CS361 Assignment 3

Last modified: Wed Oct 1 15:57:12 CDT 2008

Due: Monday, October 13, by midnight

Deliverables: You will be submitting your code electronically to the TA. Submission instructions are on the class website. Make sure that you clearly identify the members of your team (one or two people). Include a README file so that the TA will understand clearly how to compile and run your code.

Make sure that all members of your team understand the code and contribute equally, as much as possible, to its development.

Background

A question that often arises in secure system design is whether a particular collection of security mechanisms is sufficient to implement a given policy. For example, if you had a Bell and LaPadula system, could you use it to enforce Biba integrity by a clever assignment of labels? (I'm not sure.) This is essentially the problem that Lipner considered in his system: given BLP and Biba rules that were designed for military security, can we use that to enforce a policy designed for a commercial environment?

In your career, you could be asked to implement a policy on top of some existing security mechanisms. This assignment gives you the opportunity to practice doing this in a benign setting.

This is a two part assignment. In part I, you will be implementing a basic Biba Strict Integrity system, where the hierarchical levels and categories are arbitrary. In part II, you'll build upon part I to implement a secure lending library. Build your system such that you can run in either BIBA mode or LIBRARY mode according to a flag that can be entered at the command level.

Part I: Basic Biba

Emulate a Biba Strict Integrity system. Your system should accept the following commands from standard input (which could be from a file): 
   LEVELS  L1 L2 ... Lk
   CATEGORIES C1 C2 ... Cj
   OBJECT obj_name obj_level
   SUBJECT subj_name subj_level
   ADD-CAT obj_name cat_name
   REMOVE-CAT obj_name cat_name
   READ subj_name obj_name
   WRITE subj_name obj_name val
A level can be input as K + 1 separate fields, where the first is the hierarchical component and the others are the need-to-know categories, e.g., "(L1, {A, B, C})" can be entered as "L1 A B C"

The intended semantics of these commands is as follows:

  • LEVELS introduces a linearly ordered set of hierarchical labels, ordered from low to high. There should be only one of these commands. Example: LEVELS L M H
  • CATEGORIES introduces an unordered set of need-to-know categories. Multiple CATEGORIES lines are allowed, and introduce additional categories. Example: CATEGORIES A B C
  • OBJECT introduces a new object into the system at the given level. The level should have both a hierarchical component and a set (which may be empty) of need-to-know categories. Executing another OBJECT command for the same obj_name will replace the old level by a new level. Example: OBJECT O1 L1 A B
  • SUBJECT introduces a new subject into the system at the given level. Executing another SUBJECT command on the same subj_name is not allowed.
  • READ indicates an attempt by the named subject to read the named object. The READ method can return a boolean indicating whether the access is permitted or denied. There is no need to actually implement the functionality of READ. I.e., no value is returned to the subject.
  • WRITE indicates an attempt by the named subject to write to the named object. The WRITE method can return a boolean indicating whether the access is permitted or denied. There is no need to actually store any values. That is, you're not emulating the system operation; only emulating the security checks. The val argument must be present, but can be ignored.
  • ADD-CAT and REMOVE-CAT change the level of the indicated object to add/remove the indicated category from the list of need-to-know categories associated with that object. Note that these operations are merely a convenience since we already have this functionality using the OBJECT command. The supplied category must have been previously introduced in a CATEGORIES statement.

    As usual, commands, including names, are not case sensitive. Enforce reasonable rules. In particular, a subject or object's level should not refer to hierarchical levels or categories that have not been previously introduced. There is no problem with having the same name used in multiple ways. For example, you could have a subject named FRED and an object named FRED.

    The idea is that the LEVELS, CATEGORIES, SUBJECT, OBJECT, ADD-CAT, and REMOVE-CAT commands build up the basic infrastructure for your secure system. Then the system should check whether READ and WRITE operations are permitted using the Biba Strict Integity rules.

    When run in BIBA mode, your system should be able to process an arbitrary list of legal commands and reject any that are illegal. Print a nice log of the execution of successive commands. In particular, for each READ or WRITE, print out whether or not the operation is allowed.

    Part II: Lending Library

    Assume that someone has delivered to you the basic Biba integrity system in Part I. Your task is to use this existing security system to construct a secure on-line lending library.

