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Multi Token Standard Approval Management

caution

This is part of the proposed spec NEP-245 and is subject to change.

Version 1.0.0

Summary​

A system for allowing a set of users or contracts to transfer specific tokens on behalf of an owner. Similar to approval management systems in standards like ERC-721 and ERC-1155.

Motivation​

People familiar with ERC-721 may expect to need an approval management system for basic transfers, where a simple transfer from Alice to Bob requires that Alice first approve Bob to spend one of her tokens, after which Bob can call transfer_from to actually transfer the token to himself.

NEAR's core Multi Token standard includes good support for safe atomic transfers without such complexity. It even provides "transfer and call" functionality (mt_transfer_call) which allows specific tokens to be "attached" to a call to a separate contract. For many token workflows, these options may circumvent the need for a full-blown Approval Management system.

However, some Multi Token developers, marketplaces, dApps, or artists may require greater control. This standard provides a uniform interface allowing token owners to approve other NEAR accounts, whether individuals or contracts, to transfer specific tokens on the owner's behalf.

Prior art:

Example Scenarios​

Let's consider some examples. Our cast of characters & apps:

  • Alice: has account alice with no contract deployed to it
  • Bob: has account bob with no contract deployed to it
  • MT: a contract with account mt, implementing only the Multi Token Standard with this Approval Management extension
  • Market: a contract with account market which sells tokens from mt as well as other token contracts
  • Bazaar: similar to Market, but implemented differently (spoiler alert: has no mt_on_approve function!), has account bazaar

Alice and Bob are already registered with MT, Market, and Bazaar, and Alice owns a token on the MT contract with ID="1" and a fungible style token with ID ="2" and AMOUNT ="100".

Let's examine the technical calls through the following scenarios:

  1. Simple approval: Alice approves Bob to transfer her token.
  2. Approval with cross-contract call (XCC): Alice approves Market to transfer one of her tokens and passes msg so that MT will call mt_on_approve on Market's contract.
  3. Approval with XCC, edge case: Alice approves Bazaar and passes msg again, but what's this? Bazaar doesn't implement mt_on_approve, so Alice sees an error in the transaction result. Not to worry, though, she checks mt_is_approved and sees that she did successfully approve Bazaar, despite the error.
  4. Approval IDs: Bob buys Alice's token via Market.
  5. Approval IDs, edge case: Bob transfers same token back to Alice, Alice re-approves Market & Bazaar. Bazaar has an outdated cache. Bob tries to buy from Bazaar at the old price.
  6. Revoke one: Alice revokes Market's approval for this token.
  7. Revoke all: Alice revokes all approval for this token.

1. Simple Approval​

Alice approves Bob to transfer her tokens.

High-level explanation

  1. Alice approves Bob
  2. Alice queries the token to verify

Technical calls

  1. Alice calls mt::mt_approve({ "token_ids": ["1","2"], amounts:["1","100"], "account_id": "bob" }). She attaches 1 yoctoⓃ, (.000000000000000000000001Ⓝ). Using NEAR CLI to make this call, the command would be:

    near call mt mt_approve \
      '{ "token_ids": ["1","2"], amounts: ["1","100"], "account_id": "bob" }' \
      --accountId alice --amount .000000000000000000000001

    The response:

    ''

  2. Alice calls view method mt_is_approved:

    near view mt mt_is_approved \
      '{ "token_ids": ["1", "2"], amounts:["1","100"], "approved_account_id": "bob" }'

    The response:

    true

3. Approval with cross-contract call​

Alice approves Market to transfer some of her tokens and passes msg so that MT will call mt_on_approve on Market's contract. She probably does this via Market's frontend app which would know how to construct msg in a useful way.

High-level explanation

  1. Alice calls mt_approve to approve market to transfer her token, and passes a msg
  2. Since msg is included, mt will schedule a cross-contract call to market
  3. Market can do whatever it wants with this info, such as listing the token for sale at a given price. The result of this operation is returned as the promise outcome to the original mt_approve call.

