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Reintroduce trimming of input in mbedtls_mpi_mul_int()

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Removing the trimming has significant memory impact. While it is clearly what
we want to do eventually for constant-time'ness, it should be fixed alongside
a strategy to contain the ramifications on memory usage.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
This commit is contained in:
Hanno Becker 2022-04-14 11:52:11 +01:00
parent 0dbf04a9a6
commit 3577131bb4
1 changed files with 7 additions and 3 deletions
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10
library/bignum.c
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@ -1500,8 +1500,12 @@ int mbedtls_mpi_mul_int( mbedtls_mpi *X, const mbedtls_mpi *A, mbedtls_mpi_uint
MPI_VALIDATE_RET( X != NULL );
MPI_VALIDATE_RET( A != NULL );
size_t n = A->n;
while( n > 0 && A->p[n - 1] == 0 )
--n;
/* The general method below doesn't work if b==0. */
if( b == 0 )
if( b == 0 || n == 0 )
return( mbedtls_mpi_lset( X, 0 ) );
/* Calculate A*b as A + A*(b-1) to take advantage of mbedtls_mpi_core_mla */
@ -1517,9 +1521,9 @@ int mbedtls_mpi_mul_int( mbedtls_mpi *X, const mbedtls_mpi *A, mbedtls_mpi_uint
*
* Note that calculating A*b as 0 + A*b doesn't work as-is because
* A,X can be the same. */
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, A->n + 1 ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, n + 1 ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_copy( X, A ) );
mbedtls_mpi_core_mla( X->p, X->n, A->p, A->n, b - 1 );
mbedtls_mpi_core_mla( X->p, X->n, A->p, n, b - 1 );
cleanup:
return( ret );