Post-quantum certificate anchor for QBFT

This is the code that puts a post-quantum validator certificate under the block hash in Hyperledger Besu's QBFT consensus, as an overlay on a named upstream commit.

It is published so that the claim can be checked rather than believed. Everything below that is not demonstrable from these files is marked as not demonstrable from these files.

Scope boundary, stated first and not in a footnote

Consensus on Aere Network chain 2800 is classical secp256k1 ECDSA QBFT with a post-quantum checkpoint enforced every 32 blocks. This code is what adds the checkpoint, in two steps that should not be confused:

The binding. From block 13,014,000 the block hash of every anchored header (every 32nd block) commits to a Falcon-512 certificate signed by the validators. That is a binding, not a replacement: a rewrite of anchored history must also forge the post-quantum signatures.
The enforced minimum. Since 2026-08-14 a node does not finalize an anchor block unless its certificate holds at least three valid Falcon-512 seals (f+1 of nine, the guarantee that at least one honest validator signed; eight or nine are carried in practice). Raised at block 14,961,456 (August 21, 2026): the enforced minimum is now six of nine, a full 2f+1 quorum, the same count of validators QBFT itself requires to agree on the block. The schedule step is aere.pq.anchorMinSeals=13014000:0,13034000:3,14961456:6, uniform on all nine validators; the startup guard refuses any threshold above N - f = 7, the highest count reachable with the fault budget down. Blocks between anchors carry no Falcon seals and are finalized by ECDSA alone, so the post-quantum guarantee is a checkpoint about every 16 seconds, not a per-block quorum.

Correction 2026-08-19: the fleet was also configured on 2026-08-15 with aere.falcon.forkBlock at 14,050,000, intended as a per-block blocking Falcon quorum. In the shipped code that rule (FalconSealValidationRule) is retired at aere.pq.anchorBlock (13,014,000) in favour of the anchor rules (PqAnchorConfig.legacyFalconRuleRetirementBlock), so at 14,050,000 it was already inert, and the chain itself confirms it: blocks between anchors carry no Falcon seals, which a per-block quorum rule would have rejected. The earlier statement in this repository that from 14,050,000 every block required a 2f+1 Falcon quorum to finalize is withdrawn.

"Hybrid" is the honest word only in the checkpoint sense above, and "post-quantum consensus" without that qualification overstates: block proposal, validator identity and message transport are classical elliptic-curve cryptography, and a cryptographically relevant quantum computer would break those exactly as on any other ECDSA chain. What it cannot do is rewrite history past the last anchor without also forging Falcon-512 signatures. None of this has been audited by a third party.

The threat it addresses is not "harvest now, decrypt later". A signature is public; there is nothing to harvest. The threat is retroactive rewriting: validator keys recovered later can be used to re-sign old blocks, and a chain whose history is authenticated only by ECDSA cannot distinguish the rewrite from the original. Binding a post-quantum certificate into the hash of every anchored header means a rewrite must also forge the post-quantum signatures.

The threat is not ours to claim. It is the long-range attack, and the peer-reviewed treatment is Azouvi, Danezis and Nikolaenko, "Winkle: Foiling Long-Range Attacks in Proof-of-Stake Systems" (IACR 2019/1440; AFT 2020, pp. 189-201). Two things should be said plainly about that citation, because we checked it rather than repeated it. Winkle does not mention quantum adversaries at all: it treats old validator keys becoming compromised, by any means. The quantum framing is ours. And Winkle's own defence is not ours: it adds a secondary layer of client-based validation, where clients sign a hash of the previously sequenced block. A second published defence for the same threat, Azouvi and Vukolic, "Pikachu: Securing PoS Blockchains from Long-Range Attacks by Checkpointing into Bitcoin PoW using Taproot" (arXiv 2208.05408, 2022), checkpoints into Bitcoin's proof of work.

Where this sits among the known defences

The literature calls this attack posterior corruption, also known as the founders' attack, the long-range attack, history revision, or costless simulation. Tas, Tse, Gai, Kannan, Maddah-Ali and Yu survey the mitigations in "Bitcoin-Enhanced Proof-of-Stake Security: Possibilities and Impossibilities" (arXiv 2207.08392) and count four: social consensus by a trusted committee; key-evolving signatures, where validators forget their old keys; verifiable delay functions; and timestamping onto an existing proof-of-work chain.

That paper also proves an impossibility, and it deserves to be stated correctly rather than paraphrased into something scarier or softer than it is. Slashable safety, meaning safety plus the ability to punish the attacker, is not achievable by a proof-of-stake chain without an external trusted source. Plain safety against posterior corruption is a weaker property, and key-evolving signatures are shown to provide it under an honest majority. So the impossibility is not "you cannot defend yourself"; it is "you cannot make the attacker pay, on your own."

This anchor does not claim slashable safety and does not compete with checkpointing on that ground. What it addresses is the case where key-evolving signatures stop working. Their guarantee rests on a validator's old key being gone once deleted. Against an adversary with a cryptographically relevant quantum computer, an old secp256k1 key does not need to have been kept or stolen. QBFT headers carry validator addresses rather than public keys, but every seal a validator ever wrote is an ECDSA signature from which its public key can be recovered, and those seals are in the chain forever. From the public key, a quantum adversary derives the private one. Deleting the private key buys nothing, because the chain reconstructs it. The certificate under the block hash is what still has to be forged, and forging it requires a secret the adversary cannot derive.

