HIP-410: Wrapping Ethereum Transaction Bytes in a Hedera Transaction Source

AuthorDanno Ferrin
Working GroupDanno Ferrin (@shemnon), Richard Bair (@rbair23), Dani Ivanov (@Daniel_K_Ivanov)
Discussions-Tohttps://github.com/hashgraph/hedera-improvement-proposal/discussions/411
StatusAccepted
Needs Council ApprovalYes
Review period endsTue, 19 Apr 2022 07:00:00 +0000
TypeStandards Track
CategoryCore
Created2022-03-28
Updated2022-04-21, 2022-04-28

Abstract

Add a new transaction type that will hold signed Ethereum transactions and execute them as Hedera transactions in a prescribed manner. An Ethereum transaction can transfer HBAR, create a smart contract, or invoke a smart contract. This HIP supports all three use cases.

Motivation

With HIP-26 (Besu EVM), the Hedera network is EVM-compatible with Ethereum. This means the Hedera network accepts and executes the same EVM bytecodes as any other Ethereum network. Transactions sent to the Hedera network are Hedera transactions and use the Hedera API (HAPI). The HAPI is based on gRPC and Google’s protocol buffers (protobuf). Clients of Hedera must use the HAPI for creating contracts and invoking contracts.

Standard Ethereum networks use JSON-RPC instead of gRPC, and they take Ethereum transactions encoded as RLP rather than HAPI transactions encoded in protobuf. The Ethereum ecosystem has a large and growing collection of tools and libraries that is becoming a de facto standard development stack. These tools and libraries work with RLP transactions only.

This HIP specifies a new HAPI transaction body that holds standard, Ethereum encoded and signed transactions along with the specification for how to execute those transactions. The program that prepares the Ethereum transaction, and signs it, may be different from the program that submits that transaction to the Hedera network. This would allow arbitrary Ethereum tools and libraries to prepare and sign Ethereum transactions, and then using a small amount of code the application could wrap those transactions and send them directly to the Hedera network for execution, opening Hedera for more broad adoption by the Ethereum development community.

User Stories

1. As a web3 app I want to use web3.js and an Ethereum compatible wallet to create and sign transactions that execute in the Hedera network

I am a web3 developer, and I have created a web application that currently works with Ethereum. In my application, I create transactions and send them to a browser wallet such as Brave or MetaMask for the user to sign. Using the Hedera JS SDK, I am able to take that signed transaction and submit it to the Hedera network for execution. Instead of web3.js, I might also use ethers.js.

2. As a web3 app I want to be able to pay transaction fees so my users can use zero-gas Ethereum transactions

I am a web3 developer, and I have created an application, however I do not want my users to have to hold HBAR to use my application. I will have the dApp create an Ethereum transaction with a zero gas price. My server receives that transaction and wraps it in a Hedera transaction, and my server pays for all or part of the Hedera fees needed for that transaction.

3. As a smart contract operator I want to be able to add authorizing signatures when transactions are relayed

I am a smart contract operator, and I have some transactions that require multiple approvals in the same transaction. I want to operate a server that will take an Ethereum transaction from a user, and then the relay server will decide based on its own logic if I will approve the transaction and relay it with one or more signatures. The keys for these signatures are only available to the relay service and users have to use the relay service to get authorization from those keys.

4. As a user, I want to use MetaMask to create a transaction to transfer HBAR to another account

I am a user, and I have an account with Hedera. I also use MetaMask as my wallet. By setting up a relay as my “network”, I can transfer HBAR to another account.

5. As a user, I want to use MetaMask to transfer HBAR to a contract account

I am a user, and I have an account with Hedera. I also use MetaMask as my wallet. By setting up a relay as my “network”, I can transfer HBAR to a smart contract account.

6. Operating a relay for community good or profit

I decide to operate a relay and suggest dApp developers to use my relay for sending Ethereum transactions to the Hedera network. I can operate this relay as a public service at no charge. Or, I may set up an out-of-band payment system with user accounts and credit cards, or crypto, or other means to fund the hosting of the relay. I know nothing about the contents of the transactions. I have to pay for submitting the transaction to Hedera.

