2020-09-14 07:22:13 -07:00
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::
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ZIP: 244
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2021-01-13 15:27:19 -08:00
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Title: Transaction Non-Malleability
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2020-09-14 07:22:13 -07:00
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Owners: Kris Nuttycombe <kris@electriccoin.co>
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2021-01-13 15:27:19 -08:00
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Daira Hopwood <daira@electriccoin.co>
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Status: Draft
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2020-09-14 07:22:13 -07:00
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Category: Consensus
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2021-01-13 15:27:19 -08:00
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Created: 2021-01-06
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License: MIT
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Discussions-To: <https://github.com/zcash/zips/issues/411>
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===========
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Terminology
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===========
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The key words "MUST" and "MUST NOT" in this document are to be interpreted as described in RFC 2119. [#RFC2119]_
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The terms "consensus branch", "epoch", and "network upgrade" in this document are to be interpreted as
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described in ZIP 200. [#zip-0200]_
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========
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Abstract
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========
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This proposal defines a new transaction digest algorithm for the <TBD> network upgrade
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onward, in order to introduce non-malleable transaction identifiers that commit to
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all transaction data except for attestations to transaction validity.
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This proposal also defines a new transaction digest algorithm for signature validation,
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which shares all available structure produced during the construction of transaction
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identifiers, in order to minimize redundant data hashing in validation.
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This proposal also defines new semantics for the ``hashLightClientRoot`` field of the
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block header, to enable additional commitments to be represented in this hash and to
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provide a mechanism for future extensibility of the set of commitments represented.
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==========
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Motivation
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==========
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In all cases, but particularly in order to support the use of transactions in
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higher-level protocols, any modification of the transaction that has not been
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explicitly permitted (such as via anyone-can-spend inputs) should invalidate
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attestations to spend authority or to the included outputs. Following the activation
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of this proposed change, transaction identifiers will be stable irrespective of
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any possible malleation of witness data.
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============
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Requirements
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============
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- Continue to support existing functionality of the protocol (multisig,
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signing modes for transparent inputs).
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- Allow the use of transaction ids, and pairs of the form (transaction id,
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output index) as stable identifiers.
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- A sender must be able to recognize their own transaction, even given allowed
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forms of malleability such as changes to witnesses.
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- In the case of transparent inputs, it should be possible to create a
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transaction (B) that spends the outputs from a previous transaction (A) even
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before (A) has been mined. This should also be possible in the case that the
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creator of (B) does not wait for confirmations of (A); (B) should remain
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valid so long as any variant of (A) is eventually mined.
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- It should not be possible for an attacker to malleate a transaction in a
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fashion that would result in the transaction being interpreted as a
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double-spend.
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- It should be possible in the future to upgrade the protocol in such a fashion
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that only non-malleable transactions are accepted.
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- It should be possible to use the transaction id unmodified as the value that
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is used to produce a signature hash in the case that the transaction contains
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no transparent inputs, or in the case that the ``SIGHASH_ALL`` flag is used.
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=========
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Prior Art
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=========
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- https://zips.z.cash/zip-0143
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- https://github.com/bitcoin/bips/blob/master/bip-0062.mediawiki
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- https://bitcoinclassic.com/devel/Flexible%20Transactions.html
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- SegWit
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================
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Non-requirements
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================
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In order to support backwards-compatibility with parts of the ecosystem that
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have not yet upgraded to the non-malleable transaction format, it is not an
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initial requirement that all transactions be non-malleable.
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=============
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Specification
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=============
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-------
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Digests
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-------
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All digests are personalized BLAKE2b-256 hashes. In cases where no elements are
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available for hashing (for example, if there are no transparent inputs) the resulting hash
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will be over just the personalization string, providing domain separation even for
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empty data fields.
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TxId Digest
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===========
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A new transaction digest algorithm is defined that constructs the identifier for
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a transaction from a tree of hashes. Each branch of the tree of hashes will
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correspond to a specific subset of transaction data. The overall structure of
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the hash is as follows; each name referenced here will be described in detail
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below:
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txid_digest
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├── header_digest
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├── transparent_digest
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│ ├── prevouts_digest
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│ ├── sequence_digest
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│ └── outputs_digest
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├── sprout_digest
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└── sapling_digest
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├── sapling_spends_digest
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│ ├── sapling_spends_compact_digest
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│ └── sapling_spends_noncompact_digest
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├── sapling_outputs_digest
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│ ├── sapling_outputs_compact_digest
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│ ├── sapling_outputs_memos_digest
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│ └── sapling_outputs_noncompact_digest
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└── valueBalance
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Each node written as ``snake_case`` in this tree is a BLAKE2b-256 hash of its
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children, initialized with a personalization string specific to that branch
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of the tree. Nodes that are not themselves digests are written in ``camelCase``.
