define experimental direct_scalarmul, which can be implemented as monty ladder (at least for comparison purposes at the beginning)

include/decaf.h

@@ -324,6 +324,31 @@ void decaf_448_point_scalarmul (
     const decaf_448_scalar_t scalar
 ) API_VIS NONNULL3;
 
+/**
+ * @brief Multiply a base point by a scalar: scaled = scalar*base.
+ * This function operates directly on serialized forms.
+ *
+ * @warning This function is experimental.  It may not be supported
+ * long-term.
+ *
+ * @param [out] scaled The scaled point base*scalar
+ * @param [in] base The point to be scaled.
+ * @param [in] scalar The scalar to multiply by.
+ * @param [in] allow_identity Allow the input to be the identity.
+ * @param [in] short_circuit Allow a fast return if the input is illegal.
+ *
+ * @retval DECAF_SUCCESS The scalarmul succeeded.
+ * @retval DECAF_FAILURE The scalarmul didn't succeed, because
+ * base does not represent a point.
+ */
+decaf_bool_t decaf_448_direct_scalarmul (
+    uint8_t scaled[DECAF_448_SER_BYTES],
+    const uint8_t base[DECAF_448_SER_BYTES],
+    const decaf_448_scalar_t scalar,
+    decaf_bool_t allow_identity,
+    decaf_bool_t short_circuit
+) API_VIS NONNULL3 WARN_UNUSED;
+
 /**
  * @brief Precompute a table for fast scalar multiplication.
  * Some implementations do not include precomputed points; for
@@ -367,7 +392,9 @@ void decaf_448_precomputed_scalarmul (
  * @param [in] scalar1 A first scalar to multiply by.
  * @param [in] base2 A second point to be scaled.
  * @param [in] scalar2 A second scalar to multiply by.
+ *
  * @TODO: test
+ * @TODO: define vartime/precomp version of this for performance??
  */
 void decaf_448_point_double_scalarmul (
     decaf_448_point_t combo,

src/decaf.c

@@ -778,6 +778,24 @@ void decaf_448_precompute (
     decaf_448_point_copy(a->p[0],b);
 }
 
+decaf_bool_t decaf_448_direct_scalarmul (
+    uint8_t scaled[DECAF_448_SER_BYTES],
+    const uint8_t base[DECAF_448_SER_BYTES],
+    const decaf_448_scalar_t scalar,
+    decaf_bool_t allow_identity,
+    decaf_bool_t short_circuit
+) {
+    decaf_448_point_t basep;
+    decaf_bool_t succ = decaf_448_point_decode(basep, base, allow_identity);
+    /* FIXME: compiler can probably reorder this to something non-consttime even if
+     * !short_circuit.
+     */
+    if (short_circuit && ~succ) return succ;
+    decaf_448_point_scalarmul(basep, basep, scalar);
+    decaf_448_point_encode(scaled, basep);
+    return succ;
+}
+
 void decaf_448_precomputed_scalarmul (
     decaf_448_point_t a,
     const decaf_448_precomputed_s *b,

src/decaf_fast.c

@@ -809,3 +809,22 @@ void decaf_448_precomputed_scalarmul (
 ) {
     decaf_448_point_scalarmul(a,b->p[0],scalar);
 }
+
+decaf_bool_t decaf_448_direct_scalarmul (
+    uint8_t scaled[DECAF_448_SER_BYTES],
+    const uint8_t base[DECAF_448_SER_BYTES],
+    const decaf_448_scalar_t scalar,
+    decaf_bool_t allow_identity,
+    decaf_bool_t short_circuit
+) {
+    decaf_448_point_t basep;
+    decaf_bool_t succ = decaf_448_point_decode(basep, base, allow_identity);
+    /* FIXME: compiler can probably reorder this to something non-consttime even if
+     * !short_circuit.
+     */
+    if (short_circuit && ~succ) return succ;
+    decaf_448_point_scalarmul(basep, basep, scalar);
+    decaf_448_point_encode(scaled, basep);
+    return succ;
+}
+

test/test_scalarmul.c

@@ -116,6 +116,8 @@ single_scalarmul_compatibility_test (
         posix_memalign((void**)&dpre, alignof_decaf_448_precomputed_s, sizeof_decaf_448_precomputed_s);
     	tw_extended_a_t ed;
         convert_tw_extensible_to_tw_extended(ed, &text);
+        uint8_t ser4[DECAF_448_SER_BYTES];
+        decaf_448_point_encode(ser4, (struct decaf_448_point_s *)ed);
         decaf_448_point_scalarmul(
             ed2,
             (struct decaf_448_point_s *)ed,
@@ -142,6 +144,7 @@ single_scalarmul_compatibility_test (
         decaf_448_point_encode(ser1, (struct decaf_448_point_s *)ed);
         decaf_448_point_encode(ser2, ed2);
         decaf_448_point_encode(ser3, ed3);
+        (void)decaf_448_direct_scalarmul(ser4, ser4, (struct decaf_448_scalar_s *)scalar, -1, -1);
 
         /* check consistency mont vs window */
         consistent &= field_eq(mont, ct);
@@ -150,6 +153,7 @@ single_scalarmul_compatibility_test (
         consistent &= field_eq(mont, sced);
         consistent &= memcmp(ser1,ser2,sizeof(ser1)) ? 0 : -1;
         consistent &= memcmp(ser1,ser3,sizeof(ser1)) ? 0 : -1;
+        consistent &= memcmp(ser1,ser4,sizeof(ser1)) ? 0 : -1;
     }
     
     /* check consistency mont vs combs */