#include <stdio.h>

// this program uses brute force hacking to decrypt a file 
// encrypted using the standard "crypt" command in UNIX. It
// assumes that the key is a 4 ASCII character key which
// contains only characters a to z and numbers 0 to 9. The 
// decrypted file is stored in temp.txt each time and if the
// first 3 characters match [a-z0-9] then its stored in the
// file named decryptedfile%key%.txt, where %key% is the key 
// used for that particular decryption.
int main(int argc, char *argv[])	{
	if(argc!=2)	{
		printf("Usage: BruteForceHacker encrypted_filename");
		exit(1);
	}
	int i,j,k,l,w,x,y,z;
	int keybase[36];	// contains the characters a-z and numbers 0-9
	for(i=0;i<26;i++)
		keybase[i] = i+97;
	for(i=26;i<36;i++)
		keybase[i] = i+22;

	char command[50], key[5];	// command contains the crypt command each time around
								// key contains the key each time around

	// looping which takes care of all possible combinations of characters in the keybase
	for(w=0;w<36;w++)
	  for(x=0;x<36;x++)
	    for(y=0;y<36;y++)
	      for(z=0;z<36;z++)	{
		key[0] = keybase[w];
		key[1] = keybase[x];
		key[2] = keybase[y];
		key[3] = keybase[z];
		key[4] = '\0';
		printf("%s\n",key);
		sprintf(command,"crypt %s < %s > temp.txt",key,argv[1]);
		//printf("%s\n",command);
		system(command);	// the decrypted characters are placed in temp.txt
	
		// reading the decrypted file
		char decryptedtext[1000];
		char onechar[1];
		int dtptr = 0;

		FILE *fp;
		fp = fopen("temp.txt","r");
		while(1)	{
			if(fread(onechar,1,1,fp)==0)
				break;
			decryptedtext[dtptr++] = onechar[0];
		}
		fclose(fp);
	
		int flag;
		// checking whether the first 3 characters of the decrypted file contains english characters only
		for(i=0;i<3;i++)	{
			flag = 0;
			for(j=0;j<36;j++)	{
				if(decryptedtext[i]==keybase[j] || decryptedtext[i]==' ')	{
					flag = 1;
					break;
				}
			}
			if(flag==0)	{
				flag = 2;
				break;
			}
		}
		if(flag!=2)	{// there was no mismatch
			printf("\n\ngotcha!\t\t%s\n",key);
			char movecommand[40];
			// store the possible decrypted file in decryptedfile%key%.txt
			sprintf(movecommand,"mv temp.txt decryptedfile%s.txt",key);
			system(movecommand);
		}
	      }
}