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.

Author: guggero

tappsbt/decode.go

@@ -289,6 +289,10 @@ func (o *VOutput) decode(pOut psbt.POutput, txOut *wire.TxOut) error {
 			key:     PsbtKeyTypeOutputTapProofDeliveryAddress,
 			decoder: urlDecoder(&o.ProofDeliveryAddress),
 		},
+		{
+			key:     PsbtKeyTypeOutputTapAssetProofSuffix,
+			decoder: proofDecoder(&o.ProofSuffix),
+		},
 	}
 
 	for idx := range mapping {

tappsbt/encode.go

@@ -289,6 +289,10 @@ func (o *VOutput) encode(coinType uint32) (psbt.POutput, *wire.TxOut, error) {
 			key:     PsbtKeyTypeOutputTapProofDeliveryAddress,
 			encoder: urlEncoder(o.ProofDeliveryAddress),
 		},
+		{
+			key:     PsbtKeyTypeOutputTapAssetProofSuffix,
+			encoder: proofEncoder(o.ProofSuffix),
+		},
 	}
 
 	for idx := range mapping {

tappsbt/interface.go

@@ -54,6 +54,7 @@ var (
 	PsbtKeyTypeOutputTapAnchorTapscriptSibling             = []byte{0x78}
 	PsbtKeyTypeOutputTapAssetVersion                       = []byte{0x79}
 	PsbtKeyTypeOutputTapProofDeliveryAddress               = []byte{0x7a}
+	PsbtKeyTypeOutputTapAssetProofSuffix                   = []byte{0x7b}
 )
 
 // The following keys are used as custom fields on the BTC level anchor
@@ -528,6 +529,13 @@ type VOutput struct {
 	// ProofDeliveryAddress is the address to which the proof of the asset
 	// transfer should be delivered.
 	ProofDeliveryAddress *url.URL
+
+	// 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.
+	ProofSuffix *proof.Proof
 }
 
 // SplitLocator creates a split locator from the output. The asset ID is passed
@@ -749,3 +757,19 @@ 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)
+}

tappsbt/mock.go

@@ -141,6 +141,7 @@ func RandPacket(t testing.TB) *VPacket {
 			SplitAsset:                         testOutputAsset,
 			AnchorOutputTapscriptSibling:       testPreimage1,
 			ProofDeliveryAddress:               courierAddress,
+			ProofSuffix:                        &inputProof,
 		}, {
 			Amount: 345,
 			AssetVersion: asset.Version(
@@ -502,6 +503,10 @@ func NewTestFromVOutput(t testing.TB, v *VOutput,
 		vo.ProofDeliveryAddress = v.ProofDeliveryAddress.String()
 	}
 
+	if v.ProofSuffix != nil {
+		vo.ProofSuffix = proof.NewTestFromProof(t, v.ProofSuffix)
+	}
+
 	if v.ScriptKey.TweakedScriptKey != nil {
 		bip32Derivation, trBip32Derivation := Bip32DerivationFromKeyDesc(
 			v.ScriptKey.RawKey, coinType,
@@ -582,6 +587,7 @@ type TestVOutput struct {
 	TrInternalKey                 string                   `json:"tr_internal_key"`
 	TrMerkleRoot                  string                   `json:"tr_merkle_root"`
 	ProofDeliveryAddress          string                   `json:"proof_delivery_address"`
+	ProofSuffix                   *proof.TestProof         `json:"proof_suffix"`
 }
 
 func (to *TestVOutput) ToVOutput(t testing.TB) *VOutput {
@@ -625,6 +631,10 @@ func (to *TestVOutput) ToVOutput(t testing.TB) *VOutput {
 		require.NoError(t, err)
 	}
 
+	if to.ProofSuffix != nil {
+		v.ProofSuffix = to.ProofSuffix.ToProof(t)
+	}
+
 	if len(to.Bip32Derivation) > 0 && to.TrInternalKey != "" {
 		firstDerivation := to.Bip32Derivation[0].ToBip32Derivation(t)
 		keyDesc, err := KeyDescFromBip32Derivation(firstDerivation)