HIP-583: Expand alias support in CryptoCreate & CryptoTransfer Transactions

Abstract

Formalize support for an ECDSA-derived alias, and permit all accounts to be created with an alias.

Motivation

Each account has a system-assigned account ID of the form shard.realm.num, where each value is an 8-byte number. HIP 32 defines a flow where, when transferring HBAR to an account, the “sender” can send to an alias for the account instead of the account number, where the alias is an encoding of the public key of the account. If there is an account with that alias, then the transfer succeeds. If the account does not exist, then it will be auto-created with the public key of the account matching that of the alias and the transfer succeeds.

With HIP 482, Hedera now supports an integration path with Ethereum-native wallets, where those wallets can talk with the JSON-RPC Relay, which talks to the Hedera network. These wallets work under the assumption that all accounts are identified by a 20-byte number. If a user is using an Ethereum-native wallet such as MetaMask, they cannot send hbars to a shard.realm.num account ID, they must send it to a 20-byte address.

Every account in Hedera has an Ethereum compatible 20-byte alias (known as the “long zero” alias since it is prefixed by many zeros), and an Ethereum-native wallet using the JSON-RPC relay can use that to send value to existing accounts with no modification to the existing accounts. However, what if a user wants to transfer value (hbars or tokens) to an account that doesn’t yet exist? The Ethereum-native wallet cannot use the Hedera-specific key encoding defined in HIP-32 because it is Hedera-specific. Rather, they need support for a new type of alias.

In Ethereum, addresses are derived from an ECDSA secp256k1 public key. Transferring value from one account to another involves a crypto transfer from one derived account address to another, and the target account is “auto created” if it did not previously exist. The holder of the ECDSA secp256k1 key that corresponds to that derived address can use that new account since any transaction they sign will be signed by that key.

This HIP allows any new account on Hedera to have an ECDSA derived alias, compatible with Ethereum addresses, and permits the use of this alias for all operations supported by alias today. This HIP expands support in the auto-create flow for ECDSA derived addresses to be used as the alias. It also adds support to CryptoCreate transactions for creation with an ECDSA derived alias.

Rationale

Fundamental in the design of the Hedera network are the concepts of accounts, account identifiers, and keys. These are fundamentally different entities. Today, a user Account must have a key. The key for the account may be a simple key (typically, an ED25519 public key), or it may be a compound key, such as a list of keys all of which must sign each transaction, or a threshold key such that N of the keys must be signed. These may be nested to create very rich and powerful accounts requiring multiple-signatures. Further, an Account may change the keys on the account over time.

As previously mentioned, an Account also has a system-assigned account identifier of the form shard.realm.num, where each value is an 8-byte number. An Account may also be referred to with an identifier of the form shard.realm.alias, where the alias is of one of the following forms:

1. account-num alias - This is the Hedera account number styled as an Ethereum account hex string. The Hedera account number is hex encoded and the value is prefixed with zeros to form a 20 byte address. This format cannot be used to auto-create an account, but can be used wherever an alias can be used. This is sometimes referred to as the “long-zero” alias, since the leftmost 12 bytes are 0.
2. key alias - This format is created by using the bytes of a simple cryptographic public key supported by Hedera (such as ED25519 or ECDSA secp256k1). It is not necessary for the account’s key to match the alias, only that the account owner has asserted cryptographic proof of ownership of the account alias key in addition to the account key. The key is protobuf-encoded and those raw bytes are included in the alias.
3. evm address alias - This format is created by using the rightmost 20 bytes of the 32 byte Keccak-256 hash of an ECDSA secp256k1 public key. This calculation is in the manner described by the Ethereum Yellow Paper. Note, that the recovery id is not formally part of the public key and not included in the hash. This is calculated on the consensus nodes using the ECDSA key bytes in the alias provided in the auto create CryptoTransfer transaction. While both evm address alias and account-num alias are 20 byte values, they can be differentiated by the fact that account-num alias is always prefixed with 12 bytes. The likelihood of randomly generated ECDSA keys, being hashed, having 12 sequential zeros is very low. Those keys, should they be found, cannot be used on the network.
4. All accounts will have a derived account-num alias but all account may have a key alias or evm address alias.

From the perspective of the Ethereum ecosystem, the account-num alias and evm address alias are both directly usable when prefixed with 0x. Examples of each format are noted below: (Please NOTE: the alias is a binary format when encoded on-wire for protobuf. We represent the bytes as hexidecimal ASCII for clarity in the documentation, but in reality these are raw bytes)

It is not possible to specify an account-num alias in either crypto transfer or account creation transactions. Any attempt to specify the alias using the account-num alias format will be detected and rejected by the system. The account-num alias is automatically assigned to an account based on its account number, and this number is automatically assigned by the system when the entity is created. Thus, the account-num alias cannot be specified.

