795 lines
27 KiB
C
795 lines
27 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 "mbedtls/ssl_internal.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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#if defined(MBEDTLS_BASE64_C)
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#include "constant_time_invasive.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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/* 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_MODES_USE_MAC)
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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_MODES_USE_MAC */
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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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#if defined(MBEDTLS_BASE64_C)
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/* Return 0xff if low <= c <= high, 0 otherwise.
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*
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* Constant flow with respect to c.
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*/
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MBEDTLS_STATIC_TESTABLE
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unsigned char mbedtls_ct_uchar_mask_of_range(unsigned char low,
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unsigned char high,
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unsigned char c)
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{
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/* low_mask is: 0 if low <= c, 0x...ff if low > c */
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unsigned low_mask = ((unsigned) c - low) >> 8;
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/* high_mask is: 0 if c <= high, 0x...ff if c > high */
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unsigned high_mask = ((unsigned) high - c) >> 8;
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return ~(low_mask | high_mask) & 0xff;
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}
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#endif /* MBEDTLS_BASE64_C */
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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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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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}
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#endif /* MBEDTLS_BIGNUM_C */
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#if defined(MBEDTLS_BASE64_C)
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unsigned char mbedtls_ct_base64_enc_char(unsigned char value)
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{
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unsigned char digit = 0;
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/* For each range of values, if value is in that range, mask digit with
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* the corresponding value. Since value can only be in a single range,
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* only at most one masking will change digit. */
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digit |= mbedtls_ct_uchar_mask_of_range(0, 25, value) & ('A' + value);
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digit |= mbedtls_ct_uchar_mask_of_range(26, 51, value) & ('a' + value - 26);
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digit |= mbedtls_ct_uchar_mask_of_range(52, 61, value) & ('0' + value - 52);
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digit |= mbedtls_ct_uchar_mask_of_range(62, 62, value) & '+';
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digit |= mbedtls_ct_uchar_mask_of_range(63, 63, value) & '/';
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return digit;
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}
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signed char mbedtls_ct_base64_dec_value(unsigned char c)
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{
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unsigned char val = 0;
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/* For each range of digits, if c is in that range, mask val with
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* the corresponding value. Since c can only be in a single range,
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* only at most one masking will change val. Set val to one plus
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* the desired value so that it stays 0 if c is in none of the ranges. */
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val |= mbedtls_ct_uchar_mask_of_range('A', 'Z', c) & (c - 'A' + 0 + 1);
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val |= mbedtls_ct_uchar_mask_of_range('a', 'z', c) & (c - 'a' + 26 + 1);
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val |= mbedtls_ct_uchar_mask_of_range('0', '9', c) & (c - '0' + 52 + 1);
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val |= mbedtls_ct_uchar_mask_of_range('+', '+', c) & (c - '+' + 62 + 1);
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val |= mbedtls_ct_uchar_mask_of_range('/', '/', c) & (c - '/' + 63 + 1);
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/* At this point, val is 0 if c is an invalid digit and v+1 if c is
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* a digit with the value v. */
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return val - 1;
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}
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#endif /* MBEDTLS_BASE64_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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}
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for (i = 0; i < total; i++) {
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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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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_MODES_USE_MAC)
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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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}
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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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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
|
|
* 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 1.0-1.2 only support SHA-384, SHA-256, SHA-1, MD-5,
|
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* all of which have the same block size except SHA-384. */
|
|
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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|
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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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|
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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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|
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MD_CHK(mbedtls_md_setup(&aux, ctx->md_info, 0));
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|
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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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/* Fill the hash buffer in advance with something that is
|
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* not a valid hash (barring an attack on the hash and
|
|
* deliberately-crafted input), in case the caller doesn't
|
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* check the return status properly. */
|
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memset(output, '!', hash_size);
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|
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/* For each possible length, compute the hash up to that point */
|
|
for (offset = min_data_len; offset <= max_data_len; offset++) {
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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 */
|
|
mbedtls_ct_memcpy_if_eq(output, aux_out, hash_size,
|
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offset, data_len_secret);
|
|
|
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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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|
|
/* The context needs to finish() before it starts() again */
|
|
MD_CHK(mbedtls_md_finish(ctx, aux_out));
|
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|
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/* Now compute HASH(okey + inner_hash) */
|
|
MD_CHK(mbedtls_md_starts(ctx));
|
|
MD_CHK(mbedtls_md_update(ctx, okey, block_size));
|
|
MD_CHK(mbedtls_md_update(ctx, output, hash_size));
|
|
MD_CHK(mbedtls_md_finish(ctx, output));
|
|
|
|
/* Done, get ready for next time */
|
|
MD_CHK(mbedtls_md_hmac_reset(ctx));
|
|
|
|
#undef MD_CHK
|
|
|
|
cleanup:
|
|
mbedtls_md_free(&aux);
|
|
return ret;
|
|
}
|
|
|
|
#endif /* MBEDTLS_SSL_SOME_MODES_USE_MAC */
|
|
|
|
#if defined(MBEDTLS_BIGNUM_C)
|
|
|
|
#define MPI_VALIDATE_RET(cond) \
|
|
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.)
|
|
*/
|
|
#if defined(_MSC_VER) && defined(_M_ARM64) && (_MSC_FULL_VER < 193131103)
|
|
/*
|
|
* MSVC miscompiles this function if it's inlined prior to Visual Studio 2022 version 17.1. See:
|
|
* https://developercommunity.visualstudio.com/t/c-compiler-miscompiles-part-of-mbedtls-library-on/1646989
|
|
*/
|
|
__declspec(noinline)
|
|
#endif
|
|
int mbedtls_mpi_safe_cond_assign(mbedtls_mpi *X,
|
|
const mbedtls_mpi *Y,
|
|
unsigned char assign)
|
|
{
|
|
int ret = 0;
|
|
size_t i;
|
|
mbedtls_mpi_uint limb_mask;
|
|
MPI_VALIDATE_RET(X != NULL);
|
|
MPI_VALIDATE_RET(Y != NULL);
|
|
|
|
/* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
|
|
limb_mask = mbedtls_ct_mpi_uint_mask(assign);;
|
|
|
|
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(X, Y->n));
|
|
|
|
X->s = (int) mbedtls_ct_uint_if(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 would 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 = (int) mbedtls_ct_uint_if(swap, Y->s, X->s);
|
|
Y->s = (int) mbedtls_ct_uint_if(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(int mode,
|
|
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];
|
|
|
|
if (mode == MBEDTLS_RSA_PRIVATE) {
|
|
/* 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;
|
|
}
|
|
} else {
|
|
/* Decode EMSA-PKCS1-v1_5 padding: 0x00 || 0x01 || PS || 0x00
|
|
* where PS must be at least 8 bytes with the value 0xFF. */
|
|
bad |= input[1] ^ MBEDTLS_RSA_SIGN;
|
|
|
|
/* Read the whole buffer. Set pad_done to nonzero if we find
|
|
* the 0x00 byte and remember the padding length in pad_count.
|
|
* If there's a non-0xff byte in the padding, the padding is bad. */
|
|
for (i = 2; i < ilen; i++) {
|
|
pad_done |= mbedtls_ct_uint_if(input[i], 0, 1);
|
|
pad_count += mbedtls_ct_uint_if(pad_done, 0, 1);
|
|
bad |= mbedtls_ct_uint_if(pad_done, 0, input[i] ^ 0xFF);
|
|
}
|
|
}
|
|
|
|
/* 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 */
|