zips/zip-0211.rst

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ZIP: 211
Title: Disabling Addition of New Value to the Sprout Chain Value Pool
Owners: Daira Emma Hopwood <daira@electriccoin.co>
Credits: Sean Bowe
Status: Final
Category: Consensus
Created: 2019-03-29
License: MIT
Terminology
===========
The key words "MUST", "SHOULD", and "OPTIONAL" in this document are to be interpreted
as described in BCP 14 [#BCP14]_ when, and only when, they appear in all capitals.
The term "network upgrade" in this document is to be interpreted as described in ZIP 200
[#zip-0200]_.
The term "Sprout shielded protocol" in this document refers to the shielded payment protocol
defined at the launch of the Zcash network.
The term "Sapling shielded protocol" in this document refers to the shielded payment protocol
introduced in the Sapling network upgrade [#zip-0205]_ [#protocol]_.
The term "Sprout chain value pool balance" in this document is to be interpreted as described
in ZIP 209 [#zip-0209]_.
Abstract
========
This proposal disables the ability to add new value to the Sprout chain value pool balance.
This takes a step toward being able to remove the Sprout shielded protocol, thus reducing
the overall complexity and attack surface of Zcash.
Motivation
==========
The first iteration of the Zcash network, called Sprout, provided a shielded payment
protocol that was relatively closely based on the original Zerocash proposal. [#zerocash]_
The Sapling network upgrade [#zip-0205]_ introduced significant efficiency and
functionality improvements for shielded transactions. It is expected that over time,
the use of Sapling will replace the use of Sprout in shielded transactions.
The Sapling and Sprout shielded protocols employ different cryptographic designs.
Since an adversary could potentially exploit any vulnerability in either design,
supporting both presents additional risk over supporting only the newer Sapling shielded
protocol.
For example, a vulnerability was discovered in the zero-knowledge proving system
originally used by Zcash that could have allowed counterfeiting [#counterfeiting]_.
While this particular vulnerability was addressed (also for Sprout shielded transactions)
by the Sapling upgrade, and we are not aware of others at the time of writing, the
possibility of other cryptographic weaknesses cannot be entirely ruled out.
In addition, the Zcash specification and implementation incurs complexity and
"technical debt" from the requirement to support and test both shielded payment
protocols.
Removing the ability to add to the Sprout chain value pool balance, is a first step
toward reducing this complexity and potential risk. This does not prevent extracting value
held in Sprout addresses and sending it to transparent addresses, or to Sapling addresses
via the migration tool [#zip-0308]_.
Specification
=============
Consensus rule: From the relevant activation height, the ``vpub_old`` field of each
JoinSplit description MUST be zero.
When this proposal is activated, nodes and wallets MUST disable any facilities to create
transactions that have both one or more outputs to Sprout addresses, and one or more
inputs from non-Sprout addresses. This SHOULD be made clear in user interfaces and API
documentation.
Notes:
* The requirement on wallets is intentionally slightly stronger than that enforced
by the consensus rule. It is possible to construct a mixed transaction with inputs
from both Sprout and non-Sprout addresses, in which all ``vpub_old`` fields are zero,
but there are nevertheless sufficient funds from Sprout inputs to balance the Sprout
outputs. This is prohibited for usability reasons, but not by consensus.
* The facility to send to Sprout addresses, even before activation of this proposal,
is OPTIONAL for a particular node or wallet implementation.
Rationale
=========
This design does not require any change to the JoinSplit circuit, thus minimizing
the risk of security regressions, and avoiding the need for a new ceremony to generate
circuit parameters.
The code changes needed are very small and simple, and their security is easy to
analyse.
During the development of this proposal, alternative designs were considered that
would have removed some fields of a JoinSplit description. These alternatives were
abandoned for several reasons:
* Privacy concerns raised as a consequence of preventing the use of internal change
between JoinSplits, and/or change sent back to the input Sprout addresses. This
would have required the total value of the input Sprout notes (or, for some considered
designs, the total value of the two Sprout inputs to each JoinSplit) to be leaked.
As it is, there is an unavoidable leak of the total value extracted from the Sprout
value pool, but not of the sum of values of particular subsets of notes.
* Modifications would have been needed to the design of the Sprout to Sapling migration
procedure described in [#zip-0308]_.
* A new transaction version would have been required.
Security and Privacy Considerations
===================================
The security motivations for making this change are described in the Motivation section.
Privacy concerns that led to the current design are discussed in the Rationale section.
Since all clients change their behaviour at the same time from this proposal's activation
height, there is no additional client distinguisher.
Deployment
==========
This proposal will be deployed with the Canopy network upgrade. [#zip-0251]_
Reference Implementation
========================
https://github.com/zcash/zcash/pull/4489
References
==========
.. [#BCP14] `Information on BCP 14 — "RFC 2119: Key words for use in RFCs to Indicate Requirement Levels" and "RFC 8174: Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words" <https://www.rfc-editor.org/info/bcp14>`_
.. [#protocol] `Zcash Protocol Specification, Version 2021.2.16 or later <protocol/protocol.pdf>`_
.. [#zip-0200] `ZIP 200: Network Upgrade Mechanism <zip-0200.rst>`_
.. [#zip-0205] `ZIP 205: Deployment of the Sapling Network Upgrade <zip-0205.rst>`_
.. [#zip-0209] `ZIP 209: Prohibit Negative Shielded Value Pool <zip-0209.rst>`_
.. [#zip-0251] `ZIP 251: Deployment of the Canopy Network Upgrade <zip-0251.rst>`_
.. [#zip-0308] `ZIP 308: Sprout to Sapling Migration <zip-0308.rst>`_
.. [#zerocash] `Zerocash: Decentralized Anonymous Payments from Bitcoin (extended version) <https://eprint.iacr.org/2014/349>`_
.. [#counterfeiting] `Zcash Counterfeiting Vulnerability Successfully Remediated <https://electriccoin.co/blog/zcash-counterfeiting-vulnerability-successfully-remediated/>`_