psbt: allow empty bip32 derivation path

btcutil/psbt/bip32.go

@@ -40,8 +40,11 @@ func (s Bip32Sorter) Less(i, j int) bool {
 // little endian encodings of uint32 values, the first of which is the
 // masterkeyfingerprint and the remainder of which are the derivation path.
 func readBip32Derivation(path []byte) (uint32, []uint32, error) {
-
-	if len(path)%4 != 0 || len(path)/4-1 < 1 {
+	// BIP-0174 defines the derivation path being encoded as
+	//   "<32-bit uint> <32-bit uint>*"
+	// with the asterisk meaning 0 to n times. Which in turn means that an
+	// empty path is valid, only the key fingerprint is mandatory.
+	if len(path)%4 != 0 {
 		return 0, nil, ErrInvalidPsbtFormat
 	}
 

btcutil/psbt/psbt_test.go

@@ -11,10 +11,10 @@ import (
 	"encoding/hex"
 	"testing"
 
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcd/btcutil"
 	"github.com/davecgh/go-spew/spew"
 )
 
@@ -63,6 +63,7 @@ var validPsbtHex = map[int]string{
 	3: "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",
 	4: "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",
 	5: "70736274ff01003f0200000001ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff0000000000ffffffff010000000000000000036a010000000000000a0f0102030405060708090f0102030405060708090a0b0c0d0e0f0000",
+	6: "70736274ff01003f0200000001ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff0000000000ffffffff010000000000000000036a010000000000002206030d097466b7f59162ac4d90bf65f2a31a8bad82fcd22e98138dcf279401939bd104ffffffff0a0f0102030405060708090f0102030405060708090a0b0c0d0e0f0000",
 }
 
 // These are all invalid PSBTs for the indicated