Zcash is about to freeze $16 million in its oldest privacy pool

Zcash holders have less than a day to move about $16 million out of the network’s oldest privacy pool before an upgrade makes the remaining coins unspendable, even for owners who still have their private keys.

About 12,226 ZEC remained in the network’s Sprout pool on Thursday morning, down from roughly 22,263 a week earlier, according to ZecStats’ historical records. Most of that reduction came Sunday, when about 9,094 ZEC left. Just 40 ZEC followed Wednesday.

The Zcash network lets users hide who paid whom and how much. Its private balances sit in shielded pools, which can be thought of as a separate compartment of the blockchain that uses a different software generation of privacy technology. Sprout launched with the network in October 2016 and has since been superseded by newer pools.

The deadline arrives with NU7, a network upgrade scheduled for block 3,543,405.

The chain stood at block 3,542,391 at 05:24 UTC Thursday, according to Blockchair. At the current target of one block every 75 seconds, that left roughly 21 hours, putting activation near 9:30 p.m. ET Thursday. The exact time depends on how quickly miners find blocks.

NU7 will reject version-4 transactions, the only transaction format capable of spending Sprout coins, according to ZIP 2003, the specification defining the change. Any balance still in the pool will become unspendable under the new rules.

At $1,300 per ZEC, the remaining balance is worth about $16 million, though further withdrawals before activation would reduce the amount frozen.

The public pool balance cannot identify the owners or establish how much belongs to wallets whose keys have been lost. Sprout addresses begin with “zc,” a detail that can help people checking old wallet records recognize whether the deadline affects them.

The coins will remain part of Zcash’s issued supply. ZIP 2003 leaves open the possibility of restoring access through a future upgrade, while warning that spending may never be re-enabled.

Keeping Sprout running also carries a cost. Its older transaction machinery adds complexity and security risks to the software checking Zcash payments, despite the pool’s limited use, according to ZIP 2003.

NU7 also cuts the target time between blocks from 75 seconds to 25, shortening the expected wait for transaction confirmations.