| /** |
| * Constant-time functions |
| * |
| * Copyright The Mbed TLS Contributors |
| * SPDX-License-Identifier: Apache-2.0 |
| * |
| * Licensed under the Apache License, Version 2.0 (the "License"); you may |
| * not use this file except in compliance with the License. |
| * You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, software |
| * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT |
| * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| * See the License for the specific language governing permissions and |
| * limitations under the License. |
| */ |
| |
| /* |
| * The following functions are implemented without using comparison operators, as those |
| * might be translated to branches by some compilers on some platforms. |
| */ |
| |
| #include "common.h" |
| #include "constant_time_internal.h" |
| #include "mbedtls/constant_time.h" |
| #include "mbedtls/error.h" |
| #include "mbedtls/platform_util.h" |
| |
| #if defined(MBEDTLS_BIGNUM_C) |
| #include "mbedtls/bignum.h" |
| #endif |
| |
| #if defined(MBEDTLS_SSL_TLS_C) |
| #include "ssl_misc.h" |
| #endif |
| |
| #if defined(MBEDTLS_RSA_C) |
| #include "mbedtls/rsa.h" |
| #endif |
| |
| #if defined(MBEDTLS_BASE64_C) |
| #include "constant_time_invasive.h" |
| #endif |
| |
| #include <string.h> |
| |
| int mbedtls_ct_memcmp( const void *a, |
| const void *b, |
| size_t n ) |
| { |
| size_t i; |
| volatile const unsigned char *A = (volatile const unsigned char *) a; |
| volatile const unsigned char *B = (volatile const unsigned char *) b; |
| volatile unsigned char diff = 0; |
| |
| for( i = 0; i < n; i++ ) |
| { |
| /* Read volatile data in order before computing diff. |
| * This avoids IAR compiler warning: |
| * 'the order of volatile accesses is undefined ..' */ |
| unsigned char x = A[i], y = B[i]; |
| diff |= x ^ y; |
| } |
| |
| return( (int)diff ); |
| } |
| |
| unsigned mbedtls_ct_uint_mask( unsigned value ) |
| { |
| /* MSVC has a warning about unary minus on unsigned, but this is |
| * well-defined and precisely what we want to do here */ |
| #if defined(_MSC_VER) |
| #pragma warning( push ) |
| #pragma warning( disable : 4146 ) |
| #endif |
| return( - ( ( value | - value ) >> ( sizeof( value ) * 8 - 1 ) ) ); |
| #if defined(_MSC_VER) |
| #pragma warning( pop ) |
| #endif |
| } |
| |
| #if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC) |
| |
| size_t mbedtls_ct_size_mask( size_t value ) |
| { |
| /* MSVC has a warning about unary minus on unsigned integer types, |
| * but this is well-defined and precisely what we want to do here. */ |
| #if defined(_MSC_VER) |
| #pragma warning( push ) |
| #pragma warning( disable : 4146 ) |
| #endif |
| return( - ( ( value | - value ) >> ( sizeof( value ) * 8 - 1 ) ) ); |
| #if defined(_MSC_VER) |
| #pragma warning( pop ) |
| #endif |
| } |
| |
| #endif /* MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC */ |
| |
| #if defined(MBEDTLS_BIGNUM_C) |
| |
| mbedtls_mpi_uint mbedtls_ct_mpi_uint_mask( mbedtls_mpi_uint value ) |
| { |
| /* MSVC has a warning about unary minus on unsigned, but this is |
| * well-defined and precisely what we want to do here */ |
| #if defined(_MSC_VER) |
| #pragma warning( push ) |
| #pragma warning( disable : 4146 ) |
| #endif |
| return( - ( ( value | - value ) >> ( sizeof( value ) * 8 - 1 ) ) ); |
| #if defined(_MSC_VER) |
