A Zcash network upgrade due to activate on Thursday, 5 November, is set to leave about 12,224 ZEC permanently stuck in the network’s oldest privacy pool.
The upgrade, known as NU7, activates at block 3,543,405, expected on Thursday evening US time. At recent prices, the coins left in the pool are worth about $15 million.
Why the coins get stuck
Sprout was the privacy pool Zcash launched with in 2016. It has been closed to new deposits for years, and has since been replaced by newer systems including Sapling, Orchard and Ironwood.
NU7 removes version 4 transactions, the only type that can move coins out of Sprout. Once the upgrade is live, anything left in the pool can no longer be spent.
“NU7 (block 3,543,405) makes v4 transactions invalid, so what remains in Sprout is unspendable: frozen, not burned,” according to ZecStats, which tracks Zcash data.
The coins will still count towards Zcash’s total supply figures. A future upgrade could in theory restore access, but Zcash’s technical specification warns that spending these funds may never be allowed again.
It is not known how much of the remaining balance belongs to active users and how much sits in wallets whose owners lost their keys, because the blockchain does not show who owns the coins.
Why developers are removing it
Developers want to retire Sprout because supporting outdated technology adds complexity and security risks. Sprout’s original privacy system also needed a lot of computing power.
Faster blocks
NU7 also cuts Zcash’s target time between blocks from 75 seconds to 25 seconds, so first confirmations should take about a third as long, according to the Zcash Foundation.
What holders need to know
According to Zcash’s technical specification, anyone still holding coins in Sprout needs to move them out through a compatible withdrawal method before the upgrade activates, or risk losing access to them. Sprout addresses start with “zc.”
Holders using modern, supported wallets do not need to move anything, but should keep their wallet software up to date.
What happens next
Once NU7 is live, the Sprout balance will be frozen at whatever remains at activation. The faster block times will be the first visible change for users.
