Information Assurance

Clay Shields

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Information Assurance

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For this assignment, you will be writing a program that provides password access to a file on the class linux system. You should use the programming skills you currently have, and avoid researching secure computing topics unless needed.

Like unix, linix uses the suid bit on files to provide a method of changing domains. Programs that need to run at a privilege level that is higher that that of an individual user are often owned by root and have the suid bit set, so that when they run, they run with root's privilege level. It is possible for others to provide access to their user domain using suid programs as well. (Check out page 145 of the text for more on suid programs).

For this project, what you are going to do is write a suid program that will allow others to access a file in your account on ia-class.

  • Your program should be named <user_name>.exe. Even though linix ignores the .exe suffix, we will use that to identify which is the executable.
  • It should read a password from a user who runs the program.
  • If the password is valid, print the contents of an existing file named <user_name>.txt.
  • Your program will also log who accessed <user_name>.txt in another file named <user_name>.log
  • Rather than have a single password for file access, you will have separate passwords that can be issued to different possible users.
  • You can use only passwords as identifiers, or you can assign each user a name at your discretion.
  • Provide a mechanism for adding and removing users who may access the file. This does not have to be fancy, and can be part of your suid program, or can be some simple mechanism like hand-editing a configuration file.
  • If you use a configuration file, it should be named <user_name>.cfg.
Each of you should write your program in either C or C++ on ia-class.

To test your program, you can suid it to your own name and run it yourself. To do this, you have been given another account on the machine, called your secretid account. You can look in your current account, and in there will be a file named with the name of your secretid account. Your password for that account is again your GoCard number.

To make your program suid:

  • Make sure it is executable by doing chmod a+x <user_name>.exe
  • Make it suid by doing chmod +s <user_name>.exe
See the chmod man page for more information.

When done testing it, I recommend that you unset the suid bit by doing chmod -s <user_name>.exe. This is more secure for the files in your account.

What to turn in:

First, you will create a separate directory in your account on ia-class named NETID-project2 where NETID is your login. Place in this directory a copy of the source code, the executable, the text file to be accessed, and any configuration files required. Do not include any object files you create.

Second, You will mail the instructor a tar file of this directory. You can create this by using the command:

tar -cf NETID.tar NETID-project2

Projects are due before class on February 8, 2007.