Zcash activates NU7, cutting target block time to 25 seconds

Zcash has activated NU7, a mainnet upgrade that cuts the target time between blocks from 75 seconds to 25 seconds, aiming to make private payments confirm sooner. The upgrade went live at block 3,543,405; CoinDesk’s report on the activation says that block was mined at 02:34 UTC on Friday.

The shorter target can reduce the wait for a first confirmation to about 25 seconds on average. A service that waits for three confirmations could see its expected wait fall from nearly four minutes to about 75 seconds. But those are averages based on the new target, not guarantees: when CoinDesk checked at 04:59 UTC, blocks were averaging about 39 seconds apart as mining difficulty adjusted. Merchants, exchanges and other services set their own confirmation requirements, so the time before a payment is usable will vary.

How does NU7 speed up private payments?

NU7 increases block frequency while limiting shielded activity so that wallets do not face an unbounded increase in the work needed to find incoming payments. The official NU7 specification, ZIP 259, says the upgrade sets limits on shielded actions per pool and across the network. CoinDesk reports that the limits reduce the worst-case daily download burden for wallets from 271 megabytes to 169 megabytes.

Wallets scan blockchain data to identify payments addressed to them. More frequent blocks could mean more data to check; the limits are designed to constrain that burden while allowing more private transactions through. The specification estimates that capacity for simple private payments could rise from about three per second to more than six. That is a potential network capacity, not a promise that every wallet or payment service will process transactions at that rate.

The change also requires coordination from the infrastructure that produces and checks blocks. The Zcash Foundation said node operators needed to install NU7-compatible software before activation to remain on the consensus chain. Its Zebra 7.0.0 release notes say the upgrade does not require significant changes for most wallets, though exchanges, node operators and other services must ensure their software handles NU7’s rules.

What happens to fees and old Sprout funds?

NU7 changes how transaction fees support the network: miners keep 40% of fees, while the other 60% is removed from circulation and held in a public reserve. Under the specification, reserve reissuance is scheduled to begin at block 8,939,664. The Zcash Foundation says the revised block rewards and halving intervals preserve the existing halving schedule and the 21 million ZEC supply cap.

The upgrade also disallows version 4 transactions. Sprout, Zcash’s original shielded pool, can use only that transaction version, so any ZEC still there became unspendable at activation. CoinDesk reports that about 12,224 ZEC remained in Sprout. The specification confirms the funds cannot be transferred after NU7 through a supported replacement format.

What should users watch next?

The near-term measure is whether block production moves closer to the 25-second target as mining difficulty adjusts, and how quickly the services people use adopt the new pace in their confirmation policies. For the fee reserve, the milestone set in the specification is block 8,939,664, when gradual reissuance is scheduled to start.

Sources

• CoinDesk’s report on the activation — coindesk.com
• official NU7 specification, ZIP 259 — zips.z.cash