- Split Input: an Action input used to ensure that the output note of that Action is of a validly issued :math:`\mathsf{AssetBase}` when there is no corresponding real input note, in situations where the number of outputs are larger than the number of inputs. See formal definition in `Split Notes`_.
None of the currently deployed Zcash transfer protocols support Custom Assets. Enabling
multi-asset support on the Zcash chain will open the door for a host of applications, and
enhance the ecosystem with application developers and Asset custody institutions for
issuance and bridging purposes.
Overview
========
In order to be able to represent different Assets, we need to define a data field that uniquely represents the Asset in question, which we call the Asset Identifier :math:`\mathsf{AssetId}`.
This Asset Identifier maps to an Asset Base :math:`\mathsf{AssetBase}^{\mathsf{Orchard}}` that is stored in Orchard-based ZSA notes.
These terms are formally defined in ZIP 227 [#zip-0227]_.
The Asset Identifier (via means of the Asset Digest and Asset Base) will be used to enforce that the balance of an Action Description [#protocol-actions]_ is preserved across Assets (see the Orchard Binding Signature [#protocol-binding]_), and by extension the balance of an Orchard transaction. That is, the sum of all the :math:`\mathsf{value^{net}}` from each Action Description, computed as :math:`\mathsf{value^{old}-value^{new}}`, must be balanced **only with respect to the same Asset Identifier**. This is especially important since we will allow different Action Descriptions to transfer notes of different Asset Identifiers, where the overall balance is checked without revealing which (or how many distinct) Assets are being transferred.
As was initially proposed by Jack Grigg and Daira Hopwood [#initial-zsa-issue]_[#generalized-value-commitments]_, we propose to make this happen by changing the value base point, :math:`\mathcal{V}^{\mathsf{Orchard}}`, in the Homomorphic Pedersen Commitment that derives the value commitment, :math:`\mathsf{cv^{net}}`, of the *net value* in an Orchard Action.
Because in a single transaction all value commitments are balanced, there must be as many different value base points as there are Asset Identifiers for a given shielded protocol used in a transaction. We propose to make the Asset Base :math:`\mathsf{AssetBase}^{\mathsf{Orchard}}` an auxiliary input to the proof for each Action statement [#protocol-actionstatement]_, represented already as a point on the Pallas curve. The circuit then should check that the same :math:`\mathsf{AssetBase}^{\mathsf{Orchard}}` is used in the old note commitment and the new note commitment [#protocol-concretesinsemillacommit]_, **and** as the base point :math:`\mathcal{V}^\mathsf{Orchard}` in the value commitment [#protocol-concretevaluecommit]_. This ensures (1) that the input and output notes are of the same :math:`\mathsf{AssetBase}^{\mathsf{Orchard}}`, and (2) that only Actions with the same Asset Base will balance out in the Orchard binding signature.
In order to ensure the security of the transfers, and as we will explain below, we are redefining input dummy notes [#protocol-dummynotes]_ for Custom Assets, as we need to enforce that the :math:`\mathsf{AssetBase}^{\mathsf{Orchard}}` of the output note of that Split Action is the output of a valid :math:`\mathsf{ZSAValueBase^{Orchard}}` computation defined in ZIP 227 [#zip-0227]_.
Finally, in this ZIP we also describe the *burn* mechanism, which is a direct extension of the transfer mechanism. The burn process uses a similar mechanism to what is used in Orchard to unshield ZEC, by using the :math:`\mathsf{valueBalance}` of the Asset in question. Burning Assets is useful for many purposes, including bridging of Wrapped Assets and removing supply of Assets.
Specification
=============
Most of the protocol is kept the same as the Orchard protocol released with NU5, except for the following.
For every new Asset, there must be a new and unique Asset Identifier. Every Asset is defined by an *Asset description*, :math:`\mathsf{asset\_desc}`, which is a global byte string (scoped across all future versions of Zcash). From this Asset description and the issuance key of the issuer, the specific Asset Identifier, :math:`\mathsf{AssetId}`, the Asset Digest, and the Asset Base (:math:`\mathsf{AssetBase}^{\mathsf{Orchard}}` for the Orchard-based ZSA protocol) are derived as defined in ZIP 227 [#zip-0227]_.
