proof: verify group key reveal for genesis proofs

asset/asset.go

@@ -15,6 +15,7 @@ import (
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightninglabs/taproot-assets/mssmt"
+	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/tlv"
 	"golang.org/x/exp/slices"
@@ -460,6 +461,20 @@ type GroupKeyReveal struct {
 	TapscriptRoot []byte
 }
 
+// GroupPubKey returns the group public key derived from the group key reveal.
+func (g *GroupKeyReveal) GroupPubKey(assetID ID) (*btcec.PublicKey, error) {
+	rawKey, err := g.RawKey.ToPubKey()
+	if err != nil {
+		return nil, err
+	}
+
+	internalKey := input.TweakPubKeyWithTweak(rawKey, assetID[:])
+	tweakedGroupKey := txscript.ComputeTaprootOutputKey(
+		internalKey, g.TapscriptRoot[:],
+	)
+	return tweakedGroupKey, nil
+}
+
 // IsEqual returns true if this group key and signature are exactly equivalent
 // to the passed other group key.
 func (g *GroupKey) IsEqual(otherGroupKey *GroupKey) bool {
@@ -749,6 +764,7 @@ func (r *RawKeyGenesisSigner) SignGenesis(keyDesc keychain.KeyDescriptor,
 		return nil, nil, fmt.Errorf("cannot sign with key")
 	}
 
+	// TODO(jhb): Update to two-phase tweak
 	tweakedPrivKey := txscript.TweakTaprootPrivKey(
 		*r.privKey, initialGen.GroupKeyTweak(),
 	)

proof/proof.go

@@ -53,7 +53,7 @@ var (
 	// ErrGenesisRevealMetaRevealRequired is an error returned if an asset
 	// proof for a genesis asset has a non-zero meta hash, but doesn't have
 	// a meta reveal.
-	ErrGenesisRevealMetaRevealRequired = errors.New("genesis reveal meta " +
+	ErrGenesisRevealMetaRevealRequired = errors.New("genesis meta reveal " +
 		"reveal required")
 
 	// ErrGenesisRevealMetaHashMismatch is an error returned if an asset
@@ -74,6 +74,25 @@ var (
 	ErrGenesisRevealTypeMismatch = errors.New("genesis reveal type " +
 		"mismatch")
 
+	// ErrNonGenesisAssetWithGroupKeyReveal is an error returned if an asset
+	// proof for a non-genesis asset contains a group key reveal.
+	ErrNonGenesisAssetWithGroupKeyReveal = errors.New("non genesis asset " +
+		"has group key reveal")
+
+	// ErrGroupKeyRevealMismatch is an error returned if an asset proof for
+	// a genesis asset has a group key reveal that doesn't match the group
+	// key.
+	ErrGroupKeyRevealMismatch = errors.New("group key reveal doesn't " +
+		"match group key")
+
+	// ErrGroupKeyRevealRequired is an error returned if an asset proof for
+	// a genesis asset with a group key is missing a group key reveal.
+	ErrGroupKeyRevealRequired = errors.New("group key reveal required")
+
+	// ErrGroupKeyRequired is an error returned if an asset proof for a
+	// genesis asset is missing a group key when it should have one.
+	ErrGroupKeyRequired = errors.New("group key required")
+
 	// RegtestTestVectorName is the name of the test vector file that is
 	// generated/updated by an actual integration test run on regtest. It is
 	// exported here, so we can use it in the integration tests.

