Partial Notes

Partial notes are a concept that allows users to commit to an encrypted value, and allows a counterparty to update that value without knowing the specific details of the encrypted value.

Use cases

Why is this useful?

Consider the case where a user wants to pay for a transaction fee, using a fee-payment contract and they want to do this privately. They can't be certain what the transaction fee will be because the state of the network will have progressed by the time the transaction is processed by the sequencer, and transaction fees are dynamic. So the user can commit to a value for the transaction fee, publicly post this commitment, the fee payer can update the public commitment, deducting the final cost of the transaction from the commitment and returning the unused value to the user.

So, in general, the user is:

• doing some computation in private
• encrypting/compressing that computation with a point
• passing that point as an argument to a public function

And the fee payer is:

• updating that point in public
• treating/emitting the result(s) as a note hash(es)

The idea of committing to a value and allowing a counterparty to update that value without knowing the specific details of the encrypted value is a powerful concept that can be used in many different applications. For example, this could be used for updating timestamp values in private, without revealing the exact timestamp, which could be useful for many defi applications.

To do this, we leverage the following properties of elliptic curve operations:

1. x_1 * G + x_2 * G equals (x_1 + x_2) * G and
2. f(x) = x * G being a one-way function.

Property 1 allows us to be continually adding to a point on elliptic curve and property 2 allows us to pass the point to a public realm without revealing anything about the point preimage.

Before getting to partial notes let's recap what is the flow of standard notes.

Note lifecycle recap

The standard note flow is as follows:

1. Create a note in your contract,
2. compute the note hash,
3. emit the note hash,
4. emit the note (note hash preimage) as an encrypted note log,
5. sequencer picks up the transaction, includes it in a block (note hash gets included in a note hash tree) and submits the block on-chain,
6. nodes and PXEs following the network pick up the new block, update its internal state and if they have accounts attached they search for relevant encrypted note logs,
7. if a users PXE finds a log it stores the note in its database,
8. later on when we want to spend a note, a contract obtains it via oracle and stores a note hash read request within the function context (note hash read request contains a newly computed note hash),
9. based on the note and a nullifier secret key a nullifier is computed and emitted,
10. protocol circuits check that the note is a valid note by checking that the note hash read request corresponds to a real note in the note hash tree and that the new nullifier does not yet exist in the nullifier tree,
11. if the conditions in point 10. are satisfied the nullifier is inserted into the nullifier tree and the note is at the end of its life.

Now let's do the same for partial notes.

Partial notes life cycle

1. Create a partial/unfinished note in a private function of your contract --> partial here means that the values within the note are not yet considered finalized (e.g. amount in a TokenNote),
2. compute a note hiding point of the partial note using a multi scalar multiplication on an elliptic curve. For TokenNote this would be done as G_amt * amount0 + G_npk * npk_m_hash + G_rnd * randomness + G_slot * slot, where each G_ is a generator point for a specific field in the note,
3. pass the note hiding point to a public function,
4. in a public function determine the value you want to add to the note (e.g. adding a value to an amount) and add it to the note hiding point (e.g. NOTE_HIDING_POINT + G_amt * amount),
5. get the note hash by finalizing the note hiding point (the note hash is the x coordinate of the point),
6. emit the note hash,
7. manually construct the note in your application and add it to your node (PXE) --> this currently has to be done manually and not automatically via encrypted note logs because we have not yet implemented partial notes delivery (tracked in issue #8238)
8. from this point on the flow of partial notes is the same as for normal notes.

Private Fee Payment Example

Alice wants to use a fee-payment contract for fee abstraction, and wants to use private balances. That is, she wants to pay the FPC (fee-payment contract) some amount in an arbitrary token privately (e.g. a stablecoin), and have the FPC pay the transaction_fee.

Alice also wants to get her refund privately in the same token (e.g. the stablecoin).

The trouble is that the FPC doesn't know if Alice is going to run public functions, in which case it doesn't know what refund is due until the end of public execution.

And we can't use the normal flow to create a transaction fee refund note for Alice, since that demands we have Alice's address in public.

So we define a new type of note with its compute_note_hiding_point defined as:

$$\text{amount}*G_{amount} + \text{address}*G_{address} + \text{randomness}*G_{randomness} + \text{slot}*G_{slot}$$

Suppose Alice is willing to pay up to a set amount in stablecoins for her transaction. (Note, this amount gets passed into public so that when transaction_fee is known the FPC can verify that it isn't losing money. Wallets are expected to choose common values here, e.g. powers of 10).

