surjectionproof: make sure that n_used_pubkeys > 0 in generate

If the proof was generated with surjectionproof_initialize (as mandated by the
API docs), then n_used_pubkeys can never be 0. Without this commit, compilers
will (rightfully) warn that borromean_s[ring_input_index] is not initialized in
surjectionproof_generate. Therefore, this commit makes sure that n_used_pubkeys
is greater than 0 which ensures that the array is initialized at
ring_input_index.

Author: jonasnick

src/modules/surjection/main_impl.h

@@ -322,6 +322,10 @@ int secp256k1_surjectionproof_generate(const secp256k1_context* ctx, secp256k1_s
     /* Compute public keys */
     n_total_pubkeys = secp256k1_surjectionproof_n_total_inputs(ctx, proof);
     n_used_pubkeys = secp256k1_surjectionproof_n_used_inputs(ctx, proof);
+
+    /* This must be true if the proof was created with surjectionproof_initialize */
+    ARG_CHECK(n_used_pubkeys > 0);
+
     if (n_used_pubkeys > n_total_pubkeys || n_total_pubkeys != n_ephemeral_input_tags) {
         return 0;
     }

src/modules/surjection/tests_impl.h

@@ -173,31 +173,45 @@ static void test_surjectionproof_api(void) {
     CHECK(secp256k1_surjectionproof_verify(vrfy, &proof, ephemeral_input_tags, n_inputs, NULL) == 0);
     CHECK(ecount == 16);
 
+    /* Test how surjectionproof_generate fails when the proof was not created
+     * with surjectionproof_initialize */
+    ecount = 0;
+    CHECK(secp256k1_surjectionproof_generate(sign, &proof, ephemeral_input_tags, n_inputs, &ephemeral_output_tag, 0, input_blinding_key[0], output_blinding_key) == 1);
+    {
+        secp256k1_surjectionproof tmp_proof = proof;
+        tmp_proof.n_inputs = 0;
+        CHECK(secp256k1_surjectionproof_generate(sign, &tmp_proof, ephemeral_input_tags, n_inputs, &ephemeral_output_tag, 0, input_blinding_key[0], output_blinding_key) == 0);
+    }
+    CHECK(ecount == 1);
+
+    CHECK(secp256k1_surjectionproof_generate(sign, &proof, ephemeral_input_tags, n_inputs, &ephemeral_output_tag, 0, input_blinding_key[0], output_blinding_key) == 1);
+
     /* Check serialize */
+    ecount = 0;
     serialized_len = sizeof(serialized_proof);
     CHECK(secp256k1_surjectionproof_serialize(none, serialized_proof, &serialized_len, &proof) != 0);
-    CHECK(ecount == 16);
+    CHECK(ecount == 0);
     serialized_len = sizeof(serialized_proof);
     CHECK(secp256k1_surjectionproof_serialize(none, NULL, &serialized_len, &proof) == 0);
-    CHECK(ecount == 17);
+    CHECK(ecount == 1);
     serialized_len = sizeof(serialized_proof);
     CHECK(secp256k1_surjectionproof_serialize(none, serialized_proof, NULL, &proof) == 0);
-    CHECK(ecount == 18);
+    CHECK(ecount == 2);
     serialized_len = sizeof(serialized_proof);
     CHECK(secp256k1_surjectionproof_serialize(none, serialized_proof, &serialized_len, NULL) == 0);
-    CHECK(ecount == 19);
+    CHECK(ecount == 3);
 
     serialized_len = sizeof(serialized_proof);
     CHECK(secp256k1_surjectionproof_serialize(none, serialized_proof, &serialized_len, &proof) != 0);
     /* Check parse */
     CHECK(secp256k1_surjectionproof_parse(none, &proof, serialized_proof, serialized_len) != 0);
-    CHECK(ecount == 19);
+    CHECK(ecount == 3);
     CHECK(secp256k1_surjectionproof_parse(none, NULL, serialized_proof, serialized_len) == 0);
-    CHECK(ecount == 20);
+    CHECK(ecount == 4);
     CHECK(secp256k1_surjectionproof_parse(none, &proof, NULL, serialized_len) == 0);
-    CHECK(ecount == 21);
+    CHECK(ecount == 5);
     CHECK(secp256k1_surjectionproof_parse(none, &proof, serialized_proof, 0) == 0);
-    CHECK(ecount == 21);
+    CHECK(ecount == 5);
 
     secp256k1_context_destroy(none);
     secp256k1_context_destroy(sign);