Merge #196: surjectionproof: fail to generate proofs when an input equals the output

d1175d2 surjectionproof: use secp256k1_memcmp_var rather than bare memcmp (Andrew Poelstra)
bf18ff5 surjectionproof: fix generation to fail when any input == the output (Andrew Poelstra)
4ff6e42 surjectionproof: add test for existing behavior on input=output proofs (Andrew Poelstra)

Pull request description:

  If any ephemeral input tag equals the ephemeral output tag (i.e. an input asset is exactly equal to the output asset), verification will fail due to an unexpected interaction between our surjectionproof logic and the underlying borromean ring siganture logic. However, our generation code still allows creating proofs like this, "succeeding" in creating bad proofs.

  Since we cannot fix the verification side without hardforking Liquid, fix the generation side to fail in this situation.

ACKs for top commit:
  real-or-random:
    utACK d1175d2

Tree-SHA512: c15e130de028d6c1f705543fe2774ec23016c71f9d6b38ef0708820a517d156e2126f8369e94f16f9fd1855c29cd907d406f6ea26c95499a9ae1ce0dd92f77b2

Author: real-or-random

src/modules/surjection/main_impl.h

@@ -307,10 +307,14 @@ int secp256k1_surjectionproof_generate(const secp256k1_context* ctx, secp256k1_s
     if (overflow) {
         return 0;
     }
-    /* The only time the input may equal the output is if neither one was blinded in the first place,
-     * i.e. both blinding keys are zero. Otherwise this is a privacy leak. */
-    if (secp256k1_scalar_eq(&tmps, &blinding_key) && !secp256k1_scalar_is_zero(&blinding_key)) {
-        return 0;
+    /* If any input tag is equal to an output tag, verification will fail, because our ring
+     * signature logic would receive a zero-key, which is illegal. This is unfortunate but
+     * it is deployed on Liquid and cannot be fixed without a hardfork. We should review
+     * this at the same time that we relax the max-256-inputs rule. */
+    for (i = 0; i < n_ephemeral_input_tags; i++) {
+        if (secp256k1_memcmp_var(ephemeral_input_tags[i].data, ephemeral_output_tag->data, sizeof(ephemeral_output_tag->data)) == 0) {
+            return 0;
+        }
     }
     secp256k1_scalar_negate(&tmps, &tmps);
     secp256k1_scalar_add(&blinding_key, &blinding_key, &tmps);

src/modules/surjection/tests_impl.h

@@ -524,6 +524,31 @@ void test_bad_parse(void) {
     CHECK(secp256k1_surjectionproof_parse(ctx, &proof, serialized_proof2, sizeof(serialized_proof2)) == 0);
 }
 
+void test_input_eq_output(void) {
+    secp256k1_surjectionproof proof;
+    secp256k1_fixed_asset_tag fixed_tag;
+    secp256k1_generator ephemeral_tag;
+    unsigned char blinding_key[32];
+    unsigned char entropy[32];
+    size_t input_index;
+
+    secp256k1_testrand256(fixed_tag.data);
+    secp256k1_testrand256(blinding_key);
+    secp256k1_testrand256(entropy);
+
+    CHECK(secp256k1_surjectionproof_initialize(ctx, &proof, &input_index, &fixed_tag, 1, 1, &fixed_tag, 100, entropy) == 1);
+    CHECK(input_index == 0);
+
+    /* Generation should fail */
+    CHECK(secp256k1_generator_generate_blinded(ctx, &ephemeral_tag, fixed_tag.data, blinding_key));
+    CHECK(!secp256k1_surjectionproof_generate(ctx, &proof, &ephemeral_tag, 1, &ephemeral_tag, input_index, blinding_key, blinding_key));
+
+    /* ...even when the blinding key is zero */
+    memset(blinding_key, 0, 32);
+    CHECK(secp256k1_generator_generate_blinded(ctx, &ephemeral_tag, fixed_tag.data, blinding_key));
+    CHECK(!secp256k1_surjectionproof_generate(ctx, &proof, &ephemeral_tag, 1, &ephemeral_tag, input_index, blinding_key, blinding_key));
+}
+
 void test_fixed_vectors(void) {
     const unsigned char tag0_ser[] = {
         0x0a,
@@ -672,6 +697,7 @@ void test_fixed_vectors(void) {
 
 void run_surjection_tests(void) {
     test_surjectionproof_api();
+    test_input_eq_output();
     test_fixed_vectors();
 
     test_input_selection(0);