An asynchronous payment channel implemented in Func with a command line interface in Fift.

It allows two parties to send payments off-chain wihout trusting each other. “Asynchronous” means that the payments do not need receipt confirmations, are allowed to arrive in any order, and can even get lost without affecting the final settlement, as long as the last payment in each direction arrives or the party that sent it participates in the closing procedure.

You may want to start by following the Tutorial in `doc/Tutorial.md` or going through the protocol specification in `doc/Payment-channel.md`.

Cheers!

We would also like to present our second submission, which we unfortunately failed to properly submit to the bot in time (even though it was ready by the deadline).

It is a DSL for Fift Assembler embedded into Haskell. It heavily uses Haskell’s advanced type system to guarantee that the stack is correctly typed after executing each instruction.

Then we use this eDSL to implement a copy of the standard wallet and a multisig wallet contract.

You can find the source code here: https://github.com/serokell/fift-asm-dsl/tree/georgeee/signed-nonce

Just to top it off, a bunch of PRs that we made:

https://github.com/ton-blockchain/ton/pull/51
https://github.com/ton-blockchain/ton/pull/55
https://github.com/ton-blockchain/ton/pull/58

(and there are others to be made, but, of course, these won’t be part of our submission)

During our internal audit of the payment channel contract (which, unfortunately, we performed only after submitting the solution due to limited time) we discovered an issue with the cli. Since no one has reported it yet, we would like to self-report it here. The issue is that the cli tool does not compare the wallet address in the IOU message with the one that it uses to locate the state of the wallet. This is an issue with the cli, not with the contract and not with the protocol (the specification does not specify how the cli locates the state; we could have chosen not to ask the user to provide the address when receiving a payment and then this issue would not be possible). You can find more details here: https://github.com/serokell/ton-paychan/pull/19.

A great work actually.
But upvote spamming is meh.. (:

Multisig wallet
plus:
A very interesting infrastructure is written for the task. DSL for TVM assembly looks very beautiful and promising.
The resulting code is very readable despite being essentially an assembly code.
Garbage collect scans only expired entries.
Our test (one transfer with n=16, k=10) works.

minus:
Message by default is sent without flag 2.
Sadly, the resulting smart contract is quite slow. Not sure why. (0.125G for a transfer with n=16, k=10)
Not all features are implemented (merge signatures).

Async channel:
plus:
Greate documentation.
Readable and well-structured source code.
Non-obvious guarantees like "Reliable settlement for honest parties".
The message contains signed address of smart contract (protection from replay in other smart contracts)
Some bugs were fixed after the end of the contest.
Actually works on the blockchain.

minus:
Scripts do not check if the share is enough to make a transaction.
Some (minor) bugs were found after the end of the contest.

Run into issue with make:

[ 1][t 0][1571260814.111481190][words.cpp:2856] Fift.fif:1: //: -?

[ 1][t 0][1571260814.111542225][fift-main.cpp:172] Error interpreting standard preamble file `Fift.fif`: error interpreting included file `Fift.fif` : Fift.fif:1: //: -?

Check that correct include path is set by -I or by FIFTPATH environment variable, or disable standard preamble by -n.

make: *** [compile] Error 2

Nevertheless, build/paychan.asm.fif was generated.

Got the following error during "chan close".
It fails during mkCloseMsgBody.
My guess, it is because of Alice got no iou from Bob.

----
runalice paychan chan close $addr

+ cd alice
+ paychan chan close 0-dd9af4e8f088b9bc56552312ae11142ba5c4dfe083b44f812989fbe887984859

Loading secret key from file `alice.sk`

[ 1][t 0][1572268456.244788170][words.cpp:2906] Main.fif:17: cmdline_main: integer does not fit into cell

[ 1][t 0][1572268456.244848251][fift-main.cpp:196] Error interpreting file `/Users/arseny30/Downloads/contest/entry404/ton-paychan/src/cli/Main.fif`: error interpreting included file `Main.fif` : Main.fif:17: cmdline_main: integer does not fit into cell

About multisig. I tried to send a message with 0 grams. The transaction failed during action_phase with no_funds error.

And also this message was replayed several times, so now account balance is almost zero. Probably it is because you've sent an internal message without a +2 flag.

The address is kQBKB4lY_t7S6VtURW-6wYi9JnkkQdbKsPFzKpz_zycqAXAn

Am I right that there is no way to get more than fine if the other party disappears?

It would be reasonable to expect that if I have Iou from the other party, I would be able to receive at least that much after the account is closed.