proof: add AltLeaves to Proof

In this commit, we add an AltLeaves field to Proof, and update Proof
en/decode to support this optional field. With a small test change, we
also check that Proof.Verify() functions for Proofs that accounted for
any present AltLeaves correctly.

Author: jharveyb

proof/proof.go

@@ -309,6 +309,12 @@ type Proof struct {
 	// require only a valid signature for the previously revealed group key.
 	GroupKeyReveal asset.GroupKeyReveal
 
+	// AltLeaves represent data used to construct an Asset commitment, that
+	// was inserted in the output anchor Tap commitment. These data-carrying
+	// leaves are used for a purpose distinct from representing individual
+	// Taproot Assets.
+	AltLeaves []asset.AltLeaf[asset.Asset]
+
 	// UnknownOddTypes is a map of unknown odd types that were encountered
 	// during decoding. This map is used to preserve unknown types that we
 	// don't know of yet, so we can still encode them back when serializing.
@@ -329,7 +335,7 @@ func (p *Proof) OutPoint() wire.OutPoint {
 
 // EncodeRecords returns the set of known TLV records to encode a Proof.
 func (p *Proof) EncodeRecords() []tlv.Record {
-	records := make([]tlv.Record, 0, 15)
+	records := make([]tlv.Record, 0, 16)
 	records = append(records, VersionRecord(&p.Version))
 	records = append(records, PrevOutRecord(&p.PrevOut))
 	records = append(records, BlockHeaderRecord(&p.BlockHeader))
@@ -369,6 +375,9 @@ func (p *Proof) EncodeRecords() []tlv.Record {
 			&p.GroupKeyReveal,
 		))
 	}
+	if len(p.AltLeaves) > 0 {
+		records = append(records, AltLeavesRecord(&p.AltLeaves))
+	}
 
 	// Add any unknown odd types that were encountered during decoding.
 	return asset.CombineRecords(records, p.UnknownOddTypes)
@@ -392,6 +401,7 @@ func (p *Proof) DecodeRecords() []tlv.Record {
 		BlockHeightRecord(&p.BlockHeight),
 		GenesisRevealRecord(&p.GenesisReveal),
 		GroupKeyRevealRecord(&p.GroupKeyReveal),
+		AltLeavesRecord(&p.AltLeaves),
 	}
 }
 

proof/proof_test.go

@@ -283,6 +283,18 @@ func genRandomGenesisWithProof(t testing.TB, assetType asset.Type,
 		},
 	)
 	require.NoError(t, err)
+
+	// Include 1 or more alt leaves in the anchor output Tap commitment.
+	// Since this is also used for generating the test vectors, we don't
+	// actually want to have zero alt leaves.
+	innerAltLeaves := asset.RandAltLeaves(t, true)
+
+	altCommitment, err := commitment.NewAssetCommitment(innerAltLeaves...)
+	require.NoError(t, err)
+
+	err = tapCommitment.Upsert(altCommitment)
+	require.NoError(t, err)
+	altLeaves := asset.ToAltLeaves(innerAltLeaves)
 	genesisAsset := assets[0]
 	_, commitmentProof, err := tapCommitment.Proof(
 		genesisAsset.TapCommitmentKey(),
@@ -355,6 +367,7 @@ func genRandomGenesisWithProof(t testing.TB, assetType asset.Type,
 		AdditionalInputs: nil,
 		GenesisReveal:    genReveal,
 		GroupKeyReveal:   groupKeyReveal,
+		AltLeaves:        altLeaves,
 	}, genesisPrivKey
 }
 

proof/records.go

@@ -27,6 +27,7 @@ const (
 	BlockHeightType      tlv.Type = 22
 	GenesisRevealType    tlv.Type = 23
 	GroupKeyRevealType   tlv.Type = 25
+	AltLeavesType        tlv.Type = 27
 
 	TaprootProofOutputIndexType     tlv.Type = 0
 	TaprootProofInternalKeyType     tlv.Type = 2
@@ -53,7 +54,7 @@ var KnownProofTypes = fn.NewSet(
 	TxMerkleProofType, AssetLeafType, InclusionProofType,
 	ExclusionProofsType, SplitRootProofType, MetaRevealType,
 	AdditionalInputsType, ChallengeWitnessType, BlockHeightType,
-	GenesisRevealType, GroupKeyRevealType,
+	GenesisRevealType, GroupKeyRevealType, AltLeavesType,
 )
 
 // KnownTaprootProofTypes is a set of all known Taproot proof TLV types. This
@@ -393,3 +394,18 @@ func GroupKeyRevealRecord(reveal *asset.GroupKeyReveal) tlv.Record {
 		GroupKeyRevealDecoder,
 	)
 }
+
+func AltLeavesRecord(leaves *[]asset.AltLeaf[asset.Asset]) tlv.Record {
+	sizeFunc := func() uint64 {
+		var buf bytes.Buffer
+		err := asset.AltLeavesEncoder(&buf, leaves, &[8]byte{})
+		if err != nil {
+			panic(err)
+		}
+		return uint64(len(buf.Bytes()))
+	}
+	return tlv.MakeDynamicRecord(
+		AltLeavesType, leaves, sizeFunc, asset.AltLeavesEncoder,
+		asset.AltLeavesDecoder,
+	)
+}