I still use very old Office 2000 SR3 with its 2007 compatbility pack.
I couldn't open password protected docx files in the past. It is a
known issue. See my
http://answers.microsoft.com/en-us/o...1-952b1cb52568
forum thread and the official MS' notes in
http://office.microsoft.com/en-us/wo...010370115.aspx
... I also tried LibreOffice and OpenOffice back in 2013 which
failed to open and people mentioned this. I don't know if that was
ever fixed today.

First it is a mtter of the Office 2007 Compatibility Pack.

Then it is a matter of the encryption algorithm. For Office 2000 to
open an encypted document, it would have to be encrypted by a method
available to Office 2000 such as "Microsoft Base Cryptograhic Provider
v1.0". That was before to Windows CryptoAPI which is used Today.
Microsoft Base Cryptograhic Provider v1.0 is now a part of the Windows
CryptoAPI but a later version of MS Office would choose an enhanced
Cryptograhic routine with a higher bit strength.

Office 2000 could not decrypt the file because the routine ( algorithm )
was not avaliable 15+ years ago.

Interesting and thanks!
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