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Transaction Permission Layer EVM Package

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EVM package including TPL jurisdiction implementation and an ERC20-enforced TPL.

TPL is a method for assigning metadata (or “attributes”) to Ethereum addresses. These attributes then form the basis for designing systems that enforce permissions when performing certain transactions. For instance, using TPL, securities tokens can require that attributes be present and have an appropriate value every time a token is sent or received. This allows projects to remain compliant with regulations by validating every single exchange between participants, beyond just the initial offering.

At the core of TPL is the jurisdiction — a single smart contract that links attributes to addresses. It implements an Attribute Registry interface, where attributes are registered to addresses as a key-value pair with a single canonical value. Implementing tokens then use this interface to request attributes that will inform whether to permit or reject the token transfer. Furthermore, implementers do not need to know any additional information on who set the attribute or how, and can check for the attribute value in a straightforward and efficient manner.

This jurisdiction does not set attributes itself, but rather defines a valid set of attribute types and designates validators that are approved to issue specific attribute types. The validators then either add attributes directly, or sign off-chain attribute approvals that can be relayed to the jurisdiction by the attribute holder or a designated third party. Considerable focus is also paid to ensuring that the jurisdiction and validators can revoke attributes, or entire categories of attributes, when necessary.

TPL is designed to be flexible enough for a wide variety of use-cases beyond just securities tokens, and promotes a distributed architecture where information is shared between multiple jurisdictions with their own specialties. It does so by allowing jurisdictions to specify secondary sources for any type of attribute, delegating the query to another jurisdiction or other attribute registry. A basic jurisdiction is also available that implements a smaller subset of these features.

Table of Contents

Usage

To deploy an upgradeable instance of TPL contracts using ZeppelinOS, you can link this EVM package and create a new instance. This will reuse the pre-deployed logic contracts on either mainnet, ropsten, rinkeby, or kovan, saving both gas and time.

$ npm install -g zos
$ zos init YourProject
$ zos link tpl-contracts-eth
$ zos push --network rinkeby # if working on a local network, add --deploy-dependencies
> Connecting to dependency tpl-contracts-eth
$ zos create tpl-contracts-eth/BasicJurisdiction --args "$OWNER" --network rinkeby --from $ADMIN
> Instance created at JURISDICTION_ADDRESS

Note: Make sure to pick different addresses for OWNER and ADMIN. The ADMIN will be the account that controls the upgradeability of the jurisdiction, and as such, is not allowed to interact with it except for upgrading. It is recommended to use a non-default account (ie not the first one listed in your node) for ADMIN.

Once you have created a jurisdiction, you can interact with it. As an example, we can create a new attribute type that we will use for receiving tokens, and assign the OWNER of the jurisdiction as an appointed validator for it. Fire up a new console via npx truffle console --network rinkeby and run the following, replacing JURISDICTION_ADDRESS and OWNER as needed:

const OWNER = // The owner address as set above
const JURISDICTION_ADDRESS = // The address of the jurisdiction as returned by the zos create call, also found under "proxies" in the zos.rinkeby.json file
const BasicJurisdictionABI = require('tpl-contracts-eth/build/contracts/BasicJurisdiction.json').abi;
const jurisdiction = new web3.eth.Contract(BasicJurisdictionABI, JURISDICTION_ADDRESS);
const ATTRIBUTE_ID = 1;
const TX_OPTS = { from: OWNER, gas: 1000000 };

jurisdiction.methods.addAttributeType(ATTRIBUTE_ID, "RECEIVE").send(TX_OPTS);
jurisdiction.methods.addValidator(OWNER, "OWNER").send(TX_OPTS);
jurisdiction.methods.addValidator(OWNER, "OWNER").send(TX_OPTS);
jurisdiction.methods.addValidatorApproval(OWNER, ATTRIBUTE_ID).send(TX_OPTS);

We can use this jurisdiction to set up a TPL restricted token. This is a regular ERC20 token, but requires that every recipient on every transfer has been listed with ATTRIBUTE_ID by a validator in the jurisdiction. This way, the jurisdiction can control who is whitelisted to control this asset.

API

NOTE: This documentation is not yet complete. See the relevant contract source code for additional information on available methods.

