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Fix multiplication producing a negative zero

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Fix mbedtls_mpi_mul_mpi() when one of the operands is zero and the
other is negative. The sign of the result must be 1, since some
library functions do not treat {-1, 0, NULL} or {-1, n, {0}} as
representing the value 0.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-06-10 15:51:54 +02:00
parent b4347d859b
commit f4998b0a20
3 changed files with 23 additions and 3 deletions
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12
library/bignum.c
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@ -1658,7 +1658,17 @@ int mbedtls_mpi_mul_mpi( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
for( ; j > 0; j-- )
mpi_mul_hlp( i, A->p, X->p + j - 1, B->p[j - 1] );
X->s = A->s * B->s;
/* If the result is 0, we don't shortcut the operation, which reduces
* 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 ) ) )
{
X->s = 1;
}
else
X->s = A->s * B->s;
cleanup: