Add safety checks

(cherry picked from commit 1bd9b6abdca6bb1fbb60835cae58c5c5caad2683)

swap/src/protocol/bob/state.rs

@@ -495,6 +495,30 @@ impl State4 {
         self.b.encsign(self.S_a_bitcoin, tx_redeem.digest())
     }
 
+    pub async fn check_for_tx_redeem(
+        &self,
+        bitcoin_wallet: &bitcoin::Wallet,
+    ) -> Result<State5> {
+        let tx_redeem =
+            bitcoin::TxRedeem::new(&self.tx_lock, &self.redeem_address, self.tx_redeem_fee);
+        let tx_redeem_encsig = self.b.encsign(self.S_a_bitcoin, tx_redeem.digest());
+
+        let tx_redeem_candidate = bitcoin_wallet.get_raw_transaction(tx_redeem.txid()).await?;
+
+        let tx_redeem_sig =
+            tx_redeem.extract_signature_by_key(tx_redeem_candidate, self.b.public())?;
+        let s_a = bitcoin::recover(self.S_a_bitcoin, tx_redeem_sig, tx_redeem_encsig)?;
+        let s_a = monero::private_key_from_secp256k1_scalar(s_a.into());
+
+        Ok(State5 {
+            s_a,
+            s_b: self.s_b,
+            v: self.v,
+            tx_lock: self.tx_lock.clone(),
+            monero_wallet_restore_blockheight: self.monero_wallet_restore_blockheight,
+        })
+    }
+
     pub async fn watch_for_redeem_btc(&self, bitcoin_wallet: &bitcoin::Wallet) -> Result<State5> {
         let tx_redeem =
             bitcoin::TxRedeem::new(&self.tx_lock, &self.redeem_address, self.tx_redeem_fee);

swap/src/protocol/bob/swap.rs

@@ -185,7 +185,9 @@ async fn next_state(
         BobState::XmrLocked(state) => {
             let tx_lock_status = bitcoin_wallet.subscribe_to(state.tx_lock.clone()).await;
 
-            if let ExpiredTimelocks::None { .. } = state.expired_timelock(bitcoin_wallet).await? {
+            if let Ok(state5) = state.check_for_tx_redeem(bitcoin_wallet).await {
+                BobState::BtcRedeemed(state5)
+            } else if let ExpiredTimelocks::None { .. } = state.expired_timelock(bitcoin_wallet).await? {
                 // Alice has locked Xmr
                 // Bob sends Alice his key