ZIP: 244
Title: Transaction Identifier Non-Malleability
Owners: Kris Nuttycombe <kris@electriccoin.co>
Daira Emma Hopwood <daira@electriccoin.co>
Jack Grigg <str4d@electriccoin.co>
Status: Final
Category: Consensus
Created: 2021-01-06
Discussions-To: <https://github.com/zcash/zips/issues/411>

## 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. 3

The term "field encoding" refers to the binary serialized form of a Zcash transaction field, as specified in section 7.1 of the Zcash protocol specification 2.

## Abstract

This proposal defines a new transaction digest algorithm for the NU5 network upgrade onward, in order to introduce non-malleable transaction identifiers that commit to all transaction data except for attestations to transaction validity.

This proposal also defines a new transaction digest algorithm for signature validation, which shares all available structure produced during the construction of transaction identifiers, in order to minimize redundant data hashing in validation.

This proposal also defines a new name and semantics for the hashLightClientRoot field of the block header, to enable additional commitments to be represented in this hash and to provide a mechanism for future extensibility of the set of commitments represented.

## Motivation

In all cases, but particularly in order to support the use of transactions in higher-level protocols, any modification of the transaction that has not been explicitly permitted (such as via anyone-can-spend inputs) should invalidate attestations to spend authority or to the included outputs. Following the activation of this proposed change, transaction identifiers will be stable irrespective of any possible malleation of "witness data" such as proofs and transaction signatures.

In addition, by specifying a transaction identifier and signature algorithm that is decoupled from the serialized format of the transaction as a whole, this change makes it so that the wire format of transactions is no longer consensus-critical.

## Requirements

• Continue to support existing functionality of the protocol (multisig, signing modes for transparent inputs).
• Allow the use of transaction ids, and pairs of the form (transaction id, output index) as stable identifiers.
• A sender must be able to recognize their own transaction, even given allowed forms of malleability such as recomputation of transaction signatures.
• In the case of transparent inputs, it should be possible to create a transaction (B) that spends the outputs from a previous transaction (A) even before (A) has been mined. This should also be possible in the case that the creator of (B) does not wait for confirmations of (A). That is, (B) should remain valid so long as any variant of (A) is eventually mined.
• It should not be possible for an attacker to malleate a transaction in a fashion that would result in the transaction being interpreted as a double-spend.
• It should be possible in the future to upgrade the protocol in such a fashion that only non-malleable transactions are accepted.
• It should be possible to use the transaction id unmodified as the value that is used to produce a signature hash in the case that the transaction contains no transparent inputs.

## Non-requirements

In order to support backwards-compatibility with parts of the ecosystem that have not yet upgraded to the non-malleable transaction format, it is not an initial requirement that all transactions be non-malleable.

It is not required that legacy (Sapling V4 and earlier) transaction formats support construction of non-malleable transaction identifiers, even though they may continue to be accepted by the network after the NU5 upgrade.

## Specification

### Digests

All digests are personalized BLAKE2b-256 hashes. In cases where no elements are available for hashing (for example, if there are no transparent transaction inputs or no Orchard actions), a personalized hash of the empty byte array will be used. The personalization string therefore provides domain separation for the hashes of even empty data fields.

The notation BLAKE2b-256(personalization_string, []) is used to refer to hashes constructed in this manner.

#### TxId Digest

A new transaction digest algorithm is defined that constructs the identifier for a transaction from a tree of hashes. Each branch of the tree of hashes will correspond to a specific subset of transaction data. The overall structure of the hash is as follows; each name referenced here will be described in detail below:

txid_digest
├── transparent_digest
│   ├── prevouts_digest
│   ├── sequence_digest
│   └── outputs_digest
├── sapling_digest
│   ├── sapling_spends_digest
│   │   ├── sapling_spends_compact_digest
│   │   └── sapling_spends_noncompact_digest
│   ├── sapling_outputs_digest
│   │   ├── sapling_outputs_compact_digest
│   │   ├── sapling_outputs_memos_digest
│   │   └── sapling_outputs_noncompact_digest
│   └── valueBalance
└── orchard_digest
├── orchard_actions_compact_digest
├── orchard_actions_memos_digest
├── orchard_actions_noncompact_digest
├── flagsOrchard
├── valueBalanceOrchard
└── anchorOrchard

