208 lines
9.6 KiB
ReStructuredText
208 lines
9.6 KiB
ReStructuredText
::
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ZIP: 213
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Title: Shielded Coinbase
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Owners: Jack Grigg <jack@z.cash>
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Status: Final
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Category: Consensus
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Created: 2019-03-30
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License: MIT
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Terminology
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===========
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The key words "MUST" and "MAY" in this document are to be interpreted as described in
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RFC 2119. [#RFC2119]_
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The term "network upgrade" in this document is to be interpreted as described in ZIP 200
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[#zip-0200]_.
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The term "Sapling" in this document is to be interpreted as described in ZIP 205
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[#zip-0205]_.
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The terms "Founders' Reward" and "funding stream" in this document are to be interpreted
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as described in ZIP 207 [#zip-0207]_.
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Abstract
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========
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This proposal defines modifications to the Zcash consensus rules that enable coinbase
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funds to be mined to Sapling (and later Orchard) addresses. It does not disable the use
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of transparent addresses in coinbase transactions.
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Motivation
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==========
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Zcash inherited the concept of "coinbase transactions" from Bitcoin: special transactions
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inside each block that are allowed to have no inputs. These transactions are created by
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miners during block creation, and collect the block reward and transaction fees into new
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transparent outputs that can then be spent. They are also leveraged in Zcash for the
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Founders' Reward (and potentially for funding streams [#zip-0207]_).
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On the path to deprecating and removing Bitcoin-inherited transparent addresses within the
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Zcash network, a required step is to be able to create coinbase transactions that have no
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transparent outputs. However, Zcash was launched with a consensus rule preventing coinbase
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transactions from containing shielded outputs, instead enforcing that coinbase funds could
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not be spent in transactions with transparent outputs. This was partly in order to reduce
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the complexity of the original Zcash modifications to the Bitcoin Core codebase, but also
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because at the time, shielded transactions required significant memory and CPU resources
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to create.
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The Sapling network upgrade [#zip-0205]_ deployed architectural changes and performance
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improvements that make shielding funds directly in the coinbase transaction feasible. In
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order to reduce the complexity of the Sapling network upgrade, the existing consensus
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rules preventing coinbase transactions from containing shielded outputs were extended to
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cover Sapling outputs. Therefore, it is now necessary to modify the consensus rules in
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order to enable miners to start using Sapling addresses. It will also be possible for
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miners to use Orchard addresses starting from activation of the NU5 upgrade [#zip-0252]_.
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Specification
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=============
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Prior to activation of the Heartwood network upgrade, this existing consensus rule on
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coinbase transactions is enforced:
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A coinbase transaction MUST NOT have any JoinSplit descriptions, Spend descriptions,
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or Output descriptions.
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Once the Heartwood network upgrade activates:
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- Coinbase transactions MAY contain Sapling outputs. More precisely, the above existing
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consensus rule is modified to:
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A coinbase transaction MUST NOT have any JoinSplit descriptions or Spend
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descriptions.
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[Pre-Heartwood] A coinbase transaction also MUST NOT have any Output descriptions.
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- The consensus rules applied to ``valueBalance``, ``vShieldedOutput``, and ``bindingSig``
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in non-coinbase transactions MUST also be applied to coinbase transactions.
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- Only transparent outputs in coinbase transactions are subject to the existing
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restrictions on spending coinbase funds. More precisely:
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- The existing consensus rule requiring transactions that spend coinbase outputs to have
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an empty ``vout``, is amended to only apply to transactions that spend transparent
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coinbase outputs.
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- The existing consensus rule requiring coinbase outputs to have 100 confirmations
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before they may be spent (coinbase maturity), is amended to only apply to transparent
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coinbase outputs. Specifically, it becomes:
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A transaction MUST NOT spend a transparent output of a coinbase transaction from a
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block less than 100 blocks prior to the spend. Note that outputs of coinbase
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transactions include Founders’ Reward outputs [and potentially funding stream
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outputs].
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- Full note decryption of shielded coinbase outputs MUST succeed using the all-zero
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outgoing viewing key. More precisely, all shielded coinbase outputs MUST have valid
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note commitments when recovered using a sequence of 32 zero bytes as the outgoing
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viewing key.
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Once the NU5 network upgrade activates:
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- Coinbase transactions MAY contain Sapling and/or Orchard outputs.
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This does not require any change to the consensus rule given above; it is a consequence
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of other rules as of NU5 activation.