Technical calls

  1. Using near-cli:

    near call mt mt_approve '{
      "token_ids": ["1","2"],
      "amounts": ["1", "100"],
      "account_id": "market",
      "msg": "{\"action\": \"list\", \"price\": [\"100\",\"50\"],\"token\": \"nDAI\" }"
    }' --accountId alice --amount .000000000000000000000001

    At this point, near-cli will hang until the cross-contract call chain fully resolves, which would also be true if Alice used a Market frontend using near-api-js. Alice's part is done, though. The rest happens behind the scenes.

  2. mt schedules a call to mt_on_approve on market. Using near-cli notation for easy cross-reference with the above, this would look like:

    near call market mt_on_approve '{ "token_ids": ["1","2"], "amounts": ["1","100"], "owner_id": "alice", "approval_ids": ["4","5"], "msg": "{\"action\": \"list\", \"price\": [\"100\",\"50\"], \"token\": \"nDAI\" }" }' --accountId mt

  3. market now knows that it can sell Alice's tokens for 100 nDAI and 50 nDAI, and that when it transfers it to a buyer using mt_batch_transfer, it can pass along the given approval_ids to ensure that Alice hasn't changed her mind. It can schedule any further cross-contract calls it wants, and if it returns these promises correctly, Alice's initial near-cli call will resolve with the outcome from the final step in the chain. If Alice actually made this call from a Market frontend, the frontend can use this return value for something useful.

3. Approval with cross-contract call, edge case​

Alice approves Bazaar and passes msg again. Maybe she actually does this via near-cli, rather than using Bazaar's frontend, because what's this? Bazaar doesn't implement mt_on_approve, so Alice sees an error in the transaction result.

Not to worry, though, she checks mt_is_approved and sees that she did successfully approve Bazaar, despite the error. She will have to find a new way to list her token for sale in Bazaar, rather than using the same msg shortcut that worked for Market.

High-level explanation

  1. Alice calls mt_approve to approve bazaar to transfer her token, and passes a msg.
  2. Since msg is included, mt will schedule a cross-contract call to bazaar.
  3. Bazaar doesn't implement mt_on_approve, so this call results in an error. The approval still worked, but Alice sees an error in her near-cli output.
  4. Alice checks if bazaar is approved, and sees that it is, despite the error.

Technical calls

  1. Using near-cli:

    near call mt mt_approve '{ "token_ids": ["1"], "amounts: ["1000"], "account_id": "bazaar", "msg": "{\"action\": \"list\", \"price\": \"100\", \"token\": \"nDAI\" }" }' --accountId alice --amount .000000000000000000000001

  2. mt schedules a call to mt_on_approve on market. Using near-cli notation for easy cross-reference with the above, this would look like:

    near call bazaar mt_on_approve '{ "token_ids": ["1"], "amounts": ["1000"], "owner_id": "alice", "approval_ids": [3], "msg": "{\"action\": \"list\", \"price\": \"100\", \"token\": \"nDAI\" }" }' --accountId mt

  3. 💥 bazaar doesn't implement this method, so the call results in an error. Alice sees this error in the output from near-cli.

  4. Alice checks if the approval itself worked, despite the error on the cross-contract call:

    near view mt mt_is_approved \
      '{ "token_ids": ["1","2"], "amounts":["1","100"], "approved_account_id": "bazaar" }'

    The response:

    true

4. Approval IDs​

Bob buys Alice's token via Market. Bob probably does this via Market's frontend, which will probably initiate the transfer via a call to ft_transfer_call on the nDAI contract to transfer 100 nDAI to market. Like the MT standard's "transfer and call" function, Fungible Token's ft_transfer_call takes a msg which market can use to pass along information it will need to pay Alice and actually transfer the MT. The actual transfer of the MT is the only part we care about here.

High-level explanation

  1. Bob signs some transaction which results in the market contract calling mt_transfer on the mt contract, as described above. To be trustworthy and pass security audits, market needs to pass along approval_id so that it knows it has up-to-date information.

