feat: use a generated intermediate address as the forwarder

middleware/packet-forward-middleware/proto/router/v1/genesis.proto

@@ -43,4 +43,5 @@ message InFlightPacket {
   int32 retries_remaining = 10;
   uint64 timeout = 11;
   bool nonrefundable = 12;
+  string fee_payer = 13;
 }

middleware/packet-forward-middleware/router/ibc_middleware.go

@@ -3,6 +3,7 @@ package router
 import (
 	"encoding/json"
 	"fmt"
+	"github.com/cosmos/cosmos-sdk/types/address"
 	"strings"
 	"time"
 
@@ -143,6 +144,8 @@ func (im IBCMiddleware) OnRecvPacket(
 	packet channeltypes.Packet,
 	relayer sdk.AccAddress,
 ) ibcexported.Acknowledgement {
+	var err error
+
 	var data transfertypes.FungibleTokenPacketData
 	if err := transfertypes.ModuleCdc.UnmarshalJSON(packet.GetData(), &data); err != nil {
 		return channeltypes.NewErrorAcknowledgement(err)
@@ -156,12 +159,20 @@ func (im IBCMiddleware) OnRecvPacket(
 	)
 
 	d := make(map[string]interface{})
-	err := json.Unmarshal([]byte(data.Memo), &d)
+	err = json.Unmarshal([]byte(data.Memo), &d)
 	if err != nil || d["forward"] == nil {
 		// not a packet that should be forwarded
 		im.keeper.Logger(ctx).Debug("packetForwardMiddleware OnRecvPacket forward metadata does not exist")
 		return im.app.OnRecvPacket(ctx, packet, relayer)
 	}
+
+	// originalReceiver will pay fees for the forwarded packet
+	originalReceiver := data.Receiver
+	// Ignore the receiver on the packet and use a generated address instead
+	data.Receiver, err = DeriveIntermediateAddress(packet.SourcePort, packet.SourceChannel, data.Sender)
+	if err != nil {
+		return channeltypes.NewErrorAcknowledgement(err)
+	}
 	m := &types.PacketMetadata{}
 	err = json.Unmarshal([]byte(data.Memo), m)
 	if err != nil {
@@ -183,6 +194,11 @@ func (im IBCMiddleware) OnRecvPacket(
 	// underlying app, otherwise the transfer module's OnRecvPacket callback could be invoked more than once
 	// which would mint/burn vouchers more than once
 	if !processed {
+		// Replace the packet receiver with the address of the generated intermediate account
+		packet.Data, err = transfertypes.ModuleCdc.MarshalJSON(&data)
+		if err != nil {
+			return channeltypes.NewErrorAcknowledgement(err)
+		}
 		ack := im.app.OnRecvPacket(ctx, packet, relayer)
 		if ack == nil || !ack.Success() {
 			return ack
@@ -220,7 +236,7 @@ func (im IBCMiddleware) OnRecvPacket(
 		retries = im.retriesOnTimeout
 	}
 
-	err = im.keeper.ForwardTransferPacket(ctx, nil, packet, data.Sender, data.Receiver, metadata, token, retries, timeout, []metrics.Label{}, nonrefundable)
+	err = im.keeper.ForwardTransferPacket(ctx, nil, packet, data.Sender, originalReceiver, data.Receiver, metadata, token, retries, timeout, []metrics.Label{}, nonrefundable)
 	if err != nil {
 		return channeltypes.NewErrorAcknowledgement(err)
 	}
@@ -336,3 +352,11 @@ func (im IBCMiddleware) GetAppVersion(
 ) (string, bool) {
 	return im.keeper.GetAppVersion(ctx, portID, channelID)
 }
+
+func DeriveIntermediateAddress(port, channel, originalSender string) (string, error) {
+	senderStr := fmt.Sprintf("%s/%s/%s", port, channel, originalSender)
+	senderHash32 := address.Hash(types.IntermediateAddrPrefix, []byte(senderStr))
+	addr := sdk.AccAddress(senderHash32[:])
+	bech32Prefix := sdk.GetConfig().GetBech32AccountAddrPrefix()
+	return sdk.Bech32ifyAddressBytes(bech32Prefix, addr)
+}

