Terminology

The key words "MUST", "SHOULD", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 1 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 3.

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 4 2.

The term "Sprout chain value pool balance" in this document is to be interpreted as described in ZIP 209 5.

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. 8

The Sapling network upgrade 4 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 9. 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 7.

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 7.
• 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. 6

Reference Implementation

https://github.com/zcash/zcash/pull/4489

References