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Security Policy Implementation

This weekend has been taken up almost exclusively by coding (yet again :)). This time, I’m working on a project for ELE 386, also known as Cybersecurity. The task is to develop a working implementation of a well-known security policy in a way that can be used to update permissions and check permissions. I am implementing the Clark-Wilson model and building a multi-platform, command-line interface. Considering I started this about 36 hours ago, I think I’ve made pretty good progress. Normally, I’d write something like this in Java, just due to the ease of using built-in data structures (TreeMaps, HashSets, etc.) but the assignment kinda leaned towards C/C++ (yes, I know I can write my own, but I don’t have the time or patience to..). Seeing as I’ve just picked up C this semester, I decided to go with that as a challenge.

Yesterday, I hacked together the basics of control flow and some I/O (C89 doesn’t allow function overloading. what. -_-). The challenge here is that we are to assume that the user will be malicious; in fact, we will be trying to break each other’s implementations in the next part of the assignment. So, every single input has to be extremely meticulously validated before anything happens. In fact, I’ve completely decided against using scanf() or even fgets() for user-related I/O; I need finer control. So, I’ve decided to literally getchar() each character by hand, doing multiple checks on bounds and legality of inputs. Every input has a pre-defined upper bound and so far, it seems to be watertight. One problem that I encountered was figuring out how to get rid of extraneous characters in the stdin buffer (for example, if the user is asked for at most 3 characters and replies with 46 characters).  I tried a lot of different things and ended up with this approach, a slight adaptation from here:

void flushInput()
{
   char c = ' ';
   ungetc(c, stdin);
   scanf("%c%*[^\n]%*c", &c);
}

Essentially, this little function first creates an arbitrary characters (a space in this example), uses ungetc() to push the space onto the stdin buffer. Then, using a bit of formatted string ninja, scanf() reads in the space, skips everything that’s not a newline characters, and then skips the character right after that (the newline itself). I know it’s not the prettiest piece of code, but it’s faster than doing getchar() in a loop and checking for EOF.

Once I had crossed that hurdle, I was able to quickly get through a lot of other small issues. One source file quickly become 3 which quickly became like 9. The layout and refactoring that I started doing early really helps to keep it all sane. Last night, I also finally was able to glue in an MD5 hashsum implementation with my code, as I only want to be storing the hashes of the passwords instead of the passwords themselves. That ended up working pretty nicely and made my implementation stronger.

This morning I was able to write the database importer that loads the file into memory. I ended up going with an array of struct pointers, which in turn hold a linked list of struct pointers as my primary data structure. So far it’s working well and I’ll have to serialize this abstraction and write the exporter of the database back onto disk. Should be fun. In the meantime, I’ve gotta write some more code to validate queries, which can be rather long and complex. Did I mention I need to get this working by Tuesday? It’s gonna be a long night..

EDIT: One last note. Found an error in the Bryant & O’Hallaron book on page 611. The function prototype for execve() is listed as:

#include <unistd.h>

int execve(char *filename, char *argv[], char *envp);

However, the correct prototype is actually:

#include <unistd.h>

/* Note the braces on envp */
int execve(char *filename, char *argv[], char *envp[]);

Not a huge deal, but it’s an important function, so just wanted to point that one out.

 
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Posted by on April 18, 2010 in Uncategorized

 

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