Task: Build an Algorand Smart Contract

ryanRfox · December 17, 2019, 11:39pm

Hash Time Lock Contract (HTLC) for Algorand Standard Assets (ASA)

Functionality

This contract implements a hashed timelock contract (HTLC) for holding Algorand Standard Assets (ASA). It is intended for use in cross-chain atomic swaps of an ASA for an asset on a foreign chain using the same preimage.

The contract will approve transactions from itself under three scenarios:

If txn is an “opt-in” transaction for TMPL_ASA_ID.
If txn.FirstValid is greater than TMPL_TIMEOUT, then funds may be closed out to TMPL_OWN.
If an argument arg_0 is passed to the script such that TMPL_HASHFN(arg_0) is equal to TMPL_HASHIMG, then funds may be closed out to TMPL_RCV.

Code overview

( (Scenario 1) OR  ( (        Scenario 2           ) OR  (      Scenario 3     ) ) AND Fee)
( (  Opt-in  ) bnz ( ( Time Lock: ( ASA || Algos ) ) bnz ( Hash Lock: ASA Only ) ) && Fee )

TEAL Template Code

github.com

ryanRfox/go-algorand/blob/template-asa-htlc/tools/teal/templates/asa-htlc.teal.tmpl

// Implements a hashed timelock contract (HTLC) for Algorand Standard Assets (ASA).
//
// This is a contract account comprised of three validation scenarios:
//
// 1. An "opt-in" transaction is required to initialize this account to hold the designated `TMPL_ASA_ID`
// Money is released under two circumstances:
// 2. To TMPL_RCV if TMPL_HASHFN(arg_0) = TMPL_HASHIMG
// 3. To TMPL_OWN if txn.FirstValid > TMPL_TIMEOUT
//
// Parameters:
//  - `TMPL_RCV`:     the address to send funds to when the preimage is supplied
//  - `TMPL_HASHFN`:  the specific hash function (`sha256` or `keccak256`) to use
//  - `TMPL_HASHIMG`: the image of the hash function for which knowing the preimage under `TMPL_HASHFN` 
//                    will release funds
//  - `TMPL_TIMEOUT`: the round after which funds may be closed out to `TMPL_OWN`
//  - `TMPL_OWN`:     the address to refund funds to on timeout
//  - `TMPL_ASA_ID`:  the assetid of the ASA the contract may hold
//  - `TMPL_FEE`:     maximum fee of any transactions approved by this contract
//
// Code Overview:

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Template Details and Walkthrough

github.com

ryanRfox/go-algorand/blob/template-asa-htlc/tools/teal/templates/docs/asa-htlc.teal.md

# Hash Time Lock Contract (HTLC) for Algorand Standard Assets (ASA)

## Functionality

This contract implements a hashed timelock contract (HTLC) for holding Algorand Standard Assets (ASA). It is intended to be used as a "contract only" account, not as a "delegated contract" account. In other words, this contract should not be signed by a spending key. 

The contract will approve transactions from itself under three circumstances:

  1. If `txn` is an "opt-in" transaction for `TMPL_ASA_ID`.
  2. If `txn.FirstValid` is greater than `TMPL_TIMEOUT`, then funds may be closed out to `TMPL_OWN`.
  3. If an argument `arg_0` is passed to the script such that `TMPL_HASHFN(arg_0)` is equal to `TMPL_HASHIMG`, then funds may be closed out to `TMPL_RCV`. 
  
The idea is that by knowing the preimage to `TMPL_HASHIMG`, ASA funds may be released to `TMPL_RCV` (Scenario 3). Alternatively, after some timeout round `TMPL_TIMEOUT`, all remaining funds may be closed back to their original owner, `TMPL_OWN` (Scenario 2). Note that Scenario 3 may occur up until Scenario 2 occurs, even if `TMPL_TIMEOUT` has already passed. As a "contract only" account, Scenario 1 is required to initialize the account by performing an "opt-in" transaction prior to funding it with an amount of `TMPL_ASA_ID`.

## Parameters

  - `TMPL_RCV`: the address to send funds to when the preimage is supplied
  - `TMPL_HASHFN`: the specific hash function (`sha256` or `keccak256`) to use
  - `TMPL_HASHIMG`: the image of the hash function for which knowing the preimage under `TMPL_HASHFN` will release funds
  - `TMPL_TIMEOUT`: the round after which funds may be closed out to `TMPL_OWN`

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Next Steps

Please make use of the template and provide feedback. Pull Requests are welcome

Footnotes

All code is offered “AS-IS” and “WITHOUT WARRANTY” into the PUBLIC DOMAIN.

The only caveat I am aware of is overfunding the escrow account with Algos, as anyone may send an “opt-in” transaction which could consume all Algos upto the minimum balance requirement. Either party may send Algos to the escrow along with their HTLC transaction to pay for fees just-in-time, so the contract cannot be halted.

Eligible account to receive TEAL Badge:

FT43JWVPZQ4U3M5BL5DFENHG3LXWXD7Q34YZP6SWWQCEB7OBREMDM3RKC4

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