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Change mbedtls_mpi_cmp_mpi_ct to check less than

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The signature of mbedtls_mpi_cmp_mpi_ct() meant to support using it in
place of mbedtls_mpi_cmp_mpi(). This meant full comparison functionality
and a signed result.

To make the function more universal and friendly to constant time
coding, we change the result type to unsigned. Theoretically, we could
encode the comparison result in an unsigned value, but it would be less
intuitive.

Therefore we won't be able to represent the result as unsigned anymore
and the functionality will be constrained to checking if the first
operand is less than the second. This is sufficient to support the
current use case and to check any relationship between MPIs.

The only drawback is that we need to call the function twice when
checking for equality, but this can be optimised later if an when it is
needed.
This commit is contained in:
Janos Follath 2019-10-11 14:21:53 +01:00 committed by Simon Butcher
parent 81c9fe5f2c
commit 8faf1d627b
5 changed files with 71 additions and 70 deletions
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11
include/mbedtls/bignum.h
View File

@ -560,23 +560,22 @@ int mbedtls_mpi_cmp_abs( const mbedtls_mpi *X, const mbedtls_mpi *Y );
int mbedtls_mpi_cmp_mpi( const mbedtls_mpi *X, const mbedtls_mpi *Y );
/**
* \brief Compare two MPIs in constant time.
* \brief Check if an MPI is less than the other in constant time.
*
* \param X The left-hand MPI. This must point to an initialized MPI
* with the same allocated length as Y.
* \param Y The right-hand MPI. This must point to an initialized MPI
* with the same allocated length as X.
* \param ret The result of the comparison:
* \c 1 if \p X is greater than \p Y.
* \c -1 if \p X is lesser than \p Y.
* \c 0 if \p X is equal to \p Y.
* \c 1 if \p X is less than \p Y.
* \c 0 if \p X is greater than or equal to \p Y.
*
* \return 0 on success.
* \return MBEDTLS_ERR_MPI_BAD_INPUT_DATA if the allocated length of
* the two input MPIs is not the same.
*/
int mbedtls_mpi_cmp_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y,
int *ret );
int mbedtls_mpi_lt_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y,
unsigned *ret );
/**
* \brief Compare an MPI with an integer.

68
library/bignum.c
View File

@ -1071,7 +1071,8 @@ int mbedtls_mpi_cmp_mpi( const mbedtls_mpi *X, const mbedtls_mpi *Y )
return( 0 );
}
static int ct_lt_mpi_uint( const mbedtls_mpi_uint x, const mbedtls_mpi_uint y )
static unsigned ct_lt_mpi_uint( const mbedtls_mpi_uint x,
const mbedtls_mpi_uint y )
{
mbedtls_mpi_uint ret;
mbedtls_mpi_uint cond;
@ -1098,16 +1099,11 @@ static int ct_lt_mpi_uint( const mbedtls_mpi_uint x, const mbedtls_mpi_uint y )
return ret;
}
static int ct_bool_get_mask( unsigned int b )
{
return ~( b - 1 );
}
/*
* Compare signed values in constant time
*/
int mbedtls_mpi_cmp_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y,
int *ret )
int mbedtls_mpi_lt_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y,
unsigned *ret )
{
size_t i;
unsigned int cond, done, sign_X, sign_Y;
@ -1120,45 +1116,49 @@ int mbedtls_mpi_cmp_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y,
return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
/*
* if( X->s > 0 && Y->s < 0 )
* {
* *ret = 1;
* done = 1;
* }
* else if( Y->s > 0 && X->s < 0 )
* {
* *ret = -1;
* done = 1;
* }
* Get sign bits of the signs.
*/
sign_X = X->s;
sign_X = sign_X >> ( sizeof( unsigned int ) * 8 - 1 );
sign_Y = Y->s;
cond = ( ( sign_X ^ sign_Y ) >> ( sizeof( unsigned int ) * 8 - 1 ) );
*ret = ct_bool_get_mask( cond ) & X->s;
sign_Y = sign_Y >> ( sizeof( unsigned int ) * 8 - 1 );
/*
* If the signs are different, then the positive operand is the bigger.
* That is if X is negative (sign bit 1), then X < Y is true and it is false
* if X is positive (sign bit 0).
*/
cond = ( sign_X ^ sign_Y );
*ret = cond & sign_X;
/*
* 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( ( X->p[i - 1] > Y->p[i - 1] ) && !done )
* {
* done = 1;
* *ret = X->s;
* }
* If Y->p[i - 1] < X->p[i - 1] and both X and Y are negative, then
* X < Y.
*
* Again even if we can make a decision, we just mark the result and
* the fact that we are done and continue looping.
*/
cond = ct_lt_mpi_uint( Y->p[i - 1], X->p[i - 1] );
*ret |= ct_bool_get_mask( cond & ( 1 - done ) ) & X->s;
cond = ct_lt_mpi_uint( Y->p[i - 1], X->p[i - 1] ) & sign_X;
*ret |= cond & ( 1 - done );
done |= cond & ( 1 - done );
/*
* if( ( X->p[i - 1] < Y->p[i - 1] ) && !done )
* {
* done = 1;
* *ret = -X->s;
* }
* If X->p[i - 1] < Y->p[i - 1] and both X and Y are positive, then
* X < Y.
*
* Again even if we can make a decision, we just mark the result and
* the fact that we are done and continue looping.
*/
cond = ct_lt_mpi_uint( X->p[i - 1], Y->p[i - 1] );
*ret |= ct_bool_get_mask( cond & ( 1 - done ) ) & -X->s;
cond = ct_lt_mpi_uint( X->p[i - 1], Y->p[i - 1] ) & ( 1 - sign_X );
*ret |= cond & ( 1 - done );
done |= cond & ( 1 - done );
}

