Blockchain & Smart Contracts

Hash chains · Consensus · Smart contracts

1:00

How hash chains, cryptographic signatures, and consensus protocols let mutually distrusting parties share a tamper-evident ledger with no central authority — and how smart contracts extend that with programmable, immutable on-chain logic.

By the end, you can

Explain why a hash-linked chain is tamper-evident and trace how a single edit cascades to all later blocks.
Distinguish what digital signatures prove (authorization) from what they cannot prove (ordering), and explain why consensus is necessary.
Describe how proof-of-work mining works (nonce guessing, hash target, longest-chain rule) and what a 51% attack requires.
Explain the proof-of-stake incentive design and why Ethereum's Merge reduced energy use by about 99.95%.
Construct and verify a Merkle inclusion proof, and calculate how many sibling hashes are needed for n transactions.
Define gas, explain why it is required, and describe what happens when a transaction runs out.
Identify the reentrancy bug in a vulnerable withdraw() function and apply the checks-effects-interactions pattern to fix it.
Name the three vertices of the scalability trilemma and give an example of a trade-off.
Decide when blockchain is and is not the right tool, citing the cost of full-network re-execution.

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