Specification

A “relay” takes an Ethereum transaction created by a “sender” and submits it to the Hedera network. This is done by wrapping the Ethereum transaction with a HAPI protobuf object called an EthereumTransactionBody. The relay can be the client app itself, a backend server, or it can be any third party relay. Special care is taken to account for payment of transaction submission separately from payment of transaction execution to be consistent with the Hedera security model. The relay pays for transaction submission, while the sender pays for the execution of the transaction.

The sender key that signs the Ethereum transaction must have a public key (ECDSA secp256K1) registered as an alias to an account on the Hedera network, and the relay must also have an account on the Hedera network.

App as the Relay

The above diagram provides a flow where the app itself is the relay. In this flow, the app creates the Ethereum transaction and sends it to MetaMask or another wallet for signing (1), retrieves those signed bytes (2), and using the JS SDK creates a HAPI transaction in which the Ethereum transaction is wrapped and sent to the Hedera network for execution. The user that signed the transaction has an account on Hedera, and the application itself has an account on Hedera.

Bridge as the Relay

The above diagram proposes an alternative flow where the application itself does not know about Hedera and the transaction is submitted by the wallet to the relay directly, assuming that the relay implements the necessary Ethereum JSON-RPC APIs. The user that signs the transaction has an account on Hedera, and the relay has an account on Hedera, but the application itself does not. The relay may charge the application or client out-of-band, supporting a variety of economic models. The structure and form of a relay, and its relationship to any wallets or other software, is not specified by this HIP.

Economics

Fees in the Hedera network are charged to the “payer” account in the transaction. In a simple crypto transfer case where Alice sends money to Bob, Alice is usually the payer and the sender – she creates the transaction, signs the transaction, and pays for all fees associated with the transaction.

Likewise, when invoking a smart contract, the client that invokes the smart contract pays fees, including gas fees. Gas is used and then converted into HBAR and charged to the payer account.

Fees in Hedera are predictable and computed based on fiat (USD). This HIP preserves this invariant.

There are two actors defined by this HIP: the “sender” which is the account that signed the Ethereum transaction, and the “relay” which is the account that wrapped the Ethereum transaction with a Hedera transaction. Every such wrapped transaction thus has two signatures – one on the Hedera transaction and one on the Ethereum transaction.

The relay is responsible for paying the fees for submitting a transaction to the network. The sender is responsible for paying the fees for executing the transaction. The relay may have an out-of-band relationship with the sender and receive reimbursement for submitting transactions.

The relay creates a standard Hedera transaction and signs it in the standard way and pays fees in the standard way. The fee prices are determined by the governing council and not set by this HIP.

The sender pays for fees using the mechanisms defined by Ethereum in the Ethereum transaction, which was signed by the user. The total cost of executing the transaction, including all gas fees and other fees, are paid for by the account signing the Ethereum transaction and as authorized by that transaction.

There is one exception. The relay may specify a maxGasAllowance which is used to cover the costs of executing the transaction if, and only if, the Ethereum transaction cannot cover the costs itself. The value by default is 0, meaning that the relay will not pay any of the execution cost.

The only mechanism Ethereum provides for paying for a transaction is gas. Therefore, all fees related to executing an Ethereum transaction on Hedera must be denoted by gas. The minimum gas used by any transaction is 21,000. Bytes transferred as part of the contract create or contract call increase the amount of gas used. The price per unit of gas in US dollars will be fixed to a value determined by the Governing Council. The price per unit of gas in HBAR will float based on the HBAR <-> USD exchange rate, as it does today. The current price per unit of gas in HBAR will be made available in mirror nodes, so an application can determine the amount of HBAR to be used in a transaction.

Ethereum defines two different payment models, one for “Type 1” and legacy transactions, and one for “Type 2” transactions defined in the London hard fork. In both models there is the concept of a maximum price the caller is willing to pay.

Type 2 Payments

EIP-1559 introduced a major revision to the pricing scheme for Ethereum transactions. The following table defines the different variables that are used for computing the charges associated with a transaction.

Field Defined By Notes
base_fee_per_gas Block Base per gas price in wei
max_priority_fee_per_gas Transaction Tip per gas to pay the miner in wei
max_fee_per_gas Transaction Max per gas willing to pay in wei
gas_limit Transaction Max gas willing to use
gas_used Miner Amount of gas actually

In Ethereum, the absolute most that a single transaction is willing to pay can be computed by max_fee_per_gas * gas_limit. If less gas is used, then less is paid. The miner earns max_priority_fee_per_gas * gas_used. The base_fee_per_gas is variable depending on congestion pricing (the price is higher when the network is busy and lower when it is quieter).