| #pragma warning( pop ) |
| #endif |
| } |
| |
| #endif /* MBEDTLS_BIGNUM_C */ |
| |
| #if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC) |
| |
| /** Constant-flow mask generation for "less than" comparison: |
| * - if \p x < \p y, return all-bits 1, that is (size_t) -1 |
| * - otherwise, return all bits 0, that is 0 |
| * |
| * This function can be used to write constant-time code by replacing branches |
| * with bit operations using masks. |
| * |
| * \param x The first value to analyze. |
| * \param y The second value to analyze. |
| * |
| * \return All-bits-one if \p x is less than \p y, otherwise zero. |
| */ |
| static size_t mbedtls_ct_size_mask_lt( size_t x, |
| size_t y ) |
| { |
| /* This has the most significant bit set if and only if x < y */ |
| const size_t sub = x - y; |
| |
| /* sub1 = (x < y) ? 1 : 0 */ |
| const size_t sub1 = sub >> ( sizeof( sub ) * 8 - 1 ); |
| |
| /* mask = (x < y) ? 0xff... : 0x00... */ |
| const size_t mask = mbedtls_ct_size_mask( sub1 ); |
| |
| return( mask ); |
| } |
| |
| size_t mbedtls_ct_size_mask_ge( size_t x, |
| size_t y ) |
| { |
| return( ~mbedtls_ct_size_mask_lt( x, y ) ); |
| } |
| |
| #endif /* MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC */ |
| |
| #if defined(MBEDTLS_BASE64_C) |
| |
| /* Return 0xff if low <= c <= high, 0 otherwise. |
| * |
| * Constant flow with respect to c. |
| */ |
| MBEDTLS_STATIC_TESTABLE |
| unsigned char mbedtls_ct_uchar_mask_of_range( unsigned char low, |
| unsigned char high, |
| unsigned char c ) |
| { |
| /* low_mask is: 0 if low <= c, 0x...ff if low > c */ |
| unsigned low_mask = ( (unsigned) c - low ) >> 8; |
| /* high_mask is: 0 if c <= high, 0x...ff if c > high */ |
| unsigned high_mask = ( (unsigned) high - c ) >> 8; |
| return( ~( low_mask | high_mask ) & 0xff ); |
| } |
| |
| #endif /* MBEDTLS_BASE64_C */ |
| |
| unsigned mbedtls_ct_size_bool_eq( size_t x, |
| size_t y ) |
| { |
| /* diff = 0 if x == y, non-zero otherwise */ |
| const size_t diff = x ^ y; |
| |
| /* MSVC has a warning about unary minus on unsigned integer types, |
| * but this is well-defined and precisely what we want to do here. */ |
| #if defined(_MSC_VER) |
| #pragma warning( push ) |
| #pragma warning( disable : 4146 ) |
| #endif |
| |
| /* diff_msb's most significant bit is equal to x != y */ |
| const size_t diff_msb = ( diff | (size_t) -diff ); |
| |
| #if defined(_MSC_VER) |
| #pragma warning( pop ) |
| #endif |
| |
| /* diff1 = (x != y) ? 1 : 0 */ |
| const unsigned diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 ); |
| |
| return( 1 ^ diff1 ); |
| } |
| |
| #if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT) |
| |
| /** Constant-flow "greater than" comparison: |
| * return x > y |
| * |
| * This is equivalent to \p x > \p y, but is likely to be compiled |
| * to code using bitwise operation rather than a branch. |
| * |
| * \param x The first value to analyze. |
| * \param y The second value to analyze. |
| * |
| * \return 1 if \p x greater than \p y, otherwise 0. |
| */ |
| static unsigned mbedtls_ct_size_gt( size_t x, |
| size_t y ) |
| { |
| /* Return the sign bit (1 for negative) of (y - x). */ |
| return( ( y - x ) >> ( sizeof( size_t ) * 8 - 1 ) ); |
| } |
| |
| #endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */ |
| |
| #if defined(MBEDTLS_BIGNUM_C) |
