Bitcoin: Synchronization andSharing of Transactions

Roger Wattenhofer

ETH Zurich – Distributed Computing Group – www.disco.ethz.ch

20102011

20122013

20142015

0

200

400

600

800

1000

1200

Pri

ce [

USD

/BTC

]

Exchange Rate USD/BTC

What is Bitcoin?

+ + =

Bitcoin Basics

The Bank of Bitcoin

User BalanceA 2B 5C 8

TXB −→ A

User BalanceA 2 4B 5 3C 8

The Bank of Bitcoin

User BalanceA 2B 5C 8

TXB −→ A

User BalanceA 2 4B 5 3C 8

The Bank of Bitcoin

User BalanceA 2B 5C 8

TXB −→ A

User BalanceA 2 4B 5 3C 8

The Bank of Bitcoin

User BalanceA 2B 5C 8

TXB −→ A

User BalanceA 2 4B 5 3C 8

Opening an Account in Bitcoin

Private Key Public Key Address

Transferring Bitcoins

TX: 41b221

B0.1

A4.798

A4.899

Inputs OutputsFee

0.001

Prev. TX:a1a53743

4.899

0

C...

1

|Outputs

Transferring Bitcoins

TX: 41b221

B0.1

A4.798

A4.899

Inputs OutputsFee

0.001

Prev. TX:a1a53743

4.899

0

C...

1

|Outputs

Transferring Bitcoins

TX: 41b221

B0.1

A4.798

A4.899

Inputs OutputsFee

0.001

Prev. TX:a1a53743

4.899

0

C...

1

|Outputs

Transferring Bitcoins

TX: 41b221

B0.1

A4.798

A4.899

Inputs OutputsFee

0.001

Prev. TX:a1a53743

4.899

0

C...

1

|Outputs

Transferring Bitcoins

TX: 41b221

B0.1

A4.798

A4.899

Inputs Outputs

Fee

0.001

Prev. TX:a1a53743

4.899

0

C...

1

|Outputs

Transferring Bitcoins

TX: 41b221

B0.1

A4.798

A4.899

Inputs OutputsFee

0.001

Prev. TX:a1a53743

4.899

0

C...

1

|Outputs

Transferring Bitcoins

TX: 41b221

B0.1

A4.798

A4.899

Inputs OutputsFee

0.001

Prev. TX:a1a53743

4.899

0

C...

1

|Outputs

Distributing the Bank

User BalanceA 2B 5C 8

TX

TX

Distributing the Bank

TX

Distributing the Bank

TX

Distributing the Bank

TX

Distributing the Bank

TX

Distributing the Bank

TX

Let’s Buy a Snack

[Bamert, Decker, Elsen, W, Welten, 2013]

Doublespending

TX B1

A

1

1

Inputs

Outputs

TX’ A1

1

�

Doublespending

TX B1

A

1

1

Inputs

Outputs

TX’ A1

1

�

Doublespending

TX B1

A

1

1

Inputs

Outputs

TX’ A1

1

�

Transaction Conflicts

TX

TX

Transaction Conflicts

TX

TX

Transaction Conflicts

TX

TX

Transaction Conflicts

TX

TX

Resolving Conflicts

Green!

Resolving Conflicts

Green!

Resolving Conflicts

Green!

How to Choose a Leader?

Proof-of-Work

Block

H(Previous Block)

TX TX TX TX

I H(Block|0) → 094d66aa7c844a9dbb516a41259b5877. . .

I H(Block|1) → f2496854af8bf989171587a9259f634f. . .

I H(Block|2) → aec87c0ca2e5eb3f23111092f1089ada. . .

I H(Block|3) → 777f75b2a8ecfdc8026c236fc1d2ffa0. . ....

I H(Block|961127) → 0000014823419622d4c133672a7d657e. . .

Proof-of-Work

Block

H(Previous Block)

TX TX TX TX

I H(Block|0) → 094d66aa7c844a9dbb516a41259b5877. . .

I H(Block|1) → f2496854af8bf989171587a9259f634f. . .

I H(Block|2) → aec87c0ca2e5eb3f23111092f1089ada. . .

I H(Block|3) → 777f75b2a8ecfdc8026c236fc1d2ffa0. . ....

I H(Block|961127) → 0000014823419622d4c133672a7d657e. . .

Proof-of-Work

Block

H(Previous Block) TX TX TX TX

I H(Block|0) → 094d66aa7c844a9dbb516a41259b5877. . .

