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\section*{Handout 4 (Access Control)}

Access control is essentially about deciding whether to grant
access to a resource or deny it. Sounds easy, no? Well it
turns out that things are not as simple as they seem at first
glance. Let us first look, as a case-study, at how access
control is organised in Unix-like systems (Windows systems
have similar access controls, although the details might be
quite different). Then we have a look at how secrecy and
integrity can be ensured in a system, and finally have a look
at shared access control in multi-agent systems.


\subsubsection*{Unix-Style Access Control}

Following the Unix-philosophy that everything is considered as
a file, even memory, ports and so on, access control in Unix
is organised around 11 Bits that specify how a file can be
accessed. These Bits are sometimes called the \emph{permission
attributes} of a file. There are typically three modes for
access: \underline{\textbf{r}}ead, \underline{\textbf{w}}rite
and e\underline{\textbf{x}}ecute. Moreover there are three
user groups to which the modes apply: the owner of the file,
the group the file is associated with and everybody else. This
relatively fine granularity seems to cover many useful
scenarios of access control. A typical example of some files
with permission attributes is as follows:

{\small\lstinputlisting[language={}]{../slides/lst}}

\noindent The leading \pcode{d} in Lines 2 and 6 indicate that
the file is a directory, whereby in the Unix-tradition the
\pcode{.} points to the directory itself. The \pcode{..}
points at the directory ``above'', or parent directory. The
second to fourth letter specify how the owner of the file can
access the file. For example Line 3 states that \pcode{ping}
can read and write \pcode{manual.txt}, but cannot execute it.
The next three letters specify how the group members of the
file can access the file. In Line 4, for example, all students
can read and write the file \pcode{report.txt}. Finally the
last three letters specify how everybody else can access a
file. This should all be relatively familiar and
straightforward. No?

There are already some special rules for directories and
links. If the execute attribute of a directory is \emph{not}
set, then one cannot change into the directory and one cannot
access any file inside it. If the write attribute is
\emph{not} set, then one can change existing files (provide
they are changeable), but one cannot create new files. If the
read attribute is \emph{not} set, one cannot search inside the
directory (\pcode{ls -la} does not work) but one can access an
existing file, provided one knows its name. Links to files
never depend on the permission of the link, but the file they
are pointing to. Otherwise one could easily change access
rights to files.

While the above might sound already moderately complicated,
the real complications with Unix-style file permissions
involve the setuid and setgid attributes. For example the file
\pcode{microedit} in Line 5 has the setuid attribute set
(indicated by the \pcode{s} in place of the usual \pcode{x}).
The purpose of setuid and setgid is to solve the following
puzzle: The program \pcode{passwd} allows users to change
their passwords. Therefore \pcode{passwd} needs to have write
access to the file \pcode{/etc/passwd}. But this file cannot
be writable for every user, otherwise anyone can set anyone
else's password. So changing securely passwords cannot be
achieved with the simple Unix access rights discussed so far.
While this situation might look like an anomaly, it is in fact
an often occurring problem. For example looking at current
active processes with \pcode{/bin/ps} requires access to
internal data structures of the operating system, which only
root should be allowed to. In fact any of the following
actions cannot be configured for single users, but need
privileged root access

\begin{itemize}
\item changing system databases (users, groups, routing tables
and so on)
\item opening a network port below 1024
\item interacting with peripheral hardware, such as printers, 
harddisk etc
\item overwriting operating system facilities, like
process scheduling and memory management
\end{itemize}

\noindent This will typically involve quite a lot of programs
on a Unix system. I counted 90 programs with the setuid
attribute set on my bog-standard Mac OSX system (including the
program \pcode{/usr/bin/login} for example). The problem is
that if there is a security problem with only one of them, be
it a buffer overflow for example, then malicious users can
gain root access (and for outside attackers it is much easier
to take over a system). Unfortunately it is rather easy to
cause a security problem since the handling of elevating and
dropping access rights in such programs rests entirely with
the programmer.

