Slides for the talk given at CodeFreeze Meetup, Dec 2016, Saint-Petersburg

1. CodeFreeze Meetup, Dec `16
Blockchain For Developers
Alexander Chepurnoy
(aka kushti)
@chepurnoy
IOHK Research

2. Background
● Nxt core developer
● smartcontract.com cofounder (left)
● Scorex since late 2014
● IOHK Research

3. The talk is about
● How a developer can view a blockchain system
● What are open problems in blockchains

4. Blockchain
● Public append-log
● Readable by all
● Writeable by all
● Tamper-resistant
● Authenticated content

5. Use Cases
● E-Cash
● Titles
● Stocks settlement

6. Digital Signature
● generate(seed): PublicKey → PrivateKey
● sign(message, privateKey): Signature
● verify(publicKey, signature, message): Boolean
We assume a scheme doesn't allow to get a signature for
a new message using old message→signature pairs
Examples: RSA, ECDSA, EdDSA

7. Cryptographic Hash Function
hash(preimage) → value
● arbitrary-sized preimage
● fixed-sized random output
● Infeasible to find a preimage or a collision

8. Merkle Tree

9. Background: E-Cash
● Alice is sending a coin to Bob
● A coin is just a bitstring
● How to prevent double-spend?
● How to make money transfers untrackable?

10. Background: E-Cash
● David Chaum, mid-1980s
● E-Cash
● Anonymous
● Central Bank required
● DigiCash (bankruptcy in 1998)
● a lot of academic research (not turned out to be practical)

11. Distributed Consensus
● Fixed number of known parties
● 0/1 inputs
Properties:
● Agreement – all the parties agree on the same output
● Validity – an output must be some node's input
● Termination – the protocol somewhen finishes

12. Failure Models
● Crash-Stop
● Crash-Recovery
● Byzantine – arbitrary behaviour

13. Impossibility Results
● FLP theorem (Fischer et al, 1983) – a deterministic
consensus protocol doesn't have a guarantee of
termination in case of asynchronous messaging and a
possibility of a single node to fail
● Byzantine Generals problem - it there are t Byzantine
parties amongst n, agreement is possible only if n > 3t and
the communication is synchronous (bounded delay)

14. Environment
● P2P network
● No central party
● Many parties are possibly malicious
● Probabilistic broadcast

15. P2P network
● Each node has own view
● The goal is to have replicated subset of it across the
network
● In the presence of Byzantine adversaries!
● (so only honest nodes agree on the state)
● (and only eventually)

16. A transaction
● Atomic state modifier
● Authenticated

17. Minimal State
● Can answer the question „whether a transaction is valid
and so applicable“
● apply(min_state, tx): (MinimalState | Error)
● apply(apply(min_state, tx), tx) is always Error
● In Bitcoin UTXO set

18. Minimal State
● Transaction application is deterministic
● There's some initial (genesis) state hardcoded
● By applying the same sequence of transactions to the
genesis state, two honest nodes got the same minimal
state
● Thus we need for a guarantee every pair of honest nodes
is eventually applying the same sequence of transactions!

19. Blockchain
● Transactions packed into blocks
● Blocks are linked

20. Blockchain

21. Block Generator Election
● random party
● sybil-resistant
● efficient (min communication)
solution
● each party has limited queries to an oracle function
● random oracle answers „yes“ with adjustable probability
● replace random oracle with a hash function

22. Block Header
02000000b6ff0b1b1680a2862a30ca44d346d9e8910d334beb48ca0
c00000000000000009d10aa52ee949386ca9385695f04ede270dd
a20810decd12bc9b048aaab3147124d95a5430c31b18fe9f0864
● version (2)
● hash of a previous header
● Merkle root hash of transactions
● timestamp
● target
● nonce

23. Bitcoin's Proof-of-Work
● hash(blockheader) < target
● target = 1 / difficulty
● difficulty recalculated every M blocks
● difficulty Di
= Di-1
* M * texp
/ Tobserved
● texp
= 10 minutes
● M = 2016

24. Rewards
● Why to perform work?
● Miner creates a transaction to self with a fixed amount
● This is the 1st transaction in the Merkle tree (coinbase tx)

25. Race For a Block
● Everyone can participate
● Chance to win is hashpower(mine) / hashpower(total)
(well, not really)
● Collisions are possible (two blocks at about same time)
● Eventually longest chain wins
(well, not really)

26. Break!
● Questions?

27. GKL Model
● „The Bitcoin Backbone Protocol:Analysis and Applications“
by Garay / Kiayias / Leonardos
● slides: https://bitcoinschool.gr/slides/session2.pdf
Bitcoin consensus protocol properties:
● Common Prefix
● Chain Quality
● Chain Growth