When submitted to Hedera, the max_priority_fee_per_gas is ignored because nodes are not miners and are not rewarded in this way. The max_fee_per_gas is used in exactly the same way for Hedera as for Ethereum, as are gas_limit and gas_used. The base_fee_per_gas is determined by the Hedera network by taking the fixed price per gas in USD and converting it to the price per gas in HBAR. This value can be read from mirror nodes. The user is only charged for base_fee_per_gas * gas_used.

Type 1 Payments

The legacy, or Type 1 transaction, has a gas_price and gas_limit. The application creates a transaction with values for gas_price and gas_limit and the user signs that transaction. The maximum they will pay will be gas_price * gas_limit. There is no right or wrong answer for gas_price, it is literally an auction, where each transaction says what it is willing to pay and the miner collects all of gas_price * gas_used.

When Hedera handles a Type 1 transaction, it verifies that gas_price is greater than or equal to the network’s base_fee_per_gas, and that the gas_limit is at least 21,000. Unlike Ethereum, Hedera will only collect base_fee_per_gas * gas_used. If gas_price was higher than base_fee_per_gas then the user keeps the extra.

Protobuf changes

Ethereum Transaction

A new transaction body EthereumTransactionBody will be introduced to hold the Ethereum transaction data.

message EthereumTransactionBody {

  // required 
  bytes ethereumData = 1;

  // optional 
  FileID callData = 2;

  // optional
  int64 maxGasAllowance = 3;
}

The Ethereum transaction data will be converted into a ByteString. If the Ethereum transaction is small enough to fit into a Hedera transaction body then ethereumData will be the entire raw and unmodified Ethereum transaction. Hedera transactions are limited to a total of 6KB.

To support larger Ethereum transactions we will allow the data section of the transaction (also referred to as callData or inputData) to be stored as a Hedera file. In these instances the transaction body contains an edited version of the Ethereum transaction, replacing the data field in the RLP with a zero length string and storing the call data in a Hedera File as a hex string. When the transaction is executed and before the signatures are extracted the Ethereum transaction bytes will be reconstituted to include the bytes from the reference file.

Some Ethereum transactions will be too large for a single Hedera transaction. The relay will receive the full raw Ethereum transaction and remove the callData or inputData (the data section of the Ethereum transaction) and upload it separately as a Hedera file in the Hedera File System (HFS) as a hex encoded string. The remaining bytes will be set as the ethereumData with a zero-length string where the data had been, and the File ID of the uploaded data will be set as the callData field in the protobuf object.

Large Transactions

When the transaction is executed, and before the signatures are extracted, the Ethereum transaction bytes will be restored to include the bytes from the reference file.

Some further minor additions are needed to some protobuf objects (as detailed below). The ethereumNonce will be reported in the AccountInfo message in the response to CrytpoGetInfoQuery. The ContractCallLocalQuery will also get a senderId so that the sender account of the view calls can be set. Similarly, the senderId will be added to ContractFunctionResults recording the account that the Ethereum signature resolved to.

// basic_types.proto 
enum HederaFunctionality {
  //... 
  EthereumTransaction = 84; // enum number may be different once finalized 
}

// smart_contract_service.proto 
service SmartContractService {
  //... 
  rpc contractCallEthereum (Transaction) returns (TransactionResponse);
}

// transaction_body.proto 
message TransactionBody {
  //... 
  oneof data {
    //...
    EthereumTransactionBody ethereumTransaction = 51; // field number may change
    //... 
  }
  //... 
}

// transaction_record.proto
message TransactionRecord {
  //...
  bytes ethereum_hash = 19;
}

// crypto_get_info.proto
message CryptoGetInfoResponse {
  //...
  message AccountInfo {
    //...
    int64 ethereumNonce = 24;
  }
  //...
}

// contract_call_local.proto
//...
message ContractFunctionResult {
  //...
  AccountID senderID = 13;
}

//...
message ContractCallLocalQuery {
  //...
  AccountID senderID = 6;
}

// crypto_get_info.proto
message CryptoGetInfoResponse {
  //...
  message AccountInfo {
    //...
    int64 ethereumNonce = 24;
  }
  //...
}

ContractCreateTransaction

An Ethereum transaction can be used to create a new smart contract. When used in this way, an EthereumTransactionBody is used, just as with HBAR transfers and contract call invocations. When the contract is created, a corresponding record must be created in the record stream. This requires some modifications to the ContractCreateTransactionBody. These modifications also permit an optimization in traditional HAPI contract create calls, detailed here.

