Oh, right, so that's the name of it. Thanks to that I can now find the exact duplicate:

https://www.codewars.com/kata/simple-fun-number-31-is-substitution-cipher

By the way, this is called a simple substitution cipher. In the output, each character in the string can be replaced with a different character consistently based on a linear mapping. If this function returns false, then a cipherkey cannot exist.

Note that your kata is very similar to https://www.codewars.com/kata/562846dd1f77ab7c00000033, and one more kata that I couldn't find right now. So it's a duplicate ;-)

I guess I might word the challenge as "every character in the reference string must be represented by a character in the translation string, and that representation must be consistent for all instances of the same letter," then proceed to give a couple examples. At least that way nobody might get confused. :)

Thank goodness I'm not the only one that thought Test8 was smoking something.

Oh, you don't have to lowercase the strings. They're guaranteed lowercase :)