contracts/chequebook: add chequebook contract wrapper

The wrapper code is adapted from the swarm/services/chequebook package
with the following mostly cosmetic changes:

* The code now uses the new Go API interfaces to query balances. Some
  minor functional changes were required to make this work.
* The package no longer depends on swarm/services/swap/swap. References
  to swap.Promise are replaced by interface{}, the base type of Promise.
  This is temporary.
* The contract wrapper has been regenerated with latest abigen
  and solc v0.3.6.
* There is a new generator that creates the 'deployed code' variable.
* Documentation comments now follow the recommended godoc style.
* [CHEQUEBOOK] log prefixes are gone.
* LGPL license headers have been added to all files.

1 files changed, 643 insertions, 0 deletions

diff --git a/contracts/chequebook/cheque.go b/contracts/chequebook/cheque.go
new file mode 100644
index 000000000..8199904ef
--- /dev/null
+++ b/contracts/chequebook/cheque.go
@@ -0,0 +1,643 @@
+// Copyright 2016 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+// Package chequebook package wraps the 'chequebook' Ethereum smart contract.
+//
+// The functions in this package allow using chequebook for
+// issuing, receiving, verifying cheques in ether; (auto)cashing cheques in ether
+// as well as (auto)depositing ether to the chequebook contract.
+package chequebook
+
+//go:generate abigen --sol contract/chequebook.sol --pkg contract --out contract/chequebook.go
+//go:generate go run ./gencode.go
+
+import (
+	"bytes"
+	"crypto/ecdsa"
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"math/big"
+	"os"
+	"sync"
+	"time"
+
+	"github.com/ethereum/go-ethereum/accounts/abi/bind"
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/contracts/chequebook/contract"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/crypto"
+	"github.com/ethereum/go-ethereum/logger"
+	"github.com/ethereum/go-ethereum/logger/glog"
+	"golang.org/x/net/context"
+)
+
+// TODO(zelig): watch peer solvency and notify of bouncing cheques
+// TODO(zelig): enable paying with cheque by signing off
+
+// Some functionality require interacting with the blockchain:
+// * setting current balance on peer's chequebook
+// * sending the transaction to cash the cheque
+// * depositing ether to the chequebook
+// * watching incoming ether
+
+var (
+	gasToCash = big.NewInt(2000000) // gas cost of a cash transaction using chequebook
+	// gasToDeploy = big.NewInt(3000000)
+)
+
+// Backend wraps all methods required for chequebook operation.
+type Backend interface {
+	bind.ContractBackend
+	TransactionReceipt(ctx context.Context, txHash common.Hash) (*types.Receipt, error)
+	BalanceAt(ctx context.Context, address common.Address, blockNum *big.Int) (*big.Int, error)
+}
+
+// Cheque represents a payment promise to a single beneficiary.
+type Cheque struct {
+	Contract    common.Address // address of chequebook, needed to avoid cross-contract submission
+	Beneficiary common.Address
+	Amount      *big.Int // cumulative amount of all funds sent
+	Sig         []byte   // signature Sign(Keccak256(contract, beneficiary, amount), prvKey)
+}
+
+func (self *Cheque) String() string {
+	return fmt.Sprintf("contract: %s, beneficiary: %s, amount: %v, signature: %x", self.Contract.Hex(), self.Beneficiary.Hex(), self.Amount, self.Sig)
+}
+
+type Params struct {
+	ContractCode, ContractAbi string
+}
+
+var ContractParams = &Params{contract.ChequebookBin, contract.ChequebookABI}
+
+// Chequebook can create and sign cheques from a single contract to multiple beneficiaries.
+// It is the outgoing payment handler for peer to peer micropayments.
