science

Shared secret

Shared secret is a 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 Shared secret rather than just read about it. In short: In cryptography, a shared secret is a piece of data, known only to the parties involved, in a secure communication. This usually refers to the key of a symmetric cryptosystem.

Key takeaways

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

Reference excerpt

In cryptography, a shared secret is a piece of data, known only to the parties involved, in a secure communication. This usually refers to the key of a symmetric cryptosystem. The shared secret can be a PIN code, a password, a passphrase, a big number, or an array of randomly chosen bytes. The shared secret is either shared beforehand between the communicating parties, in which case it can also be called a pre-shared key, or it is created at the start of the communication session by using a key-agreement protocol, for instance using public-key cryptography such as Diffie–Hellman or using symmetric-key cryptography such as Kerberos. The shared secret can be used for authentication (for instance when logging in to a remote system) using methods such as challenge–response or it can be fed to a key derivation function to produce one or more keys to use for encryption and/or MACing of messages. To make unique session and message keys the shared secret is usually combined with an initialization vector (IV). An example of this is the derived unique key per transaction method. It is also often used as an authentication measure in web APIs.

See also Key stretching – a method to create a stronger key from a weak key or a weak shared secret Security question – implementation method

References Handbook of Applied Cryptography by Menezes, van Oorschot and Vanstone (2001), chapter 10 and 12.

Worked examples

Example 1 — a first encounter with Shared secret

Start with the simplest possible case. Write down what Shared secret claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In 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 Shared secret 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 Shared secret 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 Shared secret

In research: Shared secret appears in 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 Shared secret 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: Shared secret is common in secondary-school and first-year university syllabi. It links to neighbouring topics Key management, so understanding it makes those chapters shorter.
In everyday life: Look for Shared secret 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.

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How to study Shared secret in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Shared secret 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 Shared secret out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Shared secret in simple terms?

In cryptography, a shared secret is a piece of data, known only to the parties involved, in a secure communication. This usually refers to the key of a symmetric cryptosystem.

Why does Shared secret matter?

Because it connects several 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 Shared secret?

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 Shared secret.