bitcoins transaction processing yonatan sompolinsky joint work with aviv zohar

Bitcoin’s Transaction Processing

Yonatan Sompolinsky

joint work with Aviv Zohar

Bitcoin

hard to get

easy to pass

no authority, no money printing

pseudonymous

relatively secure

? high transaction volume

decentralized cryptocurrency

current protocol: conflictghost protocol: in accord

Transactions in Bitcoin

tx

tx

tx

tx

tx

tx

tx

tx

tx txtx

tx

txtx tx

Bob Calvin

? irreversible

tx’

tx’

tx’

tx’

tx’

tx’

txs:

block #81 block #82 block #83

The Blockchain

txs: txs:

~ 10 min

• consistent

• grows in time

• kept locally at each node

Time

A

B2

B1

maintain one main chain only

blocks discarded & txs cancelled

Conflicting blockchains

the”longest chain”

rule

delays => forks

Time

Block tree vs Block chain

1

3

2

5

4 6

λ:= block creation rate =

6 per hour

β:= growth rate of main chain =

4 per hour

efficiency:= β/λ=4/6<1

Time

delays => forks =>

inefficiency

Block size, Delay & Efficiency

analysis of data provided to us by Decker and Wattenhofer

λ = block creation rate

β = growth rate of main chain

0

𝛼𝑚𝑖𝑛

0.95 Efficiency =

0

Block Propagation Time

𝐵𝑙𝑜𝑐𝑘𝑆𝑖𝑧𝑒 (𝐾𝐵)Tim

e(s

ec

)

Throughput (TPS)

Transactions Processed per Second =

(block size) X (growth rate of the main chain) efficiency

2000 1/600

TPS ~ 1736

TPS < 3.3why?

Security - double spend attack

tx’

tx

Bob Calvin

tx

secret

tx

main chain

publish !

Success of attack

• computational power needed to create blocks

• share of blocks proportional to fraction of

computational power

• an attacker with computational power will eventually

fail

• conversely, an attacker with will always succeed

• security threshold is thus

Effect of increasing throughput

↑ efficiency↓

security ↓TPS ↑

delay ↑

Security limitation on TPS

attacker computational power (fraction)

TPS

Bitcoin today: secure butlow throughput

high TPS: more bandwidth needed

secure TPS

vulnerable TPS

Greedy Heaviest Observed Sub-Tree

longest chainefficiency ↓ security ↓

Greedy Heaviest Observed Sub-Tree

B

C1

A

C2

C3

D1

D2

D3 E1

tx’

B tx

main chain according to “longest” rule

main chain according to “longest” rule

104

5

1

1

1

2

6

main chain according to “ghost”

Time

GHOST throughput

• no security constraint

• hardware constraints, e.g., network congestion, limit the feasible TPS

• 8.5-fold increase of current bandwidth can withstandTPS = 2000

• in pipeline: improvement of ghost, same bandwidth more TPS

…

security

high transaction volume

https://eprint.iacr.org/2013/881.pdf