In Indonesia (and many other countries), real estate market is often inefficient.
For example, potential buyers are often faced with candidate properties that are presented
in not transparent ways: hidden address, intentionally blurry pictures to make it unsearchable
without contacting the agent, ambiguous or inaccurate description like "near place X" without
objective data. This often makes potential buyers difficult to decide whether the candidate property
is the right one. Also as a side effect of this non-transparent mechanics, the industry cannot have
clear visibility of historical sales/rents of properties in the country/locality, making it difficult
for property owners to value the properties accurately, again leading to inefficiencies in the market
as evident by properties usually sit for at least a year at the wrong pricing before getting a
closing/deal at the right price.

One of the underlying causes of this inefficiencies is in the way the game is played. An agent/broker
isn't usually able to get a property owner's confidence in being an exclusive reprensentative in
the selling effort. It usually ends up being multiple agents/brokers trying to do the job of the
owner's side (e.g. creating quality listing, photos, and other marketing materials). Counter
intuitively, multiple agents representing the owner leads to lower quality effort because each agent
thinks that spending a lot of effort is a losing strategy, because there is a high chance that
the future sale is made not being through that agent.

In the current mechanics, all stakeholders suffer:
* Potential buyers have difficulty in getting a clear list of options.
* Property owners don't have confidence in pricing their properties and having to wait for years
  in the market.
* Agents/brokers can't serve owners well due to lack of protection of their hard work reward.
* Agents/brokers can't serve potential buyers well due to the game that naturally forces them to
  hide more information rather than be transparent.
* The public not directly involved in the market, including the authority, have little transparency
  of the real estate market condition making future decision hard (e.g. making public policies).

We are introducing a system (or ecosystem) where the all the parties will benefit from.
The end goal of this system is:
* Potential buyers have a clear list of options of properties for sale/rent.
* Property owners have confidence in valuing their properties leading to quick turnover.
* Agents/brokers who represent property owners should feel protected of their effort. This means
  that the system should encourage exclusive agent representation of a property owner.
* Agents/brokers who help buyers should be protected that they should get their reward without
  resorting to intentionally hiding information from the potential buyers. This means that the system
  should have a fair and reliable way in determining that an agent (or actually anyone/anything)
  is the one who refers a buyer to a seller in a particular sale.
* The system should make it possible for the public to get clear visibility of real estate transactions
  that happened while still protecting private information (like phone numbers).

Component 1: Listing Registry

One of the components to support the system is a Listing Registry. Unlike existing property
marketplaces, this registry's role in the ecosystem should only be maintaining quality and moderated
listings that are shared in the ecosystem. The registry keeps which properties are currently in the
market (clearly identified without ambiguity, meaning addresses are transparent) and the corresponding
representatives (which may be the owner directly or an agent/broker).

To summarize, the listings in this registry:
* Are unique (there is no duplicate listings referring to the same property)
* Are each represented by a single entity, which may be the owner or an agent.
* Are publicly available, meaning no entity is monopilizing this information.
* Contain open agreement about the reward for the referrer of future closing (sale/rent). This motivates
  marketers to find the right buyers/renters.

Technically, to achieve the above, we propose the usage of blockchain technology to keep track of that information.
In order for the information to be public but also privacy protected, here is what should be available in blockchain:

* List of Listings
* For each Listing, the basic information (e.g. location/city) and the off-chain link to the detailed
  spec.
* In the detailed spec, it contains the industry-standard specs (e.g. building size, number of bedrooms)
  and the encrypted contact number of the representative. To protect privacy (e.g. from unauthorized
  collection of phone numbers) the contact number here is encrypted and only the Listing Manager
  has the key to decrypt it.
* Each interesting event should be emitted, e.g. "new Listing added", "Listing updated", "Listing
  becoming verified", "Listing rejected", "Listing removed", "Listing closing".
* Each Listing should contain the Unlock Service that is an off-chain service to unlock the encrypted
  contact number. The detail of this is elaborated in Referrral Tracking section below.

New Listing submissions and information update can be done by any Listing Manager (third-party) that
pays the blockchain network cost.
The Listing Registry controls which listings are verified and should be active at a time.

