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Centrally Banked Cryptocurrencies
George Danezis (University College London)Sarah Meiklejohn (University College London)
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who’s interested in ‘blockchain’?
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who’s interested in ‘blockchain’?
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who’s interested in ‘blockchain’?
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who’s interested in ‘blockchain’?
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who’s interested in ‘blockchain’?
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who’s interested in ‘blockchain’?
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who’s interested in ‘blockchain’?
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fully decentralized cryptocurrencies
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fully decentralized cryptocurrencies
3
fully decentralized cryptocurrencies
txtx(addrA→addrB)
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fully decentralized cryptocurrencies
tx(generate transaction ledger)
“mining”
(generate monetary supply)tx(addrA→addrB)
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fully decentralized cryptocurrencies
append-only
tx(generate transaction ledger)
“mining”
(generate monetary supply)tx(addrA→addrB)
3
fully decentralized cryptocurrencies
transparentappend-only
tx(generate transaction ledger)
“mining”
(generate monetary supply)tx(addrA→addrB)
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fully decentralized cryptocurrencies
transparentpseudonyms
append-only
tx(generate transaction ledger)
“mining”
(generate monetary supply)tx(addrA→addrB)
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issues with Bitcoin
hashing rates are out of controlincentive structure is messed up
attacks on mining
no control over monetary policy
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issues with Bitcoin
hashing rates are out of controlincentive structure is messed up
attacks on mining
no control over monetary policy
not suitable for most applications!
RSCoin
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RSCoin
5
monetary supply decentral centralcentral
RSCoin
5
monetary supplyledger centraldistributedecentral
decentral centralcentral
RSCoin
5
monetary supplyledger centraldistributedecentral
decentral centralcentral
transparent? y y (or n) n
RSCoin
5
monetary supplyledger centraldistributedecentral
decentral centralcentral
transparent? y y (or n) n
pseudonyms? y y (or n) n
RSCoin
5
monetary supplyledger centraldistributedecentral
decentral centralcentral
transparent? y y (or n) n
pseudonyms? y y (or n) n
computation high! low low
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6
bank (generate monetary supply)
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mintettemintette
mintette mintette
bank (generate monetary supply)
(generate transaction ledger)
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mintettemintette
mintette mintette
bank
user
(generate monetary supply)
(generate transaction ledger)
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mintettemintette
mintette mintette
bank
user
(generate monetary supply)
(generate transaction ledger)
6
mintettemintette
mintette mintette
bank
user
(generate monetary supply)
(generate transaction ledger)
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mintettemintette
mintette mintette
bank
user
7
mintettemintette
mintette mintette
bank
user
consensus?
7
mintettemintette
mintette mintette
bank
user
what gets sent?
consensus?
7
mintettemintette
mintette mintette
bank
user
what gets sent?
consensus?
how do mintettes collect txs?
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mintettemintette
mintette mintette
bank
user
what gets sent?
consensus?
how do mintettes collect txs?
consensus?
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user
1 2tx:
consensus
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user
1 2tx:
consensus
each address is owned by a set of mintettes
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user
1 2tx:
consensus
each address is owned by a set of mintettes
a d d r e s s e smintettemintettemintette
mintettemintettemintettemintettemintettemintette
mintettemintettemintettemintettemintettemintette
mintettemintettemintette
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mintette1
mintette1
user
1 2tx:
mintette1
1
consensus
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mintette1
mintette1
user
1 2tx:
mintette1
1
mintettes check for double spending…
…using lists of unspent transaction outputs (utxo)
consensus
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mintette1
mintette1
user
1 2tx:
✓ mintette1
✓
1
signed ‘yes’ vote (and head h)
consensus
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mintette1
mintette1
user
1 2tx:
✓ mintette1
✓
mintette2
mintette2
mintette2
1tx ✓✓
“bundle of evidence” contains ‘yes’ votes from majority of mintettes in shard
consensus
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mintette1
mintette1
user
1 2tx:
✓ mintette1
✓
mintette2
mintette2
mintette2
1tx ✓✓
mintettes check validity of bundle by checkingfor signatures from authorized mintettes…
consensus
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mintette1
mintette1
user
1 2tx:
✓ mintette1
✓
mintette2
mintette2
mintette2
1tx ✓✓tx
tx
…and if satisfied they add transaction to be committed and send back receipt
consensus
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consensus features
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consensus features
simple (adaption of Two-Phase Commit)
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consensus features
scalable!simple (adaption of Two-Phase Commit)
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consensus features
scalable!T = set of txs generated per second
Q = # mintettes per shard M = # mintettes
simple (adaption of Two-Phase Commit)
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consensus features
scalable!T = set of txs generated per second
Q = # mintettes per shard M = # mintettes
simple (adaption of Two-Phase Commit)
∑tx∈T 2(mtx+1)QMcomm. per mintette per sec =
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consensus features
scalable!T = set of txs generated per second
Q = # mintettes per shard M = # mintettes
simple (adaption of Two-Phase Commit)
∑tx∈T 2(mtx+1)Q
scales infinitely as more mintettes are added!
