I've been using LibraOffice on my KDE system and thought I would give Calligra a go. I have got a spreadsheet, created in LibraOffice, that cannot be opened without a password. When I try to open this spreadsheet in Calligra Sheets I get the following error:-

Could not open /home/blah/blah/blah.ods
Reason: Parsing error in the main document at line 1, column 1
Error message: Encountered incorrectlyencoded content.

If I remove the password from the spreadsheet it opens OK in Calligra sheets.

Now this would be OK if I could then save the spreadsheet in Calligra sheets giving it a password that will be required before the spreadsheet is opened. So far I have been unable to find out how to do this. I can protect a spreadsheet but this doesn't stop it from being opened and therefore being read.

You can go to the menu "File" -> "Document Information". At the window that will pop up, there is a "Encrypt"-button in the bottom right corner.
After clicking this button and then "OK", make changes to the document and then save it. You will be asked to select a password for encryption.

To make this work, the document has to be saved at least once before (using any program, without encryption).

However, when the document is encrypted with sheets, you will not be able to open it with LibreOffice.
So if you want to encrypt a document and need to be able to open it with Calligra and LibreOffice, you may look for some general-purpose encryption software.