Rm ecp_add() and add ecp_muladd()

ChangeLog

@@ -61,7 +61,8 @@ API Changes
 
 Removals
    * Removed mbedtls_ecp_group_read_string(). Only named groups are supported.
-   * Removed mbedtls_ecp_sub().
+   * Removed mbedtls_ecp_sub() and mbedtls_ecp_add(), use
+     mbedtls_ecp_muladd().
    * Removed individual mdX_hmac and shaX_hmac functions (use generic
      md_hmac functions from md.h)
    * Removed the PBKDF2 module (use PKCS5).

include/mbedtls/ecp.h

@@ -481,27 +481,20 @@ int mbedtls_ecp_tls_read_group( mbedtls_ecp_group *grp, const unsigned char **bu
 int mbedtls_ecp_tls_write_group( const mbedtls_ecp_group *grp, size_t *olen,
                          unsigned char *buf, size_t blen );
 
-/**
- * \brief           Addition: R = P + Q
- *
- * \param grp       ECP group
- * \param R         Destination point
- * \param P         Left-hand point
- * \param Q         Right-hand point
- *
- * \return          0 if successful,
- *                  MBEDTLS_ERR_MPI_MALLOC_FAILED if memory allocation failed
- *
- * \note            This function does not support Montgomery curves, such as
- *                  Curve25519.
- */
-int mbedtls_ecp_add( const mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
-             const mbedtls_ecp_point *P, const mbedtls_ecp_point *Q );
-
 /**
  * \brief           Multiplication by an integer: R = m * P
  *                  (Not thread-safe to use same group in multiple threads)
  *
+ * \note            In order to prevent timing attacks, this function
+ *                  executes the exact same sequence of (base field)
+ *                  operations for any valid m. It avoids any if-branch or
+ *                  array index depending on the value of m.
+ *
+ * \note            If f_rng is not NULL, it is used to randomize intermediate
+ *                  results in order to prevent potential timing attacks
+ *                  targeting these results. It is recommended to always
+ *                  provide a non-NULL f_rng (the overhead is negligible).
+ *
  * \param grp       ECP group
  * \param R         Destination point
  * \param m         Integer by which to multiply
@@ -513,21 +506,35 @@ int mbedtls_ecp_add( const mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
  *                  MBEDTLS_ERR_ECP_INVALID_KEY if m is not a valid privkey
  *                  or P is not a valid pubkey,
  *                  MBEDTLS_ERR_MPI_MALLOC_FAILED if memory allocation failed
- *
- * \note            In order to prevent timing attacks, this function
- *                  executes the exact same sequence of (base field)
- *                  operations for any valid m. It avoids any if-branch or
- *                  array index depending on the value of m.
- *
- * \note            If f_rng is not NULL, it is used to randomize intermediate
- *                  results in order to prevent potential timing attacks
- *                  targeting these results. It is recommended to always
- *                  provide a non-NULL f_rng (the overhead is negligible).
  */
 int mbedtls_ecp_mul( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
              const mbedtls_mpi *m, const mbedtls_ecp_point *P,
              int (*f_rng)(void *, unsigned char *, size_t), void *p_rng );
 
+/**
+ * \brief           Multiplication and addition of two points by integers:
+ *                  R = m * P + n * Q
+ *                  (Not thread-safe to use same group in multiple threads)
+ *
+ * \note            In contrast to ecp_mul(), this function does not guarantee
+ *                  a constant execution flow and timing.
+ *
+ * \param grp       ECP group
+ * \param R         Destination point
+ * \param m         Integer by which to multiply P
+ * \param P         Point to multiply by m
+ * \param n         Integer by which to multiply Q
+ * \param Q         Point to be multiplied by n
+ *
+ * \return          0 if successful,
+ *                  MBEDTLS_ERR_ECP_INVALID_KEY if m or n is not a valid privkey
+ *                  or P or Q is not a valid pubkey,
+ *                  MBEDTLS_ERR_MPI_MALLOC_FAILED if memory allocation failed
+ */
+int mbedtls_ecp_muladd( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
+             const mbedtls_mpi *m, const mbedtls_ecp_point *P,
+             const mbedtls_mpi *n, const mbedtls_ecp_point *Q );
+
 /**
  * \brief           Check that a point is a valid public key on this curve
  *

library/ecdsa.c

@@ -203,9 +203,9 @@ int mbedtls_ecdsa_verify( mbedtls_ecp_group *grp,
 {
     int ret;
     mbedtls_mpi e, s_inv, u1, u2;
-    mbedtls_ecp_point R, P;
+    mbedtls_ecp_point R;
 