In the network today, contracts are first uploaded as files in the Hedera File Service and then linked with the Smart Contract in a ContractCreateTransaction. This HIP extends the ContractCreateTransactionBody to support sending the contract code inline if the size of the contract is small enough to fit in a single Hedera transaction. The field initcode will be added to ContractCreateTransactionBody. Instead of uploading a file and then supplying a fileID, you can instead supply the contract code directly in the ContractCreateTransaction.

message ContractCreateTransactionBody {
  oneof initcodeSource {
    FileID fileID = 1;
    bytes initcode = 7; // field number may change in final draft 
  }
  // ... 
}

Value of gas price and value fields

For Ethereum transactions, we introduce the concept of “WeiBars”, which are 1 to 10^-18th the value of a HBAR. This is to maximize compatibility with third party tools that expect ether units to be operated on in fractions of 10^18, also known as a Wei. Thus, 1 tinyBar is 10^10 weiBars or 10 gWei. When calculating gas prices and transferred value the fractional parts below a tiny bar are dropped when converted to tinyBars.

Ethereum Requirements

The replay protection specified in EIP-155 will be required for legacy transactions. The Type 1 / EIP-2930 and Type 2 / EIP-1559 transactions also require a chainId field. Non-restricted chainIDs (where chainID is less than or equal to zero) are not supported.

For all transaction types the chainID specified must match the chainID of the network as specified in HIP-26 (0x127 for mainnet, 0x128 for testnet, 0x129 for devnet, and 0x12a for local networks).

Transaction Signing Requirements

The sender of an Ethereum transaction body will be the account with an account alias that is an ECDSA(secp256K1) public key. To identify the account the public key will be recovered from the Ethereum transaction, and converted to an Ethereum address. The transaction sender will then be set to the account whose Ethereum address of the alias key has the same Ethereum address.

This HIP does not support multiple signatures for the Ethereum transaction. If the account associated with the Ethereum transaction requires multiple signatures, then the transaction will fail.

The Hedera transaction is signed by the relay using the relay’s key. It is possible for the Hedera transaction to require multiple signatures, based on the configuration of the relay’s account.

Paying for the Transaction

In addition to the account signing the Ethereum transaction (the sender account) each Hedera transaction containing the Ethereum transaction will also need to be signed by a Hedera account (the relay account). These may be the same account or different accounts.

If the relay and sender account are the same account there are no limitations on HBAR transfers.

When the relay and sender accounts are different the following restrictions apply to HBAR transfers. There are three components to consider, first is the total amount of fees Hedera will charge the transaction. The second amount is the amount of fees the sender has validated to be spent, which is the gasLimit of the transaction multiplied by either the gasPrice for Legacy and Type 1 transactions and the max_fee_per_gas field for Type 2 transactions. These are the sender authorized fees, and the total transaction amount charged to the sender must be between the value authorized in the transaction and the value plus the sender authorized fees. The third portion is either paid by or credited to the relay.

  • When the Hedera charged fees matches the sender authorized fees then the sender will neither send nor receive HBAR. This is a rare case.
  • When the Hedera charged fees exceeds the sender authorized fees then the relay must make up the difference and will pay HBAR for the transaction.
  • When the Hedera charged fees are less than the sender authorized fees, then some fees authorized by the sender may be received by the relay such that the sender does not pay more HBAR than they authorized. The relay may not need to receive all the excess HBAR, this depends on the transaction format that the transaction was sent in.
  • For Legacy and Type 1 Ethereum transactions when the sender authorized fees exceeds the fees that Hedera charges then the relay receives the HBAR beyond the Hedera fees. There is no mechanism for the relay to receive less than the excess amount.
  • For Type 2 Ethereum transactions the amount received by the relay must not be greater than the max_priority_fee_per_gas multiplied by the gasLimit. This will often result in charging the sender less than the authorized amount. Note that the gasLimit is used instead of the calculated gasUsed to make relay calculations easier. The relay may receive less than the full amount of the priority fee if the total authorized amount does not provide enough HBAR.
  • If there is value set to be transferred, then the entire amount must come from the sender. The relay must not subsidise the transaction to the point it pays for value transfers.