In the extremely unlikely event that a user creates an account with an ECDSA key that derives to a evm address with leading zeros which makes it indistinguishable from an account-num alias, the account will not be created and the transaction will be rejected.

For example, while using an Ethereum-native wallet, I may transfer to an account-num alias address, and if that account exists, the transaction will succeed. if the account does not exist, the transfer will fail, since the account could not be auto-created.

To provide greater context for this HIP some additional terms are useful:

• Ethereum-native wallets - wallets that utilize the Ethereum JSON-RPC APIs implemented by the JSON-RPC Relay. Accounts created via these wallets are ECDSA derived keys and use the evm address alias format.
• Hedera-native wallets - wallets that utilize the Hedera SDK or gRPC HAPI calls to interact with Hedera consensus and mirror nodes. Historically, accounts created via these wallets are ED25519 based and usually identify the account via the Hedera Account Id and may support all types of alias.
• Hollow account - a partially created account which lacks a public key but does have a valid shard.realm.num ID and is auto-created with an evm address alias. Such an account may receive assets but may not take part in transactions that require its signature, until it is promoted to being a “full” account.
• auto-create - an account creation flow in which the key alias or evm address alias that a crypto-transfer is addressed to is used to automatically create a new account (if none exists) without an explicit CryptoCreate transaction being submitted. We call it hollow-create when the evm address alias is used, since it results in the creation of a Hollow account. When a user sends a transaction requiring this account, then it will be updated to a “full” account, with the account key set to match the evm address alias.

Crypto Transfers

With an Ethereum-native wallet, you can transfer the native currency (in our case, HBAR), or tokens, to an evm address that does not yet exist. If nobody in the universe has the private key for that account, the value held by that account is effectively locked until auto-renew fees drain the account or somebody is able to produce the key. If somebody does have the keys to that account, they can access the account and control it. Since Ethereum-native wallets do not understand the 0.0.10 style format of a Hedera account ID, it is necessary to support the creation of accounts with an evm address alias to enable Ethereum-native wallet workflows for auto-creation of accounts.

Smart Contract Integration

Every account is known to the EVM by a 20-byte address. That address may be the account-num alias or the evm address alias depending on how the account was created and what type of alias, if any, is on the account.

Update Account with Alias

HIP 32 specifies:

An account created normally has no alias. It can be given an alias with an account update, but only if that alias is not currently used by any other account, and only if the update transaction is signed by the private key corresponding to the public key in the alias.

This HIP SUPERSEDES HIP-32 by removing this capability. Support for updating an account with an alias introduces significant complication. If an account without an alias has already interacted with a smart contract, then it is known to the smart contract with the “long-zero” account-num alias that all accounts have. If that account were to be updated with an alias, then which evm address should be used in the EVM? For compatibility, it should use the account-num alias, at least with contracts it had already interacted with. However, this may run contrary to some expectations. While this is an important user flow we want to support, we defer this support to a future HIP. A full specification for this capability will likely require the ability for the user to select which alias is their default, and to specify on a per-call basis the alias they want to use when interacting with a smart contract.

User stories

1. As a user with an ECDSA derived account from another chain who uses an Ethereum-compatible wallet, I would like to create my Hedera account using that same ECDSA public key and associated Ethereum compatible evm address alias.
2. As a developer, I would like to create a new account with an evm address alias via the CryptoCreate transaction.
3. As a developer, I would like to transfer HBAR or tokens to a Hedera account using their evm address alias.
4. As a Hedera user with an Ethereum-native wallet, I would like to receive HBAR or tokens in my account by sharing only my evm address alias.
5. As a Hedera user with an Ethereum-native wallet, I would like to send HBAR or tokens from my account to another account on Hedera using a valid 20-byte address.
6. As a Hedera user with a Hedera-native wallet, I would like to transfer HBAR or tokens to another account using only the recipient’s evm address alias.
7. As a user with an Ethereum-native wallet, I want to be able to only share my EVM public address to an exchange, and that exchange to create a hollow account for me by transferring funds to the address.

Specification

This HIP proposes an expansion to the ways in which an account with an alias can be created and referenced to ensure compatibility with users coming from existing chains with ECDSA keys.

