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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.

kayleeFrye_onDeck
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3 Answers3

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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.

dave_thompson_085
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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.

Community
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Ella Rose
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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?.

Paul Uszak
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