feat(wallet): add TxOrdering::Custom

The deterministic sorting of transaction inputs and outputs proposed in
BIP 69 doesn't improve the privacy of transactions as originally
intended. In the search of not spreading bad practices but still provide
flexibility for possible use cases depending on particular order of the
inputs/outpus of a transaction, a new TxOrdering variant has been added
to allow the implementation of these orders through the definition of
comparison functions.

Signed-off-by: Steve Myers <steve@notmandatory.org>

crates/wallet/src/wallet/tx_builder.rs

@@ -42,10 +42,13 @@ use alloc::{boxed::Box, rc::Rc, string::String, vec::Vec};
 use core::cell::RefCell;
 use core::fmt;
 
+use alloc::sync::Arc;
+
 use bitcoin::psbt::{self, Psbt};
 use bitcoin::script::PushBytes;
 use bitcoin::{
-    absolute, Amount, FeeRate, OutPoint, ScriptBuf, Sequence, Transaction, Txid, Weight,
+    absolute, Amount, FeeRate, OutPoint, ScriptBuf, Sequence, Transaction, TxIn, TxOut, Txid,
+    Weight,
 };
 use rand_core::RngCore;
 
@@ -763,8 +766,10 @@ impl fmt::Display for AddForeignUtxoError {
 #[cfg(feature = "std")]
 impl std::error::Error for AddForeignUtxoError {}
 
+type TxSort<T> = dyn Fn(&T, &T) -> core::cmp::Ordering;
+
 /// Ordering of the transaction's inputs and outputs
-#[derive(Default, Debug, Ord, PartialOrd, Eq, PartialEq, Hash, Clone, Copy)]
+#[derive(Clone, Default)]
 pub enum TxOrdering {
     /// Randomized (default)
     #[default]
@@ -773,6 +778,24 @@ pub enum TxOrdering {
     Untouched,
     /// BIP69 / Lexicographic
     Bip69Lexicographic,
+    /// Provide custom comparison functions for sorting
+    Custom {
+        /// Transaction inputs sort function
+        input_sort: Arc<TxSort<TxIn>>,
+        /// Transaction outputs sort function
+        output_sort: Arc<TxSort<TxOut>>,
+    },
+}
+
+impl core::fmt::Debug for TxOrdering {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
+        match self {
+            TxOrdering::Shuffle => write!(f, "Shuffle"),
+            TxOrdering::Untouched => write!(f, "Untouched"),
+            TxOrdering::Bip69Lexicographic => write!(f, "Bip69Lexicographic"),
+            TxOrdering::Custom { .. } => write!(f, "Custom"),
+        }
+    }
 }
 
 impl TxOrdering {
@@ -780,14 +803,14 @@ impl TxOrdering {
     ///
     /// Uses the thread-local random number generator (rng).
     #[cfg(feature = "std")]
-    pub fn sort_tx(self, tx: &mut Transaction) {
+    pub fn sort_tx(&self, tx: &mut Transaction) {
         self.sort_tx_with_aux_rand(tx, &mut bitcoin::key::rand::thread_rng())
     }
 
     /// Sort transaction inputs and outputs by [`TxOrdering`] variant.
     ///
     /// Uses a provided random number generator (rng).
-    pub fn sort_tx_with_aux_rand(self, tx: &mut Transaction, rng: &mut impl RngCore) {
+    pub fn sort_tx_with_aux_rand(&self, tx: &mut Transaction, rng: &mut impl RngCore) {
         match self {
             TxOrdering::Untouched => {}
             TxOrdering::Shuffle => {
@@ -801,6 +824,13 @@ impl TxOrdering {
                 tx.output
                     .sort_unstable_by_key(|txout| (txout.value, txout.script_pubkey.clone()));
             }
+            TxOrdering::Custom {
+                input_sort,
+                output_sort,
+            } => {
+                tx.input.sort_unstable_by(|a, b| input_sort(a, b));
+                tx.output.sort_unstable_by(|a, b| output_sort(a, b));
+            }
         }
     }
 }
@@ -948,6 +978,117 @@ mod test {
         );
     }
 