It is thus proposed to:

• Update CryptoCreate transactions to support setting an alias using an evm address alias
• Update CryptoTransfer transactions to support hollow-account creation for accounts with an evm address alias

HAPI Changes

Add the alias property to the CryptoCreateTransactionBody proto file

message CryptoCreateTransactionBody {
	...
	/**
     * The bytes to be used as the account's alias. 
     *
     * The bytes must be formatted as the calcluated last 20 bytes of the
     * keccak-256 of the ECDSA primitive key.</li>
     *
     * All other types of keys, including but not limited to ED25519, ThresholdKey, KeyList, ContractID, and
     * delegatable_contract_id, are not supported.
     *
     * At most only one account can ever have a given alias on the network.
     */
	bytes alias = 18;
}

Add a new error code for cases where we use CryptoCreate to set the alias, but we cannot set the alias because it is already used.

  /**
   * An alias that is assigned to an account or contract cannot be assigned to another account or contract.
   */
  ALIAS_ALREADY_ASSIGNED = 332;

Setting alias with CryptoCreate

Today, an alias can only be set during a crypto transfer. This HIP adds the ability to specify the alias during account creation.

Proof of Ownership

If the alias is set during CryptoCreate, then ownership of the alias must be proven by the caller by signing the transaction with the key represented by the alias. It is not necessary for the key on the newly created account to match the alias, but it is required for the transaction to be signed by the key represented by the alias.

Signatures

The required signatures for various transactions are as enumerated in the following table.
Note that the signature of the payer is left out intentionally in the table below as it is required in all cases.

Validation of the alias

It is not possible to specify an account-num alias in CryptoCreate transactions. Any attempt to specify the alias using the account-num alias format will be detected and rejected by the system.

key alias

Creation of a new account using key alias with CrytoCreate will not be support.

evm address alias

When an attempt is made to create a new account with CryptoCreate with an alias, we check whether the key alias has exactly 20 bytes. Then after making sure it is not an account-num alias, we assume the alias is an evm address alias and:

1. If there is an account with this evm address alias (whether because the alias on the Account is this evm address alias, or because it is a key alias of type ECDSA_secp256k1 that, when derived, maps to this evm address alias), then we fail the transaction.
2. We create the account and set the alias to be the evm address alias

Since we reserve the evm address alias and since an ECDSA_secp256k1 key alias checks for collisions with the evm address alias, we effectively reserve the key alias with the corresponding ECDSA_secp256k1 key slot as well. This prevents collisions between compatible key alias and evm address alias.

CryptoTransfer to an evm address alias

When a user transfers value (hbars or tokens), they may address their destination account using the standard account ID format (shard.realm.num), or by using an alias format (shard.realm.alias). If the alias is an evm address alias, then the consensus node will first check to see if such an account exists. If it does, then it will use the normal rules for transferring the hbars or tokens (including checking receiverSigRequired, and all other rules). If the account does not exist, then the account will be auto-created with the same rules specified in HIP-32, except that no key will be present on the newly created account. An account created in this manner will be known as a Hollow Account and will only be able to receive HBAR and applicable token transfers, but cannot take part in transactions submitted by others requiring its signature.

Since Hedera accounts require keys to ensure security, an ECDSA key matching the derived evm address alias must be provided in a future signed transaction issued by the owner of the account to take control of the account. Due to the nature of ECDSA secp256k1 keys, the public key can be extracted from the signature and the signed bytes. It can then go through the derivation function to generate the associated evm address, which can be compared with the alias to prove the match and account ownership. This process will complete the creation of the account, taking it from a Hollow Account to a normal complete account with a valid public key.

Note, in this case the final value of the account alias will remain as the evm address alias and not the key alias.

EthereumTransaction transactions where value is transferred (i.e tx.to and tx.value are set) should also support this case. This will support users of Ethereum-native wallets and eth dev tools who are accustomed to creating and crediting an account’s balance in one step. In this case the tx.to value of the RLP encoded transaction would dictate the evm address alias to be used.

Finalization Algorithm in More Depth

As mentioned above, after a Hollow account has been created, it must be finalized in order for it to become a normal account. For performance reasons transaction are processed in a multiple step workflow so that steps which can be done in parallel are done so. In particular the workflow defines precheck/ingestion, prehandle and handle steps for each transaction. The details of what happens in each step is listed below. A prerequisite is that the full ECDSA key needs to be provided in order for us to be able to finalize a hollow account.