Each node written as snake_case in this tree is a BLAKE2b-256 hash of its children, initialized with a personalization string specific to that branch of the tree. Nodes that are not themselves digests are written in camelCase. In the specification below, nodes of the tree are presented in depth-first order.

##### txid_digest

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 set to:

"ZcashTxHash_" || CONSENSUS_BRANCH_ID

ZcashTxHash_ has 1 underscore character.

As in ZIP 143 6, CONSENSUS_BRANCH_ID is the 4-byte little-endian encoding of the consensus branch ID for the epoch of the block containing the transaction. Domain separation of the transaction id hash across parallel consensus branches provides replay protection: transactions targeted for one consensus branch will not have the same transaction identifier on other consensus branches.

This signature hash personalization prefix has been changed to reflect the new role of this hash (relative to ZcashSigHash as specified in ZIP 143) as a transaction identifier rather than a commitment that is exclusively used for signature purposes. The previous computation of the transaction identifier was a SHA256d hash of the serialized transaction contents, and was not personalized.

A BLAKE2b-256 hash of the following values

T.1a: version             (4-byte little-endian version identifier including overwinter flag)
T.1b: version_group_id    (4-byte little-endian version group identifier)
T.1c: consensus_branch_id (4-byte little-endian consensus branch id)
T.1d: lock_time           (4-byte little-endian nLockTime value)
T.1e: expiry_height       (4-byte little-endian block height)

The personalization field of this hash is set to:

"ZTxIdHeadersHash"
###### T.2: transparent_digest

In the case that transparent inputs or outputs are present, the transparent digest consists of a BLAKE2b-256 hash of the following values

T.2a: prevouts_digest (32-byte hash)
T.2b: sequence_digest (32-byte hash)
T.2c: outputs_digest  (32-byte hash)

The personalization field of this hash is set to:

"ZTxIdTranspaHash"

In the case that the transaction has no transparent components, transparent_digest is

BLAKE2b-256("ZTxIdTranspaHash", [])
T.2a: prevouts_digest

A BLAKE2b-256 hash of the field encoding of all outpoint field values of transparent inputs to the transaction.

The personalization field of this hash is set to:

"ZTxIdPrevoutHash"

In the case that the transaction has transparent outputs but no transparent inputs, prevouts_digest is

BLAKE2b-256("ZTxIdPrevoutHash", [])
T.2b: sequence_digest

A BLAKE2b-256 hash of the 32-bit little-endian representation of all nSequence field values of transparent inputs to the transaction.

The personalization field of this hash is set to:

"ZTxIdSequencHash"

In the case that the transaction has transparent outputs but no transparent inputs, sequence_digest is

BLAKE2b-256("ZTxIdSequencHash", [])
T.2c: outputs_digest

A BLAKE2b-256 hash of the concatenated field encodings of all transparent output values of the transaction. The field encoding of such an output consists of the encoded output amount (8-byte little endian) followed by the scriptPubKey byte array (serialized as Bitcoin script).

The personalization field of this hash is set to:

"ZTxIdOutputsHash"

In the case that the transaction has transparent inputs but no transparent outputs, outputs_digest is

BLAKE2b-256("ZTxIdOutputsHash", [])
###### T.3: sapling_digest

In the case that Sapling spends or outputs are present, the digest of Sapling components is composed of two subtrees which are organized to permit easy interoperability with the CompactBlock representation of Sapling data specified by the ZIP 307 Light Client Protocol 8.

