Some years ago I removed the passwords from dozends of Excel files (XL 97/2000). I loaded a file in a hex-editor and replaced the bytes at a certain position with a certain sequence of other bytes. The password protection was gone. I believe, Calc can open password "protected" xls by simply ignoring those bytes, but there is no way to create the right byte sequence for a given password. Password protected ODF is strongly encrypted. The encryption utilizes a public available algorithm, so other programs can encrypt and decrypt using a given password. Brute force password guessing is the only reasonable method to open a password protected ODF file when the password got lost.
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Ubuntu 16.04, OpenOffice 4.x & LibreOffice 5.x