Web3 & DApp Development Fundamentals · Lesson

Hashing in Blockchain

Immutability via hashes.

Hashing in Blockchain is a free Web3 & DApp Development Fundamentals lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Web3 & DApp Development Fundamentals learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

What Is a Hash Function?

A hash function takes any input and produces a fixed-size output called a digest or hash.

Blockchains rely on cryptographic hash functions like SHA-256 (Bitcoin) and Keccak-256 (Ethereum).

sha256("hello") =
  2cf24dba5fb0a30e26e83b2ac5b9e29e
  1b161e5c1fa7425e73043362938b9824

Deterministic Output

Hashing is deterministic: the same input always yields the same output.

This lets any node independently verify a hash and arrive at the identical result — essential for distributed agreement.

The Avalanche Effect

A tiny change in the input produces a completely different hash. This is the avalanche effect.

Notice how changing one letter changes the entire digest:

sha256("blockchain") -> ef7797e13d3a...
sha256("Blockchain") -> 3f1d8b9c2a04...
(totally different)

Fixed Output Size

No matter how large the input, the output is always the same length. SHA-256 always returns 256 bits (64 hex characters).

This makes hashes convenient, compact identifiers for blocks, transactions, and state.

Preimage Resistance

A good hash function is one-way: given a hash, you cannot feasibly find the input that produced it.

This preimage resistance is why hashes can safely commit to data without revealing it.

Collision Resistance

Collision resistance means it is infeasible to find two different inputs with the same hash.

If collisions were easy, an attacker could swap one block or transaction for another that shares the same hash — destroying the chain's integrity.

Hashes Create the Chain Link

Recall that each block stores the hash of the previous block. Because hashing is deterministic and collision-resistant, this link is secure.

Altering an old block changes its hash, breaking the chain at that point.

block.hash = keccak256(
    block.previousHash +
    block.merkleRoot +
    block.timestamp +
    block.nonce
)

Immutability Through Hashing

Immutability emerges from chained hashes. To change block 100, you must recompute block 100's hash, then 101, 102, and every block after it.

In a proof-of-work chain, that also means redoing all of their mining — practically impossible.

Hash Pointers

A hash pointer is a reference to data plus the hash of that data. If the data is tampered with, the stored hash no longer matches.

Blockchains are essentially linked lists built from hash pointers.

struct HashPointer {
    location: address-of-block
    hash:     expected-hash-of-block
}

Addresses and Hashing

Hashing also generates account addresses. An Ethereum address is derived by hashing the public key with Keccak-256 and keeping the last 20 bytes.

So hashing is woven through identity as well as integrity.

address = last20bytes(
    keccak256(publicKey)
)

Putting It Together

Cryptographic hashing gives blockchains their core guarantees: deterministic verification, tamper-evidence, compact identifiers, and immutability.

Every block link, Merkle root, and address depends on it.

Quick Check

Test your hashing knowledge.

Recap: Hashing in Blockchain

You learned that:

Hash functions are deterministic and produce fixed-size output
The avalanche effect means tiny changes alter the whole hash
Preimage and collision resistance secure the chain
Chained hashes create immutability

Next: how this data is shared across a distributed ledger.

Frequently asked questions

Is the “Hashing in Blockchain” lesson free?

Yes — the full text of “Hashing in Blockchain” is free to read here on the web, and the Web3 & DApp Development Fundamentals course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Web3 & DApp Development Fundamentals course, upgrade to CoddyKit PRO.

What will I learn in “Hashing in Blockchain”?

Immutability via hashes. You practise Web3 & DApp Development Fundamentals with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Web3 & DApp Development Fundamentals?

No prior experience is required. Web3 & DApp Development Fundamentals on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Hashing in Blockchain” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Web3 & DApp Development Fundamentals lesson?

Yes. Every Web3 & DApp Development Fundamentals lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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