I H(Block|1) → f2496854af8bf989171587a9259f634f. . .

I H(Block|2) → aec87c0ca2e5eb3f23111092f1089ada. . .

I H(Block|3) → 777f75b2a8ecfdc8026c236fc1d2ffa0. . ....

I H(Block|961127) → 0000014823419622d4c133672a7d657e. . .

Proof-of-Work

Block

H(Previous Block) TX TX TX TX

I H(Block) → fd2e2055f117bfa261b5a6c7e11df367. . .

I H(Block|0) → 094d66aa7c844a9dbb516a41259b5877. . .

I H(Block|1) → f2496854af8bf989171587a9259f634f. . .

I H(Block|2) → aec87c0ca2e5eb3f23111092f1089ada. . .

I H(Block|3) → 777f75b2a8ecfdc8026c236fc1d2ffa0. . ....

I H(Block|961127) → 0000014823419622d4c133672a7d657e. . .

Proof-of-Work

Block

H(Previous Block) TX TX TX TX Nonce

I H(Block|0) → 094d66aa7c844a9dbb516a41259b5877. . .

I H(Block|1) → f2496854af8bf989171587a9259f634f. . .

I H(Block|2) → aec87c0ca2e5eb3f23111092f1089ada. . .

I H(Block|3) → 777f75b2a8ecfdc8026c236fc1d2ffa0. . ....

I H(Block|961127) → 0000014823419622d4c133672a7d657e. . .

Proof-of-Work

Block

H(Previous Block) TX TX TX TX Nonce

I H(Block|0) → 094d66aa7c844a9dbb516a41259b5877. . .

I H(Block|1) → f2496854af8bf989171587a9259f634f. . .

I H(Block|2) → aec87c0ca2e5eb3f23111092f1089ada. . .

I H(Block|3) → 777f75b2a8ecfdc8026c236fc1d2ffa0. . ....

I H(Block|961127) → 0000014823419622d4c133672a7d657e. . .

Proof-of-Work

Block

H(Previous Block) TX TX TX TX Nonce

I H(Block|0) → 094d66aa7c844a9dbb516a41259b5877. . .

I H(Block|1) → f2496854af8bf989171587a9259f634f. . .

I H(Block|2) → aec87c0ca2e5eb3f23111092f1089ada. . .

I H(Block|3) → 777f75b2a8ecfdc8026c236fc1d2ffa0. . ....

I H(Block|961127) → 0000014823419622d4c133672a7d657e. . .

Proof-of-Work

Block

H(Previous Block) TX TX TX TX Nonce

I H(Block|0) → 094d66aa7c844a9dbb516a41259b5877. . .

I H(Block|1) → f2496854af8bf989171587a9259f634f. . .

I H(Block|2) → aec87c0ca2e5eb3f23111092f1089ada. . .

I H(Block|3) → 777f75b2a8ecfdc8026c236fc1d2ffa0. . ....

I H(Block|961127) → 0000014823419622d4c133672a7d657e. . .

The Blockchain

Time

The Blockchain

Time

Is Bitcoin stable?

The Blockchain

Time

The Blockchain

Time

Propagation Speed

0 10 20 30 40 50 60Time since first observation [s]

0.00

0.02

0.04

0.06

0.08

0.10

0.12PD

FBlock propagation

[Decker, W, 2013]http://bitcoinstats.com

Propagation Speed

0 10 20 30 40 50 60Time since first observation [s]

0.00

0.02

0.04

0.06

0.08

0.10

0.12PD

FBlock propagation

50thp

erc.[Decker, W, 2013]http://bitcoinstats.com

Propagation Speed

0 10 20 30 40 50 60Time since first observation [s]

0.00

0.02

0.04

0.06

0.08

0.10

0.12PD

FBlock propagation

50thp

erc.

95thp

erc.

[Decker, W, 2013]http://bitcoinstats.com

Propagation Speed

0 10 20 30 40 50 60Time since first observation [s]

0.00

0.02

0.04

0.06

0.08

0.10

0.12PD

FBlock propagation

[Decker, W, 2013]http://bitcoinstats.com

Blockchain Forks

180000 182000 184000 186000 188000 190000Blockchain Height

0

2

4

6

8

10

12

Fork

sBlockchain forks

1.69%

[Decker, W, 2013]

Aside: Mining Evolution

20102011

20122013

20142015

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

Hash

rate

PH

/s

Hashrate evolution

Aside: Mining Evolution

20102011

20122013

20142015

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

Hash

rate

PH

/s

Hashrate evolution

Aside: Mining Evolution

20102011

20122013

20142015

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

Hash

rate

PH

/s

Hashrate evolution

Summary

TX

Green!

