Cryptocurrency and Blockchain Research

Transcription

1 Cryptocurrency and Blockchain Research CHANATHIP NAMPREMPRE, PH.D. 1

2 Agenda Recall bitcoin consensus protocol Components of bitcoin consensus protocol Variations on the same theme Beware of snake oil Solution: secure protocol design and analysis Example from cryptography: modes of operation Conclusion 2

3 Recall bitcoin blockchain: adding a block The Times 03/Jan/2009 Chancellor on brink of second bailout for banks ph nonce H( ) tx1 tx2 txn(reward) ph nonce H( ) tx1 tx2 txn(reward) To add the next block, find nonce such that 00..0xxxxxx xxxx = H( nonce ph tx1 tx2 ) Genesis block 3

4 Recall bitcoin blockchain: forking The Times bailout for banks ph H( ) ph H( ) tx1 ph H( ) ph H( ) Genesis block ph H( ) tx1 4

5 Bitcoin consensus protocol: GOAL The Times 03/Jan/2009 Chancellor bailout for banks ph H( ) tx tx ph H( ) tx tx ph H( ) tx tx All honest participants agree on the order in which blocks are appended to the chain. 5

6 Bitcoin consensus protocol Transactions record money transfers. 1. New transactions are broadcast to all nodes. 2. Each node collects new transactions into blocks. Leader election 3. In each round, a winning node gets to add its block to the chain. 4. Other nodes accept the block only if all transactions in it are valid. 5. Nodes express their approval of the block by including its hash in the next block they create. Ledger is a blockchain. source: Bitcoin and Cryptocurrency Technology, Narayanan, et. al

7 Transaction Bitcoin Bitcoin: money transfer Transaction ID 68 in: H( ), index, signature, pk H( ), index, signature, pk H( ), index, signature, pk out: index, value, H(pk) index, value, H(pk) index, value, H(pk) referenced transaction scriptsig scriptpubsig Transaction ID 42 in: H( ), index, signature, pk H( ), index, signature, pk H( ), index, signature, pk out: index, value, H(pk) index, value, H(pk) Transaction ID 73 in: H( ), index, signature, pk H( ), index, signature, pk H( ), index, signature, pk out: index, value, H(pk) index, value, H(pk) index, value, H(pk) redeeming transaction Alternatives: smart contracts medical records college degrees referenced transaction 7 7

8 Ledger The Times 03/Jan/2009 Chancellor bailout for banks Bitcoin ph H( ) tx tx ph H( ) tx tx ph H( ) tx tx Bitcoin: chain of blocks ledger = blockchain Alternatives: individual transactions ledger = Directed Acyclic Graph (GHOST, SPECTRE, Meshcash, Hashgraph, etc.) batched transactions (IOTA) 8

9 Leader election Bitcoin To add a block to the chain, find nonce such that 00..0xxxxxx xxxx = H( nonce ph tx1 tx2 ) Bitcoin: proof-of-work Alternatives: proof-of-stake (Ouroboros, Algorand) proof-of-elapsed-time (Sawtooth Lake) proof-of-luck (Berkeley) 9

10 Leader election via Proof-of-Elasped- Time (PoET) Observation: Bitcoin proof-of-work, namely the nonce, is evidence that the winning node has taken about 10 minutes to find a nonce that works. Alternative: 00..0xxxxxx xxxx = H( nonce ph tx1 tx2 ) A node can just wait 10 minutes then can provide a proof that it has waited. 10

11 Intel SGX (Software Guard extensions) Attestation: Prove to a remote party what software was loaded on my machine Applications: Bank allows money transfer only if customer has patched his device. Gamers can join network only if their game client is unmodified. Source: Dan Boneh s course notes. 11

12 Replacing nonce with attestation Suppose P is the program wait 10 minutes P Intel SGX attestation ph attestation H( ) tx1 tx2 txn(reward) Attestation server 12

13 Participation in protocol Bitcoin Bitcoin: permissionless with dynamic join/leave Alternatives: permissioned with static membership permissioned with semi-static membership (no join/leave during each round, but ok between rounds ) 13

