computer science

Puzzle friendliness

Puzzle friendliness is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Puzzle friendliness rather than just read about it. In short: In cryptography, puzzle friendliness is a property of some cryptographic hash functions. SHA-256 is a cryptographic hash function that is thought to have this property.

Key takeaways

Puzzle friendliness belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Puzzle friendliness to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Puzzle friendliness from memory before moving on to harder problems.

Reference excerpt

In cryptography, puzzle friendliness is a property of some cryptographic hash functions. SHA-256 is a cryptographic hash function that is thought to have this property. Informally, a hash function is puzzle friendly if no solution exists which is better than just making random guesses, and the only way to find a solution is the brute force method. Although the property is very general, it is of particular importance to proof-of-work, such as in Bitcoin mining.

Definition A hash function H is said to be puzzle friendly if for every possible n-bit output value y, if k is chosen with a distribution with high min-entropy, then it is infeasible to find x such that H(k || x) = y (where the symbol "||" denotes concatenation) in time significantly less than 2n. In the above definition, the distribution has high min-entropy means that the distribution from which k is chosen is hugely distributed so that choosing some particular random value from the distribution has only a negligible probability.

Why this property is called puzzle friendliness Let H be a cryptographic hash function and let an output y be given. Let it be required to find z such that H(z) = y. Let us also assume that a part of the string z, say k, is known. Then, the problem of determining z boils down to finding x that should be concatenated with k to get z. The problem of determining x can be thought of a puzzle. It is really a puzzle only if the task of finding x is nontrivial and is nearly infeasible. Thus the puzzle friendliness property of a cryptographic hash function makes the problem of finding x closer to being a real puzzle.

Application in cryptocurrency The puzzle friendliness property of cryptographic hash functions is used in Bitcoin mining.

See also Collision resistance Collision attack Preimage attack

References

Worked examples

Example 1 — a first encounter with Puzzle friendliness

Start with the simplest possible case. Write down what Puzzle friendliness claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Puzzle friendliness before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Puzzle friendliness ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Puzzle friendliness

In research: Puzzle friendliness appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Puzzle friendliness in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Puzzle friendliness is common in secondary-school and first-year university syllabi. It links to neighbouring topics Bitcoin, Cryptographic hash functions, Hashing, so understanding it makes those chapters shorter.
In everyday life: Look for Puzzle friendliness outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Puzzle friendliness” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Puzzle friendliness in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Puzzle friendliness means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Puzzle friendliness out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Puzzle friendliness in simple terms?

In cryptography, puzzle friendliness is a property of some cryptographic hash functions. SHA-256 is a cryptographic hash function that is thought to have this property.

Why does Puzzle friendliness matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Puzzle friendliness?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Puzzle friendliness.