How do I use c's DES based crypt function in my crack program in pset 2
Whether crypt function uses DES or MD5-based algorithm is determined by what salt argument you pass to it. If the salt you pass consists of 2 alphanumeric characters, the function uses DES algorithm. You can read more about it here: http://www.gnu.org/software/libc/manual/html_node/crypt.html
Try it out in your program, just don't forget to include crypt.h at the start.