1. In precheck/ingestion we verify if the payer account for the transaction is hollow, and, if yes, we search through the sig map and try to find a matching ECDSA public key that can be used to derive the EVM address alias on the account:
   • If we find a matching key, the check passes & the transaction is accepted
   • If we don’t an INVALID_SIGNATURE status is returned
2. In prehandle, we figure out which account signatures are required (based on the transaction) At this point we can see if the required accounts-to-have-signed is a hollow account and what the account alias is If we find that there is a required account that happens to be hollow, we then go through all the ECDSA signatures on the transaction and we try to find a matching ECDSA public key that can be used to derive the EVM address alias
   • if we succeed, we prepare and provide the information about the and the to `handle`
   • if we fail to find a matching ECDSA signature, the transaction will be rejected

3. In handle, we execute the hollow accounts finalization logic, where, for each account pending finalization for which we have the information from prehandle, we update the account public key and create a synthetic CryptoUpdate transaction record

   Also, in the case of an EthereumTransaction, we verify if the raw ethereum transaction sender is a hollow account, and if yes, we finalize it with the public ECDSA key that we extract from the raw eth transaction signature.

   Note: before each finalization, we have an additional check whether we have the capacity to finalize the account in this txn (we have a limit of 3 preceeding child records)

Proof of Ownership

If the alias is used during CryptoTransfer as the target of the transfer, then there is no need to prove that anyone controls that account. If the alias is used during CryptoTransfer as the source of the transfer or f an alias is used in the AccountID of the payer, then ownership must be asserted, just as is done for any other transaction today.

Validation of the alias

It is possible to specify an account-num alias as the target of a CryptoTransfer transaction. Indeed, for Ethereum-native wallets, this is the only way to transfer value to Hedera accounts that don’t have an evm address alias. However, it is impossible to auto-create an account using the account-num alias. If the account does not exist, the transfer will fail.

key alias

The key alias is a protobuf-encoded Key object. We only support ED25519 and ECDSA_secp256k1 keys at this time. When an attempt is made to transfer value into an account using CryptoTransfer with an alias, we check whether it is a key alias by checking if there are more than 20 bytes. We then:

1. Deserialize the protobuf object. If the object is not a protobuf object, we reject the transaction.
2. If the protobuf object is not an ED25519 or ECDSA_secp256k1 Key, we reject the transaction.
3. If the Key is an ED25519 key:
   1. If the alias is already in use, we use that account and do not create a new one
   2. If the alias is not in use, we create a new account and use the given key as the key on the account
4. If the Key is an ECDSA_secp256k1 key:
   1. If the alias is already in use, we use that account and do not create a new one
   2. If the derived evm address from the key is already in use, we use that account and do not create a new one
   3. If the alias is not in use, we create a new account and use the given key as the key on the account.

If we do create a new account from CryptoTransfer, and the alias is an ECDSA_secp256k1 key, then we reserve in the aliases for both the raw protobuf-encoded Key bytes and the derived evm address. Essentially, this allows the account created with an ECDSA_secp256k1 to be addressed by that alias, the evm address alias, and the account-num alias.

evm address alias

When an attempt is made to transfer value to an account with an alias with CryptoTransfer, we check whether the alias has exactly 20 bytes. Then after making sure it is not an account-num alias, we assume the alias is an evm address alias and:

1. If there is an account with this evm address alias (whether because the alias on the Account is this evm address alias, or because it is a key alias of type ECDSA_secp256k1 that, when derived, maps to this evm address alias), then we use that account and do not create a new one.
2. Otherwise, we create a Hollow account and set the alias to be the evm address alias

Since we reserve the evm address alias and since an ECDSA_secp256k1 key alias checks for collisions with the evm address alias, we effectively reserve the key alias with the corresponding ECDSA_secp256k1 key slot as well. This prevents collisions between compatible key alias and evm address alias.

Fees and Throttles

Fees for Hollow Accounts

The fees required for the CryptoCreate and CryptoUpdate transactions are both collected at the moment of the account creation.

For HAPI transactions, we create the synthetic CryptoCreate and CryptoUpdate transactions and use the fee calculator to compute the fees required in tinybars.

For EVM transfers, we create the synthetic CryptoCreate and CryptoUpdate transactions and use the fee calculator to compute the fees required in tinybars, then we convert the tinybars to gas using the current fee schedule.

Throttles

We have the following throttles defined:

• ContractCall - 350/sec
• CryptoCreate - 2/sec
• CryptoTransfer - 10k/sec
• contracts.maxGasPerSec - 15 million
• contracts.enforceCreationThrottle - a flag that controls whether the in-EVM creations are being throttled — currently set to false

Each hollow account creation gas fee is ~600K gas, so in the worst case, with the contracts.enforceCreationThrottle set to false, the maximum hollow accounts per sec would be ~ 15 000 000 / 600 000 = ~25 hollow accounts/sec

Backwards Compatibility

Existing accounts are unaffected by the expansion and the alias field on the transaction protobuf is already supported.

CryptoTransfer auto-account creation is not impacted and will still support the provision of the public key.

