TCFS: Transparent Cryptographic File System
Suppose you have a server named foo and a client named bar and suppose you export the directory tree named /exports from server foo to client bar. For this to be true foo must have the following line in /etc/exports:
Now, login as root on bar and mount /exports by typing:
mount -t tcfs foo:/exports /mnt/tcfsThis command causes the remote file system, /exports@foo, to be mounted on the local file system, /mnt/tcfs@localhost, via a TCFS layer.
Now, suppose your login is usdm1, and you own a directory named /mnt/tcfs/usdm1. Login as usdm1 on bar and execute tcfslogin; doing so enables you to use encryption in your directory /mnt/tcfs/usdml. If tcfslogin is not issued, a permission denied error will be issued when attempting to access files with the X flag set.
In order to evaluate the overhead introduced by encryption of the data sent over the network, we performed a set of tests. We ran the test in the following framework:
The client machine running TCFS on the Linux 2.0.23 kernel is a Cyrix x686 166MHz processor
The server machine running as the NFS+xattrd file server is an Intel Pentium 133MHz processor with a 2GB fast SCSI disk.
Since encryption/decryption is a CPU-bound task, having a fast client to perform encryption results in better performance. TCFS makes use of standard VFS caches—no special caching is needed.
time dd bs=xxx if=file of=/dev/null count=n
and for the write operations:
time dd bs=xxx if=/dev/zero of=file count=n
The tables show the following results:
The overall performance of TCFS for write operations is close to NFS performances plus DES overhead. In the write, we suffer due to the lack of a cache system, since data are written directly to the server file system.
The performance of TCFS for read operations seems to hide part of the DES time, since VFS caches reduce server I/O.
Some extra cost is paid by TCFS for I/O of unencrypted files due to handling of extended attributes. In NFS several getattr calls are needed to update inode caching. In TCFS we need a getattr and a geteattr to update inode caching. This causes some extra overhead in TCFS I/O.
Use of other ciphers will result in different performances. We are planning to use IDEA, RC5 and other ciphers as optional modules for TCFS.
Ermelindo Mauriello (firstname.lastname@example.org) was born in Avellino, Italy on December 10, 1972. He is a computer science student at the Dipartimento di Informatica ed Applicazioni “Renato M. Capocelli” of the Universita' di Salerno in Italy. He has been working on the TCFS project since 1995.
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