Withdrawn Version 6 Transaction Format

Preamble

ZIP

230

Title

Withdrawn Version 6 Transaction Format

Owners

• Daira-Emma Hopwood
• Jack Grigg
• Sean Bowe
• Kris Nuttycombe
• Pablo Kogan
• Vivek Arte

Original-Authors

• Greg Pfeil
• Deirdre Connolly

Credits

Ying Tong Lai

Status

Withdrawn

Category

Consensus

Created

2023-04-18

License

MIT

Discussions-To

https://github.com/zcash/zips/issues/686

Terminology

The key words "MUST", "SHOULD", and "MAY" in this document are to be interpreted as described in BCP 14¹ when, and only when, they appear in all capitals.

The terms "Zcash Shielded Asset", "OrchardZSA", "ZSA" and "Asset" in this document are to be interpreted as described in ZIP 226².

The term "Issuance" and "Issuance Action" in this document are to be interpreted as described in ZIP 227³.

The character § is used when referring to sections of the Zcash Protocol Specification ⁴.

Abstract

This proposal defines a new Zcash peer-to-peer transaction format, which supports the changes being deployed in Network Upgrade 7⁷. It follows the same design pattern as the v5 transaction format⁸.

Motivation

The OrchardZSA protocol requires serialized data elements that are distinct from any previous Zcash transaction. Since ZIP 244 was activated in NU5, the v5 and later serialized transaction formats are not consensus-critical. Thus, this ZIP defines a format that can easily accommodate future extensions, where elements or a given pool are kept separate.

Requirements

The new format must fully support the OrchardZSA protocol.

The new format should lend itself to future extension or pruning to add or remove value pools.

The computation of the non-malleable transaction identifier hash must include all newly incorporated elements except those that attest to transaction validity.

The computation of the commitment to authorizing data for a transaction must include all newly incorporated elements that attest to transaction validity.

Non-requirements

More general forms of extensibility, such as definining a key/value format that allows for parsers that are unaware of some components, are not required.

Specification

All fields in this specification are encoded as little-endian.

The Zcash transaction format for transaction version 6 is as follows:

Transaction Format

• The fields valueBalanceSapling and bindingSigSapling are present if and only if $\mathtt{nSpendsSapling} + \mathtt{nOutputsSapling} > 0$. If valueBalanceSapling is not present, then $\mathsf{v^{balanceSapling}}$ is defined to be $0$.
• The field anchorSapling is present if and only if $\mathtt{nSpendsSapling} > 0$.
• The elements of vSpendProofsSapling and vSpendAuthSigsSapling have a 1:1 correspondence to the elements of vSpendsSapling and MUST be ordered such that the proof or signature at a given index corresponds to the SpendDescriptionV6 at the same index.
• The elements of vOutputProofsSapling have a 1:1 correspondence to the elements of vOutputsSapling and MUST be ordered such that the proof at a given index corresponds to the OutputDescriptionV6 at the same index.
• The fields valueBalanceOrchard and bindingSigOrchard are present if and only if $\mathtt{nActionGroupsOrchard} > 0$. If valueBalanceOrchard is not present, then $\mathsf{v^{balanceOrchard}}$ is defined to be $0$.
• The fields issueAuthSigLength and issueAuthSig are present if and only if $\mathtt{nIssueActions} > 0$. The fields issuerLength, issuer, nIssueActions, and vIssueActions are always present. If $\mathtt{nIssueActions} = 0$ then issuerLength MUST be set to $0$ (issuer and vIssueActions will be empty in this case).
• For coinbase transactions, the enableSpendsOrchard and enableZSAs bits MUST be set to $0$.
• The fields nMemoChunks, pruned, and vMemoChunks are present if and only if fAllPruned == 0.
• If fAllPruned == 0, then:
  • nonceOrHash contains the ZIP 231 nonce for deriving encryption keys.
  • Each bit of pruned, in little-endian order, indicates the type of the corresponding entry in vMemoChunks.
    • A bit value of 0 indicates that the entry will be of type byte[272] representing an encrypted memo chunk.
    • A bit value of 1 indicates the entry will be a byte[32] and contains the ZIP 246 memo_chunk_digest for a pruned chunk.
• If fAllPruned == 1, then:
  • nonceOrHash contains the ZIP 246 memo_digest for the pruned memo bundle.