Technical calls

Using near-cli notation for consistency:

near call mt mt_transfer '{
  "receiver_id": "bob",
  "token_id": "1",
  "amount": "1",
  "approval_id": 2,
}' --accountId market --amount .000000000000000000000001

5. Approval IDs, edge case​

Bob transfers same token back to Alice, Alice re-approves Market & Bazaar, listing her token at a higher price than before. Bazaar is somehow unaware of these changes, and still stores approval_id: 3 internally along with Alice's old price. Bob tries to buy from Bazaar at the old price. Like the previous example, this probably starts with a call to a different contract, which eventually results in a call to mt_transfer on bazaar. Let's consider a possible scenario from that point.

High-level explanation

Bob signs some transaction which results in the bazaar contract calling mt_transfer on the mt contract, as described above. To be trustworthy and pass security audits, bazaar needs to pass along approval_id so that it knows it has up-to-date information. It does not have up-to-date information, so the call fails. If the initial mt_transfer call is part of a call chain originating from a call to ft_transfer_call on a fungible token, Bob's payment will be refunded and no assets will change hands.

Technical calls

Using near-cli notation for consistency:

near call mt mt_transfer '{
  "receiver_id": "bob",
  "token_id": "1",
  "amount": "1",
  "approval_id": 3,
}' --accountId bazaar --amount .000000000000000000000001

6. Revoke one​

Alice revokes Market's approval for this token.

Technical calls

Using near-cli:

near call mt mt_revoke '{
  "account_id": "market",
  "token_ids": ["1"],
}' --accountId alice --amount .000000000000000000000001

Note that market will not get a cross-contract call in this case. The implementors of the Market app should implement cron-type functionality to intermittently check that Market still has the access they expect.

7. Revoke all​

Alice revokes all approval for these tokens

Technical calls

Using near-cli:

near call mt mt_revoke_all '{
  "token_ids": ["1", "2"],
}' --accountId alice --amount .000000000000000000000001

Again, note that no previous approvers will get cross-contract calls in this case.

Reference-level explanation​

The TokenApproval structure returned by mt_token_approvals returns approved_account_ids field, which is a map of account IDs to Approval and approval_owner_id which is the associated account approved for removal from. The amount field though wrapped in quotes and treated like strings, the number will be stored as an unsigned integer with 128 bits. in approval is Using TypeScript's Record type notation:

+ type Approval = {
+   amount: string 
+   approval_id: string
+ }
+
+ type TokenApproval = {
+  approval_owner_id: string,
+  approved_account_ids: Record<string, Approval>,
+ };

Example token approval data:

[{
  "approval_owner_id": "alice.near",
  "approved_account_ids": {
    "bob.near": {
      "amount": "100",
      "approval_id":1,
    },
    "carol.near": {
      "amount":"2",
      "approval_id": 2,
    }
  }
}]

What is an "approval ID"?​

This is a unique number given to each approval that allows well-intentioned marketplaces or other 3rd-party MT resellers to avoid a race condition. The race condition occurs when:

  1. A token is listed in two marketplaces, which are both saved to the token as approved accounts.
  2. One marketplace sells the token, which clears the approved accounts.
  3. The new owner sells back to the original owner.
  4. The original owner approves the token for the second marketplace again to list at a new price. But for some reason the second marketplace still lists the token at the previous price and is unaware of the transfers happening.
  5. The second marketplace, operating from old information, attempts to again sell the token at the old price.

Note that while this describes an honest mistake, the possibility of such a bug can also be taken advantage of by malicious parties via front-running.

To avoid this possibility, the MT contract generates a unique approval ID each time it approves an account. Then when calling mt_transfer, mt_transfer_call, mt_batch_transfer, or mt_batch_transfer_call the approved account passes approval_id or approval_ids with this value to make sure the underlying state of the token(s) hasn't changed from what the approved account expects.