Finally, if an Ethereum transaction reaches consensus and is invalid and cannot be processed (for critical failures such as a bad ECDSA signature, an account not matching the sender, an invalid Ethereum transaction, an incorrect nonce, insufficient balance, or any other such error) the relay is responsible for all fees relating to bringing the transaction to consensus.

Account Nonce

Ethereum transactions have a different deconfliction and transaction enablement mechanism than Hedera, but it is an inseparable part of Ethereum toolchains. To that end we will support transaction sequence numbers as an additional transaction validity check.

Each Hedera account will track a new value ethereumNonce, defaulting to zero.

When an Ethereum transaction is validated during pre-check the transaction will fail if the account’s ethereumNonce is not equal to the nonce in the Ethereum transaction bytes.

If the transaction passes all validations and EVM execution (or equivalent processing) starts then the ethereumNonce in the user account is incremented by one. If the transaction fails for any reason prior to EVM execution the nonce is not incremented.

Nonce Management

Using Ethereum nonces introduces the nonce management issue for clients wishing to use Ethereum Transactions. Most ecosystem tools already have nonce management built in using JSON-RPC APIs. JSON-RPC Bridges and other tools utilizing this feature will need to keep a few considerations in mind.

First, Hedera has no memory pool. The notion of cancelling transactions out of the memory pool with a higher priced transaction of the same nonce won’t work. Once submitted to a node and accepted by that node the transaction will be executed and cannot be cancelled. Fast finality helps alleviate this problem as a whole.

Nonces only increase and never go down. Thus, when relays are presented with a transaction that has a nonce that is lower than the accounts current nonce it should always reject that transaction and not relay it.

Gaps in nonce are not accepted. If a transaction comes to consensus with a nonce greater than the expected nonce it will fail execution. The Ethereum transaction may be resubmitted in the future when the nonce in the transaction is the same as the nonce expected by the account.

Executing the Transaction

Ethereum transactions can have a number of possible handling routes. All of them are driven by the to field in the Ethereum transaction.

If the to field is empty, zero length, or all zeros, it will be converted into a ContractCreate transaction. If the transaction has a callData value in the transaction it will be placed in the ContractCreates fileID field, otherwise the data goes in the initcode field.

For non-zero non-empty to fields the address will first be decoded as a standard Hedera mapping (bytes 1-4 are shard, bytes 5-12 are realm, bytes 13-20 are id, big endian encoded). If this does not map to an exiting entity then the to address will be processed as an alias, which currently can map to Crypto Accounts and Smart Contract Accounts.

If the to field still does not map to an entity the transaction fails with a Hedera specific error. The relay will be charged for this transaction.

If the to address points to a Contract account the body of the Ethereum transaction will be converted into a ContractCall and executed.

If the to address corresponds to a Token ID the call will initially be handled as a crypto or token transfer. The implementation may choose to stand up the EVM to execute these operations, or have a fast-path to use the layer-1 call paths.

If the to address points to a Topic ID or File ID the call will be handled as an EVM, and converted into a ContractCall. If precompile support for the consensus and file service is developed then there may be similar handling as token calls.

Mirror Node

Mirror nodes will require a number of changes to expose Ethereum transaction information via its APIs.

Accounts REST API

The /api/v1/accounts/{idOrAlias} REST API will be updated to also accept an EVM address as a path parameter in lieu of an ID or alias that will be calculated and stored on account creation. An ethereum_nonce and evm_address will be added to the response of /api/v1/accounts/{idOrAliasOrAddress} and /api/v1/accounts. ethereum_nonce will be extracted from the ethereumData RLP bytes in the EthereumTransactionBody and associated with the account in the mirror node.

