bridge

(contracts only)

bridge between two ethereum blockchains, home and foreign.

current functionality

the bridge allows users to deposit ether into a smart contract on home and get it on foreign in form of a token balance. it also allows users to withdraw their tokens on foreign and get the equivalent ether on home. on foreign users can freely transfer tokens between each other.

foreign is assumed to use PoA (proof of authority) consensus. relays between the chains happen in a byzantine fault tolerant way using the authorities of foreign.

next steps

1. deploy to bridge ethereum and kovan with the kovan authorities being the immutable set of bridge authorities
2. make bridge work with contract-based dynamic validator set
3. after kovan hardfork 2: deploy to kovan again with dynamic validator set

eventual goals

connect ethereum to polkadot

deposit ether into HomeBridge and get it in form of a token balance on ForeignBridge

sender deposits value into HomeBridge. the HomeBridge fallback function emits Deposit(sender, value). for each Deposit event on HomeBridge every authority makes a transaction ForeignBridge.deposit(sender, value, transactionHash). once there are ForeignBridge.requiredSignatures such transactions with identical arguments and from distinct authorities then ForeignBridge.balances(sender) is increased by value and ForeignBridge.Deposit(sender, value) is emitted.

withdraw balance on ForeignBridge and get it as ether on home chain

sender executes ForeignBridge.transferHomeViaRelay(recipient, value) which checks and reduces ForeignBridge.balances(sender) by value and emits ForeignBridge.Withdraw(recipient, value). for each ForeignBridge.Withdraw every bridge authority creates a message containg value, recipient and the transactionHash of the transaction containing the ForeignBridge.Withdraw event, signs the message and makes a transaction ForeignBridge.submitSignature(signature, message). this collection of signatures on foreign is necessary because transactions are free for authorities on foreign, since they are the authorities of foreign, but not free on home. once ForeignBridge.requiredSignatures signatures by distinct authorities are collected a ForeignBridge.CollectedSignatures(authorityThatSubmittedLastSignature, messageHash) event is emitted. everyone (usually authorityThatSubmittedLastSignature) can then call ForeignBridge.message(messageHash) and ForeignBridge.signature(messageHash, 0..requiredSignatures) to look up the message and signatures and execute HomeBridge.withdraw(vs, rs, ss, message) and complete the withdraw.

transfer on foreign

sender executes ForeignBridge.transferLocal(recipient, value) which checks and reduces ForeignBridge.balances(sender) and increases ForeignBridge.balances(recipient) by value.

build

cargo build -p bridge-cli --release

cli options

Ethereum-Kovan bridge.
    Copyright 2017 Parity Technologies (UK) Limited

Usage:
    bridge --config <config> --database <database>
    bridge -h | --help

Options:
    -h, --help           Display help message and exit.

• --config - location of the configuration file. configuration file must exist
• --database - location of the database file. if there is no file at specified location, new bridge contracts will be deployed and new database will be created

configuration file example

estimated_gas_cost_of_withdraw = 100000

[home]
account = "0x006e27b6a72e1f34c626762f3c4761547aff1421"
ipc = "/Users/marek/Library/Application Support/io.parity.ethereum/jsonrpc.ipc"
required_confirmations = 0

[home.contract]
bin = "contracts/EthereumBridge.bin"

[foreign]
account = "0x006e27b6a72e1f34c626762f3c4761547aff1421"
ipc = "/Users/marek/Library/Application Support/io.parity.ethereum/jsonrpc.ipc"
required_confirmations = 0

[foreign.contract]
bin = "contracts/KovanBridge.bin"

[authorities]
accounts = [
	"0x006e27b6a72e1f34c626762f3c4761547aff1421",
	"0x006e27b6a72e1f34c626762f3c4761547aff1421",
	"0x006e27b6a72e1f34c626762f3c4761547aff1421"
]
required_signatures = 2

[transactions]
home_deploy = { gas = 500000 }
foreign_deploy = { gas = 500000 }

options

• estimated_gas_cost_of_withdraw - how much gas a transaction to HomeBridge.withdraw consumes (required)
  • currently recommended value: 100000
  • run tools/estimate_gas_costs.sh to compute an estimate
  • see recipient pays relay cost to relaying authority for why this config option is needed

home options

• home.account - authority address on the home (required)
• home.ipc - path to home parity ipc handle (required)
• home.contract.bin - path to the compiled bridge contract (required)
• home.required_confirmations - number of confirmation required to consider transaction final on home (default: 12)
• home.poll_interval - specify how often home node should be polled for changes (in seconds, default: 1)
• home.request_timeout - specify request timeout (in seconds, default: 5)