The encodings of tx_in, and tx_out are as in prior transaction versions (i.e. unchanged since Zcash launch).¹⁴¹⁵

The encodings of SpendDescriptionV6, OutputDescriptionV6, ActionGroupDescription, AssetBurn and IssueAction are described below.

The encoding of Sapling Spends and Outputs has changed relative to prior versions in order to better separate data that describe the effects of the transaction from the proofs of and commitments to those effects, and for symmetry with this separation in the Orchard-related parts of the transaction format.

Sapling Output Description (OutputDescriptionV6)

The encodings of each of these elements are defined in §7.4 ‘Output Description Encoding and Consensus’¹⁶.

OrchardZSA Action Group Description

The OrchardZSA Action Group Description is encoded in a transaction as an instance of an ActionGroupDescription type:

The encodings of OrchardZSAAction and AssetBurn are described below.

• The proofs aggregated in proofsOrchardZSA, and the elements of vSpendAuthSigsOrchard, each have a 1:1 correspondence to the elements of vActionsOrchard and MUST be ordered such that the proof or signature at a given index corresponds to the OrchardZSAAction at the same index.
• If the value of nAGExpiryHeight is nonzero, it MUST be consistent with the nExpiryHeight of the overall transaction.
• In NU7, nExpiryHeight MUST be set to 0; this restriction is expected to be lifted in a future network upgrade.

Rationale for nAGExpiryHeight

We introduce the nAGExpiryHeight field in this transaction format in order to be forward compatible with Swaps over ZSAs, as proposed in ZIP 228¹⁹. For the OrchardZSA protocol, which does not make use of an additional expiry height for transactions, we set the value of nAGExpiryHeight to be 0 by consensus. This serves as a default value to represent the situation where there is no expiry, analogous to the convention adopted for nExpiryHeight in ZIP 203 [#zip-0203].

Rationale for including Burn fields inside OrchardZSA Action Groups

Note that the v6 transaction format includes the burn fields of the transaction inside the OrchardZSA Action Group rather than at the transaction level. This is a design choice that considers the future scenario where Action Groups may be generated by different parties before being bundled together into a transaction. In such a scenario, the individual parties can burn Assets of their choice in their corresponding Action Groups. Maintaining the burn fields at the transaction level would provide the ability to burn Assets only to the party performing the bundling of the Action Groups.

OrchardZSA Action Description (OrchardZSAAction)

The encodings of each of these elements are defined in §7.5 ‘Action Description Encoding and Consensus’²⁰.

OrchardZSA Asset Burn Description

An OrchardZSA Asset Burn description is encoded in a transaction as an instance of an AssetBurn type:

The encodings of each of these elements are defined in ZIP 226²².

Transparent Sighash Information (TransparentSighashInfo)

Sapling Signature (SaplingSignature)

Orchard Signature (OrchardSignature)

Issuance Authorization Signature (IssueAuthSignature)

Issuance Action Description (IssueAction)

An issuance action, IssueAction, is the instance of issuing a specific Custom Asset, and contains the following fields:

The encoding of IssueNoteDescription is described below. Note that we allow the number of notes (represented by nNotes) to be zero. This allows for issuers to create Issuance Actions to only finalize an issued Asset, without needing them to simultaneously issue more of that Asset.

Issue Note Description (IssueNoteDescription)

An issuance note description, IssueNoteDescription contains the following fields:

Note Plaintexts

New note plaintext formats using lead byte {{LEADBYTE}} are introduced for Sapling and Orchard, in order to support memo bundles and (for Orchard) ZSAs and quantum recoverability.

The $\mathsf{leadByte}$ MUST be {{LEADBYTE}} for all note plaintexts in v6 transactions.

Sapling Note Plaintext

In order to accommodate the change needed for memo bundles ²⁸, a v6-onward Sapling note plaintext must include a 32-byte memo key \mathsf{K^{memo}} : \mathbb{B}^{\mathbb{Y}[32], rather than a 512-byte plaintext memo.

The corresponding changes to the Zcash Protocol Specification are specified in ²⁹.