| |
| unsigned mbedtls_ct_mpi_uint_lt( const mbedtls_mpi_uint x, |
| const mbedtls_mpi_uint y ) |
| { |
| mbedtls_mpi_uint ret; |
| mbedtls_mpi_uint cond; |
| |
| /* |
| * Check if the most significant bits (MSB) of the operands are different. |
| */ |
| cond = ( x ^ y ); |
| /* |
| * If the MSB are the same then the difference x-y will be negative (and |
| * have its MSB set to 1 during conversion to unsigned) if and only if x<y. |
| */ |
| ret = ( x - y ) & ~cond; |
| /* |
| * If the MSB are different, then the operand with the MSB of 1 is the |
| * bigger. (That is if y has MSB of 1, then x<y is true and it is false if |
| * the MSB of y is 0.) |
| */ |
| ret |= y & cond; |
| |
| |
| ret = ret >> ( sizeof( mbedtls_mpi_uint ) * 8 - 1 ); |
| |
| return (unsigned) ret; |
| } |
| |
| #endif /* MBEDTLS_BIGNUM_C */ |
| |
| unsigned mbedtls_ct_uint_if( unsigned condition, |
| unsigned if1, |
| unsigned if0 ) |
| { |
| unsigned mask = mbedtls_ct_uint_mask( condition ); |
| return( ( mask & if1 ) | (~mask & if0 ) ); |
| } |
| |
| #if defined(MBEDTLS_BIGNUM_C) |
| |
| /** Select between two sign values without branches. |
| * |
| * This is functionally equivalent to `condition ? if1 : if0` but uses only bit |
| * operations in order to avoid branches. |
| * |
| * \note if1 and if0 must be either 1 or -1, otherwise the result |
| * is undefined. |
| * |
| * \param condition Condition to test. |
| * \param if1 The first sign; must be either +1 or -1. |
| * \param if0 The second sign; must be either +1 or -1. |
| * |
| * \return \c if1 if \p condition is nonzero, otherwise \c if0. |
| * */ |
| static int mbedtls_ct_cond_select_sign( unsigned char condition, |
| int if1, |
| int if0 ) |
| { |
| /* In order to avoid questions about what we can reasonably assume about |
| * the representations of signed integers, move everything to unsigned |
| * by taking advantage of the fact that if1 and if0 are either +1 or -1. */ |
| unsigned uif1 = if1 + 1; |
| unsigned uif0 = if0 + 1; |
| |
| /* condition was 0 or 1, mask is 0 or 2 as are uif1 and uif0 */ |
| const unsigned mask = condition << 1; |
| |
| /* select uif1 or uif0 */ |
| unsigned ur = ( uif0 & ~mask ) | ( uif1 & mask ); |
| |
| /* ur is now 0 or 2, convert back to -1 or +1 */ |
| return( (int) ur - 1 ); |
| } |
| |
| void mbedtls_ct_mpi_uint_cond_assign( size_t n, |
| mbedtls_mpi_uint *dest, |
| const mbedtls_mpi_uint *src, |
| unsigned char condition ) |
| { |
| size_t i; |
| |
| /* MSVC has a warning about unary minus on unsigned integer types, |
| * but this is well-defined and precisely what we want to do here. */ |
| #if defined(_MSC_VER) |
| #pragma warning( push ) |
| #pragma warning( disable : 4146 ) |
| #endif |
| |
| /* all-bits 1 if condition is 1, all-bits 0 if condition is 0 */ |
| const mbedtls_mpi_uint mask = -condition; |
| |
| #if defined(_MSC_VER) |
| #pragma warning( pop ) |
| #endif |
| |
| for( i = 0; i < n; i++ ) |
| dest[i] = ( src[i] & mask ) | ( dest[i] & ~mask ); |
| } |
| |
| #endif /* MBEDTLS_BIGNUM_C */ |
| |
| #if defined(MBEDTLS_BASE64_C) |
| |
| unsigned char mbedtls_ct_base64_enc_char( unsigned char value ) |
| { |
| unsigned char digit = 0; |
| /* For each range of values, if value is in that range, mask digit with |
| * the corresponding value. Since value can only be in a single range, |
