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"Voting Software"
xkcd.com/2030/
Cryptocurrencies & Security on the Blockchain
- Prof. Tom Austin
San José State University
"Voting Software" xkcd.com/2030/ Cryptocurrencies & - - PowerPoint PPT Presentation
"Voting Software" xkcd.com/2030/ Cryptocurrencies & - - PowerPoint PPT Presentation
"Voting Software" xkcd.com/2030/ Cryptocurrencies & Security on the Blockchain The Blockchain Prof. Tom Austin San Jos State University Lab Review Byzantine General's Problem (Lamport et al. 1982) Generals decide
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Lab Review
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Byzantine General's Problem
(Lamport et al. 1982)
- Generals decide whether to attack a
city.
- If all (or most) attack, victory is sure.
- If insufficient forces attack they will
be crushed.
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Complications
- Some of the generals are traitors.
– Fake messages. – Lie about messages received.
- Messengers may be killed.
- Should you confirm messages?
- Should you confirm the confirmations?
- Should you confirm the confirmations of the
confirmations?
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Traitorous Lieutenant
(from Lamport et al. 1982)
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Traitorous General
(from Lamport et al. 1982)
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Previous Solutions for Byzantine Fault Tolerance
- Paxos
– Provides distributed consensus – Formal safety guarantees – Complex
- Raft
– Simplified version of Paxos – Also makes formal safety guarantees – Still complex
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James Mickens' The Saddest Moment "Whenever I go to a conference and I discover that there will be a presentation about Byzantine fault tolerance, I always feel an immediate, unshakable sense of sadness …"
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Distributed Consensus
- Anyone could lie
to us.
- With crypto-
currencies, everyone is motivated to lie.
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So what can we trust?
Greed
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Nakamoto Consensus
- Probabilistically offers Byzantine fault
tolerance.
- Solving PoW puzzle determines "leader".
– In other words, who gets to make a block.
- Incentive structure.
– More profitable to mine than double-spend.
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What is a blockchain
- Data
- Cryptographic hash linking
blocks
- Sybil resistance
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Block Information
- Version
- Timestamp
- Previous block hash
- Proof-of-work (PoW) target
- Nonce (the "proof")
- Merkle root
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Block Information
- Version
- Timestamp
- Previous block hash
- Proof-of-work (PoW) target
- Nonce (the "proof")
- Merkle root
The data
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Block Information
- Version
- Timestamp
- Previous block hash
- Proof-of-work (PoW) target
- Nonce (the "proof")
- Merkle root
The link
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Block Information
- Version
- Timestamp
- Previous block hash
- Proof-of-work (PoW) target
- Nonce (the "proof")
- Merkle root
The Sybil resistance
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What data structure is a blockchain?
- A tree
–With mostly dead branches
- Orphan blocks
–The surviving branches have names:
- Bitcoin Cash, Bitcoin Gold, etc.
- A chain of "finalized" blocks
- A tree of remaining possibilities
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Blockchain Properties
- Distributed
- Decentralized
- Immutable history
- Time-stamping
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Blockchain Limitations
- Establishes agreed-upon ordering not
necessarily the original ordering.
- Nasty stuff stored on the blockchain.
- Assumes actors are
–Rational –Incentivized by profit
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Irrational Actors
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Nation-states
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Lab: Blockchain Logger
Details in Canvas and on course website.