This :math:`\mathsf{AssetBase}^{\mathsf{Orchard}}` will be the base point of the value commitment for the specific Custom Asset. Note that the :math:`\mathsf{AssetBase}^{\mathsf{Orchard}}` of the ZEC Asset will be kept as the original value base point, :math:`\mathcal{V}^\mathsf{Orchard}`.
In future network and protocol upgrades, the same Asset description string can be carried on, potentially mapping into a different shielded pool. In that case, nodes should know how to transform the Asset Identifier, the Asset Digest, and the Asset Base from one shielded pool to another, while ensuring there are no balance violations [#zip-0209]_.
A ZSA note differs from an Orchard note [#protocol-notes]_ by additionally including the Asset Base, :math:`\mathsf{AssetBase}^{\mathsf{Orchard}}`. So a ZSA note is a tuple :math:`(\mathsf{g_d, pk_d, v, \rho, \psi, \mathsf{AssetBase}^{\mathsf{Orchard}}})`,
-:math:`\mathsf{AssetBase}^{\mathsf{Orchard}} : \mathbb{P}*` is the unique element of the Pallas group [#protocol-pallasandvesta]_ that identifies each Asset in the Orchard protocol, defined as the Asset Base in ZIP 227 [#zip-0227]_, a valid non-bottom group element that is not the identity. The byte representation of the Asset Base is defined as :math:`\mathsf{asset\_base} : \mathbb{B}^{[\ell_{\mathbb{P}}]} := \mathsf{repr}_{\mathbb{P}}(\mathsf{AssetBase}^{\mathsf{Orchard}})`.
where :math:`\mathbb{P}, \ell_{\mathbb{P}}, q_{\mathbb{P}}` are as defined for the Pallas curve [#protocol-pallasandvesta]_, and :math:`\mathsf{NoteCommit}^{\mathsf{Orchard}}.\mathsf{Trapdoor}, \mathsf{Orchard}.\mathsf{Output}` are as defined in the Zcash protocol specification [#protocol-abstractcommit]_.
Note that :math:`\mathsf{repr}_{\mathbb{P}}` and :math:`\mathsf{GroupHash}^{\mathbb{P}}` are as defined for the Pallas curve [#protocol-pallasandvesta]_, :math:`\ell^{\mathsf{Orchard}}_{\mathsf{base}}` is as defined in §5.3 [#protocol-constants]_, and :math:`\mathsf{I2LEBSP}` is as defined in §5.1 [#protocol-endian]_ of the Zcash protocol specification.
In the ZSA protocol, the instance of the note commitment scheme, :math:`\mathsf{NoteCommit^{OrchardZSA}_{rcm}}`, differs from the Orchard note commitment :math:`\mathsf{NoteCommit^{Orchard}_{rcm}}` in that for Custom Assets, the Asset Base will be added as an input to the commitment computation.
In the case where the Asset is the ZEC Asset, the commitment is computed identically to the Orchard note commitment, without making use of the ZEC Asset Base as an input.
As we will see, the nested structure of the Sinsemilla-based commitment [#protocol-concretesinsemillacommit]_ allows us to add the Asset Base as a final recursive step, and hence keep a single instance of the Sinsemilla hash function in the circuit for the note commitment verification.
The note commitment output is still indistinguishable from the original Orchard ZEC note commitments, by definition of the Sinsemilla hash function [#protocol-concretesinsemillahash]_. ZSA note commitments will therefore be added to the same Orchard Note Commitment Tree. In essence, we have:
This definition can be viewed as a generalization of the Orchard note commitment, and will allow maintaining a single commitment instance for the note commitment, which will be used both for pre-ZSA Orchard and ZSA notes.
