Authentication Witness
This page introduces the authwit library and how you can use it in your Aztec.nr smart contracts. Skip to the usage.
For a guide on using authwit in Aztec.js, read this.
Prerequisite reading
Introduction
Authentication Witness is a scheme for authentication actions on Aztec, so users can allow third-parties (eg protocols or other users) to execute an action on their behalf.
How it works logically is explained in the concepts but we will do a short recap here.
An authentication witness is defined for a specific action, such as allowing a Defi protocol to transfer funds on behalf of the user. An action is here something that could be explained as A is allowed to perform X operation on behalf of B and we define it as a hash computed as such:
authentication_witness_action = H(
caller: AztecAddress,
contract: AztecAddress,
selector: Field,
argsHash: Field
);
// Example action that authenticates:
// defi contract to transfer 1000 tokens to itself on behalf of alice_account
authentication_witness_action = H(
defi,
token,
transfer_selector,
H(alice_account, defi, 1000)
);
Given the action, the developer can ask the on_behalf_of account contract if the action is authenticated or not.
Note in particular that the request for a witness is done by the token contract, and the user will have to provide it to the contract before it can continue execution. Since the request is made all the way into the contract where it is to be used, we don't need to pass it along as an extra input to the functions before it which gives us a cleaner interface.
As part of AuthWit we are assuming that the on_behalf_of implements the private function:
#[private]
fn verify_private_authwit(inner_hash: Field) -> Field;
For public authwit, we have a shared registry that is used, there we are using a consume function.
Both return the value 0xabf64ad4 (IS_VALID selector) for a successful authentication, and 0x00000000 for a failed authentication. You might be wondering why we are expecting the return value to be a selector instead of a boolean. This is mainly to account for a case of selector collisions where the same selector is used for different functions, and we don't want an account to mistakenly allow a different function to be called on its behalf - it is hard to return the selector by mistake, but you might have other functions returning a bool.
The AuthWit library.
As part of Aztec.nr, we are providing a library that can be used to implement authentication witness for your contracts.
This library also provides a basis for account implementations such that these can more easily implement authentication witness.
For our purposes here (not building a wallet), the most important part of the library is the auth utility which exposes a couple of helper methods for computing the action hash, retrieving witnesses, validating them and emitting the nullifier.
General utilities
The primary general utility is the compute_authwit_message_hash_from_call function which computes the action hash from its components. This is useful for when you need to generate a hash that is not for the current call, such as when you want to update a public approval state value that is later used for authentication in public. You can view the implementation of this function here (GitHub link).
TypeScript utilities
To make it convenient to compute the message hashes in TypeScript, the aztec.js package includes a computeAuthWitMessageHash function that you can use. Implementation here (GitHub link).
Utilities for private calls
For private calls where we allow execution on behalf of others, we generally want to check if the current call is authenticated by on_behalf_of. To easily do so, we can use the assert_current_call_valid_authwit which fetches information from the current context without us needing to provide much beyond the on_behalf_of.
This function will then make a call to on_behalf_of to execute the verify_private_authwit function which validates that the call is authenticated.
The on_behalf_of should assert that we are indeed authenticated and then return the IS_VALID selector. If the return value is not as expected, we throw an error. This is to cover the case where the on_behalf_of might implemented some function with the same selector as the verify_private_authwit that could be used to authenticate unintentionally.
Example
if (!from.eq(context.msg_sender())) {
assert_current_call_valid_authwit(&mut context, from);
} else {
assert(nonce == 0, "invalid nonce");
}
Source code: noir-projects/noir-contracts/contracts/token_contract/src/main.nr#L422-L428
Utilities for public calls
Very similar to the above, we have variations that work in the public domain (assert_current_call_valid_authwit_public). These functions are wrapped to give a similar flow for both cases, but behind the scenes the logic is slightly different since the public goes to the auth registry, while the private flow calls the account contract.
Example
if (!from.eq(context.msg_sender())) {
assert_current_call_valid_authwit_public(&mut context, from);
} else {
assert(nonce == 0, "invalid nonce");
}
Source code: noir-projects/noir-contracts/contracts/token_contract/src/main.nr#L259-L265
Usage
Ok, enough talking, how do we use this?
Importing it
To add it to your project, add the authwit library to your Nargo.toml file.
[dependencies]
aztec = { git="https://github.com/AztecProtocol/aztec-packages/", tag="aztec-packages-v0.62.0", directory="noir-projects/aztec-nr/aztec" }
authwit = { git="https://github.com/AztecProtocol/aztec-packages/", tag="aztec-packages-v0.62.0", directory="noir-projects/aztec-nr/authwit"}
Then you will be able to import it into your contracts as follows.
use dep::authwit::auth::{
assert_current_call_valid_authwit, assert_current_call_valid_authwit_public,
compute_authwit_nullifier,
};
Source code: noir-projects/noir-contracts/contracts/token_contract/src/main.nr#L47-L52
Private Functions
Checking if the current call is authenticated
Based on the diagram earlier on this page let's take a look at how we can implement the transfer function such that it checks if the tokens are to be transferred from the caller or needs to be authenticated with an authentication witness.
