SCP: A Computationally Scalable Byzantine
Consensus Protocol for Blockchains
Loi Luu, Viswesh Narayanan, Kunal Baweja, Chaodong Zheng,
Seth Gilbert, Prateek Saxena
National University of Singapore

Bitcoin doesn’t scale• Hard coded parameters– 1 block per 10 minutes– 1 MB block size
– 7 TXs per second• Today– 1‐2 TXs per second– VISA: 10, 000 TXs per secondOur solution: SCP• Scale up throughput several orders of magnitude– Without degrading any security guarantee• Several blocks in each epoch– No. of blocks ≈ network computation capacity• Require minimum amount of network bandwidth– Broadcast only one block header

Byzantine consensus problem• Problem– N nodes, f are malicious– Propose and agree on one value

Block i‐2

• Byzantine consensus for blockchains– Set of valid TXs per epoch
Block i‐1

0/1 iBlockClassical byzantine consensus protocol
 Intensive research Can tolerate f < n/2

x Assumption of known identity set
x Bandwidth limited– O(n2) messages (e.g. PBFT)– Work for a small network (e.g n < 1000)

Nakamoto consensus protocol
 Work for network of any size
 Select leader by proof of work Linear message complexity
x Does not scale well in practicex One block per epochx Bandwidth = O(block size)x Reparameterization is not a long term solutionReparameterization: reducing epoch time
• Setup• Using Amazon EC2• Run over 5 regions• Results• TX rate increases
until some threshold• Drops at 12 second
epoch timeProblem• Secure & scalable consensus protocol– Compete with VISA?

SCP overview• Adjust throughput based on network mining power– Split the network into several committees– Committees propose blocks in parallel– No. of committees ≈ F(network mining capacity)• Data needed for reaching consensus is minimal– Consensus data != transactional data
– Verify block without block data
– Selectively download block data

SCP protocol

12Consensus Blk

101010101101
Data Blk

Blk Header

35
Data Blk

4

Step 1: Identity establishment• Solve PoW– SHA2(EpochRandomness || IP || pubkey || nonce) < DID PoWIPPubkey

100001…

a.b.c.d ABC…

200001…

a.b.c.e DEF…

..

…

…

….

Step 2: Assigning committees• Randomly & uniformly distribute identities to committees– Based on the last k bits of PoW

ID
1
234..

PoW
00001…0000000…0100000…1000001…11…

00001…0000000…10…

00000…1100001…01…

Size of a committee C• Decide the probability of majority honest– P(error) reduces exponentially with C• f = N/3, C = 400, p(error) ≈ 10‐12• f = N/3, C = 100, p(error) ≈ 0.0004• Why majority honest within a committee?
– Run practical authenticated BFT– Allow others to verify committee’s block without block’s data
• At least 1 member is honest in any (C/2 + 1) members

Step 3: Propose a block within a committee• Run a classical Byzantine consensus protocol– Members agree & sign on one valid data block– No. of messages ≈ O(C2)• TX sets included in data blocks are disjoint– Include TXs with a specific prefix
Block

TX’s IDS

Data Block 1 00…
Data Block 2 01…
Data Block 3 10…
Data Block 4 11…

Step 4: Final committee unions all results

00001…00
00000…00

Propose aconsensus block…

Header of
Data Block 1

Header of
Data Block 2

Header of
Data Block 300001…00100000…1000000…1100000…10100001…1000001…11…

…

…

SCP blockchain
Consensus
block i‐1

Data block 1

Consensus
block i

Data block 2

Consensus
block i+1

Data block 3

Step 5: Generate an epoch randomness• Goal– Generate a fresh randomness– Adversary cannot control or predict• Common approach: Use consensus block hash
– Problem: adversary can predict the consensus block early• Our approach: Users can have different randomness

Commit Ri inSHA2(Ri) when join
the committee

Agree on theconsensus
blockBroadcast theblock headerand Ri

Use any c/2 Ri
as the epochrandomness

Implement a SCP‐based cryptocurrency• Challenges– How to form committees efficiently• Too many new identities in each epoch• Epoch time may be long to prevent conflict– Double spending transactions• Without previous block data?

Input 0x123..

Input 0x123…

Forming committees efficiently• Approach: Reuse identities from previous epoch– Elect one new member and remove the oldest one– Number of new identities ≈ number of committees

1st Epoch2nd Epoch 3rd Epoch0

12…

C‐2

C‐1CC+1

Avoid double spending• Approach:– Split double spending check into both miners and users(recipients)
Checked bycommittee members

Within ablock

Doublespending

Checked byrecipients*

Acrossblocks*: Proof‐of‐publication

Checking double spending across blocks• Merkle tree of TX inputs– An input is spent in a block• Proof of size log(N)– An input is not spent in a block• Proof of size 2*log(N)Prove that
5 is notincluded?
All leaves are sorted

Checking double spending across blocks (2)• Sender proves that the TX’s input is not spent elsewhere– The proof of size L*log(N)– Can be optimized• Recipient checks by using only consensus block headers– Actively support SPV clients without a trusted third party– Support 1‐confirmation TXs

Conclusion• SCP scales almost linearly with network mining capacity– More mining power, higher transaction rate
– Reduced network bandwidth– Secure• Applicable to several applications– Cryptocurrency, decentralized database, etc

Q&ALoi Luu
loiluu@comp.nus.edu.sg
www.comp.nus.edu.sg/~loiluu

Future work
• Incentive structure– Incentivize committee members and other parties• Prevent DoS attack by sending invalid TXs– Users can send arbitrary TXs to the blockchain now• Rollback solution– P(error) != 0

Related work
• Bitcoin‐NG & Ghost
 Allow more blocks
x Does not separate consensus plane and data plane• Lighting network Allows more micro transactions
x Does not solve scalability problem• Sidechains Good for experimenting new blockchainsx Does not make Bitcoin scalable

Adjusts number of committees frequently• Similar to how Bitcoin adjusts the block difficulty– T: the expected epoch time
– T’: the averaged epoch time of the most 1000 recent blocks– S: Current number of committees– S’: adjusted number of committees

TS 'log(S ')  S log(S)T'Data Block 1

Consensus BlockPreviousConsensus
BlkMerkle root
commitment ofTXs

Previous Block Hash

Timestamp

Block hashNo. of TXs

Committee
signatures

Global
Merkle RootCommittee
signatures

Timestamp

Data block commitments
No.

Data Block’shash

Merkle rootof TXs

10x123abc…

…

20x123456…

…

Included TXs

Data Block 2
PreviousConsensus
BlkMerkle root
commitment ofTXs

Block hashNo. of TXs

Committeesignatures

Timestamp

Included TXs

SCP properties• Number of data blocks ≈ network mining power– Frequent adjustment of no. of blocks• Data broadcast to the network is minimal– Broadcast data is independent of block size

• Secure against adaptive adversary w.h.p.
– Can reparameterize c to secure against stronger adversary