I dont know the password, how do I unlock Protected Cells?

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Btw, there's now a How to remove a Password Protected Sheet Flag FAQ.

While the question was about a specific case, let me answer generally on the title question: how to unprotect a sheet which has been protected by password, but otherwise not unreadable (so not competely encrypted by a password).

Cell protection password is just a friendly hint that we shouldn't edit the cells. Still, LibreOffice do not seem to offer to disable this hint, so you need a bit of hacking. Nothing really serious, but basic computer knowledge is required, and unix/linux tools help tremendously.

Take the ODS file, and simply unzip it into a directory.

You will see lots of stuff, among them the content.xml file, which contains all the data for the sheet. This file contains also the protection, in the form of XML attributes. We simply have to mess with them. The laziest way is to replace the protection flag:

sed -e 's/table:protected="true"/table:protected="false"/g' < content.xml > tmp ; mv tmp content.xml

then zip it again and rename to something.ods. When loading it LibreOffice will complain that the file is invalid and should be fixed, so let loffice do the work. The result is a sheet with no protection and hopefully all data intact.

If you're not the unix type you should find some program which can replace the string in the content.xml while not breaking its structure (you can reformat it, it's basically a text file, but don't break words). zipping and unzipping generally works everywhere.

I was able to solve this. Here is how I did it. I am using Ubuntu 18.04.

Assume that the file whose password protection we want to remove is AttendanceReport.ods. Assume also that it is in the current directory. All commands below should be made in the directory where AttendanceReport.ods is present.

First create a copy of the file in a temporary directory.

 $ mkdir WorkArea
 $ cp AttendanceReport.ods WorkArea/
 $ cd WorkArea/

Unzip contents of AttendanceReport.ods

 $ unzip AttendanceReport.ods

Examine the contents of the file "content.xml" and search for the string "table:protected"

 $ less content.xml

 Within the less command press '/table:protected' (Do not include the single quotes.)

 You should find a long string containing something like the following. Copy and paste it to a convenient open editor.

 table:protected="true" table:protection-key="KFLVezSiI9yL3/pl1z6FnxYrTbQ=" table:protection-key-digest-algorithm="http://docs.oasis-open.org/office/ns/table/legacy-hash-excel"

 Yours will look different but the idea is to copy the attributes table:protected, table:protection-key, table:protection-key-digest-algorithm.

 Press q to leave the 'less' command.

Remove the attributes that protect the spreadsheet. You should still be in WorkArea/. Give the following command.

 $ sed -i 's|table:protected="true" table:protection-key="KFLVezSiI9yL3/pl1z6FnxYrTbQ=" table:protection-key-digest-algorithm="http://docs.oasis-open.org/office/ns/table/legacy-hash-excel"||g' content.xml

Freshen the file AttendanceReport.ods with the new content.xml

 $ zip -f AttendanceReport.ods content.xml

The file AttendanceReport.ods in WorkArea/ should now have their cells unprotected.

If a spreadsheet has been protected by a password then that password is specific to that spreadsheet. You have not set one in any global settings and forgotten it.

There are two modes for cell protection. One without setting a password is so that cells are not accidentally changed. When setting protection in this way you leave the password fields blank. When you unset protection you will not be asked for a password.

The other way when you want to use a password you set up the password when you set protection and require this password to unprotect.

In your case it is unlikely that Amazon will have set a password without telling you what it is. Select one of the protected cells and go to Tools > Protect Document > Sheet and if there is no request for a password the cells should be unprotected. It is confusing that whether you are protecting or unprotecting the Menu always shows Protect Document even if protection is on and you want to unprotect it.