| * only at most one masking will change digit. */ |
| digit |= mbedtls_ct_uchar_mask_of_range( 0, 25, value ) & ( 'A' + value ); |
| digit |= mbedtls_ct_uchar_mask_of_range( 26, 51, value ) & ( 'a' + value - 26 ); |
| digit |= mbedtls_ct_uchar_mask_of_range( 52, 61, value ) & ( '0' + value - 52 ); |
| digit |= mbedtls_ct_uchar_mask_of_range( 62, 62, value ) & '+'; |
| digit |= mbedtls_ct_uchar_mask_of_range( 63, 63, value ) & '/'; |
| return( digit ); |
| } |
| |
| signed char mbedtls_ct_base64_dec_value( unsigned char c ) |
| { |
| unsigned char val = 0; |
| /* For each range of digits, if c is in that range, mask val with |
| * the corresponding value. Since c can only be in a single range, |
| * only at most one masking will change val. Set val to one plus |
| * the desired value so that it stays 0 if c is in none of the ranges. */ |
| val |= mbedtls_ct_uchar_mask_of_range( 'A', 'Z', c ) & ( c - 'A' + 0 + 1 ); |
| val |= mbedtls_ct_uchar_mask_of_range( 'a', 'z', c ) & ( c - 'a' + 26 + 1 ); |
| val |= mbedtls_ct_uchar_mask_of_range( '0', '9', c ) & ( c - '0' + 52 + 1 ); |
| val |= mbedtls_ct_uchar_mask_of_range( '+', '+', c ) & ( c - '+' + 62 + 1 ); |
| val |= mbedtls_ct_uchar_mask_of_range( '/', '/', c ) & ( c - '/' + 63 + 1 ); |
| /* At this point, val is 0 if c is an invalid digit and v+1 if c is |
| * a digit with the value v. */ |
| return( val - 1 ); |
| } |
| |
| #endif /* MBEDTLS_BASE64_C */ |
| |
| #if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT) |
| |
| /** Shift some data towards the left inside a buffer. |
| * |
| * `mbedtls_ct_mem_move_to_left(start, total, offset)` is functionally |
| * equivalent to |
| * ``` |
| * memmove(start, start + offset, total - offset); |
| * memset(start + offset, 0, total - offset); |
| * ``` |
| * but it strives to use a memory access pattern (and thus total timing) |
| * that does not depend on \p offset. This timing independence comes at |
| * the expense of performance. |
| * |
| * \param start Pointer to the start of the buffer. |
| * \param total Total size of the buffer. |
| * \param offset Offset from which to copy \p total - \p offset bytes. |
| */ |
| static void mbedtls_ct_mem_move_to_left( void *start, |
| size_t total, |
| size_t offset ) |
| { |
| volatile unsigned char *buf = start; |
| size_t i, n; |
| if( total == 0 ) |
| return; |
| for( i = 0; i < total; i++ ) |
| { |
| unsigned no_op = mbedtls_ct_size_gt( total - offset, i ); |
| /* The first `total - offset` passes are a no-op. The last |
| * `offset` passes shift the data one byte to the left and |
| * zero out the last byte. */ |
| for( n = 0; n < total - 1; n++ ) |
| { |
| unsigned char current = buf[n]; |
| unsigned char next = buf[n+1]; |
| buf[n] = mbedtls_ct_uint_if( no_op, current, next ); |
| } |
| buf[total-1] = mbedtls_ct_uint_if( no_op, buf[total-1], 0 ); |
| } |
| } |
| |
| #endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */ |
| |
| #if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC) |
| |
| void mbedtls_ct_memcpy_if_eq( unsigned char *dest, |
| const unsigned char *src, |
| size_t len, |
| size_t c1, |
| size_t c2 ) |
| { |
| /* mask = c1 == c2 ? 0xff : 0x00 */ |
| const size_t equal = mbedtls_ct_size_bool_eq( c1, c2 ); |
| const unsigned char mask = (unsigned char) mbedtls_ct_size_mask( equal ); |
| |
| /* dest[i] = c1 == c2 ? src[i] : dest[i] */ |