This digest is a BLAKE2b-256 hash of the following values

T.3a: sapling_spends_digest  (32-byte hash)
T.3b: sapling_outputs_digest (32-byte hash)
T.3c: valueBalance           (64-bit signed little-endian)

The personalization field of this hash is set to:

"ZTxIdSaplingHash"

In the case that the transaction has no Sapling spends or outputs, sapling_digest is

BLAKE2b-256("ZTxIdSaplingHash", [])
T.3a: sapling_spends_digest

In the case that Sapling spends are present, this digest is a BLAKE2b-256 hash of the following values

T.3a.i:  sapling_spends_compact_digest    (32-byte hash)
T.3a.ii: sapling_spends_noncompact_digest (32-byte hash)

The personalization field of this hash is set to:

"ZTxIdSSpendsHash"

In the case that the transaction has Sapling outputs but no Sapling spends, sapling_spends_digest is

BLAKE2b-256("ZTxIdSSpendsHash", [])
T.3a.i: sapling_spends_compact_digest

A BLAKE2b-256 hash of the field encoding of all nullifier field values of Sapling shielded spends belonging to the transaction.

The personalization field of this hash is set to:

"ZTxIdSSpendCHash"
T.3a.ii: sapling_spends_noncompact_digest

A BLAKE2b-256 hash of the non-nullifier information for all Sapling shielded spends belonging to the transaction, excluding both zkproof data and spend authorization signature(s). For each spend, the following elements are included in the hash:

T.3a.ii.1: cv     (field encoding bytes)
T.3a.ii.2: anchor (field encoding bytes)
T.3a.ii.3: rk     (field encoding bytes)

In Transaction version 5, Sapling Spends have a shared anchor, which is hashed into the sapling_spends_noncompact_digest for each Spend.

The personalization field of this hash is set to:

"ZTxIdSSpendNHash"
T.3b: sapling_outputs_digest

In the case that Sapling outputs are present, this digest is a BLAKE2b-256 hash of the following values

T.3b.i:   sapling_outputs_compact_digest    (32-byte hash)
T.3b.ii:  sapling_outputs_memos_digest      (32-byte hash)
T.3b.iii: sapling_outputs_noncompact_digest (32-byte hash)

The personalization field of this hash is set to:

"ZTxIdSOutputHash"

In the case that the transaction has Sapling spends but no Sapling outputs, sapling_outputs_digest is

BLAKE2b-256("ZTxIdSOutputHash", [])
T.3b.i: sapling_outputs_compact_digest

A BLAKE2b-256 hash of the subset of Sapling output information included in the ZIP-307 8 CompactBlock format for all Sapling shielded outputs belonging to the transaction. For each output, the following elements are included in the hash:

T.3b.i.1: cmu                  (field encoding bytes)
T.3b.i.2: ephemeral_key        (field encoding bytes)
T.3b.i.3: enc_ciphertext[..52] (First 52 bytes of field encoding)

The personalization field of this hash is set to:

"ZTxIdSOutC__Hash" (2 underscore characters)
T.3b.ii: sapling_outputs_memos_digest

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

T.3b.ii.1: enc_ciphertext[52..564] (contents of the encrypted memo field)

The personalization field of this hash is set to:

"ZTxIdSOutM__Hash" (2 underscore characters)
T.3b.iii: sapling_outputs_noncompact_digest

A BLAKE2b-256 hash of the remaining subset of Sapling output information not included in the ZIP 307 8 CompactBlock format, excluding zkproof data, for all Sapling shielded outputs belonging to the transaction. For each output, the following elements are included in the hash:

T.3b.iii.1: cv                    (field encoding bytes)
T.3b.iii.2: enc_ciphertext[564..] (post-memo Poly1305 AEAD tag of field encoding)
T.3b.iii.3: out_ciphertext        (field encoding bytes)

The personalization field of this hash is set to:

"ZTxIdSOutN__Hash" (2 underscore characters)
###### T.4: orchard_digest

In the case that Orchard actions are present in the transaction, this digest is 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"