Keeping with the example above, say the initial approval of the second marketplace generated the following approved_account_ids data:

{
  "approval_owner_id": "alice.near",
  "approved_account_ids": {
    "marketplace_1.near": {
      "approval_id": 1,
      "amount": "100",
    },
    "marketplace_2.near": 2,
    "approval_id": 2,
     "amount": "50",
  }
}

But after the transfers and re-approval described above, the token might have approved_account_ids as:

{
  "approval_owner_id": "alice.near",
  "approved_account_ids": {
    "marketplace_2.near": {
      "approval_id": 3,
      "amount": "50",
    }
  }
}

The marketplace then tries to call mt_transfer, passing outdated information:

# oops!
near call mt-contract.near mt_transfer '{"account_id": "someacct", "amount":"50", "approval_id": 2 }'

Interface​

The MT contract must implement the following methods:

/******************/
/* CHANGE METHODS */
/******************/

// Add an approved account for a specific set of tokens.
//
// Requirements
// * Caller of the method must attach a deposit of at least 1 yoctoⓃ for
//   security purposes
// * Contract MAY require caller to attach larger deposit, to cover cost of
//   storing approver data
// * Contract MUST panic if called by someone other than token owner
// * Contract MUST panic if addition would cause `mt_revoke_all` to exceed
//   single-block gas limit. See below for more info.
// * Contract MUST increment approval ID even if re-approving an account
// * If successfully approved or if had already been approved, and if `msg` is
//   present, contract MUST call `mt_on_approve` on `account_id`. See
//   `mt_on_approve` description below for details.
//
// Arguments:
// * `token_ids`: the token ids for which to add an approval
// * `account_id`: the account to add to `approved_account_ids`
// * `amounts`: the number of tokens to approve for transfer, wrapped in quotes and treated
//    like an array of string, although the numbers will be stored as an array of
//    unsigned integer with 128 bits.  

// * `msg`: optional string to be passed to `mt_on_approve`
//
// Returns void, if no `msg` given. Otherwise, returns promise call to
// `mt_on_approve`, which can resolve with whatever it wants.
function mt_approve(
  token_ids: [string],
  amounts: [string],
  account_id: string,
  msg: string|null,
): void|Promise<any> {}

// Revoke an approved account for a specific token.
//
// Requirements
// * Caller of the method must attach a deposit of 1 yoctoⓃ for security
//   purposes
// * If contract requires >1yN deposit on `mt_approve`, contract
//   MUST refund associated storage deposit when owner revokes approval
// * Contract MUST panic if called by someone other than token owner
//
// Arguments:
// * `token_ids`: the token for which to revoke approved_account_ids
// * `account_id`: the account to remove from `approvals`
function mt_revoke(
  token_ids: [string],
  account_id: string
) {}

// Revoke all approved accounts for a specific token.
//
// Requirements
// * Caller of the method must attach a deposit of 1 yoctoⓃ for security
//   purposes
// * If contract requires >1yN deposit on `mt_approve`, contract
//   MUST refund all associated storage deposit when owner revokes approved_account_ids
// * Contract MUST panic if called by someone other than token owner
//
// Arguments:
// * `token_ids`: the token ids with approved_account_ids to revoke
function mt_revoke_all(token_ids: [string]) {}

/****************/
/* VIEW METHODS */
/****************/

// Check if tokens are approved for transfer by a given account, optionally
// checking an approval_id
//
// Requirements:
// * Contract MUST panic if `approval_ids` is not null and the length of
//   `approval_ids` is not equal to `token_ids`
//
// Arguments:
// * `token_ids`: the tokens for which to check an approval
// * `approved_account_id`: the account to check the existence of in `approved_account_ids`
// * `amounts`: specify the positionally corresponding amount for the `token_id`
//    that at least must be approved. The number of tokens to approve for transfer,
//    wrapped in quotes and treated like an array of string, although the numbers will be
//    stored as an array of unsigned integer with 128 bits.
// * `approval_ids`: an optional array of approval IDs to check against
//    current approval IDs for given account and `token_ids`.
//
// Returns:
// if `approval_ids` is given, `true` if `approved_account_id` is approved with given `approval_id` 
// and has at least the amount specified approved  otherwise, `true` if `approved_account_id` 
// is in list of approved accounts and has at least the amount specified approved
// finally it returns false for all other states
function mt_is_approved(
  token_ids: [string],
  approved_account_id: string,
  amounts: [string],
  approval_ids: number[]|null
): boolean {}

