Async payment channel for TON

Our team made a sync channel with all wrappers. However, we did not finished yet with the same wrappers to async channel, we decided to apply only with funC smart contract and docs about it.

Description

State

Variables	Size	Description
init	1 bit	flag to initialize contract
timeout	32 bits	timeout in second for exit
timestamp	48 bits	unix time in seconds
pubkA	256 bits	pub key of user A
pubkB	256 bits	pub key of user B
fA	1 bit	flag registry for A
fB	1 bit	flag registry for B
X_a	214 bits	nanograms registry for A
X_b	214 bits	nanograms registry for B

The total size is 1023 and it is enough to store in 1 Cell. 214 bits is enough to rotate all balance of TON network for 5e45 times, so, it is safe.

State evolution

Deploy

The first the state is (0, timeout, 0, pubkA, pubkB, 0, 0, 0, 0)

Setup

Onchain

Users can deposit and withdraw grams and store intermediate values into X_a and X_b.

Offchain

Users decide, which amounts U_a and U_b each user needs to deposit to open the channel and which shares a and b will belong to each user as initial.

At first, A sign and send to B signed(A, (contract_address, b/2)) and B send to A signed(B, (contract_address, a/2)), where c=a+b

After that A and B sign both and exchanges with(contract_address, 0, U_a, U_b)

Onchain

Any user send to the contract (contract_address, U_a, U_b), the contract checks, that X_a and X_b more or equal then U_a and U_b, after that init flag is set to 1, remained grams returns to owners, X_a and X_b set to zero.

Protocol

Each user stores current value for coins received by another user. When the user needs to send some more value, the user signs new integral value of all amount received by the counterparty as signed(user, (contract_address, message_code, x)) and send the data to another party. The procedure is unblockable. Technically, the total user's balance may become negative. We consider negative balance as zero balance.

Closing

Let's user A decide to close the channel.

• A send to the protocol tuple of separately signed values (X_a, X_b) into the contract. The contract set X_a, X_b, timestamp:=now+timeout, and set F_b:=1. This means two things:

  • F_b is fixed and nobody can change it
  • if anybody preset signed by A F_b more then presented by A, the contract send all money to B.

• B can process the same procedure with one restriction. Both values sent by B must be more or equal than the same sent by A.

• If now > timestamp || F_a && F_b, the contract provide withdrawal procedure for the users.

Withdrawal

# Consider C is the total amount, A and B value for users A and B to withdraw

a := min(C, max(0, X_a - X_b + C/2))
b := C - a

Force exit

Send to special method the message (contract_address, exit_code, a), signed by both users, the contract send a and total_amount-a grams to receivers and close the channel.

Requirements

• have func and fift available in $PATH
• have ton lite node running
• have wallets for Alice and Bob
• have alice.addr, alice.pk, bob.addr, bob.pk in the current dir

Deployment

• func stdlib.fc lightning.fc > lightning.fif - this file is already commited to the repo
• ./deploy.fif <alice addr> <bob addr> <challenge timeout in sec>
• send some test grams to the displayed address
• in ton node sendfile deploy.boc

Notes

See test dir for more usage examples. It demonstrates work of all functions.

you can compile using stdlib_test.fc to replace my_address() and get_balance() calls with mocks but unfortunately it does not work for now() because if compiler bug (function name collides with its opcode), you'll need to disable those calls in code manually to avoid errors.

Authors

Nick Kozlov - CTO and Co-founder of BUTTON Wallet (@enormousrage, nk@buttonwallet.com)

Kirill Kuznecov - Co-founder of BUTTON Wallet (@krboktv, kk@buttonwallet.com)

Alexey Prazdnikov - Fullstack developer at BUTTON Wallet (@noprazd, ap@buttonwallet.com)

Max Spiridonov - Lead Backend developer at BUTTON Wallet (@maxSpt, ms@buttonwallet.com)

Ivan Frolov - Backend developer at BUTTON Wallet (@ivnkhh, if@buttonwallet.com)

Artem Vorobev - Lead Developer at BUTTON Wallet ( @artall64 artem@buttonwallet.com )

Roman Semenov - One of founders of Copperbits community, co-author of Tornado.cash Ethereum mixer (@poma, semenov.roma@gmail.com)

Igor Gulamov Blockchain Researcher and Entrepreneur (@igor_gulamov, igor.gulamov@gmail.com)