group: add gej_eq_var

Author: jonasnick

src/bench_ecmult.c

@@ -84,9 +84,7 @@ static void bench_ecmult_teardown_helper(bench_data* data, size_t* seckey_offset
         }
     }
     secp256k1_ecmult_gen(&data->ctx->ecmult_gen_ctx, &tmp, &sum_scalars);
-    secp256k1_gej_neg(&tmp, &tmp);
-    secp256k1_gej_add_var(&tmp, &tmp, &sum_output, NULL);
-    CHECK(secp256k1_gej_is_infinity(&tmp));
+    CHECK(secp256k1_gej_eq_var(&tmp, &sum_output));
 }
 
 static void bench_ecmult_setup(void* arg) {

src/group.h

@@ -97,6 +97,9 @@ static void secp256k1_gej_set_infinity(secp256k1_gej *r);
 /** Set a group element (jacobian) equal to another which is given in affine coordinates. */
 static void secp256k1_gej_set_ge(secp256k1_gej *r, const secp256k1_ge *a);
 
+/** Check two group elements (jacobian) for equality in variable time. */
+static int secp256k1_gej_eq_var(const secp256k1_gej *a, const secp256k1_gej *b);
+
 /** Compare the X coordinate of a group element (jacobian). */
 static int secp256k1_gej_eq_x_var(const secp256k1_fe *x, const secp256k1_gej *a);
 

src/group_impl.h

@@ -236,6 +236,13 @@ static void secp256k1_gej_set_ge(secp256k1_gej *r, const secp256k1_ge *a) {
    secp256k1_fe_set_int(&r->z, 1);
 }
 
+static int secp256k1_gej_eq_var(const secp256k1_gej *a, const secp256k1_gej *b) {
+    secp256k1_gej tmp;
+    secp256k1_gej_neg(&tmp, a);
+    secp256k1_gej_add_var(&tmp, &tmp, b, NULL);
+    return secp256k1_gej_is_infinity(&tmp);
+}
+
 static int secp256k1_gej_eq_x_var(const secp256k1_fe *x, const secp256k1_gej *a) {
     secp256k1_fe r, r2;
     VERIFY_CHECK(!a->infinity);

src/tests.c

@@ -3872,6 +3872,22 @@ void run_gej(void) {
         test_gej_cmov(&a, &b);
         test_gej_cmov(&b, &a);
     }
+
+    /* Tests for secp256k1_gej_eq_var */
+    for (i = 0; i < count; i++) {
+        secp256k1_fe fe;
+        random_gej_test(&a);
+        random_gej_test(&b);
+        CHECK(!secp256k1_gej_eq_var(&a, &b));
+
+        b = a;
+        random_field_element_test(&fe);
+        if (secp256k1_fe_is_zero(&fe)) {
+            continue;
+        }
+        secp256k1_gej_rescale(&a, &fe);
+        CHECK(secp256k1_gej_eq_var(&a, &b));
+    }
 }
 
 void test_ec_combine(void) {
@@ -4077,17 +4093,12 @@ void run_ecmult_chain(void) {
                 0xB95CBCA2, 0xC77DA786, 0x539BE8FD, 0x53354D2D,
                 0x3B4F566A, 0xE6580454, 0x07ED6015, 0xEE1B2A88
             );
-
-            secp256k1_gej_neg(&rp, &rp);
-            secp256k1_gej_add_var(&rp, &rp, &x, NULL);
-            CHECK(secp256k1_gej_is_infinity(&rp));
+            CHECK(secp256k1_gej_eq_var(&rp, &x));
         }
     }
     /* redo the computation, but directly with the resulting ae and ge coefficients: */
     secp256k1_ecmult(&x2, &a, &ae, &ge);
-    secp256k1_gej_neg(&x2, &x2);
-    secp256k1_gej_add_var(&x2, &x2, &x, NULL);
-    CHECK(secp256k1_gej_is_infinity(&x2));
+    CHECK(secp256k1_gej_eq_var(&x, &x2));
 }
 
 void test_point_times_order(const secp256k1_gej *point) {
@@ -4380,33 +4391,25 @@ void test_ecmult_multi(secp256k1_scratch *scratch, secp256k1_ecmult_multi_func e
         /* only G scalar */
         secp256k1_ecmult(&r2, &ptgj, &szero, &sc[0]);
         CHECK(ecmult_multi(&ctx->error_callback, scratch, &r, &sc[0], ecmult_multi_callback, &data, 0));
-        secp256k1_gej_neg(&r2, &r2);
-        secp256k1_gej_add_var(&r, &r, &r2, NULL);
-        CHECK(secp256k1_gej_is_infinity(&r));
+        CHECK(secp256k1_gej_eq_var(&r, &r2));
 