28. Common Prefix
no matter the strategy of the adversary, the chains of two
honest parties will fork in the last k blocks with probability
exponentially decreasing with k

29. Chain Quality
any sequence of blocks in an honest party’s chain will
contain some number of honest blocks with overwhelming
probability

30. Chain Growth
honest party's chain grows with some minimal pace with
an overwhelming probability

31. Bitcoin
● digital cash
● transaction is a set of token transfers

32. Bitcoin: Transaction

33. Bitcoin Script
output: OP_DUP OP_HASH160 <pubKeyHash>
OP_EQUALVERIFY OP_CHECKSIG
input: <sig> <pubKey>

34. Bitcoin Addresses
● There are no any accounts in Bitcoin
● In most cases address is PKH (160 bits) + checksum +
additional info, in Base58 encoding

35. Bitcoin: UTXO set
● unspent outputs set
● enough to validate any transaction
● application is about removing outputs spent and add new
ones

36. Memory Pool
● contains unconfirmed transaction
● inconsistent across a network

37. Vault (Wallet)
● node-specific information
● e.g. transactions for selected pubkeys

38. Node view
(MinimalState, Blockchain, MemoryPool, Wallet)
(MinimalState, Blockchain) is eventually the same for all
the honest nodes

39. What app developer should know
● Rollbacks are possible!
● Transaction is always visible before inclusion
● Frontrunning / replay attacks
● Signature and transaction malleability

40. Modifications
● alternative consensus protocols (Proof-of-Stake etc)
● richer transactional models (NameCoin, Ethereum, ZCash)
● alternative log structures (Bitcoin-NG, GHOST,
TwinsChain)
● incentivization of certain activities (Permacoin, Rollerchain)

41. Scalability

42. Basic assumption
● It should be possible to run a fullnode on a commodity
hardware
● HDD
● 1-2 GB RAM
● 1 Mbps at most
● Ethereum lost
● Bitcoin is doing hard to hold the assumption

43. Bitcoin's Troughput (TPS)
● 7 ??? no
● 2-3 in fact
● 1/600 in worse case
https://www.reddit.com/r/Bitcoin/comments/3cgft7/largest_trans

44. Better throughput
● Bitcoin-NG
● GHOST/SPECTRE

45. Blockchain Pruning

46. Rollerchain
● Chepurnoy, Larangeira, Ozhiganov
http://arxiv.org/abs/1603.07926
„Rollerchain, a Blockchain Secure in the Rational Storage
Setting“

47. Rollerchain
● Only last n full blocks to be stored collectively
and n state snapshots
● Each miner stores k state snapshots

48. Rollerchain
● New node can download a historical snapshot
● Fullblocks not needed for mining could be thrown away
● Blockheaders are to be stored forever, so must be small

49. Unload the chain
● Move things off-chain
● Sidechains
● Avoid all the transactions execution(RsCoin)

50. Offchain
● Lightning Network
● Offchain contracts(SMC/SMP)

51. Smart Contracts
● Ethereum isn't scalable
● Hawk
● Enigma

52. Dark side of a public blockchain

53. Slowing down processing
● Bitcoin: CVE-2013-2293 (fetching outputs from hdd)
● Ethereum: most of recent attacks (fetching account states)

54. Asymmetric schemes
● Not neccessary to hold the whole state
● Full security guarantees
● Reyzin, Meshkov, Chepurnoy, Ivanov
„Improving Authenticated Dynamic Dictionaries, with
Applications to Cryptocurrencies“
https://eprint.iacr.org/2016/994

55. Asymmetric vs Symmetric

56. AVL+ trees vs Ethereum‘s tries

57. Rational Behaviour
● Why store blocks for years after processing?
● Why to validate blocks (in PoW)?
● Why to work on a single chain (in PoS)?

58. Validationless (SPV) mining
● Start to mine on a header
● Trust other nodes regarding transactions
● https://bitcoin.org/en/alert/2015-07-04-spv-mining - 6
blocks starting with an invalid one

59. If no block reward
Carlsten, Kalodner, Weinberg, Narayan
„On the Instability of Bitcoin Without the Block Reward“
http://randomwalker.info/publications/mining_CCS.pdf

60. Advices
● Don‘t roll your own crypto
● Don‘t roll own blockchain

61. Scorex
● Scorex
● Utterly abstract (no even a blockchain in the bottom)
● GitHub
https://github.com/ScorexFoundation/Scorex
● Manual (not fully ready):
https://github.com/ScorexFoundation/ScorexTutorial

62. Scorex

63. Questions?
Twitter: @chepurnoy
Mail: kushti@protonmail.ch