From de9f953b9f3b32f94bb8ea1769bfd2e86f9b83ef Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Manuel=20P=C3=A9gouri=C3=A9-Gonnard?= <mpg2@elzevir.fr>
Date: Fri, 23 Oct 2015 15:50:37 +0200
Subject: [PATCH] Optimize more common cases in ecp_muladd()

---
 library/ecp.c | 43 +++++++++++++++++++++++++++++++++----------
 1 file changed, 33 insertions(+), 10 deletions(-)

diff --git a/library/ecp.c b/library/ecp.c
index 4abc87c61..19bb4882e 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -1683,8 +1683,39 @@ cleanup:
 }
 #endif /* ECP_SHORTWEIERSTRASS */
 
+/*
+ * R = m * P with shortcuts for m == 1 and m == -1
+ * NOT constant-time - ONLY for short Weierstrass!
+ */
+static int mbedtls_ecp_mul_shortcuts( mbedtls_ecp_group *grp,
+                                      mbedtls_ecp_point *R,
+                                      const mbedtls_mpi *m,
+                                      const mbedtls_ecp_point *P )
+{
+    int ret;
+
+    if( mbedtls_mpi_cmp_int( m, 1 ) == 0 )
+    {
+        MBEDTLS_MPI_CHK( mbedtls_ecp_copy( R, P ) );
+    }
+    else if( mbedtls_mpi_cmp_int( m, -1 ) == 0 )
+    {
+        MBEDTLS_MPI_CHK( mbedtls_ecp_copy( R, P ) );
+        if( mbedtls_mpi_cmp_int( &R->Y, 0 ) != 0 )
+            MBEDTLS_MPI_CHK( mbedtls_mpi_sub_mpi( &R->Y, &grp->P, &R->Y ) );
+    }
+    else
+    {
+        MBEDTLS_MPI_CHK( mbedtls_ecp_mul( grp, R, m, P, NULL, NULL ) );
+    }
+
+cleanup:
+    return( ret );
+}
+
 /*
  * Linear combination
+ * NOT constant-time
  */
 int mbedtls_ecp_muladd( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
              const mbedtls_mpi *m, const mbedtls_ecp_point *P,
@@ -1698,16 +1729,8 @@ int mbedtls_ecp_muladd( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
 
     mbedtls_ecp_point_init( &mP );
 
-    /* Optimize some simple special cases */
-    if( mbedtls_mpi_cmp_int( m, 1 ) == 0 )
-        MBEDTLS_MPI_CHK( mbedtls_ecp_copy( &mP, P ) );
-    else
-        MBEDTLS_MPI_CHK( mbedtls_ecp_mul( grp, &mP, m, P, NULL, NULL ) );
-
-    if( mbedtls_mpi_cmp_int( n, 1 ) == 0 )
-        MBEDTLS_MPI_CHK( mbedtls_ecp_copy( R, Q ) );
-    else
-        MBEDTLS_MPI_CHK( mbedtls_ecp_mul( grp, R,   n, Q, NULL, NULL ) );
+    MBEDTLS_MPI_CHK( mbedtls_ecp_mul_shortcuts( grp, &mP, m, P ) );
+    MBEDTLS_MPI_CHK( mbedtls_ecp_mul_shortcuts( grp, R,   n, Q ) );
 
     MBEDTLS_MPI_CHK( ecp_add_mixed( grp, R, &mP, R ) );
     MBEDTLS_MPI_CHK( ecp_normalize_jac( grp, R ) );