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Implement a Defensive Security System

This assignment will help you understand security mechanisms. You will be guided through the steps of creating a reference monitor using the security layer functionality in Repy V2. A reference monitor is an access control concept that refers to an abstract machine that mediates all access to objects by subjects. This can be used to allow, deny, or change the behavior of any set of calls. While not a perfect way of validating your reference monitor, it is useful to create test cases to see whether your security layer will work as expected. (The test cases may be turned in as part of the next assignment.)

This assignment is intended to reinforce concepts about access control and reference monitors in a hands-on manner.


In this assignment you will create a security layer which keeps a backup copy of a file in case it is written incorrectly. This is a common technique for things like firmware images where a system may not be able to recover if the file is written incorrectly. For this assignment, every `correct' file must start with the character 'S' and end with the character 'E'. If any other characters (including lowercase 's', 'e', etc.) are the first or last characters, then the file is considered invalid.

However, you must permit the application to write information into the file.
The application should not be blocked from performing any writeat() operation, because when it chooses it may later write 'S' at the start and 'E' at the end. Note that checking if the file starts with 'S' and ends with 'E' is only performed when close is called.

You may store two copies of A/B files on disk, one that is the valid backup (which is used for reading) and the other that is written to. When an app calls ABopenfile(), this indicates that the A/B files, which you should name filename.a and filename.b, should be opened.
When the app calls readat(), all reads must be performed on the valid file. Similarly, when the app calls writeat(), all writes must be performed on the invalid file. If the app uses ABopenfile() to create a file that does not exist (by setting create=True when calling ABopenfile()), the reference monitor will create a new file 'SE' in filename.a and an empty file called filename.b. When close() is called on the file, if a file is not valid, it is discarded. if both files are valid, the older one is discarded.

Note that the behavior of other file system calls should remain unchanged. This means listfiles(), removefile(), and calls to files accessed with openfile() instead of ABopenfile() remain unchanged by this reference monitor.

Three design paradigms are at work in this assignment: accuracy, efficiency, and security.

  • Accuracy: The security layer should only stop certain actions from being blocked. All other actions should be allowed. For example, if an app tries to read data from a valid file, this must succeed as per normal and must not be blocked. All situations that are not described above must match that of the underlying API.

  • Efficiency: The security layer should use a minimum number of resources, so performance is not compromised. For example, keeping a complete copy of every file on disk in memory would be forbidden.

  • Security: The attacker should not be able to circumvent the security layer. Hence, if the attacker can cause an invalid file to be read or can write to a valid file, then the security is compromised, for example.

Getting Python and RepyV2

Please refer to the SeattleTestbed Build Instructions for details.

Once you have built RepyV2 into a directory of your choice, change into that directory. Use the command below in order to run your RepyV2 programs:

python restrictions.default encasementlib.r2py [security_layer].r2py [program].r2py

(Replace [security_layer].r2py and [program].r2py by the names of the security layers and program that you want to run.)

In order to test whether or not these steps worked, please copy and paste the code found below for the sample security layer and sample attack.

If you got an error, please go through the troubleshooting section found below.

Troubleshooting Repy code

If you can't get Repy files to run, some of the following common errors may have occurred:

  • using print instead of log:

Repy is a subset of Python, but its syntax is slightly different. For example, Python's print statement cannot be used; Repy has log for that. For a full list of acceptable syntax please see []

  • command line errors:

files are missing: In the above command line call, you must have, restrictions.default, encasementlib.r2py, the security layer and the program you want to run in the current working directory. If any or all of the above files are not in that directory then you will not be able to run repy files.

Tutorials for Repy and Python

Now that you have Repy and Python, you may need a refresher on how to use them. The following tutorials provide this information.

Building the security layer

The following program is a basic and incomplete sample code for you to get an idea about writing security layer. Remember, you have no idea how the attacker will try to penetrate your security layer, so it is important that you leave nothing to chance!

A basic (and inadequate) defense

Time to start coding! Let's inspect a basic security layer.

This security layer inadequately handles A/B storage for files in RepyV2.

    This security layer uses encasementlib.r2py, restrictions.default, and Python
    Also you need to give it an application to run.
    python restrictions.default encasementlib.r2py [security_layer].r2py [attack_program].r2py 

class ABFile():
  def __init__(self,filename,create):
    # globals
    mycontext['debug'] = False   
    # local (per object) reference to the underlying file
    self.Afn = filename+'.a'
    self.Bfn = filename+'.b'

    # make the files and add 'SE' to the readat file...
    if create:
      self.Afile = openfile(self.Afn,create)
      self.Bfile = openfile(self.Bfn,create)

  def writeat(self,data,offset):
    # Write the requested data to the B file using the sandbox's writeat call
  def readat(self,bytes,offset):
    # Read from the A file using the sandbox's readat...
    return self.Afile.readat(bytes,offset)

  def close(self):

def ABopenfile(filename, create):
  return ABFile(filename,create)

# The code here sets up type checking and variable hiding for you.  You
# should not need to change anything below here.
sec_file_def = {"obj-type":ABFile,

CHILD_CONTEXT_DEF["ABopenfile"] = {TYPE:OBJC,ARGS:(str,bool),EXCP:Exception,RETURN:sec_file_def,TARGET:ABopenfile}

# Execute the user code

Testing your security layer

In this part of the assignment you will pretend to be an attacker. Remember the attacker's objective is to bypass the A/B restrictions or cause the security layer to act in a disallowed manner. By understanding how the attacker thinks, you will be able to write better security layers.

An example of an attack is found below:

if "testfile.txt.a" in listfiles():
if "testfile.txt.b" in listfiles():
myfile=ABopenfile("testfile.txt",True)  #Create an AB file

# I should get 'SE' when reading an empty file...
assert('SE' == myfile.readat(None,0))

# put some valid data in the file.

# I should still get 'SE' because the file wasn't closed.
assert('SE' == myfile.readat(None,0))

#Close the file

Note: All attacks should be written as Repy V2 files, using the .r2py extension.

Choice of File Names

It is important to keep in mind that only lowercase file names are allowed. So in the above code, specifically:

# Open a file

look.txt is a valid file name, however Look.txt and LOOK.TXT are not. Examples of other invalid files names are, _look.txt, look/.txt, and look().txt. Essentially all non-alphanumeric characters are not allowed.

Running your security layer

Finally, type the following commands at the terminal to run your security layer with your attack program

python restrictions.default encasementlib.r2py [security_layer].r2py [attack_program].r2py

Make sure you went through the "How to get RepyV2" section!

Notes and Resources

python restrictions.default encasementlib.r2py [security_layer1].r2py [security_layer2].r2py [security_layer3].r2py [program].r2py

**Your security layer must produce no output!! **

  • In repy log replaces print from python. This may be helpful when testing if Repy installed correctly.

Extra Credit

For extra credit, program that keeps all old versions of files and allows read from any of them. Writing to any old file creates a new (empty) version of that file. Do not submit this code inside your assignment. Submit a separate copy for extra credit.

What to turn in?

  • Turn in a repy file called reference_monitor_[ netid ].r2py with all letters in lowercase.

  • Never raise unexpected errors or produce any output. Your program must produce no output when run normally.

  • For extra credit turn in a second repy file called extra_credit_[netid].r2py You must turn in separate files for the normal assignment and extra credit