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The word "rotate" in respect to "key-rotation" has connotations of looping/repeating variable values like a rotor, but as far as I can tell there isn't a single definition available for "key-rotation" that involves re-using a key that was previously replaced.

So why don't we call it key-replacement or something else like that? i.e., why did the cryptography communities decide the more ambiguous word "rotation" was the more appropriate term to describe the act of changing keys?

It just seems a strange phrase to use, especially if you're not a native English speaker.

The only thing I could think of was some sort of an anachronism in reference to oldschool cryptographic key-generators, like the Enigma Machine, or your typical multi-rotor-combo-locks popular for use with bikes & storage lockers.

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    $\begingroup$ good question probably because there was a codebook and you selected the code of the day, and when the month changed you "looped" back to page 1 $\endgroup$
    – Richie Frame
    Commented Sep 19, 2019 at 0:43
  • $\begingroup$ I never heard of this looping-back thing for oldschool codebooks! I just assumed you used a new one instead of re-using an old one :/ $\endgroup$
    – kayleeFrye_onDeck
    Commented Sep 19, 2019 at 1:15
  • $\begingroup$ @RichieFrame if you can show an example of this in Answer-format, I will accept it. $\endgroup$
    – kayleeFrye_onDeck
    Commented Sep 19, 2019 at 1:27
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    $\begingroup$ was more of a speculative comment, as before modern encryption, codebooks were used. Even the German enigma rotor settings came from a codebook, I just do not know how they were managed $\endgroup$
    – Richie Frame
    Commented Sep 19, 2019 at 2:53

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Purely speculation, but given most cipher usage historically was military, diplomatic and espionage, I wonder if it might be related to the longtime common practice of the military (certainly US, and I believe many others), and to an extent the diplomatic corps at least US, of changing the assignments of individuals to jobs, posts, or duties, and for the military of whole units to missions, on a regular schedule automatically (i.e. not requested by the people involved). This practice is usually called rotation, and a particular (limited-time) assignment is metonymically called a rotation -- e.g. a soldier might speak of 'doing a 3-month rotation in \$job1 and then being moved to \$job2'. Random example To a military comms specialist -- and their commanding officer -- the periodic, scheduled replacement of one key by another would certainly seem similar.

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  • $\begingroup$ This is the only speculative answer that actually makes sense when held against scrutiny. It explains the word's specific usage historically and how it was adopted conceptually by modern cryptographic jargon, given the symbiotic and historical relationship between militaries and cryptography. It helps me actually visualize the process both for keys and for people. Not bad for pure speculation :) That said if someone drops in with some actual reference material I would feel obliged to grant them the Accepted Answer; but for now this more than suffices! :) $\endgroup$
    – kayleeFrye_onDeck
    Commented Sep 19, 2019 at 23:51
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The only thing I could think of was some sort of an anachronism in reference to oldschool cryptographic key-generators, like the Enigma Machine, or your typical multi-rotor-combo-locks popular with bike & storage locks.

It is because of the English language and not because of cryptography.

In English, a single word can have multiple definitions.

The definition of rotation is: (emphasis added)

Definition of rotation

1 a (1): the action or process of rotating on or as if on an axis or center

...

2a: return or succession in a series

rotation of the seasons

b: CROP ROTATION

Key rotation is the succession of a series of keys. That rotor machines are related to the idea of keys is purely a coincidence. In this context, the series is not necessarily periodic.

If one were to say "I have a vehicle rotation strategy that extends the lifespan of my cars", nobody would be confused about the meaning because of all of the rotating parts on cars. "rotation" in this context clearly does not mean "rotation about an axis".

Similarly, if we say "I have a key rotation strategy that extends the lifespan of my users", you should not get confused because things that rotate have been used in the history of cryptography.

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  • $\begingroup$ I appreciate you spending time on the English lesson, but it doesn't address the big, "why are we calling it rotation instead of change" question, meaning, is there some significance to that word's usage that "change"/"replacement" does not have, and if not why did we choose this less specific word to begin with? For example, dave_thompson_085 does this in their answer. $\endgroup$
    – kayleeFrye_onDeck
    Commented Sep 19, 2019 at 23:46
  • $\begingroup$ I updated the question. Sorry if my lack of specificity wasted some of your time :( $\endgroup$
    – kayleeFrye_onDeck
    Commented Sep 20, 2019 at 0:12
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Short answer = semantics.

Rotation in this grammatical sense does not infer anything rotary. It simply means 'changing'. See What's the purpose of key-rotation?.

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  • $\begingroup$ For me you can't arbitrarily deem connotations of rotors when mentioning cryptography and rotation together as somehow invalid given historical context of things like rotor-type locks and the Enigma machine. Also, the linked-to answer doesn't actually answer the question, just addresses the concept of key-rotation, which in itself doesn't bother to define the "rotatation" part of key-rotation. Which then begs the question, why isn't it called key-changing then if that's the actual intent of the phrase? Furthermore, checking Google's NGram viewer shows the earliest use in respect to rotors. $\endgroup$
    – kayleeFrye_onDeck
    Commented Sep 19, 2019 at 1:22
  • $\begingroup$ Just for clarification, I'm not disagreeing with your answer, just not the logic of this term's mutation and phraseology for its intended meaning IRT modern cryptography. $\endgroup$
    – kayleeFrye_onDeck
    Commented Sep 19, 2019 at 1:24
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    $\begingroup$ @kayleeFrye_onDeck I know what you mean. "key-replacement" is exactly what we're talking about. You have to allow some latitude for semantic nuances in this particular domain. AES is not a typical mathematical permutation, rotation is not a stackoverflow bitwise rotation and contemporary encryption does not involve a metal rotor going round and round. I suggest that "mutation" is also a bit programy. Quantum key distribution networks 'change' the key every few minutes as per your 'key-rotation' tag. And Enigma of course didn't 'rotate' the key, it 'rotated' a metal disc. $\endgroup$
    – Paul Uszak
    Commented Sep 19, 2019 at 2:14

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