AttributeRegistryInterface

function hasAttribute

AttributeRegistryInterface.hasAttribute(account, attributeTypeID) view 4b5f297a

Check if an attribute of the type with ID attributeTypeID has been assigned to the account at account and is still valid.

Inputs

type name description
address account address The account to check for a valid attribute.
uint256 attributeTypeID uint256 The ID of the attribute type to check for.

Outputs

type name description
bool undefined

function getAttributeValue

AttributeRegistryInterface.getAttributeValue(account, attributeTypeID) view cd6c8343

Retrieve the value of the attribute of the type with ID attributeTypeID on the account at account, assuming it is valid.

Inputs

type name description
address account address The account to check for the given attribute value.
uint256 attributeTypeID uint256 The ID of the attribute type to check for.

Outputs

type name description
uint256 undefined

function countAttributeTypes

AttributeRegistryInterface.countAttributeTypes() view d71710e0

Count the number of attribute types defined by the registry.

Outputs

type name description
uint256 undefined

function getAttributeTypeID

AttributeRegistryInterface.getAttributeTypeID(index) view 0e62fde6

Get the ID of the attribute type at index index.

Inputs

type name description
uint256 index uint256 The index of the attribute type in question.

Outputs

type name description
uint256 undefined

BasicJurisdiction

function getAttributeTypeID

BasicJurisdiction.getAttributeTypeID(index) view 0e62fde6

Get the ID of the attribute type at index index.

Inputs

type name description
uint256 index uint256 The index of the attribute type in question.

Outputs

type name description
uint256 undefined

function getAttributeValidator

BasicJurisdiction.getAttributeValidator(account, attributeTypeID) view 17cf31d8

Find the validator that issued the attribute of the type with ID attributeTypeID on the account at account and determine if the validator is still valid.

if no attribute of the given attribute type exists on the account, the function will return (address(0), false).

Inputs

type name description
address account address The account that contains the attribute be checked.
uint256 attributeTypeID uint256 The ID of the attribute type in question.

Outputs

type name description
address validator undefined
bool isStillValid undefined

function getValidatorInformation

BasicJurisdiction.getValidatorInformation(validator) view 35b8ef26

Get a description of the validator at account validator.

Inputs

type name description
address validator address The account of the validator in question.

Outputs

type name description
string description undefined

function removeValidator

BasicJurisdiction.removeValidator(validator) nonpayable 40a141ff

Remove the validator at address validator from the jurisdiction.

Any attributes issued by the validator will become invalid upon their removal. If the validator is reinstated, those attributes will become valid again. Any approvals to issue attributes of a given type will need to be set from scratch in the event a validator is reinstated.

Inputs

type name description
address validator address The account of the validator to remove.

function issueAttribute

BasicJurisdiction.issueAttribute(account, attributeTypeID, value) payable 50135c3a

Issue an attribute of the type with ID attributeTypeID and a value of value to account if message.caller.address() is approved validator.

Existing attributes of the given type on the address must be removed in order to set a new attribute. Be aware that ownership of the account to which the attribute is assigned may still be transferable - restricting assignment to externally-owned accounts may partially alleviate this issue.

Inputs

type name description
address account address The account to issue the attribute on.
uint256 attributeTypeID uint256 The ID of the attribute type to issue.
uint256 value uint256 An optional value for the issued attribute.

function addValidator

BasicJurisdiction.addValidator(validator, description) nonpayable 63e2a232

Add account validator as a validator with a description description who can be approved to set attributes of specific types.

Note that the jurisdiction can add iteslf as a validator if desired.

Inputs

type name description
address validator address The account to assign as the validator.
string description string A description of the validator.

function getAttributeTypeInformation

BasicJurisdiction.getAttributeTypeInformation(attributeTypeID) view 6b600462

Get a description of the attribute type with ID attributeTypeID.

Inputs

type name description
uint256 attributeTypeID uint256 The ID of the attribute type to check for.

Outputs

type name description
string description undefined

function addValidatorApproval

BasicJurisdiction.addValidatorApproval(validator, attributeTypeID) nonpayable 7756588c

Approve the validator at address validator to issue attributes of the type with ID attributeTypeID.

