aere-node/anchor

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.

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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:

1. 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.
2. 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The earlier statement in this repository that from 14,050,000 every block required a 2f+1 Falcon quorum to finalize is withdrawn.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 the case where a validator's old signing keys reach an adversary, 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.

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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.

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What is in here

Updated 2026-08-29. The first publication of this directory, on 2026-08-11, was staged by hand and was never refreshed: by 29 August, 63 of the 78 files here had changed in the tree we actually run and 50 files had never been published at all. That is fixed at the root rather than by one copy — stage-anchor.cjs in our repository derives this directory from the authoritative overlay, refuses to run if the two files this README tells you to read first are missing, deletes what we removed rather than letting the published set grow monotonically, and stops on the secret-scanner's verdict instead of around it. The per-file SHA-256 list is in MANIFEST-sha256.txt.

Three of the files in the overlay are not here, and it is the same three every time: the negative-control harnesses. They plant a defect on purpose to prove a guard can fail, so they are a recipe for disabling a guard rather than a description of one. Everything they prove is stated in "What is proven, and by what" below, and every guard they exercise is here.

What is armed on chain 2800, and what is only present

This matters more than the file list, so it is stated before it.

Layer	In this directory	Armed on chain 2800
Anchor certificate under the block hash	yes	yes, since block 13,014,000
Enforced minimum seals at an anchor height	yes	yes, 6 of 9 since 14,961,456
Legacy per-block Falcon rule (aere.falcon.forkBlock)	yes	no — retired at the anchor block
Post-quantum seal on PREPARE, emission	yes	no — no node sets the property
Post-quantum seal on PREPARE, enforcement	yes	no — no node sets the property
Post-quantum seal on the PROPOSAL, emission	yes	no — no node sets the property
Post-quantum seal on the PROPOSAL, enforcement	yes	no — no node sets the property
Hybrid Falcon + SLH-DSA certificate	yes	no — needs new keys, not generated

Everything in the "no" rows is disarmed by absence, not by a flag: the properties that arm them are unset, and unset means never. Each refuses loudly on a value it cannot parse rather than booting a node that believes itself armed — a node that disarms itself because of a mistyped character looks exactly like a correctly configured one, right up to the day it matters. The tests for that behaviour are in this directory and they are the ones to read if you doubt the claim.

The PREPARE layer is newer than the anchor and stronger where it applies: an armed node that refuses unsealed PREPAREs never reaches the prepared state, so it never sends COMMIT at all. That also means it has no safety net during an activation, which is why it is not armed anywhere and why its activation height is a decision that has not been taken.

The PROPOSAL layer (added 2026-08-30) closes the remaining hot-path gap, and its claim is stated precisely because the imprecise version oversells: an adversary who breaks ECDSA cannot finalize anything while the PREPARE layer is armed, since "prepared" needs a full quorum of PREPAREs. What forged proposals could still do is OPEN rounds and waste them. With proposal enforcement armed, a proposal without a valid Falcon seal from its own proposer does not open a round. The seal signs its own domain over (chainId, height, round, digest), so an honest proposal seal cannot be replayed as a vote nor a vote seal as a proposal - both directions are tested. What remains classical: the ROUND-CHANGE message itself (its embedded justifications are already coupled to PREPARE enforcement) and node-level devp2p authentication.

The files

• 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.

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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.
• The PREPARE layer has not run on a live network. It has been exercised on a test network, including a mixed run against a second, independent client implementation, and it has not been armed on chain 2800 or anywhere else that carries value. Test-network evidence is evidence about a test network.
• The hybrid Falcon + SLH-DSA certificate has never been signed with a real key. The scheme layer is here and a second algorithm passes through the same consensus code untouched, which is what the tests measure. Generating hybrid validator keys is a separate decision that has not been taken, so no hybrid certificate exists on any chain.

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.

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Terminology, used precisely

A certificate at an anchor height is signed by a quorum of six of nine validators, since block 14,961,456 (August 21, 2026). It was f+1 before that, and this paragraph said so. The distinction is worth keeping in view rather than deleting, because it is the difference between two genuinely different claims. f+1 signatures guarantee only that at least one honest validator signed. A quorum of 2f+1 is the same count QBFT itself requires to agree on the block, so from 14,961,456 the post-quantum certificate at an anchor height is backed by as many validators as the block itself.

Two things follow, and both should be said rather than left to be discovered. The word "quorum" applies only at anchor heights - about every 32nd block; the blocks between them carry no Falcon seals at all and are finalized by ECDSA alone, so "a post-quantum quorum on every block" would be false. And the higher threshold costs liveness margin: at K=6 of nine, three late validators are enough to stall an anchor block, where K=3 tolerated six. That trade was made deliberately, and it is the reason the threshold is a configured schedule rather than a constant.

Anyone counting will notice which of the two regimes a given height falls in, and they should. The schedule is in the header of this document and on every node.

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.

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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.