For developers and engineers

Under the hood

The numbers behind the machines: vsize, fee rates, retargets, descriptors and attack probabilities.

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Advanced levelThe Life of a Transaction
Read the fee and fee rate straight from inputs, outputs and virtual size.

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Optional challenge Inputs total 137,250 sat, outputs 135,558 sat, and the transaction is 141 vB. What is the fee rate? 10 sat/vB 12 sat/vB 1,692 sat/vB
Advanced levelThe UTXO Machine
Weigh the fee and privacy cost of consolidating coins.

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Optional challenge A transaction weighs 561 weight units. What is its virtual size? 140 vB 141 vB 561 vB
Advanced levelThe Blockspace Market
Use CPFP to pull a stuck parent into the next block.

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Optional challenge A parent of 200 vB pays 400 sat; its child of 100 vB pays 2,600 sat. What is the package fee rate? 2 sat/vB 10 sat/vB 26 sat/vB
Advanced levelThe Difficulty Engine
Compute a retarget, including the 4× clamp.

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Optional challenge The last retarget window took 7 days instead of 14. How does the target change? It halves (difficulty doubles) It doubles (difficulty halves) It is clamped to a quarter
Advanced levelThe Multisignature Vault
See why a multisig backup needs the descriptor, not just the seeds.

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Optional challenge Why must a 2-of-3 multisig backup include the output descriptor? It contains the private keys Rebuilding the addresses needs every cosigner's xpub and the policy, which no single seed holds Nodes require it to validate spends
Advanced levelThe Bitcoin Security Arena
Compare simulated success rates with the exact race odds and the whitepaper's figure.

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Optional challenge Whitepaper §11: an attacker with 10 % of hash rate, the merchant waits 5 blocks. The catch-up probability is about… 0.09 % 0.9 % 9 %

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