| for( size_t i = 0; i < len; i++ ) |
| dest[i] = ( src[i] & mask ) | ( dest[i] & ~mask ); |
| } |
| |
| void mbedtls_ct_memcpy_offset( unsigned char *dest, |
| const unsigned char *src, |
| size_t offset, |
| size_t offset_min, |
| size_t offset_max, |
| size_t len ) |
| { |
| size_t offsetval; |
| |
| for( offsetval = offset_min; offsetval <= offset_max; offsetval++ ) |
| { |
| mbedtls_ct_memcpy_if_eq( dest, src + offsetval, len, |
| offsetval, offset ); |
| } |
| } |
| |
| int mbedtls_ct_hmac( mbedtls_md_context_t *ctx, |
| const unsigned char *add_data, |
| size_t add_data_len, |
| const unsigned char *data, |
| size_t data_len_secret, |
| size_t min_data_len, |
| size_t max_data_len, |
| unsigned char *output ) |
| { |
| /* |
| * This function breaks the HMAC abstraction and uses the md_clone() |
| * extension to the MD API in order to get constant-flow behaviour. |
| * |
| * HMAC(msg) is defined as HASH(okey + HASH(ikey + msg)) where + means |
| * 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. |
| * |
| * We'll first compute inner_hash = HASH(ikey + msg) by hashing up to |
| * minlen, then cloning the context, and for each byte up to maxlen |
| * finishing up the hash computation, keeping only the correct result. |
| * |
| * Then we only need to compute HASH(okey + inner_hash) and we're done. |
| */ |
| const mbedtls_md_type_t md_alg = mbedtls_md_get_type( ctx->md_info ); |
| /* TLS 1.2 only supports SHA-384, SHA-256, SHA-1, MD-5, |
| * all of which have the same block size except SHA-384. */ |
| const size_t block_size = md_alg == MBEDTLS_MD_SHA384 ? 128 : 64; |
| const unsigned char * const ikey = ctx->hmac_ctx; |
| const unsigned char * const okey = ikey + block_size; |
| const size_t hash_size = mbedtls_md_get_size( ctx->md_info ); |
| |
| unsigned char aux_out[MBEDTLS_MD_MAX_SIZE]; |
| mbedtls_md_context_t aux; |
| size_t offset; |
| int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED; |
| |
| mbedtls_md_init( &aux ); |
| |
| #define MD_CHK( func_call ) \ |
| do { \ |
| ret = (func_call); \ |
| if( ret != 0 ) \ |
| goto cleanup; \ |
| } while( 0 ) |
| |
| MD_CHK( mbedtls_md_setup( &aux, ctx->md_info, 0 ) ); |
| |
| /* After hmac_start() of hmac_reset(), ikey has already been hashed, |
| * so we can start directly with the message */ |
| MD_CHK( mbedtls_md_update( ctx, add_data, add_data_len ) ); |
| MD_CHK( mbedtls_md_update( ctx, data, min_data_len ) ); |
| |
| /* For each possible length, compute the hash up to that point */ |
| for( offset = min_data_len; offset <= max_data_len; offset++ ) |
| { |
| MD_CHK( mbedtls_md_clone( &aux, ctx ) ); |
| MD_CHK( mbedtls_md_finish( &aux, aux_out ) ); |
| /* Keep only the correct inner_hash in the output buffer */ |
| mbedtls_ct_memcpy_if_eq( output, aux_out, hash_size, |
| offset, data_len_secret ); |
| |
| if( offset < max_data_len ) |
| MD_CHK( mbedtls_md_update( ctx, data + offset, 1 ) ); |
| } |
| |
| /* The context needs to finish() before it starts() again */ |
| MD_CHK( mbedtls_md_finish( ctx, aux_out ) ); |
| |
| /* 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_SUITES_USE_TLS_CBC */ |
| |
| #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.) |
| */ |
| 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 = 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 */ |
