Document that infinity must not be passed to ecmult_const

src/ecmult_const.h

@@ -11,10 +11,11 @@
 #include "group.h"
 
 /**
- * Multiply: R = q*A (in constant-time)
+ * Multiply: R = q*A (in constant-time).
+ * A must not be the point at infinity.
  * Here `bits` should be set to the maximum bitlength of the _absolute value_ of `q`, plus
  * one because we internally sometimes add 2 to the number during the WNAF conversion.
  */
-static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, const secp256k1_scalar *q, int bits);
+static void secp256k1_ecmult_const_no_infinity(secp256k1_gej *r, const secp256k1_ge *a, const secp256k1_scalar *q, int bits);
 
 #endif /* SECP256K1_ECMULT_CONST_H */

src/ecmult_const_impl.h

@@ -134,7 +134,7 @@ static int secp256k1_wnaf_const(int *wnaf, const secp256k1_scalar *scalar, int w
     return skew;
 }
 
-static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, const secp256k1_scalar *scalar, int size) {
+static void secp256k1_ecmult_const_no_infinity(secp256k1_gej *r, const secp256k1_ge *a, const secp256k1_scalar *scalar, int size) {
     secp256k1_ge pre_a[ECMULT_TABLE_SIZE(WINDOW_A)];
     secp256k1_ge tmpa;
     secp256k1_fe Z;
@@ -174,6 +174,7 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons
      * that the Z coordinate was 1, use affine addition formulae, and correct
      * the Z coordinate of the result once at the end.
      */
+    VERIFY_CHECK(!a->infinity);
     secp256k1_gej_set_ge(r, a);
     secp256k1_ecmult_odd_multiples_table_globalz_windowa(pre_a, &Z, r);
     for (i = 0; i < ECMULT_TABLE_SIZE(WINDOW_A); i++) {

src/modules/ecdh/main_impl.h

@@ -50,7 +50,7 @@ int secp256k1_ecdh(const secp256k1_context* ctx, unsigned char *output, const se
     overflow |= secp256k1_scalar_is_zero(&s);
     secp256k1_scalar_cmov(&s, &secp256k1_scalar_one, overflow);
 
-    secp256k1_ecmult_const(&res, &pt, &s, 256);
+    secp256k1_ecmult_const_no_infinity(&res, &pt, &s, 256);
     secp256k1_ge_set_gej(&pt, &res);
 
     /* Compute a hash of the point */

src/tests.c

@@ -2630,7 +2630,7 @@ void ecmult_const_random_mult(void) {
         0xb84e4e1b, 0xfb77e21f, 0x96baae2a, 0x63dec956
     );
     secp256k1_gej b;
-    secp256k1_ecmult_const(&b, &a, &xn, 256);
+    secp256k1_ecmult_const_no_infinity(&b, &a, &xn, 256);
 
     CHECK(secp256k1_ge_is_valid_var(&a));
     ge_equals_gej(&expected_b, &b);
@@ -2646,12 +2646,12 @@ void ecmult_const_commutativity(void) {
     random_scalar_order_test(&a);
     random_scalar_order_test(&b);
 
-    secp256k1_ecmult_const(&res1, &secp256k1_ge_const_g, &a, 256);
-    secp256k1_ecmult_const(&res2, &secp256k1_ge_const_g, &b, 256);
+    secp256k1_ecmult_const_no_infinity(&res1, &secp256k1_ge_const_g, &a, 256);
+    secp256k1_ecmult_const_no_infinity(&res2, &secp256k1_ge_const_g, &b, 256);
     secp256k1_ge_set_gej(&mid1, &res1);
     secp256k1_ge_set_gej(&mid2, &res2);
-    secp256k1_ecmult_const(&res1, &mid1, &b, 256);
-    secp256k1_ecmult_const(&res2, &mid2, &a, 256);
+    secp256k1_ecmult_const_no_infinity(&res1, &mid1, &b, 256);
+    secp256k1_ecmult_const_no_infinity(&res2, &mid2, &a, 256);
     secp256k1_ge_set_gej(&mid1, &res1);
     secp256k1_ge_set_gej(&mid2, &res2);
     ge_equals_ge(&mid1, &mid2);
@@ -2667,13 +2667,13 @@ void ecmult_const_mult_zero_one(void) {
     secp256k1_scalar_negate(&negone, &one);
 
     random_group_element_test(&point);
-    secp256k1_ecmult_const(&res1, &point, &zero, 3);
+    secp256k1_ecmult_const_no_infinity(&res1, &point, &zero, 3);
     secp256k1_ge_set_gej(&res2, &res1);
     CHECK(secp256k1_ge_is_infinity(&res2));
-    secp256k1_ecmult_const(&res1, &point, &one, 2);
+    secp256k1_ecmult_const_no_infinity(&res1, &point, &one, 2);
     secp256k1_ge_set_gej(&res2, &res1);
     ge_equals_ge(&res2, &point);
-    secp256k1_ecmult_const(&res1, &point, &negone, 256);
+    secp256k1_ecmult_const_no_infinity(&res1, &point, &negone, 256);
     secp256k1_gej_neg(&res1, &res1);
     secp256k1_ge_set_gej(&res2, &res1);
     ge_equals_ge(&res2, &point);
@@ -2699,7 +2699,7 @@ void ecmult_const_chain_multiply(void) {
     for (i = 0; i < 100; ++i) {
         secp256k1_ge tmp;
         secp256k1_ge_set_gej(&tmp, &point);
-        secp256k1_ecmult_const(&point, &tmp, &scalar, 256);
+        secp256k1_ecmult_const_no_infinity(&point, &tmp, &scalar, 256);
     }
     secp256k1_ge_set_gej(&res, &point);
     ge_equals_gej(&res, &expected_point);

src/tests_exhaustive.c

@@ -172,7 +172,7 @@ void test_exhaustive_ecmult(const secp256k1_context *ctx, const secp256k1_ge *gr
                 ge_equals_gej(&group[(i * r_log + j) % order], &tmp);
 
                 if (i > 0) {
-                    secp256k1_ecmult_const(&tmp, &group[i], &ng, 256);
+                    secp256k1_ecmult_const_no_infinity(&tmp, &group[i], &ng, 256);
                     ge_equals_gej(&group[(i * j) % order], &tmp);
                 }
             }