         /* 1-point */
         secp256k1_ecmult(&r2, &ptgj, &sc[0], &szero);
         CHECK(ecmult_multi(&ctx->error_callback, scratch, &r, &szero, ecmult_multi_callback, &data, 1));
-        secp256k1_gej_neg(&r2, &r2);
-        secp256k1_gej_add_var(&r, &r, &r2, NULL);
-        CHECK(secp256k1_gej_is_infinity(&r));
+        CHECK(secp256k1_gej_eq_var(&r, &r2));
 
         /* Try to multiply 1 point, but callback returns false */
         CHECK(!ecmult_multi(&ctx->error_callback, scratch, &r, &szero, ecmult_multi_false_callback, &data, 1));
 
         /* 2-point */
         secp256k1_ecmult(&r2, &ptgj, &sc[0], &sc[1]);
         CHECK(ecmult_multi(&ctx->error_callback, scratch, &r, &szero, ecmult_multi_callback, &data, 2));
-        secp256k1_gej_neg(&r2, &r2);
-        secp256k1_gej_add_var(&r, &r, &r2, NULL);
-        CHECK(secp256k1_gej_is_infinity(&r));
+        CHECK(secp256k1_gej_eq_var(&r, &r2));
 
         /* 2-point with G scalar */
         secp256k1_ecmult(&r2, &ptgj, &sc[0], &sc[1]);
         CHECK(ecmult_multi(&ctx->error_callback, scratch, &r, &sc[1], ecmult_multi_callback, &data, 1));
-        secp256k1_gej_neg(&r2, &r2);
-        secp256k1_gej_add_var(&r, &r, &r2, NULL);
-        CHECK(secp256k1_gej_is_infinity(&r));
+        CHECK(secp256k1_gej_eq_var(&r, &r2));
     }
 
     /* Check infinite outputs of various forms */
@@ -4491,9 +4494,7 @@ void test_ecmult_multi(secp256k1_scratch *scratch, secp256k1_ecmult_multi_func e
 
         secp256k1_ecmult(&r2, &r, &sc[0], &szero);
         CHECK(ecmult_multi(&ctx->error_callback, scratch, &r, &szero, ecmult_multi_callback, &data, 20));
-        secp256k1_gej_neg(&r2, &r2);
-        secp256k1_gej_add_var(&r, &r, &r2, NULL);
-        CHECK(secp256k1_gej_is_infinity(&r));
+        CHECK(secp256k1_gej_eq_var(&r, &r2));
     }
 
     /* Check random scalars, constant point */
@@ -4514,9 +4515,7 @@ void test_ecmult_multi(secp256k1_scratch *scratch, secp256k1_ecmult_multi_func e
         secp256k1_gej_set_ge(&p0j, &pt[0]);
         secp256k1_ecmult(&r2, &p0j, &rs, &szero);
         CHECK(ecmult_multi(&ctx->error_callback, scratch, &r, &szero, ecmult_multi_callback, &data, 20));
-        secp256k1_gej_neg(&r2, &r2);
-        secp256k1_gej_add_var(&r, &r, &r2, NULL);
-        CHECK(secp256k1_gej_is_infinity(&r));
+        CHECK(secp256k1_gej_eq_var(&r, &r2));
     }
 
     /* Sanity check that zero scalars don't cause problems */
@@ -4578,9 +4577,7 @@ void test_ecmult_multi(secp256k1_scratch *scratch, secp256k1_ecmult_multi_func e
 
                         secp256k1_ecmult(&expected, &ptgj, &tmp1, &szero);
                         CHECK(ecmult_multi(&ctx->error_callback, scratch, &actual, &szero, ecmult_multi_callback, &data, 2));
-                        secp256k1_gej_neg(&expected, &expected);
-                        secp256k1_gej_add_var(&actual, &actual, &expected, NULL);
-                        CHECK(secp256k1_gej_is_infinity(&actual));
+                        CHECK(secp256k1_gej_eq_var(&actual, &expected));
                     }
                 }
             }
@@ -4750,9 +4747,7 @@ int test_ecmult_multi_random(secp256k1_scratch *scratch) {
     CHECK(ecmult_multi(&ctx->error_callback, scratch, &computed, g_scalar_ptr, ecmult_multi_callback, &data, filled));
     mults += num_nonzero + g_nonzero;
     /* Compare with expected result. */
-    secp256k1_gej_neg(&computed, &computed);
-    secp256k1_gej_add_var(&computed, &computed, &expected, NULL);
-    CHECK(secp256k1_gej_is_infinity(&computed));
+    CHECK(secp256k1_gej_eq_var(&computed, &expected));
     return mults;
 }