Blockchain Technology Flashcards

A distributed ledger where records (transactions) are grouped into blocks, each cryptographically linked to the previous block via its hash. No single party controls or maintains it. Tamper-evident: changing any block breaks all subsequent blocks.

A record of transactions replicated across many independent nodes simultaneously. No central server holds the authoritative copy — all nodes hold equal copies and continuously verify consistency. Eliminates single points of failure and control.

1. Transaction data (the actual records: who sent what to whom). 2. Timestamp (when the block was created). 3. Hash of the previous block (the cryptographic link that creates the chain). Also contains the block's own hash and (in PoW) a nonce.

Each block includes the hash of the previous block. This links every block to all preceding history. If any past block is modified, its hash changes, invalidating every subsequent block — the tamper is immediately detectable by all nodes comparing hashes.

The first block in a blockchain — it has no "previous block" to reference. All subsequent blocks trace their cryptographic chain back to it. It is the foundation of the entire chain's immutability.

The protocol by which all nodes agree on which blocks are valid without a central authority. Ensures all honest nodes maintain the same version of the ledger. Common types: Proof of Work (PoW) and Proof of Stake (PoS).

Nodes (miners) compete to solve a computational puzzle — finding a nonce that produces a valid hash. The winner adds the next block and earns a reward. Computationally expensive to produce but easy to verify. Security comes from the enormous computational cost of forging history.

Validators are selected to create blocks in proportion to their staked (locked) cryptocurrency. Cheating loses your stake — economic punishment replaces computational difficulty. More energy-efficient than PoW. Used by Ethereum (since 2022) and others.

An attack where one entity controls more than 50% of a blockchain's consensus power (hash rate in PoW, stake in PoS). This majority allows overriding consensus: approving fraudulent transactions, double-spending, or rewriting recent history. Practically infeasible against large networks.

To modify a past block, an attacker must recompute every subsequent block (because hash chaining breaks) AND do this faster than the legitimate network is producing new blocks. Against a large network with massive combined computing power, this race is computationally unwinnable.

A self-executing program stored on a blockchain that automatically enforces agreement terms when conditions are met. No intermediary needed — the code runs automatically and transparently. Example: automatic insurance payout when a flight delay is confirmed by a trusted data feed.

Public blockchain: open to anyone — anyone can join, validate, and view the full ledger (Bitcoin, Ethereum). Private/permissioned blockchain: restricted to known authorized parties — used in enterprise and government settings where full public transparency isn't needed but tamper-resistance is valuable.

In Proof of Work, a number that miners vary when computing a block's hash. Miners increment the nonce until they find a hash that meets the difficulty target (e.g., starts with a required number of zeros). Finding the correct nonce requires massive computation; verifying it requires a single hash check.

Each step in a product's journey is recorded as a blockchain transaction by each handler (manufacturer, shipper, distributor, retailer). The chain of custody is publicly auditable and tamper-evident. Counterfeits cannot be inserted without authorization. Diversion is detectable when the legitimate chain stops unexpectedly.

Spending the same digital currency twice by rewriting transaction history — enabled by a 51% attack. An attacker sends coins, receives goods, then rewrites the blockchain so the original transaction never happened — they keep both the goods and the coins. Prevented by waiting for multiple block confirmations.

1. Distributed: no single point of failure/control. 2. Tamper-evident: hash chaining detects any modification. 3. Transparent: all participants can verify history. 4. Immutable (effectively): changing history requires outcomputing the entire network. 5. Consensus-based: no single authority approves transactions.