Allow receiver to set minimum fee_rate

    Payjoin receiver can set minimum `fee_rate`
    in order to reject payjoin requests that contain
    a psbt which doesnt meet the minimum `fee_rate`
    threshold.

    We add this functionality to `check_broadcast_suitability`
    function which previously was called `check_can_broadcast`.

payjoin-cli/src/app.rs

@@ -282,7 +282,7 @@ impl App {
             )?;
 
         // Receive Check 1: Can Broadcast
-        let proposal = proposal.check_can_broadcast(|tx| {
+        let proposal = proposal.check_broadcast_suitability(None, |tx| {
             let raw_tx = bitcoin::consensus::encode::serialize_hex(&tx);
             let mempool_results =
                 bitcoind.test_mempool_accept(&[raw_tx]).map_err(|e| Error::Server(e.into()))?;

payjoin/src/receive/error.rs

@@ -65,6 +65,14 @@ pub(crate) enum InternalRequestError {
     /// Original PSBT input has been seen before. Only automatic receivers, aka "interactive" in the spec
     /// look out for these to prevent probing attacks.
     InputSeen(bitcoin::OutPoint),
+    /// Original `Psbt` fee rate provided by the sender 
+    /// is below minimum fee rate set by the receiver.
+    ///
+    /// First argument is the calculated fee rate of the original PSBT.
+    /// [`UncheckedProposal::psbt_fee_rate`]
+    ///
+    /// Second argument is the minimum fee rate optionaly set by the receiver.
+    PsbtBelowFeeRate(bitcoin::FeeRate, bitcoin::FeeRate),
 }
 
 impl From<InternalRequestError> for RequestError {
@@ -125,6 +133,11 @@ impl fmt::Display for RequestError {
                 write_error(f, "original-psbt-rejected", &format!("Input Type Error: {}.", e)),
             InternalRequestError::InputSeen(_) =>
                 write_error(f, "original-psbt-rejected", "The receiver rejected the original PSBT."),
+            InternalRequestError::PsbtBelowFeeRate(original_psbt_fee_rate, receiver_min_fee_rate) => write_error(
+                f,
+                "original-psbt-rejected",
+                &format!("Fee too low: {} < {}.", original_psbt_fee_rate, receiver_min_fee_rate),
+            ),
         }
     }
 }

payjoin/src/receive/mod.rs

@@ -123,7 +123,7 @@
 //! We need to know this transaction is consensus-valid.
 //!
 //! ```
-//! let checked_1 = proposal.check_can_broadcast(|tx| {
+//! let checked_1 = proposal.check_broadcast_suitability(None, |tx| {
 //!         let raw_tx = bitcoin::consensus::encode::serialize(&tx).to_hex();
 //!         let mempool_results = self
 //!             .bitcoind
@@ -295,7 +295,7 @@ pub trait Headers {
 ///
 /// If you are implementing an interactive payment processor, you should get extract the original
 /// transaction with extract_tx_to_schedule_broadcast() and schedule, followed by checking
-/// that the transaction can be broadcast with check_can_broadcast. Otherwise it is safe to
+/// that the transaction can be broadcast with check_broadcast_suitability. Otherwise it is safe to
 /// call assume_interactive_receive to proceed with validation.
 #[derive(Debug, Clone)]
 pub struct UncheckedProposal {
@@ -348,22 +348,44 @@ impl UncheckedProposal {
         self.psbt.clone().extract_tx()
     }
 
