science

MacGuffin (cipher)

MacGuffin (cipher) 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 MacGuffin (cipher) rather than just read about it. In short: In cryptography, MacGuffin is a block cipher created in 1994 by Bruce Schneier and Matt Blaze at a Fast Software Encryption workshop. It was intended as a catalyst for analysis of a new cipher structure, known as Generalized Unbalanced Feistel Networks (GUFNs).

Key takeaways

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

Reference excerpt

In cryptography, MacGuffin is a block cipher created in 1994 by Bruce Schneier and Matt Blaze at a Fast Software Encryption workshop. It was intended as a catalyst for analysis of a new cipher structure, known as Generalized Unbalanced Feistel Networks (GUFNs). The cryptanalysis proceeded very quickly, so quickly that the cipher was broken at the same workshop by Vincent Rijmen and Bart Preneel.

The algorithm Schneier and Blaze based MacGuffin on DES, their main change being that the data block is not split into equal halves in the Feistel network. Instead, 48 bits of the 64-bit data block are fed through the round function, whose output is XORed with the other 16 bits of the data block. The algorithm was experimental, intended to explore the security properties of unbalanced Feistel networks. The adjacent diagram shows one round of MacGuffin. The 64-bit data block is broken into four 16-bit words (each represented by one line). The rightmost three are XORed with subkey bits derived from the secret key. They are then fed through eight S-boxes, each of which takes six bits of input and produces two bits of output. The output (a total of 16 bits) is then recombined and XORed with the leftmost word of the data block. The new leftmost block is then rotated into the rightmost position of the resulting data block. The algorithm then continues with more rounds. MacGuffin's key schedule is a modified version of the encryption algorithm itself. Since MacGuffin is a Feistel network, decryption is easy; simply run the encryption algorithm in reverse. Schneier and Blaze recommended using 32 rounds, and specified MacGuffin with a 128-bit key.

Cryptanalysis of MacGuffin At the same workshop where MacGuffin was introduced, Rijmen and Preneel showed that it was vulnerable to differential cryptanalysis. They showed that 32 rounds of MacGuffin is weaker than 16 rounds of DES, since it took "a few hours" to get good differential characteristics for DES with good starting values, and the same time to get good differential characteristics for MacGuffin with no starting values. They found that it is possible to get the last round key with differential cryptanalysis, and from that reverse the last round; and then repeat the attack for the rest of the rounds. Rijmen and Preneel tried attacking MacGuffin with different S-boxes, taken directly from DES. This version proved to be slightly stronger, but they warn that designing an algorithm to resist only known attacks is generally not a good design principle.

References Bruce Schneier, Matt Blaze (December 1994). The MacGuffin Block Cipher Algorithm (PDF/PostScript). 2nd International Workshop on Fast Software Encryption (FSE '94). Leuven: Springer-Verlag. pp. 97–110. Retrieved 2007-08-24. Vincent Rijmen, Bart Preneel (December 1994). Cryptanalysis of McGuffin (PDF/PostScript). FSE '94. Leuven: Springer-Verlag. pp. 353–358. Retrieved 2007-08-24.{{cite conference}}: CS1 maint: miscellaneous url (link)

Illustrations

Worked examples

Example 1 — a first encounter with MacGuffin (cipher)

Start with the simplest possible case. Write down what MacGuffin (cipher) 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 MacGuffin (cipher) 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 MacGuffin (cipher) 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 MacGuffin (cipher)

In research: MacGuffin (cipher) 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 MacGuffin (cipher) 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: MacGuffin (cipher) is common in secondary-school and first-year university syllabi. It links to neighbouring topics 1994 introductions, Block ciphers, Broken block ciphers, so understanding it makes those chapters shorter.
In everyday life: Look for MacGuffin (cipher) 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 MacGuffin (cipher) in 20 minutes

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

Frequently asked questions

What is MacGuffin (cipher) in simple terms?

Why does MacGuffin (cipher) 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 MacGuffin (cipher)?

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 MacGuffin (cipher).