In the case of the ZSA protocol, the value of different Asset Identifiers in a given transaction will be committed using a **different value base point**. The value commitment becomes:
where :math:`\mathsf{v^{net}_{AssetId}} = \mathsf{v^{old}_{AssetId} - v^{new}_{AssetId}}` such that :math:`\mathsf{v^{old}_{AssetId}}` and :math:`\mathsf{v^{new}_{AssetId}}` are the values of the old and new notes of Asset Identifier :math:`\mathsf{AssetId}` respectively,
:math:`\mathsf{AssetBase}^{\mathsf{Orchard}}_{\mathsf{AssetId}}` is defined in ZIP 227 [#zip-0227]_, and
:math:`\mathcal{R}^{\mathsf{Orchard}}:=\mathsf{GroupHash^{\mathbb{P}}}\texttt{("z.cash:Orchard-cv", "r")}`, as in the Orchard protocol.
We define :math:`\mathsf{AssetBase}^{\mathsf{Orchard}}_{\mathsf{ZEC}} :=\mathcal{V}^{\mathsf{Orchard}}` so that the value commitment for ZEC notes is computed identically to the Orchard protocol deployed in NU5 [#zip-0224]_.
Rationale for Value Commitment
''''''''''''''''''''''''''''''
The Orchard Protocol uses a Homomorphic Pedersen Commitment [#protocol-concretevaluecommit]_ to perform the value commitment, with fixed base points :math:`\mathcal{V}^{\mathsf{Orchard}}` and :math:`\mathcal{R}^{\mathsf{Orchard}}` as the values represent the amount of ZEC being transferred.
The use of different value base points for different Assets enables the final balance of the transaction to be securely computed, such that each Asset Identifier is balanced independently, which is required as different Assets are not meant to be mutually fungible.
Value Balance Verification
--------------------------
In order to verify the balance of the different Assets, the verifier MUST perform exactly the same process as for the Orchard protocol [#protocol-binding]_.
For a total of :math:`n` Actions in a transfer, the prover MUST still sign the `SIGHASH` of the transaction using the binding signature key
and use it to verify the `bindingSignature` on the `SIGHASH` message.
Rationale for Value Balance Verification
''''''''''''''''''''''''''''''''''''''''
The main reason why no changes to the Orchard process are needed is that no Custom Assets can be unshielded, so all Custom Assets are contained within the shielded pool. This means that the net balance of the input and output values is zero, with only one Asset of value balance published, that of ZEC, :math:`\mathsf{v^{balanceOrchard}}`. No net amount of any other Asset will be revealed, and the number of Assets in the transaction is also hidden. The only exception to this is in the case that an Asset is *burnt*, as we will see below in the `burn mechanism`_.
As in the Orchard protocol, the binding signature verification key, :math:`\mathsf{bvk}`, will only be valid (and hence verify the signature correctly), as long as the committed values sum to zero. In contrast, in this protocol, the committed values only sum to zero **per Asset Base**, as the Pedersen commitments add up homomorphically only with respect to the same value base point.
A Split Input is a copy of a previously issued input note (that is, a note that has previously been included in the Merkle tree), with the following changes:
When the number of input notes of a particular Asset Base is smaller than the required number of output notes for the same Asset Base, the sender creates Split Inputs of the same Asset Base as padding for the input-less Actions. Note that we do not care about whether the previously issued note copied to create a Split Input is owned by the sender, or whether it was nullified before.
1. The Split Input note could be another note containing the same Asset Base that is being spent by this transaction (but not by this Split Input).
2. The Split Input note could be a different unspent note containing the same Asset Base (note that the note will not actually be spent).
3. The Split Input note could be an already spent note containing the same Asset Base (note that by zeroing the value in the circuit, we prevent double spending).
In the Orchard protocol, since each Action represents an input and an output, the transaction that wants to send one input to multiple outputs must have multiple inputs. The Orchard protocol gives *dummy spend notes*[#protocol-dummynotes]_ to the Actions that have not been assigned input notes.
The Orchard technique requires modification for the ZSA protocol with multiple Asset Identifiers, as the output note of the split Actions *cannot* contain *any* Asset Base. We must enforce it to be an actual output of a GroupHash computation (in fact, we want it to be of the same Asset Base as the original input note, but the binding signature takes care that the proper balancing is performed). Without this enforcement the prover could input a multiple (or linear combination) of an existing Asset Base, and thereby attack the network by overflowing the ZEC value balance and hence counterfeiting ZEC funds.
