From 59cc43d727000794f18dac0a502cd87c0daec22a Mon Sep 17 00:00:00 2001
From: Sebastian Falbesoner <sebastian.falbesoner@gmail.com>
Date: Fri, 14 Jun 2024 14:33:40 +0200
Subject: BIP-352: scanning: add step to skip tx if input pubkeys sum A is
 point at infinity

The input data for the test vector is taken from the signet transaction
fe788cf6578d547819def43d79e6c8f0153d4885f5a343d12bd03f34507aabd6
which spends two P2WPKH inputs with negated pubkeys (x, y) and (x, -y)
from the funding transaction 3a286147b25e16ae80aff406f2673c6e565418c40f45c071245cdebc8a94174e
(see also https://github.com/bitcoin-core/secp256k1/pull/1519#issuecomment-2143167510
and the output from the script in the previous commit message).

Co-authored-by: josibake <josibake@protonmail.com>
---
 bip-0352.mediawiki                          |  1 +
 bip-0352/reference.py                       |  4 +++
 bip-0352/send_and_receive_test_vectors.json | 42 +++++++++++++++++++++++++++++
 3 files changed, 47 insertions(+)

diff --git a/bip-0352.mediawiki b/bip-0352.mediawiki
index def4d0b..0cf63a9 100644
--- a/bip-0352.mediawiki
+++ b/bip-0352.mediawiki
@@ -335,6 +335,7 @@ A scan and spend key pair using BIP32 derivation are defined (taking inspiration
 If each of the checks in ''[[#scanning-silent-payment-eligible-transactions|Scanning silent payment eligible transactions]]'' passes, the receiving wallet must:
 
 * Let ''A = A<sub>1</sub> + A<sub>2</sub> + ... + A<sub>n</sub>'', where each ''A<sub>i</sub>'' is the public key of an input from the ''[[#inputs-for-shared-secret-derivation|Inputs For Shared Secret Derivation]]'' list
+** If ''A'' is the point at infinity, skip the transaction
 * Generate the ''input_hash'' with the smallest outpoint lexicographically and ''A'', using the method described above
 * Let ''ecdh_shared_secret = input_hash·b<sub>scan</sub>·A''
 * Check for outputs:
diff --git a/bip-0352/reference.py b/bip-0352/reference.py
index 7882ad1..b4eaf94 100755
--- a/bip-0352/reference.py
+++ b/bip-0352/reference.py
@@ -300,6 +300,10 @@ if __name__ == "__main__":
             add_to_wallet = []
             if (len(input_pub_keys) > 0):
                 A_sum = reduce(lambda x, y: x + y, input_pub_keys)
+                if A_sum.get_bytes() is None:
+                    # Input pubkeys sum is point at infinity -> skip tx
+                    assert expected["outputs"] == []
+                    continue
                 input_hash = get_input_hash([vin.outpoint for vin in vins], A_sum)
                 pre_computed_labels = {
                     (generate_label(b_scan, label) * G).get_bytes(False).hex(): generate_label(b_scan, label).hex()
diff --git a/bip-0352/send_and_receive_test_vectors.json b/bip-0352/send_and_receive_test_vectors.json
index c0288bc..264f7be 100644
--- a/bip-0352/send_and_receive_test_vectors.json
+++ b/bip-0352/send_and_receive_test_vectors.json
@@ -2713,6 +2713,48 @@
             }
         ],
         "receiving": [
+            {
+                "given": {
+                    "vin": [
+                        {
+                            "txid": "3a286147b25e16ae80aff406f2673c6e565418c40f45c071245cdebc8a94174e",
+                            "vout": 0,
+                            "scriptSig": "",
+                            "txinwitness": "024730440220085003179ce1a3a88ce0069aa6ea045e140761ab88c22a26ae2a8cfe983a6e4602204a8a39940f0735c8a4424270ac8da65240c261ab3fda9272f6d6efbf9cfea366012102557ef3e55b0a52489b4454c1169e06bdea43687a69c1f190eb50781644ab6975",
+                            "prevout": {
+                                "scriptPubKey": {
+                                    "hex": "00149d9e24f9fab4e35bf1a6df4b46cb533296ac0792"
+                                }
+                            }
+                        },
+                        {
+                            "txid": "3a286147b25e16ae80aff406f2673c6e565418c40f45c071245cdebc8a94174e",
+                            "vout": 1,
+                            "scriptSig": "",
+                            "txinwitness": "0247304402204586a68e1d97dd3c6928e3622799859f8c3b20c3c670cf654cc905c9be29fdb7022043fbcde1689f3f4045e8816caf6163624bd19e62e4565bc99f95c533e599782c012103557ef3e55b0a52489b4454c1169e06bdea43687a69c1f190eb50781644ab6975",
+                            "prevout": {
+                                "scriptPubKey": {
+                                    "hex": "00149860538b5575962776ed0814ae222c7d60c72d7b"
+                                }
+                            }
+                        }
+                    ],
+                    "outputs": [
+                        "0000000000000000000000000000000000000000000000000000000000000000"
+                    ],
+                    "key_material": {
+                        "spend_priv_key": "0000000000000000000000000000000000000000000000000000000000000001",
+                        "scan_priv_key": "0000000000000000000000000000000000000000000000000000000000000002"
+                    },
+                    "labels": []
+                },
+                "expected": {
+                    "addresses": [
+                        "sp1qqtrqglu5g8kh6mfsg4qxa9wq0nv9cauwfwxw70984wkqnw2uwz0w2qnehen8a7wuhwk9tgrzjh8gwzc8q2dlekedec5djk0js9d3d7qhnq6lqj3s"
+                    ],
+                    "outputs": []
+                }
+            }
         ]
     }
 ]
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