A common question raised from On Adding Precompiled Contracts without Hard Forks is if a transaction indicated gas price reduction, how can the client make sure that it is safe to execute and that the gas price reduction is valid. Incorrectly handling this might result in DDoS attacks in the network – whereby it would be flooded with invalid gas price reduction transactions, rendering clients too busy to spend the time to check whether they are, in fact, valid.

EVM is turing complete, so instead of validating directly on the client, we can delegate the resposibility to the transaction signer. In the networking layer, this makes it so that when a transaction with gas price reduction is broadcasted, it will also need to broadcast a supplimental piece of information in the form of a gas price reduction proof.

The Proof

The proof is a list of static call stacks with its code locations. Each call stack, specifically, is represented as:

[<pc_loc> <pc_loc> <pc_loc> ...]

The last item of the call stack must be a CALL to a non-HF precompiled contract. The client then simply follows this list of proof to check that a non-HF precompiled contract must have been called for at least N times to compensate for its gas price reduction. This check is done in linear time.

The client first goes to the code location (if it is a message call transaction, then the code is from the to address of the transaction, and if it is a contract creation transaction, then the code is from the data field of the transaction) of the first item in the call stack, and checks that it is a static call represented by the sequence below:

PUSH20(<contract address>)
SWAP1
CALL

The client then follows <contract address> and checks the second item in the call stack. For the last item, it checks whether the <contract address> is in a known list of non-HF precompiled contracts.

The Networking Layer

Instead of broadcasting the transaction with gas price reduction using the Transactions message in the Ethereum sub-protocol, we define a new message PriceReducedTransactions message with the following format:

[+0x0n: P, [[nonce: P, receivingAddress: B_20, ...], [[callStackItem: P, ...], ...]], ...]

That is, each transaction is also supplied by a list of call stack proofs to demonstrate that the gas price reduction is valid.

A new protocol version will also need to be defined because now we have a new message.

Calculating the Gas Price Reduction

Because we can only check the gas price reduction statically, it is assumed that each non-HF precompiled contract reduces the gas needed by a constant number N_<contract>. In this way we can calculate the gas price reduction as follows:

R_gas = T_gas - SUM_<contract>(N_<contract>)
R_gas * R_price = T_gas * N_price

Where T_gas is the actual gas limit specified in the transaction, R_gas is the reduced gas, R_price is the reduced gas price, and N_price is the normal gas price.

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October 17, 2017

Running SputnikVM on the Rux microkernel using Rust's no_std feature.

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September 14, 2017

The non-hard-fork approach to precompiled contracts, the first part.

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