Add an attack and solution (ethereum#908)

EIPS/eip-908.md

@@ -43,10 +43,16 @@ Not providing incentives for clients is an issue now as there is less incentive
 Note that with a supply cap (as in [EIP 960](https://github.com/ethereum/EIPs/issues/960), the issuance can be prevented from increasing indefinitely. Alternatively, it could at least be reduced (still potentially but not necessarily to zero, or to the same rate at which Ether is burnt when slashing participants, such as validators under a Casper PoS scheme or notaries under a sharding scheme), e.g. by hard forks, or as per [EIP 1015](http://eips.ethereum.org/EIPS/eip-1015), an on-chain contract governed by a decision assembly that gets signalling from other contracts that represent some set of stakeholders.
 
 ## Specification
-Add a new field to each block called `PrevBlockVerifications`, which is an arbitrary, unlimited size byte array. When a client verifies that a previous block is [valid](https://ethereum.github.io/yellowpaper/paper.pdf#subsubsection.4.3.2), the client appends a user agent to PrevBlockVerifications. The user agent is a vector with the blockhash of the block that is validated, and the index of a client address in an access list (details are below).
+Add a new field to each block called `PrevBlockVerifications`, which is an arbitrary, unlimited size byte array. When a client verifies that a previous block is [valid](https://ethereum.github.io/yellowpaper/paper.pdf#subsubsection.4.3.2), the client appends a user agent to PrevBlockVerifications via an opcode in a transaction, PREV_BLOCK_VERIF. The user agent is a vector with the  immutable fields: the blockhash of the block that is validated, and the index of a client address in an access list (details are below). A miner validates a transaction before including it in a block, however they are not able to change these fields of the vector because they're immutable.
 
 Send 0.15 ETH to the client (see the rationale below), when the block is processed. The amounts could include a proportion of transaction fees (while the miner would then receive less), which would reduce newly issued ETH. These amounts are specified in new `ClientReward` fields in the block.
 
+In order to only incentivize verifying recent blocks, assert that the block number corresponding to a blockhash is less than 400 blocks ago.
+
+### Attacks and added specifications to prevent them
+
+A miner could create a client and fill their block with transactions that only contain the PREV_BLOCK_VERIF opcode (or alternatively, arbitrarily complex transactions, still with the opcode), with previous blockhashes that they have validated and the address of their client. To prevent this, state would have to store a list containing lists, with each sublist listing the blockhashes up to 400 blocks ago that correspond to a miner, then a miner (or a proposer under Casper CBC) could have to put down a deposit, and be slashed if the proposer inserts such a transaction (that contains a blockhash which they have already proposed, not just in a block but at any time previously). Updating the state to remove blockhashes more than 400 blocks ago would add additional overhead, although you could add an extra window for older blocks, say 120,000 blocks (roughly every 3 weeks), still ignore blocks that are older than 400 blocks ago, and remove these older 120,000 blocks every 120,000 blocks. An attacker could bribe a miner/proposer to include transactions like above that contain an address of a client in the access list which they own. However, the above slashing condition should disincentivize this.
+
 ### More details on the access list
 
 The access list prevents anyone inserting any address to the first element of the vector, where there may be a way to prevent censorship and centralization of authority of who decides to register new addresses in the list, e.g. on-chain governance with signalling (possibly similar to [EIP 1015](http://eips.ethereum.org/EIPS/eip-1015), which also specifies an alternative way of sending funds) or a layer 2 proof of authority network where new addresses can be added via a smart contract. Note that there may be serious drawbacks to implementing either of these listed examples. There is a refutation of [on-chain governance](https://medium.com/@Vlad_Zamfir/against-on-chain-governance-a4ceacd040ca) as well as of [plutocracy](https://vitalik.ca/general/2018/03/28/plutocracy.html). [Proof of Authority](https://en.wikipedia.org/wiki/Proof-of-authority) isn't suitable for a public network since it doesn't distribute trust well. However, using signalling in layer 2 contracts is more acceptable, but Vlad Zamfir argues that using that to influence outcomes in the protocol can disenfranchise miners from being necessary participants in the governance process. Thus, in light of these counterpoints, having an access list may not be suitable until a decentralized, trustless way of maintaining it is implemented and ideally accepted by the majority of a random sample that represents the population of Ethereum users.
@@ -55,8 +61,6 @@ However, another alternative to managing the access list would be to have decent
 
 Additionally, it should help with being only able to read the client's address from the client, and the whole transaction could revert if the address is not in the access list. You could provide the index of the address in the access list, and then you could `assert` that the address found at that index matches that which can be read by the client (where the latter would be a read-only address).
 
-In order to only incentivize verifying recent blocks, assert that the block number corresponding to a blockhash is less than 400 blocks ago.
-
 ## Rationale
 
 <!--The rationale fleshes out the specification by describing what motivated the design and why particular design decisions were made. It should describe alternate designs that were considered and related work, e.g. how the feature is supported in other languages. The rationale may also provide evidence of consensus within the community, and should discuss important objections or concerns raised during discussion.-->