Therefore, for Custom Assets we enforce that *every* input note to an ZSA Action must be proven to exist in the set of note commitments in the note commitment tree. We then enforce this real note to be “unspendable” in the sense that its value will be zeroed in split Actions and the nullifier will be randomized, making the note not spendable in the specific Action. Then, the proof itself ensures that the output note is of the same Asset Base as the input note. In the circuit, the split note functionality will be activated by a boolean private input to the proof (aka the ``split_flag`` boolean).
This ensures that the value base points of all output notes of a transfer are actual outputs of a GroupHash, as they originate in the Issuance protocol which is publicly verified.
Note that the Orchard dummy note functionality remains in use for ZEC notes, and the Split Input technique is used in order to support Custom Assets.
Every *ZSA Action statement* is closely similar to the Orchard Action statement [#protocol-actionstatement]_, except for a few additions that ensure the security of the Asset Identifier system. We detail these changes below.
- The Asset Base, :math:`\mathsf{AssetBase}^{\mathsf{Orchard}}_{\mathsf{AssetId}}`, for the note is witnessed once, as an auxiliary input.
- In the Old note commitment integrity constraint in the Orchard Action statement [#protocol-actionstatement]_, :math:`\mathsf{NoteCommit^{Orchard}_{rcm^{old}}(repr_{\mathbb{P}}(g_d^{old}), repr_{\mathbb{P}}(pk_d^{old}), v^{old}, \rho^{old}, \psi^{old})}` is replaced with :math:`\mathsf{NoteCommit^{OrchardZSA}_{rcm^{old}}(repr_{\mathbb{P}}(g_d^{old}), repr_{\mathbb{P}}(pk_d^{old}), v^{old}, \rho^{old}, \psi^{old}, \mathsf{AssetBase}^{\mathsf{Orchard}}_{\mathsf{AssetId}})}`.
- In the New note commitment integrity constraint in the Orchard Action statement [#protocol-actionstatement]_, :math:`\mathsf{NoteCommit^{Orchard}_{rcm^{new}}(repr_{\mathbb{P}}(g_d^{new}), repr_{\mathbb{P}}(pk_d^{new}), v^{new}, \rho^{new}, \psi^{new})}` is replaced with :math:`\mathsf{NoteCommit^{OrchardZSA}_{rcm^{new}}(repr_{\mathbb{P}}(g_d^{new}), repr_{\mathbb{P}}(pk_d^{new}), v^{new}, \rho^{new}, \psi^{new}, \mathsf{AssetBase}^{\mathsf{Orchard}}_{\mathsf{AssetId}})}`.
- The fixed-base multiplication constraints between the value and the value base point of the value commitment, :math:`\mathsf{cv}`, is replaced with a variable-base multiplication between the two.
- The witness to the value base point (as defined in the `asset base`_ equation) is the auxiliary input :math:`\mathsf{AssetBase}^{\mathsf{Orchard}}_{\mathsf{AssetId}}`.
- Replace the input note value by a generic value, :math:`\mathsf{v}'`, as :math:`\mathsf{cv^{net}} = \mathsf{ValueCommit_rcv^{OrchardZSA}(v’ - v^new, \mathsf{AssetBase}^{\mathsf{Orchard}}_{\mathsf{AssetId}})}`
- Add a boolean ``split_flag`` variable as an auxiliary witness. This variable is to be activated ``split_flag = 1`` if the Action in question has a Split Input and ``split_flag = 0`` if the Action is actually spending an input note:
- If :math:`\texttt{split_flag} = 1` then constrain :math:`\mathsf{v}' = 0` otherwise constrain :math:`\mathsf{v}'=\mathsf{v^{old}}` from the auxiliary input.
- If :math:`\texttt{split_flag} = 1` then constrain :math:`\mathsf{v^{old}} \neq 0`.
- Check that (path, pos) is a valid Merkle path of depth :math:`\mathsf{MerkleDepth^Orchard}`, from :math:`\mathsf{cm^{old}}` to the anchor :math:`\mathsf{rt^{Orchard}}`.
