[Algorythm] Read-write on a shared file

From: xm (xm_at_ca.inter.net)
Date: 08/01/04 

Date: 31 Jul 2004 22:21:29 -0700

Hi there,
  I have a problem that I would like to program a solution for, though
I'm wondering about the algorythm and I was looking for your comments.

  Here's the problem:
  There one file (the file) which is stored on N different computers.
There is no central host. Only a fraction of those computers have
read and write permission and the rest only have read permission.
  The file is thus based on each and every computer in this situation.
 And let say the file contains sensitive information that Must not be
changed by someone that cannot write to it. Moreover, let say part of
that file must be unreadable or encrypted to specific individuals.
  Then let say a hacker in in front of one of the computer that only
have read access. Let say this hacker will try to sniff the
networking protocol, that he will try to crash the program and dig
into the core dump, or possibly will use some tool to look right in
memory while the program is running. Anyhow, he must not read the
portions that he's not supposed to and he must not commit changes to
the file that will change the files on the other computers.

  Here's my approach: (it might be wrong, this is where you can
comment aplenty!)
  In the file is written read and write permissions for all users (or
there may be a separate file for permissions). Some parts of the file
will be encrypted with different algorythms (or different keys) for
the different users or groups that have access to those parts.
  Now, if someone wants to read this file they must understand it's
format to change the read/write permissions (which is possible
locally) and then read the sections that the hacker wants, but they
will be encrypted with a key, that will be stored locally on the
specific computers that are allowed to decrypt these sections.
Therefore the hacker does not have a key. And sniffing the network to
catch one being sent from a trusted individual to another is
irrelevant because all communications will be SSL encrypted.
  And if the hacker wants to commit changes to the file, he must forge
himself to appear as being one of the users that have write access in
that file, then send the change via the program (there will be a
special protocol for changes) which will contact different hosts and
negotiate the change.
  These other hosts may or may not be compromised (at worst!) and they
will receive a modification from a certain user (here I'm thinking the
SSL session could be encrypted using the private key of the sender to
the public key of the receiver and those keys should corresponds to
their logins, so how would it be possible to inpersonnate someone else
without those private keys?). So a computer receives a modification
from a certain user, this computer checks to see what is the file in
question, it is an important file which contains data that himself
cannot read or change, so this computer will deny the change and tell
the computer to contact this and that computer to make the change.
Those pointed computers will be the ones with write access, and they
will look a the change, the user requesting it, the keys and will
decide if they commit or not the change.

  Now here I am with a 'decide' algorythm to make. But I'm not sure
what could prove a decision to be good or not. Possibly the
username(falsified in this example) and the public key (it is public,
so anyone can have it) plus a communication peer-to-peer encrypted
with the private key of the sender (which the hacker cannot get) and
the public key of the receiver.
  In other words, for authentication over this network, only the
peer-to-peer encrypted handshake could serve the purpose? Or maybe
that is not enough.

Thanks for any comments!
  xm

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