In the case that the transaction has no Orchard actions, orchard_digest is

BLAKE2b-256("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:

"ZTxIdOrcActCHash"
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:

"ZTxIdOrcActMHash"
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:

"ZTxIdOrcActNHash"

#### Signature Digest

A new per-input transaction digest algorithm is defined that constructs a hash that may be signed by a transaction creator to commit to the effects of the transaction. A signature digest is produced for each transparent input, each Sapling input, and each Orchard action. For transparent inputs, this follows closely the algorithms from ZIP 143 6 and ZIP 243 7. For shielded inputs, this algorithm has the exact same output as the transaction digest algorithm, thus the txid may be signed directly.

The overall structure of the hash is as follows; each name referenced here will be described in detail below:

signature_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 set to:

"ZcashTxHash_" || CONSENSUS_BRANCH_ID

ZcashTxHash_ has 1 underscore character.

This value has the same personalization as the top hash of the transaction identifier digest tree, so that what is being signed in the case that there are no transparent inputs is just the transaction id.

Identical to that specified for the transaction identifier.

###### S.2: transparent_sig_digest

If we are producing a hash for either a coinbase transaction, or a non-coinbase transaction that has no transparent inputs, the value of transparent_sig_digest is identical to the value specified in section T.2.

If we are producing a hash for a non-coinbase transaction that has transparent inputs, the value of transparent_sig_digest depends upon the value of a hash_type flag, as follows.

The construction of each component below depends upon the values of the hash_type flag bits. Each component will be described separately.

This digest is a BLAKE2b-256 hash of the following values

S.2a: hash_type                (1 byte)
S.2b: prevouts_sig_digest      (32-byte hash)
S.2c: amounts_sig_digest       (32-byte hash)
S.2d: scriptpubkeys_sig_digest (32-byte hash)
S.2e: sequence_sig_digest      (32-byte hash)
S.2f: outputs_sig_digest       (32-byte hash)
S.2g: txin_sig_digest          (32-byte hash)

The personalization field of this hash is set to:

"ZTxIdTranspaHash"
S.2a: hash_type

This is an 8-bit unsigned value. The SIGHASH encodings from the legacy script system are reused: one of SIGHASH_ALL (0x01), SIGHASH_NONE (0x02), and SIGHASH_SINGLE (0x03), with or without the SIGHASH_ANYONECANPAY flag (0x80). The following restrictions apply, which cause validation failure if violated:

• Using any undefined hash_type (not 0x01, 0x02, 0x03, 0x81, 0x82, or 0x83).
• Using SIGHASH_SINGLE without a "corresponding output" (an output with the same index as the input being verified).

If we are producing a hash for the signature over a transparent input, the value of hash_type is obtained from the input's scriptSig as encoded in the transaction. If we are producing a hash for the signature over a Sapling Spend or an Orchard Action, hash_type is set to SIGHASH_ALL.

S.2b: prevouts_sig_digest

This is a BLAKE2b-256 hash initialized with the personalization field value ZTxIdPrevoutHash.

If the SIGHASH_ANYONECANPAY flag is not set:

identical to the value of prevouts_digest as specified for the
transaction identifier in section T.2a.

otherwise:

BLAKE2b-256(ZTxIdPrevoutHash, [])
S.2c: amounts_sig_digest

If the SIGHASH_ANYONECANPAY flag is not set, the value of amounts_sig_digest is a BLAKE2b-256 hash of the concatenation of the 8-byte signed little-endian representations of all value fields 14 for the coins spent by the transparent inputs to the transaction.

The personalization field of this hash is set to:

"ZTxTrAmountsHash"

If the SIGHASH_ANYONECANPAY flag is set, amounts_sig_digest is:

BLAKE2b-256("ZTxTrAmountsHash", [])
S.2d: scriptpubkeys_sig_digest

If the SIGHASH_ANYONECANPAY flag is not set, the value of scriptpubkeys_sig_digest is a BLAKE2b-256 hash of the concatenation of the field encodings (each including a leading CompactSize) of all pk_script fields 14 for the coins spent by the transparent inputs to the transaction.