Mcomm. per mintette per sec =
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each new mintette adds≈ 75 tx/sec
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each new mintette adds≈ 75 tx/sec
compared to Bitcoin’s 7
what gets sent?
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mintettemintette
mintette mintette
bank
user
what gets sent?
how do mintettes collect txs?
(2PC)consensus?consensus?
what gets sent?
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mintettemintette
mintette mintette
bank
user
what gets sent?
how do mintettes collect txs?
(2PC)
how do mintettes collect txs?
(contacted based on shard)
consensus?
what gets sent?
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mintettemintette
mintette mintette
bank
user
what gets sent?
how do mintettes collect txs?
(2PC)
(contacted based on shard)
what gets sent?
consensus?
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security properties
no double spending (only “good” transactions get included)
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security properties
no double spending (only “good” transactions get included)
(if honest majority)
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security properties
no double spending (only “good” transactions get included)non-repudiation (mintettes are held to their promises)
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security properties
no double spending (only “good” transactions get included)non-repudiation (mintettes are held to their promises)
(because mintettes provide receipt uponcommitting transaction)
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security properties
no double spending (only “good” transactions get included)non-repudiation (mintettes are held to their promises)auditability (mintettes can’t cheat without detection)
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mintette logs
borrow ideas from Certificate Transparency to log actions
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mintette logs
borrow ideas from Certificate Transparency to log actionsmintettes create log entry every time they:
-act as mintette in first phase (Query) -act as mintette in second phase (Commit) -publish head of hash chain (CloseEpoch)
rolling hash chain of log acts as commitment to actions
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mintette logs
borrow ideas from Certificate Transparency to log actionsmintettes create log entry every time they:
-act as mintette in first phase (Query) -act as mintette in second phase (Commit) -publish head of hash chain (CloseEpoch)
rolling hash chain of log acts as commitment to actionsmintettes cross-hash chains to provide evidence of activity
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mintette logs
borrow ideas from Certificate Transparency to log actionsmintettes create log entry every time they:
-act as mintette in first phase (Query) -act as mintette in second phase (Commit) -publish head of hash chain (CloseEpoch)
rolling hash chain of log acts as commitment to actions
send logs to bank at end of every periodmintettes cross-hash chains to provide evidence of activity
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security properties
no double spending (only “good” transactions get included)non-repudiation (mintettes are held to their promises)auditability (mintettes can’t cheat without detection)
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security properties
no double spending (only “good” transactions get included)non-repudiation (mintettes are held to their promises)auditability (mintettes can’t cheat without detection)
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mintettemintette
mintette mintette
bank
user
what gets sent?
how do mintettes collect txs?
(2PC)
(contacted based on shard)
what gets sent?(cross-hashed chains)
consensus?
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mintettemintette
mintette mintette
bank
user
what gets sent?
how do mintettes collect txs?
(2PC)
(contacted based on shard)
(cross-hashed chains)
-collate transactions
consensus?
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mintettemintette
mintette mintette
bank
user
what gets sent?
how do mintettes collect txs?
(2PC)
(contacted based on shard)
(cross-hashed chains)
-collate transactions-allocate fees-audit mintettes
consensus?
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mintettemintette
mintette mintette
bank
user
what gets sent?
how do mintettes collect txs?
(2PC)
(contacted based on shard)
(cross-hashed chains)
-collate transactions-allocate fees-audit mintettes(-add coin generation)-authorize mintettes
consensus?
RSCoin
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monetary supplyledger centraldistributedecentral
decentral centralcentral
transparent? y y (or n) n
pseudonyms? y y (or n) n
computation high! low low
RSCoin
22
monetary supplyledger centraldistributedecentral
decentral centralcentral
transparent? y y (or n) n
pseudonyms? y y (or n) n
computation high! low low
Thanks! Any questions?
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