group: add ge_to_bytes and ge_from_bytes

Author: jonasnick

src/group.h

@@ -174,6 +174,14 @@ static void secp256k1_ge_storage_cmov(secp256k1_ge_storage *r, const secp256k1_g
 /** Rescale a jacobian point by b which must be non-zero. Constant-time. */
 static void secp256k1_gej_rescale(secp256k1_gej *r, const secp256k1_fe *b);
 
+/** Convert a group element that is not infinity to a 64-byte array. The output
+ *  array is platform-dependent. */
+static void secp256k1_ge_to_bytes(unsigned char *buf, secp256k1_ge *a);
+
+/** Convert a 64-byte array into group element. This function assumes that the
+ *  provided buffer correctly encodes a group element. */
+static void secp256k1_ge_from_bytes(secp256k1_ge *r, const unsigned char *buf);
+
 /** 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

@@ -7,6 +7,8 @@
 #ifndef SECP256K1_GROUP_IMPL_H
 #define SECP256K1_GROUP_IMPL_H
 
+#include <string.h>
+
 #include "field.h"
 #include "group.h"
 #include "util.h"
@@ -941,4 +943,24 @@ static int secp256k1_ge_x_frac_on_curve_var(const secp256k1_fe *xn, const secp25
      return secp256k1_fe_is_square_var(&r);
 }
 
+static void secp256k1_ge_to_bytes(unsigned char *buf, secp256k1_ge *a) {
+    secp256k1_ge_storage s;
+
+    /* We require that the secp256k1_ge_storage type is exactly 64 bytes.
+     * This is formally not guaranteed by the C standard, but should hold on any
+     * sane compiler in the real world. */
+    STATIC_ASSERT(sizeof(secp256k1_ge_storage) == 64);
+    VERIFY_CHECK(!secp256k1_ge_is_infinity(a));
+    secp256k1_ge_to_storage(&s, a);
+    memcpy(buf, &s, 64);
+}
+
+static void secp256k1_ge_from_bytes(secp256k1_ge *r, const unsigned char *buf) {
+    secp256k1_ge_storage s;
+
+    STATIC_ASSERT(sizeof(secp256k1_ge_storage) == 64);
+    memcpy(&s, buf, 64);
+    secp256k1_ge_from_storage(r, &s);
+}
+
 #endif /* SECP256K1_GROUP_IMPL_H */

src/secp256k1.c

@@ -237,25 +237,13 @@ static SECP256K1_INLINE void secp256k1_declassify(const secp256k1_context* ctx,
 }
 
 static int secp256k1_pubkey_load(const secp256k1_context* ctx, secp256k1_ge* ge, const secp256k1_pubkey* pubkey) {
-    secp256k1_ge_storage s;
-
-    /* We require that the secp256k1_ge_storage type is exactly 64 bytes.
-     * This is formally not guaranteed by the C standard, but should hold on any
-     * sane compiler in the real world. */
-    STATIC_ASSERT(sizeof(secp256k1_ge_storage) == 64);
-    memcpy(&s, &pubkey->data[0], 64);
-    secp256k1_ge_from_storage(ge, &s);
+    secp256k1_ge_from_bytes(ge, pubkey->data);
     ARG_CHECK(!secp256k1_fe_is_zero(&ge->x));
     return 1;
 }
 
 static void secp256k1_pubkey_save(secp256k1_pubkey* pubkey, secp256k1_ge* ge) {
-    secp256k1_ge_storage s;
-
-    STATIC_ASSERT(sizeof(secp256k1_ge_storage) == 64);
-    VERIFY_CHECK(!secp256k1_ge_is_infinity(ge));
-    secp256k1_ge_to_storage(&s, ge);
-    memcpy(&pubkey->data[0], &s, 64);
+    secp256k1_ge_to_bytes(pubkey->data, ge);
 }
 
 int secp256k1_ec_pubkey_parse(const secp256k1_context* ctx, secp256k1_pubkey* pubkey, const unsigned char *input, size_t inputlen) {

src/tests.c

@@ -4016,13 +4016,28 @@ static void test_add_neg_y_diff_x(void) {
     CHECK(secp256k1_gej_eq_ge_var(&sumj, &res));
 }
 
+static void test_ge_bytes(void) {
+    int i;
+
+    for (i = 0; i < COUNT; i++) {
+        unsigned char buf[64];
+        secp256k1_ge p, q;
+
+        random_group_element_test(&p);
+        secp256k1_ge_to_bytes(buf, &p);
+        secp256k1_ge_from_bytes(&q, buf);
+        CHECK(secp256k1_ge_eq_var(&p, &q));
+    }
+}
+
 static void run_ge(void) {
     int i;
     for (i = 0; i < COUNT * 32; i++) {
         test_ge();
     }
     test_add_neg_y_diff_x();
     test_intialized_inf();
+    test_ge_bytes();
 }
 
 static void test_gej_cmov(const secp256k1_gej *a, const secp256k1_gej *b) {