Inputs

type name description
address validator address The account of the validator to approve.
uint256 attributeTypeID uint256 The ID of the approved attribute type.

function removeAttributeType

BasicJurisdiction.removeAttributeType(ID) nonpayable 7aedf3e0

Remove the attribute type with ID ID from the jurisdiction.

All issued attributes of the given type will become invalid upon removal, but will become valid again if the attribute is reinstated.

Inputs

type name description
uint256 ID uint256 The ID of the attribute type to remove.

function getAttributeTypeIDs

BasicJurisdiction.getAttributeTypeIDs() view 9679c72a

Get the IDs of all available attribute types on the jurisdiction.

Outputs

type name description
uint256[] undefined

function countValidators

BasicJurisdiction.countValidators() view 97f3c806

Count the number of validators defined by the jurisdiction.

Outputs

type name description
uint256 undefined

function addAttributeType

BasicJurisdiction.addAttributeType(ID, description) nonpayable acb29172

Add an attribute type with ID ID and description description to the jurisdiction.

Once an attribute type is added with a given ID, the description of the attribute type cannot be changed, even if the attribute type is removed and added back later.

Inputs

type name description
uint256 ID uint256 The ID of the attribute type to add.
string description string A description of the attribute type.

function removeValidatorApproval

BasicJurisdiction.removeValidatorApproval(validator, attributeTypeID) nonpayable b340ec81

Deny the validator at address validator the ability to continue to issue attributes of the type with ID attributeTypeID.

Any attributes of the specified type issued by the validator in question will become invalid once the approval is removed. If the approval is reinstated, those attributes will become valid again. The approval will also be removed if the approved validator is removed.

Inputs

type name description
address validator address The account of the validator with removed approval.
uint256 attributeTypeID uint256 The ID of the attribute type to unapprove.

function getValidator

BasicJurisdiction.getValidator(index) view b5d89627

Get the account of the validator at index index.

Inputs

type name description
uint256 index uint256 The index of the validator in question.

Outputs

type name description
address undefined

function getValidators

BasicJurisdiction.getValidators() view b7ab4db5

Get the accounts of all available validators on the jurisdiction.

Outputs

type name description
address[] undefined

function countAttributeTypes

BasicJurisdiction.countAttributeTypes() view d71710e0

Count the number of attribute types defined by the jurisdiction.

Outputs

type name description
uint256 undefined

function canIssueAttributeType

BasicJurisdiction.canIssueAttributeType(validator, attributeTypeID) view f287f8fb

Determine if a validator at account validator is able to issue attributes of the type with ID attributeTypeID.

Inputs

type name description
address validator address The account of the validator.
uint256 attributeTypeID uint256 The ID of the attribute type to check.

Outputs

type name description
bool undefined

function revokeAttribute

BasicJurisdiction.revokeAttribute(account, attributeTypeID) nonpayable f9292ffb

Revoke the attribute of the type with ID attributeTypeID from account if message.caller.address() is the issuing validator.

Validators may still revoke issued attributes even after they have been removed or had their approval to issue the attribute type removed - this enables them to address any objectionable issuances before being reinstated.

Inputs

type name description
address account address The account to issue the attribute on.
uint256 attributeTypeID uint256 The ID of the attribute type to issue.

event AttributeTypeAdded

BasicJurisdiction.AttributeTypeAdded(attributeTypeID, description) e35410b0

Arguments

type name description
uint256 attributeTypeID indexed
string description not indexed

event AttributeTypeRemoved

BasicJurisdiction.AttributeTypeRemoved(attributeTypeID) 3302c92b

Arguments

type name description
uint256 attributeTypeID indexed

event ValidatorAdded

BasicJurisdiction.ValidatorAdded(validator, description) 1b7d03cc

Arguments

type name description
address validator indexed
string description not indexed

event ValidatorRemoved

BasicJurisdiction.ValidatorRemoved(validator) e1434e25

Arguments

type name description
address validator indexed

event ValidatorApprovalAdded

BasicJurisdiction.ValidatorApprovalAdded(validator, attributeTypeID) b85fe33f

Arguments

type name description
address validator not indexed
uint256 attributeTypeID indexed

event ValidatorApprovalRemoved

BasicJurisdiction.ValidatorApprovalRemoved(validator, attributeTypeID) 61556816

Arguments

type name description
address validator not indexed
uint256 attributeTypeID indexed

event AttributeAdded

BasicJurisdiction.AttributeAdded(validator, attributee, attributeTypeID, attributeValue) fc11e611

