Is it still true that AES has no known flaws?
About LibreOffice encryption
I found 2 interesting links. From the links Libreoffice seems to use AES (http://en.wikipedia.org/wiki/AES_(cipher) ) as of LibreOffice 3.5. Before that it was blowfish.
Aveesplace.wordpress
According to LO mail archive (2012 so recent) a brute force with a Windows program called Accent Office will take:
Max pwd Accent OFFICE
Length Time Estimate (same conditions)
5 27m03s
6 1d19h
7 173d3h
8 45y197d
Conclusions:
AES has no known flaws.
The only way is to provide the correct password. and you should always save an unencrypted copy on an external media like an USB disc that is kept safe in your house.