  Blockchains implement monetary systems operated by committees of nodes. The
robustness of established blockchains presents an opportunity to leverage their
infrastructure for creating expansion chains. Expansion chains can provide
additional functionality to the primary chain they leverage or implement
separate functionalities, while benefiting from the primary chain's security
and the stability of its tokens. Indeed, tools like Ethereum's EigenLayer
enable nodes to stake (deposit collateral) on a primary chain to form a
committee responsible for operating an expansion chain.
  But here is the rub. Classical protocols assume correct, well-behaved nodes
stay correct indefinitely. Yet in our case, the stake incentivizes
correctness--it will be slashed (revoked) if its owner deviates. Once a node
withdraws its stake, there is no basis to assume its correctness.
  To address the new challenge, we present Aegis, an expansion chain based on
primary-chain stake, assuming a bounded primary-chain write time. Aegis uses
references from Aegis blocks to primary blocks to define committees,
checkpoints on the primary chain to perpetuate decisions, and resets on the
primary chain to establish a new committee if the previous one becomes
obsolete. It ensures safety at all times and rapid progress when latency among
Aegis nodes is low.
