Zcash activates Ironwood upgrade, launching new shielded pool after Orchard vulnerability

Zcash’s Ironwood upgrade went live on Tuesday, introducing a new shielded pool for the privacy blockchain following concerns about the soundness of its earlier Orchard pool. 

Ironwood, which is also the name of the new pool, activated on schedule at block height 3,428,143 as part of the NU6.3 network upgrade. 

Orchard, meanwhile, will now be restricted to only support withdrawals, though removing funds will remain discretionary. Orchard was previously Zcash’s primary privacy pool, accounting for nearly 88% of the blockchain’s shielded token supply at its height, with older shield pools Sapling and Sprout making up the rest, according to The Block’s data.

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According to a migration tracker, some 40,207 ZEC have migrated into Ironwood, while about 3.6 million tokens remain in Orchard. 

ZEC, which tanked over 50% after the Orchard bug was disclosed, dropped about 9% on Tuesday to $460 before recovering to about $475, according to The Block’s price page. The token rallied hard beginning in September of last year, climbing from about $45 to a high of around $700 by year’s end.

Orchard patch

The vulnerability in Orchard, discovered in late May by Shielded Labs researcher Taylor Hornby, involved a flaw in its zero-knowledge circuit that could theoretically have allowed an attacker to create counterfeit ZEC without detection. 

While analysts have said Orchard, launched in 2022, was likely not exploited and found no evidence of it occurring, Zcash developers prioritized patching the pool and launching a more mathematically sound and secure alternative. 

Ironwood represents something of a unified effort across Zcash’s decentralized developer ecosystem, including efforts from the Zcash Open Development Lab (ZODL), Project Tachyon, Valar Group, the Zcash Foundation, and Shielded Labs. 

Shielded Labs, co-founded by renowned cryptographer Zooko Wilcox, drove the initial proposal to restore supply verifiability. ZODL Executive Director Josh Swihart said 14 ZODL engineers accounted for 82% of the merged changes to Zcash’s protocol and wallet repos as part of the Orchard fix and migration to Ironwood. 

"Orchard was fixed in 5 days and replaced in 60," Swihart said Monday on X. 

Indeed, an emergency soft fork temporarily disabled Orchard activity on June 2, with a hard fork applying corrected circuit code the following day. 

Formal verification

Ironwood reuses this fixed Orchard circuit and its Halo 2 proof system, along with some minor modifications. However, the pool has been formally verified, providing a stronger assurance it will perform as designed under all possible circumstances. 

"Soundness of our zk-SNARK would have been great on its own," Zcash co-founder Sean Bowe said on X. "But a cross-team collaboration has just changed Zcash's history forever, eliminating all sources of undetectable counterfeiting bugs from the new Ironwood protocol."

The new pool also incorporates quantum-recoverable notes under ZIP 2005, offering a future recovery path if quantum computing advances compromise current cryptography, ZODL noted. 

Notably, the upgrade also installs a "turnstile" that will limit the amount of funds that can exit Orchard and enables independent auditing of ZEC’s circulating supply, helping ensure the counterfeiting bug did not break Zcash’s 21 million coin cap.