+    #[test]
+    fn test_output_ordering_custom_but_bip69() {
+        use core::str::FromStr;
+
+        let original_tx = ordering_test_tx!();
+        let mut tx = original_tx;
+
+        let bip69_txin_cmp = |tx_a: &TxIn, tx_b: &TxIn| {
+            let project_outpoint = |t: &TxIn| (t.previous_output.txid, t.previous_output.vout);
+            project_outpoint(tx_a).cmp(&project_outpoint(tx_b))
+        };
+
+        let bip69_txout_cmp = |tx_a: &TxOut, tx_b: &TxOut| {
+            let project_utxo = |t: &TxOut| (t.value, t.script_pubkey.clone());
+            project_utxo(tx_a).cmp(&project_utxo(tx_b))
+        };
+
+        let custom_bip69_ordering = TxOrdering::Custom {
+            input_sort: Arc::new(bip69_txin_cmp),
+            output_sort: Arc::new(bip69_txout_cmp),
+        };
+
+        custom_bip69_ordering.sort_tx(&mut tx);
+
+        assert_eq!(
+            tx.input[0].previous_output,
+            bitcoin::OutPoint::from_str(
+                "0e53ec5dfb2cb8a71fec32dc9a634a35b7e24799295ddd5278217822e0b31f57:5"
+            )
+            .unwrap()
+        );
+        assert_eq!(
+            tx.input[1].previous_output,
+            bitcoin::OutPoint::from_str(
+                "0f60fdd185542f2c6ea19030b0796051e7772b6026dd5ddccd7a2f93b73e6fc2:0"
+            )
+            .unwrap()
+        );
+        assert_eq!(
+            tx.input[2].previous_output,
+            bitcoin::OutPoint::from_str(
+                "0f60fdd185542f2c6ea19030b0796051e7772b6026dd5ddccd7a2f93b73e6fc2:1"
+            )
+            .unwrap()
+        );
+
+        assert_eq!(tx.output[0].value.to_sat(), 800);
+        assert_eq!(tx.output[1].script_pubkey, ScriptBuf::from(vec![0xAA]));
+        assert_eq!(
+            tx.output[2].script_pubkey,
+            ScriptBuf::from(vec![0xAA, 0xEE])
+        );
+    }
+
+    #[test]
+    fn test_output_ordering_custom_with_sha256() {
+        use bitcoin::hashes::{sha256, Hash};
+
+        let original_tx = ordering_test_tx!();
+        let mut tx_1 = original_tx.clone();
+        let mut tx_2 = original_tx.clone();
+        let shared_secret = "secret_tweak";
+
+        let hash_txin_with_shared_secret_seed = Arc::new(|tx_a: &TxIn, tx_b: &TxIn| {
+            let secret_digest_from_txin = |txin: &TxIn| {
+                sha256::Hash::hash(
+                    &[
+                        &txin.previous_output.txid.to_raw_hash()[..],
+                        &txin.previous_output.vout.to_be_bytes(),
+                        shared_secret.as_bytes(),
+                    ]
+                    .concat(),
+                )
+            };
+            secret_digest_from_txin(tx_a).cmp(&secret_digest_from_txin(tx_b))
+        });
+
+        let hash_txout_with_shared_secret_seed = Arc::new(|tx_a: &TxOut, tx_b: &TxOut| {
+            let secret_digest_from_txout = |txin: &TxOut| {
+                sha256::Hash::hash(
+                    &[
+                        &txin.value.to_sat().to_be_bytes(),
+                        &txin.script_pubkey.clone().into_bytes()[..],
+                        shared_secret.as_bytes(),
+                    ]
+                    .concat(),
+                )
+            };
+            secret_digest_from_txout(tx_a).cmp(&secret_digest_from_txout(tx_b))
+        });
+
+        let custom_ordering_from_salted_sha256_1 = TxOrdering::Custom {
+            input_sort: hash_txin_with_shared_secret_seed.clone(),
+            output_sort: hash_txout_with_shared_secret_seed.clone(),
+        };
+
+        let custom_ordering_from_salted_sha256_2 = TxOrdering::Custom {
+            input_sort: hash_txin_with_shared_secret_seed,
+            output_sort: hash_txout_with_shared_secret_seed,
+        };
+
+        custom_ordering_from_salted_sha256_1.sort_tx(&mut tx_1);
+        custom_ordering_from_salted_sha256_2.sort_tx(&mut tx_2);
+
+        // Check the ordering is consistent between calls
+        assert_eq!(tx_1, tx_2);
+        // Check transaction order has changed
+        assert_ne!(tx_1, original_tx);
+        assert_ne!(tx_2, original_tx);
+    }
+
     fn get_test_utxos() -> Vec<LocalOutput> {
         use bitcoin::hashes::Hash;