# ERTP Introduction
ERTP uses 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.
# Creating assets with ERTP
In ERTP, mints create digital assets. Access to an asset type's mint lets you create more digital assets of that type. You can then store new assets in a payment or a purse.
- Payments: Assets you intend to send to someone else.
- Purses: Store assets until you withdraw them into a payment for use
To send assets in ERTP:
- Withdraw them from a purse.
- Put them in a payment.
- Send them to a recipient object as a message.
To receive assets in ERTP:
- Create a purse for the asset type you'll receive.
- Get access to the asset type you'll receive. Note: You do not need access to the type's mint. The Issuer (which is widely shared) provides an empty purse for the asking.
- Receive the message with the payment and put the payment in your purse.
# Security properties
ERTP purse objects have a
deposit message which takes a payment
object as its argument. It first checks that the payment object is
genuine and the same asset type as the purse (after all, quatloos
are a very unstable asset, so we wouldn't want to let people deposit
quatloos and get credit for simoleans.)
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 object is empty.
- The purse contains the payment's full content.
deposit call throws an error (i.e. something went wrong),
- The alleged payment is in the same state as before the call.
- The purse is in the same state as before the call.
# Issuers and mints
Other key ERTP objects are:
Mints: Issue new digital assets as a new Payment. Mints only issue one type of asset (quatloos, simoleons, moola, etc.). We refer to that type as the mint's Brand. So if a mint issues quatloos, it's a quatloo brand mint. Only mints can issue new digital assets. To mint new assets of a particular type, you must have a reference to that type's mint
Issuers: Create empty purses and payments and map minted assets to them when assets are added or removed. Issuers verify and move digital assets.
An issuer's special admin facet is a Mint, and that Mint and Issuer are associated with each other. With a reference to an issuer, you can check the validity of a payment in that issuer's mint's assets, as well as claim it either as a new payment to yourself or a purse you control.
Issuers (i.e. their references) should be gotten from a trusted source and then relied upon as the decider of whether an untrusted payment is valid
Note: There is a one-to-one correspondence between a brand and a mint, a mint and an issuer, and an issuer and a brand. In other words, each mint issues a unique kind of digital asset, say, quatloos, and only that kind of asset.
Amounts describe digital assets without having any value of their own. Anyone can make one, and they can be sent to anyone. They have two parts:
- Brand: An unforgeable object identity for the digital asset's type, such as an object that represents quatloos.
- Extent: How much/many of the asset. Fungible extents are natural numbers. Non-fungible extents are strings or objects representing attributes of the asset (say, a theater ticket's row and seat positions).
Note: fungible means any item in a set can be used. For example, for change for a dollar, any four quarters work. Non-fungible means specific items in a set must be used. For example, theater tickets are not all the same, and it matters if you get third row center or second balcony far left (and affects what you're willing to trade for it).
# Amount Math
Issuers must be able to deposit and withdraw assets from a purse. This
requires being able to add and subtract digital assets. They use a set
amountMath functions, which are aided by
In addition to math operations,
amountMath functions check on their
arguments' brands, throwing an error if the wrong brand was used.
# Next Steps
If Getting Started, you should go to the Introduction to Zoe.
If you've finished the Getting Started material, you should go to the ERTP Guide for a fuller explanation of ERTP concepts, including ones not covered in this Introduction.