What type of encryption is used when the encrypted phrase only uses the letters from a keyword?

For Example:

Keyword: Stack

tkc ska ktkkac skccaasca ckssak

This was encrypted using only the letters found in the word Stack. What type of scheme is this called?

Edit: Things I've thought of are that it might be some sort of Polybius Square or some modified Vigenere scheme but I don't know what I'm looking for.

I have not seen any encryptions that map many letters into one. I can't tell if the duplicate letters are meaningful or not. I can't tell if the words are padded. My guess is yes since one of the words in my encrypted texts has like 27 letters.

Edit: More Clues I noticed all of the encrypted words are multiples of 3. Is this some sort of trifid cipher with one step encryption step using only the keyword as the row and column headers???

  • $\begingroup$ This looks more like an encoding scheme with "stack" as alphabet. Part of the encoding would be re-basing (base 5 as there are 5 characters in "stack"). Also note that there are clusters of 3 characters from "stack", probably each one encodes a character... $\endgroup$ – Maarten Bodewes Feb 27 at 14:20

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