A computer system and method for securing files in a file system equipped with storage resources that are accessible to an authenticable user operating with an untrusted client device under the semi-trusted client threat model. The file to be secured is stored in one or more blocks belonging to the storage resources along with symmetric per-block key(s) KBi assigned to each of the blocks in the file. The blocks are encrypted with the symmetric per-block keys to obtain encrypted blocks. The user is assigned user key(s) and each per-block key that was used for encryption is in turn encrypted with one of the user's keys to derive wrapped key(s) for each encrypted block. Wrapped key(s) are placed in encrypted block headers and introduce a level of indirection to encrypted file(s) that is appropriate for the semi-trusted client threat model.