Talk given @ private event

1. Riga Talk
Blockchain Properties
Alexander Chepurnoy
(aka kushti)
@chepurnoy
IOHK Research

2. The talk is about
How a developer can view a blockchain system

3. Environment
● P2P network
● No central party
● Probabilistic broadcast

4. Read Books!
● Cachin et al. „Introduction to Reliable and Secure
Distributed Programming“
● Russian: „Введение в надежное и безопасное
распределенное программирование“ (Качин и др.)

5. P2P network
● Each node has its own state
● 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)

6. A transaction
● Atomic state modifier
● Authenticated

7. 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

8. 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!

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

10. Output Abstraction: Box
● minimal state is a set of closed boxes
● transaction opens some boxes and add new ones
● both UTXO and acccount model (Nxt, Ethereum)

11. Abstract Transaction Authentication
● Box is protected by a proposition (e.g. pubkey)
● To be open with a proof (e.g. a signature)
● check (proposition, proof, message)

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

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

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

15. Node view
NodeView = <
Blockchain,
MinimalState,
MemoryPool,
Vault>
(Blockchain, MinimalState) is eventually the same for all
the honest nodes

16. NodeView Modification
NodeView x Block →NodeView | Error
Blockchain x Block → <Blockchain, Opt[Rollback]> | Error
MinState x <Block, Opt[Rollback]> → MinState | Error
MemPool x <Block, Opt[Rollback]> →MemPool
Vault x <Block, Opt[Rollback]> → Vault

17. Note that:
● rollbacks exist!
● <Blockchain, MinState, Vault> MUST be consistent (after a
block application, a rollback, a crash etc)
● read-when-update semantics should be resolved
● use QuickCheck to check correctness, not unit tests

18. Laws
● Think about laws
● Check them

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