/** * 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. */ #include "common.h" #include "constant_time.h" #include "mbedtls/error.h" #if defined(MBEDTLS_BIGNUM_C) #include "mbedtls/bignum.h" #endif #if defined(MBEDTLS_SSL_TLS_C) #include "mbedtls/ssl_internal.h" #endif #include int mbedtls_cf_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 ); } /** Turn zero-or-nonzero into zero-or-all-bits-one, without branches. * * \param value The value to analyze. * \return Zero if \p value is zero, otherwise all-bits-one. */ unsigned mbedtls_cf_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 } /* * Turn a bit into a mask: * - if bit == 1, return the all-bits 1 mask, aka (size_t) -1 * - if bit == 0, return the all-bits 0 mask, aka 0 * * This function can be used to write constant-time code by replacing branches * with bit operations using masks. * * This function is implemented without using comparison operators, as those * might be translated to branches by some compilers on some platforms. */ size_t mbedtls_cf_size_mask( size_t bit ) { /* 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 -bit; #if defined(_MSC_VER) #pragma warning( pop ) #endif } /* * Constant-flow mask generation for "less than" comparison: * - if x < 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. * * This function is implemented without using comparison operators, as those * might be translated to branches by some compilers on some platforms. */ size_t mbedtls_cf_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_cf_size_mask( sub1 ); return( mask ); } /* * Constant-flow mask generation for "greater or equal" comparison: * - if x >= 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. * * This function is implemented without using comparison operators, as those * might be translated to branches by some compilers on some platforms. */ size_t mbedtls_cf_size_mask_ge( size_t x, size_t y ) { return( ~mbedtls_cf_size_mask_lt( x, y ) ); } /* * Constant-flow boolean "equal" comparison: * return x == y * * This function can be used to write constant-time code by replacing branches * with bit operations - it can be used in conjunction with * mbedtls_ssl_cf_mask_from_bit(). * * This function is implemented without using comparison operators, as those * might be translated to branches by some compilers on some platforms. */ size_t mbedtls_cf_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 size_t diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 ); return( 1 ^ diff1 ); } /** Check whether a size is out of bounds, without branches. * * This is equivalent to `size > max`, but is likely to be compiled to * to code using bitwise operation rather than a branch. * * \param size Size to check. * \param max Maximum desired value for \p size. * \return \c 0 if `size <= max`. * \return \c 1 if `size > max`. */ unsigned mbedtls_cf_size_gt( size_t size, size_t max ) { /* Return the sign bit (1 for negative) of (max - size). */ return( ( max - size ) >> ( sizeof( size_t ) * 8 - 1 ) ); } #if defined(MBEDTLS_BIGNUM_C) /** Decide if an integer is less than the other, without branches. * * \param x First integer. * \param y Second integer. * * \return 1 if \p x is less than \p y, 0 otherwise */ unsigned mbedtls_cf_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> ( sizeof( mbedtls_mpi_uint ) * 8 - 1 ); return (unsigned) ret; } #endif /* MBEDTLS_BIGNUM_C */ /** Choose between two integer values, without branches. * * This is equivalent to `cond ? if1 : if0`, but is likely to be compiled * to code using bitwise operation rather than a branch. * * \param cond Condition to test. * \param if1 Value to use if \p cond is nonzero. * \param if0 Value to use if \p cond is zero. * \return \c if1 if \p cond is nonzero, otherwise \c if0. */ unsigned mbedtls_cf_uint_if( unsigned cond, unsigned if1, unsigned if0 ) { unsigned mask = mbedtls_cf_uint_mask( cond ); return( ( mask & if1 ) | (~mask & if0 ) ); } size_t mbedtls_cf_size_if( unsigned cond, size_t if1, size_t if0 ) { size_t mask = mbedtls_cf_size_mask( cond ); return( ( mask & if1 ) | (~mask & if0 ) ); } /** * Select between two sign values in constant-time. * * This is functionally equivalent to second ? a : b but uses only bit * operations in order to avoid branches. * * \param[in] a The first sign; must be either +1 or -1. * \param[in] b The second sign; must be either +1 or -1. * \param[in] second Must be either 1 (return b) or 0 (return a). * * \return The selected sign value. */ int mbedtls_cf_cond_select_sign( int a, int b, unsigned char second ) { /* In order to avoid questions about what we can reasonnably assume about * the representations of signed integers, move everything to unsigned * by taking advantage of the fact that a and b are either +1 or -1. */ unsigned ua = a + 1; unsigned ub = b + 1; /* second was 0 or 1, mask is 0 or 2 as are ua and ub */ const unsigned mask = second << 1; /* select ua or ub */ unsigned ur = ( ua & ~mask ) | ( ub & mask ); /* ur is now 0 or 2, convert back to -1 or +1 */ return( (int) ur - 1 ); } #if defined(MBEDTLS_BIGNUM_C) /* * Conditionally assign dest = src, without leaking information * about whether the assignment was made or not. * dest and