The encoding of a v6-onward Sapling note plaintext in § 5.5 ‘Encodings of Note Plaintexts and Memo Fields’³⁰ is similarly updated and is now defined as:

$$[\mathsf{leadByte}] \,||\, \mathsf{LEBS2OSP}_{88}(\mathsf{d}) \,||\, \mathsf{I2LEOSP}_{64}(\mathsf{v}) \,||\, \mathsf{rseed} \,||\, \mathsf{K^{memo}}$$

Orchard Note Plaintext

As well as the same change to accommodate memo bundles as above, OrchardZSA notes have an additional $\mathsf{AssetBase}$ field, which needs to be included (as its byte sequence encoding $\mathsf{asset\_base}$) in a v6-onward Orchard note plaintext.

Let $\mathcal{V}^{\mathsf{Orchard}}$ be as defined in § 5.4.8.3 ‘Homomorphic Pedersen commitments (Sapling and Orchard)’ ³¹.

An Orchard note is essentially equivalent to an OrchardZSA note with $\mathsf{AssetBase} = \mathcal{V}^{\mathsf{Orchard}}$; in particular, they have the same note commitment (as defined in³²), and need not be distinguished in note plaintexts.

Each v6-onward Orchard note plaintext consists of

$$(\mathsf{leadByte} : \mathbb{B}^{\mathbb{Y}}, \mathsf{d} : \mathbb{B}^{[\ell_{\mathsf{d}}]}, \mathsf{v} : \{0 .. 2^{\ell_{\mathsf{value}}} - 1\}, \mathsf{rseed} : \mathbb{B}^{\mathbb{Y}[32]}, \mathsf{asset\_base} : \mathbb{B}^{\mathbb{Y}[\ell_{\mathbb{P}}/8]}, \mathsf{K^{memo}} : \mathbb{B}^{\mathbb{Y}[32]})$$

where (consistent with³³) $\mathsf{asset\_base}$ is $\mathsf{LEBS2OSP}_{\ell_{\mathbb{P}}}(\mathsf{repr}_{\mathbb{P}}(\mathsf{AssetBase}))$.

The encoding of a v6-onward Orchard note plaintext in § 5.5 ‘Encodings of Note Plaintexts and Memo Fields’³⁴ is similarly updated and is now defined as:

$$[\mathsf{leadByte}] \,||\, \mathsf{LEBS2OSP}_{88}(\mathsf{d}) \,||\, \mathsf{I2LEOSP}_{64}(\mathsf{v}) \,||\, \mathsf{rseed} \,||\, \mathsf{asset\_base} \,||\, \mathsf{K^{memo}}$$

The encodings of $\mathsf{d}$, $\mathsf{v}$, and $\mathsf{rseed}$ remain unchanged from the previous specification in §5.5 ‘Encodings of Note Plaintexts and Memo Fields’ of the Zcash Protocol Specification³⁵.

Non-normative note: The use of the $\mathsf{rseed}$ in Orchard note decryption changes, but its type and encoding do not.

Rationale for requiring the lead byte to be {{LEADBYTE}} in v6

It was decided to synchronize the changes to note encryption required for quantum recoverability, ZSA support, and memo bundles with a change to the transaction format, for two reasons:

• This was necessary because the note ciphertexts must be a different size to those in v5 transactions — to accommodate the ZSA $\mathsf{AssetBase}$ field for Orchard, and due to changing the 512-byte $\mathsf{memo}$ field to a 32-byte $\mathsf{K^{memo}}$ field for both Sapling and Orchard. In fact the latter change makes it impossible to retain the lead-byte $\mathtt{0x02}$ format unchanged, because the note ciphertext is not large enough (in either Sapling or Orchard outputs) for a 512-byte memo.
• The quantum recoverability change did not necessarily have to be done at the same time as the ZSA and memo bundle changes. However, it would have significantly increased overall protocol complexity for there to be a separate note plaintext format for quantum-recoverable notes. Also, doing it at the same time minimizes the risk that non-conformant wallets would create outputs after this change that could not be successfully decrypted by conformant wallets. That could potentially result in inaccessible funds, as was observed with the previous note plaintext change at the Canopy upgrade³⁶. (As noted above, it is still possible for funds to be temporarily stuck if a receiving wallet does not fully support v6 transactions, but this is less problematic because upgrading the wallet will unstick them.)