14 Participation in leader election protocol Bitcoin Bitcoin: all mining nodes Alternatives: a fixed subset (e.g., Honey Badger) a dynamic subset [e.g., endorsers (Ouroboros) committee members (Algorand) validators (Tendermint) trusted quorums (Stellar), ] 14

15 Participation in leader election protocol Bitcoin Bitcoin: all mining nodes Alternatives: a fixed subset (e.g., Honey Badger) a dynamic subset [e.g., endorsers (Ouroboros) committee members (Algorand) validators (Tendermint) trusted quorums (Stellar), ] 15

16 Failure modes Bitcoin Bitcoin: Byzantine Alternatives: fail-stop (crash only) honest-but-curious honest-but-lazy (in this model, Algorand can tolerate failure of majority of nodes!) 16

17 Clocks Bitcoin Bitcoin: loosely synchronized (NTP) Alternatives: strongly synchronized same speed (Algorand) 17

18 Message delivery model Bitcoin Bitcoin: unreliable Alternatives: reliable with known delay (msgs are delivered correctly and exactly once within a certain amount of time) reliable with unknown delay dropped/ modified/ injected 18

19 Timing model Bitcoin fast slow Bitcoin: asynchronous (msgs can take arbitrarily long to arrive, and processor speeds can be vastly different) (different from the A in AJAX) Alternatives: synchronous (msg delays are bounded, and differences in processor speeds are bounded) eventually synchronous weakly synchronous strongly synchronous 19

20 Timing model [Dwork and Lynch, 1988] In a synchronous system, there is a known fixed upper bound A on the time required for a message to be sent from one processor to another and a known fixed upper bound B on the relative speeds of different processors. In an asynchronous system, no fixed upper bounds A and B exist. In an eventually synchronous system, the system is synchronous most of the time but may experience periods of asynchrony. 20

21 Consensus protocol example: Algorand Transaction: any Ledger: blockchain Leader election: proof-of-stake Participation: permissionless Participation in leader election: committee Failure modes: byzantine Message delivery: Timing: Clock: reliable (gossip protocol) strong synchrony (liveness) weak synchrony (safety) same speed 21

22 Recall Bitcoin consensus protocol 1. New transactions are broadcast to all nodes. 2. Each node collects new transactions into blocks. 3. In each round, a winning node gets to add its block to the chain. 4. Other nodes accept the block only if all transactions in it are valid. 5. Nodes express their approval of the block by extending the chain from it. a. Select committee members based on stakes b. Committee members vote on winner based on stakes 22

23 Algorand sortition a. Select committee members based on stakes Cannot be done deterministically Cannot be done in multiple steps Must be done in secret committee members may be targeted! b. Committee members vote on winner based on stakes 23

24 Verifiable Random Function (VRF) VRF = (F, VF) such that, for any public-secret key pair (pk,sk), (h,p) = F(sk,x) On input x, the algorithm F returns a hash and a proof. The hash h is random to anyone who does not know sk. b = VF(pk,x,h,p) On input pk, x, h, and p, the algorithm VF returns a bit. The proof p lets anyone with pk verify that h corresponds to x. 24

25 Implementing VRF with digital signatures VRF = (F, VF) such that, for any public-secret key pair (pk,sk), (h,p) = F(sk,x) p Sign( sk, x ) ; h H( p ) On input x, the algorithm F returns a hash and a proof. The hash h is random to anyone who does not know sk. b = VF(pk,x,h,p) Vf( pk, x, p ) h == H( p ) On input pk, x, h, and p, the algorithm VF returns a bit. The proof p lets anyone with pk verify that h corresponds to x. 25

26 Algorand (much simplified) a. Select committee members based on stakes Cannot be done deterministically Cannot be done in multiple steps Must be done in secret (h,p) VRF(sk, seed role) seed 0 agreed through distributed random number generation seed r = h r b. Committee members vote on winner based on stakes 26

27 Algorand (much simplified) a. Select committee members based on stakes Cannot be done deterministically Cannot be done in multiple steps Must be done in secret (h,p,j) Sortition(sk, seed,role,w,w) b. Committee members vote on winner based on stakes 27

