roadie

Roadie

Roadie facilitates an atomic swap between two parties that would like to exchange siacoins for ether. Roadie was created to help set the stage for Sia.

(The screencast has been sped up and edited for clarity.)

How It Works

Alice has ether on the Ethereum blockchain and would like to trade with Bob, who has siacoins on the Sia blockchain. Bob is running a server, which he has registered with the roadie smart contract on Ethereum. Alice looks up the address of Bob's server and connects to it with a request detailing how many siacoins she would like to buy. She receives an offer and decides to accept it.

Sia uses Schnorr signatures, which have the nice property that they support "native multisig". This means that Alice and Bob can each create a fresh public/private key pair and combine the public keys to create a 2-of-2 multisig Sia address. (Sia also has built-in support for multisig, but this feature is not used here.) Bob transfers siacoins to this address, but not before creating a refund transaction and having it signed by Alice. This refund transaction will transfer all siacoins back to Bob, but is timelocked and can only be used in the near future. Should anything go wrong with the atomic swap, Bob can simply wait and then publish the refund transaction.

The atomic swap itself makes uses of an "adaptor" signature scheme (as described in "Flipping the scriptless script on Schnorr"). Bob prepares a transaction which moves all siacoins from the 2-of-2 address to Alice. But instead of signing and publishing it in the regular way - which would allow Alice to just take the coins - he instead creates an adaptor public/private key pair and publishes an adaptor signature for the payout transaction. Alice cannot use this signature directly, but she can verify that if she were to learn the adaptor secret, she would be able to claim the coins.

Alice now locks up her ether in the roadie smart contract, where it can be released only under one of two conditions. The first is a timelock again - if anything goes wrong, Alice can simply wait and then reclaim her ether. The second condition is for Bob to reveal the adaptor secret and receive the payment.

Bob now sends the adaptor secret to the smart contract. The smart contract checks that it is indeed the correct secret (using ed25519-solidity, an implemention of the necessary elliptic curve math in Solidity to check that the adaptor secret matches the adaptor public key) and releases the ether payment to Bob. Alice uses the adaptor secret to build a valid payout transaction and claims the siacoins from the 2-of-2 multisig. The atomic swap is complete.

Installation

Roadie needs a Sia node with an unlocked wallet and an Ethereum node (for example geth). Your distribution might have packages for this software. It is all written in Go, so you can also use go get to install it. In this case the software will - by default - be installed in ~/go/bin/, so make sure that this directory is on your $PATH:

$ export PATH=$PATH:~/go/bin

Installing and running Sia:

$ go get -u gitlab.com/NebulousLabs/Sia/...     # '...' is important
$ mkdir sia; cd sia
~/sia$ siad
~/sia$ siac wallet init
~/sia$ siac wallet unlock

Installing and running geth:

$ go get -u github.com/ethereum/go-ethereum/cmd/geth
$ geth --syncmode light

Installing roadie:

$ go get -u github.com/javgh/roadie/cmd/roadie

Usage

Roadie uses its own Ethereum wallet. After initialization, the wallet will be stored in ~/.config/roadie/keystore with the password set to blank. If necessary, it can also be read by geth and many other Ethereum wallets.

$ roadie init
Creating new Ethereum keystore ~/.config/roadie/keystore
Ethereum address: <new Ethereum address here>
Ethereum balance: 0.000000 ETH

Please deposit funds into the address listed above.
A minimum of 0.010000 ETH is needed to proceed.

After funding the wallet, you are ready to buy siacoins:

$ roadie buy 1

See roadie help for additional options. The command roadie serve is currently for advanced users only. Not all aspects of running a server are documented yet. It will also probably be necessary to implement a custom pricing strategy - see FixedPremiumTrader in trader/trader.go for an example.

Sequence Diagram

Alice                                   Bob
-----                                   ---
                                        register server on blockchain
request non-binding offer for X SC
                                        send non-binding offer including:
                                        message, availability, X ether, X anti-spam fee
decide on offer
pick fresh anti-spam id
make anti-spam payment to
  hash(anti-spam id) and
  wait for confirmations
request binding offer with
  anti-spam id
                                        verify anti-spam payment
                                        lock in binding offer
                                        if lock successful: remember anti-spam id
                                        send binding offer including:
                                        message, availability, X ether
decide on offer
accept offer and
  send alicePubKey
                                        build funding tx
                                        build refund tx
                                          with timelock 4 hours
                                        send bobPubkey, fundingOutputID,
                                          bobRefundUnlockHash, timelock,
                                          bobRefundNoncePoint
check timelock
build refund tx
sign refund tx
send aliceRefundNoncePoint,
  refundSigAlice
                                        sign refund tx
                                        verify refund tx
                                        sign funding tx
                                        broadcast funding tx
                                        keep refund tx for potential rollback
                                        send funding tx id
wait for funding tx id
verify funding tx
build claim tx
send aliceClaimUnlockHash,
  aliceClaimNoncePoint
                                        build claim tx
                                        generate adaptor
                                        adaptor sign claim tx
                                        send bobClaimNoncePoint,
                                          adaptorPubKey, adaptorSigBob,
                                          depositRecipient
verify adaptor sig
deposit ether for adaptor
  with timelock 2 hours
  and burn anti-spam id
remember details for
  potential rollback
wait for deposit tx
announce deposit
                                        check deposit
                                        claim deposit and publish adaptor
                                        send ok
wait for adaptor
adaptor sign claim tx
combine sigs and adaptor
broadcast claim tx

Gas Cost

The smart contract unfortunately does require a fair amount of gas. Especially performing the necessary elliptic curve math can use more than 1 000 000 gas in some cases. It should be possible to optimize this further. Ethereum might add additional precompiles for these kinds of operations or it might be possible to port the optimized code at http://ed25519.cr.yp.to/ to Solidity.

Acknowledgments

This project makes heavy use of prior work done by the Hyperspace developers on a manual atomic swap.