Use TX IDs for Bitcoin Eventualities

They're a bit more binding, smaller, provided by the Rust bitcoin library,
sane, and we don't have to worry about malleability since all of our inputs are
SegWit.

coins/bitcoin/src/rpc.rs

@@ -149,11 +149,11 @@ impl Rpc {
 
   /// Get the hash of a block by the block's number.
   pub async fn get_block_hash(&self, number: usize) -> Result<[u8; 32], RpcError> {
-    let mut hash = *self
+    let mut hash = self
       .rpc_call::<BlockHash>("getblockhash", json!([number]))
       .await?
       .as_raw_hash()
-      .as_byte_array();
+      .to_byte_array();
     // bitcoin stores the inner bytes in reverse order.
     hash.reverse();
     Ok(hash)

coins/bitcoin/src/wallet/send.rs

@@ -13,6 +13,7 @@ use k256::{elliptic_curve::sec1::ToEncodedPoint, Scalar};
 use frost::{curve::Secp256k1, Participant, ThresholdKeys, FrostError, sign::*};
 
 use bitcoin::{
+  hashes::Hash,
   sighash::{TapSighashType, SighashCache, Prevouts},
   absolute::LockTime,
   script::{PushBytesBuf, ScriptBuf},
@@ -245,6 +246,13 @@ impl SignableTransaction {
     })
   }
 
+  /// Returns the TX ID of the transaction this will create.
+  pub fn txid(&self) -> [u8; 32] {
+    let mut res = self.tx.txid().to_byte_array();
+    res.reverse();
+    res
+  }
+
   /// Returns the outputs this transaction will create.
   pub fn outputs(&self) -> &[TxOut] {
     &self.tx.output

coins/bitcoin/tests/wallet.rs

@@ -279,6 +279,7 @@ async_sequential! {
       FEE
     ).unwrap();
     let needed_fee = tx.needed_fee();
+    let expected_id = tx.txid();
     let tx = sign(&keys, tx);
 
     assert_eq!(tx.output.len(), 3);
@@ -322,6 +323,7 @@ async_sequential! {
     let mut hash = *tx.txid().as_raw_hash().as_byte_array();
     hash.reverse();
     assert_eq!(tx, rpc.get_transaction(&hash).await.unwrap());
+    assert_eq!(expected_id, hash);
   }
 
   async fn test_data() {

processor/src/networks/bitcoin.rs

@@ -21,7 +21,7 @@ use bitcoin_serai::{
     consensus::{Encodable, Decodable},
     script::Instruction,
     address::{NetworkChecked, Address as BAddress},
-    OutPoint, TxOut, Transaction, Block, Network as BNetwork,
+    Transaction, Block, Network as BNetwork,
   },
   wallet::{
     tweak_keys, address_payload, ReceivedOutput, Scanner, TransactionError,
@@ -37,7 +37,7 @@ use bitcoin_serai::bitcoin::{
   sighash::{EcdsaSighashType, SighashCache},
   script::{PushBytesBuf, Builder},
   absolute::LockTime,
-  Sequence, Script, Witness, TxIn, Amount as BAmount,
+  Amount as BAmount, Sequence, Script, Witness, OutPoint, TxOut, TxIn,
   transaction::Version,
 };
 
@@ -206,32 +206,22 @@ impl TransactionTrait<Bitcoin> for Transaction {
 }
 
 #[derive(Clone, PartialEq, Eq, Debug)]
-pub struct Eventuality {
-  // We need to bind to the plan. While we could bind to the plan ID via an OP_RETURN, plans will
-  // use distinct inputs and this is accordingly valid as a binding to a specific plan.
-  plan_binding_input: OutPoint,
-  outputs: Vec<TxOut>,
-}
+pub struct Eventuality([u8; 32]);
 
