How Does Bitcoin Work?
Bitcoin is the original cryptocurrency and the largest digital asset by market capitalization (~$1.4 trillion). It operates a proof-of-work blockchain secured by over 1 ZH/s of hashrate, making it the most computationally secure network on Earth. Bitcoin's B+ risk grade reflects 17 years of continuous operation without a consensus-level exploit, the simplest mechanism profile of any major blockchain, no oracle dependencies, and fully decentralized governance with no admin keys or freeze capabilities. The primary risk drivers are scale exposure (largest crypto asset, high-value target) and emerging concerns around mining centralization and the long-term security budget as block rewards halve. Its A- store-of-value grade reflects a perfect scarcity score (21M hard cap), unmatched liquidity depth, and institutional adoption via spot ETFs, offset only by continued price volatility relative to traditional stores of value.
TVL
—
Sector
L1
Risk Grade
B
Value Grade
A-
Core Mechanisms
5.1.1
SHA-256 Proof-of-Work (Nakamoto consensus) with difficulty adjustment every 2,016 blocks (~2 weeks)
The original PoW implementation, live since January 2009. Hashrate exceeds 1 ZH/s. Nakamoto consensus is the most battle-tested consensus mechanism in crypto. All other PoW chains are derivatives.
1.1.2
Halving emission schedule — block reward halves every 210,000 blocks (~4 years), from 50 BTC (2009) to current 3.125 BTC (2024). Next halving ~March 2028 to 1.5625 BTC.
Hardcoded in consensus rules. The most predictable supply schedule in crypto, reaching 21M hard cap by ~2140. Currently ~19.8M BTC mined (94.3% of total supply).
1.3.1
UTXO-based transaction model with dynamic fee market based on mempool congestion and block weight
The original UTXO model. SegWit (2017) introduced witness discount, Taproot (2021) improved efficiency and privacy. Ordinals/BRC-20/Runes created new demand for block space since 2023.
5.2.1
Difficulty adjustment algorithm — recalibrates every 2,016 blocks to maintain ~10-minute block times regardless of hashrate changes
Smooth adjustment prevents hashrate oscillation. Proven resilient through major hashrate disruptions including China's 2021 mining ban (~50% hashrate drop recovered within months).
7.1.1
BIP soft-fork governance — consensus changes via Bitcoin Improvement Proposals with miner signaling, node adoption, and rough social consensus
Intentionally conservative governance. Major upgrades (SegWit 2017, Taproot 2021) take years of deliberation. No admin keys, no multisig control, no foundation with override power. The most decentralized governance in crypto.
How the Pieces Interact
As block rewards decline through successive halvings, miners must rely increasingly on transaction fees. If fee revenue does not scale to replace lost subsidies (~$450M/year at current prices post-2028 halving), hashrate may decline, reducing network security. The 2024 halving was absorbed due to price appreciation and Ordinals fee revenue, but this is not guaranteed for future halvings.
Pool operators controlling >60% of hashrate collectively have power over transaction selection. While individual miners can switch pools, inertia and convenience create structural concentration. Stratum V2 separates transaction selection from hashrate aggregation but adoption remains early (Ocean Pool, DEMAND).
A cryptographically relevant quantum computer (millions of stable qubits) could derive private keys from exposed public keys using Shor's algorithm. ~25% of Bitcoin supply (~6-7M BTC, ~$470B) sits in vulnerable P2PK and reused P2PKH addresses. Consensus timeline: experts estimate 50%+ probability of CRQCs by 2030-2035. BIP-360 post-quantum migration is in early proposal stage.
~4MB block weight cap limits throughput to ~7 TPS. Demand spikes (Ordinals, BRC-20) cause fee surges that price out small-value transactions and make dust UTXOs uneconomical to spend. Lightning Network scales throughput but diverts fee revenue from base layer, potentially worsening the security budget concern.
Contentious protocol upgrades can fragment the mining community, as seen in the 2017 Bitcoin/Bitcoin Cash fork. The conservative governance process means critical upgrades (like post-quantum cryptography) may take years to activate, creating a window of vulnerability if quantum computing advances faster than expected.
What Could Go Wrong
- Bitcoin's security budget depends on block rewards that halve every four years. After the April 2024 halving cut rewards to 3.125 BTC/block, transaction fees cover only 5-15% of miner revenue. The next halving (~March 2028) drops rewards to 1.5625 BTC. If fee revenue does not scale to replace lost subsidies, hashrate may decline, reducing the cost of a 51% attack.
- Mining pool centralization: Foundry USA, AntPool, and F2Pool collectively control over 60% of Bitcoin's hashrate. While individual miners can switch pools, concentrated pool operators have significant power over transaction ordering and inclusion, creating censorship risk. Stratum V2 adoption is still nascent.
- Quantum computing poses a long-term threat to Bitcoin's ECDSA signatures. Approximately 25% of all Bitcoin (~6-7 million BTC) is in addresses with exposed public keys (P2PK and reused P2PKH), vulnerable to a cryptographically relevant quantum computer. BIP-360 post-quantum upgrade proposals are in progress but require consensus activation.
- Bitcoin's base layer processes ~7 transactions per second with 10-minute block times. During demand spikes, fees can surge to $50+ per transaction. Layer 2 solutions like the Lightning Network address throughput but introduce their own trust assumptions and may redirect fee revenue away from base-layer miners.
Security Budget Exhaustion via Halving Cascade
TailTrigger: Post-2028 halving (reward drops to 1.5625 BTC), if average daily fee revenue remains below 50 BTC/day AND BTC price does not compensate sufficiently (e.g., stays below $100K).
- 1.Block reward drops to 1.5625 BTC at the ~March 2028 halving — Total daily miner revenue (subsidy + fees) falls below operational costs for marginal miners
- 2.Unprofitable miners shut down, hashrate declines 20-40% — Difficulty adjusts downward after 2,016 blocks, but during adjustment period block times lengthen to 12-15 minutes
- 3.Reduced hashrate lowers the cost of a 51% attack proportionally — Attack cost drops from $50M+/day to potentially $20-30M/day, making state-level or well-funded attacks theoretically feasible
- 4.Market perceives reduced security guarantees — Exchange confirmation requirements increase, settlement finality degrades, institutional confidence weakens
Risk Profile at a Glance
Overall: B (21/100)
Lower score = safer