group: Add constant-time secp256k1_ge_set_all_gej

This is a dump mechanical translation of secp256k1_ge_set_all_gej_var
that assumes that inputs are not infinity.

src/group.h

@@ -80,7 +80,11 @@ static void secp256k1_ge_set_gej(secp256k1_ge *r, secp256k1_gej *a);
 /** Set a group element equal to another which is given in jacobian coordinates. */
 static void secp256k1_ge_set_gej_var(secp256k1_ge *r, secp256k1_gej *a);
 
-/** Set a batch of group elements equal to the inputs given in jacobian coordinates */
+/** Set group elements r[0:len] (affine) equal to group elements a[0:len] (jacobian).
+ * None of the group elements in a[0:len] may be infinity. Constant time. */
+static void secp256k1_ge_set_all_gej(secp256k1_ge *r, const secp256k1_gej *a, size_t len);
+
+/** Set group elements r[0:len] (affine) equal to group elements a[0:len] (jacobian). */
 static void secp256k1_ge_set_all_gej_var(secp256k1_ge *r, const secp256k1_gej *a, size_t len);
 
 /** Bring a batch of inputs to the same global z "denominator", based on ratios between

src/group_impl.h

@@ -195,6 +195,52 @@ static void secp256k1_ge_set_gej_var(secp256k1_ge *r, secp256k1_gej *a) {
     SECP256K1_GE_VERIFY(r);
 }
 
+static void secp256k1_ge_set_all_gej(secp256k1_ge *r, const secp256k1_gej *a, size_t len) {
+    secp256k1_fe u;
+    size_t i;
+    size_t last_i = SIZE_MAX;
+#ifdef VERIFY
+    for (i = 0; i < len; i++) {
+        SECP256K1_GEJ_VERIFY(&a[i]);
+        VERIFY_CHECK(!secp256k1_gej_is_infinity(&a[i]));
+    }
+#endif
+
+    for (i = 0; i < len; i++) {
+        /* Use destination's x coordinates as scratch space */
+        if (last_i == SIZE_MAX) {
+            r[i].x = a[i].z;
+        } else {
+            secp256k1_fe_mul(&r[i].x, &r[last_i].x, &a[i].z);
+        }
+        last_i = i;
+    }
+    if (last_i == SIZE_MAX) {
+        return;
+    }
+    secp256k1_fe_inv(&u, &r[last_i].x);
+
+    i = last_i;
+    while (i > 0) {
+        i--;
+        secp256k1_fe_mul(&r[last_i].x, &r[i].x, &u);
+        secp256k1_fe_mul(&u, &u, &a[last_i].z);
+        last_i = i;
+    }
+    VERIFY_CHECK(!a[last_i].infinity);
+    r[last_i].x = u;
+
+    for (i = 0; i < len; i++) {
+        secp256k1_ge_set_gej_zinv(&r[i], &a[i], &r[i].x);
+    }
+
+#ifdef VERIFY
+    for (i = 0; i < len; i++) {
+        SECP256K1_GE_VERIFY(&r[i]);
+    }
+#endif
+}
+
 static void secp256k1_ge_set_all_gej_var(secp256k1_ge *r, const secp256k1_gej *a, size_t len) {
     secp256k1_fe u;
     size_t i;

src/tests.c

@@ -3822,14 +3822,27 @@ static void test_ge(void) {
 
     /* Test batch gej -> ge conversion without known z ratios. */
     {
+        secp256k1_ge *ge_set_all_var = (secp256k1_ge *)checked_malloc(&CTX->error_callback, (4 * runs + 1) * sizeof(secp256k1_ge));
         secp256k1_ge *ge_set_all = (secp256k1_ge *)checked_malloc(&CTX->error_callback, (4 * runs + 1) * sizeof(secp256k1_ge));
-        secp256k1_ge_set_all_gej_var(ge_set_all, gej, 4 * runs + 1);
+        secp256k1_ge_set_all_gej_var(&ge_set_all_var[0], &gej[0], 4 * runs + 1);
         for (i = 0; i < 4 * runs + 1; i++) {
             secp256k1_fe s;
             testutil_random_fe_non_zero(&s);
             secp256k1_gej_rescale(&gej[i], &s);
-            CHECK(secp256k1_gej_eq_ge_var(&gej[i], &ge_set_all[i]));
+            CHECK(secp256k1_gej_eq_ge_var(&gej[i], &ge_set_all_var[i]));
         }
+
+        /* Skip infinity at &gej[0]. */
+        secp256k1_ge_set_all_gej(&ge_set_all[1], &gej[1], 4 * runs);
+        for (i = 1; i < 4 * runs + 1; i++) {
+            secp256k1_fe s;
+            testutil_random_fe_non_zero(&s);
+            secp256k1_gej_rescale(&gej[i], &s);
+            CHECK(secp256k1_gej_eq_ge_var(&gej[i], &ge_set_all[i]));
+            CHECK(secp256k1_ge_eq_var(&ge_set_all_var[i], &ge_set_all[i]));
+        }
+
+        free(ge_set_all_var);
         free(ge_set_all);
     }