The consequences of faults and intrusions in computer networks are of ever-increasing importance; new efficient techniques are needed to deal with them. The SATURNE [DES 86] project aims at solutions for architectures based on the concept of fault- and intrusion-tolerance rather than avoidance techniques. The subject of the paper concerns the security of files. Files are partitioned and then fragmented before being stored in different, distributed archive sites in order to tolerate intrusions into some of these sites. The choice among different fragmentation cryptographic methods is discussed, related to different kinds of possible attacks. A simple and fast solution is proposed that nevertheless ensures a high level of file security and authenticity.