Arguments

type name description
address validator not indexed
address attributee indexed
uint256 attributeTypeID not indexed
uint256 attributeValue not indexed

event AttributeRemoved

BasicJurisdiction.AttributeRemoved(validator, attributee, attributeTypeID) aa5b822d

Arguments

type name description
address validator not indexed
address attributee indexed
uint256 attributeTypeID not indexed

OrganizationsValidator

function getJurisdiction

OrganizationsValidator.getJurisdiction() view 1fa1087c

Get the account of the utilized jurisdiction.

Outputs

type name description
address undefined

function unpauseIssuance

OrganizationsValidator.unpauseIssuance() nonpayable 2585a270

Unpause issuance of new attributes by organizations.

function addOrganization

OrganizationsValidator.addOrganization(organization, maximumIssuableAttributes, name) nonpayable 35357c7c

Add an organization at account organization and with an initial allocation of issuable attributes of maximumIssuableAttributes.

Inputs

type name description
address organization address The account to assign to the organization.
uint256 maximumIssuableAttributes uint256 The number of issuable accounts.
string name undefined

function countOrganizations

OrganizationsValidator.countOrganizations() view 379c31bf

Count the number of organizations defined by the validator.

Outputs

type name description
uint256 undefined

function getOrganization

OrganizationsValidator.getOrganization(index) view 4526f690

Get the account of the organization at index index.

Inputs

type name description
uint256 index uint256 The index of the organization in question.

Outputs

type name description
address organization undefined

function issuanceIsPaused

OrganizationsValidator.issuanceIsPaused() view 6c823242

Determine if attribute issuance is currently paused.

Outputs

type name description
bool undefined

function getOrganizationInformation

OrganizationsValidator.getOrganizationInformation(organization) view 83235a0a

Get information about the organization at account account.

Note that an organization issuing numerous attributes may cause the function to fail, as the dynamic array could grow beyond a returnable size.

Inputs

type name description
address organization address The account of the organization in question.

Outputs

type name description
bool exists undefined
uint256 maximumAccounts undefined
string name undefined
address[] issuedAccounts undefined

function getOrganizations

OrganizationsValidator.getOrganizations() view 9754a3a8

Get the accounts of all available organizations.

Outputs

type name description
address[] accounts undefined

function getValidAttributeTypeID

OrganizationsValidator.getValidAttributeTypeID() view 98a11d8c

Get the ID of the attribute type that the validator can issue.

Outputs

type name description
uint256 undefined

function setMaximumIssuableAttributes

OrganizationsValidator.setMaximumIssuableAttributes(organization, maximumIssuableAttributes) nonpayable a2a71da5

Modify an organization at account organization to change the number of issuable attributes to maximumIssuableAttributes.

Note that the maximum number of accounts cannot currently be set to a value less than the current number of issued accounts. This feature, coupled with the ability to revoke attributes, will *prevent an organization from being 'frozen' since the organization can remove an address and then add an arbitrary address in its place. Options to address this include a dedicated method for freezing organizations, or a special exception to the requirement below that allows the maximum to be set to 0 which will achieve the intended effect.

Inputs

type name description
address organization address The account assigned to the organization.
uint256 maximumIssuableAttributes uint256 The number of issuable attributes.

function issueAttribute

OrganizationsValidator.issueAttribute(account) nonpayable c828b82b

Add an attribute to account account.

This function would need to be made payable to support jurisdictions that require fees in order to set attributes.

Inputs

type name description
address account address The account to issue the attribute to.

function initialize

OrganizationsValidator.initialize(jurisdiction, validAttributeTypeID) nonpayable cd6dc687

The initializer function for the Organinzations Validator, with owner and pauser roles initially assigned to contract creator (message.caller.address()), and with an associated jurisdiction at jurisdiction and an assignable attribute type with ID validAttributeTypeID.

Note that it may be appropriate to require that the referenced jurisdiction supports the correct interface via EIP-165 and that the validator has been approved to issue attributes of the specified type when initializing the contract - it is not currently required.

