science

Running key cipher

Running key 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 Running key cipher rather than just read about it. In short: In classical cryptography, the running key cipher is a type of polyalphabetic substitution cipher in which a text, typically from a book, is used to provide a very long keystream. The earliest description of such a cipher was given in 1892 by French mathematician Arthur Joseph Hermann (better known for founding Éditions Hermann).

Key takeaways

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

Reference excerpt

In classical cryptography, the running key cipher is a type of polyalphabetic substitution cipher in which a text, typically from a book, is used to provide a very long keystream. The earliest description of such a cipher was given in 1892 by French mathematician Arthur Joseph Hermann (better known for founding Éditions Hermann). Usually, the book to be used would be agreed ahead of time, while the passage to be used would be chosen randomly for each message and secretly indicated somewhere in the message. The term running key was introduced by William F. Friedman; it is distinct from progressive key (David Kahn's term for systems such as that of Trithemius, which cycle through every alphabet in a fixed order before repeating) or progressing key (Gaines' term for the same concept). An early cryptanalysis of a running-text key was carried out by Étienne Bazeries in 1899, when he read the enciphered dispatches of the conspirators around the Duke of Orléans. The dispatches were written in four-figure groups, each pair of digits standing for a letter through a Beaufort table, and the key was a continuous text: successive lines of Alfred de Musset's poem La Nuit de décembre, or a written-out date, selected according to the calendar date. Because the key did not repeat over the short telegrams, their apparent periodicity was a chance effect, and Bazeries recovered the messages by guessing probable words.

Example The key text used is a portion of The C Programming Language (1978 edition), and the tabula recta is the tableau. The plaintext here is "Flee at once". Page 63, line 1 is selected as the running key:

errors can occur in several places. A label has... The running key is then written under the plaintext:

The message is then sent as "JCVSR LQNPS". However, unlike a Vigenère cipher, if the message is extended, the key is not repeated; the key text itself (the text from The C Programming Language) is used as the key and can be extended for any arbitrary length. If the message is extended, such as, "Flee at once. We are discovered", then the running key continues as before:

To determine where to find the running key, a fake block of five ciphertext characters is subsequently added, with three denoting the page number, and two the line number, using A=0, B=1 etc. to encode digits. Such a block is called an indicator block. The indicator block will be inserted as the second last of each message. (Many other schemes are possible for hiding indicator blocks.) Thus page 63, line 1 encodes as "AGDAB" (06301). This yields a final message of "JCVSR LQNPS YGUIM QAWXS AGDAB MECTO".

Variants Modern variants of the running key cipher often replace the traditional tabula recta with bitwise exclusive or, operate on whole bytes rather than alphabetic letters, and derive their running keys from large files. Apart from possibly greater entropy density of the files, and the ease of automation, there is little practical difference between such variants and traditional methods. The American Cryptogram Association uses "Running Key" for a constrained puzzle form, in which a 40–50-letter plaintext is split in half and its first half is used as the key for its second half. Methods for solving this form are set out in the association's lessons.

Gromark cipher The "Gromark cipher" ("Gronsfeld cipher with mixed alphabet and running key") uses a running numerical key formed by adding successive pairs of digits. The VIC cipher uses a similar lagged Fibonacci generator.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Running key cipher

Start with the simplest possible case. Write down what Running key 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 Running key 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 Running key 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 Running key cipher

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

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

Frequently asked questions

What is Running key cipher in simple terms?

Why does Running key 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 Running key 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 Running key cipher.