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ZIP: 225
Title: Version 5 Transaction Format
Owners: Daira Hopwood <[email protected]>
        Jack Grigg <[email protected]>
        Sean Bowe <[email protected]>
        Kris Nuttycombe <[email protected]>
        Ying Tong Lai <[email protected]>
Status: Proposed
Category: Consensus
Created: 2021-02-28
License: MIT
Discussions-To: <https://github.com/zcash/zips/issues/440>

Terminology

The key words "MUST" and "MAY" in this document are to be interpreted as described in RFC 2119. [1]

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

Abstract

This proposal defines an update to the Zcash peer-to-peer transaction format to include support for data elements required to support the Orchard protocol [2]. The new transaction format defines well-bounded regions of the serialized form to serve each of the existing pools of funds, and adds and describes a new region containing Orchard-specific elements.

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 [7].

Motivation

The new Orchard shielded pool requires serialized data elements that are distinct from any previous Zcash transaction. In addition, with the activation of ZIP 244, the serialized transaction format will no longer be consensus-critical. It makes sense at this point to define a format that can readily accommodate future extension in a systematic fashion, where elements required for support for a given pool are kept separate.

Requirements

The new format must fully support the Orchard 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

The Zcash transaction format for transaction version 5 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]
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 SpendDescriptionV5[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 OutputDescriptionV5[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 Transaction Fields
varies nActionsOrchard compactSize The number of Orchard Action descriptions in vActionsOrchard.
884 * nActionsOrchard vActionsOrchard OrchardAction[nActionsOrchard] A sequence of Orchard Action descriptions, encoded per §7.5 ‘Action Description Encoding and Consensus’.
1 flagsOrchard byte
An 8-bit value representing a set of flags. Ordered from LSB to MSB:
  • enableSpendsOrchard
  • enableOutputsOrchard
  • 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 sizeProofsOrchard compactSize Length in bytes of proofsOrchard.
sizeProofsOrchard proofsOrchard byte[sizeProofsOrchard] Encoding of aggregated zk-SNARK proofs for Orchard Actions.
64 * nActionsOrchard vSpendAuthSigsOrchard byte[64 * nActionsOrchard] Authorizing signatures for each Orchard Action.
64 bindingSigOrchard byte[64] An Orchard binding signature on the SIGHASH transaction hash.
  • 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, sizeProofsOrchard, proofsOrchard, 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 SpendDescriptionV5 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 OutputDescriptionV5 at the same index.
  • The proofs aggregated in proofsOrchard, 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 OrchardAction at the same index.
  • For coinbase transactions, the enableSpendsOrchard bit 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 SpendDescriptionV5, OutputDescriptionV5 and OrchardAction 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 (SpendDescriptionV5)

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 (OutputDescriptionV5)

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 Action Description (OrchardAction)

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
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 §7.5 ‘Action Description Encoding and Consensus’ of the Zcash Protocol Specification [5].

Modifications to ZIP 244

TxId Digest

The tree of hashes defined by ZIP 244 [7] is re-structured to include a new branch for Orchard hashes. The orchard_digest branch is the only new addition to the tree; header_digest, transparent_digest, and sapling_digest are as in ZIP 244:

txid_digest
├── header_digest
├── transparent_digest
├── sapling_digest
└── orchard_digest
txid_digest

The top hash of the txid_digest tree is modified from the ZIP 244 structure to be a BLAKE2b-256 hash of the following values

T.1: header_digest      (32-byte hash output)
T.2: transparent_digest (32-byte hash output)
T.3: sapling_digest     (32-byte hash output)
T.4: orchard_digest     (32-byte hash output)

The personalization field of this hash is unmodified from ZIP 244.

T.4: orchard_digest

A BLAKE2b-256 hash of the following values

T.4a: orchard_actions_compact_digest      (32-byte hash output)
T.4b: orchard_actions_memos_digest        (32-byte hash output)
T.4c: orchard_actions_noncompact_digest   (32-byte hash output)
T.4d: flagsOrchard                        (1 byte)
T.4e: valueBalanceOrchard                 (64-bit signed little-endian)
T.4f: anchorOrchard                       (32 bytes)

The personalization field of this hash is set to:

"ZTxIdOrchardHash"
T.4a: orchard_actions_compact_digest

A BLAKE2b-256 hash of the subset of Orchard Action information intended to be included in an updated version of the ZIP-307 [8] CompactBlock format for all Orchard Actions belonging to the transaction. For each Action, the following elements are included in the hash:

T.4a.i  : nullifier            (field encoding bytes)
T.4a.ii : cmx                  (field encoding bytes)
T.4a.iii: ephemeralKey         (field encoding bytes)
T.4a.iv : encCiphertext[..52]  (First 52 bytes of field encoding)

The personalization field of this hash is set to:

"ZTxIdOrcOutCHash"
T.4b: orchard_actions_memos_digest

A BLAKE2b-256 hash of the subset of Orchard shielded memo field data for all Orchard Actions belonging to the transaction. For each Action, the following elements are included in the hash:

T.4b.i: encCiphertext[52..564] (contents of the encrypted memo field)

The personalization field of this hash is set to:

"ZTxIdOrcOutMHash"
T.4c: orchard_actions_noncompact_digest

A BLAKE2b-256 hash of the remaining subset of Orchard Action information not intended for inclusion in an updated version of the the ZIP 307 [8] CompactBlock format, for all Orchard Actions belonging to the transaction. For each Action, the following elements are included in the hash:

T.4c.i  : cv                    (field encoding bytes)
T.4c.ii : rk                    (field encoding bytes)
T.4c.iii: encCiphertext[564..]  (post-memo suffix of field encoding)
T.4c.iv : outCiphertext         (field encoding bytes)

The personalization field of this hash is set to:

"ZTxIdOrcOutNHash"

Signature Digest

The signature digest creation algorithm defined by ZIP 244 [7] is modified to include a new branch for Orchard hashes. The orchard_digest branch is the only new addition to the tree; header_digest, transparent_sig_digest, and sapling_digest are as in ZIP 244:

signature_digest
├── header_digest
├── transparent_sig_digest
├── sapling_digest
└── orchard_digest
signature_digest

A BLAKE2b-256 hash of the following values

S.1: header_digest            (32-byte hash output)
S.2: transparent_sig_digest   (32-byte hash output)
S.3: sapling_digest           (32-byte hash output)
S.4: orchard_digest           (32-byte hash output)

The personalization field of this hash is unmodified from ZIP 244

S.4: orchard_digest

Identical to that specified for the transaction identifier.

Authorizing Data Commitment

The tree of hashes defined by ZIP 244 [7] for authorizing data commitments is re-structured to include a new branch for Orchard Actions. The orchard_auth_digest branch is the only new addition to the tree; transparent_scripts_digest, and sapling_auth_digest are as in ZIP 244:

auth_digest
├── transparent_scripts_digest
├── sapling_auth_digest
└── orchard_auth_digest
auth_digest

A BLAKE2b-256 hash of the following values:

A.1: transparent_scripts_digest (32-byte hash output)
A.2: sapling_auth_digest        (32-byte hash output)
A.3: orchard_auth_digest        (32-byte hash output)

The personalization field of this hash is unmodified from ZIP 244.

A.3: orchard_auth_digest

A BLAKE2b-256 hash of the field encoding of the zkProofsOrchard, spendAuthSigsOrchard, and bindingSigOrchard fields of the transaction:

A.3a: proofsOrchard            (field encoding bytes)
A.3b: vSpendAuthSigsOrchard    (field encoding bytes)
A.3c: bindingSigOrchard        (field encoding bytes)

The personalization field of this hash is set to:

"ZTxAuthOrchaHash"

Alternatives

The original version of ZIP-225 included Sprout-related fields nJoinSplit, vJoinSplit, joinSplitPubKey, and joinSplitSig in the V5 transaction format. The Electric Coin Company and Zcash Foundation teams have elected to remove these fields from the V5 transaction format as part of the continuing process of deprecation of the Sprout shielded pool. As a consequence of these fields being removed:

  • This effectively prohibits migration transactions that would directly move funds from the Sprout pool to the Orchard pool. Sprout -> Transparent and Sprout -> Sapling migration transactions will still be supported when using the V4 transaction format.

Removing these fields reduces the complexity of the NU5 upgrade in the following ways:

  • V5 parsing and serialization code does not need to take these fields into account.
  • ZIP 244 [7] transaction identifier, signature hash, and authorizing data commitment computations are simplified by excluding consideration of these fields.

Removal of these fields means that that in the future, removing the support for the V4 transaction format will also effectively end support for Sprout transactions on the Zcash network, though it might be possible to restore limited support for migration via a future ZIP 222 [6] extension or by other means not yet determined.

The original definitions for the transaction fields that have been removed are:

Sprout Transaction Fields
varies nJoinSplit compactSize The number of JoinSplit descriptions in vJoinSplit.
1698 * nJoinSplit vJoinSplit JSDescriptionGroth16[nJoinSplit] A sequence of JoinSplit descrptions using Groth16 proofs, encoded per §7.2 ‘JoinSplit Description Encoding and Consensus’.
32 joinSplitPubKey byte[32] An encoding of a JoinSplitSig public validating key.
64 joinSplitSig byte[64] A signature on a prefix of the transaction encoding, to be verfied using joinSplitPubKey as specified in §4.11 ‘Non-malleability (Sprout)’.
  • The joinSplitPubKey and joinSplitSig fields were specified to be present if and only if \mathtt{nJoinSplit} > 0.

Reference implementation

TBD

References

[1]RFC 2119: Key words for use in RFCs to Indicate Requirement Levels
[2](1, 2) Zcash Protocol Specification, Version 2021.1.24 or later [NU5 proposal]
[3]Zcash Protocol Specification, Version 2021.1.24 [NU5 proposal]. Section 4.4: Spend Descriptions
[4]Zcash Protocol Specification, Version 2021.1.24 [NU5 proposal]. Section 4.5: Output Descriptions
[5]Zcash Protocol Specification, Version 2021.1.24 [NU5 proposal]. Section 4.6: Action Descriptions
[6]ZIP 222: Transparent Zcash Extensions
[7](1, 2, 3, 4, 5) ZIP 244: Transaction Identifier Non-Malleability
[8](1, 2) ZIP 307: Light Client Protocol for Payment Detection