Inputs

type name description
address jurisdiction address The account of the associated jurisdiction.
uint256 validAttributeTypeID uint256 The ID of the attribute type to issue.

function revokeAttribute

OrganizationsValidator.revokeAttribute(account) nonpayable da15b9bd

Revoke an attribute from account account.

Organizations may still revoke attributes even after new issuance has been paused. This is the intended behavior, as it allows them to correct attributes they have issued that become compromised or otherwise erroneous.

Inputs

type name description
address account address The account to revoke the attribute from.

function pauseIssuance

OrganizationsValidator.pauseIssuance() nonpayable df23cbb1

Pause all issuance of new attributes by organizations.

event OrganizationAdded

OrganizationsValidator.OrganizationAdded(organization, name) 99387386

Arguments

type name description
address organization not indexed
string name not indexed

event AttributeIssued

OrganizationsValidator.AttributeIssued(organization, attributee) 1f8ea1fa

Arguments

type name description
address organization indexed
address attributee not indexed

event AttributeRevoked

OrganizationsValidator.AttributeRevoked(organization, attributee) 2f7805b6

Arguments

type name description
address organization indexed
address attributee not indexed

event IssuancePaused

OrganizationsValidator.IssuancePaused() df34d30c

event IssuanceUnpaused

OrganizationsValidator.IssuanceUnpaused() 940c41df

TPLRestrictedReceiverTokenInterface [outdated]

function getRegistry

TPLTokenInterface.getRegistry() view 5ab1bd53

Get the account of the utilized attribute registry.

Outputs

type name description
address undefined

function canTransfer

TPLTokenInterface.canTransfer(to, value) view d45e09c1

Check if an account is approved to transfer an amount of value to a recipient at account to.

Consider also returning a status code, e.g. EIP-1066

Inputs

type name description
address to address The account of the recipient.
uint256 value uint256 the amount to be transferred.

Outputs

type name description
bool undefined

function canTransferFrom

TPLTokenInterface.canTransferFrom(from, to, value) view f37d11cc

Check if an account is approved to transfer an amount of value to a recipient at account to on behalf of a sender at account from.

Consider also returning a status code, e.g. EIP-1066

Inputs

type name description
address from address The account of the sender.
address to address The account of the recipient.
uint256 value uint256 the amount to be transferred.

Outputs

type name description
bool undefined

Additional Information

NOTE: This section is out-of-date and is included for now for the sake of completeness.

  • An attribute registry is any smart contract that implements an interface containing a small set of external methods related to determining the existence of attributes. It enables implementing tokens and other contracts to avoid much of the complexity inherent in attribute validation and assignment by instead retrieving information from a trusted source. Attributes can be considered a lightweight alternative to claims as laid out in EIP-735.

  • The standard jurisdiction is implemented as a single contract that stores validated attributes for each participant, where each attribute is a uint256 => uint256 key-value pair. It implements an AttributeRegistry interface along with associated EIP-165 support, allowing other contracts to identify and confirm attributes recognized by the jurisdiction. It also implements additional basic and extended interfaces with methods and events that provide further context regarding actions within the jurisdiction.

  • A jurisdiction defines attribute types, or permitted attribute groups, with the following fields (with optional fields set to 0 | false | 0x | "" depending on the field's type):

    • an arbitrary uint256 attributeID field, unique to each attribute type within the jurisdiction, for accessing the attribute,
    • an optional bool isRestricted field which prevents attributes of the given type from being removed by the participant directly when set,
    • an optional bool onlyPersonal field which prevents attributes of the given type from being added by third-party operator,
    • an optional address secondarySource field which designates an external attribute registry that will be checked if an attribute has not been assigned locally,
    • an optional uint256 secondaryId field which designates the attribute ID to check when calling into the external attribute registry in question,
    • an optional uint256 minimumRequiredStake field, which requires that attributes of the given type must lock a minimum amount of ether in the jurisdiction in order to be added,
    • an optional uint256 jursdictionFee field to be paid upon assignment of any attribute of the given type, and
    • an optional string description field for including additional context on the given attribute type.
    • NOTE: one additional field not currently included in TPL attribute types but under active consideration is an optional bytes extraData field to support forward-compatibility.