Contract Logs by Address REST API

The existing /api/v1/contracts/{address}/results/logs API will be enhanced to support pagination. The existing index and transaction parameters will be used as the multi-column key and be set for subsequent requests in the next link. The index parameter cannot be present without the timestamp parameter. The index only applies within a given timestamp, not across timestamps.

The REST API currently supports logical AND for topics by passing multiple topic query parameters (e.g. topic0=0x01&topic01=0x02). To support logical OR operations, the API will be enhanced to allow repeated parameters so that topic0=0x01&topic0=0x02&topic1=0x03 means “(topic 0 is 0x01 or 0x02) and (topic 1 is 0x03)”.

Contract Logs REST API

A new /api/v1/contracts/results/logs API will be added with the same query parameters and response as /api/v1/contracts/{address}/results/logs but with the ability to search across contracts. It will not support address as it’s expected users use the existing API if they need logs for a specific address. The same rules around not exceeding maxTimestampRange will still apply and allow it to stay performant. Pagination will be possible using a combination of the timestamp and index query parameters.

Contract Results REST API

The existing /api/v1/contracts/results/{transactionId} will be updated to accept the 32 byte Ethereum transaction hash as a path parameter in addition to the transaction ID that it supports now.

Its response, as well as the similar /api/v1/contracts/{idOrAddress}/results/{timestamp}, will be updated to add the following new EthereumTransaction fields:

{
  "access_list": "0xabcd...",
  "block_gas_used": 564684,
  "chain_id": "0x0127",
  "gas_price": "0xabcd...",
  "max_fee_per_gas": "0xabcd...",
  "max_priority_fee_per_gas": "0xabcd...",
  "nonce": 1,
  "r": "0x84f0...",
  "s": "0x5e03...",
  "transaction_index": 1,
  "type": 2,
  "v": 0
}

Note: Existing fields omitted for brevity.

Network Fee Schedule REST API

Mirror node will add a new fee schedule REST API /api/v1/network/fees that returns the feeSchedules.json. The API will be the direction conversion of the FeeSchedule protobuf to JSON similar to how it appears in the services feeSchedules.json. Note that because it is a JSON conversion of a protobuf file and not normalized to a database like most other APIs, the mirror node can make no guarantee of API stability.

{
  "expiry_time": 1633392000,
  "fee_schedule": [{
    "hederaFunctionality": "CryptoCreate",
    "fees": [{
      "subType": "DEFAULT",
      "nodedata": {
        "constant": 11461413665,
        "bpt": 508982,
        "vpt": 1272455960,
        "rbh": 339,
        "sbh": 25,
        "gas": 3393,
        "bpr": 508982,
        "sbpr": 12725,
        "min": 0,
        "max": 1000000000000000
      },
      "networkdata": {
        "constant": 229228273302,
        "bpt": 10179648,
        "vpt": 25449119198,
        "rbh": 6786,
        "sbh": 509,
        "gas": 67864,
        "bpr": 10179648,
        "sbpr": 254491,
        "min": 0,
        "max": 1000000000000000
      },
      "servicedata": {
        "constant": 229228273302,
        "bpt": 10179648,
        "vpt": 25449119198,
        "rbh": 6786,
        "sbh": 509,
        "gas": 67864,
        "bpr": 10179648,
        "sbpr": 254491,
        "min": 0,
        "max": 1000000000000000
      }
    }]
  }]
}

It will support the following query parameters:

  • current - Return results for the current or next fee schedule. Defaults to true.
  • limit - The maximum number of results to return. Defaults to 25 and max 100 allowed.
  • order - Sort by type. Values of asc or desc with a default of asc.
  • type - Filter the results to exact transaction type.

Exchange Rate REST API

A new exchange rate REST API /api/v1/network/exchangerate will be added that returns the exchange rate network file stored in 0.0.112.

{
  "current_rate": {
    "cent_equivalent": 596987
    "expiration_time": 1649689200
    "hbar_equivalent": 30000
  },
  "next_rate": {
    "cent_equivalent": 594920
    "expiration_time": 1649692800
    "hbar_equivalent": 30000
  },
  "timestamp": "1649689200.123456789"
}

Query Parameters:

  • timestamp - Show the exchange rate as of a certain timestamp or timestamp range. Supports eq, gt, gte, lt, lte operators, but note that it will only return the most recent exchange rate within the given timestamp range.