-    /// Call after checking that the Original PSBT can be broadcast.
+    /// The Sender's Original PSBT fee rate.
+    pub fn psbt_fee_rate(&self) -> Result<FeeRate, Error> {
+        if let Ok(original_psbt_fee) = self.psbt.fee() {
+            let original_transaction = self.psbt.clone().extract_tx();
+            Ok(original_psbt_fee / original_transaction.weight())
+        } else {
+            Err(Error::BadRequest(InternalRequestError::OriginalPsbtNotBroadcastable.into()))
+        }
+    }
+
+    /// Check that the Original PSBT can be broadcasted.
     ///
     /// Receiver MUST check that the Original PSBT from the sender
-    /// can be broadcast, i.e. `testmempoolaccept` bitcoind rpc returns { "allowed": true,.. }
-    /// for `extract_tx_to_sheculed_broadcast()` before calling this method.
+    /// can be broadcast, i.e. `testmempoolaccept` bitcoind rpc returns { "allowed": true,.. }.
+    ///
+    /// Receiver can optionaly set a minimum feerate that will be enforced on the Original PSBT.
+    /// This can be used to prevent probing attacks and make it easier to deal with
+    /// high feerate environments.
     ///
     /// Do this check if you generate bitcoin uri to receive Payjoin on sender request without manual human approval, like a payment processor.
     /// Such so called "non-interactive" receivers are otherwise vulnerable to probing attacks.
     /// If a sender can make requests at will, they can learn which bitcoin the receiver owns at no cost.
     /// Broadcasting the Original PSBT after some time in the failure case makes incurs sender cost and prevents probing.
     ///
     /// Call this after checking downstream.
-    pub fn check_can_broadcast(
+    pub fn check_broadcast_suitability(
         self,
+        min_feerate: Option<FeeRate>,
         can_broadcast: impl Fn(&bitcoin::Transaction) -> Result<bool, Error>,
     ) -> Result<MaybeInputsOwned, Error> {
+        let original_psbt_fee_rate = self.psbt_fee_rate()?;
+        if let Some(min_feerate) = min_feerate {
+            if original_psbt_fee_rate < min_feerate {
+                return Err(Error::BadRequest(
+                    InternalRequestError::PsbtBelowFeeRate(original_psbt_fee_rate, min_feerate).into(),
+                ));
+            }
+        }
         if can_broadcast(&self.psbt.clone().extract_tx())? {
             Ok(MaybeInputsOwned { psbt: self.psbt, params: self.params })
         } else {
@@ -894,4 +916,10 @@ mod test {
 
         assert!(payjoin.is_ok(), "Payjoin should be a valid PSBT");
     }
+
+    #[test]
+    fn psbt_fee_rate() {
+        let proposal = proposal_from_test_vector(None).unwrap();
+        assert_eq!(proposal.psbt_fee_rate().unwrap().to_sat_per_vb_floor(), 2);
+    }
 }

payjoin/tests/integration.rs

@@ -12,7 +12,7 @@ mod integration {
     use payjoin::bitcoin::base64;
     use payjoin::receive::Headers;
     use payjoin::send::{Request, RequestBuilder};
-    use payjoin::{bitcoin, Error, Uri};
+    use payjoin::{bitcoin, Uri};
 
     #[test]
     fn integration_test() {
@@ -143,8 +143,24 @@ mod integration {
         let _to_broadcast_in_failure_case = proposal.extract_tx_to_schedule_broadcast();
 
         // Receive Check 1: Can Broadcast
+        // Here we set receiver min feerate to be higher than the proposal's psbt feerate.
+        let original_psbt_fee_rate = proposal.psbt_fee_rate().unwrap();
+        let receiver_min_feerate = original_psbt_fee_rate.checked_mul(10); 
+        assert!(proposal
+            .clone()
+            .check_broadcast_suitability(receiver_min_feerate, |tx| {
+                Ok(receiver
+                    .test_mempool_accept(&[bitcoin::consensus::encode::serialize_hex(&tx)])
+                    .unwrap()
+                    .first()
+                    .unwrap()
+                    .allowed)
+            })
+            .is_err());
+        // Here we set receiver min feerate to be lower than the proposal's psbt feerate
+        let receiver_min_feerate = original_psbt_fee_rate.checked_div(10).unwrap(); 
         let proposal = proposal
-            .check_can_broadcast(|tx| {
+            .check_broadcast_suitability(Some(receiver_min_feerate), |tx| {
                 Ok(receiver
                     .test_mempool_accept(&[bitcoin::consensus::encode::serialize_hex(&tx)])
                     .unwrap()