  • The jurisdiction also designates validators (analogous to Certificate Authorities), which are addresses that can:

    • add or remove attributes of participants in the jurisdiction directly, assuming they have been approved to issue them by the validator,
    • sign off-chain approvals for adding attributes that can then be relayed by prospective attribute holders, and
    • modify their signingKey, an address corresponding to a private key used to sign approvals.

  • Validators then issue attributes to participants, which have the following properties:

    • a uint256 value field for attributes that require an associated quantity,
    • a uint256 stake amount greater than or equal to the minimum required by the attribute's type that, together with any jurisdictionFee (specified by the attribute type) and/or validatorFee (specified in the validator's approval signature), must be provided in msg.value when submitting a transaction to add the attribute, and
    • a valid or invalid state, contingent on the state of the issuing validator, the attribute type, or the validator's approval to issue attributes of that type.

  • The jurisdiction owner is an address (such as an a DAO, a multisig contract, or simply a standard externally-owned account) that can:

    • add or remove attribute types to the jurisdiction,
    • add or remove validators to the jurisdiction,
    • add or remove approvals for validators to assign attribute types, and
    • remove attributes from participants as required.

  • The TPLRestrictedReceiverToken is a standard OpenZeppelin ERC20 token that enforces attribute checks during every token transfer. For this implementation, the token checks the jurisdiction's registry for an attribute used to whitelist valid token recipients. The additional overhead for each transaction in the minimum-case is 4156 gas, with 1512 used to execute jurisdiction contract logic and 2644 for general "plumbing" (the overhead of checking against an external call to the registry that simply returns true). (NOTE: the attributes defined in the jurisdiction and required by TPLToken have been arbitrarily defined for this PoC, and are not intended to serve as a proposal for the attributes that will be used for validating transactions.)

Attribute scope

Issued attributes exist in the scope of the issuing validator - if a validator is removed, all attributes issued by that validator become invalid and must be renewed. Furthermore, an attribute exists in the scope of it's attribute type, and if the attribute type is removed from the jurisdiction the associated attributes will become invalid. Finally, each attribute type that a validator is approved to add has a scope, and if a validator has its approval for issuing attributes of a particular type, all attributes it has issued with the given type will become invalid.

The validator that issued an attribute to a given address can be found by calling getAttributeValidator, but most contracts that implement a jurisdiction as the primary registry for performing transaction permission logic should not have to concern themselves with the validators at all - indeed, much of the point of the jurisdiction is to allow for tokens and other interfacing contracts to delegate managing validators and attributes to the jurisdiction altogether.

Off-chain attribute approvals

Validators may issue and revoke attributes themselves on-chain (and, indeed, this may be the preferred method for validators who are in turn smart contracts and wish to implement their own on-chain attribute approval / revokation logic or fee structure), but they have another option at their disposal - they may sign an approval off-chain and let the participant, or an approved operator designated by the approval, submit the transaction. This has a number of beneficial properties:

  • Validators do not have to pay transaction fees in order to assign attributes,
  • Participants or operators may decide when they want to add the attribute, enhancing privacy and saving on fees when attributes are not ultimately required, and
  • Participants or operators can optionally be required to stake some ether when assigning the attribute, which will go toward paying the transaction fee should the validator or jurisdiction owner need to revoke the attribute in the future.
  • Furthermore, participants and operators can optionally be required to include additional fees to the jurisdiction owner and/or to the validator, as required in the attribute type or signed attribute approval, respectively.

To sign an attribute approval, a validator may use the following (with appropriate arguments):

var Web3 = require('web3')
var web3 = new Web3('ws://localhost:8545')  // replace with desired web3 provider

function getAttributeApprovalHash(
  jurisdictionAddress,
  assigneeAddress,
  operatorAddress, // set to 0 when assigned personally
  fundsRequired, // stake + jurisdiction fee + validator fee
  validatorFee,
  attributeID,
  attributeValue
) {
  if (operatorAddress === 0) {
    operatorAddress = '0x0000000000000000000000000000000000000000'
  }
  return web3.utils.soliditySha3(
    {t: 'address', v: jurisdictionAddress},
    {t: 'address', v: assigneeAddress},
    {t: 'address', v: operatorAddress},
    {t: 'uint256', v: fundsRequired},
    {t: 'uint256', v: validatorFee},
    {t: 'uint256', v: attributeID},
    {t: 'uint256', v: attributeValue}
  )
}

async function signValidation(
  validatorSigningKey,
  jurisdictionAddress,
  assigneeAddress,
  operatorAddress,
  fundsRequired, // stake + jurisdiction fee + validator fee
  validatorFee,
  attributeID,
  attributeValue
) {
  return web3.eth.sign(
    getAttributeApprovalHash(
      jurisdictionAddress,
      assigneeAddress,
      operatorAddress,
      fundsRequired,
      validatorFee,
      attributeID,
      attributeValue
    ),
    validatorSigningKey
  )
}

