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ZIP: 245
Title: Transaction Identifier Digests & Signature Validation for Transparent Zcash Extensions
Owners: Kris Nuttycombe <kris@electriccoin.co>
Status: Draft
Category: Consensus
Created: 2021-01-13
License: MIT
Discussions-To: <https://github.com/zcash/zips/issues/384>

Terminology

The key words "MUST" and "MUST NOT" in this document are to be interpreted as described in RFC 2119. [1]

The terms "consensus branch", "epoch", and "network upgrade" in this document are to be interpreted as described in ZIP 200. [2]

Abstract

This proposal defines changes to ZIP 244 [4] transaction id and signature digest algorithms to accommodate the inclusion of transparent Zcash extensions (TZEs) as defined in ZIP 222 [3].

Specification

TxId Digest

The tree of hashes defined by ZIP 244 [4] is re-structured to include a new branch for TZE hashes. The tze_digest branch is the only new addition to the tree; header_digest, transparent_digest, sprout_digest, and sapling_digest are as in ZIP 244:

txid_digest
├── header_digest
├── transparent_digest
├── tze_digest
│   ├── tzein_digest
│   └── tzeout_digest
├── sprout_digest
└── sapling_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: tze_digest         (32-byte hash output)
T.4: sprout_digest      (32-byte hash output)
T.5: sapling_digest     (32-byte hash output)

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

2: tze_digest

A BLAKE2b-256 hash of the following values

T.2a: tzein_digest  (32-byte hash)
T.2b: tzeout_digest (32-byte hash)

The personalization field of this hash is set to:

"ZTxIdTZE____Hash" (4 underscore characters)
2a: tzein_digest

A BLAKE2b-256 hash of all TZE inputs to the transaction, excluding witness data. For each TZE input, the following values are appended to this hash:

2a.i:  extension_id (CompactSize field encoding)
2a.ii: mode         (CompactSize field encoding)

The personalization field of this hash is set to:

"ZTxIdTZEIns_Hash" (1 underscore character)
2a: tzeout_digest

A BLAKE2b-256 hash of the field encoding of all TZE outputs belonging to the transaction.

The personalization field of this hash is set to:

"ZTxIdTzeOutsHash"

Signature Digest

The signature digest creation algorithm defined by ZIP 244 [4] is modified to include a new branch for TZE hashes. The tze_digest branch is the only new addition to the tree; header_digest, transparent_digest, sprout_digest, and sapling_digest are as in ZIP 244:

signature_digest
├── header_digest
├── transparent_digest
├── tze_digest
│   ├── tzein_digest
│   └── tzeout_digest
├── sprout_digest
└── sapling_digest

signature_digest

A BLAKE2b-256 hash of the following values

S.1: header_digest      (32-byte hash output)
S.2: transparent_digest (32-byte hash output)
S.3: tze_digest         (32-byte hash output)
S.4: sprout_digest      (32-byte hash output)
S.5: sapling_digest     (32-byte hash output)

The personalization field of this hash is set to:

"ZcashTxHash_" || CONSENSUS_BRANCH_ID

ZcashTxHash_ has 1 underscore character.

This value must have the same personalization as the top hash of the transaction identifier digest tree, in order to make it possible to sign the transaction id in the case that there are no transparent inputs.

S.3: tze_digest

This digest is a BLAKE2b-256 hash of the following values of the TZE input being signed:

S.3a: prevout_digest (field encoding bytes)
S.3b: extension_id   (CompactSize field encoding)
S.3c: mode           (CompactSize field encoding)
S.3d: payload        (arbitrary bytes)
S.3e: value          (8-byte little endian value of the output spent by this input)

The personalization field of this hash is set to:

"Zcash__TzeInHash" (2 underscore characters)

Authorizing Data Commitment

The tree of hashes defined by ZIP 244 [4] for authorizing data commitments is re-structured to include a new branch for TZE hashes. The tze_witnesses_digest branch is the only new addition to the tree; transparent_auth_digest, sprout_auth_digest, and sapling_auth_digest are as in ZIP 244:

auth_digest
├── transparent_scripts_digest
├── tze_witnesses_digest
├── sprout_auth_digest
└── sapling_auth_digest

auth_digest

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

A.1: transparent_scripts_digest (32-byte hash output)
A.2: tze_witnesses_digest       (32-byte hash output)
A.3: sprout_auth_digest         (32-byte hash output)
A.4: sapling_auth_digest        (32-byte hash output)

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

2: tze_witnesses_digest

A BLAKE2b-256 hash of the field encoding of the witness payload data associated with each TZE input belonging to the transaction.

The personalization field of this hash is set to:

"ZTxAuthTZE__Hash" (2 underscore characters)

Reference implementation

References

[1]RFC 2119: Key words for use in RFCs to Indicate Requirement Levels
[2]ZIP 200: Network Upgrade Mechanism
[3]ZIP 222: Transparent Zcash Extensions
[4](1, 2, 3, 4) ZIP 244: Transaction Identifier Non-Malleability