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ZIP: 230
Title: Version 6 Transaction Format
Owners: Daira-Emma Hopwood <daira-emma@electriccoin.co>
        Jack Grigg <jack@electriccoin.co>
        Sean Bowe <sean@electriccoin.co>
        Kris Nuttycombe <kris@electriccoin.co>
        Pablo Kogan <pablo@qed-it.com>
        Vivek Arte <vivek@qed-it.com>
Original-Authors: Greg Pfeil
                  Deirdre Connolly
Credits: Ying Tong Lai
Status: Draft
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 14 [1] when, and only when, they appear in all capitals.

The character § is used when referring to sections of the Zcash Protocol Specification [2].

Abstract

This proposal defines a new Zcash peer-to-peer transaction format, which includes data that supports the Orchard-ZSA protocol and all operations related to Zcash Shielded Assets (ZSAs). The new format adds and describes new fields containing ZSA-specific elements. Like the existing v5 transaction format, it keeps well-bounded regions of the serialized form to serve each pool of funds.

This ZIP also depends upon and defines modifications to the computation of the values TxId Digest, Signature Digest, and Authorizing Data Commitment defined by ZIP 244 [8].

Motivation

The Orchard-ZSA 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 format that can easily accommodate future extensions, where elements or a given pool are kept separate.

Requirements

The new format must fully support the Orchard-ZSA 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

Bytes Name Data Type Description
Common Transaction Fields
4 header uint32
Contains:
  • fOverwintered flag (bit 31, always set)
  • version (bits 30 .. 0) – transaction version.
4 nVersionGroupId uint32 Version group ID (nonzero).
4 nConsensusBranchId uint32 Consensus branch ID (nonzero).
4 lock_time uint32 Unix-epoch UTC time or block height, encoded as in Bitcoin.
4 nExpiryHeight uint32 A block height in the range {1 .. 499999999} after which the transaction will expire, or 0 to disable expiry. [ZIP-203]
8 fee int64 The fee to be paid by this transaction, in zatoshis.
Transparent Transaction Fields
varies tx_in_count compactSize Number of transparent inputs in tx_in.
varies tx_in tx_in Transparent inputs, encoded as in Bitcoin.
varies tx_out_count compactSize Number of transparent outputs in tx_out.
varies tx_out tx_out Transparent outputs, encoded as in Bitcoin.
Sapling Transaction Fields
varies nSpendsSapling compactSize Number of Sapling Spend descriptions in vSpendsSapling.
96 * nSpendsSapling vSpendsSapling SpendDescriptionV6[nSpendsSapling] A sequence of Sapling Spend descriptions, encoded per protocol §7.3 ‘Spend Description Encoding and Consensus’.
varies nOutputsSapling compactSize Number of Sapling Output Decriptions in vOutputsSapling.
756 * nOutputsSapling vOutputsSapling OutputDescriptionV6[nOutputsSapling] A sequence of Sapling Output descriptions, encoded per protocol §7.4 ‘Output Description Encoding and Consensus’.
8 valueBalanceSapling int64 The net value of Sapling Spends minus Outputs
32 anchorSapling byte[32] A root of the Sapling note commitment tree at some block height in the past.
192 * nSpendsSapling vSpendProofsSapling byte[192 * nSpendsSapling] Encodings of the zk-SNARK proofs for each Sapling Spend.
64 * nSpendsSapling vSpendAuthSigsSapling byte[64 * nSpendsSapling] Authorizing signatures for each Sapling Spend.
192 * nOutputsSapling vOutputProofsSapling byte[192 * nOutputsSapling] Encodings of the zk-SNARK proofs for each Sapling Output.
64 bindingSigSapling byte[64] A Sapling binding signature on the SIGHASH transaction hash.
Orchard-ZSA Transaction Fields
varies nActionsOrchard compactSize The number of Orchard-ZSA Action descriptions in vActionsOrchard.
852 * nActionsOrchard vActionsOrchard OrchardZsaAction[nActionsOrchard] A sequence of Orchard-ZSA Action descriptions, encoded per the Orchard-ZSA Action Description Encoding.
1 flagsOrchard byte
An 8-bit value representing a set of flags. Ordered from LSB to MSB:
  • enableSpendsOrchard
  • enableOutputsOrchard
  • enableZSAs
  • The remaining bits are set to 0\!.
8 valueBalanceOrchard int64 The net value of Orchard spends minus outputs.
32 anchorOrchard byte[32] A root of the Orchard note commitment tree at some block height in the past.
varies sizeProofsOrchardZSA compactSize Length in bytes of proofsOrchardZSA. Value is (TO UPDATE) 2720 + 2272 \cdot \mathtt{nActionsOrchard}\!.
sizeProofsOrchardZSA proofsOrchardZSA byte[sizeProofsOrchardZSA] Encoding of aggregated zk-SNARK proofs for Orchard-ZSA Actions.
64 * nActionsOrchard vSpendAuthSigsOrchard byte[64 * nActionsOrchard] Authorizing signatures for each Orchard-ZSA Action.
64 bindingSigOrchard byte[64] An Orchard binding signature on the SIGHASH transaction hash.
Orchard-ZSA Burn Fields
varies nAssetBurn compactSize The number of Assets burnt.
40 * nAssetBurn vAssetBurn AssetBurn[nAssetBurn] A sequence of Asset Burn descriptions, encoded per Orchard-ZSA Asset Burn Description.
Orchard-ZSA Issuance Fields
varies nIssueActions compactSize The number of issuance actions in the bundle.
IssueActionSize * nIssueActions vIssueActions IssueAction[nIssueActions] A sequence of issuance action descriptions, where IssueActionSize is the size, in bytes, of an IssueAction description.
32 ik byte[32] The issuance validating key of the issuer, used to validate the signature.
64 issueAuthSig byte[64] The signature of the transaction SIGHASH, signed by the issuer, validated as in Issuance Authorization Signature Scheme [7].
  • 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 fields flagsOrchard, valueBalanceOrchard, anchorOrchard, sizeProofsOrchardZSA, proofsOrchardZSA, and bindingSigOrchard are present if and only if \mathtt{nActionsOrchard} > 0\!. If valueBalanceOrchard is not present, then \mathsf{v^{balanceOrchard}} is defined to be 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 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.
  • For coinbase transactions, the enableSpendsOrchard and enableZSAs bits MUST be set to 0\!.

