feat(consensus): enforce transaction version 3 after HF5

After HF5 activation, only version 3 transactions are accepted.
Before HF5, version 3 is rejected. Matches C++ check_tx_semantic
hardfork gating logic.

Co-Authored-By: Charon <charon@lethean.io>

consensus/errors.go

@@ -20,6 +20,8 @@ var (
 	ErrInvalidOutput     = errors.New("consensus: invalid output")
 	ErrDuplicateKeyImage = errors.New("consensus: duplicate key image in transaction")
 	ErrInvalidExtra      = errors.New("consensus: invalid extra field")
+	ErrTxVersionInvalid  = errors.New("consensus: invalid transaction version for current hardfork")
+	ErrPreHardforkFreeze = errors.New("consensus: non-coinbase transaction rejected during pre-hardfork freeze")
 
 	// Transaction economic errors.
 	ErrInputOverflow  = errors.New("consensus: input amount overflow")

consensus/tx.go

@@ -18,6 +18,11 @@ func ValidateTransaction(tx *types.Transaction, txBlob []byte, forks []config.Ha
 	hf1Active := config.IsHardForkActive(forks, config.HF1, height)
 	hf4Active := config.IsHardForkActive(forks, config.HF4Zarcanum, height)
 
+	// 0. Transaction version for current hardfork.
+	if err := checkTxVersion(tx, forks, height); err != nil {
+		return err
+	}
+
 	// 1. Blob size.
 	if uint64(len(txBlob)) >= config.MaxTransactionBlobSize {
 		return fmt.Errorf("%w: %d bytes", ErrTxTooLarge, len(txBlob))
@@ -122,6 +127,29 @@ func checkOutputs(tx *types.Transaction, hf1Active, hf4Active bool) error {
 	return nil
 }
 
+// checkTxVersion validates that the transaction version is correct for the
+// current hardfork era. After HF5, version must be 3. Before HF5, version 3
+// is rejected.
+func checkTxVersion(tx *types.Transaction, forks []config.HardFork, height uint64) error {
+	hf5Active := config.IsHardForkActive(forks, config.HF5, height)
+
+	if hf5Active {
+		// After HF5: must be version 3.
+		if tx.Version != types.VersionPostHF5 {
+			return fmt.Errorf("%w: got version %d, require %d after HF5",
+				ErrTxVersionInvalid, tx.Version, types.VersionPostHF5)
+		}
+	} else {
+		// Before HF5: version 3 is not allowed.
+		if tx.Version >= types.VersionPostHF5 {
+			return fmt.Errorf("%w: version %d not allowed before HF5",
+				ErrTxVersionInvalid, tx.Version)
+		}
+	}
+
+	return nil
+}
+
 func checkKeyImages(tx *types.Transaction) error {
 	seen := make(map[types.KeyImage]struct{})
 	for _, vin := range tx.Vin {

consensus/tx_version_test.go

@@ -0,0 +1,112 @@
+// Copyright (c) 2017-2026 Lethean (https://lt.hn)
+//
+// Licensed under the European Union Public Licence (EUPL) version 1.2.
+// SPDX-License-Identifier: EUPL-1.2
+
+//go:build !integration
+
+package consensus
+
+import (
+	"testing"
+
+	"forge.lthn.ai/core/go-blockchain/config"
+	"forge.lthn.ai/core/go-blockchain/types"
+)
+
+// validV2Tx returns a minimal valid v2 (Zarcanum) transaction for testing.
+func validV2Tx() *types.Transaction {
+	return &types.Transaction{
+		Version: types.VersionPostHF4,
+		Vin: []types.TxInput{
+			types.TxInputZC{
+				KeyImage: types.KeyImage{1},
+			},
+		},
+		Vout: []types.TxOutput{
+			types.TxOutputZarcanum{StealthAddress: types.PublicKey{1}},
+			types.TxOutputZarcanum{StealthAddress: types.PublicKey{2}},
+		},
+	}
+}
+
+// validV3Tx returns a minimal valid v3 (HF5) transaction for testing.
+func validV3Tx() *types.Transaction {
+	return &types.Transaction{
+		Version:    types.VersionPostHF5,
+		HardforkID: 5,
+		Vin: []types.TxInput{
+			types.TxInputZC{
+				KeyImage: types.KeyImage{1},
+			},
+		},
+		Vout: []types.TxOutput{
+			types.TxOutputZarcanum{StealthAddress: types.PublicKey{1}},
+			types.TxOutputZarcanum{StealthAddress: types.PublicKey{2}},
+		},
+	}
+}
+
+func TestCheckTxVersion_Good(t *testing.T) {
+	tests := []struct {
+		name   string
+		tx     *types.Transaction
+		forks  []config.HardFork
+		height uint64
+	}{
+		// v1 transaction before HF4 — valid.
+		{"v1_before_hf4", validV1Tx(), config.MainnetForks, 5000},
+		// v2 transaction after HF4, before HF5 — valid.
+		{"v2_after_hf4_before_hf5", validV2Tx(), config.TestnetForks, 150},
+		// v3 transaction after HF5 — valid.
+		{"v3_after_hf5", validV3Tx(), config.TestnetForks, 250},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := checkTxVersion(tt.tx, tt.forks, tt.height)
+			if err != nil {
+				t.Errorf("checkTxVersion returned unexpected error: %v", err)
+			}
+		})
+	}
+}
+
+func TestCheckTxVersion_Bad(t *testing.T) {
+	tests := []struct {
+		name   string
+		tx     *types.Transaction
+		forks  []config.HardFork
+		height uint64
+	}{
+		// v2 transaction after HF5 — must be v3.
+		{"v2_after_hf5", validV2Tx(), config.TestnetForks, 250},
+		// v3 transaction before HF5 — too early.
+		{"v3_before_hf5", validV3Tx(), config.TestnetForks, 150},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := checkTxVersion(tt.tx, tt.forks, tt.height)
+			if err == nil {
+				t.Error("expected ErrTxVersionInvalid, got nil")
+			}
+		})
+	}
+}
+
+func TestCheckTxVersion_Ugly(t *testing.T) {
+	// v3 at exact HF5 activation boundary (height 201 on testnet, HF5.Height=200).
+	tx := validV3Tx()
+	err := checkTxVersion(tx, config.TestnetForks, 201)
+	if err != nil {
+		t.Errorf("v3 at HF5 activation boundary should be valid: %v", err)
+	}
+
+	// v2 at exact HF5 activation boundary — should be rejected.
+	tx2 := validV2Tx()
+	err = checkTxVersion(tx2, config.TestnetForks, 201)
+	if err == nil {
+		t.Error("v2 at HF5 activation boundary should be rejected")
+	}
+}