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In the specification below, nodes of the tree are presented in depth-first order.
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``txid_digest``
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--------------
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A BLAKE2b-256 hash of the following values ::
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* ``header_digest`` (32-byte hash output)
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* ``transparent_digest`` (32-byte hash output)
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* ``sprout_digest (32-byte hash output)
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* ``sapling_digest (32-byte hash output)
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The personalization field of this hash is set to::
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"ZTxIdHeadersHash"
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1: ``transparent_digest``
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`````````````````````````
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A BLAKE2b-256 hash of the following values ::
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* 1a. ``prevouts_digest`` (32-byte hash)
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* 1b. ``sequence_digest`` (32-byte hash)
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* 1c. ``outputs_digest`` (32-byte hash)
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The personalization field of this hash is set to::
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"ZTxIdTranspaHash"
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1a: ``prevouts_digest``
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'''''''''''''''''''''''
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A BLAKE2b-256 hash of the field encoding of all ``outpoint``
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field values of transparent inputs to the transaction.
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The personalization field of this hash is set to::
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"ZTxIdPrevoutHash"
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1b: ``sequence_digest``
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'''''''''''''''''''''''
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A BLAKE2b-256 hash of the 32-bit little-endian representation of all ``nSequence``
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field values of transparent inputs to the transaction.
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The personalization field of this hash is set to::
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"ZTxIdSequencHash"
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1c: ``outputs_digest``
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''''''''''''''''''''''
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A BLAKE2b-256 hash of the field encoding of all transparent outputs
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belonging to the transaction.
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The personalization field of this hash is set to::
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"ZTxIdOutputsHash"
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2: ``sprout_digest``
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`````````````````````````
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A BLAKE2b-256 hash of the field encoding of all Sprout ``JoinSplit`` components of the
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transaction followed by the field encoding of the ``joinSplitPubKey`` value. This is
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equivalent to the internal hash that is specified by ZIP 143 [#zip-0143]_ over the Sprout
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data, with the distinction that the unmodified hash finalized immediately after
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initialization with the personalization string is used in the case that no ``JoinSplit``
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components exist, rather than the ``uint256`` of ``0x0000...0000``.
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The personalization field of this hash is set to::
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"ZTxIdJSplitsHash"
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3: ``sapling_digest``
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`````````````````````
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The digest of Sapling components is composed of two subtrees which are organized to
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permit easy interoperability with the ``CompactBlock`` representation of Sapling data
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specified by the ZIP 307 Light Client Protocol [#zip-0307]_.
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This digest is a BLAKE2b-256 hash of the following values ::
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* 3a. ``sapling_spends_digest`` (32-byte hash)
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* 3b. ``sapling_outputs_digest`` (32-byte hash)
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* 3c. ``valueBalance`` (64-bit signed little-endian)
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The personalization field of this hash is set to::
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"ZTxIdSaplingHash"
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3a: ``sapling_spends_digest``
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''''''''''''''''''''''''''''''
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This digest is a BLAKE2b-256 hash of the following values ::
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* 3a.i. ``sapling_spends_compact_digest`` (32-byte hash)
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* 3b.ii. ``sapling_spends_noncompact_digest`` (32-byte hash)
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The personalization field of this hash is set to::
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"ZTxIdSSpendsHash"
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3a.i: ``sapling_spends_compact_digest``
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.......................................
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A BLAKE2b-256 hash of the field encoding of all nullifier field
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values of Sapling shielded spends belonging to the transaction.
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The personalization field of this hash is set to::
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"ZTxIdSSpendCHash"
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3a.ii: ``sapling_spends_noncompact_digest``
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...........................................
