The Playground
Every interactive demo from the course, gathered on one page so you can poke at the ideas back-to-back. Hash something, mine a block, forge a key, sign a message, tamper with a chain — the real algorithms, running right in your browser.
Hashing
A one-way fingerprint. Switch between SHA-256 and RIPEMD-160 — Bitcoin uses both — and change a single character to see the avalanche effect.
Private → public keys (secp256k1)
A public key is derived from a private key by elliptic-curve multiplication. Instant forwards; reversing it is the discrete-log problem no computer can solve.
———Key → address pipeline
The real byte-by-byte derivation of a legacy address: public key → SHA-256 → RIPEMD-160 → version byte → checksum → Base58Check.
Digital signatures (ECDSA)
Sign a message with your private key; anyone can verify it against your public key. Change one character and it fails — the guarantee behind every Bitcoin spend.
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——Mining (Proof of Work)
Spin the nonce until the hash starts with enough zeros. Hard to find, trivial to check — the asymmetry that secures the whole ledger.
0000…0…Anatomy of a transaction
No balances — only coins. Spend whole inputs, create outputs, and the fee is whatever’s left over.
Inputs — coins you own
Outputs — where it goes
Under the hood — the raw transaction
Merkle tree
How a block’s transactions are hashed together, pair by pair, up to a single Merkle root. Edit a transaction and watch its path to the root change.
The blockchain (tamper-evident)
Edit a block and every block after it turns red. Crank the difficulty and feel how expensive rewriting history becomes.
Peer-to-peer consensus
Three peers each hold the chain. Tamper with one and it falls out of consensus; re-mine it and it forks; the majority is what the network trusts. Change the same block on two of three to see a 51% attack.
Bitcoin Script simulator
Pick a script template — including 2-of-3 multisig — and step through the stack machine. The signature checks are real ECDSA.
Script — unlocking, then locking
Stack — top highlighted
Lightning payment channel
Send instant off-chain payments between Alice and Bob. The off-chain count climbs without limit while the on-chain count stays at 1 — until you close the channel.
HTLC routing (multi-hop)
Alice pays Carol through Bob with no direct channel. Step through the Hash Time-Locked Contract — the secret stays hidden until Carol reveals it, then cascades back. The hash is a real SHA-256.
Dollar-cost-averaging calculator
Pick a monthly amount and a start date and see what steady buying since 2015 would be worth today — real historical prices, live valuation.
Illustrative, real monthly BTC/USD closes since 2015; today’s value uses the live price. Past performance is not a promise of future results.
The scale of big numbers
Lottery odds, the world economy, grains of sand, atoms — ranked against the 2²⁵⁶ Bitcoin key-space. Tap any row to measure everything else against it.
Tap a row to measure everything against it. Bars are on a logarithmic scale.
Halving & the 21M supply curve
Drag the year: supply climbs toward 21 million as the block reward halves and inflation falls to nothing.
Difficulty adjustment
Change the hashrate and blocks drift off the 10-minute target; re-target (every 2016 blocks) to pull them back.
The fee market
Block space is an auction. Set your fee rate and see if you make the next block; add congestion, mine to clear the top bidders.
The next block — filling with the highest-paying transactions first
RSA key pair
Pick two secret primes; the whole key pair falls out. Encrypt a number with the public key and decrypt it with the private one — real modular arithmetic, tiny primes.
Purchasing power over time
Pick a year and see what $100 saved back then is worth today — as cash, gold or bitcoin.
What $100 saved back then is worth today
Fractional-reserve money multiplier
One $1,000 deposit becomes many times that as it is lent and re-lent — then trigger a bank run.
Multisig vault
Pick an M-of-N policy and toggle which keys you hold — the vault opens only when enough are present.
Annual dilution (stock-to-flow)
How fast new supply is created: gold, the dollar, and Bitcoin halving toward zero.
Confirmation depth
Stack confirmations and watch a transaction get buried — coffee needs zero, a house six.
Money-properties radar
Score gold, fiat, seashells and bitcoin against the properties of money — toggle to compare shapes.