aboutsummaryrefslogtreecommitdiffstats
path: root/contracts/chequebook/cheque.go
blob: 5ece1391b5d9c55b3cad2a87bf110fd6a0f70216 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
// 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"
    "github.com/ethereum/go-ethereum/swarm/services/swap/swap"
    "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) (swap.Promise, error) {
    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 swap.Promise) (*big.Int, error) {
    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
}