+type Chequebook struct {
+	path     string                      // path to chequebook file
+	prvKey   *ecdsa.PrivateKey           // private key to sign cheque with
+	lock     sync.Mutex                  //
+	backend  Backend                     // blockchain API
+	quit     chan bool                   // when closed causes autodeposit to stop
+	owner    common.Address              // owner address (derived from pubkey)
+	contract *contract.Chequebook        // abigen binding
+	session  *contract.ChequebookSession // abigen binding with Tx Opts
+
+	// persisted fields
+	balance      *big.Int                    // not synced with blockchain
+	contractAddr common.Address              // contract address
+	sent         map[common.Address]*big.Int //tallies for beneficiarys
+
+	txhash    string   // tx hash of last deposit tx
+	threshold *big.Int // threshold that triggers autodeposit if not nil
+	buffer    *big.Int // buffer to keep on top of balance for fork protection
+}
+
+func (self *Chequebook) String() string {
+	return fmt.Sprintf("contract: %s, owner: %s, balance: %v, signer: %x", self.contractAddr.Hex(), self.owner.Hex(), self.balance, self.prvKey.PublicKey)
+}
+
+// NewChequebook creates a new Chequebook.
+func NewChequebook(path string, contractAddr common.Address, prvKey *ecdsa.PrivateKey, backend Backend) (self *Chequebook, err error) {
+	balance := new(big.Int)
+	sent := make(map[common.Address]*big.Int)
+
+	chbook, err := contract.NewChequebook(contractAddr, backend)
+	if err != nil {
+		return nil, err
+	}
+	transactOpts := bind.NewKeyedTransactor(prvKey)
+	session := &contract.ChequebookSession{
+		Contract:     chbook,
+		TransactOpts: *transactOpts,
+	}
+
+	self = &Chequebook{
+		prvKey:       prvKey,
+		balance:      balance,
+		contractAddr: contractAddr,
+		sent:         sent,
+		path:         path,
+		backend:      backend,
+		owner:        transactOpts.From,
+		contract:     chbook,
+		session:      session,
+	}
+
+	if (contractAddr != common.Address{}) {
+		self.setBalanceFromBlockChain()
+		glog.V(logger.Detail).Infof("new chequebook initialised from %s (owner: %v, balance: %s)", contractAddr.Hex(), self.owner.Hex(), self.balance.String())
+	}
+	return
+}
+
+func (self *Chequebook) setBalanceFromBlockChain() {
+	balance, err := self.backend.BalanceAt(context.TODO(), self.contractAddr, nil)
+	if err != nil {
+		glog.V(logger.Error).Infof("can't get balance: %v", err)
+	} else {
+		self.balance.Set(balance)
+	}
+}
+
+// LoadChequebook loads a chequebook from disk (file path).
+func LoadChequebook(path string, prvKey *ecdsa.PrivateKey, backend Backend, checkBalance bool) (self *Chequebook, err error) {
+	var data []byte
+	data, err = ioutil.ReadFile(path)
+	if err != nil {
+		return
+	}
+
+	self, _ = NewChequebook(path, common.Address{}, prvKey, backend)
+
+	err = json.Unmarshal(data, self)
+	if err != nil {
+		return nil, err
+	}
+	if checkBalance {
+		self.setBalanceFromBlockChain()
+	}
+
+	glog.V(logger.Detail).Infof("loaded chequebook (%s, owner: %v, balance: %v) initialised from %v", self.contractAddr.Hex(), self.owner.Hex(), self.balance, path)
+
+	return
+}
+
+// chequebookFile is the JSON representation of a chequebook.
+type chequebookFile struct {
+	Balance  string
+	Contract string
+	Owner    string
+	Sent     map[string]string
+}
+
+// UnmarshalJSON deserialises a chequebook.
+func (self *Chequebook) UnmarshalJSON(data []byte) error {
+	var file chequebookFile
+	err := json.Unmarshal(data, &file)
+	if err != nil {
+		return err
+	}
+	_, ok := self.balance.SetString(file.Balance, 10)
+	if !ok {
+		return fmt.Errorf("cumulative amount sent: unable to convert string to big integer: %v", file.Balance)
+	}
+	self.contractAddr = common.HexToAddress(file.Contract)
+	for addr, sent := range file.Sent {
+		self.sent[common.HexToAddress(addr)], ok = new(big.Int).SetString(sent, 10)
+		if !ok {
+			return fmt.Errorf("beneficiary %v cumulative amount sent: unable to convert string to big integer: %v", addr, sent)
+		}
+	}
+	return nil
+}
+
+// MarshalJSON serialises a chequebook.
