Searchable symmetric encryption

Key takeaways

Searchable symmetric encryption 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 Searchable symmetric encryption to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Searchable symmetric encryption from memory before moving on to harder problems.

Reference excerpt

Searchable symmetric encryption (SSE) is a form of encryption that allows efficient searching over a collection of encrypted documents or files without requiring decryption. SSE can be used to outsource files to an untrusted cloud storage server without revealing the files in plaintext, while preserving the server's ability to perform searches over them.

Description A searchable symmetric encryption scheme is a symmetric-key encryption scheme that encrypts a collection of documents D = ( D 1 , … , D n ) {\displaystyle \mathbf {D} =(\mathrm {D_{1}} ,\dots ,\mathrm {D_{n}} )} , where each document D i ⊆ W {\displaystyle \mathrm {D_{i}} \subseteq \mathbb {W} } is viewed as a set of keywords from a keyword space W {\displaystyle \mathbb {W} } . Given the encryption key K {\displaystyle K} and a keyword w ∈ W {\displaystyle w\in \mathbb {W} } , one can generate a search token t k {\displaystyle tk} with which the encrypted data collection can be searched for w {\displaystyle w} . The result of the search is the subset of encrypted documents that contain the keyword w {\displaystyle w} .

Static SSE A static SSE scheme consists of three algorithms S S E = ( S e t u p , T o k e n , S e a r c h ) {\displaystyle {\mathsf {SSE=(Setup,Token,Search)}}} that work as follows:

S e t u p {\displaystyle {\mathsf {Setup}}} takes as input a security parameter k {\displaystyle k} and a document collection D {\displaystyle \mathbf {D} } and outputs a symmetric key K {\displaystyle K} , an encrypted index I {\displaystyle \mathbf {I} } , and an encrypted document collection E D {\displaystyle \mathbf {ED} }

T o k e n {\displaystyle {\mathsf {Token}}} takes as input the secret key K {\displaystyle K} and a keyword w {\displaystyle w} and outputs a search token t k {\displaystyle tk}

S e a r c h {\displaystyle {\mathsf {Search}}} takes as input the encrypted index I {\displaystyle \mathbf {I} } , the encrypted document collection E D {\displaystyle \mathbf {ED} } and a search token t k {\displaystyle tk} and outputs a set of encrypted documents R ⊆ E D {\displaystyle \mathbf {R} \subseteq \mathbf {ED} }

A static SSE scheme is used by a client and an untrusted server as follows: the client encrypts its data collection using the S e t u p {\displaystyle {\mathsf {Setup}}} algorithm which returns a secret key K {\displaystyle K} , an encrypted index I {\displaystyle \mathbf {I} } , and an encrypted document collection E D {\displaystyle \mathbf {ED} } . The client keeps K {\displaystyle K} secret and sends E D {\displaystyle \mathbf {ED} } and I {\displaystyle \mathbf {I} } to the untrusted server. To search for a keyword w {\displaystyle w} , the client runs the T o k e n {\displaystyle {\mathsf {Token}}} algorithm on K {\displaystyle K} and w {\displaystyle w} to generate a search token t k {\displaystyle tk} which it sends to the server. The server runs Search with E D {\displaystyle \mathbf {ED} } , I {\displaystyle \mathbf {I} } , and t k {\displaystyle tk} and returns the resulting encrypted documents back to the client.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Searchable symmetric encryption

Start with the simplest possible case. Write down what Searchable symmetric encryption 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 Searchable symmetric encryption 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 Searchable symmetric encryption 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 Searchable symmetric encryption

In research: Searchable symmetric encryption 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 Searchable symmetric encryption 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: Searchable symmetric encryption is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cryptographic primitives, so understanding it makes those chapters shorter.
In everyday life: Look for Searchable symmetric encryption 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.

How to study Searchable symmetric encryption in 20 minutes

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

Frequently asked questions

What is Searchable symmetric encryption in simple terms?

Searchable symmetric encryption (SSE) is a form of encryption that allows efficient searching over a collection of encrypted documents or files without requiring decryption. SSE can be used to outsource files to an untrusted cloud storage server without revealing the files in plaintext, while prese…

Why does Searchable symmetric encryption 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 Searchable symmetric encryption?

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 Searchable symmetric encryption.