Then we can subtract the set amount from Alice's balance of private stablecoins, and create a point in private like:

$$P_a' := \text{alice address}*G_{address} + \text{rand}_a*G_{randomness} + \text{Alice note slot}*G_{slot}$$

We also need to create a point for the owner of the FPC (whom we call Bob) to receive the transaction fee, which will also need randomness.

So in the contract we compute \text{rand}_b := h(\text{rand}_a, \text{msg_sender}).

warning

We need to use different randomness for Bob's note here to avoid potential privacy leak (see description of setup_refund function)

$$P_b' := \text{bob address}*G_{address} + \text{rand}_b*G_{randomness} + \text{Bob note slot}*G_{slot}$$

Here, the $P'$s "partially encode" the notes that we are going to create for Alice and Bob. So we can use points as "Partial Notes".

We pass these points and the funded amount to public, and at the end of public execution, we compute tx fee point $P_{fee} := (\text{transaction fee}) * G_{amount}$ and refund point P_{refund} := (\text{funded_amount - transaction_fee}) * G_{amount}

Then, we arrive at the point that corresponds to the complete note by

$$P_a := P_a'+P_{refund} = (\text{funded amount} - \text{transaction fee})*G_{amount} + \text{alice address}*G_{address} +\text{rand}_a*G_{randomness} + \text{Alice note slot}*G_{slot}$$ $$P_b := P_b'+P_{fee} = (\text{transaction fee})*G_{amount} + \text{bob address}*G_{address} +\text{rand}_b*G_{randomness} + \text{Bob note slot}*G_{slot}$$

Then we just emit P_a.x and P_b.x as a note hashes, and we're done! (Now Alice and Bob need to manually add the notes to their PXEs since issue #8238 remains to be implemented.)

Private Fee Payment Implementation

NoteInterface.nr implements compute_note_hiding_point, which takes a note and computes the point "hides" it.

This is implemented in the example token contract:

compute_note_hiding_point

fn compute_note_hiding_point(self) -> Point {
    // We use the unsafe version because the multi_scalar_mul will constrain the scalars.
    let amount_scalar = from_field_unsafe(self.amount.to_integer());
    let npk_m_hash_scalar = from_field_unsafe(self.npk_m_hash);
    let randomness_scalar = from_field_unsafe(self.randomness);
    let slot_scalar = from_field_unsafe(self.header.storage_slot);
    // We compute the note hiding point as:
    //     `G_amt * amount + G_npk * npk_m_hash + G_rnd * randomness + G_slot * slot`
    // instead of using pedersen or poseidon2 because it allows us to privately add and subtract from amount
    // in public by leveraging homomorphism.
    multi_scalar_mul(
        [G_amt, G_npk, G_rnd, G_slot],
        [amount_scalar, npk_m_hash_scalar, randomness_scalar, slot_scalar]
    )
}

^{_{Source code: noir-projects/noir-contracts/contracts/token_contract/src/types/token_note.nr#L50-L66}}

Those G_x are generators that generated here. Anyone can use them for separating different fields in a "partial note".

We can see the complete implementation of creating and completing partial notes in an Aztec contract in the setup_refund and complete_refund functions.

setup_refund

setup_refund

#[aztec(private)]
fn setup_refund(
    fee_payer: AztecAddress, // Address of the entity which will receive the fee note.
    user: AztecAddress, // A user for which we are setting up the fee refund.
    funded_amount: Field, // The amount the user funded the fee payer with (represents fee limit).
    user_randomness: Field, // A randomness to mix in with the generated refund note for the sponsored user.
    fee_payer_randomness: Field // A randomness to mix in with the generated fee note for the fee payer.
) {
    // 1. This function is called by fee paying contract (fee_payer) when setting up a refund so we need to support
    // the authwit flow here and check that the user really permitted fee_payer to set up a refund on their behalf.
    assert_current_call_valid_authwit(&mut context, user);

    // 2. Get all the relevant keys
    let fee_payer_npk_m_hash = get_current_public_keys(&mut context, fee_payer).npk_m.hash();
    let user_keys = get_current_public_keys(&mut context, user);
    let user_npk_m_hash = user_keys.npk_m.hash();

