Simple Summary

An extension to the ERC-20 standard token that allows tokens to be put on hold. This guarantees a future transfer and makes the held tokens unavailable for transfer in the mean time. Holds are similar to escrows in that are firm and lead to final settlement.

Actors

Operator

An account which has been approved by an account to create holds on its behalf.

Hold issuer

The account, which creates a hold. This can be the account owner itself, or any account, which has been approved as an operator for the account.

Notary

The account which decides if a hold should be executed.

Abstract

A hold specifies a payer, a payee, a maximum amount, a notary and an expiration time. When the hold is created, the specified token balance from the payer is put on hold. A held balance cannot be transferred until the hold is either executed or released. The hold can only be executed by the notary, which triggers the transfer of the tokens from the payer to the payee. If a hold is released, either by the notary at any time, or by anyone after the expiration, no transfer is carried out and the amount is available again for the payer.

A hold can be partially executed, if the execution specifies an amount less than the maximum amount. In this case the specified amount is transferred to the payee and the remaining amount is available again to the payer.

Holds can be specified to be perpetual. In this case, the hold cannot be released upon expiration, and thus can only be executed by the notary or released by the notary or payee.

Motivation

A hold has to be used in different scenarios where a immediate transfer between accounts is not possible or has to be guaranteed beforehand:

1. A regulated token may not allow to do a token transfer between accounts without verifying first, that it follows all the regulations. In this case a clearable transfer has to be used. During the clearing process a hold is created to ensure, that the transfer is successful after all checks have passed. If the transfer violates any of the regulations, it is cleared and not further processed.

2. In certain business situations a payment has to be guaranteed before its services can be used. For example: When checking in a hotel, the hotel will put a hold on the guest's account to ensure that enough balance is available to pay for the room before handing over the keys.

3. In other occasions a payment has to be guaranteed without knowing the exact amount beforehand. To stay with the hotel example: The hotel can put a hold on the guest's account as a guarantee for any possible extras, like room service. When the guest checks out the hold is partially executed and the remaining amount is available again on the guest's account.

The ERC-20 approve function provides some of the necessary functionality for the use cases above. The main difference to holds, is that approve does not ensure a payment, as the approved money is not blocked and can be transferred at any moment.

Specification

interface IHoldable /* is ERC-20 */ {
    enum HoldStatusCode {
        Nonexistent,
        Ordered,
        Executed,
        ReleasedByNotary,
        ReleasedByPayee,
        ReleasedOnExpiration
    }

    function hold(string calldata operationId, address to, address notary, uint256 value, uint256 timeToExpiration) external returns (bool); 
    function holdFrom(string calldata operationId, address from, address to, address notary, uint256 value, uint256 timeToExpiration) external returns (bool);
    function releaseHold(string calldata operationId) external returns (bool);
    function executeHold(string calldata operationId, uint256 value) external returns (bool);
    function renewHold(string calldata operationId, uint256 timeToExpiration) external returns (bool);
    function retrieveHoldData(string calldata operationId) external view returns (address from, address to, address notary, uint256 value, uint256 expiration, HoldStatusCode status);

    function balanceOnHold(address account) external view returns (uint256);
    function netBalanceOf(address account) external view returns (uint256);
    function totalSupplyOnHold() external view returns (uint256);

    function authorizeHoldOperator(address operator) external returns (bool);
    function revokeHoldOperator(address operator) external returns (bool);
    function isHoldOperatorFor(address operator, address from) external view returns (bool);

    event HoldCreated(address indexed holdIssuer, string  operationId, address from, address to, address indexed notary, uint256 value, uint256 expiration);
    event HoldExecuted(address indexed holdIssuer, string operationId, address indexed notary, uint256 heldValue, uint256 transferredValue);
    event HoldReleased(address indexed holdIssuer, string operationId, HoldStatusCode status);
    event HoldRenewed(address indexed holdIssuer, string operationId, uint256 oldExpiration, uint256 newExpiration);
    event AuthorizedHoldOperator(address indexed operator, address indexed account);
    event RevokedHoldOperator(address indexed operator, address indexed account);
}

Functions

hold

Creates a hold on behalf of the msg.sender in favor of the payee. It specifies a notary who is responsible to either execute or release the hold. The function must revert if the operation ID has been used before.

Parameter	Description
operationId	The unique ID to identify the hold
to	The address of the payee, to whom the tokens are to be transferred if executed
notary	The address of the notary who is going to determine whether the hold is to be executed or released
value	The amount to be transferred. Must be less or equal than the balance of the payer.
timeToExpiration	The duration until the hold is expired. If it is '0' the hold must be perpetual.

holdFrom

Creates a hold on behalf of the payer in favor of the payee. The from account has to approve beforehand, that another account can issue holds on its behalf by calling approveToHold. The function must revert if the operation ID has been used before.

Parameter	Description
operationId	The unique ID to identify the hold
from	The address of the payer, from whom the tokens are to be taken if executed
to	The address of the payee, to whom the tokens are to be transferred if executed
notary	The address of the notary who is going to determine whether the hold is to be executed or released
value	The amount to be transferred. Must be less or equal than the balance of the payer.
timeToExpiration	The duration until the hold is expired. If it is '0' the hold must be perpetual.

releaseHold

Releases a hold. Release means that the transfer is not executed and the held amount is available again for the payer. Until a hold has expired it can only be released by the notary or the payee. After it has expired it can be released by anyone.