- Replace the :math:`\psi_{old}` value with a generic :math:`\psi'` as :math:`\mathsf{nf_{old}} = \mathsf{DeriveNullifier_{nk}}(\rho^\mathsf{old}, \psi', \mathsf{cm^{old}})`
- if :math:`\mathtt{split\_flag} = 0` then constrain :math:`\psi' = \psi^{old}`. (Otherwise :math:`\psi'` should be sampled uniformly at random on :math:`\mathbb{F}_{q_{\mathbb{P}}}`.)
The input note in the old note commitment integrity check must either include an Asset Base (ZSA note) or not (pre-ZSA Orchard note). If the note is a pre-ZSA Orchard note, the note commitment is computed in the original Orchard fashion [#protocol-abstractcommit]_. If the note is a ZSA note, the note commitment is computed as defined in the `Note Structure & Commitment`_ section.
The burn mechanism is a transparent extension to the transfer protocol that enables a specific amount of any Asset Identifier to be "destroyed". The burn mechanism does NOT send Assets to a non-spendable address, it simply reduces the total number of units of a given Custom Asset in circulation at the consensus level. It is enforced at the consensus level, by using an extension of the value balance mechanism used for ZEC Assets.
The sender includes a :math:`\mathsf{v^{AssetBase}}` variable for every Asset Identifier that is being burnt. As we will show in the `ZSA Transaction Structure`_, this is separate from the regular :math:`\mathsf{valueBalance^Orchard}` that is the default transparent value for the ZEC Asset.
For every Custom Asset that is burnt, we add to the :math:`\mathsf{assetBurn}` vector the tuple :math:`(\mathsf{AssetBase},\mathsf{v^{AssetBase}})` such that the validator of the transaction can compute the value commitment with the corresponding value base point of that Asset. This ensures that the values are all balanced out with respect to the Asset Identifiers in the transfer.
The value balances for each Asset Identifier in :math:`\mathsf{assetBurn}` represents the amount of that Asset that is being burnt. In the case of ZEC, the value balance represents either the transaction fee, or the amount of ZEC changing pools (eg: to Sapling or Transparent).
The validator needs to verify the Balance and Binding Signature by adding the value balances for all Assets, as committed using their respective :math:`\mathsf{AssetBase}` as the value base point of the Pedersen Commitment. This is done as follows
In the case that the balance of all the Action values related to a specific Asset will be zero, there will be no value added to the vector. This way, neither the number of Assets nor their Asset Identifiers will be revealed, except in the case that an Asset is burnt.
1. We require that :math:`\forall (\mathsf{AssetBase},\mathsf{v^{AssetBase}}) \in \mathsf{assetBurn}\ ,\ \mathsf{AssetBase} \neq \mathcal{V}^{\mathsf{Orchard}}`. That is, ZEC or TAZ is not allowed to be burnt.
2. We require that for every :math:`\forall (\mathsf{AssetBase},\mathsf{v^{AssetBase}}) \in \mathsf{assetBurn}\ ,\ \mathsf{v^{AssetBase}} \neq 0`.
3. We require that there be no duplication of Custom Assets in the :math:`\mathsf{assetBurn}` set. That is, every :math:`\mathsf{AssetBase}` has at most one entry in :math:`\mathsf{assetBurn}`.
**Note:** Even if this mechanism allows having transparent ↔ shielded Asset transfers in theory, the transparent protocol will not be changed with this ZIP to adapt to a multiple Asset structure. This means that unless future consensus rules changes do allow it, unshielding will not be possible for Custom Assets.
The transaction format is similar to the version 5 transaction format described in the Zcash specification [#protocol-transactionstructure]_, with the following additions to the Orchard bundle:
In terms of the Action size, the ZSA action size differs from the Orchard action size by 32 bytes (due to the addition of the :math:`\mathsf{AssetBase}`). This implies that the size goes from 820 bytes in the Orchard action to 852 bytes in the ZSA Action.
| 32 | AssetBase | byte[32] | For the Orchard-based ZSA protocol, this is the encoding of the Asset Base :math:`\mathsf{AssetBase}^{\mathsf{Orchard}}`. |
- The notes generated via the ZSA protocol are different from those generated via the Orchard protocol. As with any protocol upgrade, it will be possible to distinguish between notes generated by each protocol. However, all ZEC notes will be fully spendable with the ZSA protocol transaction structure due to the built-in backward compatibility.