+func (self *Chequebook) MarshalJSON() ([]byte, error) {
+	var file = &chequebookFile{
+		Balance:  self.balance.String(),
+		Contract: self.contractAddr.Hex(),
+		Owner:    self.owner.Hex(),
+		Sent:     make(map[string]string),
+	}
+	for addr, sent := range self.sent {
+		file.Sent[addr.Hex()] = sent.String()
+	}
+	return json.Marshal(file)
+}
+
+// Save persists the chequebook on disk, remembering balance, contract address and
+// cumulative amount of funds sent for each beneficiary.
+func (self *Chequebook) Save() (err error) {
+	data, err := json.MarshalIndent(self, "", " ")
+	if err != nil {
+		return err
+	}
+	glog.V(logger.Detail).Infof("saving chequebook (%s) to %v", self.contractAddr.Hex(), self.path)
+
+	return ioutil.WriteFile(self.path, data, os.ModePerm)
+}
+
+// Stop quits the autodeposit go routine to terminate
+func (self *Chequebook) Stop() {
+	defer self.lock.Unlock()
+	self.lock.Lock()
+	if self.quit != nil {
+		close(self.quit)
+		self.quit = nil
+	}
+}
+
+// Issue creates a cheque signed by the chequebook owner's private key. The
+// signer commits to a contract (one that they own), a beneficiary and amount.
+func (self *Chequebook) Issue(beneficiary common.Address, amount *big.Int) (ch *Cheque, err error) {
+	defer self.lock.Unlock()
+	self.lock.Lock()
+
+	if amount.Cmp(common.Big0) <= 0 {
+		return nil, fmt.Errorf("amount must be greater than zero (%v)", amount)
+	}
+	if self.balance.Cmp(amount) < 0 {
+		err = fmt.Errorf("insufficent funds to issue cheque for amount: %v. balance: %v", amount, self.balance)
+	} else {
+		var sig []byte
+		sent, found := self.sent[beneficiary]
+		if !found {
+			sent = new(big.Int)
+			self.sent[beneficiary] = sent
+		}
+		sum := new(big.Int).Set(sent)
+		sum.Add(sum, amount)
+
+		sig, err = crypto.Sign(sigHash(self.contractAddr, beneficiary, sum), self.prvKey)
+		if err == nil {
+			ch = &Cheque{
+				Contract:    self.contractAddr,
+				Beneficiary: beneficiary,
+				Amount:      sum,
+				Sig:         sig,
+			}
+			sent.Set(sum)
+			self.balance.Sub(self.balance, amount) // subtract amount from balance
+		}
+	}
+
+	// auto deposit if threshold is set and balance is less then threshold
+	// note this is called even if issueing cheque fails
+	// so we reattempt depositing
+	if self.threshold != nil {
+		if self.balance.Cmp(self.threshold) < 0 {
+			send := new(big.Int).Sub(self.buffer, self.balance)
+			self.deposit(send)
+		}
+	}
+
+	return
+}
+
+// Cash is a convenience method to cash any cheque.
+func (self *Chequebook) Cash(ch *Cheque) (txhash string, err error) {
+	return ch.Cash(self.session)
+}
+
+// data to sign: contract address, beneficiary, cumulative amount of funds ever sent
+func sigHash(contract, beneficiary common.Address, sum *big.Int) []byte {
+	bigamount := sum.Bytes()
+	if len(bigamount) > 32 {
+		return nil
+	}
+	var amount32 [32]byte
+	copy(amount32[32-len(bigamount):32], bigamount)
+	input := append(contract.Bytes(), beneficiary.Bytes()...)
+	input = append(input, amount32[:]...)
+	return crypto.Keccak256(input)
+}
+
+// Balance returns the current balance of the chequebook.
+func (self *Chequebook) Balance() *big.Int {
+	defer self.lock.Unlock()
+	self.lock.Lock()
+	return new(big.Int).Set(self.balance)
+}
+
+// Owner returns the owner account of the chequebook.
+func (self *Chequebook) Owner() common.Address {
+	return self.owner
+}
+
+// Address returns the on-chain contract address of the chequebook.
+func (self *Chequebook) Address() common.Address {
+	return self.contractAddr
+}
+
+// Deposit deposits money to the chequebook account.
+func (self *Chequebook) Deposit(amount *big.Int) (string, error) {
+	defer self.lock.Unlock()
+	self.lock.Lock()
+	return self.deposit(amount)
+}
+
+// deposit deposits amount to the chequebook account.
