Barrapunto

No habría estado de más que en la 3.5, cuando grabas un documento cifrado con la versión anterior, te diera opción a mantener el cifrado antiguo. Después de todo, tienen que implementado el algoritmo antiguo para poder leer documentos antiguos.

Encryption
In LibreOffice 3.5 a different encryption will be introduced. LibreOffice 3.4.5 enables you to open those files.
See the release notes page.

Different Encryption Algorithm
In LibreOffice 3.5, a different and more secure encryption algorithm (AES-256) is used for password protected files in the ODF 1.2 and ODF 1.2 Extended file formats. The new cipher is activated by default. Blowfish encryption in ODF 1.2 files is considered deprecated
-LibreOffice versions up to 3.4.4 don't support the AES-256 cipher.
-LibreOffice 3.4.5 can read ODF files encrypted with the AES-256 cipher, but on writing a password protected file will fall-back to the Blowfish cipher.
-LibreOffice 3.5.0 can read and write ODF files encrypted with the AES-256 (and Blowfish) cipher. -For files in the ODF 1.2 and ODF 1.2 Extended formats, AES-256 is the default cipher. New as well as existing password encrypted documents in these formats are always encrypted with the AES-256 cipher on writing them to disk. There's no UI to configure the cipher.
-Hint: LibreOffice versions up to 3.4.4 cannot open password protected files in the ODF 1.2 and ODF 1.2 Extended formats written by LibreOffice 3.5. All users of the LibreOffice 3.3 and 3.4 series are advised to update to LibreOffice 3.4.5 or later.