Zcash's First Shielded Pool Is Closing
The NU7 upgrade retires the v4 transaction format Sprout depends on. Move your ZEC now, or watch it freeze — possibly forever.
By The crypto-money Desk
Zcash launched Sprout in 2016 as the first large-scale deployment of general-purpose zero-knowledge proofs — a genuine technical feat at the time. [2] Nine years later, the network is moving to close it. The NU7 upgrade, expected to activate at mainnet block 3,543,405 around Nov. 5th, will retire the v4 transaction format that Sprout requires. [1] Any ZEC still sitting in the Sprout pool when that block lands will be frozen — not burned, but effectively inaccessible.
The mechanism is straightforward. ZIP 2003 disallows all v4 transactions from NU7 onward. [2] Because Sprout transfers can only be represented in v4 transactions, this proposal also disables the ability to spend Sprout funds. [2] The v5 format introduced in the NU5 upgrade and the v6 format introduced in NU6.3 do not support Sprout. [2] There is no migration path built into either of those formats; the only exit is to move funds out of the Sprout pool before activation.
The ZIP is careful on one point: it is not proposed to unissue, burn, or otherwise make Sprout funds permanently unavailable. [2] A future consensus change could, in theory, restore a retrieval mechanism. [2] For anyone holding ZEC in Sprout, that theoretical future rescue is not a plan — it is a footnote.
The retirement of a founding technology is always more than a version number change. It marks the moment a community decides that the weight of the past costs more than any remaining value it carries.
Sprout's obsolescence has been accumulating for years. The Sapling and Orchard shielded protocols made substantial optimizations and added new functionality such as viewing keys and diversified addresses. [2] As a result, Sprout is essentially unused at this point. [2] The deprecation of zcashd, just prior to the NU6.3 upgrade, also removed the only maintained software that still provided wallet functionality for Sprout. [2] By the time NU7 activates, there will be no maintained wallet to help a Sprout holder move funds anyway — the warning is already overdue.
Keeping Sprout alive has carried a real, ongoing cost. The parts of Sprout not shared with Sapling — including the JoinSplit circuit and the handling of Sprout nullifiers and note commitments — still impose a substantial burden on live transaction-acceptance paths before NU7. [2] Verification of transactions involving Sprout JoinSplit descriptions has to be taken into account for denial-of-service attacks against mempool and new-block processing. [2] Because of Sprout's lack of use, there is little incentive for node implementations to optimize its verification, which may lead to such attacks imposing greater cost than they otherwise would. [2]
The NU7 upgrade comes with a compressed timeline that adds urgency for node operators. NU7 skips Zcash's usual roughly three-month runway between new node software and activation, and ZIP 259 warns that stragglers could keep running on the old, pre-NU7 chain. [1] Operators who do not update in time risk a chain split — running on an orphaned fork while the rest of the network moves forward.
Protocol upgrades are, at their core, acts of collective editing. The community decides what stays in the story and what gets cut — and the cuts are rarely clean, rarely painless, and rarely without someone left holding something they did not know was about to become worthless.
There is a privacy dimension worth noting for anyone planning to exit Sprout. Moving value out of the Sprout pool changes its publicly visible chain value pool balance through the vpub_old and vpub_new fields. [2] The amount moved by a transaction is therefore visible. [2] A short or poorly coordinated exit period could also make transaction timing and distinctive amounts easier to correlate with particular holders. [2] Ironically, the act of leaving the privacy pool may reveal more than staying ever did — though staying now means losing access entirely.
Beyond the Sprout deadline, NU7 brings 25-second blocks and a fee-recycling Network Sustainability Mechanism. [1] The Sprout chain value pool balance as of activation remains issued and is included in IssuedSupply for the purpose of calculations in Network Sustainability Mechanism ZIPs that affect issuance. [2] In other words, even frozen Sprout funds count toward the network's supply accounting — a ghost balance that the protocol must keep carrying, visible in the ledger, inaccessible to its owners.
The Sprout shielded protocol was the first large-scale deployment of general-purpose zero-knowledge proofs, but suffered from long proving times and high memory requirements that limited its practicality. [2] Its retirement is not a dismissal of that history; particular credit goes to the four people who were put under a unique kind of stress during Sapling's development — Ariel Gabizon, Sean Bowe, Nathan Wilcox, and Zooko Wilcox. [2] The work that built Sprout is the foundation that made Sapling and Orchard possible. What NU7 closes is the scaffolding, not the building.
There is something clarifying about a hard deadline. It forces a decision that soft deprecations never quite manage — the slow fade replaced by a specific block height, a specific date, a specific consequence.
If you have ZEC in a Sprout address, the deadline is not approximate. Block 3,543,405 is the line. [2] The network is not waiting for consensus on whether to move on. It already has.
The ability to spend Sprout funds may never be re-enabled.ZIP 2003