+// The caller must hold self.lock.
+func (self *Chequebook) deposit(amount *big.Int) (string, error) {
+	// since the amount is variable here, we do not use sessions
+	depositTransactor := bind.NewKeyedTransactor(self.prvKey)
+	depositTransactor.Value = amount
+	chbookRaw := &contract.ChequebookRaw{Contract: self.contract}
+	tx, err := chbookRaw.Transfer(depositTransactor)
+	if err != nil {
+		glog.V(logger.Warn).Infof("error depositing %d wei to chequebook (%s, balance: %v, target: %v): %v", amount, self.contractAddr.Hex(), self.balance, self.buffer, err)
+		return "", err
+	}
+	// assume that transaction is actually successful, we add the amount to balance right away
+	self.balance.Add(self.balance, amount)
+	glog.V(logger.Detail).Infof("deposited %d wei to chequebook (%s, balance: %v, target: %v)", amount, self.contractAddr.Hex(), self.balance, self.buffer)
+	return tx.Hash().Hex(), nil
+}
+
+// AutoDeposit (re)sets interval time and amount which triggers sending funds to the
+// chequebook. Contract backend needs to be set if threshold is not less than buffer, then
+// deposit will be triggered on every new cheque issued.
+func (self *Chequebook) AutoDeposit(interval time.Duration, threshold, buffer *big.Int) {
+	defer self.lock.Unlock()
+	self.lock.Lock()
+	self.threshold = threshold
+	self.buffer = buffer
+	self.autoDeposit(interval)
+}
+
+// autoDeposit starts a goroutine that periodically sends funds to the chequebook
+// contract caller holds the lock the go routine terminates if Chequebook.quit is closed.
+func (self *Chequebook) autoDeposit(interval time.Duration) {
+	if self.quit != nil {
+		close(self.quit)
+		self.quit = nil
+	}
+	// if threshold >= balance autodeposit after every cheque issued
+	if interval == time.Duration(0) || self.threshold != nil && self.buffer != nil && self.threshold.Cmp(self.buffer) >= 0 {
+		return
+	}
+
+	ticker := time.NewTicker(interval)
+	self.quit = make(chan bool)
+	quit := self.quit
+	go func() {
+	FOR:
+		for {
+			select {
+			case <-quit:
+				break FOR
+			case <-ticker.C:
+				self.lock.Lock()
+				if self.balance.Cmp(self.buffer) < 0 {
+					amount := new(big.Int).Sub(self.buffer, self.balance)
+					txhash, err := self.deposit(amount)
+					if err == nil {
+						self.txhash = txhash
+					}
+				}
+				self.lock.Unlock()
+			}
+		}
+	}()
+	return
+}
+
+// Outbox can issue cheques from a single contract to a single beneficiary.
+type Outbox struct {
+	chequeBook  *Chequebook
+	beneficiary common.Address
+}
+
+// NewOutbox creates an outbox.
+func NewOutbox(chbook *Chequebook, beneficiary common.Address) *Outbox {
+	return &Outbox{chbook, beneficiary}
+}
+
+// Issue creates cheque.
+func (self *Outbox) Issue(amount *big.Int) (interface{}, error) {
+	// TODO(fjl): the return type should be more descriptive.
+	return self.chequeBook.Issue(self.beneficiary, amount)
+}
+
+// AutoDeposit enables auto-deposits on the underlying chequebook.
+func (self *Outbox) AutoDeposit(interval time.Duration, threshold, buffer *big.Int) {
+	self.chequeBook.AutoDeposit(interval, threshold, buffer)
+}
+
+// Stop helps satisfy the swap.OutPayment interface.
+func (self *Outbox) Stop() {}
+
+// String implements fmt.Stringer.
+func (self *Outbox) String() string {
+	return fmt.Sprintf("chequebook: %v, beneficiary: %s, balance: %v", self.chequeBook.Address().Hex(), self.beneficiary.Hex(), self.chequeBook.Balance())
+}
+
+// Inbox can deposit, verify and cash cheques from a single contract to a single
+// beneficiary. It is the incoming payment handler for peer to peer micropayments.
