I was looking for a source to determine the safety of password protection in libre office. The thread at ask.libreoffice.org suggest its save to use. However the thread is rather old and it is hosted on a site not primarily concerned with security. So the question is are there any known vulnerabilities in password protection for *.odt files in libre office 18.104.22.168?
According to the OpenDocument Specification, these files use a strong encryption with no known flaws.
...This digest is used to produce a derived key by undergoing key stretching with PBKDF2
...ODF 1.0 and 1.1 use Blowfish in 8-bit cipher feedback mode, while ODF 1.2 considers it a legacy algorithm and allows Triple DES and AES
However, like almost every encryption algorithms, odt files are vulnerable to bruteforce but, according to this post it's not very efficient and if your password is strong enough you should be fine.