How we enable in-store purchases using crypto

Background: STACK has partnered with STK to provide a cryptocurrency wallet inside the existing STACK app.

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STK lets you make instant payments at points of sale directly from your cryptocurrency wallet. Now you can add your cryptocurrency wallet alongside your other currency wallets in STACK. When you tap to pay through the STACK app, you can make purchases at any retail location that supports credit or debit cards. STK opens a bridge between the Ethereum blockchain and traditional credit card payment rails.

Almost a decade after the introduction of cryptocurrencies, and despite their promise of immense profit, none of the established payment providers have rolled out a convenient way to pay at stores or online with Bitcoin or Ether. In this article I’m going to explain the challenges of creating a cryptocurrency payment protocol at point of sale, and how we resolved them.

Traditional payment rails

Most of the world’s retail transactions (shopping), as well as banking go through a handful of core payment rails. The best known of these rails are run by major credit card companies. They enable banks and other financial institutions to transmit money to each other, securely and reliably.

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At STK we focused on consumer-to-retail payments, like those you make at grocery stores, convenience stores, restaurants and other retail locations.

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Understanding Blockchains Intuitively, part 2

This is part two of an ongoing set of articles, designed to help you understand the motivations and concepts behind blockchains, in an intuitive and non-technical way. You can read the first part here.

To briefly recap, there is an island in the Pacific called Yap, whose inhabitants, rather than holding their money in their pocket, simply agree amongst each other how much money each person has, and update these records every time a sale or transaction is made. This provides a good analogy for blockchains. In modern blockchains, computers the network connections between them stand in place of the islanders of Yap.


No discussion of blockchains would be complete without explaining Smart Contracts. The most popular Smart Contract blockchain in use right now is Ethereum, a blockchain similar to Bitcoin but with an added twist. To help explain this twist, let’s return once again to our inhabitants of Yap.

Peering into the brain of one of the Yapese we can see a long history of transfers of stone coins. These go in order, starting from the oldest that he or she remembers up till the most recent.

SmartContract (3)

One day, this islander is chatting with a friend of hers named Peter. Peter is worried about the damaging effect of annual hurricanes on his crops. He wishes to buy crop insurance to protect himself in case of disaster. Peter has therefore decided to enter into an insurance contract with another islander, Sumesh. Here’s how that contract goes:

Continue reading “Understanding Blockchains Intuitively, part 2”

Technological disruptions vs societal disruptions

The fairy-tale of technological disruption is a familiar one to us all. It is the recurring parable of our modern society. A plucky entrepreneur, filled with determination and the tiny but invincible seed of a vision, sets out to overturn our assumptions, confronts and overcomes the guardians of the decaying establishment, and brings his new gift to the world. The invention of the iPod and the iPhone, the birth of Google and AirBnB, and countless other entrepreneurial stories are enshrined this way in our culture.

If you’re involved in tech, or even peripheral to it, you’re likely familiar with the idea of technological disruption. Ever since Christensen first wrote about it in The Innovator’s Dilemma, it has been the dominant model used for analyzing new companies and emergent trends.

The pattern is typically as follows: a novel technology facilitates or makes more convenient what was previously tedious and expensive. Germinating in niche or low-end markets, and spreading though unorthodox sales channels, it eventually overtakes established industries and becomes the new norm or mode of life. Uber and AirBnb are poster children of this wave of technological revolution; but by no means the only ones.

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Understanding Blockchains Intuitively, part 1

This is the first part of a two part series. This part explains the basics of the blockchain through an analogy. The second part builds on this and discusses the idea of smart contracts.

There is an island in the Pacific called Yap. The people of this island have a strange and unintuitive type of money.


Their money consists of large stone coins called rai. Many of these coins are taller than a person. They are heavy and difficult to move. Nevertheless, these large stones are what they use as money.*


*In recent decades they have gradually moved away from using rai to using the US dollar, and the stone coins are reserved for ceremonial occasions.

How do you get your hands on, or even spend one of these coins? Your first guess might be that when you get a coin it gets shipped to your house and put in your front yard. That way you can know who owns what coin and, as an added bonus, you can make your neighbours jealous.

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Intuitions in Machine Learning: Bias and overfitting

We are constantly trying to predict the future. Understanding how our actions affect what will happen helps us make better decisions. Since none of us can actually see the future, we use our past experience to make connections between two or more events, then use that to predict what will happen.

For instance, the open screen door seems to be connected to flying bugs coming into my house:

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“Hmmm… When I leave the front door open, bugs tend to fly in.”

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Intuitions in Machine Learning: Understanding ROC Curves

Have you tried to teach yourself Machine Learning online, and got overwhelmed by the first flood of mathematical terms and equations?

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What you get when you look up ROC curves and Confusion Matrices

My goal in these articles is to give you an intuition for the math, by connecting the math to your personal experience. I’ll cover Machine Learning topics through concrete, accessible examples.

I’ll start the series by covering a simple topic: ROC curves*. In this article, I’ll explain what they are, how they work, and why and when to use them. In order to explain ROC curves we’ll also cover these related concepts: Confusion Matrices, sensitivity, and specificity.

(*ROC is short for “Receiver Operating Characteristic”. This term is a holdover from when it was used in Electrical Engineering, and you don’t need to understand it to use it now.)

The Prospector

Let’s start by creating an imagined scenario. You’re a gold prospector back in the Old West. These were entrepreneurs who explored the California frontier looking for gold during the gold rush.

This is you.

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