Blast is an L2 based on Optimism that modifies the native ether token to be rebasing. Blast also provides a rebasing ERC-20 stablecoin called USDB. Blast users and smart contracts can earn ~4% from beacon chain staking yield and ~5% from US treasury bill yield, and those yields are reflected in their accounts automatically without them sending any transactions. Additionally, Blast also redirects sequencer fees to the dapps that induced them, allowing smart contract developers to have an additional source of revenue.

Blast features

Rebasing ETH balances on the L2

• Accounts have three options for their rebasing mode
  • Disabled — ETH balance never changes; no yield is earned
  • Automatic — native ETH balance rebases (increasing only)
  • Claimable — ETH balance never changes; yield accumulates separately
• Yield reports
  • On positive yield — L2 balances go up
  • On negative yield — L2 balances stay the same, and L1 withdrawals are discounted

Sequencer fees to dapps

• Contracts have two options for their gas mode
  • Void — base + priority fees go to the sequencer operator
  • Claimable — base + priority fees spent on this contract can be claimed by the contract
• Nested contract calls distribute the incurred fees to the contracts that incurred the gas; not to the top-level contract
• Contracts pay a variable tax rate to the sequencer operator when they claim their accumulated gas
• This tax rate starts at 100% and decreases to 0% over time as the accumulated gas vests in the gas manager
• The sequencer operator can step in and claim all of a contract’s accumulated gas to mitigate incentive issues in extreme circumstances

Withdrawals

• User goes through three stages: L2 burn, L1 proof, and L1 finalize
• Bridge operator asynchronously does: L1 unstake, L1 withdrawal confirmation
• Users can only do their L1 finalize after a corresponding L1 withdrawal confirmation
• If negative yields have accumulated, the withdrawal amount will be discounted relative to the nominal withdrawal amount

High level implementation overview

This overview will assume a working understanding of OP stack based L2s and focus on Blast’s unique changes.

Depositing ETH

When ETH is bridged from the L1, it ultimately ends up in the Blast YieldManager contract. This contract is responsible for staking the ETH in yield bearing protocols like Lido. On the L2, ETH is represented in terms of shares, such that the global share price multiplied by the number of shares equals the ETH value that was bridged over. The account node corresponding to the recipient of this ETH stores the number of shares and not the number of wei.