DUE DATE: Friday, November 15, 2002, 5:00 pm

(no late submissions accepted)

Summary

This project introduces students to mixed-language programming, specifically integrating C and assembly code, data encryption, and dynamic libraries.

Your task is to write a program that would encrypt and decrypt data files. The choice of which cipher to use is left up to you. At the least, you should do a simple XOR cipher. (XOR data with a key to encrypt, then XOR it again with the same key to decrypt.) Alternatively, you may want to implement a more secure algorithm, such as DES, Blowfish, or even RSA.

Your program should take four arguments: a switch specifying whether to encrypt or to decrypt the file (-e or -d, respectively), the cipher to use (see below), the source file name and the destination file name. It should then prompt user for a key, e.g. a secret password. After reading the data from the source file and encrypting it with a key, your program should write the encrypted data to the destination file.

For example:

        $ ./crypt -e xor secrets.txt secrets.enc
        Enter the key: 
        $ ./crypt -d xor secrets.enc secrets.txt 
        Enter the key:

Notes

• Your encryption and decryption routine should be written in assembly for speed, and your main program should be written in C for portability. The C program will call the assembly routines, so make sure to follow C calling conventions. The prototypes for the assembly routines should look like this:

          int encrypt(unsigned char *key, unsigned char *data, int size);
          int decrypt(unsigned char *key, unsigned char *data, int size);
  

  That is, both functions should take a pointer to the secret key, a pointer to the data being encrypted or decrypted, and the size of the data in bytes. The return value should be the number of bytes processed, or a negative value on error. That is, upon success the return value will be equal to the size argument.

• Your encryption and decryption routine should be placed in a separate dynamically-loaded library file. That way, it becomes extremely easy to add capability for new ciphers - simply get another cipher file and specify it on the command line, no recompilation or relinking necessary. Recall that the dynamic libraries have a .so extension, so a library that implements simple XOR cipher (as shown in the example above) should be named xor.so.

• Your main program should be able to support any cipher that implements the above two functions. It should read the cipher name from the command line, add the .so extension, and attempt to dynamically load the file. You need to use the dl interface, of course.

• For this project, creating a Makefile is required.

Extra Credit

• (20 points) Implement a more secure cipher, such as DES, 3DES, AES, Blowfish, Twofish, RSA, DSA, etc.

What to turn in

Please submit your source code, Makefile, object file, and executable to your submit directory for this class. (located under /home/submit/cs210.) In addition please submit a printout of your source code to the instructor.