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Merge pull request #278 from ARMmbed/dev/yanesca/iotcrypt-767-ecdsa-timing-side-channel

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ECDSA timing side channel due to non-constant-time integer comparison
This commit is contained in:
Gilles Peskine 2019-11-12 11:44:13 +01:00 committed by GitHub
commit e80c7e49e7
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GPG Key ID: 4AEE18F83AFDEB23
5 changed files with 240 additions and 3 deletions
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20
include/mbedtls/bignum.h
View File

@ -185,7 +185,7 @@ extern "C" {
*/
typedef struct mbedtls_mpi
{
int s; /*!< integer sign */
int s; /*!< Sign: -1 if the mpi is negative, 1 otherwise */
size_t n; /*!< total # of limbs */
mbedtls_mpi_uint *p; /*!< pointer to limbs */
}
@ -594,6 +594,24 @@ 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 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 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_lt_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y,
unsigned *ret );
/**
* \brief Compare an MPI with an integer.
*

101
library/bignum.c
View File

@ -1148,6 +1148,107 @@ int mbedtls_mpi_cmp_mpi( const mbedtls_mpi *X, const mbedtls_mpi *Y )
return( 0 );
}
/** 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
*/
static unsigned ct_lt_mpi_uint( 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 >> ( biL - 1 );
return (unsigned) 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 = ct_lt_mpi_uint( 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 = ct_lt_mpi_uint( X->p[i - 1], Y->p[i - 1] );
*ret |= cond & ( 1 - done ) & ( 1 - X_is_negative );
done |= cond;
}
return( 0 );
}
/*
* Compare signed values
*/

10
library/ecp.c
View File

@ -2803,6 +2803,7 @@ int mbedtls_ecp_gen_privkey( const mbedtls_ecp_group *grp,
{
/* SEC1 3.2.1: Generate d such that 1 <= n < N */
int count = 0;
unsigned cmp = 0;
/*
* Match the procedure given in RFC 6979 (deterministic ECDSA):
@ -2827,9 +2828,14 @@ int mbedtls_ecp_gen_privkey( const mbedtls_ecp_group *grp,
*/
if( ++count > 30 )
return( MBEDTLS_ERR_ECP_RANDOM_FAILED );
ret = mbedtls_mpi_lt_mpi_ct( d, &grp->N, &cmp );
if( ret != 0 )
{
goto cleanup;
}
}
while( mbedtls_mpi_cmp_int( d, 1 ) < 0 ||
mbedtls_mpi_cmp_mpi( d, &grp->N ) >= 0 );
while( mbedtls_mpi_cmp_int( d, 1 ) < 0 || cmp != 1 );
}
#endif /* ECP_SHORTWEIERSTRASS */

