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Write a donations contract

In this tutorial we'll create two contracts related to crowdfunding:

  • A crowdfunding contract with two core components
    • Fully private donations
    • Verifiable withdrawals to the operator
  • A reward contract for anyone else to anonymously reward donors

Along the way you will:

  • Install Aztec developer tools
  • Setup a new Noir contract project
  • Add base Aztec dependencies
  • Call between private and public contexts
  • Wrap an address with its interface (token)
  • Create custom private value notes

Setup

Install tools

Please ensure that the you already have Installed the Sandbox.

And if using VSCode, see here to install Noir LSP, where you'll benefit from syntax highlighting, profiling, and more.

Create an Aztec project

Use aztec-nargo in a terminal to create a new Aztec contract project named "crowdfunding":

aztec-nargo new --contract crowdfunding

Inside the new crowdfunding directory you will have a base to implement the Aztec smart contract.

Use aztec-nargo --help to see other commands.

Private donations

  1. An "Operator" begins a Crowdfunding campaign (contract), specifying:
  • an existing token address
  • their account address
  • a deadline timestamp
  1. Any address can donate (in private context)
  • private transfer token from sender to contract
  • transaction receipts allow private claims via another contract
  1. Only the operator can withdraw from the fund

1. Create a campaign

Initialize

Open the project in your preferred editor. If using VSCode and the LSP, you'll be able to select the aztec-nargo binary to use (instead of nargo).

In main.nr, rename the contract from Main, to Crowdfunding.

empty-contract
contract Crowdfunding {
Source code: noir-projects/noir-contracts/contracts/crowdfunding_contract/src/main.nr#L1-L3

Replace the example functions with an initializer that takes the required campaign info as parameters. Notice use of #[aztec(...)] macros inform the compiler that the function is a public initializer.

#[aztec(public)]
#[aztec(initializer)]
fn init(donation_token: AztecAddress, operator: AztecAddress, deadline: u64) {
//...
}

More about initializers here.

Dependencies

When you compile the contracts by running aztec-nargo compile in your project directory, you'll notice it cannot resolve AztecAddress. (Or hovering over in VSCode)

#[aztec(public)]
#[aztec(initializer)]
fn init(donation_token: AztecAddress, operator: AztecAddress, deadline: u64) {
//...
}

Add the required dependency by going to your project's Nargo.toml file, and adding aztec from the aztec-nr framework. It resides in the aztec-packages mono-repo:

[dependencies]
aztec = { git="https://github.com/AztecProtocol/aztec-packages/", tag="aztec-packages-v0.43.0", directory="noir-projects/aztec-nr/aztec" }

A word about versions:

  • Choose the aztec packages version to match your aztec sandbox version
  • Check that your compiler_version in Nargo.toml is satisified by your aztec compiler - aztec-nargo -V

More about versions here.

Inside the Crowdfunding contract definition, use the dependency that defines the address type AztecAddress (same syntax as Rust)

use dep::aztec::protocol_types::address::AztecAddress;

The aztec::protocol_types can be browsed here. And like rust dependencies, the relative path inside the dependency corresponds to address::AztecAddress.

Storage

To retain the initializer parameters in the contract's Storage, we'll need to declare them in a preceding Storage struct:

storage
#[aztec(storage)]
struct Storage {
// Token used for donations (e.g. DAI)
donation_token: SharedImmutable<AztecAddress>,
// Crowdfunding campaign operator
operator: SharedImmutable<AztecAddress>,
// End of the crowdfunding campaign after which no more donations are accepted
deadline: PublicImmutable<u64>,
// Notes emitted to donors when they donate (can be used as proof to obtain rewards, eg in Claim contracts)
donation_receipts: PrivateSet<ValueNote>,
}
Source code: noir-projects/noir-contracts/contracts/crowdfunding_contract/src/main.nr#L30-L42

The ValueNote type is in the top-level of the Aztec.nr framework, namely noir-projects/aztec-nr. Like before, you'll need to add the crate to Nargo.toml

(See here for common dependencies).


Back in main.nr, reference use of the type

use dep::value_note::value_note::ValueNote;

Now complete the initializer by setting the storage variables with the parameters:

init
#[aztec(public)]
#[aztec(initializer)]
fn init(donation_token: AztecAddress, operator: AztecAddress, deadline: u64) {
storage.donation_token.initialize(donation_token);
storage.operator.initialize(operator);
storage.deadline.initialize(deadline);
}
Source code: noir-projects/noir-contracts/contracts/crowdfunding_contract/src/main.nr#L44-L57

You can compile the code so far with aztec-nargo compile.

2. Taking private donations

Checking campaign duration against the timestamp

To check that the donation occurs before the campaign deadline, we must access the public timestamp. It is one of several Public Global Variables.