    The idea is as follows. The library contains various read-only documents (objects): D1, ..., Dm. There is also a pool of potential borrowers (subjects): B1, ..., Bn. Each document has an associated list of allowed borrowers, which is an arbitrary subset of the pool of subjects. For example, it might be that document D19 is only accessible by B3, B7 and B12, while D82 might be accessible only by B7 and B15. The idea of Part II is to implement a system that provides this functionality. Don't implement this from scratch, but use your existing Biba integrity system (Part I) as a platform.

    Your library system will have the following commands, read from standard input: 

       READER reader_name
   DOCUMENT doc_name reader_name1 ... reader_namek
   CHECKOUT doc_name reader_name
   RETURN doc_name reader_name
   REVOKE doc_name reader_name
    These commands have the following interpretation:
  • READER introduces a new potential borrower (subject) into the system.
  • DOCUMENT introduces a new document (object) into the system and authorizes the listed readers to have access to it. Multiple DOCUMENT commands for the same doc_name may augment the permission list with additional readers. They do not revoke any permissions.
  • CHECKOUT delivers the document to the requesting reader if permitted (if the requesting reader is on the list of allowed readers for the document). Otherwise, it does nothing. Your system should keep a record of which documents are checked out to which readers. Multiple readers can have a document checked out at the same time. No reader may check out a document he already holds.
  • RETURN removes the book from the reader's record of items checked out.
  • REVOKE removes the reader's permission to check out that item in the future. However, it does not do anything to seize the item if it is currently checked out by that subject. Keep a record of which documents are checked out to which readers. This is an electronic system so a given document can be checked out to multiple readers at the same time.

    My intention for the way to implement this is as follows. Let's refer to our two systems as "Biba" and "Library". We'll implement the Library system by "compiling" commands into programs for the underlying Biba system.

    Each document/borrower at the Library level will be implemented as an object/subject in the Biba level. Each will have a label that will permit exactly the accesses required. Let's say that document GoneWithTheWind can only be checked out by Fred and Ethel. Then we give the document the label (low, {fred, ethel}). Fred has label (low, {fred}) and Ethel has label (low, {ethel}). Notice that the object level dominates the subject level for Fred and for Ethel, but for no other possible subjects. Following this scheme and the Biba rules, only Fred and Ethel could ever successfully execute a READ instruction on this document.

    To get started, we execute the following command on the underlying BIBA system: 

     LEVELS low
    This is the only hierarchical level required. The work is all done with need-to-know categories.

    Then, each command in the Library system is "compiled" into commands for the underlying Biba system along with some additional recordkeeping. For example, the Library system command 

       READER fred
    is implemented by the Biba commands: 
       CATEGORIES fred
   SUBJECT fred low fred
    This creates a subject named Fred at the Biba level and gives it the appropriate level (low, {fred}).

    The library system commands: 

       DOCUMENT MobyDick fred ethel ricky
   DOCUMENT MobyDick lucy
    are implemented by the Biba commands: 
       OBJECT MobyDick low fred ethel ricky
   ADD-CAT MobyDick lucy
    This creates an object named MobyDick at the Biba level and gives it the integrity label (low, {fred, ethel, ricky, lucy}).

    The library system command: 

       CHECKOUT MobyDick ethel
    is implemented by invoking the Biba command 
       READ MobyDick ethel
    and recording that Ethel has this document checked-out (assuming that the current permissions allow this READ).

    Your task is to devise implementations for the other commands and to implement this system. You will need to maintain a database of the subjects and objects of the system. For a subject, you'll store the name, level, and documents checked out. For an object you'll store the name and level. Note that the labels of objects change over time. The levels of subjects should not change, once established.

    Design your Library system to be useful and fairly user-friendly. Print out a reasonable commentary on what's happening with each instruction executed. Don't worry about covert channels.

    Construct your system such that it can be run in either BIBA mode or LIBRARY mode. In BIBA mode, it accepts only Biba commands; in LIBRARY mode it accepts only Library commands. In Library mode, the BIBA interaction is hidden from the user by the Library command interface. Note that no command in your LIBRARY implementation should ever invoke the BIBA WRITE command. That is the way that you enforce the read-only nature of all the documents.