# Ironwood (NU6.3)

Ironwood is a new shielded pool introduced by the NU6.3 network upgrade. It
replaces Orchard for payments: after activation, wallets automatically route
shielded payments (including payments to Orchard receivers in unified
addresses) into Ironwood. The two pools use the same underlying cryptography,
but Orchard becomes withdraw-only, and wallets are encouraged to migrate user
funds away from Orchard as soon as possible. Transactions that use Ironwood
require the V6 transaction format.

Ironwood's purpose is threefold: provide a shielded pool with a credibly
sound supply after the Orchard counterfeiting vulnerability
(https://zodl.com/orchard-vulnerability-successfully-remediated/), provide
evidence that no counterfeiting occurred in the Orchard pool, and
re-establish a bound on the circulating supply of ZEC.

## Assurance and formal verification

The Ironwood pool starts fresh at NU6.3 on the patched Orchard protocol with
minor modifications. Its primary security guarantee is no undetectable
counterfeiting bugs: only the protocol's mathematics could hide a
counterfeiting bug that leaves no evidence of exploitation, and Project
Tachyon is working to eliminate that entire bug class, up to the stated
cryptographic assumptions, by analyzing the math with proof assistants such
as Lean (https://tachyon.z.cash/blog/detecting-counterfeiting-in-zcash/).
The now-patched Orchard soundness bug could theoretically have allowed
undetectable counterfeiting; there is no evidence it was exploited, but
exploitation cannot be ruled out retrospectively.

The stack has also undergone intense audit scrutiny. Shielded Labs, who
discovered the Orchard counterfeiting vulnerability
(https://shieldedlabs.net/the-orchard-counterfeiting-vulnerability/),
subjected Orchard to their own in-house analysis and obtained an audit
report from Anthropic's frontier Mythos AI. Valar Group and Project Tachyon
engaged zkSecurity, OtterSec, and Zellic to analyze Ironwood and Orchard,
and OpenAI assisted Project Tachyon with access to models of similar
capability to Mythos.

## Supply audits and turnstiles

After NU6.3 activation the Orchard pool is withdraw-only: payments cannot
occur within it, payments to Orchard receivers route to Ironwood, and all
funds leaving Orchard cross the protocol's turnstile, which accounts for
them. As users migrate funds from Orchard to Ironwood through the turnstile,
the upper bound on any hypothetical counterfeiting exploit shrinks; given
enough migration, there are few plausible explanations except that no
counterfeiting occurred. Meanwhile, because payments cannot take place in
the old pool and no new funds can enter it, circulating ZEC is bounded
above by 21M: every coin is either secured by shielded pools with no known
or exploited counterfeiting bugs, or forcibly subjected to a turnstile.

## Quantum recoverability

Ironwood implements ZIP 2005 by default, so Ironwood funds are quantum
recoverable: if advances in quantum computers are made, the pool can be
suspended and users can retrieve their funds without a quantum computer
being used to steal or counterfeit with the pool.

NU6.3 is scheduled for mainnet block **3428143**, expected around 28 July
2026. The block does not exist yet, so there is no explorer page to link.
The deployment parameters are in
https://github.com/zcash/zips/blob/main/zips/zip-0258.md. Verify the current
chain status before describing the upgrade as activated.

The dedicated audit status page tracks formal verification, third-party audits,
finding-level remediation, publication status, and additional assurance:
https://zakura.com/ironwood/audits/ (markdown:
https://zakura.com/ironwood/audits.md).

The resources page collects explainers, the normative specifications, and
the library and node releases that carry Ironwood support:
https://zakura.com/ironwood/resources/ (markdown:
https://zakura.com/ironwood/resources.md).

The history page tells the story of how Ironwood came to be, from the
vulnerability's discovery through the NU6.3 implementation effort:
https://zakura.com/ironwood/history/ (markdown:
https://zakura.com/ironwood/history.md).

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Canonical HTML version: https://zakura.com/ironwood/