87
tests/suites/test_suite_mpi.data
View File

@ -175,6 +175,93 @@ 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_lt_mpi_ct #1
mbedtls_mpi_lt_mpi_ct:1:"2B5":1:"2B5":0:0
Base test mbedtls_mpi_lt_mpi_ct #2
mbedtls_mpi_lt_mpi_ct:1:"2B5":1:"2B4":0:0
Base test mbedtls_mpi_lt_mpi_ct #3
mbedtls_mpi_lt_mpi_ct:1:"2B5":1:"2B6":1:0
Base test mbedtls_mpi_lt_mpi_ct (Negative values) #1
mbedtls_mpi_lt_mpi_ct:1:"-2":1:"-2":0:0
Base test mbedtls_mpi_lt_mpi_ct (Negative values) #2
mbedtls_mpi_lt_mpi_ct:1:"-2":1:"-3":0:0
Base test mbedtls_mpi_lt_mpi_ct (Negative values) #3
mbedtls_mpi_lt_mpi_ct:1:"-2":1:"-1":1:0
Base test mbedtls_mpi_lt_mpi_ct (Mixed values) #1
mbedtls_mpi_lt_mpi_ct:1:"-3":1:"2":1:0
Base test mbedtls_mpi_lt_mpi_ct (Mixed values) #2
mbedtls_mpi_lt_mpi_ct:1:"2":1:"-3":0:0
Base test mbedtls_mpi_lt_mpi_ct (Mixed values) #3
mbedtls_mpi_lt_mpi_ct:2:"-2":2:"1C67967269C6":1:0
Base test mbedtls_mpi_lt_mpi_ct (X is longer in storage)
mbedtls_mpi_lt_mpi_ct:3:"2B5":2:"2B5":0:MBEDTLS_ERR_MPI_BAD_INPUT_DATA
Base test mbedtls_mpi_lt_mpi_ct (Y is longer in storage)
mbedtls_mpi_lt_mpi_ct:3:"2B5":4:"2B5":0:MBEDTLS_ERR_MPI_BAD_INPUT_DATA
Base test mbedtls_mpi_lt_mpi_ct (corner case - 64 bit) #1
mbedtls_mpi_lt_mpi_ct:2:"7FFFFFFFFFFFFFFF":2:"FF":0:0
Base test mbedtls_mpi_lt_mpi_ct (corner case - 64 bit) #2
mbedtls_mpi_lt_mpi_ct:2:"8000000000000000":2:"7FFFFFFFFFFFFFFF":0:0
Base test mbedtls_mpi_lt_mpi_ct (corner case - 64 bit) #3
mbedtls_mpi_lt_mpi_ct:2:"8000000000000000":2:"1":0:0
Base test mbedtls_mpi_lt_mpi_ct (corner case - 64 bit) #4
mbedtls_mpi_lt_mpi_ct:2:"8000000000000000":2:"0":0:0
Base test mbedtls_mpi_lt_mpi_ct (corner case - 64 bit) #5
mbedtls_mpi_lt_mpi_ct:2:"FFFFFFFFFFFFFFFF":2:"FF":0:0
Base test mbedtls_mpi_lt_mpi_ct (corner case - 32 bit) #1
mbedtls_mpi_lt_mpi_ct:1:"7FFFFFFF":1:"FF":0:0
Base test mbedtls_mpi_lt_mpi_ct (corner case - 32 bit) #2
mbedtls_mpi_lt_mpi_ct:1:"80000000":1:"7FFFFFFF":0:0
Base test mbedtls_mpi_lt_mpi_ct (corner case - 32 bit) #3
mbedtls_mpi_lt_mpi_ct:1:"80000000":1:"1":0:0
Base test mbedtls_mpi_lt_mpi_ct (corner case - 32 bit) #4
mbedtls_mpi_lt_mpi_ct:1:"80000000":1:"0":0:0
Base test mbedtls_mpi_lt_mpi_ct (corner case - 32 bit) #5
mbedtls_mpi_lt_mpi_ct:1:"FFFFFFFF":1:"FF":0:0
Multi-limb mbedtls_mpi_lt_mpi_ct (X<Y, zero vs non-zero MS limb)
mbedtls_mpi_lt_mpi_ct:2:"0FFFFFFFFFFFFFFFF":2:"1FFFFFFFFFFFFFFFF":1:0
Multi-limb mbedtls_mpi_lt_mpi_ct (X>Y, equal MS limbs)
mbedtls_mpi_lt_mpi_ct:2:"-EEFFFFFFFFFFFFFFF1":2:"-EEFFFFFFFFFFFFFFFF":0:0
Multi-limb mbedtls_mpi_lt_mpi_ct (X=Y)
mbedtls_mpi_lt_mpi_ct:2:"EEFFFFFFFFFFFFFFFF":2:"EEFFFFFFFFFFFFFFFF":0:0
Multi-limb mbedtls_mpi_lt_mpi_ct (X=-Y)
mbedtls_mpi_lt_mpi_ct:2:"-EEFFFFFFFFFFFFFFFF":2:"EEFFFFFFFFFFFFFFFF":1:0
Multi-limb mbedtls_mpi_lt_mpi_ct (Alternating limbs) #1
mbedtls_mpi_lt_mpi_ct:2:"11FFFFFFFFFFFFFFFF":2:"FF1111111111111111":1:0
Multi-limb mbedtls_mpi_lt_mpi_ct (Alternating limbs) #2
mbedtls_mpi_lt_mpi_ct:2:"FF1111111111111111":2:"11FFFFFFFFFFFFFFFF":0:0
Multi-limb mbedtls_mpi_lt_mpi_ct (Alternating limbs) #3
mbedtls_mpi_lt_mpi_ct:2:"-11FFFFFFFFFFFFFFFF":2:"-FF1111111111111111":0:0
Multi-limb mbedtls_mpi_lt_mpi_ct (Alternating limbs) #4
mbedtls_mpi_lt_mpi_ct:2:"-FF1111111111111111":2:"-11FFFFFFFFFFFFFFFF":1:0
Base test mbedtls_mpi_cmp_abs #1
mbedtls_mpi_cmp_abs:10:"693":10:"693":0

25
tests/suites/test_suite_mpi.function
View File

@ -587,6 +587,31 @@ exit:
}
/* END_CASE */
/* BEGIN_CASE */
void mbedtls_mpi_lt_mpi_ct( int size_X, char * input_X,
int size_Y, char * input_Y,
int input_ret, int input_err )
{
unsigned ret;
unsigned input_uret = input_ret;
mbedtls_mpi X, Y;
mbedtls_mpi_init( &X ); mbedtls_mpi_init( &Y );
TEST_ASSERT( mbedtls_mpi_read_string( &X, 16, input_X ) == 0 );
TEST_ASSERT( mbedtls_mpi_read_string( &Y, 16, input_Y ) == 0 );
mbedtls_mpi_grow( &X, size_X );
mbedtls_mpi_grow( &Y, size_Y );
TEST_ASSERT( mbedtls_mpi_lt_mpi_ct( &X, &Y, &ret ) == input_err );
if( input_err == 0 )
TEST_ASSERT( ret == input_uret );
exit:
mbedtls_mpi_free( &X ); mbedtls_mpi_free( &Y );
}
/* END_CASE */
/* BEGIN_CASE */
void mbedtls_mpi_cmp_abs( int radix_X, char * input_X, int radix_Y,
char * input_Y, int input_A )