From 77be35915adb3e7669010478ee2423f6832cfb92 Mon Sep 17 00:00:00 2001
From: spacebear <git@spacebear.dev>
Date: Wed, 14 Aug 2024 12:01:56 -0400
Subject: [PATCH] Implement disable_output_substitution check correctly

When disable_output_substitition is true, the receiver may not change
the script pubkey or decrease the value of their original output, but is
allowed to increase the amount or add new outputs.
---
 payjoin/src/receive/mod.rs | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/payjoin/src/receive/mod.rs b/payjoin/src/receive/mod.rs
index 8a67c34d..f139bb82 100644
--- a/payjoin/src/receive/mod.rs
+++ b/payjoin/src/receive/mod.rs
@@ -360,8 +360,22 @@ impl WantsOutputs {
         match outputs {
             Some(o) => {
                 if self.params.disable_output_substitution {
-                    // TODO: only fail if the original output's amount decreased or its script pubkey is not in `outputs`
-                    return Err(Error::Server("Output substitution is disabled.".into()));
+                    // Receiver may not change the script pubkey or decrease the value of the
+                    // their output, but can still increase the amount or add new outputs.
+                    // https://github.com/bitcoin/bips/blob/master/bip-0078.mediawiki#payment-output-substitution
+                    let original_output = &self.psbt.unsigned_tx.output[self.owned_vouts[0]];
+                    match o.iter().find(|txo| txo.script_pubkey == original_output.script_pubkey) {
+                        Some(txo) if txo.value < original_output.value => {
+                            return Err(Error::Server(
+                                "Decreasing the receiver output value is not allowed".into(),
+                            ));
+                        }
+                        None =>
+                            return Err(Error::Server(
+                                "Changing the receiver output script pubkey is not allowed".into(),
+                            )),
+                        _ => log::info!("Receiver is augmenting outputs"),
+                    }
                 }
                 let mut replacement_outputs = o.into_iter();
                 let mut outputs = vec![];