Block

H(Previous Block) TX TX TX TX Nonce

Time

How to Lose $500M

Addressing Transaction Malleability: MtGox has detectedunusual activity on its Bitcoin wallets and performedinvestigations during the past weeks.

The MtGox Incident

I July 2010: First trade on MtGox

I May 2011: Transaction malleability identified as low priority issue

I February 7, 2014: MtGox halts withdrawals

I February 10, 2014: MtGox announces loss of 850,000 bitcoins (620millio USD) and cites transaction malleability as root cause

I February 28, 2014: MtGox files for bankruptcy

I March 7 2014: MtGox finds 200,000 bitcoins

I August 2015: MtGox CEO is arrested

Signatures

00 00

61 af bb 4d e9 f8 b8 74 86 1e

There are multiple ways to serialize a signature:

I Multiple push operations (1 byte, 2 byte, 4 byte)

I Non-canonical DER encodings

I Padding

I . . .

Signatures

00 00 61 af bb 4d e9 f8 b8 74 86 1e

There are multiple ways to serialize a signature:

I Multiple push operations (1 byte, 2 byte, 4 byte)

I Non-canonical DER encodings

I Padding

I . . .

Transaction Malleability Attack

TX

TX

TX

TX

TX

Red!

TX?Refund

Transaction Malleability Attack

TX

TX

TX

TX

TX

Red!

TX?Refund

Transaction Malleability Attack

TX

TX

TX

TX

TX

Red!

TX?Refund

Transaction Malleability Attack

TX

TX

TX

TX

TX

Red!

TX?Refund

Transaction Malleability Attack

TX

TX

TX

TX

TX

Red!

TX?

Refund

Transaction Malleability Attack

TX

TX

TX

TX

TX

Red!

TX?

Refund

Incident Timeline

Feb 01 2014

Feb 04 2014

Feb 07 2014

Feb 10 2014

Feb 13 2014

Feb 16 2014

Feb 19 2014

Feb 22 2014

Feb 25 2014

Feb 28 20140

50000

100000

150000

200000

250000

300000

350000

bit

coin

s

0

5000

10000

15000

20000

25000

30000

transa

ctio

ns

1st

Pre

ss R

ele

ase

2nd P

ress

Rele

ase

Cumulative malleable doublespends

value

number

386 BTC

[Decker, W, 2014]

Incident Timeline

Feb 01 2014

Feb 04 2014

Feb 07 2014

Feb 10 2014

Feb 13 2014

Feb 16 2014

Feb 19 2014

Feb 22 2014

Feb 25 2014

Feb 28 20140

50000

100000

150000

200000

250000

300000

350000

bit

coin

s

0

5000

10000

15000

20000

25000

30000

transa

ctio

ns

1st

Pre

ss R

ele

ase

2nd P

ress

Rele

ase

Cumulative malleable doublespends

value

number

386 BTC

[Decker, W, 2014]

Is Bitcoin Secure?

Securing Your Bitcoins

[Bamert, Decker, W, 2013]

Does Bitcoin Scale?

The Bitcoin Ecosystem is Growing

20102011

20122013

20142015

0

20000

40000

60000

80000

100000

120000

140000

Tra

nsa

ctio

ns

Daily Transaction Volume

Scalability Limits

I Disk space: < 500 transactions per second

I Processing power: < 200 transactions per second

I Network bandwidth: < 100 transactions per second

I Artificial 1MB limit: < 3 transactions per second

Today:

I Bitcoin: 1 transaction per second

I Credit Cards: > 10, 000 transactions per second

Scalability Limits

I Disk space: < 500 transactions per second

I Processing power: < 200 transactions per second

I Network bandwidth: < 100 transactions per second

I Artificial 1MB limit: < 3 transactions per second

Today:

I Bitcoin: 1 transaction per second

I Credit Cards: > 10, 000 transactions per second

Scalability Limits

I Disk space: < 500 transactions per second

I Processing power: < 200 transactions per second

I Network bandwidth: < 100 transactions per second

I Artificial 1MB limit: < 3 transactions per second

Today:

