Terms used across this documentation, defined once.
Chain and consensus
Anchor block. A block whose header carries a post-quantum certificate over its parent, and whose block hash covers a commitment to that certificate. Every 32nd block since block 20,746,736 (height % 32 == 16); every 128th from block 17,225,968 to block 20,746,736, on a grid offset from block 17,225,968.
Certificate. The set of validator seals attached to an anchor header, encoded and committed to by a 32-byte digest in vanityData.
Deterministic finality. A committed block does not reorganise. One confirmation is final. This is a property of QBFT, not a probabilistic assumption.
f. The number of faulty validators the consensus tolerates. On chain 2800, f = 3 with ten validators.
Quorum. The number of validators that must agree for a block to commit: seven of ten.
QBFT. The Byzantine fault tolerant consensus protocol of chain 2800, run on two clients: a Besu-derived client at nine validators and, since 2026-09-11, a Nethermind-derived client at the tenth (corrected 2026-10-27: this entry named the engine from Hyperledger Besu alone).
Seal. One validator's signature. A classical seal is secp256k1; a post-quantum seal is Falcon-512 or SLH-DSA-SHA2-128s.
extraData. The header field carrying QBFT's consensus data. On chain 2800 it has an AERE sixth element, and element 0, vanityData, carries the anchor digest.
Cryptography
Falcon-512, Falcon-1024. Lattice-based post-quantum signature schemes from the NIST process. Signatures are compressed and therefore variable length.
ML-DSA-44. FIPS 204, the standardised lattice-based signature scheme, formerly Dilithium.
SLH-DSA-SHA2-128s. FIPS 205, the standardised hash-based signature scheme, formerly SPHINCS+. Large and slow, and dependent on nothing but the hash function.
SHAKE256. FIPS 202 extendable-output hash function.
Hybrid certificate. A certificate carrying seals from two different signature families at once, so that a break in one family does not break the certificate. Live from block 17,047,600.
HNDL, harvest now decrypt later. A threat to encrypted data, not to signatures. It does not apply to a blockchain signature, which is public the moment it is made. See Threat model.
Long-range attack. Rewriting old history using validator keys recovered later. This is the threat the anchor addresses.
Positive control. A check, run in the same execution as a measurement, that a thing known to be present is found. Without it, a zero from a broken tool is indistinguishable from a zero that was earned. This term appears throughout our documentation because it is the difference between a measurement and a guess.
Operations
Anchor interval. How often an anchor block occurs. 32 until block 17,225,968, then 128 until block 20,746,736, then 32 again for Falcon-512, with SLH-DSA seals every 128th block.
Enforcement height. The block number at which a consensus rule begins to be required.
Follower. A node that follows the chain without producing blocks.
Registry. The list of validator post-quantum public keys, indexed, against which certificate seals are verified. Its hash is committed in the genesis config for the epochs up to the one of block 18,082,816; the epoch in force since block 20,836,228 (version 3, post-quantum ids, the same ten keys) is pinned by its hash in the node configuration, not in the genesis and not in a contract.
State window. The number of recent blocks for which an endpoint keeps state. 512 on the public RPC; beyond it state reads are refused with an error (-32000 World state unavailable; eth_call answers -32603), measured on both public endpoints on 2026-10-15. Until 2026-09-10 eth_getTransactionCount answered a false 0x0 there, and another provider can still do so (corrected 2026-10-27: this sentence dated the end of the false zero to the measurement of 2026-10-15).