Remove another local MPI from ecp_normalize_jac_many()
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/library/ecp.c b/library/ecp.c
index c441e10..7834265 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -1279,7 +1279,7 @@
#else
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
size_t i;
- mbedtls_mpi *c, u, Zi;
+ mbedtls_mpi *c, t;
if( ( c = mbedtls_calloc( T_size, sizeof( mbedtls_mpi ) ) ) == NULL )
return( MBEDTLS_ERR_ECP_ALLOC_FAILED );
@@ -1287,7 +1287,7 @@
for( i = 0; i < T_size; i++ )
mbedtls_mpi_init( &c[i] );
- mbedtls_mpi_init( &u ); mbedtls_mpi_init( &Zi );
+ mbedtls_mpi_init( &t );
/*
* c[i] = Z_0 * ... * Z_i
@@ -1299,33 +1299,40 @@
}
/*
- * u = 1 / (Z_0 * ... * Z_n) mod P
+ * c[n] = 1 / (Z_0 * ... * Z_n) mod P
*/
- MPI_ECP_INV( &u, &c[T_size-1] );
+ MPI_ECP_INV( &c[T_size-1], &c[T_size-1] );
for( i = T_size - 1; ; i-- )
{
- /*
- * Zi = 1 / Z_i mod p
- * u = 1 / (Z_0 * ... * Z_i) mod P
+ /* At the start of iteration i (note that i decrements), we have
+ * - c[j] = Z_0 * .... * Z_j for j < i,
+ * - c[j] = 1 / (Z_0 * .... * Z_j) for j == i,
+ *
+ * This is maintained via
+ * - c[i-1] <- c[i] * Z_i
+ *
+ * We also derive 1/Z_i = c[i] * c[i-1] for i>0 and use that
+ * to do the actual normalization. For i==0, we already have
+ * c[0] = 1 / Z_0.
*/
- if( i == 0 )
+
+ if( i > 0 )
{
- MPI_ECP_MOV( &Zi, &u );
+ /* Compute 1/Z_i and establish invariant for the next iteration. */
+ MPI_ECP_MUL( &t, &c[i], &c[i-1] );
+ MPI_ECP_MUL( &c[i-1], &c[i], &T[i]->Z );
}
else
{
- MPI_ECP_MUL( &Zi, &u, &c[i-1] );
- MPI_ECP_MUL( &u, &u, &T[i]->Z );
+ MPI_ECP_MOV( &t, &c[0] );
}
- /*
- * proceed as in normalize()
- */
- MPI_ECP_MUL( &T[i]->Y, &T[i]->Y, &Zi );
- MPI_ECP_MUL( &Zi, &Zi, &Zi );
- MPI_ECP_MUL( &T[i]->X, &T[i]->X, &Zi );
- MPI_ECP_MUL( &T[i]->Y, &T[i]->Y, &Zi );
+ /* Now t holds 1 / Z_i; normalize as in ecp_normalize_jac() */
+ MPI_ECP_MUL( &T[i]->Y, &T[i]->Y, &t );
+ MPI_ECP_MUL( &t, &t, &t );
+ MPI_ECP_MUL( &T[i]->X, &T[i]->X, &t );
+ MPI_ECP_MUL( &T[i]->Y, &T[i]->Y, &t );
/*
* Post-precessing: reclaim some memory by shrinking coordinates
@@ -1343,7 +1350,7 @@
cleanup:
- mbedtls_mpi_free( &u ); mbedtls_mpi_free( &Zi );
+ mbedtls_mpi_free( &t );
for( i = 0; i < T_size; i++ )
mbedtls_mpi_free( &c[i] );
mbedtls_free( c );