Although the ZSA and quantum recoverability-related note plaintext changes do not apply to Sapling, the change from $\mathsf{memo}$ to $\mathsf{K^{memo}}$ justifies changing the lead byte for both Sapling and Orchard.

Implications for Mining

ZIP 235³⁷ requires that from activation of that ZIP, which is scheduled for NU7, all coinbase transactions created by miners and mining pools are v6 transactions.

The specification of the Stratum protocol in³⁸ does not need any updates for v6 transactions, but implementations might need to be updated.

Implications for Wallets

Sending v6 transactions

All Zcash wallets SHOULD, without undue delay, switch to sending only v6 transactions once they are allowed on the network. This applies to all transactions regardless of whether they transact in native or non-native assets, or whether they use other v6 features.

Rationale for sending v6 transactions

Click to show/hide

This obtains the maximum possible privacy for the wallet's users, maximizes the overall rate of increase of the anonymity set of OrchardZSA commitments, and ensures that users' funds are quantum-recoverable.

Because v5 transactions cannot spend or create non-native assets, a wallet using Orchard with v5 transactions is revealing more information than a wallet using it with v6 transactions — i.e. that all of the Orchard notes spent and output by those transactions are ZEC.

Only OrchardZSA outputs of v6 transactions are quantum-recoverable, as described and motivated in³⁹.

Finally, if wallets continue to send v5 transactions then users will not benefit from other v6 transaction features, such as the larger memo sizes enabled by memo bundles⁴⁰, or the benefits of making the fee field explicit⁴¹.

Support for receiving funds in v6 transactions

Zcash wallets MUST support parsing and processing v6 transactions by the time they are allowed on the network (scheduled for NU7 activation). This includes detecting and decrypting lead-byte {{LEADBYTE}} note plaintexts and memo bundles.

The necessary changes to note decryption are specified in ZIP 231⁴², ZIP 226⁴³, and ZIP 2005⁴⁴ (which defines the necessary changes to the Zcash protocol specification). Note that the changes arising from⁴⁵ apply also to Sapling outputs.

In the event that a wallet implementation, contrary to the above requirement, does not support decrypting outputs of v6 transactions (or some subset of them) immediately once they are allowed on the network, it MUST rescan those outputs once that support is added.

Rationale for being ready to receive v6 transactions at their activation

Note plaintexts with lead byte {{LEADBYTE}}, which are required for Orchard notes in v6 transactions, can be sent to any Orchard address.

These notes use a different computation of $\mathsf{rcm}$ and $\text{ψ}$ from $\mathsf{rseed}$, which is necessary in order to achieve quantum recoverability as explained in⁴⁶. They also have a new $\mathsf{AssetBase}$ field used to support ZSAs. Therefore, they cannot be validated or spent without making all of the specified changes. A wallet attempting to non-conformantly use the note decryption algorithm for lead byte $\mathtt{0x02}$ would compute a different note commitment $\mathsf{cm}_x$ and would reject the note.

If wallets do not support v6 transactions and lead-byte {{LEADBYTE}} note decryption immediately, then funds may be sent to them that they cannot receive. This affects both the existing Orchard functionality, and receiving non-native ZSA assets.

This situation is different from previous upgrades that introduced a new shielded pool, such as the Sapling and NU5 upgrades, because in those cases the new pool used an entirely new address type, and existing wallets would not have given out addresses of those types.

Furthermore, as described in [Sending v6 transactions]_, other wallets can be expected to immediately start sending v6 transactions as soon as they are allowed, and so all wallets need to be ready to receive them at that point.

