A Turtle, a Blockchain, and a Lesson in Consensus

2025/06/25

I was about to write that the turtle I met this morning by the lake reminded me of my progress learning crypto. But then I realized:

1. The turtle actually knew exactly what it was doing - it was on a clear mission to cross a dangerous pedestrian path swarming with predator-grade humans.
2. When I got close, it bolted so quickly that I barely managed to snap a photo.

My journey doesn’t look like that at all: no clear direction, no fire under me (i.e., no real sense of urgency or specific motivation).
But let’s carry on with the questions I raised here.

Blockchain: The Foundation Beneath It All

The blockchain is the backbone of crypto - absolutely everything is built on top of it. At its core, it’s a list of blocks. Each block contains:

• Some data (like a transaction),
• A hash of that data (a kind of fingerprint),
• And the hash of the previous block in the chain.

A hash is a unique fingerprint for that block. When you apply a known public hash function to a block’s data, it always gives the same result. And this is the cornerstone of crypto: you can’t fake or quietly swap one block for another. For example:

1. I send $100 to my neighbor to cover beer. That gets recorded in Block 1, along with its Hash 1.
2. My neighbor immediately sends that $100 to his wife to buy a lamb pastry - this becomes Block 2, which includes both its own data and Hash 1 from Block 1.

All of this is duplicated across the network - every computer running this blockchain sees the same story.

The Consequences of Immutability

This setup leads to two big consequences:

1. Since the blockchain is public, I can see that my neighbor didn’t buy beer - he sent the money to his wife instead.
2. But I can’t do anything about it. If I regret sending the money and try to quietly edit my Block 1 to say I invested it on Bybit instead - tough luck. The hash of my Block 1 would change, and it wouldn’t match what’s already out there in the rest of the network. That would make my version of the blockchain invalid.

Unless I control 51% of the network’s power — either computing (in Bitcoin) or staked coins (in Ethereum) - and can rewrite history by force. That’s what’s known as a 51% attack.

A Note on Ethereum

In Ethereum’s Proof-of-Stake system, validators sign the blocks they propose. This adds an identity layer to block creation - if a validator misbehaves, they can be punished. But the hash structure still secures the data’s integrity across the chain.

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