Add psbt signature verification

x/btcbridge/keeper/msg_server.go

@@ -178,6 +178,9 @@ func (m msgServer) SubmitWithdrawSignatures(goCtx context.Context, msg *types.Ms
 	}
 
 	// verify the signatures
+	if !types.VerifyPsbtSignatures(packet) {
+		return nil, types.ErrInvalidSignatures
+	}
 
 	// Set the signing request status to signed
 	request := m.GetSigningRequest(ctx, msg.Txid)

x/btcbridge/types/signature.go

@@ -1,6 +1,64 @@
 package types
 
-func (m *BitcoinSigningRequest) ValidateSignatures(sigs []string) bool {
+import (
+	secp256k1 "github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/ecdsa"
+	"github.com/btcsuite/btcd/btcutil/psbt"
+	"github.com/btcsuite/btcd/txscript"
+)
 
-	return false
+// VerifyPsbtSignatures verifies the signatures of the given psbt
+// Note: assume that the psbt is valid and all inputs are native segwit
+func VerifyPsbtSignatures(p *psbt.Packet) bool {
+	// build previous output fetcher
+	prevOutputFetcher := txscript.NewMultiPrevOutFetcher(nil)
+
+	for i, txIn := range p.UnsignedTx.TxIn {
+		prevOutput := p.Inputs[i].WitnessUtxo
+		if prevOutput == nil {
+			return false
+		}
+
+		prevOutputFetcher.AddPrevOut(txIn.PreviousOutPoint, prevOutput)
+	}
+
+	// verify signatures
+	for i := range p.Inputs {
+		output := p.Inputs[i].WitnessUtxo
+		hashType := p.Inputs[i].SighashType
+
+		witness := p.Inputs[i].FinalScriptWitness
+		if len(witness) < 72+33 {
+			return false
+		}
+
+		sigBytes := witness[0 : len(witness)-33]
+		pkBytes := witness[len(witness)-33:]
+
+		if sigBytes[len(sigBytes)-1] != byte(hashType) {
+			return false
+		}
+
+		sig, err := ecdsa.ParseDERSignature(sigBytes[0 : len(sigBytes)-1])
+		if err != nil {
+			return false
+		}
+
+		pk, err := secp256k1.ParsePubKey(pkBytes)
+		if err != nil {
+			return false
+		}
+
+		sigHash, err := txscript.CalcWitnessSigHash(output.PkScript, txscript.NewTxSigHashes(p.UnsignedTx, prevOutputFetcher),
+			hashType, p.UnsignedTx, i, output.Value)
+		if err != nil {
+			return false
+		}
+
+		if !sig.Verify(sigHash, pk) {
+			return false
+		}
+	}
+
+	return true
 }