Make ecp_mod_p521 a bit faster
diff --git a/library/ecp.c b/library/ecp.c
index 78b05c4..33081a0 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -547,12 +547,12 @@
/*
* Size of p521 in terms of t_uint
*/
-#define P521_SIZE_INT ( 521 / CHAR_BIT / sizeof( t_uint ) + 1 )
+#define P521_SIZE_INT ( 521 / 8 / sizeof( t_uint ) + 1 )
/*
* Bits to keep in the most significant t_uint
*/
-#if defined(POLARSS_HAVE_INT8)
+#if defined(POLARSSL_HAVE_INT8)
#define P521_MASK 0x01
#else
#define P521_MASK 0x01FF
@@ -560,26 +560,36 @@
/*
* Fast quasi-reduction modulo p521 (FIPS 186-3 D.2.5)
+ * Write N as A1 + 2^521 A0, return A0 + A1
*/
static int ecp_mod_p521( mpi *N )
{
int ret;
- t_uint Mp[P521_SIZE_INT];
+ size_t i;
mpi M;
+ t_uint Mp[P521_SIZE_INT+1];
+ /* Worst case for the size of M is when sizeof( t_uint ) == 16:
+ * we need to hold bits 513 to 1056, which is 34 limbs, that is
+ * P521_SIZE_INT + 1. Otherwise P521_SIZE is enough. */
if( N->n < P521_SIZE_INT )
return( 0 );
- memset( Mp, 0, P521_SIZE_INT * sizeof( t_uint ) );
- memcpy( Mp, N->p, P521_SIZE_INT * sizeof( t_uint ) );
- Mp[P521_SIZE_INT - 1] &= P521_MASK;
-
+ /* M = A1 */
M.s = 1;
- M.n = P521_SIZE_INT;
+ M.n = N->n - ( P521_SIZE_INT - 1 );
+ if( M.n > P521_SIZE_INT + 1 )
+ M.n = P521_SIZE_INT + 1;
M.p = Mp;
+ memcpy( Mp, N->p + P521_SIZE_INT - 1, M.n * sizeof( t_uint ) );
+ MPI_CHK( mpi_shift_r( &M, 521 % ( 8 * sizeof( t_uint ) ) ) );
- MPI_CHK( mpi_shift_r( N, 521 ) );
+ /* N = A0 */
+ N->p[P521_SIZE_INT - 1] &= P521_MASK;
+ for( i = P521_SIZE_INT; i < N->n; i++ )
+ N->p[i] = 0;
+ /* N = A0 + A1 */
MPI_CHK( mpi_add_abs( N, N, &M ) );
cleanup: