# ERTP Guide

ERTP (Electronic Rights Transfer Protocol) is Agoric's token standard for transferring tokens and other digital assets in JavaScript. Using the ERTP API, you can easily create and use digital assets, all of which are transferred exactly the same way and with exactly the same security properties.

ERTP uses OCaps (object capabilities) to enforce access control. If your program has a reference to an object, it can call methods on that object. If it doesn't have a reference, it can't. For more on object capabilities, see Chip Morningstar's post (opens new window).

# ERTP Concepts Overview

# Asset

There are three kinds of assets: fungible, non-fungible, and semi-fungible.

Fungible assets are interchangeable. For example, if you have 100 one-dollar bills and need to pay someone 5 dollars, it doesn't matter which five of your one-dollar bills you give them.

Non-fungible assets have the same brand, but are not interchangeable. For example, you might have 100 theater tickets. But someone wanting to buy even a General Admission ticket from you will want one for a specific date and time. This might also affect the price; you'll want to charge more for a Friday evening ticket than a Wednesday matinee ticket, even if it's for the same show.

Semi-fungible assets have distinct forms which are not interchangeable with each other, but in which instances of a single form may interchangeable with other instances of the same form. For example, theater tickets for a single show might be partitioned into General Admission and Balcony sections, where a holder may sit in any seat of the respective section.

# Amount

Assets are described by Amount records consisting of a brand and a value.

  • Brand: An asset's type. You can think of this as the answer to the question "What is it?" about an asset.
  • Value: An asset's size. You can think of this as the answer to the questions "how many?" or "how much?" about an asset.

Important: Amounts are descriptions of digital assets, not the actual assets. They have no economic scarcity or intrinsic value.

So, using the fictional currency Quatloos, you could have an asset described as being "400 Quatloos", where 400n is the value and Quatloos is the brand. For now, we'll just look at fungible assets whose values have to be non-negative integers represented as BigInts (thus the appended "n" on that value).

# AmountMath

ERTP uses the AmountMath library for operations such as adding, subtracting, and comparing amount values (such as when depositing to or withdrawing assets from a purse).

# Brand

Most ERTP objects have a permanent constraint to working with or on one specific Brand established at their creation. If one is initially associated with Quatloos, it always associated with Quatloos and Quatloos only. In particular, a brand and its mint and its issuer are all in unchangeable respective one-to-one relationships with each other.

  • Mint: The unique creator of digital assets of a particular brand.
  • Issuer: The source of truth of how many digital assets each purse and payment holds. An issuer is used to validate payments received from untrusted parties for the brand with which it is associated.

ERTP object relationships

Let's look at an example. Suppose there is the "Quatloos" brand. That means there is also:

  • A "Quatloos mint" that is the only ERTP mint that can ever create new Quatloos assets.
  • A "Quatloos issuer" that is the only issuer that can create a new purse to contain Quatloos and operate on a payment containing Quatloos.

ERTP object relationships 2

# Purses and Payments

We've already mentioned our final two concepts:

  • Purse: An object for holding digital assets of a specific brand.
  • Payment: An object for transferring digital assets of a specific brand to another party.

Similar to other component instances, a purse and a payment only work with one brand. So a purse or payment that holds Quatloos cannot hold an asset of brand Moola or vice versa. You cannot change the brand a purse or payment was originally associated with. Once you create a Quatloos purse or Quatloos payment, it can never hold anything other than Quatloos.

However, these are not one-to-one relationships. There can be thousands or more purses or payments that hold Quatloos or any other brand.

# Method Naming Structure

ERTP methods use a template for their names. Knowing what a particular method name prefix represents can help you when reading code. For consistency, you may want to also use this template for your Agoric code.

  • make<Foo>(): Creates a new Foo object and returns only that object.
  • make<Foo>Kit(): Creates a new Foo object as well as other things. It returns some combination of useful things, usually including the new Foo object. But not always; sometimes Foo is conceptual, and, for example, instead of a single object, two facets are returned.
  • create<Foo>(): Creates a new Foo, but doesn't return it.
  • get<Foo>(): Returns a Foo that already exists.
  • provide<Foo>(): If Foo already exists, returns it. If not, it creates a new Foo and returns that.

# Life of Assets

Let's look at some asset operation lifecycles. While it's very rare for an asset to be destroyed, as opposed to being redistributed, these lifecycles show assets from their creation through common usage patterns. These are deliberately stripped down to their basic, core, functionality. Optional parameters and non-core operations are not shown, nor are some significant concepts which would make this introduction more confusing. Those are covered on the component-specific pages.

# Asset creation and storage

Asset creation

const {
  issuer: quatloosIssuer,
  mint: quatloosMint,
  brand: quatloosBrand,
} = makeIssuerKit('quatloos');

First, you pass a string naming a new brand to makeIssuerKit(). As noted above, a make<Foo>Kit() method creates both a new Foo, in this case an issuer, and some other things. Here it also creates a new mint and formal brand for the argument, and returns all three new objects. The mint, issuer, and brand are all in unchangeable one-to-one relationships with each other.

In this case, you used the string 'quatloos' to name the brand.

  const quatloosSeven = AmountMath.make(quatloosBrand, 7n);

Here you use AmountMath to make a new amount description of the asset you want to create. You need to specify what you want for the value of the new amount, in this case 7n, as well as what brand it will be.

This returns an amount description stored in quatloosSeven. Remember, an amount is only a description of an asset, not an asset itself. quatloosSeven has no intrinsic value.

  const quatloosPayment = quatloosMint.mintPayment(quatloosSeven);

This mints a new asset of 7 Quatloos. In this case, since it's a mint operation, you are creating a new digital asset of 7 Quatloos. It's returned as a payment, so you want a place to store it for the longer term.

const quatloosPurse = quatloosIssuer.makeEmptyPurse();
quatloosPurse.deposit(quatloosPayment);

For long term storage, we prefer using a purse. payments are generally used to transfer assets rather than hold them for extended periods. First you create a new empty purse for Quatloos using the Quatloos associated issuer. Then you deposit the payment into the purse. When this happens, the payment is automatically consumed and the 7 Quatloos are now resident in the purse. If you'd used an existing purse that contained, say, 17 Quatloos, these 7 would have been added to them so the purse balance would be 24 Quatloos.

# Transferring an asset

Asset transfer Start with your quatloosPurse that holds 7 Quatloos. You decide you want to send 5 Quatloos to another party named Alice.

  const quatloosFive = AmountMath.make(quatloosBrand, 5n);

First you create a new Quatloos branded amount with a value of 5 to describe what you want to withdraw. Remember, an amount is just a description of assets, not the actual assets.

  const myQuatloosPayment = quatloosPurse.withdraw(quatloosFive);

Now you tell your Quatloos containing purse that you want to withdraw the specified amount from it. The withdrawn 5 Quatloos goes into a payment

You've got your payment for 5 Quatloos, but how do you get it to Alice? She needs to do some work first so there's somewhere for her to put it and a way of getting it to her rather than someone else.

  const aliceQuatloosDepositFacet = aliceQuatloosPurse.getDepositFacet();

Assume Alice already has a Quatloos purse of her own. To let other parties safely deposit Quatloos into it, she creates a deposit facet for that purse. Anyone who has access to a deposit facet can deposit assets to its purse but cannot either make a withdrawal from the purse or get its balance. It's like being able to send money to a friend via their email address; you can't then take money out of your friend's accounts or find out how much is in them.

const aliceQuatloosDepositFacetId = await E(board).getId(
  aliceQuatloosDepositFacet,
);

Alice puts her deposit facet on Agoric's Board, a key-value "bulletin board" that lets her make it generally available for use.

The Board is a basic bulletin board type system where users can post an Id for a value and others can get the value just by knowing the Id. Alice can make her Id(s) known by any communication method she likes; private email, an email blast to a mailing list or many individuals, buying an ad on a website, tv program, or newspaper, listing it on her website, etc.

const depositFacet = E(board).getValue(aliceQuatloosDepositFacetId);
await E(depositFacet).receive(myQuatloosPayment);

Remember, ERTP's use of OCaps requires that you have access to an object in order to run methods on it. So someone who wants to use Alice's deposit facet has to first get it off the Board.

Alice tells you the Board Id associated with her Quatloos purse deposit facet. You get the Id associated value, which gives you the reference to that deposit facet. You then just tell the facet to receive your payment of 5 Quatloos.

Things end up with your Quatloos purse having 2 Quatloos (7 - 5), Alice's Quatloos purse having 5 more Quatloos in it, and the 5 Quatloos payment consumed when the transfer happened.

The E() notation is a local "bridge" that lets you invoke methods on remote objects. It takes a local representative (a presence) for a remote object as an argument and sends messages to the remote object. This is explained in more detail at the E() section in the Distributed JavaScript page.

# Creating and using non-fungible assets

Say you own the Agoric Theatre and want to sell tickets to seats for a play. Tickets are non-fungible assets, as they refer to a specific seat for a specific show at a specific time and date. It matters to buyers which ticket they get.

The Agoric Theatre has 1114 seats numbered 1 to 1114. An object representing a valid ticket has the properties:

const startDateString = new Date(2019, 11, 9, 20, 30).toISOString();

const ticketValues = Array(1114)
  .fill()
  .map((_, i) => ({
    seat: i + 1,
    show: 'Hamilton',
    start: startDateString,
  }));

To create tickets, you first create JavaScript objects that each represent a ticket. Then, because you need to specify the amount of digital assets to be minted, you can use AmountMath to make an amount. In this case, you're making tickets for one performance of Hamilton.

const {
  mint: agoricTheatreTicketMint,
  brand: agoricTheatreTicketBrand,
} = makeIssuerKit('Agoric Theater tickets', AssetKind.SET);

As before, you use makeIssuerKit() to create a mint that can create Agoric Theatre ticket assets. The difference from when you created a fungible asset is that you have to use a second argument, in this case AssetKind.SET.

There are two AssetKinds. Each one polymorphically implements the same set of methods.

  • AssetKind.NAT: Works with natural number values and fungible assets. Default value for makeIssuerKit().
  • AssetKind.SET: Used with non-fungible assets, operates on an array of records (objects) with keys and values.
const ticketAmounts = ticketValues.map(ticketValue =>
  AmountMath.make(agoricTheatreTicketBrand, harden([ticketValue])),
);
const agoricTheatreTicketPayments = ticketAmounts.map(ticketAmount =>
  agoricTheatreTicketMint.mintPayment(ticketAmount),
);

First you define an amount description for each ticket you want to issue.

Then you use your mint for the appropriate brand to create an asset for each ticket. Each ticket asset is a separate payment. You can transfer and deposit a non-fungible asset payment just like a fungible one.

# Amounts are not assets

IMPORTANT: Despite how it may seem, an amount is not an asset in and of itself. It merely describes assets along the two axes of what they are and how much there are (brand and value). Amounts are used to negotiate without sending/sharing actual assets until a deal is made.

For example, if I want to make you an offer to buy an asset, let's say a magic sword in a game, I'll send you an amount describing the asset of 5 Quatloos I'm willing to trade for your sword. I don't send you the actual 5 Quatloos; that only happens when we agree on the trade terms and I send you a payment of 5 Quatloos, the actual asset. If you reject my offer, I can change it so the amount I specify is for 10 Quatloos. I haven't added actual assets of 5 Quatloos to what I send you, only the description of assets in the offer I'm making for the sword.

Making a new amount does not create any new assets. Nor does adding two amounts; since an amount is immutable, the addition just creates a new amount while the original two still exist. ERTP assets can only be created by their mint returning a new payment containing them. Since an amount is just a description of an asset, it's like a drawing of a ten dollar bill, while an asset is analogous to an actual ten dollar bill printed by an authorized facility with value derived from its government backing.

In other words, I don't make you an offer that I'll sell you a ticket to Hamilton for $300 by sending you an actual ticket any more than you'd send me $300 before finding out what I'm willing to give you for it. Instead, I make you an offer by sending a description of what I'm willing to swap ("I will exchange a Hamilton ticket for $300"). If the offer is accepted, then I send you the actual asset (enjoy the show!) and you send me the actual $300 (I'll enjoy spending it!). In the Agoric stack, assets of the exchange are escrowed with Zoe.

# Object capabilities and ERTP

ERTP uses object capabilities. You can only use an object and issue commands to it if you have access to that object, not just its human-readable name or similar. For example, I might know (or make a good guess), that the mint that makes Quatloos has the human-understandable alleged name of 'quatloos-mint'. But unless I have the actual mint object associated with the quatloos brand object, I can't use it to create a million Quatloos and bet them all on Captain Kirk to win his gladiatorial match on Triskelion (opens new window).

# Security properties

ERTP purses have a deposit method which takes a payment as its argument. It first checks that the payment is genuine and the same asset brand as the purse

If everything passes the checks, the asset moves from the payment to the purse. If there's a problem, it throws an error.

After a successful deposit, ERTP guarantees:

  • The payment is deleted from its issuer's records and no longer has any assets associated with it.
  • Its issuer no longer recognizes that payment.
  • The purse contains all digital assets that were in the payment.

When the deposit call throws an error (i.e. something went wrong), ERTP guarantees that neither the purse nor the alleged payment are affected by it.

In addition, you can create a deposit facet for any purse. This is an object associated with a specific purse that can be sent to another party instead of a reference to the purse. The security advantage is that the other party can only use the deposit facet to make deposits to the associated purse. They cannot use it to make a withdrawal from or ask about the balance of a purse.

# Promises

Several ERTP methods are asynchronous and instead of immediately returning their expected value, return a promise for that value.

JavaScript implements Promise objects, and recently added the two keywords async and await to simplify working with them. For general, and extensive, information about JavaScript's implementation, see javascript.info (opens new window) or MDN (opens new window).