Under this scheme, handling the management of signing keys in an effective manner takes on critical importance. Validators can specify an address associated with a signing key, which the jurisdiction will enforce via ecrecover using OpenZeppelin's ECRecovery library (with direct EIP-1271 support also under consideration). Management of keys and revokations can then be handled seperately (potentially via a validator contract, which would not be able to sign approvals due to the lack of an associated private key on contracts) from the actual signing of attribute approvals. If a signing key is then lost or compromised, the validator can modify the key, which will invalidate any unsubmitted attribtue approvals signed using the old key, but any existing attributes issued using the old key will remain valid. Attribute approvals may also be invalidated by the issuing validator or by the jurisdiction owner by passing the result of getAttributeApprovalHash and the signature above (in the case of validators - the owner can disregard the signature field) into invalidateAttributeApproval.

NOTE: a requirement not currently included in TPL but under active consideration is the submission of a bytes proof field when modifying a key - there is a requirement for signing keys to be unique so that they point back to a specific validator, which creates an opportunity for existing validators to set their "signing key" as the address of a contract under consideration for addition as a new validator, blocking the addition of said validator, as the signing key is initially set to the validator's address. Requiring a signature proving that the validator controls the associated private key would prevent this admittedly obscure attack.

Staked attributes & Revocations

When approving attributes for participants to relay off-chain, validators may specify a required stake to be included in msg.value of the transaction relaying the signed attribute approval. This required stake must be greater or equal to the minimunRequiredStake specified by the jurisdiction in the attribute type, and may easily be set to 0 as long as minimumRequiredStake is also set to 0. In that event, participants do not need to include any stake - they won't even need to provide an extra argument with a value of 0, as msg.value is included by default in every transaction.

Should a validator elect to require a staked amount, they or the jurisdiction will receive a transaction rebate, up to the staked value, for removing the attribute in question. This value is calculated by multiplying an estimate of the transaction's gas usage (currently set to 37700) with tx.gasPrice. Any additional stake will be returned to whatever address locked the funds originally - this enables the jurisdiction to receive transaction rebates for removing attributes set by the validator if required. Should the jurisdiction assign multiple validators to an attribute, market forces should cause the staked requirement to move towards equilibrium with expected gas requirements for removing the attribute in question. Validators may also perform risk analysis on participants as part of their attribute approval process and offer signed attribute approvals with a variable required stake that is catered to the reliability of the participant in question.

Care should be taken when determining the estimated gas usage of the attribute revocation, as setting the value too high will incentivize spurious revokations. Additionally, if there is a profit to be made by the revoker, they may elect to set as high a tx.gasPrice as possible to improve their profit margin at the expense of wasting any additional staked ether that would otherwise be returned to the staker. The actual gas usage will also depend on the attribute in question, as attributes with more data in contract storage will provide a larger gas rebate at the end of the transaction, and using gasLeft() to calculate gas usage will fail to account for this rebate. It is recommended to set this estimate to a conservative value, so as to provide the maximum possible transaction rebate without creating any cases where the rebate will exceed the realized transaction cost.

Feature proposals

Some features of this implementation, and others that are not included as part of this implementation, are still under consideration for inclusion in TPL. Some of the most pressing open questions include:

  • the degree of support for various Ethereum Improvement Proposals (with a tradeoff cross-compatibility vs over-generalization & complexity),
  • enabling batched attribute assignment and removal to facilitate both cost savings by validators and simultaneous assignment of multiple related attributes by participants, and
  • the possibility of integrating a native token for consistent internal accounting irregardless of external inputs (though this option is likely unneccessary and needlessly complex).

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MIT © 2018 Transaction Permission Layer

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Upgradeable on-chain version of TPL contracts

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