psbt: add new PSBT_GLOBAL_GENERIC_SINGED_MESSAGE field

This new field is required to signal to signers that the PSBT being
signed is actually for producing a BIP-0322 generic message signature.

Author: guggero

btcutil/psbt/psbt.go

@@ -142,6 +142,11 @@ type Packet struct {
 	// derived.
 	XPubs []XPub
 
+	// GenericSignedMessage contains a BIP-0322 generic message to be
+	// signed. An empty message is a valid message, that's why this field
+	// is a nillable string.
+	GenericSignedMessage *string
+
 	// Unknowns are the set of custom types (global only) within this PSBT.
 	Unknowns []*Unknown
 }
@@ -240,8 +245,9 @@ func NewFromRawBytes(r io.Reader, b64 bool) (*Packet, error) {
 	// Next we parse any unknowns that may be present, making sure that we
 	// break at the separator.
 	var (
-		xPubSlice    []XPub
-		unknownSlice []*Unknown
+		xPubSlice            []XPub
+		genericSignedMessage *string
+		unknownSlice         []*Unknown
 	)
 	for {
 		keyint, keydata, err := getKey(r)
@@ -275,6 +281,12 @@ func NewFromRawBytes(r io.Reader, b64 bool) (*Packet, error) {
 
 			xPubSlice = append(xPubSlice, *xPub)
 
+		case GenericSignedMessageType:
+			// Golang's default string encoding is UTF-8, so we
+			// don't need to worry about the encoding here.
+			messageString := string(value)
+			genericSignedMessage = &messageString
+
 		default:
 			keyintanddata := []byte{byte(keyint)}
 			keyintanddata = append(keyintanddata, keydata...)
@@ -313,11 +325,12 @@ func NewFromRawBytes(r io.Reader, b64 bool) (*Packet, error) {
 
 	// Populate the new Packet object.
 	newPsbt := Packet{
-		UnsignedTx: msgTx,
-		Inputs:     inSlice,
-		Outputs:    outSlice,
-		XPubs:      xPubSlice,
-		Unknowns:   unknownSlice,
+		UnsignedTx:           msgTx,
+		Inputs:               inSlice,
+		Outputs:              outSlice,
+		XPubs:                xPubSlice,
+		GenericSignedMessage: genericSignedMessage,
+		Unknowns:             unknownSlice,
 	}
 
 	// Extended sanity checking is applied here to make sure the
@@ -369,6 +382,17 @@ func (p *Packet) Serialize(w io.Writer) error {
 		}
 	}
 
+	// Serialize the generic signed message.
+	if p.GenericSignedMessage != nil {
+		msgBytes := []byte(*p.GenericSignedMessage)
+		err := serializeKVPairWithType(
+			w, uint8(GenericSignedMessageType), nil, msgBytes,
+		)
+		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 p.Unknowns {

btcutil/psbt/psbt_test.go

@@ -107,6 +107,8 @@ var validPsbtBase64 = map[int]string{
 	7: "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",
 	// Case: PSBT with `PSBT_GLOBAL_XPUB`.
 	8: "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",
+	// Case: PSBT wich `PSBT_GLOBAL_GENERIC_SIGNED_MESSAGE`.
+	9: "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",
 }
 
 // These are all invalid PSBTs for the indicated reasons.

btcutil/psbt/types.go

@@ -30,6 +30,13 @@ const (
 	// extended public key.
 	XPubType GlobalType = 1
 
+	// GenericSignedMessageType is used to house a BIP-0322 generic signed
+	// message, encoded as UTF-8.
+	//
+	// The key is ({0x09}|{message}), and the value is the UTF-8 encoded
+	// message.
+	GenericSignedMessageType GlobalType = 0x09
+
 	// VersionType houses the global version number of this PSBT. There is
 	// no key (only contains the byte type), then the value if omitted, is
 	// assumed to be zero.