foreign options

• foreign.account - authority address on the foreign (required)
• foreign.ipc - path to foreign parity ipc handle (required)
• foreign.contract.bin - path to the compiled bridge contract (required)
• foreign.required_confirmations - number of confirmation required to consider transaction final on foreign (default: 12)
• foreign.poll_interval - specify how often home node should be polled for changes (in seconds, default: 1)
• foreign.request_timeout - specify request timeout (in seconds, default: 5)

authorities options

• authorities.account - all authorities (required)
• authorities.required_signatures - number of authorities signatures required to consider action final (required)

transaction options

• transaction.home_deploy.gas - specify how much gas should be consumed by home contract deploy
• transaction.home_deploy.gas_price - specify gas price for home contract deploy
• transaction.foreign_deploy.gas - specify how much gas should be consumed by foreign contract deploy
• transaction.foreign_deploy.gas_price - specify gas price for foreign contract deploy
• transaction.deposit_relay.gas - specify how much gas should be consumed by deposit relay
• transaction.deposit_relay.gas_price - specify gas price for deposit relay
• transaction.withdraw_confirm.gas - specify how much gas should be consumed by withdraw confirm
• transaction.withdraw_confirm.gas_price - specify gas price for withdraw confirm
• transaction.withdraw_relay.gas - specify how much gas should be consumed by withdraw relay
• transaction.withdraw_relay.gas_price - specify gas price for withdraw relay

database file format

home_contract_address = "0x49edf201c1e139282643d5e7c6fb0c7219ad1db7"
foreign_contract_address = "0x49edf201c1e139282643d5e7c6fb0c7219ad1db8"
home_deploy = 100
foreign_deploy = 101
checked_deposit_relay = 120
checked_withdraw_relay = 121
checked_withdraw_confirm = 121

all fields are required

• home_contract_address - address of the bridge contract on home chain
• foreign_contract_address - address of the bridge contract on foreign chain
• home_deploy - block number at which home contract has been deployed
• foreign_deploy - block number at which foreign contract has been deployed
• checked_deposit_relay - number of the last block for which an authority has relayed deposits to the foreign
• checked_withdraw_relay - number of the last block for which an authority has relayed withdraws to the home
• checked_withdraw_confirm - number of the last block for which an authirty has confirmed withdraw

example run

./target/release/bridge --config examples/config.toml --database db.toml

• example run requires a parity instance running
• this parity instance can be started by running examples/parity_start.sh
• it connects to this parity instance twice. one connection treats the node as home, whereas the other as foreign
• by default, parity tries to unlock account generates from seedphrase this is sparta - 0x006e27b6a72e1f34c626762f3c4761547aff1421
• this is just an example. the 'real world' bridge needs to connect to the two different parity instances

deposit

withdraw

truffle tests

requires yarn to be installed

cd truffle
yarn test

recipient pays relay cost to relaying authority

a bridge authority has to pay for gas (cost) to execute HomeBridge.withdraw when withdrawing value from foreign chain to home chain. value - cost is transferred to the recipient. cost is transferred to the authority executing HomeBridge.withdraw. the recipient pays the relaying authority for the execution of the transaction. that shuts down an attack that enabled exhaustion of authorities funds on home.

read on for a more thorough explanation.

parity-bridge connects a value-bearing ethereum blockchain home (initally the ethereum foundation chain) to a non-value-bearing PoA ethereum blockchain foreign (initally the kovan testnet).

value-bearing means that the ether on that chain has usable value in the sense that in order to obtain it one has to either mine it (trade in electricity) or trade in another currency. non-value-bearing means that one can easily obtain a large amount of ether on that chain for free. through a faucet in the case of testnets for example.

the bridge authorities are also the validators of the foreign PoA chain. transactions by the authorities are therefore free (gas price = 0) on foreign.

to execute a transaction on home a bridge authority has to spend ether to pay for the gas.

this opened up an attack where a malicious user could deposit a very small amount of wei on HomeBridge, get it relayed to ForeignBridge, then spam ForeignBridge.transfer with 1 wei withdraws. it would cost the attacker very little home chain wei and essentially free foreign testnet wei to cause the authorities to spend orders of magnitude more wei to relay the withdraw to home by executing HomeBridge.withdraw. an attacker was able to exhaust bridge authorities funds on home.

to shut down this attack HomeBridge.withdraw was modified so value - cost is transferred to the recipient and cost is transferred to the authority doing the relay. this way the recipient pays the relaying authority for the execution of the withdraw transaction.

if the value withdrawn is too low to pay for the relay at current gas prices then bridge authorities will ignore it. one can think of it as value getting spent entirely on paying the relay with no value left to pay out the recipient.

HomeBridge.withdraw is currently the only transaction bridge authorities execute on home. care must be taken to secure future functions that bridge authorities will execute on home in similar ways.