+type Inbox struct {
+	lock        sync.Mutex
+	contract    common.Address              // peer's chequebook contract
+	beneficiary common.Address              // local peer's receiving address
+	sender      common.Address              // local peer's address to send cashing tx from
+	signer      *ecdsa.PublicKey            // peer's public key
+	txhash      string                      // tx hash of last cashing tx
+	abigen      bind.ContractBackend        // blockchain API
+	session     *contract.ChequebookSession // abi contract backend with tx opts
+	quit        chan bool                   // when closed causes autocash to stop
+	maxUncashed *big.Int                    // threshold that triggers autocashing
+	cashed      *big.Int                    // cumulative amount cashed
+	cheque      *Cheque                     // last cheque, nil if none yet received
+}
+
+// NewInbox creates an Inbox. An Inboxes is not persisted, the cumulative sum is updated
+// from blockchain when first cheque is received.
+func NewInbox(prvKey *ecdsa.PrivateKey, contractAddr, beneficiary common.Address, signer *ecdsa.PublicKey, abigen bind.ContractBackend) (self *Inbox, err error) {
+	if signer == nil {
+		return nil, fmt.Errorf("signer is null")
+	}
+	chbook, err := contract.NewChequebook(contractAddr, abigen)
+	if err != nil {
+		return nil, err
+	}
+	transactOpts := bind.NewKeyedTransactor(prvKey)
+	transactOpts.GasLimit = gasToCash
+	session := &contract.ChequebookSession{
+		Contract:     chbook,
+		TransactOpts: *transactOpts,
+	}
+	sender := transactOpts.From
+
+	self = &Inbox{
+		contract:    contractAddr,
+		beneficiary: beneficiary,
+		sender:      sender,
+		signer:      signer,
+		session:     session,
+		cashed:      new(big.Int).Set(common.Big0),
+	}
+	glog.V(logger.Detail).Infof("initialised inbox (%s -> %s) expected signer: %x", self.contract.Hex(), self.beneficiary.Hex(), crypto.FromECDSAPub(signer))
+	return
+}
+
+func (self *Inbox) String() string {
+	return fmt.Sprintf("chequebook: %v, beneficiary: %s, balance: %v", self.contract.Hex(), self.beneficiary.Hex(), self.cheque.Amount)
+}
+
+// Stop quits the autocash goroutine.
+func (self *Inbox) Stop() {
+	defer self.lock.Unlock()
+	self.lock.Lock()
+	if self.quit != nil {
+		close(self.quit)
+		self.quit = nil
+	}
+}
+
+// Cash attempts to cash the current cheque.
+func (self *Inbox) Cash() (txhash string, err error) {
+	if self.cheque != nil {
+		txhash, err = self.cheque.Cash(self.session)
+		glog.V(logger.Detail).Infof("cashing cheque (total: %v) on chequebook (%s) sending to %v", self.cheque.Amount, self.contract.Hex(), self.beneficiary.Hex())
+		self.cashed = self.cheque.Amount
+	}
+	return
+}
+
+// AutoCash (re)sets maximum time and amount which triggers cashing of the last uncashed
+// cheque if maxUncashed is set to 0, then autocash on receipt.
+func (self *Inbox) AutoCash(cashInterval time.Duration, maxUncashed *big.Int) {
+	defer self.lock.Unlock()
+	self.lock.Lock()
+	self.maxUncashed = maxUncashed
+	self.autoCash(cashInterval)
+}
+
+// autoCash starts a loop that periodically clears the last check
+// if the peer is trusted. Clearing period could be 24h or a week.
+//
+// The caller must hold self.lock.
+func (self *Inbox) autoCash(cashInterval time.Duration) {
+	if self.quit != nil {
+		close(self.quit)
+		self.quit = nil
+	}
+	// if maxUncashed is set to 0, then autocash on receipt
+	if cashInterval == time.Duration(0) || self.maxUncashed != nil && self.maxUncashed.Cmp(common.Big0) == 0 {
+		return
+	}
+
+	ticker := time.NewTicker(cashInterval)
+	self.quit = make(chan bool)
+	quit := self.quit
+	go func() {
+	FOR:
+		for {
+			select {
+			case <-quit:
+				break FOR
+			case <-ticker.C:
+				self.lock.Lock()
+				if self.cheque != nil && self.cheque.Amount.Cmp(self.cashed) != 0 {
+					txhash, err := self.Cash()
+					if err == nil {
+						self.txhash = txhash
+					}
+				}
+				self.lock.Unlock()
+			}
+		}
+	}()
+	return
+}
+
+// Receive is called to deposit the latest cheque to the incoming Inbox.