The fundamental idea behind the setuid attribute is that a
file will be able to run not with the callers access rights,
but with the rights of the owner of the file. So
\pcode{/usr/bin/login} will always be running with root access
rights, no matter who invokes this program. The problem is
that this entails a rather complicated semantics of what the
identity of a process (that runs the program) is. One would
hope there is only one such ID, but in fact Unix distinguishes
three(!):

\begin{itemize}
\item \emph{real identity}\\ 
This is the ID of the user who creates 
the process; can only be changed to something else by root. 
\item \emph{effective identity}\\ 
This is the ID that is used to 
grant or deny access to a resource; can be changed to either
the real identity or saved identity by users, can be changed 
to anything by root.
\item \emph{saved identity}\\
If the setuid bit set in a file then the process is started
with the real identity of the user who started the program,
and the identity of the owner of the program as effective and
saved identity. If the setuid bit is not set, then the
saved identity will be the real identity. 
\end{itemize}

\noindent As an example consider again the \pcode{passwd}
program. When started by, say the user \pcode{foo}, it has at
the beginning the identities:

\begin{itemize}
\item \emph{real identity}: \pcode{foo}\\
\emph{effective identity}: \pcode{foo}\\ 
\emph{saved identity}: \pcode{root}
\end{itemize}

\noindent It is then allowed to change the effective
identity to the saved identity to have

\begin{itemize}
\item \emph{real identity}: \pcode{foo}\\
\emph{effective identity}: \pcode{root}\\ 
\emph{saved identity}: \pcode{root}
\end{itemize}

\noindent It can now read and write the file
\pcode{/etc/passwd}. After finishing the job it is supposed to
drop the effective identity back to \pcode{foo}. This is the
responsibility of the programmers who wrote \pcode{passwd}.
Notice that the effective identity is not automatically
elevated to \pcode{root}, but the program itself must make
this change. After it has done the work, the effective
identity should go back to the real identity.


Despite this complicated semantics, Unix-style access control
is of no use in a number of situations. For example it cannot
be used to exclude some subset of people, but otherwise have
files readable by everybody else (say you want to restrict
access to a file such that your office mates cannot access 
a file). You could try setting the group of the file to this
subset and then restrict access accordingly. But this does not
help, because users can drop membership in groups. If one
needs such fine-grained control over who can access a file,
one needs more powerful \emph{mandatory access controls}
as described next.


\subsubsection*{Secrecy and Integrity}

Often you need to keep information secret within a system or
organisation, or secret from the ``outside world''. An example
would be to keep insiders from leaking information to
competitors. An instance of such an access control system is
the secrecy levels used in the military. There you distinguish
usually four secrecy levels:

\begin{itemize}
\item top secret
\item secret
\item confidential
\item unclassified
\end{itemize}

The idea is that the secrets classified as top-secret are most
closely guarded and only accessible to people who have a
special clearance. The unclassified category is the lowest
level not needing any clearance. While the idea behind these
security levels is quite straightforward, there are some
interesting phenomenons that you need to think about when
realising such a system. First this kind of access control
needs to be \emph{mandatory} as opposed to
\emph{discretionary}. With discretionary access control, the
users can decide how to restrict or grant access to resources.
With mandatory access control, the access to resources is
enforced ``system-wide'' and cannot be controlled by the user.
There would be no point to let users set the secrecy level,
because if they want to leak information they would set it to
the lowest. Even if there is no malicious intent, it could
happen that somebody by accident sets the secrecy level too
low for a document. Note also that the secrecy levels are in
tension with the Unix-style access controls. There root is
allowed to do everything, but in a system enforcing secrecy,
you might not like to give root such powers. 

There are also some interesting rules for reading and writing
a resource that need to be enforced: 


\begin{itemize}
\item {\bf Read Rule}: a principal $P$ can read a resource $O$
      provided $P$'s security level is at least as high as
      $O$'s
\item {\bf Write Rule}: a principal $P$ can write a resource
      $O$ provided $O$'s security level is at least as high as
      $P$'s 
\end{itemize} 

\noindent The first rule implies that a principal with secret
clearance can read secret documents or lower, but not
documents classified top-secret. The second rule for writing
needs to be the other way around: someone with secret
clearance can write secret or top-secret documents---no
information is leaked in these cases. In contrast the
principal cannot write confidential documents, because then
information can be leaked to lower levels. These rules about
enforcing secrecy with multi-level clearances are often called
\emph{Bell/LaPadula} model, named after two people who studied
such systems.

A problem with this kind of access control system is when two
people want to talk to each other but are assigned different
security clearances, say secret and confidential. In these
situations, the people with the higher clearance have to lower
their security level and are not allowed to take any document
from the higher level with them to the lower level (otherwise
information could be leaked). In actual systems, this
might mean that people need to log out and log into the system
again---this time with credentials for the lower level.

