asset: turn Pederson key into xpub for hardware wallet support

Hardware wallets that support miniscript policies only accept a
pk() fragment that specifies an extended key, for example with:
pk(@1/<0;1>/*). When signing the transaction the actual derivation path
to use (for example 0/0) is defined in the PSBT itself. So the signing
policy is generic and the user only has to accept it once.
A child key derived from an extended NUMS key is still a NUMS key, as
it's just tweaked again. The initial private key is still unknown.

asset/asset.go

@@ -20,6 +20,7 @@ import (
 	"github.com/btcsuite/btcd/btcec/v2/schnorr"
 	"github.com/btcsuite/btcd/btcutil/hdkeychain"
 	"github.com/btcsuite/btcd/btcutil/psbt"
+	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
@@ -1142,19 +1143,13 @@ func NewNonSpendableScriptLeaf(version NonSpendLeafVersion,
 	// For the Pedersen commitment based version, we'll use a single
 	// OP_CEHCKSIG with an un-spendable key.
 	case PedersenVersion:
-		var msg [sha256.Size]byte
-		copy(msg[:], data)
-
-		// Make a Pedersen opening that uses no mask (we don't carry on
-		// the random value, as we don't care about hiding here). We'll
-		// also use the existing NUMs point.
-		op := pedersen.Opening{
-			Msg: msg,
+		_, commitPoint, err := TweakedNumsKey(data)
+		if err != nil {
+			return txscript.TapLeaf{}, fmt.Errorf("failed to "+
+				"derive tweaked NUMS key: %w", err)
 		}
-		commitPoint := pedersen.NewCommitment(op).Point()
-
-		commitBytes := schnorr.SerializePubKey(&commitPoint)
 
+		commitBytes := schnorr.SerializePubKey(commitPoint)
 		builder = txscript.NewScriptBuilder().AddData(commitBytes).
 			AddOp(txscript.OP_CHECKSIG)
 
@@ -1202,6 +1197,68 @@ func NewGKRCustomSubtreeRootRecord(root *chainhash.Hash) tlv.Record {
 	return tlv.MakePrimitiveRecord(GKRCustomSubtreeRoot, (*[32]byte)(root))
 }
 
+// NumsXPub turns the given NUMS key into an extended public key (using the x
+// coordinate of the public key as the chain code), then derives the actual key
+// to use from the derivation path 0/0. The extended key always has the mainnet
+// version, but can be converted to any network on demand by the caller with
+// CloneWithVersion().
+func NumsXPub(numsKey *btcec.PublicKey) (*hdkeychain.ExtendedKey,
+	*btcec.PublicKey, error) {
+
+	keyBytes := numsKey.SerializeCompressed()
+	chainCode := keyBytes[1:]
+
+	// We use a depth of 3, emulating BIP44/49/84/86 style derivation for
+	// xpubs. We also always use mainnet to not require the caller to pass
+	// in the net params. Converting to another network is possible with
+	// CloneWithVersion().
+	const depth = 3
+	extendedNumsKey := hdkeychain.NewExtendedKey(
+		chaincfg.MainNetParams.HDPublicKeyID[:], keyBytes, chainCode,
+		[]byte{0, 0, 0, 0}, depth, 0, false,
+	)
+
+	// Derive the actual key to use from the xpub.
+	changeBranch, err := extendedNumsKey.Derive(0)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	indexBranch, err := changeBranch.Derive(0)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	actualKey, err := indexBranch.ECPubKey()
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return extendedNumsKey, actualKey, nil
+}
+
+// TweakedNumsKey derives the NUMS key from the given data, then creates the
+// extended key from it and derives the actual (derived child) key to use from
+// the derivation path 0/0. The extended key always has the mainnet version, but
+// can be converted to any network on demand by the caller with
+// CloneWithVersion().
+func TweakedNumsKey(data []byte) (*hdkeychain.ExtendedKey, *btcec.PublicKey,
+	error) {
+
+	var msg [sha256.Size]byte
+	copy(msg[:], data)
+
+	// Make a Pedersen opening that uses no mask (we don't carry on
+	// the random value, as we don't care about hiding here). We'll
+	// also use the existing NUMs point.
+	op := pedersen.Opening{
+		Msg: msg,
+	}
+	commitPoint := pedersen.NewCommitment(op).Point()
+
+	return NumsXPub(&commitPoint)
+}
+
 // GroupKeyRevealTapscript holds data used to derive the tapscript root, which
 // is then used to calculate the asset group key.
 //
@@ -1273,7 +1330,9 @@ func NewGKRCustomSubtreeRootRecord(root *chainhash.Hash) tlv.Record {
 //   - One that uses a normal OP_CHECKSIG operator where the pubkey
 //     argument is a key that cannot be signed with. We generate this
 //     special public key using a Pedersen commitment, where the message is
-//     the asset ID.
+//     the asset ID. To achieve hardware wallet support, that key is then turned
+//     into an extended key (xpub) and a child key at path 0/0 is used as the
+//     actual public key that goes into the OP_CHECKSIG script.
 //
 // If `custom_root_hash` is not provided, then there is no sibling to the asset
 // ID leaf, meaning the tree only has a single leaf. This makes it possible to
@@ -1300,7 +1359,9 @@ func NewGKRCustomSubtreeRootRecord(root *chainhash.Hash) tlv.Record {
 //   - One that uses a normal OP_CHECKSIG operator where the pubkey
 //     argument is a key that cannot be signed with. We generate this
 //     special public key using a Pedersen commitment, where the message is
-//     the asset ID.
+//     the asset ID. To achieve hardware wallet support, that key is then turned
+//     into an extended key (xpub) and a child key at path 0/0 is used as the
+//     actual public key that goes into the OP_CHECKSIG script.
 type GroupKeyRevealTapscript struct {
 	// version is the version of the group key reveal that determines how
 	// the non-spendable leaf is created.