Blockchain Last-Minute Revision

30 quick revision notes for Blockchain

1. Each block stores the previous block's hash, so changing old data breaks every later link and is easy to detect.
2. Bitcoin uses the UTXO model while Ethereum uses the account model with balances and nonces per account.
3. Bitcoin targets ten-minute blocks, adjusts difficulty every 2016 blocks and pays 3.125 BTC per block after the 2024 halving.
4. Ethereum moved to Proof of Stake in the Merge in September 2022 and produces a block every 12 seconds.
5. An Ethereum epoch has 32 slots, and checkpoints finalise after about two epochs, roughly 13 minutes.
6. Slashing punishes double proposals and conflicting attestations; simply being offline only causes small inactivity penalties.
7. BFT protocols need 3f+1 nodes to tolerate f malicious nodes and give immediate deterministic finality.
8. An Ethereum address is the last 20 bytes of the Keccak-256 hash of the public key derived from the private key.
9. Ethereum uses Keccak-256, which differs in padding from the final NIST SHA3-256 standard.
10. A Merkle proof needs only about log2(n) hashes to prove that an item belongs to a set of n items.
11. BIP-39 turns entropy into a seed phrase and BIP-32 and BIP-44 derive many accounts from that single seed.
12. EIP-1559 splits fees into a burned base fee and a priority tip, with the base fee changing up to 12.5 percent per block.
13. A simple ETH transfer costs 21,000 gas and writing a new storage slot costs about 20,000 gas.
14. A function selector is the first four bytes of the Keccak-256 hash of the function signature.
15. delegatecall runs another contract's code using the caller's storage, balance and msg.sender, which is how proxies work.
16. CREATE2 addresses depend on the deployer, a salt and the init code hash, so they are known before deployment.
17. EIP-4844 in March 2024 added blob transactions, cutting Layer 2 data costs, and blobs are pruned after about 18 days.
18. EIP-7702 from the Pectra upgrade lets an externally owned account delegate to smart contract code while keeping its address.
19. Optimistic rollups use a challenge window of about seven days, while zk-rollups prove each batch with validity proofs.
20. Solidity 0.8 reverts on overflow by default, and unchecked blocks disable these checks only where overflow is impossible.
21. The EIP-170 limit caps deployed contract runtime code at 24,576 bytes, so large contracts must be split.
22. Use msg.sender, never tx.origin, for authorisation, because tx.origin enables phishing attacks through malicious contracts.
23. Checks-effects-interactions means validating, then updating state, then making external calls to prevent reentrancy.
24. Private variables are still readable by anyone from storage, so never store secrets in a contract.
25. Block values such as timestamp and prevrandao are not safe randomness; use a VRF or commit-reveal instead.
26. Spot prices from one DEX pool can be manipulated with flash loans, so use robust oracles or TWAPs.
27. A constant product AMM keeps x times y equal to k, and impermanent loss is about 5.7 percent when price doubles.
28. A health factor below 1 in a lending protocol means the position can be liquidated.
29. Upgradeable contracts use initialisers instead of constructors and must keep storage layout append-only.
30. In India, gains from virtual digital assets are taxed at 30 percent, with 1 percent TDS on transfers.

Night-before checklist

• Revise your saved questions on this page once, aloud.
• Read the quick notes and mark any you cannot explain in one line.
• Rehearse a two-minute self-introduction and two project stories.
• Keep CV, ID, laptop charger and interview link ready; sleep on time.