- We prevent a potential malleability attack on the Asset Identifier by ensuring the output notes receive an Asset Base that exists on the global state.
- Wallets need to communicate the names of the Assets in a non-confusing way to users, since the byte representation of the Asset Identifier would be hard to read for an end user. Possible solutions are the use of a petname system or a list of well-known Assets.
- One proposal for a petname system for the zcashd wallet is the use of an additional configuration file that stores a one-to-one mapping of names to Asset Identifiers. This allows clients to rename the Assets in a way they find useful. Default versions of this file with well-known Assets listed can be made available online as a starting point for clients.
In order to have backward compatibility with the ZEC notes, we have designed the circuit to support both ZEC and ZSA notes. As we specify above, there are three main reasons we can do this:
- Note commitments for ZEC notes will remain the same, while note commitments for Custom Assets will be computed taking into account the :math:`AssetBase` value as well.
- The existing Orchard shielded pool will continue to be used for the new ZSA notes post the upgrade.
- The value commitment is abstracted to allow for the value base-point as a variable private input to the proof.
- The ZEC-based Actions will still include dummy input notes, whereas the ZSA-based Actions will include both dummy and split input notes.
..[#zip-0226]`ZIP 226: Transfer and Burn of Zcash Shielded Assets <zip-0226.html>`_
..[#zip-0227]`ZIP 227: Issuance of Zcash Shielded Assets <zip-0227.html>`_
..[#protocol-notes]`Zcash Protocol Specification, Version 2022.3.8. Section 3.2: Notes <protocol/protocol.pdf#notes>`_
..[#protocol-notept]`Zcash Protocol Specification, Version 2022.3.8. Section 5.5: Encodings of Note Plaintexts and Memo Fields <protocol/protocol.pdf#notept>`_
..[#protocol-actions]`Zcash Protocol Specification, Version 2022.3.8. Section 3.7: Action Transfers and their Descriptions <protocol/protocol.pdf#actions>`_
..[#protocol-abstractcommit]`Zcash Protocol Specification, Version 2022.3.8. Section 4.1.8: Commitment <protocol/protocol.pdf#abstractcommit>`_
..[#protocol-binding]`Zcash Protocol Specification, Version 2022.3.8. Section 4.14: Balance and Binding Signature (Orchard) <protocol/protocol.pdf#orchardbalance>`_
..[#protocol-commitmentsandnullifiers]`Zcash Protocol Specification, Version 2022.3.8. Section 4.16: Note Commitments and Nullifiers <protocol/protocol.pdf#commitmentsandnullifiers>`_
..[#protocol-endian]`Zcash Protocol Specification, Version 2022.3.8. Section 5.1: Integers, Bit Sequences, and Endianness <protocol/protocol.pdf#endian>`_
..[#protocol-constants]`Zcash Protocol Specification, Version 2022.3.8. Section 5.3: Constants <protocol/protocol.pdf#constants>`_
..[#protocol-pallasandvesta]`Zcash Protocol Specification, Version 2022.3.8. Section 5.4.9.6: Pallas and Vesta <protocol/protocol.pdf#pallasandvesta>`_
..[#protocol-transactionstructure]`Zcash Protocol Specification, Version 2022.3.8. Section 7.1: Transaction Encoding and Consensus (Transaction Version 5) <protocol/protocol.pdf#>`_
..[#initial-zsa-issue]`User-Defined Assets and Wrapped Assets <https://github.com/str4d/zips/blob/zip-udas/drafts/zip-user-defined-assets.rst>`_
..[#generalized-value-commitments]`Comment on Generalized Value Commitments <https://github.com/zcash/zcash/issues/2277#issuecomment-321106819>`_
..[#zip-0317b]`ZIP 317: Proportional Transfer Fee Mechanism - Pull Request #667 for ZSA Protocol ZIPs <https://github.com/zcash/zips/pull/667>`_
