science

SNOW

SNOW 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 SNOW rather than just read about it. In short: SNOW is a family of word-based synchronous stream ciphers developed by Thomas Johansson and Patrik Ekdahl at Lund University. They have a 512-bit linear feedback shift register at their core, followed by a non-linear output state machine with a few additional words of state.

Key takeaways

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

Reference excerpt

SNOW is a family of word-based synchronous stream ciphers developed by Thomas Johansson and Patrik Ekdahl at Lund University. They have a 512-bit linear feedback shift register at their core, followed by a non-linear output state machine with a few additional words of state. SNOW 1.0, SNOW 2.0, and SNOW 3G use a shift register of 16 32-bit words, and a 32-bit add-rotate-XOR (ARX) output transformation with 2 or 3 words of state. Each iteration advances the shift register by 32 bits and produces 32 bits of output. SNOW-V and SNOW-Vi use a shift register of 32 16-bit words (designed to be implemented as 4 128-bit SIMD registers) which is advanced by 16 bits per iteration. 8 LFSR iterations can be performed simultaneously using SIMD operations, after which one output transformation step is performed, producing 128 bits of output. The output transformation uses the Advanced Encryption Standard (AES) round function (commonly implemented in hardware on recent processors), and maintains 2 additional 128-bit words of state.

History SNOW 1.0, originally simply SNOW, was submitted to the NESSIE project. The cipher has no known intellectual property or other restrictions. The cipher works on 32-bit words and supports both 128- and 256-bit keys. The cipher consists of a combination of a LFSR and a finite-state machine (FSM) where the LFSR also feeds the next state function of the FSM. The cipher has a short initialization phase and very good performance on both 32-bit processors and in hardware. During the evaluation, weaknesses were discovered and as a result, SNOW was not included in the NESSIE suite of algorithms. The authors have developed a new version, version 2.0 of the cipher, that solves the weaknesses and improves the performance. During ETSI SAGE evaluation, the design was further modified to increase its resistance against algebraic attacks with the result named SNOW 3G. It has been found that related keys exist both for SNOW 2.0 and SNOW 3G, allowing attacks against SNOW 2.0 in the related-key model.

Use SNOW has been used in the ESTREAM project as a reference cipher for the performance evaluation. SNOW 2.0 is one out of stream ciphers chosen for ISO/IEC standard ISO/IEC 18033-4. SNOW 3G is chosen as the stream cipher for the 3GPP encryption algorithms UEA2 and UIA2. SNOW-V was an extensive redesign published in 2019, designed to match 5G cellular network speeds by generating 128 bits of output per iteration. SNOW-Vi was tweaked for even higher speed using small changes to the LFSR; the output transformation is identical.

Sources

External links The Lund Crypto and Security group website Archived 2021-09-20 at the Wayback Machine

Worked examples

Example 1 — a first encounter with SNOW

Start with the simplest possible case. Write down what SNOW 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 SNOW 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 SNOW 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 SNOW

In research: SNOW 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 SNOW 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: SNOW is common in secondary-school and first-year university syllabi. It links to neighbouring topics Stream ciphers, so understanding it makes those chapters shorter.
In everyday life: Look for SNOW 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 SNOW in 20 minutes

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

Frequently asked questions

What is SNOW in simple terms?

Why does SNOW 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 SNOW?

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 SNOW.