withdraws can be burned by relayers through setting very high gas price

[snd]

thanks to @bjornwgnr for finding this

problem

given a ForeignBridge.CollectedSignatures event that has not yet
been relayed i.e. doesn't have a matching HomeBridge.Withdraw.

anyone can call HomeBridge.withdraw to relay the withdraw.
this is by design. HomeBridge.withdraw only trusts the list of authorities.
signatures of those authorities (on the message) are passed to withdraw.

as with any transaction the sender of withdraw can freely choose the gas price.
currently (introduced by #77) estimatedGasCostOfWithdraw * tx.gasPrice
is subtracted from the withdrawn value and transfered to sender to pay
for the relay.

since the transaction sender can freely choose the gas price they can burn a users withdraw by setting an extremely high gas price.

solution

add additional param homeGasPrice to
ForeignBridge.transferHomeViaRelay so the interface looks like
ForeignBridge.transferHomeViaRelay(recipient, value, homeGasPrice).

end user can easily find out a gas price on something like https://ethgasstation.info/.

add homeGasPrice to message that is signed and relayed.

then in HomeBridge.withdraw(vs, rs, ss, message)
require require(tx.gasPrice == homeGasPrice) forcing the sender of
the withdraw to use the user specified gas price.

this solution has other nice properties:

we now know the homeGasPrice on foreign since it's provided by the enduser.
so we can require require(estimatedGasCostOfWithdraw * homeGasPrice > value)
in Foreign.transferHomeViaRelay and revert such withdraws that
don't have enough value to pay for the relay.
this improves UX since such withdraws are currently ignored by bridge processes
which, while being well documented and sensible, can lead to confusion.

the homeGasPrice is documented in the ForeignBridge.Withdraw event
so it's transparent if the relay transaction doesn't get mined on home
because of too low a gas price.

end users now have control over the cost/latency tradeoff of their relay through
the homeGasPrice.

edge case

provided gas price is too low and transaction doesn't get mined.

in HomeBridge.withdraw we could allow only recipient to change the gas price
by changing the require to require(recipient == msg.sender || tx.gasPrice == homeGasPrice).
now the recipient can manually rescue by providing a new gas price and paying it
themselves.

[tomusdrw]

Looks good! I really like the edge case solution as well.

[Ri2k18]

Hi I have Eth address for parity signer, how can I use my eth there if I forgot my recovery phrase
Plese help me. Thanks

[snd]

hey @Ri2k18, please ask your question again in the right repo https://github.com/paritytech/parity-signer