Bridging
Bridging - Deposit Transactions
Deposit transactions don’t exist on L1s, and are how transactions on an L2 can be initiated from the L1. Importantly, this is how bridge applications can get L1 ETH or tokens into an L2 OP Stack chain. You can read more on deposit transactions here.Bridging - Withdrawal Transactions and Fault Proofs
Withdrawal transactions are how the state of the L2 rollup can be proven to the L1. Often this involves users withdrawing tokens or ETH to the L1. Fault proofs are the mechanism by which withdrawal transactions are currently proven to the L1. You can read more about fault proofs here.Opcodes
Address aliasing
Address aliasing is an important security feature that impacts the behavior of transactions sent from L1 to L2 by smart contracts.
Make sure to read this section carefully if you are working with cross-chain transactions.
Note that the
CrossChainMessenger contracts will handle address aliasing internally on your behalf.CREATE opcode, it is possible to create a contract on both L1 and on L2 that share the same address but have different bytecode.
Even though these contracts share the same address, they are fundamentally two different smart contracts and cannot be treated as the same contract.
As a result, the sender of a transaction sent from L1 to L2 by a smart contract cannot be the address of the smart contract on L1 or the smart contract on L1 could act as if it were the smart contract on L2 (because the two contracts share the same address).
To prevent this sort of impersonation, the sender of a transaction is slightly modified by the OptimismPortal when a smart contract calls depositTransaction to send a transaction from L1 to L2.
This alias is applied when the caller has code, and that code is not a valid EIP-7702 delegation.
When the address has no code, or the code present at the address is a valid EIP-7702 delegation, the sender is not modified.
Note that ephemeral contracts (those that are created and self-destructed in the same transaction) WILL be aliased, despite having no code at the start or end of transaction execution.
Instead of appearing to be sent from the actual L1 contract address, the L2 transaction is sent from an “aliased” version of the L1 contract address.
This aliased address is produced by treating the address as a 160-bit integer, adding a constant offset with overflows allowed, and then truncating the result to a 20-byte address.
The address hashing function ensures that aliased address will never conflict with any other address on L2.
The simple offset original L1 address can easily be recovered from the aliased address.
The OptimismPortal applies this change only to L2 transactions sent by L1 smart contracts.
In all other cases, the transaction sender address is set according to the same rules used by Ethereum.
