The Law of Archives (1955) stipulates that all archival material will be open to the public, but restrictions can be formulated by decree. Here's a link to the law.
The Archives Decree of 2010 sets periods of restriction for varying types of documents and information. Most documents will be opened after a waiting period of 15 years, but some materials will remain sealed for 25, 30, 50 or in some cases even 70 years. The main reasons not to open files are the need to protect the security of the state, or the privacy of individuals. A special committee of Cabinet ministers may seal specific files beyond the waiting term stipulated in the decree.
Here's a link to the decree
Although such a process is not mandated in law, the State Archivist will at times look into cases where researchers wish to appeal the sealing of specific files.