-    mbedtls_ecp_point_init( &R ); mbedtls_ecp_point_init( &P );
+    mbedtls_ecp_point_init( &R );
     mbedtls_mpi_init( &e ); mbedtls_mpi_init( &s_inv ); mbedtls_mpi_init( &u1 ); mbedtls_mpi_init( &u2 );
 
     /* Fail cleanly on curves such as Curve25519 that can't be used for ECDSA */
@@ -249,9 +249,7 @@ int mbedtls_ecdsa_verify( mbedtls_ecp_group *grp,
      * Since we're not using any secret data, no need to pass a RNG to
      * mbedtls_ecp_mul() for countermesures.
      */
-    MBEDTLS_MPI_CHK( mbedtls_ecp_mul( grp, &R, &u1, &grp->G, NULL, NULL ) );
+    MBEDTLS_MPI_CHK( mbedtls_ecp_muladd( grp, &R, &u1, &grp->G, &u2, Q ) );
-    MBEDTLS_MPI_CHK( mbedtls_ecp_mul( grp, &P, &u2, Q, NULL, NULL ) );
-    MBEDTLS_MPI_CHK( mbedtls_ecp_add( grp, &R, &R, &P ) );
 
     if( mbedtls_ecp_is_zero( &R ) )
     {
@@ -275,7 +273,7 @@ int mbedtls_ecdsa_verify( mbedtls_ecp_group *grp,
     }
 
 cleanup:
-    mbedtls_ecp_point_free( &R ); mbedtls_ecp_point_free( &P );
+    mbedtls_ecp_point_free( &R );
     mbedtls_mpi_free( &e ); mbedtls_mpi_free( &s_inv ); mbedtls_mpi_free( &u1 ); mbedtls_mpi_free( &u2 );
 
     return( ret );

library/ecp.c

@@ -1048,24 +1048,6 @@ cleanup:
     return( ret );
 }
 
-/*
- * Addition: R = P + Q, result's coordinates normalized
- */
-int mbedtls_ecp_add( const mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
-             const mbedtls_ecp_point *P, const mbedtls_ecp_point *Q )
-{
-    int ret;
-
-    if( ecp_get_type( grp ) != ECP_TYPE_SHORT_WEIERSTRASS )
-        return( MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE );
-
-    MBEDTLS_MPI_CHK( ecp_add_mixed( grp, R, P, Q ) );
-    MBEDTLS_MPI_CHK( ecp_normalize_jac( grp, R ) );
-
-cleanup:
-    return( ret );
-}
-
 /*
  * Randomize jacobian coordinates:
  * (X, Y, Z) -> (l^2 X, l^3 Y, l Z) for random l
@@ -1684,6 +1666,32 @@ cleanup:
 }
 #endif /* ECP_SHORTWEIERSTRASS */
 
+/*
+ * Linear combination
+ */
+int mbedtls_ecp_muladd( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
+             const mbedtls_mpi *m, const mbedtls_ecp_point *P,
+             const mbedtls_mpi *n, const mbedtls_ecp_point *Q )
+{
+    int ret;
+    mbedtls_ecp_point mP;
+
+    if( ecp_get_type( grp ) != ECP_TYPE_SHORT_WEIERSTRASS )
+        return( MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE );
+
+    mbedtls_ecp_point_init( &mP );
+
+    MBEDTLS_MPI_CHK( mbedtls_ecp_mul( grp, &mP, m, P, NULL, NULL ) );
+    MBEDTLS_MPI_CHK( mbedtls_ecp_mul( grp, R,   n, Q, NULL, NULL ) );
+    MBEDTLS_MPI_CHK( ecp_add_mixed( grp, R, &mP, R ) );
+    MBEDTLS_MPI_CHK( ecp_normalize_jac( grp, R ) );
+
+cleanup:
+    mbedtls_ecp_point_free( &mP );
+
+    return( ret );
+}
+
 
 #if defined(ECP_MONTGOMERY)
 /*