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mbedtls/library/constant_time.c
Tautvydas Žilys 61156f8a6a Cap the workaround for mbedtls_mpi_safe_cond_assign on MSVC/ARM64 to MSVC versions prior to 17.1. 
Signed-off-by: Tautvydas Žilys <tautvydas.zilys@gmail.com>
2022-01-31 13:37:47 -08:00

827 lines
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/**
* 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 "mbedtls/ssl_internal.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 1.0-1.2 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_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.)
*/
#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 = 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( 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 */
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