6
library/ecp.c
View File

@ -2724,7 +2724,7 @@ int mbedtls_ecp_gen_privkey( const mbedtls_ecp_group *grp,
{
/* SEC1 3.2.1: Generate d such that 1 <= n < N */
int count = 0;
int cmp = 0;
unsigned cmp = 0;
/*
* Match the procedure given in RFC 6979 (deterministic ECDSA):
@ -2750,13 +2750,13 @@ int mbedtls_ecp_gen_privkey( const mbedtls_ecp_group *grp,
if( ++count > 30 )
return( MBEDTLS_ERR_ECP_RANDOM_FAILED );
ret = mbedtls_mpi_cmp_mpi_ct( d, &grp->N, &cmp );
ret = mbedtls_mpi_lt_mpi_ct( d, &grp->N, &cmp );
if( ret != 0 )
{
goto cleanup;
}
}
while( mbedtls_mpi_cmp_int( d, 1 ) < 0 || cmp >= 0 );
while( mbedtls_mpi_cmp_int( d, 1 ) < 0 || cmp != 1 );
}
#endif /* ECP_SHORTWEIERSTRASS */

44
tests/suites/test_suite_mpi.data
View File

@ -163,38 +163,38 @@ mbedtls_mpi_cmp_mpi:10:"2":10:"-3":1
Base test mbedtls_mpi_cmp_mpi (Mixed values) #6
mbedtls_mpi_cmp_mpi:10:"-2":10:"31231231289798":-1
Base test mbedtls_mpi_cmp_mpi_ct #1
mbedtls_mpi_cmp_mpi_ct:1:10:"693":1:10:"693":0:0
Base test mbedtls_mpi_lt_mpi_ct #1
mbedtls_mpi_lt_mpi_ct:1:10:"693":1:10:"693":0:0
Base test mbedtls_mpi_cmp_mpi_ct #2
mbedtls_mpi_cmp_mpi_ct:1:10:"693":1:10:"692":1:0
Base test mbedtls_mpi_lt_mpi_ct #2
mbedtls_mpi_lt_mpi_ct:1:10:"693":1:10:"692":0:0
Base test mbedtls_mpi_cmp_mpi_ct #3
mbedtls_mpi_cmp_mpi_ct:1:10:"693":1:10:"694":-1:0
Base test mbedtls_mpi_lt_mpi_ct #3
mbedtls_mpi_lt_mpi_ct:1:10:"693":1:10:"694":1:0
Base test mbedtls_mpi_cmp_mpi_ct (Negative values) #1
mbedtls_mpi_cmp_mpi_ct:1:10:"-2":1:10:"-2":0:0
Base test mbedtls_mpi_lt_mpi_ct (Negative values) #1
mbedtls_mpi_lt_mpi_ct:1:10:"-2":1:10:"-2":0:0
Base test mbedtls_mpi_cmp_mpi_ct (Negative values) #2
mbedtls_mpi_cmp_mpi_ct:1:10:"-2":1:10:"-3":1:0
Base test mbedtls_mpi_lt_mpi_ct (Negative values) #2
mbedtls_mpi_lt_mpi_ct:1:10:"-2":1:10:"-3":0:0
Base test mbedtls_mpi_cmp_mpi_ct (Negative values) #3
mbedtls_mpi_cmp_mpi_ct:1:10:"-2":1:10:"-1":-1:0
Base test mbedtls_mpi_lt_mpi_ct (Negative values) #3
mbedtls_mpi_lt_mpi_ct:1:10:"-2":1:10:"-1":1:0
Base test mbedtls_mpi_cmp_mpi_ct (Mixed values) #4
mbedtls_mpi_cmp_mpi_ct:1:10:"-3":1:10:"2":-1:0