The encodings of tx_in, and tx_out are as in a version 4 transaction (i.e. unchanged from Canopy). The encodings of SpendDescriptionV6, OutputDescriptionV6 , OrchardZsaAction, 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 Spend Description (SpendDescriptionV6)

Bytes Name Data Type Description
32 cv byte[32] A value commitment to the net value of the input note.
32 nullifier byte[32] The nullifier of the input note.
32 rk byte[32] The randomized validating key for the element of spendAuthSigsSapling corresponding to this Spend.

The encodings of each of these elements are defined in §7.3 ‘Spend Description Encoding and Consensus’ of the Zcash Protocol Specification [3].

Sapling Output Description (OutputDescriptionV6)

Bytes Name Data Type Description
32 cv byte[32] A value commitment to the net value of the output note.
32 cmu byte[32] The u\!-coordinate of the note commitment for the output note.
32 ephemeralKey byte[32] An encoding of an ephemeral Jubjub public key.
580 encCiphertext byte[580] The encrypted contents of the note plaintext.
80 outCiphertext byte[80] The encrypted contents of the byte string created by concatenation of the transmission key with the ephemeral secret key.

The encodings of each of these elements are defined in §7.4 ‘Output Description Encoding and Consensus’ of the Zcash Protocol Specification [4].

Orchard-ZSA Action Description (OrchardZsaAction)

Bytes Name Data Type Description
32 cv byte[32] A value commitment to the net value of the input note minus the output note.
32 nullifier byte[32] The nullifier of the input note.
32 rk byte[32] The randomized validating key for the element of spendAuthSigsOrchard corresponding to this Action.
32 cmx byte[32] The x\!-coordinate of the note commitment for the output note.
32 ephemeralKey byte[32] An encoding of an ephemeral Pallas public key
612 encCiphertext byte[580] The encrypted contents of the note plaintext.
80 outCiphertext byte[80] The encrypted contents of the byte string created by concatenation of the transmission key with the ephemeral secret key.

The encodings of each of these elements are defined in §7.5 ‘Action Description Encoding and Consensus’ of the Zcash Protocol Specification [5].

Orchard-ZSA Asset Burn Description

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

Bytes Name Data Type Description
32 AssetBase byte[32] For the Orchard-based ZSA protocol, this is the encoding of the Asset Base \mathsf{AssetBase^{Orchard}}\!.
8 valueBurn \{1 .. 2^{64} - 1\} The amount being burnt.

The encodings of each of these elements are defined in ZIP 226 [6].

Issuance Action Description (IssueAction)

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

Bytes Name Data Type Description
2 assetDescSize byte The length of the asset description string in bytes.
assetDescSize asset_desc byte[assetDescSize] A byte sequence of length assetDescSize bytes which SHOULD be a well-formed UTF-8 code unit sequence according to Unicode 15.0.0 or later.
varies nNotes compactSize The number of notes in the issuance action.
noteSize * nNotes vNotes Note[nNotes] A sequence of note descriptions within the issuance action, where noteSize is the size, in bytes, of a Note.
1 flagsIssuance byte
An 8-bit value representing a set of flags. Ordered from LSB to MSB:
  • \mathsf{finalize}
  • The remaining bits are set to 0\!.

The encodings of each of these elements are defined in ZIP 227 [7].

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]Zcash Protocol Specification, Version 2023.4.0 or later [NU5 proposal]
[3]Zcash Protocol Specification, Version 2023.4.0 [NU5 proposal]. Section 4.4: Spend Descriptions
[4]Zcash Protocol Specification, Version 2023.4.0 [NU5 proposal]. Section 4.5: Output Descriptions
[5]Zcash Protocol Specification, Version 2023.4.0 [NU5 proposal]. Section 4.6: Action Descriptions
[6]ZIP 226: Transfer and Burn of Zcash Shielded Assets
[7](1, 2) ZIP 227: Issuance of Zcash Shielded Assets
[8]ZIP 244: Transaction Identifier Non-Malleability