Add mod_p256k1
Makes secp256k1 about 4x faster
diff --git a/library/ecp_curves.c b/library/ecp_curves.c
index 4ebe31c..366f7fa 100644
--- a/library/ecp_curves.c
+++ b/library/ecp_curves.c
@@ -643,15 +643,20 @@
#if defined(POLARSSL_ECP_DP_SECP521R1_ENABLED)
static int ecp_mod_p521( mpi * );
#endif
-#if defined(POLARSSL_ECP_DP_M255_ENABLED)
-static int ecp_mod_p255( mpi * );
-#endif
#define NIST_MODP( P ) grp->modp = ecp_mod_ ## P;
#else
#define NIST_MODP( P )
#endif /* POLARSSL_ECP_NIST_OPTIM */
+/* Additional forward declarations */
+#if defined(POLARSSL_ECP_DP_M255_ENABLED)
+static int ecp_mod_p255( mpi * );
+#endif
+#if defined(POLARSSL_ECP_DP_SECP256K1_ENABLED)
+static int ecp_mod_p256k1( mpi * );
+#endif
+
#define LOAD_GROUP_A( G ) ecp_group_load( grp, \
G ## _p, sizeof( G ## _p ), \
G ## _a, sizeof( G ## _a ), \
@@ -755,6 +760,7 @@
#if defined(POLARSSL_ECP_DP_SECP256K1_ENABLED)
case POLARSSL_ECP_DP_SECP256K1:
+ grp->modp = ecp_mod_p256k1;
return( LOAD_GROUP_A( secp256k1 ) );
#endif /* POLARSSL_ECP_DP_SECP256K1_ENABLED */
@@ -1195,7 +1201,7 @@
/*
* Fast quasi-reduction modulo p255 = 2^255 - 19
- * Write N as A1 + 2^255 A1, return A0 + 19 * A1
+ * Write N as A0 + 2^255 A1, return A0 + 19 * A1
*/
static int ecp_mod_p255( mpi *N )
{
@@ -1232,4 +1238,80 @@
}
#endif /* POLARSSL_ECP_DP_M255_ENABLED */
+#if defined(POLARSSL_ECP_DP_SECP256K1_ENABLED)
+
+/* Size of p256k1 in terms of t_uint */
+#define P256K1_WIDTH ( 256 / 8 / sizeof( t_uint ) )
+
+/* Value of R (see below) */
+static t_uint p256k1_r_p[] = {
+ BYTES_TO_T_UINT_8( 0xD1, 0x03, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00 ),
+};
+#define P256K1_R_WIDTH ( sizeof( p256k1_r_p ) / sizeof( t_uint ) )
+
+/*
+ * Fast quasi-reduction modulo p256k1 = 2^256 - R,
+ * with R = 2^32 + 2^9 + 2^8 + 2^7 + 2^6 + 2^4 + 1 = 4294968273
+ *
+ * Write N as A0 + 2^256 A1, return A0 + R * A1.
+ * Actually do two passes, since R is big.
+ */
+static int ecp_mod_p256k1( mpi *N )
+{
+ int ret;
+ size_t i;
+ mpi M, R;
+ t_uint Mp[P256K1_WIDTH + P256K1_R_WIDTH];
+
+ if( N->n < P256K1_WIDTH )
+ return( 0 );
+
+ /* Init R */
+ R.s = 1;
+ R.p = p256k1_r_p;
+ R.n = P256K1_R_WIDTH;
+
+ /* Common setup for M */
+ M.s = 1;
+ M.p = Mp;
+
+ /* M = A1 */
+ M.n = N->n - P256K1_WIDTH;
+ if( M.n > P256K1_WIDTH )
+ M.n = P256K1_WIDTH;
+ memset( Mp, 0, sizeof Mp );
+ memcpy( Mp, N->p + P256K1_WIDTH, M.n * sizeof( t_uint ) );
+ M.n += R.n; /* Make room for multiplication by R */
+
+ /* N = A0 */
+ for( i = P256K1_WIDTH; i < N->n; i++ )
+ N->p[i] = 0;
+
+ /* N = A0 + R * A1 */
+ MPI_CHK( mpi_mul_mpi( &M, &M, &R ) );
+ MPI_CHK( mpi_add_abs( N, N, &M ) );
+
+ /* Second pass */
+
+ /* M = A1 */
+ M.n = N->n - P256K1_WIDTH;
+ if( M.n > P256K1_WIDTH )
+ M.n = P256K1_WIDTH;
+ memset( Mp, 0, sizeof Mp );
+ memcpy( Mp, N->p + P256K1_WIDTH, M.n * sizeof( t_uint ) );
+ M.n += R.n; /* Make room for multiplication by R */
+
+ /* N = A0 */
+ for( i = P256K1_WIDTH; i < N->n; i++ )
+ N->p[i] = 0;
+
+ /* N = A0 + R * A1 */
+ MPI_CHK( mpi_mul_mpi( &M, &M, &R ) );
+ MPI_CHK( mpi_add_abs( N, N, &M ) );
+
+cleanup:
+ return( ret );
+}
+#endif /* POLARSSL_ECP_DP_SECP256K1_ENABLED */
+
#endif