While secrecy is one property you often want to enforce,
integrity is another. This property ensures that nobody
without adequate clearance can change, or tamper with,
systems. An example for this property is a \emph{fire-wall},
which isolates a local system from threads from the 
Internet, for example. The rule for such a system is
that somebody from inside the fire-wall can write resources
outside the firewall, but you cannot write a resource inside 
the fire-wall from outside. Otherwise an outside can just
tamper with a system in order to break in. In contrast
we can read resources from inside the fire-wall, for example
web-pages. But we cannot read anything from outside the 
fire-wall. Lest we might introduce a virus into the system
(behind the fire-wall). In effect in order to ensure
integrity the read and write rules are reversed from the
case of secrecy:

\begin{itemize}
\item {\bf Read Rule}: a principal $P$ can read a resource $O$
      provided $P$'s security level is lower or equal than
      $O$'s
\item {\bf Write Rule}: a principal $P$ can write a resource
      $O$ provided $O$'s security level is lower or equal than
      $P$'s 
\end{itemize} 

\noindent This kind of access control system is called
\emph{Biba} model, named after Kenneth Biba. Its purpose is to
prevent data modification by unauthorised principals.

The somewhat paradoxical result of the different reading and
writing rules in the \emph{\mbox{Bell}/LaPadula} and
\emph{Biba} models is that we cannot have secrecy and
integrity at the same time in a system, or they need to be
enforced by different means.

\subsubsection*{Multi-Agent Access Control}

In military or banking, for example, very critical decisions
need to be made using a \emph{two-people rule}. This means such
decisions need to be taken by two people together, so that no
single person can defraud a bank or start a nuclear war (you
will know what I mean if you have seen the classic movie ``Dr
Strangelove or: How I Learned to Stop Worrying and Love the
Bomb''\footnote{\url{http://en.wikipedia.org/wiki/Dr._Strangelove}}).
Translating the two-people rule into a software system seems not
as straightforward as one might think.

Let us assume we want to implement a system where CEOs can
make decisions on their own, for example whether or not to
sell assets, but two managing directors (MDs) need to come
together to make the same decision. If ``lowly'' directors
(Ds) want to take this decision, three need to come together.
Remember cryptographic keys are just sequences of bits. A
naive solution to the problem above is to split the necessary
key into $n$ parts according to the ``level'' where the
decision is taken. For example one complete key for a CEO, 
halves of the key for the MDs and thirds for the Ds. The
problem with this kind of sharing a key is that there might be
many hundreds MDs and Ds in your organisations. Simple-minded
halving or devision by three of the key just does not work.

A much more clever solution was proposed by Blakley and Shamir
in 1979. This solution is inspired by some simple geometric
laws. Suppose a three-dimentional axis system. We can, clearly,
specify a point on the $z$-axis, say, by specifying its
coordinates. But we could equally specify this point by a line
that intersects the $z$-axis in this point. How can a line be
specified? Well, by giving two points in space. But as you
might remember from school days, we can specify the point also
by a plane intersecting the $z$-axis and a plane can be
specified by three points in space. This could be pictured as
follows:

\begin{center}
\includegraphics[scale=0.45]{../pics/pointsplane.jpg}
\end{center}

\noindent The idea is to use the points as keys for each level
of shared access. The CEO gets the point directly. The MDs get
keys lying on a line and the Ds get keys lying on the plane.
Clever, no? Scaling this idea to more dimensions allows for
even more levels of access control and more interesting access
rules, like one MD and 2 Ds can take a decision together.

Is such a shared access control used in practice? Well
military command-chains are obviously organised like this.
But in software systems often need to rely on data that might
not be entirely accurate. So the CEO-level would correspond
to the in-house data-source that you can trust completely.
The MD-level would correspond to simple errors where you need
three inputs and you decide on what to do next according to
what at least two data-sources agree (the third source 
is then disregarded, because it is assumed it contains an 
error). If your data contains not just simple errors, you
need levels corresponding to Ds.


\subsubsection*{Further Information}

If you want to know more about the intricacies of the
``simple'' Unix access control system you might find the
relatively readable paper about ``Setuid Demystified'' 
useful.

\begin{center}\small
\url{http://www.cs.berkeley.edu/~daw/papers/setuid-usenix02.pdf}
\end{center}

\noindent About secrecy and integrity, and shared access
control I recommend to read the chapters on ``Nuclear Command
and Control'' and ``Multi-Level Security'' in Ross Anderson's
Security Engineering book (whose first edition is free).

\end{document}

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