Parameter	Description
operationId	The unique ID to identify the hold

executeHold

Executes a hold. Execute means that the specified value is transferred from the payer to the payee. If the specified value is less than the hold value the remaining amount is available again to the payer. The implementation must verify that only the notary is able to successfully call the function.

Parameter	Description
operationId	The unique ID to identify the hold
value	The amount to be transferred. This amount has to be less or equal than the hold value

renewHold

Renews a hold. The new expiration time must be the block timestamp plus the given timeToExpiration, independently if the hold was perpetual or not before that. Furthermore a hold must be made perpetual if timeToExpiration is '0'. The implementation must verify that only the payer or operator are able to successfully call the function. Furthermore the only a hold, which has not yet expired can be successfully renewed.

Parameter	Description
operationId	The unique ID to identify the hold
timeToExpiration	The new duration until the hold is expired.

retrieveHoldData

Retrieves all the information available for a particular hold.

Parameter	Description
operationId	The unique ID to identify the hold

balanceOnHold

Retrieves how much of the balance is currently held and therefore not available for transfer.

Parameter	Description
account	The address which held balance should be returned

netBalanceOf

Retrieves the net balance, which is the sum of balanceOf and balanceOnHold.

Parameter	Description
account	The address which net balance should be returned

totalSupplyOnHold

Retrieves the total sum of how many tokens are on hold.

Parameter	Description
-	-

authorizeHoldOperator

Approves an operator to issue holds on behalf of msg.sender.

Parameter	Description
operator	The address to be approved as operator of holds

revokeHoldOperator

Revokes the approval to issue holds on behalf of msg.sender.

Parameter	Description
operator	The address to be revoked as operator of holds

isHoldOperatorFor

Retrieves if an operator is approved to create holds on behalf of from.

Parameter	Description
operator	The address to be a operator of holds
from	The address on which the holds would be created

balanceOf

The standard implementation of ERC-20 has to be changed in order to deduct the held balance from the ERC-20 balance.

transfer

The standard implementation of ERC-20 has to be changed in order to deduct the held balance from the ERC-20 balance. Any amount that is held must not be transferred.

transferFrom

The standard implementation of ERC-20 has to be changed in order to deduct the held balance from the ERC-20 balance. Any amount that is held must not be transferred.

Events

HoldCreated

Emitted when a hold has been created.

Parameter	Description
holdIssuer	The address of the hold issuer of the hold
operationId	The unique ID to identify the hold
from	The address of the payer, from whom the tokens are to be taken if executed
to	The address of the payee, to whom the tokens are to be paid if executed
notary	The address of the notary who is going to determine whether the hold is to be executed or released
value	The amount to be transferred. Must be less or equal than the balance of the payer.
expiration	The unix timestamp when the hold is expired

HoldExecuted

Emitted when a hold has been executed.

Parameter	Description
holdIssuer	The address of the hold issuer of the hold
operationId	The unique ID to identify the hold
notary	The address of the notary who executed the hold
heldValue	The amount which was put on hold during creation
transferredValue	The amount which was used for the transfer

HoldReleased

Emitted when a hold has been released.

Parameter	Description
holdIssuer	The address of the hold issuer of the hold
operationId	The unique ID to identify the hold
status	Can be one of the following values: ReleasedByNotary, ReleasedByPayee, ReleasedOnExpiration

HoldRenewed

Emitted when a hold has been renewed.

Parameter	Description
holdIssuer	The address of the hold issuer of the hold
operationId	The unique ID to identify the hold
oldExpiration	The expiration time before the renewal
newExpiration	The expiration time after the renewal

AuthorizedHoldOperator

Emitted when an operator has been approved to create holds on behalf of another account.

Parameter	Description
operator	The address to be a operator of holds
account	Address on which behalf holds will potentially be created

RevokedHoldOperator

Emitted when an operator has been revoked from creating holds on behalf of another account.

Parameter	Description
operator	The address to be a operator of holds
account	Address on which behalf holds could potentially be created

Rationale

This standards provides a functionality, to guarantee future payments, which is needed for many business cases where transfers have to be guaranteed.

It goes a step further than the ERC-20 approve function by ensuring that the held balance will be available when the transfer is done. Something that can not be done with approve, as the approved amount is only a maximum spending amount, but never guaranteed to be available.

While not requiring it, the naming of the functions authorizeHoldOperator, revokeHoldOperator and isHoldOperatorFor follows the naming convention of ERC-777.

The operationId is a string and not something more gas efficient to allow easy traceability of the hold and allow human readable ids. It is up to the implementer if the string should be stored on-chain or only its hash, as it is enough to identify a hold.

The operationId is a competitive resource. It is recommended, but nor required, that the hold issuers used a unique prefix to avoid collisions.

Backwards Compatibility

This EIP is fully backwards compatible as its implementation extends the functionality of ERC-20.

Implementation

The GitHub repository IoBuilders/holdable-token contains the reference implementation.

Contributors

This proposal has been collaboratively implemented by adhara.io and io.builders.

Copyright

Copyright and related rights waived via CC0.