Zcash’s Sprout is about to become a $16 million cryptographic attic 🔒

Zcash’s NU7 upgrade is set to freeze approximately $15 million to $16 million in its oldest privacy pool, leaving some coins inaccessible even to holders with valid passwords and private keys. About 12,224 $ZEC remained in Sprout on Thursday, according to ZecStats data updated Thursday, while historical records showed about 12,226 $ZEC earlier that day. At $1,300 per $ZEC, the remaining balance was worth about $16 million before further withdrawals.

The balance was down from roughly 22,263 $ZEC a week earlier, with about 9,094 $ZEC leaving the pool Sunday and just 40 $ZEC departing Wednesday. NU7 will reject version-4 transactions, the only format capable of spending Sprout coins, according to ZIP 2003, the specification defining the change. Any balance still in the pool when the upgrade activates will be unspendable under the new rules.

The upgrade is scheduled for block 3,543,405, expected Thursday evening in the United States. The chain stood at block 3,542,391 at 05:24 UTC Thursday, according to Blockchair. At the target of one block every 75 seconds, activation was expected near 9:30 p.m. ET Thursday, although the exact time depends on how quickly miners find blocks and could shift to early Asian hours Friday. NU7 also reduces the target block interval from 75 seconds to 25 seconds, shortening the expected wait for transaction confirmations.

Sprout launched with Zcash in October 2016 and uses zero-knowledge proofs to verify transactions without revealing transaction amounts or addresses. It has since been superseded by Sapling, Orchard and Ironwood. “NU7 (block 3,543,405) makes v4 transactions invalid, so what remains in Sprout is unspendable: frozen, not burned,” Zecstats says in its information card for Sprout. The coins will continue counting toward Zcash’s circulating issuance calculations, and ZIP 2003 leaves open the possibility that a future upgrade could restore access while warning that spending may never be re-enabled.

Legacy holders whose $ZEC remains in Sprout must use a compatible withdrawal mechanism before NU7 activates. Sprout addresses begin with “zc,” which can help users identify affected records in old wallets. Regular $ZEC holders using modern supported wallets do not need to migrate coins out of Sprout. ZIP 2003 states that retaining Sprout’s older transaction machinery adds complexity and security risks despite its limited use.