#include <stdio.h>

#include <string.h>



#define KEYSIZE 61



unsigned char key_enc[KEYSIZE] =

	"*+*This file encrypted with SecurityPlus! (C)SoftByte Labs*+*";

unsigned char key_dec[KEYSIZE];  // to be read from the encrypted file

unsigned char pwd[KEYSIZE+1];

int pwdlen;



int

password_check(void)

{

	int i, j;

	unsigned char sub1, sub2, sub3, sub4;

	unsigned char key_chk[KEYSIZE];



		// we copy the key before modifying it: this routine can be called

		// more than once

	memcpy(key_chk, key_dec, KEYSIZE);



		// init the four subtractors

	sub1 = pwd[0];

	sub2 = pwd[pwdlen-1];

	sub3 = KEYSIZE;

	sub4 = pwd[0];



		// encoding loops

	for (i=0, j=pwdlen; i<KEYSIZE; i++) {

		key_chk[I] -= sub1 + sub2 + sub3 + sub4;



			// update for next loop

		if (++j >= pwdlen) j = 0;

		sub1 = pwd[j];

		sub2 = key_dec[I];

		sub3 = KEYSIZE - 1 - i;

		sub4 += 1 + j;

		}

	return memcmp(key_chk, key_enc, KEYSIZE) == 0;

}



void

main(int argc, char **argv)

{

	int i;

	FILE *fp;

	unsigned char p, p0, tmp;



	if (argc != 2) {

		printf("usage: %s filename.sp$\n", argv[0]);

		return;

		}



	fp = fopen(argv[1], "rb";

		// skip first chunk

	fseek(fp, KEYSIZE, SEEK_SET);

		// bring key_dec[] in memory

	fread(key_dec, KEYSIZE, 1, fp);



		// we use the equation in password_check() loop with i==1 to find the

		// first character of the password

	p0 = key_dec[1] - key_dec[0] - key_enc[1]/*'+'*/ - (KEYSIZE-1) - 1;

	p0 /= 2;

	pwd[0] = p0;

	p0++;  // take into account the '1' above



		// we use the following equations to find the following characters

		// assuming the worst case of a password with length KEYSIZE

	for (i=2; i<KEYSIZE; i++) {

		p0 += i;

		p = key_dec[I] - key_dec[i-1] - key_enc[I] - (KEYSIZE-i) - p0;

		pwd[i-1] = p;

		}



		// we use the equation in password_check() loop with i==0 to find the

		// password length

	for (i=1; i<KEYSIZE; i++) {

		p = key_dec[0] - 2*pwd[0] - pwd[I] - KEYSIZE;

		if (key_enc[0] == p) {

			pwdlen = i+1;

			tmp = pwd[pwdlen];

			pwd[pwdlen] = 0;

				// we need a complete check before exiting: the equation could

				// be satisfied for more than one value

			if (password_check()) break;

				// failure: restore the character replaced by the null

			pwd[pwdlen] = tmp;

			}

		}



	printf("password: \"%s\"\n", pwd);

}