    // 3. Deduct the funded amount from the user's balance - this is a maximum fee a user is willing to pay
    // (called fee limit in aztec spec). The difference between fee limit and the actual tx fee will be refunded
    // to the user in the `complete_refund(...)` function.
    let change = subtract_balance(
        &mut context,
        storage,
        user,
        U128::from_integer(funded_amount),
        INITIAL_TRANSFER_CALL_MAX_NOTES
    );
    storage.balances.at(user).add(user_keys.npk_m, change).emit(
        encode_and_encrypt_note_with_keys_unconstrained(&mut context, user_keys.ovpk_m, user_keys.ivpk_m, user)
    );

    // 4. We create the partial notes for the fee payer and the user.
    // --> Called "partial" because they don't have the amount set yet (that will be done in `complete_refund(...)`).
    let fee_payer_partial_note = TokenNote {
        header: NoteHeader {
            contract_address: AztecAddress::zero(),
            nonce: 0,
            storage_slot: storage.balances.at(fee_payer).set.storage_slot,
            note_hash_counter: 0
        },
        amount: U128::zero(),
        npk_m_hash: fee_payer_npk_m_hash,
        randomness: fee_payer_randomness
    };
    let user_partial_note = TokenNote {
        header: NoteHeader {
            contract_address: AztecAddress::zero(),
            nonce: 0,
            storage_slot: storage.balances.at(user).set.storage_slot,
            note_hash_counter: 0
        },
        amount: U128::zero(),
        npk_m_hash: user_npk_m_hash,
        randomness: user_randomness
    };

    // 5. Now we get the note hiding points.
    let mut fee_payer_point = fee_payer_partial_note.to_note_hiding_point();
    let mut user_point = user_partial_note.to_note_hiding_point();

    // 6. Set the public teardown function to `complete_refund(...)`. Public teardown is the only time when a public
    // function has access to the final transaction fee, which is needed to compute the actual refund amount.
    context.set_public_teardown_function(
        context.this_address(),
        comptime {
        FunctionSelector::from_signature("complete_refund(((Field,Field,bool)),((Field,Field,bool)),Field)")
    },
        [
        fee_payer_point.inner.x, fee_payer_point.inner.y, fee_payer_point.inner.is_infinite as Field, user_point.inner.x, user_point.inner.y, user_point.inner.is_infinite as Field, funded_amount
    ]
    );
}

^{_{Source code: noir-projects/noir-contracts/contracts/token_contract/src/main.nr#L510-L583}}

The setup_refund function sets the complete_refund function to be called at the end of the public function execution (set_public_teardown_function). This ensures that the partial notes will be completed and the fee payer will be paid and the user refund will be issued.

complete_refund

complete_refund

#[aztec(public)]
#[aztec(internal)]
fn complete_refund(
    // TODO(#7771): the following makes macros crash --> try getting it work once we migrate to metaprogramming
    // mut fee_payer_point: TokenNoteHidingPoint,
    // mut user_point: TokenNoteHidingPoint,
    fee_payer_point_immutable: TokenNoteHidingPoint,
    user_point_immutable: TokenNoteHidingPoint,
    funded_amount: Field
) {
    // TODO(#7771): nuke the following 2 lines once we have mutable args
    let mut fee_payer_point = fee_payer_point_immutable;
    let mut user_point = user_point_immutable;

    // TODO(#7728): Remove the next line
    let funded_amount = U128::from_integer(funded_amount);
    let tx_fee = U128::from_integer(context.transaction_fee());

    // 1. We check that user funded the fee payer contract with at least the transaction fee.
    // TODO(#7796): we should try to prevent reverts here
    assert(funded_amount >= tx_fee, "funded amount not enough to cover tx fee");

    // 2. We compute the refund amount as the difference between funded amount and tx fee.
    let refund_amount = funded_amount - tx_fee;

    // 3. We add fee to the fee payer point and refund amount to the user point.
    fee_payer_point.add_amount(tx_fee);
    user_point.add_amount(refund_amount);

    // 4. We finalize the hiding points to get the note hashes.
    let fee_payer_note_hash = fee_payer_point.finalize();
    let user_note_hash = user_point.finalize();

    // 5. At last we emit the note hashes.
    context.push_note_hash(fee_payer_note_hash);
    context.push_note_hash(user_note_hash);
    // --> Once the tx is settled user and fee recipient can add the notes to their pixies.
}

^{_{Source code: noir-projects/noir-contracts/contracts/token_contract/src/main.nr#L587-L626}}

Future work

This pattern of making public commitments to notes that can be modified by another party, privately, can be generalized to work with different kinds of applications. The Aztec labs team is working on adding libraries and tooling to make this easier to implement in your own contracts.