The personalization field of this hash is set to:

"ZTxTrScriptsHash"

If the SIGHASH_ANYONECANPAY flag is set, scriptpubkeys_sig_digest is:

BLAKE2b-256("ZTxTrScriptsHash", [])
S.2e: sequence_sig_digest

This is a BLAKE2b-256 hash initialized with the personalization field value ZTxIdSequencHash.

If the SIGHASH_ANYONECANPAY flag is not set:

identical to the value of sequence_digest as specified for the
transaction identifier in section T.2b.

otherwise:

BLAKE2b-256(ZTxIdSequencHash, [])
S.2f: outputs_sig_digest

This is a BLAKE2b-256 hash initialized with the personalization field value ZTxIdOutputsHash.

If the sighash type is neither SIGHASH_SINGLE nor SIGHASH_NONE:

identical to the value of outputs_digest as specified for the
transaction identifier in section T.2c.

If the sighash type is SIGHASH_SINGLE and the signature hash is being computed for the transparent input at a particular index, and a transparent output appears in the transaction at that index:

the hash is over the transaction serialized form of the transparent output at that
index

otherwise:

BLAKE2b-256(ZTxIdOutputsHash, [])
S.2g: txin_sig_digest

If we are producing a hash for the signature over a transparent input, the value of txin_sig_digest is a BLAKE2b-256 hash of the following properties of the transparent input being signed, initialized with the personalization field value Zcash___TxInHash (3 underscores):

S.2g.i:   prevout      (field encoding)
S.2g.ii:  value        (8-byte signed little-endian)
S.2g.iii: scriptPubKey (field encoding)
S.2g.iv:  nSequence    (4-byte unsigned little-endian)

Notes:

• value is defined in the consensus rules to be a nonnegative value <= MAX_MONEY, but all existing implementations parse this value as signed and enforce the nonnegative constraint as a consensus check. It is defined as signed here for consistency with those existing implementations.
• scriptPubKey is the field encoding (including a leading CompactSize) of the pk_script field 14 for the coin spent by the transparent input. For P2SH coins, this differs from the redeemScript committed to in ZIP 243 7.

If we are producing a hash for the signature over a Sapling Spend or an Orchard Action, txin_sig_digest is:

BLAKE2b-256("Zcash___TxInHash", [])
###### S.3: sapling_digest

Identical to that specified for the transaction identifier.

###### S.4: orchard_digest

Identical to that specified for the transaction identifier.

#### Authorizing Data Commitment

A new transaction digest algorithm is defined that constructs a digest which commits to the authorizing data of a transaction from a tree of BLAKE2b-256 hashes. For v5 transactions, the overall structure of the hash is as follows:

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

Each node written as snake_case in this tree is a BLAKE2b-256 hash of authorizing data of the transaction.

For transaction versions before v5, a placeholder value consisting of 32 bytes of 0xFF is used in place of the authorizing data commitment. This is only used in the tree committed to by hashAuthDataRoot, as defined in Block Header Changes.

The pair (Transaction Identifier, Auth Commitment) constitutes a commitment to all the data of a serialized transaction that may be included in a block.

##### 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 set to:

"ZTxAuthHash_" || CONSENSUS_BRANCH_ID

ZTxAuthHash_ has 1 underscore character.

###### A.1: transparent_scripts_digest

In the case that the transaction contains transparent inputs, this is a BLAKE2b-256 hash of the field encoding of the concatenated values of the Bitcoin script values associated with each transparent input belonging to the transaction.

