What is the role of cryptography in blockchain technology?

Cryptography gives blockchain its two core properties: proof that data hasn't been altered, and proof that a transaction came from the account that claims to have sent it. Bitcoin and Ethereum both link blocks using SHA-256 or Keccak-256 hashing, where changing a single transaction in an old block changes every hash that follows it, and both rely on ECDSA to sign transactions with a private key that only the account holder controls.

How does cryptography secure the chain itself?

Every block header stores the hash of the previous block, so altering historical data means recalculating every hash after it, which becomes computationally impractical once a chain has enough blocks behind it. Within each block, transactions are organized into a Merkle tree: each transaction is hashed, paired hashes are hashed again, and the process repeats up to a single Merkle root. That structure lets a lightweight client verify one transaction by checking a handful of hashes instead of downloading the entire block.

Where does this show up in practice?

Wallet addresses are derived from a public key generated through elliptic-curve cryptography, using the secp256k1 curve in Bitcoin and Ethereum, and every outgoing transaction carries an ECDSA signature that nodes verify against that public key before accepting it into a block. Smart contract calls, token transfers, and multi-signature wallets all depend on that same signature scheme to confirm the sender holds the private key without ever exposing it.

What doesn't cryptography guarantee?

None of this makes a blockchain application secure by default. Cryptography guarantees that on-chain data hasn't been tampered with and that a signature is valid, but it says nothing about whether a smart contract's logic is correct or whether a private key was stored safely off-chain, which is where most real-world blockchain thefts actually happen.