[1/13] It may be initially confusing to fully grasp how deposits and withdrawals from L1 to @optimismPBC are actually implemented, and it helps to see the on-chain action of what is happening behind the scenes.

[2/13] Initial setup (simplified): on L1 we have SyntheticBridgeToOptimism from Synthetic and OVM_L1CrossDomainManager from Optimism contracts. On L2 we have SynthetixBridgeToBase and OVM_L2_CrossDomainManager contracts.

[3/13] Additionally we have Sequencer (L2 mining node) that verifies all L2 transactions and submits them in batches to L1 for future reference and Relayer that is responsible for relaying messages from L2 —> L1

[4/13] Step 1 - Alice wants to deposit $SNX to L2. To do that she calls initiateDeposit() method on the L1 SyntheticBridgeToOptimism which takes her $SNX, puts it in the escrow and calls OVM_L1CrossDomainManager sendMessage() method.

[5/13] The CrossDomainManager puts this request to CanonicalTransactionChain (this is an “official” and “unmutable” list of all L2 transactions on L1). As a consequence the Sequencer (L2 mining node) will need to execute this transaction on L2.

1/ If, after reading https://t.co/IzzATArtZl you are still confused how Alpha Homora and IronBank were hacked, here's how the hack was conceived

2/ Normally when you borrow funds from AH bank, your debtShare and totalDebt increases. Specifically if you want to borrow x tokens, your debt share will be calculated as:

share = x * totalShare / totalDebt

and it is added to totalShare

3/ All these numbers are very big integers (as token precisions are 18 digits) and the calculation is correct, but when totalShare = 1 (think 1 wei) and x < totalDebt, new debt share will be 0 (integer division)

4/ So if you manage to have AH bank with totalShare = 1, and some totalDebt you can repeatedly borrow less than the totalDebt (ideally totalDebt - 1) effectively doubling totalDebt in each iteration. You can do it as many times as you want, while totalShare will remain 1

5/ Eventually Iron Bank that supplies funds to AH will run out of funds, so when amounts get big enough (you are doubling each time), also make sure to replenish it with flash loan