The personalization field of this hash is set to:

"ZTxAuthTransHash"

In the case that the transaction has no transparent inputs, transparent_scripts_digest is

BLAKE2b-256("ZTxAuthTransHash", [])
###### A.2: sapling_auth_digest

In the case that Sapling Spends or Sapling Outputs are present, this is a BLAKE2b-256 hash of the field encoding of the Sapling zkproof value of each Sapling Spend Description, followed by the field encoding of the spend_auth_sig value of each Sapling Spend Description belonging to the transaction, followed by the field encoding of the zkproof field of each Sapling Output Description belonging to the transaction, followed by the field encoding of the binding signature:

A.2a: spend_zkproofs           (field encoding bytes)
A.2b: spend_auth_sigs          (field encoding bytes)
A.2c: output_zkproofs          (field encoding bytes)
A.2d: binding_sig              (field encoding bytes)

The personalization field of this hash is set to:

"ZTxAuthSapliHash"

In the case that the transaction has no Sapling Spends or Sapling Outputs, sapling_auth_digest is

BLAKE2b-256("ZTxAuthSapliHash", [])
###### A.3: orchard_auth_digest

In the case that Orchard Actions are present, this is 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"

In the case that the transaction has no Orchard Actions, orchard_auth_digest is

BLAKE2b-256("ZTxAuthOrchaHash", [])

The nonmalleable transaction identifier specified by this ZIP will be used in the place of the current malleable transaction identifier within the Merkle tree committed to by the hashMerkleRoot value. However, this change now means that hashMerkleRoot is not sufficient to fully commit to the transaction data, including witnesses, that appear within the block.

As a consequence, we now need to add a new commitment to the block header. This commitment will be the root of a Merkle tree having leaves that are transaction authorizing data commitments, produced according to the Authorizing Data Commitment part of this specification. The insertion order for this Merkle tree MUST be identical to the insertion order of transaction identifiers into the Merkle tree that is used to construct hashMerkleRoot, such that a path through this Merkle tree to a transaction identifies the same transaction as that path reaches in the tree rooted at hashMerkleRoot.

This new commitment is named hashAuthDataRoot and is the root of a binary Merkle tree of transaction authorizing data commitments having height $$\mathsf{ceil(log_2(tx\_count))}$$ , padded with leaves having the "null" hash value [0u8; 32]. Note that $$\mathsf{log_2(tx\_count)}$$ is well-defined because $$\mathsf{tx\_count} > 0$$ , due to the coinbase transaction in each block. Non-leaf hashes in this tree are BLAKE2b-256 hashes personalized by the string "ZcashAuthDatHash".

Changing the block header format to allow space for an additional commitment is somewhat invasive. Instead, the name and meaning of the hashLightClientRoot field, described in ZIP 221 4, is changed.

hashLightClientRoot is renamed to hashBlockCommitments. The value of this hash is the BLAKE2b-256 hash personalized by the string "ZcashBlockCommit" of the following elements:

hashLightClientRoot (as described in ZIP 221)
hashAuthDataRoot    (as described below)
terminator          [0u8;32]

This representation treats the hashBlockCommitments value as a linked list of hashes terminated by arbitrary data. In the case of protocol upgrades where additional commitments need to be included in the block header, it is possible to replace this terminator with the hash of a newly defined structure which ends in a similar terminator. Fully validating nodes MUST always use the entire structure defined by the latest activated protocol version that they support.

The linked structure of this hash is intended to provide extensibility for use by light clients which may be connected to a third-party server that supports a later protocol version. Such a third party SHOULD provide a value that can be used instead of the all-zeros terminator to permit the light client to perform validation of the parts of the structure it needs.

Unlike the hashLightClientRoot change, the change to hashBlockCommitments happens in the block that activates this ZIP.

The block header byte format and version are not altered by this ZIP.

## Rationale

In S.2, we use the same personalization strings for fields that have matching fields in T.2, in order to facilitate reuse of their digests. In particular, the "no transparent inputs or outputs" case of S.2 is identical to the equivalent case in T.2; thus for fully shielded transactions, signature_digest is equal to txid_digest.