Deployment

Version 6 transactions are proposed to be allowed on the network starting from Network Upgrade 7.⁴⁷

Reference implementation

TODO

References

---

1. Information on BCP 14 — "RFC 2119: Key words for use in RFCs to Indicate Requirement Levels" and "RFC 8174: Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"↩︎

2. ZIP 226: Transfer and Burn of Zcash Shielded Assets↩︎

3. ZIP 227: Issuance of Zcash Shielded Assets↩︎

4. Zcash Protocol Specification, Version 2025.6.2 or later [NU6.1]↩︎

5. ZIP 229: Version 6 Transaction Format↩︎

6. ZIP 2005: Quantum Recoverability↩︎

7. draft-arya-deploy-nu7: Deployment of the NU7 Network Upgrade↩︎

8. ZIP 225: Version 5 Transaction Format↩︎

9. ZIP 203: Transaction Expiry↩︎

10. ZIP 233: Network Sustainability Mechanism: Removing Funds From Circulation↩︎

11. ZIP 227: Issuance of Zcash Shielded Assets — Issuer Identifier↩︎

12. ZIP 227: Issuance of Zcash Shielded Assets — Issuance Authorization Signature Scheme↩︎

13. ZIP 231: Memo Bundles — Memo Encryption↩︎

14. Zcash Protocol Specification, Version 2025.6.2 [NU6.1]. Section 7.1: Transaction Encoding and Consensus↩︎

15. ZIP 225: Version 5 Transaction Format. Specification: Transaction Format↩︎

16. Zcash Protocol Specification, Version 2025.6.2 [NU6.1]. Section 4.5: Output Descriptions↩︎

17. Zcash Protocol Specification, Version 2025.6.2 [NU6.1]. Section 7.1: Transaction Encoding and Consensus↩︎

18. Zcash Protocol Specification, Version 2025.6.2 [NU6.1]. Section 7.1: Transaction Encoding and Consensus↩︎

19. ZIP 228: Asset Swaps for Zcash Shielded Assets↩︎

20. Zcash Protocol Specification, Version 2025.6.2 [NU6.1]. Section 4.6: Action Descriptions↩︎

21. ZIP 226: Transfer and Burn of Zcash Shielded Assets - Burn Mechanism↩︎

22. ZIP 226: Transfer and Burn of Zcash Shielded Assets - Burn Mechanism↩︎

23. ZIP 246: Digests for the Withdrawn Version 6 Transaction Format↩︎

24. ZIP 246: Digests for the Withdrawn Version 6 Transaction Format↩︎

25. ZIP 246: Digests for the Withdrawn Version 6 Transaction Format↩︎

26. ZIP 246: Digests for the Withdrawn Version 6 Transaction Format↩︎

27. ZIP 227: Issuance of Zcash Shielded Assets — Issuance Authorization Signature Scheme↩︎

28. ZIP 231: Memo Bundles — Memo Encryption↩︎

29. ZIP 231: Memo Bundles — Changes to the Zcash Protocol Specification↩︎

30. Zcash Protocol Specification, Version 2025.6.2 [NU6.1]. Section 5.5: Encodings of Note Plaintexts and Memo Fields↩︎

31. Zcash Protocol Specification, Version 2025.6.2 [NU6.1]. Section 5.4.8.3: Homomorphic Pedersen commitments (Sapling and Orchard)↩︎

32. ZIP 226: Transfer and Burn of Zcash Shielded Assets — Note Structure and Commitment↩︎

33. ZIP 226: Transfer and Burn of Zcash Shielded Assets — Note Structure and Commitment↩︎

34. Zcash Protocol Specification, Version 2025.6.2 [NU6.1]. Section 5.5: Encodings of Note Plaintexts and Memo Fields↩︎

35. Zcash Protocol Specification, Version 2025.6.2 [NU6.1]. Section 5.5: Encodings of Note Plaintexts and Memo Fields↩︎

36. ZIP 212: Allow Recipient to Derive Ephemeral Secret from Note Plaintext↩︎

37. ZIP 235: Remove 60% of Transaction Fees From Circulation↩︎

38. ZIP 301: Zcash Stratum Protocol↩︎

39. ZIP 2005: Quantum Recoverability↩︎

40. ZIP 231: Memo Bundles — Motivation↩︎

41. ZIP 231: Explicit Fees — Motivation↩︎

42. ZIP 231: Memo Bundles↩︎

43. ZIP 226: Transfer and Burn of Zcash Shielded Assets — OrchardZSA Transaction Structure↩︎

44. ZIP 2005: Quantum Recoverability↩︎

45. ZIP 231: Memo Bundles↩︎

46. ZIP 2005: Quantum Recoverability — Security Analysis↩︎

47. draft-arya-deploy-nu7: Deployment of the NU7 Network Upgrade↩︎