 impl EventualityTrait for Eventuality {
   fn lookup(&self) -> Vec<u8> {
-    let mut buf = Vec::with_capacity(32 + 4);
-    self.plan_binding_input.consensus_encode(&mut buf).unwrap();
-    buf
+    self.0.to_vec()
   }
 
   fn read<R: io::Read>(reader: &mut R) -> io::Result<Self> {
-    let plan_binding_input = OutPoint::consensus_decode(reader)
-      .map_err(|_| io::Error::other("couldn't decode outpoint in eventuality"))?;
-    let outputs = Vec::<TxOut>::consensus_decode(reader)
-      .map_err(|_| io::Error::other("couldn't decode outputs in eventuality"))?;
-    Ok(Eventuality { plan_binding_input, outputs })
+    let mut id = [0; 32];
+    reader
+      .read_exact(&mut id)
+      .map_err(|_| io::Error::other("couldn't decode ID in eventuality"))?;
+    Ok(Eventuality(id))
   }
   fn serialize(&self) -> Vec<u8> {
-    let mut buf = Vec::with_capacity(32 + 4 + 4 + (self.outputs.len() * (8 + 32)));
-    self.plan_binding_input.consensus_encode(&mut buf).unwrap();
-    self.outputs.consensus_encode(&mut buf).unwrap();
-    buf
+    self.0.to_vec()
   }
 }
 
@@ -653,23 +643,7 @@ impl Network for Bitcoin {
       res: &mut HashMap<[u8; 32], (usize, Transaction)>,
     ) {
       for tx in &block.txdata[1 ..] {
-        let input = &tx.input[0].previous_output;
-        let mut lookup = Vec::with_capacity(4 + 32);
-        input.consensus_encode(&mut lookup).unwrap();
-        if let Some((plan, eventuality)) = eventualities.map.remove(&lookup) {
-          // Sanity, as this is guaranteed by how the lookup is performed
-          assert_eq!(input, &eventuality.plan_binding_input);
-          // If the multisig is honest, then the Eventuality's outputs should match the outputs of
-          // this transaction
-          // This panic is fine as this multisig being dishonest will require intervention on
-          // Substrate to trigger a slash, and then an update to the processor to handle the exact
-          // adjustments needed
-          // Panicking here is effectively triggering the halt we need to perform anyways
-          assert_eq!(
-            tx.output, eventuality.outputs,
-            "dishonest multisig spent input on distinct set of outputs"
-          );
-
+        if let Some((plan, _)) = eventualities.map.remove(tx.id().as_slice()) {
           res.insert(plan, (eventualities.block_number, tx.clone()));
         }
       }
@@ -744,13 +718,8 @@ impl Network for Bitcoin {
           RecommendedTranscript::new(b"Serai Processor Bitcoin Transaction Transcript");
         transcript.append_message(b"plan", plan_id);
 
-        let plan_binding_input = *inputs[0].output.outpoint();
-        let outputs = signable.outputs().to_vec();
-
-        (
-          SignableTransaction { transcript, actual: signable },
-          Eventuality { plan_binding_input, outputs },
-        )
+        let eventuality = Eventuality(signable.txid());
+        (SignableTransaction { transcript, actual: signable }, eventuality)
       },
     ))
   }
@@ -785,8 +754,7 @@ impl Network for Bitcoin {
   }
 
   fn confirm_completion(&self, eventuality: &Self::Eventuality, tx: &Transaction) -> bool {
-    (eventuality.plan_binding_input == tx.input[0].previous_output) &&
-      (eventuality.outputs == tx.output)
+    eventuality.0 == tx.id()
   }
 
   #[cfg(test)]

tests/full-stack/src/tests/mint_and_burn.rs

@@ -35,7 +35,7 @@ async fn mint_and_burn_test() {
 
       // Helper to mine a block on each network
       async fn mine_blocks(
-        handles: &Vec<Handles>,
+        handles: &[Handles],
         ops: &DockerOperations,
         producer: &mut usize,
         count: usize,