tappsbt: add ProofSuffix field to VOutput

We need a way to transport proofs that are complete in terms of MS-SMT
proofs committing to the correct asset but incomplete in terms of the
asset witness (in the ASSET_VERSION_V1 case).
The virtual transaction output is as good a place as any other. This
will also allow us to simplify some of the data structures around
passive assets, as currently we need to carry those proofs around in a
separate map.

tappsbt/decode.go

@@ -278,11 +278,15 @@ func (o *VOutput) decode(pOut psbt.POutput, txOut *wire.TxOut) error {
 			),
 		},
 		{
-			key: PsbtKeyTypeOutputAssetVersion,
+			key: PsbtKeyTypeOutputTapAssetVersion,
 			decoder: tlvDecoder(
 				&o.AssetVersion, vOutputAssetVersionDecoder,
 			),
 		},
+		{
+			key:     PsbtKeyTypeOutputTapAssetProofSuffix,
+			decoder: tlvDecoder(&o.ProofSuffix, tlv.DVarBytes),
+		},
 	}
 
 	for idx := range mapping {

tappsbt/encode.go

@@ -277,11 +277,15 @@ func (o *VOutput) encode(coinType uint32) (psbt.POutput, *wire.TxOut, error) {
 			),
 		},
 		{
-			key: PsbtKeyTypeOutputAssetVersion,
+			key: PsbtKeyTypeOutputTapAssetVersion,
 			encoder: tlvEncoder(
 				&o.AssetVersion, vOutputAssetVersionEncoder,
 			),
 		},
+		{
+			key:     PsbtKeyTypeOutputTapAssetProofSuffix,
+			encoder: tlvEncoder(&o.ProofSuffix, tlv.EVarBytes),
+		},
 	}
 
 	for idx := range mapping {

tappsbt/interface.go

@@ -50,7 +50,8 @@ var (
 	PsbtKeyTypeOutputTapAsset                              = []byte{0x76}
 	PsbtKeyTypeOutputTapSplitAsset                         = []byte{0x77}
 	PsbtKeyTypeOutputTapAnchorTapscriptSibling             = []byte{0x78}
-	PsbtKeyTypeOutputAssetVersion                          = []byte{0x79}
+	PsbtKeyTypeOutputTapAssetVersion                       = []byte{0x79}
+	PsbtKeyTypeOutputTapAssetProofSuffix                   = []byte{0x7a}
 )
 
 // The following keys are used as custom fields on the BTC level anchor
@@ -529,6 +530,14 @@ type VOutput struct {
 	// serialized, this will be stored in the TaprootInternalKey and
 	// TaprootDerivationPath fields of the PSBT output.
 	ScriptKey asset.ScriptKey
+
+	// ProofSuffix is the optional new transition proof blob that is created
+	// once the asset output was successfully committed to the anchor
+	// transaction referenced above. The proof suffix is not yet complete
+	// since the header information needs to be added once the anchor
+	// transaction was confirmed in a block. This field cannot be of type
+	// proof.Proof directly because that would cause a circular dependency.
+	ProofSuffix []byte
 }
 
 // SplitLocator creates a split locator from the output. The asset ID is passed
@@ -750,3 +759,19 @@ func deserializeTweakedScriptKey(pOut psbt.POutput) (*asset.TweakedScriptKey,
 		Tweak:  tweak,
 	}, nil
 }
+
+// Encode encodes the virtual packet into a byte slice.
+func Encode(vPkt *VPacket) ([]byte, error) {
+	var buf bytes.Buffer
+	err := vPkt.Serialize(&buf)
+	if err != nil {
+		return nil, err
+	}
+
+	return buf.Bytes(), nil
+}
+
+// Decode decodes a virtual packet from a byte slice.
+func Decode(encoded []byte) (*VPacket, error) {
+	return NewFromRawBytes(bytes.NewReader(encoded), false)
+}