To highlight the design decision of choosing blockchain over traditional database + API, here
are the benefits that support our goals:
* Listings are publicly available and always will be. There is no way that the Listing Registry
  betrays the mission and turns it into a closed monopolized data, due to the nature of blockchain
  immutability.
* Blockchain is highly reliable due to its decentralized nature. This is very important as the Listing
  Registry should be the "operating system" of the industry and has low tolerance to downtime.
  Being available in blockchain makes it easy for everybody, especially marketers to sync data
  with the Listing Registry.

Component 2: Marketers

The listing registry alone is not useful without entities who look them up and do active effort
in finding matching buyers/renters. These entities are called Marketers.

Marketers are incentivized by the reward agreement that the Listing representatives explicitly state
in the Listing Registry. We propose the standard compensation of 0.5% for sale and 1% for rent.

Unlike the traditional ecosystem where marketers have to prove their involvement by becoming an intermediary
(forcing them to hide information rather than be transparent),
our proposed system is able to determine the Marketers involvement as being referrers and maintaining
high transparency to potential buyers. The Referral Tracking system is transparent, verifiable, and privacy-protected.

Component 3: Referral Tracking System

Marketer registration:
* A Marketer registers themself to the blockchain contract. Once registered, it should contain
  the information about the Marketer:
  * Name: can be agent/broker name if individual, or company name if a business entity.
  * Contact Number: This is publicly available, so this should be business phone number instead
    of personal.
  * The public key: This will be used for cryptographic purposes, for example verification of
    signed messages.
* The Marketer pays only the blockchain network cost and no registration cost.

The registered Marketer may participate in the Verifiable Transparent Log for referral tracking:
* The Marketer displays the Listing to a potential buyer/renter. The Listing is sourced from Listing
  Registry and the Marketer has its own way of reaching the buyer, for example by becoming a search
  engine, on online content creator, a personal real estate broker, or even just a friend or
  acquintance sending a link to a potential buyer.
* The contact number of the Listing is not shown, but the potential buyer can unlock it at real time
  if they are interested to proceed to continue the interaction with the Listing representative.
* When the potential buyer clicks the "Unlock Contact", it should open the Listing Registry's
  page that displays the interface. The interface authenticates the potential buyer's phone number
  and present the user confirmation that they (the owner of that authenticated phone number) is
  unlocking the contact number of that Listing through the mentioned Marketer.
* Once the potential buyer confirms, the Listing Registry unlock interface returns a signed log
  that the Marketer should then post to the blockchain. This unlock log entry in the blockchain
  contains:
  * The sha256 hash of the Marketer's random number + phone number ID (assigned by Listing Registry as blinding factor) + phone number.
    The purpose of the Marketer's random number is so that this log appears unique in the blockchain log
    and does not repeat the hash of another log with the same phone number (the log is public so this
    is important to protect privacy). And also the Marketer's
    random number will be revealed by the Marketer to proof that this log corresponds to the phone
    number of the buyer when closing happens in the future.
  * The Marketer id
  * The listing id
  * The unlocked contact number, this time encrypted using the Marketer's public key so that the Marketer can decrypt it
    using their private key.
  * The timestamp
* Once the Marketer posts this to blockchain, they are sure that their involvement is recorded and
  cannot be cheated. They should then display the decrypted contact number to the potential buyer
  to continue interaction with Listing representative.

At closing:
* Closing is reported by either Listing representative, the buyer/renter, or a third party.
* Once the Listing Registry confirms the truth, the closing is logged to the blockchain containing:
  * The hash of phone number ID + phone number as the buyer identity
  * The closing amount: publicly available.
* Once this appears in the blockchain, an event is emitted so that Marketers may check whether
  they are involved in the referral. The Marketer then checks their log if there is a match
  with their Unlock log and only the Marketer can make this confirmation because of the random
  number that they attached to the Unlock log.
* If found, the Marketer then posts a Claim to the blockchain by mentioning:
  * The Unlock log id
  * The random number prefix: will be checked to verify whether they have the random number that
    when hashed with phone number id + phone number of the potential buyer is equal to the
    Closing log's buyer/renter.
* After given some time, all Claims are already cryptographically confirmed to be true without
  revealing private information.
* Listing Registry then proceeds to send reward obligation document to the Listing representative
  and the Claimers.

Conclusion

The mechanics above provide a way so that each participant is incentivized to do their roles
the best ways with maintaining transparency, ensuring efficiency, and providing useful public
information to the public that over time will strengthen the industry.