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Fix multiplication with negative result and a low-order 0 limb

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Fix a bug introduced in "Fix multiplication producing a negative zero" that
caused the sign to be forced to +1 when A > 0, B < 0 and B's low-order limb
is 0.

Add a non-regression test. More generally, systematically test combinations
of leading zeros, trailing zeros and signs.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine
2021-06-15 21:44:32 +02:00
parent 0759cadddf
commit d65b50063a
2 changed files with 103 additions and 5 deletions
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10
library/bignum.c
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@ -1641,6 +1641,7 @@ int mbedtls_mpi_mul_mpi( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
size_t i, j;
mbedtls_mpi TA, TB;
int result_is_zero = 0;
MPI_VALIDATE_RET( X != NULL );
MPI_VALIDATE_RET( A != NULL );
MPI_VALIDATE_RET( B != NULL );
@ -1653,10 +1654,14 @@ int mbedtls_mpi_mul_mpi( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
for( i = A->n; i > 0; i-- )
if( A->p[i - 1] != 0 )
break;
if( i == 0 && ( A->n == 0 || A->p[0] == 0 ) )
result_is_zero = 1;
for( j = B->n; j > 0; j-- )
if( B->p[j - 1] != 0 )
break;
if( j == 0 && ( B->n == 0 || B->p[0] == 0 ) )
result_is_zero = 1;
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, i + j ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_lset( X, 0 ) );
@ -1668,11 +1673,8 @@ int mbedtls_mpi_mul_mpi( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
* but does not eliminate side channels leaking the zero-ness. We do
* need to take care to set the sign bit properly since the library does
* not fully support an MPI object with a value of 0 and s == -1. */
if( ( i == 0 && ( A->n == 0 || A->p[0] == 0 ) ) ||
( j == 0 && ( B->n == 0 || B->p[0] == 0 ) ) )
{
if( result_is_zero )
X->s = 1;
}
else
X->s = A->s * B->s;