bridge

(contracts only)

parity-bridge is currently an ERC20 token contract on one ethereum-based blockchain that is backed by ether on another ethereum-based blockchain.

eventually parity-bridge will be able to pass arbitrary messages between two ethereum-based blockchains. in the future you'll be able to build the current ether-ERC20 bridge and any other cross-chain application on top of the message passing bridge.

currently users can convert ether on one chain into the same amount of ERC20 tokens on the other and back. the bridge securely relays these conversions.

the bridge can mitigate scaling issues: by deploying a proof-of-authority network and bridging it to the Ethereum Foundation network ('mainnet') users can convert their mainnet ether into ERC20 tokens on the PoA chain and there transfer them with much lower transaction fees, faster block times and unaffected by mainnet congestion.

the users can withdraw their tokens worth of ether on the mainnet at any point.

parity is using the bridge project to prototype the system that will eventually connect ethereum and other non-parachains to polkadot.

current functionality

the bridge connects two chains home and foreign.

when users deposit ether into the HomeBridge contract on home they get the same amount of ERC20 tokens on foreign.

they can use ForeignBridge as they would use any ERC20 token.

to convert their foreign ERC20 into ether on home users can always call ForeignBridge.transferHomeViaRelay(homeRecipientAddress, value, homeGasPrice).

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.

high level explanation of home ether -> foreign ERC20 relay

sender deposits value into HomeBridge. the HomeBridge fallback function emits Deposit(sender, value).

for each Deposit event on HomeBridge every authority executes ForeignBridge.deposit(sender, value, transactionHash).

once there are ForeignBridge.requiredSignatures such transactions with identical arguments and from distinct authorities then ForeignBridge.balanceOf(sender) is increased by value.

high level explanation of foreign ERC20 -> home ether relay

sender executes ForeignBridge.transferHomeViaRelay(recipient, value, homeGasPrice) which checks and reduces ForeignBridge.balances(sender) by value and emits ForeignBridge.Withdraw(recipient, value, homeGasPrice).

for every ForeignBridge.Withdraw, every bridge authority creates a message containing value, recipient and the transactionHash of the transaction referenced by the ForeignBridge.Withdraw event; signs that message and executes ForeignBridge.submitSignature(signature, message). this collection of signatures is on foreign because transactions are free for the authorities on 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.

HomeBridge.withdraw(vs, rs, ss, message) recovers the addresses from the signatures, checks that enough authorities in its authority list have signed and finally transfers value ether (minus the relay gas costs) to recipient.

deploy

read our deployment guide

run truffle smart contract tests

requires yarn to be $PATH. installation instructions

cd truffle
yarn test

build

requires rust and cargo: installation instructions.

requires solc: installation instructions.

assuming you've cloned the bridge (git clone git@github.com:paritytech/parity-bridge.git) and are in the project directory (cd parity-bridge) run:

cargo build -p parity-bridge --release

to install copy target/release/parity-bridge into a folder that's in your $PATH.

run

env RUST_LOG=info parity-bridge --config config.toml --database db.toml

• --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

the bridge is configured through a configuration file.

here's an example configuration file: integration-tests/bridge_config.toml

following is a detailed explanation of all config options. all fields are required unless marked with optional.

options

• estimated_gas_cost_of_withdraw - an upper bound on the gas a transaction to HomeBridge.withdraw consumes
  • currently recommended value: "200000"
  • must be a string because the toml crate can't parse numbers greater max i64
  • 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
• max_total_home_contract_balance - reject deposits that would increase HomeBridge.balance beyond this value
  • security feature:
    • limits the total amount of home/mainnet ether that can be lost if the bridge is faulty or compromised in any way!
  • set to "0" to disable.
  • recommended for test deployment: 10 ether = "10000000000000000000"
  • must be a string because the toml crate can't parse numbers greater max i64 and this value frequently is greater
• max_single_deposit_value - reject deposits whose msg.value is higher than this value
  • security feature
  • set to 0 to disable
  • recommended for test deployment: 1 ether = "1000000000000000000"
  • must be a string because the toml crate can't parse numbers greater max i64 and this value frequently is greater

home options

• home.account - address of this bridge authority on home chain
• home.ipc - path to the ipc socket of a parity node that has home.account unlocked
• home.contract.bin - path to the compiled HomeBridge contract
  • required for initial deployment
  • run tools/compile_contracts.sh to compile contracts into dir compiled_contracts
  • then set this to compiled_contracts/HomeBridge.bin
• home.required_confirmations - number of confirmations required to consider transaction final on home.ipc
  • optional, default: 12
• home.poll_interval - specify how frequently (seconds) home.ipc should be polled for changes
  • optional, default: 1
• home.request_timeout - how many seconds to wait for responses from home.ipc before timing out
  • optional, default: 5

foreign options

• foreign.account - address of this bridge authority on home chain
  • usually the same as home.account
• foreign.ipc - path to the ipc socket of a parity node that has foreign.account unlocked
• foreign.contract.bin - path to the compiled ForeignBridge contract
  • required for initial deployment
  • run tools/compile_contracts.sh to compile contracts into dir compiled_contracts
  • then set this to compiled_contracts/ForeignBridge.bin
• foreign.required_confirmations - number of confirmations required to consider transaction final on foreign.ipc
  • optional, default: 12
• foreign.poll_interval - specify how frequently (seconds) foreign.ipc should be polled for changes
  • optional, default: 1
• foreign.request_timeout - how many seconds to wait for responses from foreign.ipc before timing out
  • optional, default: 5

authorities options

• authorities.account - array of addresses of authorities
• authorities.required_signatures - number of authorities signatures required to consider action final

transaction options

gas and gas_price to use for the specific transactions. these are all optional and default to 0.

look into the [transactions] section in integration-tests/bridge_config.toml for recommendations on provided gas.

these happen on home:

• transaction.home_deploy.gas
• transaction.home_deploy.gas_price
• transaction.withdraw_relay.gas
• transaction.withdraw_relay.gas_price

these happen on foreign:

• transaction.foreign_deploy.gas
• transaction.foreign_deploy.gas_price
• transaction.deposit_relay.gas
• transaction.deposit_relay.gas_price
• transaction.withdraw_confirm.gas
• transaction.withdraw_confirm.gas_price

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 authority 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

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 (initially the ethereum foundation chain) to a non-value-bearing PoA ethereum blockchain foreign (initially 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.transferHomeViaRelay 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.

relayers can set the gas price for HomeBridge.withdraw. they could set a very high gas price resulting in a very high cost through which they could burn large portions of value. to shut down this attack the homeGasPrice param was added to ForeignBridge.transferHomeViaRelay. end users have control over the cost/latency tradeoff of their relay transaction through the homeGasPrice. relayers have to set gas price to homeGasPrice when calling HomeBridge.withdraw. the recipient for value is the exception and can freely choose any gas price. see paritytech#112 for more details.

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.