is 0 */
|
|
limb_mask = mbedtls_ct_mpi_uint_mask( swap );
|
|
|
|
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, Y->n ) );
|
|
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( Y, X->n ) );
|
|
|
|
s = X->s;
|
|
X->s = mbedtls_ct_cond_select_sign( swap, Y->s, X->s );
|
|
Y->s = mbedtls_ct_cond_select_sign( swap, s, Y->s );
|
|
|
|
|
|
for( i = 0; i < X->n; i++ )
|
|
{
|
|
tmp = X->p[i];
|
|
X->p[i] = ( X->p[i] & ~limb_mask ) | ( Y->p[i] & limb_mask );
|
|
Y->p[i] = ( Y->p[i] & ~limb_mask ) | ( tmp & limb_mask );
|
|
}
|
|
|
|
cleanup:
|
|
return( ret );
|
|
}
|
|
|
|
/*
|
|
* Compare signed values in constant time
|
|
*/
|
|
int mbedtls_mpi_lt_mpi_ct( const mbedtls_mpi *X,
|
|
const mbedtls_mpi *Y,
|
|
unsigned *ret )
|
|
{
|
|
size_t i;
|
|
/* The value of any of these variables is either 0 or 1 at all times. */
|
|
unsigned cond, done, X_is_negative, Y_is_negative;
|
|
|
|
MPI_VALIDATE_RET( X != NULL );
|
|
MPI_VALIDATE_RET( Y != NULL );
|
|
MPI_VALIDATE_RET( ret != NULL );
|
|
|
|
if( X->n != Y->n )
|
|
return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
|
|
|
|
/*
|
|
* Set sign_N to 1 if N >= 0, 0 if N < 0.
|
|
* We know that N->s == 1 if N >= 0 and N->s == -1 if N < 0.
|
|
*/
|
|
X_is_negative = ( X->s & 2 ) >> 1;
|
|
Y_is_negative = ( Y->s & 2 ) >> 1;
|
|
|
|
/*
|
|
* If the signs are different, then the positive operand is the bigger.
|
|
* That is if X is negative (X_is_negative == 1), then X < Y is true and it
|
|
* is false if X is positive (X_is_negative == 0).
|
|
*/
|
|
cond = ( X_is_negative ^ Y_is_negative );
|
|
*ret = cond & X_is_negative;
|
|
|
|
/*
|
|
* This is a constant-time function. We might have the result, but we still
|
|
* need to go through the loop. Record if we have the result already.
|
|
*/
|
|
done = cond;
|
|
|
|
for( i = X->n; i > 0; i-- )
|
|
{
|
|
/*
|
|
* If Y->p[i - 1] < X->p[i - 1] then X < Y is true if and only if both
|
|
* X and Y are negative.
|
|
*
|
|
* Again even if we can make a decision, we just mark the result and
|
|
* the fact that we are done and continue looping.
|
|
*/
|
|
cond = mbedtls_ct_mpi_uint_lt( Y->p[i - 1], X->p[i - 1] );
|
|
*ret |= cond & ( 1 - done ) & X_is_negative;
|
|
done |= cond;
|
|
|
|
/*
|
|
* If X->p[i - 1] < Y->p[i - 1] then X < Y is true if and only if both
|
|
* X and Y are positive.
|
|
*
|
|
* Again even if we can make a decision, we just mark the result and
|
|
* the fact that we are done and continue looping.
|
|
*/
|
|
cond = mbedtls_ct_mpi_uint_lt( X->p[i - 1], Y->p[i - 1] );
|
|
*ret |= cond & ( 1 - done ) & ( 1 - X_is_negative );
|
|
done |= cond;
|
|
}
|
|
|
|
return( 0 );
|
|
}
|
|
|
|
#endif /* MBEDTLS_BIGNUM_C */
|
|
|
|
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
|
|
|
|
int mbedtls_ct_rsaes_pkcs1_v15_unpadding( unsigned char *input,
|
|
size_t ilen,
|
|
unsigned char *output,
|
|
size_t output_max_len,
|
|
size_t *olen )
|
|
{
|
|
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
|
|
size_t i, plaintext_max_size;
|
|
|
|
/* The following variables take sensitive values: their value must
|
|
* not leak into the observable behavior of the function other than
|
|
* the designated outputs (output, olen, return value). Otherwise
|
|
* this would open the execution of the function to
|
|
* side-channel-based variants of the Bleichenbacher padding oracle
|
|
* attack. Potential side channels include overall timing, memory
|
|
* access patterns (especially visible to an adversary who has access
|
|
* to a shared memory cache), and branches (especially visible to
|
|
* an adversary who has access to a shared code cache or to a shared
|
|
* branch predictor). */
|
|
size_t pad_count = 0;
|
|
unsigned bad = 0;
|
|
unsigned char pad_done = 0;
|
|
size_t plaintext_size = 0;
|
|
unsigned output_too_large;
|
|
|
|
plaintext_max_size = ( output_max_len > ilen - 11 ) ? ilen - 11
|
|
: output_max_len;
|
|
|
|
/* Check and get padding length in constant time and constant
|
|
* memory trace. The first byte must be 0. */
|
|
bad |= input[0];
|
|
|
|
|
|
/* Decode EME-PKCS1-v1_5 padding: 0x00 || 0x02 || PS || 0x00
|
|
* where PS must be at least 8 nonzero bytes. */
|
|
bad |= input[1] ^ MBEDTLS_RSA_CRYPT;
