From bf18ff5a8c6295cb7db6e2989aefd6a78df7720f Mon Sep 17 00:00:00 2001 From: Andrew Poelstra Date: Tue, 26 Jul 2022 17:14:49 +0000 Subject: [PATCH] surjectionproof: fix generation to fail when any input == the output Verification will fail in this case, so don't "succeed" in generating a bad proof. --- src/modules/surjection/main_impl.h | 12 ++++++++---- src/modules/surjection/tests_impl.h | 6 ++---- 2 files changed, 10 insertions(+), 8 deletions(-) diff --git a/src/modules/surjection/main_impl.h b/src/modules/surjection/main_impl.h index b8f145e5..343b86ae 100644 --- a/src/modules/surjection/main_impl.h +++ b/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 (memcmp(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); diff --git a/src/modules/surjection/tests_impl.h b/src/modules/surjection/tests_impl.h index 70955760..ac7c269b 100644 --- a/src/modules/surjection/tests_impl.h +++ b/src/modules/surjection/tests_impl.h @@ -543,12 +543,10 @@ void test_input_eq_output(void) { 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)); - /* It succeeds when the blinding factor is 0... (will fix this in the next commit) */ + /* ...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)); - /* ...but verification doesn't */ - CHECK(!secp256k1_surjectionproof_verify(ctx, &proof, &ephemeral_tag, 1, &ephemeral_tag)); + CHECK(!secp256k1_surjectionproof_generate(ctx, &proof, &ephemeral_tag, 1, &ephemeral_tag, input_index, blinding_key, blinding_key)); } void test_fixed_vectors(void) {