Put plainly: this is the post-quantum analogue of what key-evolving signatures give classically, for the setting in which their central assumption no longer holds. It is narrower than checkpointing and it borrows nothing from another chain, and both of those are true at the same time.

The problem this solves, which is not obvious

In QBFT, extraData is an RLP list. The block hash is not computed over the stored bytes. It is computed over a re-encoding of the decoded list, with the seal fields removed, so that every node agrees on a hash before the seals exist.

That has a consequence that is easy to miss: anything the decoder does not know about is dropped before hashing. Append a certificate as a new element and it survives in storage, travels between nodes, and is entirely absent from the hash. Two nodes can hold different certificates for the same block and both consider it valid. The certificate would be decoration.

Four obvious designs were tried and all four died on that:

Attempt	Why it died
new RLP element after the seals	dropped by the re-encode, never reaches keccak
extend the seal list	changes the seal encoding, so every existing node rejects the header
a second `extraData`-like field	not in the header schema; a header with it is not a header
commit in the state root	the state root is computed before the certificate exists

The design that works uses a field that is already under keccak: vanityData, element 0 of the list, 32 bytes, historically arbitrary. At an anchor height it carries the digest of the certificate instead of the usual vanity string. The certificate itself still rides outside the hash, but it is now pinned: change one byte of it and the digest no longer matches, and the header is rejected.

PqAnchor.java and PqAnchorDigestRule.java are where that lives. Read those two first.

What is in here

consensus/common/.../bft/ — the anchor itself: configuration, the digest, the seal cache and store, the producer that attaches seals, the Falcon registry that maps a validator to a key.
consensus/qbft/.../headervalidationrules/ — the validation rules: the digest must match, the seals must verify, and the rules must actually be wired into the validator chain.
app/.../controller/ — where the rules are built and where the node refuses to start on a configuration that would produce headers its own fleet rejects.
tests — including the negative controls. A test that cannot fail is not a test, and several of these exist specifically to prove the guards can fail.

BASE.txt names the upstream commit. Applying these files to any other tree overwrites whatever upstream added since, silently. That is stated there in more detail because it is a real hazard.

One build file changes, and it is one line. consensus/common/build.gradle gains implementation 'org.bouncycastle:bcprov-jdk18on'. Falcon verification happens inside the consensus module, so the module needs the library on its own compile path. No version is stated, because upstream pins it: platform/build.gradle declares bcprov-jdk18on:1.83, and four upstream modules already take the dependency in exactly this versionless form. So the line adds a compile-path entry and no new artifact, and it does not move any version. That is the entire build change, and it is called out here rather than left to be found in the diff.

No cryptography is implemented in this overlay. Falcon signature verification calls Bouncy Castle's implementation; what is ours is the framing, the registry that maps a validator to a key, the digest, and the validation rules.

What is proven, and by what

The certificate is byte-identical across nodes. Measured on a test network, six nodes.
A stripped certificate is rejected. 12 attempts, 12 rejections.
The guards compile and pass together, and the negative control was run: with the guards removed the proofs go red. A guard that has never failed cannot be trusted, so each was made to fail on purpose.
Arming does not halt the chain across a validator-set change. On a test network of ten processes, 1,110 blocks were produced across the arming height while four validator-set votes were driven through it, and the block rate did not change. The negative control for that run was separate and blunt: three nodes restarted without their Falcon key produced zero blocks in ninety seconds while every node was alive; with the keys restored, eighty-nine.
The wiring is tested. An earlier version of this code registered a rule that could not be seen from outside, because Besu's BlockHeaderValidator.Builder wraps detached rules in a lambda. The rule was present and untested for that reason alone. QbftAnchorRuleWiringTest exists because of that, and its negative control is measured: comment out the registration line and it goes red.

What is not proven here

This overlay has not been audited by a third party. No external security review of this code exists. If you are reading it as an auditor, you are the first.
A rehearsal with a deliberately un-upgraded node has not been run. Every rehearsal so far upgraded every node.
Nothing here demonstrates what is configured on any live network. These files show what the code does when armed. They are not evidence about any running fleet, and should not be read as any.

One claim we retracted, on purpose

An earlier version of our public material said that no public chain has a block hash covering a post-quantum validator certificate. That does not survive a hostile reading. Cellframe's ESBoCS signs blocks with keys that resolve to Dilithium, Falcon or SPHINCS+, and hashes the block with the signatures attached. The capability exists in their code today.

The defensible statement is narrower: no public chain has a post-quantum validator certificate under the block hash that is proven and independently verifiable. This repository is our half of that sentence. Someone else has to do the verifying, which is why it is here.

Terminology, used precisely

A certificate here is signed by f+1 validators, not by a quorum. With f Byzantine faults tolerated, f+1 signatures guarantee that at least one honest validator signed. That is a real property and it is not the same as a quorum, and we do not call it one. Anyone counting will notice, and they should.

The seal threshold is a floor, not a cap: nodes attach as many verified seals as arrive in time, which is at least the threshold and often more. A separate cap bounds how many are written, because each seal costs bytes in every header forever.

Licence

Apache 2.0, matching upstream Hyperledger Besu. See ../LICENSE and ../NOTICE. Files that modify upstream carry the change notice required by section 4(b); files that are new are ours.

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