Base test mbedtls_mpi_lt_mpi_ct (Mixed values) #4
mbedtls_mpi_lt_mpi_ct:1:10:"-3":1:10:"2":1:0
Base test mbedtls_mpi_cmp_mpi_ct (Mixed values) #5
mbedtls_mpi_cmp_mpi_ct:1:10:"2":1:10:"-3":1:0
Base test mbedtls_mpi_lt_mpi_ct (Mixed values) #5
mbedtls_mpi_lt_mpi_ct:1:10:"2":1:10:"-3":0:0
Base test mbedtls_mpi_cmp_mpi_ct (Mixed values) #6
mbedtls_mpi_cmp_mpi_ct:2:10:"-2":2:10:"31231231289798":-1:0
Base test mbedtls_mpi_lt_mpi_ct (Mixed values) #6
mbedtls_mpi_lt_mpi_ct:2:10:"-2":2:10:"31231231289798":1:0
Base test mbedtls_mpi_cmp_mpi_ct (X is longer in storage) #7
mbedtls_mpi_cmp_mpi_ct:3:10:"693":2:10:"693":0:MBEDTLS_ERR_MPI_BAD_INPUT_DATA
Base test mbedtls_mpi_lt_mpi_ct (X is longer in storage) #7
mbedtls_mpi_lt_mpi_ct:3:10:"693":2:10:"693":0:MBEDTLS_ERR_MPI_BAD_INPUT_DATA
Base test mbedtls_mpi_cmp_mpi_ct (Y is longer in storage) #8
mbedtls_mpi_cmp_mpi_ct:3:10:"693":4:10:"693":0:MBEDTLS_ERR_MPI_BAD_INPUT_DATA
Base test mbedtls_mpi_lt_mpi_ct (Y is longer in storage) #8
mbedtls_mpi_lt_mpi_ct:3:10:"693":4:10:"693":0:MBEDTLS_ERR_MPI_BAD_INPUT_DATA
Base test mbedtls_mpi_cmp_abs #1
mbedtls_mpi_cmp_abs:10:"693":10:"693":0

12
tests/suites/test_suite_mpi.function
View File

@ -538,10 +538,12 @@ exit:
/* END_CASE */
/* BEGIN_CASE */
void mbedtls_mpi_cmp_mpi_ct( int size_X, int radix_X, char * input_X, int size_Y,
int radix_Y, char * input_Y, int input_ret, int input_err )
void mbedtls_mpi_lt_mpi_ct( int size_X, int radix_X, char * input_X,
int size_Y, int radix_Y, char * input_Y,
int input_ret, int input_err )
{
int ret;
unsigned ret;
unsigned input_uret = input_ret;
mbedtls_mpi X, Y;
mbedtls_mpi_init( &X ); mbedtls_mpi_init( &Y );
@ -551,9 +553,9 @@ void mbedtls_mpi_cmp_mpi_ct( int size_X, int radix_X, char * input_X, int size_Y
mbedtls_mpi_grow( &X, size_X );
mbedtls_mpi_grow( &Y, size_Y );
TEST_ASSERT( mbedtls_mpi_cmp_mpi_ct( &X, &Y, &ret ) == input_err );
TEST_ASSERT( mbedtls_mpi_lt_mpi_ct( &X, &Y, &ret ) == input_err );
if( input_err == 0 )
TEST_ASSERT( ret == input_ret );
TEST_ASSERT( ret == input_uret );
exit:
mbedtls_mpi_free( &X ); mbedtls_mpi_free( &Y );