// Get a the list of approvals for a given token_id and account_id 
//
// Arguments:
// * `token_id`: the token for which to check an approval
// * `account_id`: the account to retrieve approvals for 
//
// Returns a TokenApproval object, as described in Approval Management standard
function mt_token_approval(
  token_id: string,
  account_id: string,
): TokenApproval {}


// Get a list of all approvals for a given token_id 
//
// Arguments:
// * `from_index`: a string representing an unsigned 128-bit integer,
//    representing the starting index of tokens to return
// * `limit`: the maximum number of tokens to return
//
// Returns an array of TokenApproval objects, as described in Approval Management standard, and an empty array if there are no approvals
function mt_token_approvals(
  token_id: string,
  from_index: string|null, // default: "0"
  limit: number|null,
): TokenApproval[] {}

Why must mt_approve panic if mt_revoke_all would fail later?​

In the description of mt_approve above, it states:

Contract MUST panic if addition would cause `mt_revoke_all` to exceed
single-block gas limit.

What does this mean?

First, it's useful to understand what we mean by "single-block gas limit". This refers to the hard cap on gas per block at the protocol layer. This number will increase over time.

Removing data from a contract uses gas, so if an MT had a large enough number of approvals, mt_revoke_all would fail, because calling it would exceed the maximum gas.

Contracts must prevent this by capping the number of approvals for a given token. However, it is up to contract authors to determine a sensible cap for their contract (and the single block gas limit at the time they deploy). Since contract implementations can vary, some implementations will be able to support a larger number of approvals than others, even with the same maximum gas per block.

Contract authors may choose to set a cap of something small and safe like 10 approvals, or they could dynamically calculate whether a new approval would break future calls to mt_revoke_all. But every contract MUST ensure that they never break the functionality of mt_revoke_all.

Approved Account Contract Interface​

If a contract that gets approved to transfer MTs wants to, it can implement mt_on_approve to update its own state when granted approval for a token:

// Respond to notification that contract has been granted approval for a token.
//
// Notes
// * Contract knows the token contract ID from `predecessor_account_id`
//
// Arguments:
// * `token_ids`: the token_ids to which this contract has been granted approval
// * `amounts`: the ositionally corresponding amount for the token_id 
//    that at must be approved. The number of tokens to approve for transfer, 
//    wrapped in quotes and treated like an array of string, although the numbers will be 
//    stored as an array of unsigned integer with 128 bits.  
// * `owner_id`: the owner of the token
// * `approval_ids`: the approval ID stored by NFT contract for this approval.
//    Expected to be a number within the 2^53 limit representable by JSON.
// * `msg`: specifies information needed by the approved contract in order to
//    handle the approval. Can indicate both a function to call and the
//    parameters to pass to that function.
function mt_on_approve(
  token_ids: [TokenId],
  amounts: [string],
  owner_id: string,
  approval_ids: [number],
  msg: string,
) {}

Note that the MT contract will fire-and-forget this call, ignoring any return values or errors generated. This means that even if the approved account does not have a contract or does not implement mt_on_approve, the approval will still work correctly from the point of view of the MT contract.

Further note that there is no parallel mt_on_revoke when revoking either a single approval or when revoking all. This is partially because scheduling many mt_on_revoke calls when revoking all approvals could incur prohibitive gas fees. Apps and contracts which cache MT approvals can therefore not rely on having up-to-date information, and should periodically refresh their caches. Since this will be the necessary reality for dealing with mt_revoke_all, there is no reason to complicate mt_revoke with an mt_on_revoke call.

No incurred cost for core MT behavior​

MT contracts should be implemented in a way to avoid extra gas fees for serialization & deserialization of approved_account_ids for calls to mt_* methods other than mt_tokens. See near-contract-standards implementation of ft_metadata using LazyOption as a reference example.