Rationale

Embedding Ethereum Transaction Directly

The signature in the Ethereum transaction is the most critical piece of data, and must be validated on the exact bytes passed into the signing software. Attempting to re-constitute the signed bytes or “vouch” for a signature (such as custodial bridges) introduce categories of software that diminish the value of a decentralized system.

A Single Transaction Type

Adding Ethereum data to supported transactions adds more complexity to transactions that need to track multiple modes. Similarly, a parallel series of transactions increases the API surface in ways that will be hard to maintain.

A Wrapped “Meta” Transaction

There are critical concepts in Hedera transaction processing, such as paying the node that posted the transaction and transaction validity windows, that have no equivalent in Ethereum and cannot easily be added to the transaction. Hence, simply storing the “raw” Ethereum transaction would not provide enough information to process the transaction.

By wrapping the Ethereum transaction in a Hedera transaction those values can be added as appropriate to the Ethereum transaction at the discretion of the transaction signer. Signing the transaction is also needed to seal in the choices and ensure they are not mutable.

Not Repeating Explicit and Implied Values

In order to save transaction space, no data that can be read directly from the RLP (such as the nonce) or that is implied (such as the public key of the signature) will be stored elsewhere in the Hedera transaction.

Backwards Compatibility

This HIP introduces a new transaction type, and does not directly impact other transactions other than ContractCreateTransaction. This HIP does increase the number of call paths that depend on particular behavior of the smart contract and HTS precompile, however the impact is expected to increase the fidelity and consistency of actions. Part of this will be ensured by reusing all the transition logic from dependant transactions.

There are no backwards incompatible changes.

Security Implications

The Ethereum transaction will change some validation logic for authorization of actions. Instead of requiring signatures just at the Hedera transaction level, the body of the Ethereum transaction will contain a signature that will authorize actions within the scope of the transaction (value transfer and contract/precompile calls). Security logic will need to be altered to also check Ethereum transaction data for authorization. Because security logic in Hedera is to “deny” by default, any implementation errors will be expected to prevent legitimate actions but not enable illegitimate actions.

How to Teach This

This is not a transaction dApps and integration software will typically use unless they specifically need Ethereum style transactions. Typical apps using this would be apps bridging JSON-RPC calls or apps integrating with other Ethereum tooling as a downstream consumer.

The documentation for this transaction should emphasize that normal HAPI APIs provide greater control and functionality than the functionality exposed via Ethereum transactions. If the application has control over the key signing functionality, this transaction should be avoided.

Reference Implementation

An incomplete prototype implementation is at https://github.com/hashgraph/hedera-services/tree/rlp . A production version will follow.

Rejected Ideas

Foreign Transaction Data

An internal prototype piloted the idea of adding foreign transaction data to all possible transactions, and then populating a normal Hedera transaction with the data from the foreign bytes. Transition logic would need to be updated to either validate the data matches the foreign bytes or to reject any transaction with foreign data.

This was rejected because it introduced a large amount of data duplication. Parsing of RLP data is very quick in comparison to other cryptographic validations and the weight on the record stream was deemed too much.

Enhanced Transaction Types

An idea that was work-shopped was to create separate ‘Ethereum’ variants of each of the transaction types, populating each with the Ethereum transaction and doing similar field copying semantics.

This idea was rejected because the explosion in number of transaction types was deemed a risk to software maintenance.

Storing just the Ethereum Transaction

There was brief discussion on the idea of storing the Ethereum transaction without any Hedera wrappers, and having the transaction execution logic detect the transaction and handle it directly.

This was rejected because of the inability to deconflict and deduplicate transactions in the manner that is required for the Hedera network structure. The node also was unable to be identified for proper credit in bringing the transaction to consensus.

Open Issues

None.

References

  • EIP-155 - Simple replay attack protection
  • EIP-2930 - Optional access lists
  • EIP-1559 - Fee market change for ETH 1.0 chain
  • Ethereum Yellow Paper
  • HIP-26 - Migrate Smart Contract Service EVM to Hyperledger Besu EVM
  • HIP-32 - Auto Account Creation
  • HIP-206 - Hedera Token Service Precompiled Contract for Hedera SmartContract Service

Copyright/license

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