Zcash activates NU7, cutting target block time to 25 seconds

Zcash activated its NU7 network upgrade at block 3,543,405 at 02:34 UTC on Nov. 6, cutting target block spacing from 75 seconds to 25. The shorter interval could make private payments usable sooner, while new limits are designed to keep wallet scanning and synchronization manageable.

According to CoinDesk’s report on the activation, the network produced 225 more blocks by 04:59 UTC, averaging about 39 seconds between blocks as mining difficulty adjusted. The 25-second figure is the new target, not a guarantee that every block or payment will arrive on that schedule. Merchants, exchanges and other services also set their own confirmation rules.

How does NU7 aim to speed up private payments?

NU7 changes the block interval and scales down each block subsidy by the same factor, so the rate of new ZEC issuance over time remains essentially unchanged. The Zcash Foundation’s Zebra 7.0.0 release notes say the expected average wait for a first confirmation falls from 75 seconds to about 25 seconds. A service that waits for three confirmations could therefore see its expected wait fall from nearly four minutes to about 75 seconds, though it decides how many confirmations to require.

The upgrade also changes how much shielded activity can fit in each block. Wallets scan the chain to find payments addressed to them, so a high volume of shielded transactions can increase the data light wallets must process. NU7 sets per-block limits for Orchard, Ironwood and Sapling activity, and allows no new Sprout JoinSplits. The Foundation says the limits raise Orchard’s potential throughput from 2.9 to 6.6 transactions per second while reducing worst-case sync bandwidth that a spammer could impose on light wallets by about 37%.

That added capacity depends on transactions fitting the protocol’s limits; it does not mean every private payment will be confirmed instantly. The faster target and the shielded activity limits work together: the first creates more blocks per unit of time, while the second constrains the scanning load each block can add.

What changes for miners and ZEC issuance?

NU7 sends 60% of each block’s transaction fees to a reserve and leaves miners with the remaining 40%. The reserve is scheduled to begin returning to block rewards at height 8,939,664, expected in 2031. Meanwhile, block subsidies are reduced to match the shorter block interval, and the halving intervals are lengthened in block count. The Foundation says this keeps the existing halving schedule and the 21 million ZEC supply cap unchanged.

The reserve mechanism addresses how the network may fund mining as block subsidies decline: fees set aside now can supplement future rewards. Its reissuance is scheduled for later, so the fee split is active now while the reserve’s contribution to rewards is not.

Which old Zcash funds can no longer be spent?

NU7 disables version 4 transactions, the only transaction format used by Zcash’s original Sprout privacy pool. As a result, ZEC left in Sprout became unspendable through normal transactions at activation. CoinDesk reported that about 12,224 ZEC remained in the pool; a possible recovery mechanism had been outlined by co-founder Sean Bowe, but no recovery plan had been approved as of Nov. 6.

For most wallets, the upgrade was not expected to require significant changes, according to the Foundation’s release. The exception is infrastructure or funds tied to the retired transaction format: node operators needed NU7-compatible software to remain on the consensus chain, and Sprout holders could no longer spend those funds normally after activation.

Sources and documents

• CoinDesk’s report on the activation — coindesk.com
• Zcash Foundation’s Zebra 7.0.0 release notes — zfnd.org