[LAU] OT: seeking help with uploading files via sftp and preventing their subsequent deletion

All, I am in a bit of a time-bind and am wondering if anyone could help me with this. Namely, I am trying to cobble a sftp system where conference participants may want to upload their proposed submissions and once they are uploaded that they are unable to delete their own or anyone else's submission. Going with commercial solutions is not an option. So, what I did so far is change /etc/ssh/sshd_config so that sftp chroots said user's home dir, and prevents access via ssh. I also created a sftponly group and added the user to it. I adjusted home dir permissions and created a subfolder "submissions" where users can submit their projects. Finally, I added umask to strip permissions from uploaded files. So, the /etc/ssh/sshd_config has the following entry Match Group sftponly ChrootDirectory /home/%u ForceCommand internal-sftp -u 0222 X11Forwarding no AllowTcpForwarding no So, everything works, except no matter what permissions assign via umask, even if I change ownership manually via a different ssh user session, sftp client can still erase the file. How is this possible? And more importantly, how can one circumvent that? And perhaps most importantly is there an easier way to do this? Below are permissions of folders in question: drwxr-xr-x 3 root USER 4096 Oct 10 15:21 . drwxr-xr-x 36 root root 4096 Oct 7 12:16 .. drwxr-xr-x 2 USER sftponly 4096 Oct 10 19:39 submissions Any idea how this can be fixed? -- Ivica Ico Bukvic, D.M.A. Associate Professor Computer Music ICAT Senior Fellow DISIS, L2Ork Virginia Tech School of Performing Arts – 0141 Blacksburg, VA 24061 (540) 231-6139 ico(a)vt.edu www.performingarts.vt.edu disis.music.vt.edu l2ork.music.vt.edu