Altcoins: Alternate Designs for

CryptocurrencyCS4501 Fall 2015

David Evans and Samee ZahurUniversity of Virginia

Elevator Pitch: Today• Blockchain Voting, Sugat Poudel, Austin J. Varshneya, Xhama Vyas• Bitcoin at Point of Sale, Elizabeth Kukla• Vending on Dark Net Markets, Collin Berman• Distributed Bitcoin Mixing with Interest, Carter Hall, Reid Bixler• Analyzing the Feasibility of a Donation Accountability System in

Bitcoin, Kienan Adams

What’s wrong with it?

• Confirmations are slow• Not completely anonymous• Not quite decentralized• Volatile price

Alternate designs• Litecoin: different proof of work

• Ripple: no proof of work

• Ethereum: distributed programs

coinmarketcap.com

Litecoin• Faster confirmations: 2.5 minutes per block on average• Proof of work: scrypt• Less valuable

• http://www.cryptocoincharts.info

www.cryptocoincharts.info

RippleFaster transactions by eliminating proof of work

“Rippling” Consensus

Byzantine Generals ProblemAll generals propose either “attack” or “retreat” as the next step. They all must reach consensus. Some generals might be corrupted

Byzantine Limit

No unconditionally secure algorithm exists if

RippleUses a variation of “Fast Byzantine Paxos”, which can tolerate Assumes a “well connected” graph.

Algorithm1. Each server makes its own “candidate list”2. They check validity3. Vote on which transactions to keep4. Any transaction with too few votes are discardedRepeat

Finally, transactions over 80% vote among neighbors are kept

Bitcoin vs Ripple

Bitcoin vs RippleNo proof of work• Anybody can join, more decentralized• Anybody can claim to be many people• Extremely fast: transactions confirmed in a few seconds

Deployment

coinmarketcap.com

Ethereum• Proof of work hash: Ethash (custom)• Mean block mining period: 12 seconds• Block reward: … complicated …• Scripting language: Turing complete

cryptorps.com

“Smart Contracts”Careful of security pitfalls!

Gas!

Transaction• Recipient• Gas price• Gas limit• Value• Signature

Mining Reward

•A static block reward for the 'winning' block, consisting of exactly 5.0 Ether •All of the gas expended within the block […]. •An extra reward for including 1/32 Uncles as part of the block

ContractsSnippets of code floating around in the network. They can send messages/money/gas to each other. Each transaction triggers at least one message.

Contracts have their own:• Account balance• Stack• Memory (integer-indexed)• Storage (key-value)

Common Pitfalls

Common Pitfalls• Everything is public• Race conditions• Limited stack

Bitcoin the ultimate• There are ways to enrich it• Make it faster• More decentralized

The end: elevator pitches

Elevator Pitch: Today• Blockchain Voting, Sugat Poudel, Austin J. Varshneya, Xhama Vyas• Bitcoin at Point of Sale, Elizabeth Kukla• Vending on Dark Net Markets, Collin Berman• Distributed Bitcoin Mixing with Interest, Carter Hall, Reid Bixler• Analyzing the Feasibility of a Donation Accountability System in

Bitcoin, Kienan Adams