Elydora Chain Hash (ECH)

The ECH provides tamper-evident chain-linking between consecutive operations from the same agent. Each chain hash incorporates the previous hash, creating an ordered, verifiable sequence.

Structure

Field	Type	Required	Description
prev_ech	string \| null	Yes	The previous chain hash in the sequence. null for the first operation.
payload_hash	string	Yes	SHA-256 hash of the current operation's canonicalized payload.
operation_id	string	Yes	UUID of the current operation.
issued_at	string	Yes	Timestamp of the current operation.
chain_hash	string	Yes	SHA-256(prev_ech \|\| payload_hash \|\| operation_id \|\| issued_at)

Computation

The chain hash is computed by concatenating the previous chain hash with the current operation's payload hash, operation ID, and timestamp, then applying SHA-256.

typescript

// Chain hash computation
function computeChainHash(
  prevEch: string | null,
  payloadHash: string,
  operationId: string,
  issuedAt: string
): string {
  const input = [
    prevEch ?? 'NULL',
    payloadHash,
    operationId,
    issuedAt
  ].join('|');

  return 'sha256:' + sha256(input);
}

Verification

To verify a chain, walk backwards from the most recent operation and recompute each chain hash. If any computed hash does not match the stored hash, the chain has been tampered with.

Gap detection — A missing operation in the chain will cause a prev_chain_hash mismatch on the following operation.
Reorder detection — Reordering operations will break the chain hash sequence.
Modification detection — Modifying any operation's payload changes its payload_hash, which breaks the chain.

Error: PREV_HASH_MISMATCH

If an agent submits an operation with a prev_chain_hash that does not match the server's expected value, the submission is rejected with PREV_HASH_MISMATCH. This typically indicates a concurrency issue or a client-side bug.