Zcash has activated its Ironwood network upgrade, introducing a new shielded pool designed to eliminate the risk of undetectable counterfeiting and begin moving funds away from the compromised Orchard system.
The Ironwood upgrade, known technically as NU6.3, went live as planned at block height 3,428,143 on Tuesday, according to Project Tachyon and the Zcash development team. Developers say the change reinstates confidence in the cryptocurrency’s supply integrity following months of concern over a critical flaw in Orchard.
Researchers had previously identified a vulnerability in Orchard’s zero-knowledge circuit that, in theory, could have allowed counterfeit ZEC to be created without public detection. Although developers insist there is no evidence the weakness was ever exploited, they concluded that Zcash’s privacy design made it impossible to prove that hidden inflation had not occurred in the past.
Orchard frozen for new activity as migration starts
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With Ironwood now active, Orchard has been placed in an exit-only mode. Users are still able to withdraw existing balances from the old shielded pool, but no new shielded transfers can be initiated there.
Data from Zcash’s migration tracker shows that about 40,207 ZEC have already been moved into the new Ironwood pool, while close to 3.6 million ZEC remain in Orchard awaiting transfer through the new accounting system.
ZEC’s price showed a brief negative reaction to the cutover. The token dropped around 9% to roughly $460 after the upgrade before recovering to about $475, according to the crypto.news price page. The asset had already endured sharp swings since the Orchard issue became public, losing more than half its value before rallying from around $45 in September last year to nearly $700 by the end of the year.
Origins of the Orchard ‘infinity’ bug
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The replacement of Orchard follows the discovery of what became known as the Orchard “infinity” bug. Shielded Labs says researcher Taylor Hornby uncovered the issue in late May inside Orchard’s zero-knowledge proof circuit.
Developers warned that, in principle, the bug could have enabled an attacker to mint counterfeit ZEC without immediate detection. They also stressed that their investigations found no indication the vulnerability had been used in practice.
In early June, emergency updates temporarily disabled Orchard, halting shielded activity in that pool while teams worked on a fix. Corrected circuit code was rolled out via the NU6.2 hard fork, but developers concluded that the privacy properties of Zcash meant no-one could definitively rule out historic hidden inflation. That concern led to the decision to build a replacement shielded pool rather than continue to rely solely on a patched version of Orchard.
How Ironwood’s new accounting works
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Ironwood introduces its own accounting layer with a separate note commitment tree, nullifier set and chain value pool, enabling nodes to track the new shielded pool independently of Orchard.
Technical documentation for the project explains that the upgrade also adds a “turnstile” – a public accounting checkpoint that allows funds to exit Orchard while ensuring that no more value can leave the old pool than was ever legitimately deposited into it.
Developers say this mechanism is intended to trap any hypothetical counterfeit ZEC inside Orchard, while giving the community an independently verifiable way to confirm that Zcash’s circulating supply continues to respect the 21 million ZEC issuance cap.
Formal proof of balance integrity
Alongside the network activation, Project Tachyon confirmed that researchers have completed a machine-checked mathematical proof for Ironwood’s balance integrity. The proof is intended to show that, under the stated cryptographic assumptions, the new shielded pool cannot generate more value than has publicly entered it.
According to Project Tachyon, the proof was written in the Lean programming language and comprises more than 2,700 theorems developed over more than a month by three teams of researchers and cryptographers.
The team says the work formally verifies the components required for balance integrity, including Ironwood’s zero-knowledge proof system, circuit rules and ledger-level accounting. They emphasised that the verification does not cover Ironwood’s privacy guarantees, which remain outside the scope of the proof.
Commenting on the cross-organisational effort, Zcash co-founder Sean Bowe said it had removed sources of undetectable counterfeiting bugs from the new Ironwood protocol.
Preparing for a quantum future
The new shielded pool also introduces ZIP 2005 quantum-recoverable notes. This feature is designed to support a potential future recovery mechanism if advances in quantum computing eventually undermine the cryptographic schemes currently protecting Zcash.
The ZIP specification explains that this capability does not make today’s Zcash transactions quantum secure. A separate recovery protocol would have to be devised and activated at a later date before any quantum-related mitigation could be used in practice.
Ironwood’s development and node transition
Ironwood is the product of a collaboration involving Shielded Labs, the Zcash Open Development Lab (ZODL), Project Tachyon, Valar Group and the Zcash Foundation.
Shielded Labs, co-founded by Zcash founder Zooko Wilcox, put forward the original proposal to replace Orchard once the vulnerability became public, arguing that verifiable supply accounting had to be restored.
ZODL Executive Director Josh Swihart stated on X that engineers from his organisation were responsible for 82% of the merged changes across Zcash’s protocol and wallet repositories during the Orchard replacement programme. Swihart also said Orchard was patched within five days, with the full replacement effort completed in around 60 days.
The NU6.3 upgrade also finalises a long-planned shift in node software. The Zcash Foundation confirmed that the long-standing zcashd node implementation no longer supports the latest consensus rules, leaving Zebra as the required node software after activation.
Infrastructure prepares for change
Before Ironwood went live, Shielded Labs Executive Director Jason McGee had warned that exchanges, wallet providers and mining pools were facing the dual task of migrating from zcashd to the new Z3 software stack while also preparing for the Ironwood upgrade itself. McGee said some infrastructure operators asked for more time, but no decision was ultimately taken to delay the network transition.
With Ironwood now in operation and Orchard closed to new activity, attention has turned to moving the remaining shielded balances through the turnstile mechanism as the network gradually retires the old pool under the new accounting framework.