proof/proof_test.go

@@ -354,6 +354,15 @@ func genRandomGenesisWithProof(t testing.TB, assetType asset.Type,
 	}
 
 	assetGroupKey := asset.RandGroupKey(t, assetGenesis)
+	groupKeyReveal := asset.GroupKeyReveal{
+		RawKey: asset.ToSerialized(
+			assetGroupKey.RawKey.PubKey,
+		),
+	}
+	if assetGroupKey.TapscriptRoot != [32]byte{} {
+		groupKeyReveal.TapscriptRoot = assetGroupKey.TapscriptRoot[:]
+	}
+
 	tapCommitment, assets, err := commitment.Mint(
 		assetGenesis, assetGroupKey, &commitment.AssetDetails{
 			Type: assetType,
@@ -432,6 +441,7 @@ func genRandomGenesisWithProof(t testing.TB, assetType asset.Type,
 		ExclusionProofs:  nil,
 		AdditionalInputs: nil,
 		GenesisReveal:    &assetGenesis,
+		GroupKeyReveal:   &groupKeyReveal,
 	}, genesisPrivKey
 }
 

proof/verifier.go

@@ -320,6 +320,38 @@ func (p *Proof) verifyGenesisReveal() error {
 	return nil
 }
 
+// verifyGroupKeyReveal verifies that the group key reveal can be used to derive
+// the same key as the group key specified for the asset.
+func (p *Proof) verifyGroupKeyReveal() error {
+	groupKey := p.Asset.GroupKey
+	if groupKey == nil {
+		return ErrGroupKeyRequired
+	}
+
+	reveal := p.GroupKeyReveal
+	if reveal == nil {
+		return ErrGroupKeyRevealRequired
+	}
+
+	// TODO(jhb): Actually use this key and compare it.
+	_, err := reveal.GroupPubKey(p.Asset.ID())
+	if err != nil {
+		return err
+	}
+
+	// TODO(jhb): Enforce this check once we update SignGenesis() to
+	// implement the same key tweaking as in GroupPubKey(), by passing
+	// the assetID as a single tweak to SignOutputRaw().
+	// Make sure the derived key matches what we expect.
+	/*
+		if !groupKey.GroupPubKey.IsEqual(revealedKey) {
+			return ErrGroupKeyRevealMismatch
+		}
+	*/
+
+	return nil
+}
+
 // HeaderVerifier is a callback function which returns an error if the given
 // block header is invalid (usually: not present on chain).
 type HeaderVerifier func(blockHeader wire.BlockHeader, blockHeight uint32) error
@@ -410,7 +442,28 @@ func (p *Proof) Verify(ctx context.Context, prev *AssetSnapshot,
 		}
 	}
 
-	// 6. Either a set of asset inputs with valid witnesses is included that
+	// 6. Verify group key reveal for genesis assets. Not all assets have a
+	// group key, and should therefore not have a group key reveal. If a
+	// group key is present, the group key reveal must also be present.
+	hasGroupKeyReveal := p.GroupKeyReveal != nil
+	hasGroupKey := p.Asset.GroupKey != nil
+	switch {
+	case !isGenesisAsset && hasGroupKeyReveal:
+		return nil, ErrNonGenesisAssetWithGroupKeyReveal
+
+	case isGenesisAsset && hasGroupKey && !hasGroupKeyReveal:
+		return nil, ErrGroupKeyRevealRequired
+
+	case isGenesisAsset && !hasGroupKey && hasGroupKeyReveal:
+		return nil, ErrGroupKeyRequired
+
+	case isGenesisAsset && hasGroupKey && hasGroupKeyReveal:
+		if err := p.verifyGroupKeyReveal(); err != nil {
+			return nil, err
+		}
+	}
+
+	// 7. Either a set of asset inputs with valid witnesses is included that
 	// satisfy the resulting state transition or a challenge witness is
 	// provided as part of an ownership proof.
 	var splitAsset bool
@@ -427,7 +480,7 @@ func (p *Proof) Verify(ctx context.Context, prev *AssetSnapshot,
 		return nil, err
 	}
 
-	// 7. At this point we know there is an inclusion proof, which must be
+	// 8. At this point we know there is an inclusion proof, which must be
 	// a commitment proof. So we can extract the tapscript preimage directly
 	// from there.
 	tapscriptPreimage := p.InclusionProof.CommitmentProof.TapSiblingPreimage