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Interaction with the Founders' Reward
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-------------------------------------
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This ZIP does not alter the existing Founders' Reward addresses.
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Rationale
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=========
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The ZIP does not require that all coinbase must be shielded immediately from activation of
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the network upgrade, so that miners and mining pools may gradually migrate from their
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existing transparent addresses to Sapling addresses. This also simplifies the consensus
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rules, because the Founders' Reward targets transparent addresses, and thus it remains
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necessary for the time being to support them. A future ZIP could require all coinbase to
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be shielded immediately.
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Enforcing coinbase maturity at the consensus level for Sapling outputs would incur
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significant complexity in the consensus rules, because it would require special-casing
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coinbase note commitments in the Sapling commitment tree. The standard recommendation when
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spending a note is to select an anchor 10 blocks back from the current chain tip, which
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acts as a de-facto 10-block maturity on all notes, coinbase included. This might be
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proposed as a consensus rule in future.
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There is another reason for shielded coinbase maturity being unnecessary: shielded
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coinbase outputs have the same effect on economic activity as regular shielded outputs.
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When a transparent address receives a coin in some "parent" transaction and later spends
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it, a tree of "direct child" transactions is created that all require the original parent
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transaction to be valid. However, most wallets treat unspent transparent outputs as a
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single "bucket of money", and select coins to spend without direct user input. This can
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create a disconnect between the economic activity of users (who might be intending to
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spend funds that they just received, creating "logical child" transactions), and their
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on-chain transaction graph.
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For example, a mining pool that successfully mines a block will receive a coinbase
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output, but their subsequent payout to miners might instead spend ZEC that they already
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had before the block was mined. If the mining pool pays out for block X, and then a
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reorg or rollback occurs that causes the transparent coinbase in block X to become
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invalid, the payout transaction might still be mined on the new chain, even though the
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funds that the miner was logically spending do not exist there.
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By contrast, when a reorg or rollback occurs that would cause a shielded coinbase output
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to disappear, it will also invalidate every shielded transaction that uses an anchor
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descending from the tree that the shielded coinbase output had been appended to. That is,
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all shielded economic activity would be rolled back in addition to the shielded coinbase
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output disappearing, ensuring that all logical child transactions are invalidated, not
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just direct child transactions. Therefore, there is no reason to make shielded coinbase a
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special case when the same behaviour already occurs in regular shielded notes.
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Requiring that note commitments are valid when recovering using a fixed outgoing viewing
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key implies that target addresses and values for all Sapling outputs within the coinbase
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are revealed. This would be necessary to correctly enforce shielded Founders' Reward or
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funding stream outputs, and it is simpler to enforce this on all outputs. Additionally,
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this maintains the ability for network observers to track miners and mining pools.
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Meanwhile, the miners and mining pools could put useful or identifying text in the memo
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fields of the outputs, instead of storing it ad-hoc elsewhere in the coinbase transaction.
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Security and Privacy Considerations
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===================================
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Sapling outputs in coinbase transactions are by design publicly viewable, in contrast to
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Sapling outputs in normal transactions. This does not introduce any privacy regressions
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relative to existing coinbase transactions, because coinbase output values and recipient
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addresses have always been public information. However, users with threat models that rely
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on keeping their Sapling address private (for example, to maintain post-quantum privacy),
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and who are also miners or mining pools, should use a coinbase-specific address when
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creating blocks.
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Revealing the coinbase output notes does not enable anyone else to detect when the note is
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spent, which removes the need for a separate shielding step like is enforced for
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transparent coinbase outputs.
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Deployment
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==========
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This proposal will be deployed with the Heartwood network upgrade. [#zip-0250]_
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Reference Implementation
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========================
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https://github.com/zcash/zcash/pull/4256
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References
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==========
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.. [#RFC2119] `RFC 2119: Key words for use in RFCs to Indicate Requirement Levels <https://www.rfc-editor.org/rfc/rfc2119.html>`_
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.. [#zip-0200] `ZIP 200: Network Upgrade Mechanism <zip-0200.rst>`_
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.. [#zip-0205] `ZIP 205: Deployment of the Sapling Network Upgrade <zip-0205.rst>`_
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.. [#zip-0207] `ZIP 207: Split Founders' Reward <zip-0207.rst>`_
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.. [#zip-0250] `ZIP 250: Deployment of the Heartwood Network Upgrade <zip-0250.rst>`_
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.. [#zip-0252] `ZIP 252: Deployment of the NU5 Network Upgrade <zip-0252.rst>`_
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