A trust machine that doesn't need anyone to be trustworthy.
What you will learn
Explain what a blockchain is in one sentence
Understand hashes as digital fingerprints
See why tampering is detectable by design
How blocks link into a tamper-evident chain
How blocks link into a tamper-evident chain
Inside a block
Inside a block
Blockchain in one sentence
A blockchain is a shared, append-only ledger where each new entry cryptographically commits to the one before it — so changing history requires redoing every subsequent entry, which the network's incentives make prohibitively expensive.
The hash: a digital fingerprint
A hash function turns any input into a fixed-length 'fingerprint.' Change one character and the hash changes completely. Hashing a block's contents gives a unique ID that proves exactly what was in it.
💡 Why tampering is obvious
Each block stores the hash of the previous block. If an attacker edits block 2, its hash changes — breaking the link to block 3, which stores the old hash. To hide the edit, they'd have to rewrite blocks 3, 4, 5… all the way to the tip, faster than everyone else adds new blocks. That's the security model.
Consensus: agreeing without a boss
Because no central authority decides the truth, the network must agree on which chain is real. Proof-of-work (compute), proof-of-stake (capital at risk), and other mechanisms align self-interest so honest participants converge on one version.
Incentives are the real security
Blockchains don't work because people are honest — they work because cheating is unprofitable. Miners/validators earn rewards for following the rules and lose money trying to rewrite history. Cryptography provides the lock; incentives provide the guard.
💡 A block, inside
A block contains: a timestamp, a list of transactions, a nonce, the previous block's hash, and its own hash. Together these make the block a sealed, timestamped, unforgeable page in the ledger.
❓ Quick check
Why is tampering with an old block detectable?
A) Blocks are encrypted
B) Each block stores the previous block's hash, so edits break the chain
C) A central authority checks
D) Blocks are very large
The hash linkage makes any edit break all subsequent blocks.
(Knowledge check — full exam is next)
Key takeaways
Blockchain = append-only ledger with cryptographic links