middleware/packet-forward-middleware/router/keeper/keeper.go

@@ -202,7 +202,8 @@ func (k *Keeper) ForwardTransferPacket(
 	inFlightPacket *types.InFlightPacket,
 	srcPacket channeltypes.Packet,
 	srcPacketSender string,
-	receiver string,
+	feePayer string,
+	intermediateSender string,
 	metadata *types.ForwardMetadata,
 	token sdk.Coin,
 	maxRetries uint8,
@@ -218,7 +219,7 @@ func (k *Keeper) ForwardTransferPacket(
 
 	// pay fees
 	if feeAmount.IsPositive() {
-		hostAccAddr, err := sdk.AccAddressFromBech32(receiver)
+		hostAccAddr, err := sdk.AccAddressFromBech32(feePayer)
 		if err != nil {
 			return err
 		}
@@ -249,7 +250,7 @@ func (k *Keeper) ForwardTransferPacket(
 		metadata.Port,
 		metadata.Channel,
 		packetCoin,
-		receiver,
+		intermediateSender,
 		metadata.Receiver,
 		DefaultTransferPacketTimeoutHeight,
 		uint64(ctx.BlockTime().UnixNano())+uint64(timeout.Nanoseconds()),
@@ -258,7 +259,7 @@ func (k *Keeper) ForwardTransferPacket(
 
 	k.Logger(ctx).Debug("packetForwardMiddleware ForwardTransferPacket",
 		"port", metadata.Port, "channel", metadata.Channel,
-		"sender", receiver, "receiver", metadata.Receiver,
+		"sender", intermediateSender, "receiver", metadata.Receiver,
 		"amount", packetCoin.Amount.String(), "denom", packetCoin.Denom,
 	)
 
@@ -270,7 +271,7 @@ func (k *Keeper) ForwardTransferPacket(
 	if err != nil {
 		k.Logger(ctx).Error("packetForwardMiddleware ForwardTransferPacket error",
 			"port", metadata.Port, "channel", metadata.Channel,
-			"sender", receiver, "receiver", metadata.Receiver,
+			"sender", intermediateSender, "receiver", metadata.Receiver,
 			"amount", packetCoin.Amount.String(), "denom", packetCoin.Denom,
 			"error", err,
 		)
@@ -285,6 +286,7 @@ func (k *Keeper) ForwardTransferPacket(
 		inFlightPacket = &types.InFlightPacket{
 			PacketData:            srcPacket.Data,
 			OriginalSenderAddress: srcPacketSender,
+			FeePayer:              feePayer,
 			RefundChannelId:       srcPacket.DestinationChannel,
 			RefundPortId:          srcPacket.DestinationPort,
 			RefundSequence:        srcPacket.Sequence,
@@ -391,12 +393,21 @@ func (k *Keeper) RetryTimeout(
 
 	token := sdk.NewCoin(denom, amount)
 
+	// Handle backward compatibility for inflight packets right after the middleware upgrade
+	var feePayer string
+	if inFlightPacket.FeePayer == "" {
+		feePayer = data.Sender
+	} else {
+		feePayer = inFlightPacket.FeePayer
+	}
+
 	// srcPacket and srcPacketSender are empty because inFlightPacket is non-nil.
 	return k.ForwardTransferPacket(
 		ctx,
 		inFlightPacket,
 		channeltypes.Packet{},
 		"",
+		feePayer,
 		data.Sender,
 		metadata,
 		token,

middleware/packet-forward-middleware/router/module.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-
 	"github.com/cosmos/ibc-apps/middleware/packet-forward-middleware/v7/router/client/cli"
 	"github.com/cosmos/ibc-apps/middleware/packet-forward-middleware/v7/router/keeper"
 	"github.com/cosmos/ibc-apps/middleware/packet-forward-middleware/v7/router/types"