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A BLAKE2b-256 hash of the non-nullifier information for all Sapling shielded spends
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belonging to the transaction. For each spend, the following elements are included
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in the hash::
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* 3a.ii.1 ``cv`` (field encoding bytes)
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* 3a.ii.2 ``anchor`` (field encoding bytes)
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* 3a.ii.3 ``rk`` (field encoding bytes)
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* 3a.ii.4 ``zkproof`` (field encoding bytes)
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The personalization field of this hash is set to::
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"ZTxIdSSpendNHash"
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3b: ``sapling_outputs_digest``
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'''''''''''''''''''''''''''''''
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This digest is a BLAKE2b-256 hash of the following values ::
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* 3a.i. ``sapling_outputs_compact_digest`` (32-byte hash)
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* 3b.ii. ``sapling_outputs_memos_digest`` (32-byte hash)
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* 3b.iii. ``sapling_outputs_noncompact_digest`` (32-byte hash)
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The personalization field of this hash is set to::
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"ZTxIdSOutputHash"
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3b.i: ``sapling_outputs_compact_digest``
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.........................................
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A BLAKE2b-256 hash of the subset of Sapling output information included in the
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ZIP-307 ``CompactBlock`` format for all Sapling shielded outputs
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belonging to the transaction. For each output, the following elements are included
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in the hash::
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* 3b.i.1 ``cmu`` (field encoding bytes)
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* 3b.i.2 ``ephemeral_key`` (field encoding bytes)
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* 3b.i.3 ``enc_ciphertext[..52]`` (First 52 bytes of field encoding)
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The personalization field of this hash is set to::
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"ZTxIdSOutC__Hash"
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3a.ii: ``sapling_outputs_memos_digest``
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........................................
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A BLAKE2b-256 hash of the subset of Sapling shielded memo field data for all Sapling
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shielded outputs belonging to the transaction. For each output, the following elements
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are included in the hash::
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* 3b.ii.1 ``enc_ciphertext[52..564] (contents of the encrypted memo field)
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The personalization field of this hash is set to::
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"ZTxIdSOutM__Hash"
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3a.iii: ``sapling_outputs_noncompact_digest``
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..............................................
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A BLAKE2b-256 hash of the remaining subset of Sapling output information **not** included
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in the ZIP-307 ``CompactBlock`` format, for all Sapling shielded outputs belonging to the
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transaction. For each output, the following elements are included in the hash::
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* 3b.iii.1 ``cv`` (field encoding bytes)
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* 3b.iii.2 ``enc_ciphertext[564..]`` (post-memo suffix of field encoding)
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* 4b.iii.3 ``out_ciphertext`` (field encoding bytes)
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* 4b.iii.4 ``zkproof`` (field encoding bytes)
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The personalization field of this hash is set to::
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"ZTxIdSOutN__Hash"
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Witness Digest
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==============
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A new transaction digest algorithm is defined that constructs a digest which commits
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to the authorizing data of a transaction from a tree of BLAKE2b-256 hashes.
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The overall structure of the hash is as follows:
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auth_digest
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├── transparent_scripts_digest
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├── tze_witnesses_digest
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├── sprout_sigs_digest
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└── sapling_sigs_digest
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Each node written as ``snake_case`` in this tree is a BLAKE2b-256 hash of authorizing
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data of the transaction.
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``auth_digest``
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--------------
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A BLAKE2b-256 hash of the following values ::
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* ``transparent_scripts_digest`` (32-byte hash output)
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* ``tze_witnesses_digest (32-byte hash output)
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* ``sprout_sigs_digest (32-byte hash output)
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* ``sapling_sigs_digest (32-byte hash output)
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The personalization field of this hash is set to::
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"ZTxAuth_____Hash"
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1: ``transparent_scripts_digest``
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`````````````````````````````````
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A BLAKE2b-256 hash of the field encoding of the Bitcoin script associated
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with each transparent input belonging to the transaction.
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The personalization field of this hash is set to::
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"ZTxAuthTransHash"
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3: ``sprout_sigs_digest``
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```````````````````````````
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A BLAKE2b-256 hash of the field encoding of the joinsplit signature
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belonging to the transaction.
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The personalization field of this hash is set to::
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"ZTxAuthSprouHash"
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3: ``sapling_sigs_digest``
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```````````````````````````
|
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A BLAKE2b-256 hash of the field encoding of the sapling signature
|
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of each Sapling spend description belonging to the transaction, followed by the
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field encoding of the binding signature.
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The personalization field of this hash is set to::
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"ZTxAuthSapliHash"
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--------------------
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|
Block Header Changes
|
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|
--------------------
|
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|
// TODO: Need @str4d's help here.
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|
========================
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Reference implementation
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|
========================
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- https://github.com/zcash/librustzcash/pull/319/files
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==========
|
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References
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==========
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