I Bitcoin: 1 transaction per second

I Credit Cards: > 10, 000 transactions per second

Scalability Limits

I Disk space: < 500 transactions per second

I Processing power: < 200 transactions per second

I Network bandwidth: < 100 transactions per second

I Artificial 1MB limit: < 3 transactions per second

Today:

I Bitcoin: 1 transaction per second

I Credit Cards: > 10, 000 transactions per second

Scalability Limits

I Disk space: < 500 transactions per second

I Processing power: < 200 transactions per second

I Network bandwidth: < 100 transactions per second

I Artificial 1MB limit: < 3 transactions per second

Today:

I Bitcoin: 1 transaction per second

I Credit Cards: > 10, 000 transactions per second

Payment Network

Payment Network

Payment Network

Micropayment Channels

5

5

T=100 5

5

0

4

1

3

2

Micropayment Channels

5 5

T=100 5

5

0

4

1

3

2

Micropayment Channels

5 5

T=100 5

5

0

4

1

3

2

Micropayment Channels

5 5

T=100 5

5

0

4

1

3

2

Micropayment Channels

5 5

T=100 5

5

0

4

1

3

2

Micropayment Channels

5 5

T=100 5

5

0

4

1

3

2

Micropayment Channels

5 5

T=100 5

5

0

4

1

3

2

Micropayment Channels

5 5

T=100 5

5

0

4

1

3

2

Micropayment Channels

5 5

T=100 5

5

0

4

1

3

2

Atomic Multiparty Opt-In

Atomic Multiparty Opt-In

Atomic Multiparty Opt-In

Invalidating Transactions

T=100

T=99

Invalidating Transactions

T=100

T=99

Bidirectional Transfers

5 5

5

0

4 4

4

0

0

5

1

3

Bidirectional Transfers

5 5

5

0

4 4

4

0

0

5

1

3

Bidirectional Transfers

5 5

5

0

4 4

4

0

0

5

1

3

Duplex Micropayment Channels

Setup Invalidation Tree Micropayment Channels

T = 100 T = 100 T = 100

T = 99 T = 100 T = 100

T = 99 T = 100

T = 99

Duplex Micropayment Channels

Setup Invalidation Tree Micropayment Channels

T = 100 T = 100 T = 100

T = 99 T = 100 T = 100

T = 99 T = 100

T = 99

Duplex Micropayment Channels

Setup Invalidation Tree Micropayment Channels

T = 100 T = 100 T = 100

T = 99 T = 100 T = 100

T = 99 T = 100

T = 99

Duplex Micropayment Channels

Setup Invalidation Tree Micropayment Channels

T = 100 T = 100 T = 100

T = 99 T = 100 T = 100

T = 99 T = 100

T = 99

Duplex Micropayment Channels

Setup Invalidation Tree Micropayment Channels

T = 100 T = 100 T = 100

T = 99 T = 100 T = 100

T = 99 T = 100

T = 99

Summary

Red!

TX?Refund

T = 100 T = 100 T = 100

T = 99 T = 100 T = 100

T = 99 T = 100

T = 99

Thank you, questions?

Thanks to Christian Decker

ETH Zurich – Distributed Computing Group – www.disco.ethz.ch

Securing Fast Payments

Let’s Buy a Snack

[Bamert, Decker, Elsen, W, Welten, 2013]

Transaction Confidence

TX

confidence(TX ) =

Transaction Confidence

TX

confidence(TX ) =

Transaction Confidence

TX

confidence(TX ) =

Transaction Confidence

TX

confidence(TX ) =

Transaction Confidence

TX

confidence(TX ) =

Doublespend Detection

0 20 40 60 80 100Node sample size

0.0

0.2

0.4

0.6

0.8

1.0D

ete

ctio

n p

robabili

tyDoublespend detection

[Bamert, Decker, Elsen, W, Welten, 2013]

Time to Detection

20 40 60 80 100Node sample size

0

2

4

6

8

10

12

14

Tim

e [

s]Time until detection

Average time

Median time

95 percentile

99 percentile

[Bamert, Decker, Elsen, W, Welten, 2013]

Successful Doublespend

0 20 40 60 80 100Node sample size

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

Pro

babili

tyProbability of successful double spend

[Bamert, Decker, Elsen, W, Welten, 2013]

Successful Doublespend

0 20 40 60 80 100Node sample size

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

Pro

babili

tyProbability of successful double spend

0.088%

[Bamert, Decker, Elsen, W, Welten, 2013]