Mirror Node Implications

The Mirror Node can already ingest account creation transactions with alias values. This will only increase the applicable parent transactions that can cause this flow. Where it was limited to CryptoTransfer it will now include CryptoCreate, and in both cases it will now support evm address formatted aliases.

The following table describes how the alias and evm address fields will appear depending on how the alias was given during account creation. A guiding principle is that the evm address field will be shown as the address as it appears to the evm.

Wallet/Exchange Implications

With the suggested changes, the alias property of an account will become more applicable and will need greater visibility.

To provide greater web3 user scenario support Wallets & Exchanges should:

• Display the account alias property of an account
• Create an ECDSA based account with alias when full smart contract support is desired
• Accept the use of an alias as an account identifier in HBAR and token transfers
• Accept the use of an alias during account creation
• Add support to check if an account exists with a given alias

Performance Implications

CryptoCreate transactions will see extra bytes for every account created with an alias.

The size of the bytes in each case depends on the governing account key type. For an ECDSA based account the alias will always be a valid Ethereum address and will be 20 bytes long.

Overall, we anticipate no significant performance impact from this feature.

Security Implications

The hollow-create account flow requires additional checks be added to ensure transactions involving an account with only an alias and no key (i.e. Hollow accounts) are not processed. The CryptoCreate flow must verify the transaction is signed by the key represented by the alias (evm address alias).

How to Teach This

• Additional documentation on protobufs
• SDK examples should be written to highlight
  • CryptoCreate alias setting using an evm address alias
  • CryptoTransfer alias setting using an evm address alias
• Doc site tutorials utilizing SDKs should be written to highlight alias setting and the hollow account creation flow.
  • User level documentation on how to discover the 20-byte alias of any given account (either the account num alias or the evm address alias following the table above in this specification to match what the EVM would see for that account), and how to use that to transfer value using different wallets to different accounts.

Reference Implementation

The reference implementation must be complete before any HIP is given the status of “Final”. The final implementation must include test code and documentation.

Rejected Ideas

A special reserved ECDSA based aliasKey on all accounts. This would have ensured only a single key was used on contract interactions.

If an account has a single alias, and if that alias can only be set during account creation, then there is no need for an additional field. If we want to support adding an alias to an existing account, or have multiple aliases, then we have to deal with the additional complexity of deciding what alias to use with contracts. We may find there is no simple answer, and the user will want to assign different aliases for different contracts. Since there is a lot of design work to be done for this, we are taking it out of scope for this HIP, and allow a subsequent HIP to define this.

A special reserved ECDSA based aliasKey and an additional evmAlias. This would have provided complete separation of all EVM interactions with Hedera account keys.

For the same reasons as above, this is not required if we only have a single alias per account and the alias can only be set when the account is created. So we defer this discussion to a future HIP.

The ability to call CryptoCreate with just the evm address alias and not the public key.

This feature could be added, but there is not sufficient understanding at this time as to why it makes sense. You can use a CryptoTransfer to auto-create without a key, otherwise, supply the key. This feature is not harmful, but not useful either. It isn’t clear why anyone would pay to create an account that doesn’t have value. And if it does have value, it gets it through a CryptoTransfer, which auto-creates the account.

The network automatically sets the alias if the key on the account is an ECDSA key.

If the user creates an account using CryptoCreate, and the Key on the account is an ECDSA_secp256k1 key, we will not automatically set the alias on the account either as a key alias or an evm address alias. Since the setting of an alias is non-reversible, this decision should be explicit by the user. If a wallet is creating the account on behalf of the user, then the wallet can ensure that both the key and alias are set if it is desired.

For an account created via the hollow-create flow, a query call to a network node results in the key extraction and mapping via a child CryptoUpdate transaction

Only paid transactions cause a Hollow accout to be upgraded to a full account. If the paid transaction is due to a paid query, then it will cause the account to be upgraded.

The deployment of contracts with aliases via HAPI transactions

The ContractCreateTransaction HAPI call does not support an alias field and thus contracts created in this manner will not have aliases.
The only way a hollow account can be promoted to a contract is via the CREATE and CREATE2 calls available through the evm.

Creation of new accounts using key alias when calling CryptoCreate

We considered the possibility of allowing the key alias to be set on an account when calling CryptoCreate. This idea was rejected because it offered no additional utility to the end user.

Open Issues

None.

References

• Auto Account Creation
• ECRECOVER Precompiled Contract
• Ethereum Yellow Paper

Copyright/license

This document is licensed under the Apache License, Version 2.0 – see LICENSE or (https://www.apache.org/licenses/LICENSE-2.0)

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