The Blockchain (Three Properties)

Why It Matters

Strip away the markets, the jargon, and the hype, and the casebook’s definition is three properties: readable by all (anyone can inspect the ledger), writable by all (anyone can submit to it), and tamper-proof for all (no one — not even a majority of insiders — can quietly rewrite it). Micali’s image: blocks encased in bulletproof glass, chained in order. You can look, you can add, you cannot reach in and edit.

The three properties are also the test for whether you need a blockchain at all. Drop “writable by all” and a database with backups is cheaper. Drop “readable by all” and you’ve built an intranet. The technology only earns its cost when all three are required at once.

How It Works

Beginner

Think of a public guestbook in a town square, written in permanent ink, with every page numbered and laminated as soon as it’s full. Anyone may read it, anyone may add a line, and because each new page begins by quoting a fingerprint of the previous page, tearing out or altering an old page is instantly obvious to everyone. The order of entries — who signed first — is part of what’s preserved.

Intermediate

Each property has a distinct mechanism. Readable: every node holds the full ledger; transparency is the default, not a feature. Writable: anyone can broadcast a transaction; distributed consensus — not an administrator — decides what gets appended. Tamper-proof: each block embeds the hash of its predecessor, so editing history breaks every later link, in public.

The casebook then maps properties to uses: notarization and storage (a house title that can’t be quietly altered), ordering and timestamping (proving who was first — patents, priority disputes), and disintermediation (executing agreements with no trusted middleman — the road to smart contracts).

Builder

Engineering caveats behind each property. Readable: full transparency is why privacy needs separate machinery. Writable: permissionless write access is rate-limited by fees and block space — “writable by all” means no gatekeeper, not free. Tamper-proof: hash-chaining makes history tamper-evident; making it tamper-resistant is consensus’s job, and the guarantee is economic (cost of a reorg) rather than absolute — see 51% attack. Permissioned ledgers deliberately relax “writable by all” and inherit a different trust model entirely.

Examples

Bitcoin — The original instantiation of all three properties.
Notarization — Land registries and document-timestamping pilots.
Ordering — Priority proofs; first-to-file disputes resolved by chain position.
Disintermediation — From simple transfers to full smart contracts.

Tradeoffs

Strengths

A clean mental model — three properties cover what blockchains actually guarantee, no metaphysics required.
A built-in use-case filter — if an application doesn’t need all three, simpler tools win.
Composable foundation — notarization, ordering, and disintermediation are the primitives everything else builds on.

Limitations

Transparency is total — “readable by all” includes adversaries, competitors, and chain-analysis firms.
Tamper-proofing is economic — bulletproof glass is bought per-block with energy or stake, and is only as thick as the consensus securing it.
Immutable includes mistakes — wrong addresses, exploits, and illegal content are preserved with equal fidelity.

Hash Functions — The chaining that makes tampering evident
Merkle Trees — Compressing block contents into the header
Distributed Consensus — Who gets to write
Smart Contracts — Disintermediation, generalized

Sources & Last Updated

MIT BLC Module 2: Maintaining Blockchain Integrity (primary source; Micali lecture)
Vault note: The Blockchain (Three Properties) (M2 cluster)

Last updated: June 10, 2026