group: add ge_to_bytes_ext and ge_from_bytes_ext

Author: jonasnick

src/group.h

@@ -182,6 +182,14 @@ static void secp256k1_ge_to_bytes(unsigned char *buf, const secp256k1_ge *a);
  *  provided buffer correctly encodes a group element. */
 static void secp256k1_ge_from_bytes(secp256k1_ge *r, const unsigned char *buf);
 
+/** Convert a group element (that is allowed to be infinity) to a 64-byte
+ *  array. The output array is platform-dependent. */
+static void secp256k1_ge_to_bytes_ext(unsigned char *data, const secp256k1_ge *ge);
+
+/** Convert a 64-byte array into a group element. This function assumes that the
+ *  provided buffer is the output of secp256k1_ge_to_bytes_ext. */
+static void secp256k1_ge_from_bytes_ext(secp256k1_ge *ge, const unsigned char *data);
+
 /** Determine if a point (which is assumed to be on the curve) is in the correct (sub)group of the curve.
  *
  * In normal mode, the used group is secp256k1, which has cofactor=1 meaning that every point on the curve is in the

src/group_impl.h

@@ -963,4 +963,21 @@ static void secp256k1_ge_from_bytes(secp256k1_ge *r, const unsigned char *buf) {
     secp256k1_ge_from_storage(r, &s);
 }
 
+static void secp256k1_ge_to_bytes_ext(unsigned char *data, const secp256k1_ge *ge) {
+    if (secp256k1_ge_is_infinity(ge)) {
+        memset(data, 0, 64);
+    } else {
+        secp256k1_ge_to_bytes(data, ge);
+    }
+}
+
+static void secp256k1_ge_from_bytes_ext(secp256k1_ge *ge, const unsigned char *data) {
+    static const unsigned char zeros[64] = { 0 };
+    if (secp256k1_memcmp_var(data, zeros, sizeof(zeros)) == 0) {
+        secp256k1_ge_set_infinity(ge);
+    } else {
+        secp256k1_ge_from_bytes(ge, data);
+    }
+}
+
 #endif /* SECP256K1_GROUP_IMPL_H */

src/tests.c

@@ -3985,17 +3985,28 @@ static void test_add_neg_y_diff_x(void) {
 static void test_ge_bytes(void) {
     int i;
 
-    for (i = 0; i < COUNT; i++) {
+    for (i = 0; i < COUNT + 1; i++) {
         unsigned char buf[64];
         secp256k1_ge p, q;
 
-        testutil_random_ge_test(&p);
+        if (i == 0) {
+            secp256k1_ge_set_infinity(&p);
+        } else {
+            testutil_random_ge_test(&p);
+        }
 
         if (!secp256k1_ge_is_infinity(&p)) {
             secp256k1_ge_to_bytes(buf, &p);
+
             secp256k1_ge_from_bytes(&q, buf);
             CHECK(secp256k1_ge_eq_var(&p, &q));
+
+            secp256k1_ge_from_bytes_ext(&q, buf);
+            CHECK(secp256k1_ge_eq_var(&p, &q));
         }
+        secp256k1_ge_to_bytes_ext(buf, &p);
+        secp256k1_ge_from_bytes_ext(&q, buf);
+        CHECK(secp256k1_ge_eq_var(&p, &q));
     }
 }