ArticleslgStudy

computer science

Key schedule

Key schedule 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 Key schedule rather than just read about it. In short: In cryptography, the so-called product ciphers are a certain kind of cipher, where the (de-)ciphering of data is typically done as an iteration of rounds. The setup for each round is generally the same, except for round-specific fixed values called a round constant, and round-specific data derived from the cipher key called a round key.

Key schedule — main illustration
Key schedule — illustration

Key takeaways

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

Reference excerpt

In cryptography, the so-called product ciphers are a certain kind of cipher, where the (de-)ciphering of data is typically done as an iteration of rounds. The setup for each round is generally the same, except for round-specific fixed values called a round constant, and round-specific data derived from the cipher key called a round key. A key schedule is an algorithm that calculates all the round keys from the key.

Some types of key schedules Some ciphers have simple key schedules. For example, the block cipher TEA splits the 128-bit key into four 32-bit pieces and uses them repeatedly in successive rounds. DES has a key schedule in which the 56-bit key is divided into two 28-bit halves; each half is thereafter treated separately. In successive rounds, both halves are rotated left by one or two bits (specified for each round), and then 48 round key bits are selected by Permuted Choice 2 (PC-2) – 24 bits from the left half and 24 from the right. The rotations have the effect that a different set of bits is used in each round key; each bit is used in approximately 14 out of the 16 round keys. To avoid simple relationships between the cipher key and the round keys, in order to resist such forms of cryptanalysis as related-key attacks and slide attacks, many modern ciphers use more elaborate key schedules to generate an "expanded key" from which round keys are drawn. Some ciphers, such as Rijndael (AES) and Blowfish, use the same operations as those used in the data path of the cipher algorithm for their key expansion, sometimes initialized with some "nothing-up-my-sleeve numbers". Other ciphers, such as RC5, expand keys with functions that are somewhat or completely different from the encryption functions.

Notes Knudsen and Mathiassen (2004) give some experimental evidence that indicate that the key schedule plays a part in providing strength against linear and differential cryptanalysis. For toy Feistel ciphers, it was observed that those with complex and well-designed key schedules can reach a uniform distribution for the probabilities of differentials and linear hulls faster than those with poorly designed key schedules.

References Lars R. Knudsen and John Erik Mathiassen, On the Role of Key Schedules in Attacks on Iterated Ciphers, ESORICS 2004, pp322–334. Uri Blumenthal and Steven M. Bellovin, A Better Key Schedule for DES-like Ciphers, Proceedings of PRAGOCRYPT '96.

Illustrations

Key schedule: The key schedule of DES ("<<<" denotes a left rotation), showing the calculation of each round key ("Subkey").
The key schedule of DES ("<<<" denotes a left rotation), showing the calculation of each round key ("Subkey").

Worked examples

Example 1 — a first encounter with Key schedule

Start with the simplest possible case. Write down what Key schedule 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 Key schedule 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 Key schedule 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 Key schedule

In research
Key schedule 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 Key schedule 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
Key schedule is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cryptographic algorithms, so understanding it makes those chapters shorter.
In everyday life
Look for Key schedule 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 “Key schedule” →

Affiliate

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

How to study Key schedule in 20 minutes

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

Frequently asked questions

What is Key schedule in simple terms?

In cryptography, the so-called product ciphers are a certain kind of cipher, where the (de-)ciphering of data is typically done as an iteration of rounds. The setup for each round is generally the same, except for round-specific fixed values called a round constant, and round-specific data derived…

Why does Key schedule 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 Key schedule?

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 Key schedule.

Tags

  • Cryptographic algorithms

Keep exploring