src must be arrays of limbs of size n. * assign must be 0 or 1. */ void mbedtls_cf_mpi_uint_cond_assign( size_t n, mbedtls_mpi_uint *dest, const mbedtls_mpi_uint *src, unsigned char assign ) { 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 assign is 1, all-bits 0 if assign is 0 */ const mbedtls_mpi_uint mask = -assign; #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 */ /** Shift some data towards the left inside a buffer without leaking * the length of the data through side channels. * * `mbedtls_cf_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. */ void mbedtls_cf_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_cf_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_cf_uint_if( no_op, current, next ); } buf[total-1] = mbedtls_cf_uint_if( no_op, buf[total-1], 0 ); } } /* * Constant-flow conditional memcpy: * - if c1 == c2, equivalent to memcpy(dst, src, len), * - otherwise, a no-op, * but with execution flow independent of the values of c1 and c2. * * This function is implemented without using comparison operators, as those * might be translated to branches by some compilers on some platforms. */ void mbedtls_cf_memcpy_if_eq( unsigned char *dst, const unsigned char *src, size_t len, size_t c1, size_t c2 ) { /* mask = c1 == c2 ? 0xff : 0x00 */ const size_t equal = mbedtls_cf_size_bool_eq( c1, c2 ); const unsigned char mask = (unsigned char) mbedtls_cf_size_mask( equal ); /* dst[i] = c1 == c2 ? src[i] : dst[i] */ for( size_t i = 0; i < len; i++ ) dst[i] = ( src[i] & mask ) | ( dst[i] & ~mask ); } /* * Constant-flow memcpy from variable position in buffer. * - functionally equivalent to memcpy(dst, src + offset_secret, len) * - but with execution flow independent from the value of offset_secret. */ void mbedtls_cf_memcpy_offset( unsigned char *dst, const unsigned char *src_base, size_t offset_secret, size_t offset_min, size_t offset_max, size_t len ) { size_t offset; for( offset = offset_min; offset <= offset_max; offset++ ) { mbedtls_cf_memcpy_if_eq( dst, src_base + offset, len, offset, offset_secret ); } } #if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC) /* * Compute HMAC of variable-length data with constant flow. * * Only works with MD-5, SHA-1, SHA-256 and SHA-384. * (Otherwise, computation of block_size needs to be adapted.) */ int mbedtls_cf_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 1.0-1.2 only support 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_cf_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 ); /* 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 /* make sure assign is 0 or 1 in a time-constant manner */ assign = (assign | (unsigned char)-assign) >> (sizeof( assign ) * 8 - 1); /* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */ limb_mask = -assign; #if defined(_MSC_VER) #pragma warning( pop ) #endif MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, Y->n ) ); X->s = mbedtls_cf_cond_select_sign( X->s, Y->s, assign ); mbedtls_cf_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 ); /* 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 /* make sure swap is 0 or 1 in a time-constant manner */ swap = (swap | (unsigned char)-swap) >> (sizeof( swap ) * 8 - 1); /* all-bits 1 if swap is 1, all-bits 0 if swap is 0 */ limb_mask = -swap; #if defined(_MSC_VER) #pragma warning( pop ) #endif MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, Y->n ) ); MBEDTLS_MPI_CHK( mbedtls_mpi_grow( Y, X->n ) ); s = X->s; X->s = mbedtls_cf_cond_select_sign( X->s, Y->s, swap ); Y->s = mbedtls_cf_cond_select_sign( Y->s, s, swap ); 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_cf_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_cf_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_cf_rsaes_pkcs1_v15_unpadding( int mode, size_t ilen, size_t *olen, unsigned char *output, size_t output_max_len, unsigned char *buf ) { 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 = mbedtls_cf_size_if( 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 |= buf[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 |= buf[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 |= ((buf[i] | (unsigned char)-buf[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 |= buf[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_cf_uint_if( buf[i], 0, 1 ); pad_count += mbedtls_cf_uint_if( pad_done, 0, 1 ); bad |= mbedtls_cf_uint_if( pad_done, 0, buf[i] ^ 0xFF ); } } /* If pad_done is still zero, there's no data, only unfinished padding. */ bad |= mbedtls_cf_uint_if( pad_done, 0, 1 ); /* There must be at least 8 bytes of padding. */ bad |= mbedtls_cf_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_cf_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_cf_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_cf_uint_if( bad, - MBEDTLS_ERR_RSA_INVALID_PADDING, mbedtls_cf_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_cf_uint_mask( bad | output_too_large ); for( i = 11; i < ilen; i++ ) buf[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_cf_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_cf_mem_move_to_left( buf + 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, buf + 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 */