psbt: add output silent payment info

btcutil/psbt/partial_output.go

@@ -17,6 +17,7 @@ type POutput struct {
 	TaprootInternalKey     []byte
 	TaprootTapTree         []byte
 	TaprootBip32Derivation []*TaprootBip32Derivation
+	SilentPaymentInfo      *SilentPaymentInfo
 	Unknowns               []*Unknown
 }
 
@@ -144,6 +145,18 @@ func (po *POutput) deserialize(r io.Reader) error {
 				po.TaprootBip32Derivation, taprootDerivation,
 			)
 
+		case SilentPaymentV0InfoOutputType:
+			if po.SilentPaymentInfo != nil {
+				return ErrDuplicateKey
+			}
+
+			info, err := ReadSilentPaymentInfo(value)
+			if err != nil {
+				return err
+			}
+
+			po.SilentPaymentInfo = info
+
 		default:
 			// A fall through case for any proprietary types.
 			keyCodeAndData := append(
@@ -246,6 +259,16 @@ func (po *POutput) serialize(w io.Writer) error {
 		}
 	}
 
+	if po.SilentPaymentInfo != nil {
+		err := serializeKVPairWithType(
+			w, uint8(SilentPaymentV0InfoOutputType), nil,
+			SerializeSilentPaymentInfo(po.SilentPaymentInfo),
+		)
+		if err != nil {
+			return err
+		}
+	}
+
 	// Unknown is a special case; we don't have a key type, only a key and
 	// a value field
 	for _, kv := range po.Unknowns {

btcutil/psbt/silentpayments.go

@@ -200,3 +200,35 @@ func SerializeSilentPaymentDLEQ(dleq *SilentPaymentDLEQ) ([]byte, []byte) {
 
 	return keyData, dleq.Proof
 }
+
+// SilentPaymentInfo is the information needed to create a silent payment
+// recipient output.
+type SilentPaymentInfo struct {
+	// ScanKey is the silent payment recipient's scan key.
+	ScanKey []byte
+
+	// SpendKey is the silent payment recipient's spend key.
+	SpendKey []byte
+}
+
+// ReadSilentPaymentInfo deserializes a silent payment info from the given
+// value.
+func ReadSilentPaymentInfo(value []byte) (*SilentPaymentInfo, error) {
+	if len(value) != secp.PubKeyBytesLenCompressed*2 {
+		return nil, ErrInvalidPsbtFormat
+	}
+
+	return &SilentPaymentInfo{
+		ScanKey:  value[:secp.PubKeyBytesLenCompressed],
+		SpendKey: value[secp.PubKeyBytesLenCompressed:],
+	}, nil
+}
+
+// SerializeSilentPaymentInfo serializes a silent payment info to value.
+func SerializeSilentPaymentInfo(info *SilentPaymentInfo) []byte {
+	value := make([]byte, 0, secp.PubKeyBytesLenCompressed*2)
+	value = append(value, info.ScanKey...)
+	value = append(value, info.SpendKey...)
+
+	return value
+}

btcutil/psbt/types.go

@@ -208,4 +208,8 @@ const (
 	// followed by said number of 32-byte leaf hashes. The rest of the value
 	// is then identical to the Bip32DerivationInputType value.
 	TaprootBip32DerivationOutputType OutputType = 7
+
+	// SilentPaymentV0InfoOutputType is used to house the silent payment
+	// recipient information for a version 0 silent payment output.
+	SilentPaymentV0InfoOutputType OutputType = 0x09
 )