|
|
|
|
/* Read the whole buffer. Set pad_done to nonzero if we find
|
|
* the 0x00 byte and remember the padding length in pad_count. */
|
|
for( i = 2; i < ilen; i++ )
|
|
{
|
|
pad_done |= ((input[i] | (unsigned char)-input[i]) >> 7) ^ 1;
|
|
pad_count += ((pad_done | (unsigned char)-pad_done) >> 7) ^ 1;
|
|
}
|
|
|
|
|
|
/* If pad_done is still zero, there's no data, only unfinished padding. */
|
|
bad |= mbedtls_ct_uint_if( pad_done, 0, 1 );
|
|
|
|
/* There must be at least 8 bytes of padding. */
|
|
bad |= mbedtls_ct_size_gt( 8, pad_count );
|
|
|
|
/* If the padding is valid, set plaintext_size to the number of
|
|
* remaining bytes after stripping the padding. If the padding
|
|
* is invalid, avoid leaking this fact through the size of the
|
|
* output: use the maximum message size that fits in the output
|
|
* buffer. Do it without branches to avoid leaking the padding
|
|
* validity through timing. RSA keys are small enough that all the
|
|
* size_t values involved fit in unsigned int. */
|
|
plaintext_size = mbedtls_ct_uint_if(
|
|
bad, (unsigned) plaintext_max_size,
|
|
(unsigned) ( ilen - pad_count - 3 ) );
|
|
|
|
/* Set output_too_large to 0 if the plaintext fits in the output
|
|
* buffer and to 1 otherwise. */
|
|
output_too_large = mbedtls_ct_size_gt( plaintext_size,
|
|
plaintext_max_size );
|
|
|
|
/* Set ret without branches to avoid timing attacks. Return:
|
|
* - INVALID_PADDING if the padding is bad (bad != 0).
|
|
* - OUTPUT_TOO_LARGE if the padding is good but the decrypted
|
|
* plaintext does not fit in the output buffer.
|
|
* - 0 if the padding is correct. */
|
|
ret = - (int) mbedtls_ct_uint_if(
|
|
bad, - MBEDTLS_ERR_RSA_INVALID_PADDING,
|
|
mbedtls_ct_uint_if( output_too_large,
|
|
- MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE,
|
|
0 ) );
|
|
|
|
/* If the padding is bad or the plaintext is too large, zero the
|
|
* data that we're about to copy to the output buffer.
|
|
* We need to copy the same amount of data
|
|
* from the same buffer whether the padding is good or not to
|
|
* avoid leaking the padding validity through overall timing or
|
|
* through memory or cache access patterns. */
|
|
bad = mbedtls_ct_uint_mask( bad | output_too_large );
|
|
for( i = 11; i < ilen; i++ )
|
|
input[i] &= ~bad;
|
|
|
|
/* If the plaintext is too large, truncate it to the buffer size.
|
|
* Copy anyway to avoid revealing the length through timing, because
|
|
* revealing the length is as bad as revealing the padding validity
|
|
* for a Bleichenbacher attack. */
|
|
plaintext_size = mbedtls_ct_uint_if( output_too_large,
|
|
(unsigned) plaintext_max_size,
|
|
(unsigned) plaintext_size );
|
|
|
|
/* Move the plaintext to the leftmost position where it can start in
|
|
* the working buffer, i.e. make it start plaintext_max_size from
|
|
* the end of the buffer. Do this with a memory access trace that
|
|
* does not depend on the plaintext size. After this move, the
|
|
* starting location of the plaintext is no longer sensitive
|
|
* information. */
|
|
mbedtls_ct_mem_move_to_left( input + ilen - plaintext_max_size,
|
|
plaintext_max_size,
|
|
plaintext_max_size - plaintext_size );
|
|
|
|
/* Finally copy the decrypted plaintext plus trailing zeros into the output
|
|
* buffer. If output_max_len is 0, then output may be an invalid pointer
|
|
* and the result of memcpy() would be undefined; prevent undefined
|
|
* behavior making sure to depend only on output_max_len (the size of the
|
|
* user-provided output buffer), which is independent from plaintext
|
|
* length, validity of padding, success of the decryption, and other
|
|
* secrets. */
|
|
if( output_max_len != 0 )
|
|
memcpy( output, input + ilen - plaintext_max_size, plaintext_max_size );
|
|
|
|
/* Report the amount of data we copied to the output buffer. In case
|
|
* of errors (bad padding or output too large), the value of *olen
|
|
* when this function returns is not specified. Making it equivalent
|
|
* to the good case limits the risks of leaking the padding validity. */
|
|
*olen = plaintext_size;
|
|
|
|
return( ret );
|
|
}
|
|
|
|
#endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */
|