#[private]
fn transfer_from(from: AztecAddress, to: AztecAddress, amount: Field, nonce: Field) {
if (!from.eq(context.msg_sender())) {
assert_current_call_valid_authwit(&mut context, from);
} else {
assert(nonce == 0, "invalid nonce");
}
let from_ovpk_m = get_public_keys(from).ovpk_m;
let amount = U128::from_integer(amount);
// TODO: constrain encryption below - we are using unconstrained here only becuase of the following Noir issue
// https://github.com/noir-lang/noir/issues/5771
storage.balances.at(from).sub(from, amount).emit(encode_and_encrypt_note_unconstrained(
&mut context,
from_ovpk_m,
from,
from,
));
// TODO: constrain encryption below - we are using unconstrained here only becuase of the following Noir issue
// https://github.com/noir-lang/noir/issues/5771
storage.balances.at(to).add(to, amount).emit(encode_and_encrypt_note_unconstrained(
&mut context,
from_ovpk_m,
to,
from,
));
}
Source code: noir-projects/noir-contracts/contracts/token_contract/src/main.nr#L419-L453
The first thing we see in the snippet above, is that if from is not the call we are calling the assert_current_call_valid_authwit function from earlier. If the call is not throwing, we are all good and can continue with the transfer.
In the snippet we are constraining the else case such that only nonce = 0 is supported. This is not strictly necessary, but because I can't stand dangling useless values. By making it constrained, we can limit what people guess it does, I hope.
Authenticating an action in TypeScript
Cool, so we have a function that checks if the current call is authenticated, but how do we actually authenticate it? Well, assuming that we use a wallet that is following the spec, we import computeAuthWitMessageHash from aztec.js to help us compute the hash, and then we simply addAuthWitness to the wallet. Behind the scenes this will make the witness available to the oracle.
const action = asset
.withWallet(wallets[1])
.methods.transfer_from(accounts[0].address, accounts[1].address, amount, nonce);
const witness = await wallets[0].createAuthWit({ caller: accounts[1].address, action });
await wallets[1].addAuthWitness(witness);
expect(await wallets[0].lookupValidity(wallets[0].getAddress(), { caller: accounts[1].address, action })).toEqual({
isValidInPrivate: true,
isValidInPublic: false,
});
Source code: yarn-project/end-to-end/src/e2e_token_contract/transfer_private.test.ts#L88-L99
Learn more about authwits in Aztec.js by following this guide.
Public Functions
With private functions covered, how can we use this in a public function? Well, the answer is that we simply change one name of a function and then we are good to go 👀 (almost).
Checking if the current call is authenticated
#[public]
fn transfer_public(from: AztecAddress, to: AztecAddress, amount: Field, nonce: Field) {
if (!from.eq(context.msg_sender())) {
assert_current_call_valid_authwit_public(&mut context, from);
} else {
assert(nonce == 0, "invalid nonce");
}
let amount = U128::from_integer(amount);
let from_balance = storage.public_balances.at(from).read().sub(amount);
storage.public_balances.at(from).write(from_balance);
let to_balance = storage.public_balances.at(to).read().add(amount);
storage.public_balances.at(to).write(to_balance);
}
Source code: noir-projects/noir-contracts/contracts/token_contract/src/main.nr#L241-L255
Authenticating an action in TypeScript
Authenticating an action in the public domain is slightly different from the private domain, since we are executing a function on the auth registry contract to add the witness flag. As you might recall, this was to ensure that we don't need to call into the account contract from public, which is a potential DOS vector.
In the snippet below, this is done as a separate contract call, but can also be done as part of a batch as mentioned in the Accounts concepts.
const action = asset
.withWallet(wallets[1])
.methods.transfer_public(accounts[0].address, accounts[1].address, amount, nonce);
await wallets[0].setPublicAuthWit({ caller: accounts[1].address, action }, true).send().wait();
Source code: yarn-project/end-to-end/src/e2e_token_contract/transfer_public.test.ts#L50-L56
Updating approval state in Noir
We have cases where we need a non-wallet contract to approve an action to be executed by another contract. One of the cases could be when making more complex defi where funds are passed along. When doing so, we need the intermediate contracts to support approving of actions on their behalf.
This is fairly straight forward to do using the auth library which includes logic for updating values in the public auth registry. Namely, you can prepare the message_hash using compute_authwit_message_hash_from_call and then simply feed it into the set_authorized function (both are in auth library) to update the value.
When another contract later is consuming the authwit using assert_current_call_valid_authwit_public it will be calling the registry, and spend that authwit.
An example of this would be our Uniswap example which performs a cross chain swap on L1. In here, we both do private and public auth witnesses, where the public is set by the uniswap L2 contract itself. In the below snippet, you can see that we compute the action hash and update the value in the registry. When we then call the token_bridge to execute afterwards, it reads this value, burns the tokens, and consumes the authentication.
// This helper method approves the bridge to burn this contract's funds and exits the input asset to L1
// Assumes contract already has funds.
// Assume `token` relates to `token_bridge` (ie token_bridge.token == token)
// Note that private can't read public return values so created an internal public that handles everything
// this method is used for both private and public swaps.
#[public]
#[internal]
fn _approve_bridge_and_exit_input_asset_to_L1(
token: AztecAddress,
token_bridge: AztecAddress,
amount: Field,
) {
// Since we will authorize and instantly spend the funds, all in public, we can use the same nonce
// every interaction. In practice, the authwit should be squashed, so this is also cheap!
let nonce = 0xdeadbeef;
let selector = FunctionSelector::from_signature("burn_public((Field),Field,Field)");
let message_hash = compute_authwit_message_hash_from_call(
token_bridge,
token,
context.chain_id(),
context.version(),
selector,
[context.this_address().to_field(), amount, nonce],
);
// We need to make a call to update it.
set_authorized(&mut context, message_hash, true);
let this_portal_address = storage.portal_address.read_public();
// Exit to L1 Uniswap Portal !
TokenBridge::at(token_bridge)
.exit_to_l1_public(this_portal_address, amount, this_portal_address, nonce)
.call(&mut context)
}
Source code: noir-projects/noir-contracts/contracts/uniswap_contract/src/main.nr#L183-L219
Outlining more of the swap flow: this simplified diagram shows how it will look for contracts that are not wallets but also need to support authentication witnesses.
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