+// The given promise must be a *Cheque.
+func (self *Inbox) Receive(promise interface{}) (*big.Int, error) {
+	// TODO(fjl): the type of promise should be safer
+	ch := promise.(*Cheque)
+
+	defer self.lock.Unlock()
+	self.lock.Lock()
+
+	var sum *big.Int
+	if self.cheque == nil {
+		// the sum is checked against the blockchain once a check is received
+		tally, err := self.session.Sent(self.beneficiary)
+		if err != nil {
+			return nil, fmt.Errorf("inbox: error 	calling backend to set amount: %v", err)
+		}
+		sum = tally
+	} else {
+		sum = self.cheque.Amount
+	}
+
+	amount, err := ch.Verify(self.signer, self.contract, self.beneficiary, sum)
+	var uncashed *big.Int
+	if err == nil {
+		self.cheque = ch
+
+		if self.maxUncashed != nil {
+			uncashed = new(big.Int).Sub(ch.Amount, self.cashed)
+			if self.maxUncashed.Cmp(uncashed) < 0 {
+				self.Cash()
+			}
+		}
+		glog.V(logger.Detail).Infof("received cheque of %v wei in inbox (%s, uncashed: %v)", amount, self.contract.Hex(), uncashed)
+	}
+
+	return amount, err
+}
+
+// Verify verifies cheque for signer, contract, beneficiary, amount, valid signature.
+func (self *Cheque) Verify(signerKey *ecdsa.PublicKey, contract, beneficiary common.Address, sum *big.Int) (*big.Int, error) {
+	glog.V(logger.Detail).Infof("verify cheque: %v - sum: %v", self, sum)
+	if sum == nil {
+		return nil, fmt.Errorf("invalid amount")
+	}
+
+	if self.Beneficiary != beneficiary {
+		return nil, fmt.Errorf("beneficiary mismatch: %v != %v", self.Beneficiary.Hex(), beneficiary.Hex())
+	}
+	if self.Contract != contract {
+		return nil, fmt.Errorf("contract mismatch: %v != %v", self.Contract.Hex(), contract.Hex())
+	}
+
+	amount := new(big.Int).Set(self.Amount)
+	if sum != nil {
+		amount.Sub(amount, sum)
+		if amount.Cmp(common.Big0) <= 0 {
+			return nil, fmt.Errorf("incorrect amount: %v <= 0", amount)
+		}
+	}
+
+	pubKey, err := crypto.SigToPub(sigHash(self.Contract, beneficiary, self.Amount), self.Sig)
+	if err != nil {
+		return nil, fmt.Errorf("invalid signature: %v", err)
+	}
+	if !bytes.Equal(crypto.FromECDSAPub(pubKey), crypto.FromECDSAPub(signerKey)) {
+		return nil, fmt.Errorf("signer mismatch: %x != %x", crypto.FromECDSAPub(pubKey), crypto.FromECDSAPub(signerKey))
+	}
+	return amount, nil
+}
+
+// v/r/s representation of signature
+func sig2vrs(sig []byte) (v byte, r, s [32]byte) {
+	v = sig[64] + 27
+	copy(r[:], sig[:32])
+	copy(s[:], sig[32:64])
+	return
+}
+
+// Cash cashes the cheque by sending an Ethereum transaction.
+func (self *Cheque) Cash(session *contract.ChequebookSession) (string, error) {
+	v, r, s := sig2vrs(self.Sig)
+	tx, err := session.Cash(self.Beneficiary, self.Amount, v, r, s)
+	if err != nil {
+		return "", err
+	}
+	return tx.Hash().Hex(), nil
+}
+
+// ValidateCode checks that the on-chain code at address matches the expected chequebook
+// contract code. This is used to detect suicided chequebooks.
+func ValidateCode(ctx context.Context, b Backend, address common.Address) (ok bool, err error) {
+	code, err := b.CodeAt(ctx, address, nil)
+	if err != nil {
+		return false, err
+	}
+	return bytes.Equal(code, common.FromHex(contract.ContractDeployedCode)), nil
+}