Several changes in this ZIP (relative to ZIP 243 7) were made to align with BIP 341 9:

• The hash_type field is now restricted via a new consensus rule to be one of a specific set of sighash type encodings. The rationale for this change is inherited from BIP 341 10.
• Note however that we do not define SIGHASH_DEFAULT, as it is equivalent to SIGHASH_ALL, and we prefer the encodings to be canonical.
• Two new commitments (amounts_sig_digest and scriptpubkeys_sig_digest) were added, to address difficulties in the case of a hardware wallet signing transparent inputs. scriptpubkeys_sig_digest helps the hardware wallet to determine the subset of inputs belonging to it 11. amounts_sig_digest prevents the transaction creator from lying to the hardware wallet about the transaction fee 12. Without these commitments, the hardware wallet would need to be sent every transaction containing an outpoint referenced in the transaction being signed.
• The semantics of sequence_sig_digest were changed, to commit to nSequence even if SIGHASH_SINGLE or SIGHASH_NONE is set. The rationale for this change is inherited from BIP 341 13.
• The semantics of outputs_sig_digest were changed, via a new consensus rule that rejects transparent inputs for which SIGHASH_SINGLE is set without a corresponding transparent output at the same index. BIP 341 does not give a rationale for this change, but without it these inputs were effectively using SIGHASH_NONE, which is silently misleading.
• The semantics of txin_sig_digest were changed, to always commit to the scriptPubKey field of the transparent coin being spent, instead of the script actually being executed at the time signature_digest is calculated.
• This ensures that the signature commits to the entire committed script. In Taproot, this makes it possible to prove to a hardware wallet what (unused) execution paths exist 11. Alternate execution paths don't exist for P2PKH (where the executed script is scriptPubKey) or P2SH (where scriptPubKey is fully executed prior to redeemScript).
• For P2SH, this means we commit to the Hash160 digest of redeemScript instead of the actual script. Note that the Bitcoin P2SH design depends entirely on Hash160 being preimage-resistant, because otherwise anyone would be able to spend someone else's P2SH UTXO using a preimage. We do need to ensure that there is no collision attack; this holds because even if an adversary could find a Hash160 collision, it would only enable them to alter the input's scriptSig field. Doing so doesn't alter the effecting data of the transaction, which by definition means the transaction has the same effect under consensus (spends the same inputs and produces the same outputs).

Signatures over Sapling Spends or Orchard Actions, in transactions containing transparent inputs, commit to the same data that the transparent inputs do, including all of the transparent input values. Without this commitment, there would be a similar difficulty for a hardware wallet in the case where it is only signing shielded inputs, when the transaction also contains transparent inputs from a malicious other party, because that party could lie about their coins' values.

By contrast, binding signatures for shielded coinbase transactions continue to be over the transaction ID, as for non-coinbase transactions without transparent inputs. This is necessary because coinbase transactions have a single "dummy" transparent input element that has no corresponding previous output to commit to. It is also sufficient because the data in that transparent input either is already bound elsewhere (namely the block height, placed in expiry_height from NU5 activation), or does not need to be bound to the shielded outputs (e.g. miner-identifying information).

## References

1 RFC 2119: Key words for use in RFCs to Indicate Requirement Levels
3 ZIP 200: Network Upgrade Mechanism
4 ZIP 221: FlyClient - Consensus Layer Changes
5 ZIP 76: Transaction Signature Validation before Overwinter
6 ZIP 143: Transaction Signature Validation for Overwinter
7 ZIP 243: Transaction Signature Validation for Sapling
8 ZIP 307: Light Client Protocol for Payment Detection
9 BIP 341: Taproot: SegWit version 1 spending rules
10 Why reject unknown hash_type values?
11 Why does the signature message commit to the scriptPubKey?
12 Why does the signature message commit to the amounts of all transaction inputs?
13 Why does the signature message commit to all input nSequence if SIGHASH_SINGLE or SIGHASH_NONE are set?
14 Bitcoin Developer Reference. TxOut: A Transaction Output