28 Algorand (much simplified) a. Select committee members based on stakes Cannot be done deterministically Cannot be done in multiple steps Must be done in secret b. Committee members vote on winner based on stakes Block with highest priority wins. For i in {1,,j}, (h,p,j) Sortition(sk, seed,role,w,w) block priority max(block priority, H(h i)) 28

29 Algorand claims 75-page paper with tons of proofs of safety liveness other properties under various timing models, message delivery models, etc. 29

30 others? 30

31 Being able to model the system helps! Summary of consensus resilience properties, some of which use statically configured nodes with a special role. Symbols and notes: means that the protocol is resilient against the fault and that it is not;. states that no such special node exists in the protocol;? denotes that the properties cannot be assessed due to lack of information; ( ) denotes the crash of other nodes, different from the special node; + MultiChain has non-final decisions; PoET assumes trusted hardware available from only one vendor; Ripple tolerates one of the five default Ripple-operated validators (special nodes) to be subverted. Source: Blockchain consensus protocols in the wild. Christian Cachin and Marko Vukolic. IBM Zurich

32 Results from distributed systems research FLP: In an asynchronous model with reliable links, no deterministic protocol can achieve consensus when at least one party can crash. [ asynchronous parties, reliable links, f 1 in fail-stop model ] PBFT: In an eventually synchronous model with reliable links, parties running PBFT protocol can reach consensus as long as no more than one third of all parties can be malicious. [ eventually synchronous parties, reliable links, f n/3 in byzantine model ] [FLP]: Impossibility of Distributed Consensus with One Faulty Process, Fischer, Lynch, and Paterson, [PBFT]: Practical Byzantine Fault Tolerance, Castro and Liskov,

33 Meanwhile We still see claims like this: Every transaction is replicated to every node. When a majority of nodes have replicated the transaction, the transaction is committed. Kadena based on ScaleableBFT Model: asynchronous with byzantine failures 33

34 Snake oil? Unsupported, hard to verify claims abound. This situation has happened before! Look at the history of modern cryptography. 34

35 Block ciphers and modes of operation FIPS-80: DES and modes of operation Various modes proposed: ECB, CBC, CTR, OFB, CFB Assuming that the underlying block cipher is perfect, are these modes any good? 35

36 ECB encryption mode: definition Source: Symmetric Cryptography Basics, Chanathip Namprempre,

37 ECB encryption mode: secure? Source: Wikipedia 37

38 ECB insecurity 38

39 Modeling primitives and defining security definitions Block cipher What is it: family of permutations What it means to be secure: PRP-CPA, PRP-CCA Symmetric encryption scheme What is it: three algorithms satisfying a correctness condition What it means to be secure: IND-CPA, IND-CCA 39

40 Results? ECB is not IND-CPA. CBC with counter IV is not IND-CPA. CBC with random IV is IND-CPA but not IND-CCA. CTR with random IV is IND-CPA but not IND-CCA. CTR with counter IV is IND-CPA but not IND-CCA. CTR with counter IV is more secure than CTR with random IV. etc. 40

41 Theory catching up with practice Modern cryptography Block ciphers were proposed and used. Then, models and proofs were developed. Cryptocurrencies and distributed systems Bitcoin is proposed and used. Then, models and proofs are being developed! 41

42 What makes a good consensus protocol? Efficient: throughput? latency? average case? worse case? Secure: DOS? targeted attack? Prevent/eliminate forks? Liveness? Safety? 42

43 Safety and Liveness Safety: Nothing bad happens. With overwhelming probability, all nodes agree on the same order of transactions. If one honest node accepts transaction A (i.e., it appears on the chain), then with overwhelming probability any future transactions accepted by other honest nodes will be on the chain that already contains A. Liveness: Something good eventually happens. With overwhelming probability, new transactions get added to the chain. During synchronous period, with overwhelming probability honest nodes eventually add a new transaction to the chain. 43

44 State of the art Security models and proofs for blockchain systems are emerging. But the models and security definitions are complicated and hard to work with. Plenty of room for improvement! We can start with devising simpler, cleaner models and definitions. 44

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