Blockchain in fundamentals

1. Overview and History

What is Blockchain?
Blockchain is a digital ledger that records transactions in a decentralized, tamper-resistant way. Transactions are grouped into blocks, which are linked in chronological order using cryptographic hashes. Many nodes keep synchronized copies of the ledger, so no central authority is needed.

Evolution

1980s–2000s: Early work on securing digital records with cryptography. In 1991, Haber and Stornetta proposed a cryptographically secured chain of blocks for timestamping documents.
2008–2009: Satoshi Nakamoto introduced Bitcoin, the first practical blockchain application. It solved the "double-spending" problem without a trusted third party.
2015: Ethereum introduced smart contracts, moving from blockchain 1.0 (cryptocurrency) to 2.0 (decentralized apps).
2017–present: Layer 2 scaling, Proof of Stake, and interoperable platforms such as Polkadot and Cosmos.

Key Milestones

A dedicated section covers the headline events:

Bitcoin (2009): The first decentralized cryptocurrency, using Proof of Work consensus and introducing the idea of trustless digital money.
Ethereum (2015): A more flexible platform for smart contracts. Its currency, Ether (ETH), pays for transactions and computation on the network.
2017, ICO boom: Many projects raised money by selling tokens on Ethereum.
2018, DeFi emerges: Financial services began to appear without traditional intermediaries.
2020, NFTs and Layer 2 scaling: NFTs brought blockchain into mainstream culture, while the Lightning Network and Optimistic Rollups addressed scalability.
2021, Ethereum 2.0 and Proof of Stake: Upgrades aimed to improve scalability, energy efficiency, and security by moving from Proof of Work to Proof of Stake. As a note beyond the slides, the actual switch to Proof of Stake was completed in September 2022.

Blockchain vs. Traditional Databases

Point	Traditional Database	Blockchain	Example
Centralization	Managed by a central authority, such as a company or government	Decentralized, with control spread across many nodes	A bank's customer database vs. the Bitcoin network
Data structure	Data organized in tables	Data recorded in blocks linked in a chain	A spreadsheet-style table of accounts vs. a chain of transaction blocks
Immutability	Data can be edited or deleted	Nearly impossible to alter, because a change would affect every later block	An admin correcting a record vs. a Bitcoin transaction that stays permanent
Trust model	Relies on trusted administrators	Trustless, using cryptographic validation and consensus	Trusting a bank to keep your balance correct vs. the network verifying every transaction
Performance and scalability	Usually faster for large transaction volumes	Can struggle because of decentralization and consensus requirements	Visa handles far more transactions per second than Bitcoin

2. Blockchain Fundamentals

Key Components

Blocks: Each block holds a set of transactions, a timestamp, and a hash referencing the previous block. The block header carries metadata, including the hash of the block's own content and the hash of the previous block.
Chains: Because blocks are linked by hashes, altering one block's data would require changing all the blocks after it. This gives the ledger its integrity.
Nodes: Nodes are the computers that take part in the network. Full nodes store the entire blockchain history, and light nodes store only a portion. Nodes validate transactions and reach consensus on the state of the ledger.
Decentralization: There is no single point of control or failure, and transactions are verified by independent nodes. This raises security but can bring slower speeds and higher energy use.

Types of Blockchain

Public (e.g., Bitcoin, Ethereum): Anyone can take part, validate, and view transactions. It uses decentralized consensus such as Proof of Work or Proof of Stake. The pros are high transparency and security from a large number of nodes. The cons are slower transactions and higher energy consumption.
Private (e.g., Hyperledger): Access is restricted and usually managed by a single organization, and users need permission to join and validate. The pros are faster processing and suitability for enterprises. The cons are less decentralization and transparency.
Consortium: A group of organizations controls it instead of a single entity. It suits industry-specific uses such as supply chain management or inter-bank settlements. The pro is a balance between decentralization and control. The con is that it still requires some trust among members.

Real-World Use Cases

Cryptocurrency: The original and most prominent use, enabling digital currency without intermediaries.
DeFi: Blockchain-based banking, lending, and trading without centralized institutions.
Supply chain management: Tracking products through their lifecycle for transparency, origin verification, and quality assurance. IBM Food Trust, which traces food from farm to table, is the example.
Digital identity: Storing and managing identity information in a decentralized way, so individuals control more of their personal data.
NFTs: Digital assets that represent ownership of unique items such as art, music, or in-game items.
Healthcare: Securely managing patient records while protecting data integrity and privacy.