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The problem is that sftp runs as the user's id -- first, the sftp client ssh's into the target host as the given user, then runs sftp-server. Since sftp-server is running as a regular user, it has no way to "give away" a file (change owner of a file).
However, if you are able to use scp, and assign a key pair to each user, you can get around this. This involves adding a user's key to root's ~/.ssh/authorized_keys file, with a "command=" parameter to force it to run a script that sanitizes and alters the arguments of the server-side scp program. I've used this technique before to set up an anonymous scp dropbox that allowed anyone to submit a file, and ensure that no one could retrieve submitted files and also prevent overwrites.
If you are open to this technique, let me know and I'll update this post with a quick recipe.
We are interested in this scp quick recipe for our community services file sharing.
Best Regards, Gert Jan Zeilstra