Declare an Aztec function that is public and internal

#[aztec(public)]
#[aztec(internal)]
#[aztec(view)]
fn _check_deadline() {
//...
}

Read the deadline from storage and assert that the timestamp from this context is before the deadline

deadline
#[aztec(public)]
#[aztec(internal)]
#[aztec(view)]
fn _check_deadline() {
let deadline = storage.deadline.read();
assert(context.timestamp() < deadline, "Deadline has passed");
}
Source code: noir-projects/noir-contracts/contracts/crowdfunding_contract/src/main.nr#L59-L69

Since donations are to be private, the donate function will have the user's private context which has these Private Global Variables. So from the private context there is a little extra to call the (public internal) _check_deadline function.

#[aztec(private)]
fn donate(amount: u64) {
// 1) Check that the deadline has not passed
Crowdfunding::at(context.this_address())._check_deadline().enqueue_view(&mut context);
//...
}

Namely calling enqueue and passing the (mutable) context.

Now conclude adding all dependencies to the Crowdfunding contract:

all-deps
use dep::aztec::{
protocol_types::{
abis::function_selector::FunctionSelector, address::AztecAddress, traits::Serialize,
grumpkin_point::GrumpkinPoint
},
encrypted_logs::encrypted_note_emission::encode_and_encrypt,
state_vars::{PrivateSet, PublicImmutable, SharedImmutable}
};
use dep::value_note::value_note::ValueNote;
use dep::token::Token;
Source code: noir-projects/noir-contracts/contracts/crowdfunding_contract/src/main.nr#L5-L16

Like before, you can find these and other aztec::protocol_types here.

Interfacing with another contract

The token being used for donations is stored simply as an AztecAddress (named donation_token). so to easily use it as a token, we let the compiler know that we want the address to have a Token interface. Here we will use a maintained example Token contract.

Add this Token contract to Nargo.toml:

token = { git="https://github.com/AztecProtocol/aztec-packages/", tag="aztec-packages-v0.43.0", directory="noir-projects/noir-contracts/contracts/token_contract" }

With the dependency already used at the start of the contract, the token contract can be called to make the transfer from msg sender to this contract.

note

The user must have authorised this action (concept here), example use of createAuthWit in 'full donor flow' test here.

Creating and storing a private receipt note

The last thing to do is create a new value note and add it to the donation_receipts. So the full donation function is now

donate
#[aztec(private)]
fn donate(amount: u64) {
// 1) Check that the deadline has not passed
Crowdfunding::at(context.this_address())._check_deadline().enqueue_view(&mut context);

// 2) Transfer the donation tokens from donor to this contract
let donor = context.msg_sender();
Token::at(storage.donation_token.read_private()).transfer_from(donor, context.this_address(), amount as Field, 0).call(&mut context);

let header = context.get_header();
// 3) Create a value note for the donor so that he can later on claim a rewards token in the Claim
// contract by proving that the hash of this note exists in the note hash tree.
let donor_npk_m_hash = header.get_npk_m_hash(&mut context, donor);
let mut note = ValueNote::new(amount as Field, donor_npk_m_hash);
storage.donation_receipts.insert(&mut note).emit(encode_and_encrypt(&mut context, donor, donor));
}
Source code: noir-projects/noir-contracts/contracts/crowdfunding_contract/src/main.nr#L71-L92

3. Operator withdrawals

The remaining function to implement, withdraw, is reasonably straight-forward:

  1. make sure the address calling is the operator address
  2. transfer tokens from the contract to the operator
  3. reveal that an amount has been withdrawn to the operator

The last point is achieved by emitting an unencrypted event log, more here.

Copy the last function into your Crowdfunding contract:

operator-withdrawals
// Withdraws balance to the operator. Requires that msg_sender() is the operator.
#[aztec(private)]
fn withdraw(amount: u64) {
// 1) Check that msg_sender() is the operator
let operator_address = storage.operator.read_private();
assert(context.msg_sender() == operator_address, "Not an operator");

// 2) Transfer the donation tokens from this contract to the operator
Token::at(storage.donation_token.read_private()).transfer(operator_address, amount as Field).call(&mut context);

// 3) Emit an unencrypted event so that anyone can audit how much the operator has withdrawn
let event = WithdrawalProcessed { amount, who: operator_address };
context.emit_unencrypted_log(event.serialize());
}
Source code: noir-projects/noir-contracts/contracts/crowdfunding_contract/src/main.nr#L94-L109

You should be able to compile successfully with aztec-nargo compile.

Conclusion

For comparison, the full Crowdfunding contract can be found here.

Next steps?

If a new token wishes to honour donors with free tokens based on donation amounts, this is possible via the donation_receipts (a PrivateSet). See claim_contract.