#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <crypt.h>
#include <string.h>

#define ENCRYPTED_FILE "enMail.txt"

/* Turn a block of bits returned by encrypt to 8 characters */
void block2chars(char block[64], char out[8]) {
	int i, j;
	for (i=0; i<8; ++i) {
		char x = 0;
		for (j=0; j<8; ++j)
			x = (x << 1) | block[i*8+j];
		out[i] = x;
	}
}

/* Turn 8 characters to a block acceptable by encrypt */
void chars2block(char in[8], char block[64]) {
	int i, j;
	for (i=0; i<8; ++i) {
		unsigned char x = in[i];
		for (j=0; j<8; ++j) {
			block[i*8+j] = (x >> 7);
			x <<= 1;
		}
	}
}

/* Turn 8 7-bits characters to a block acceptable by setkey */
void chars2key(const char in[8], char block[64]) {
	int i;
	char passwd[8];
	for (i=0; i<8; ++i)
		passwd[i] = in[i] << 1;
	chars2block(passwd, block);
}

/* Checks whether passwd decrypt encrypted to a string 
   starting with expectedStr */
int rightPassword(char *expectedStr, char *passwd, char *encrypted) {
	char block[64], key[64], str2[8];
	chars2key(passwd, key);
	setkey(key);
	chars2block(encrypted, block);
	encrypt(block, 1);
	block2chars(block, str2);
	return strncmp(str2, expectedStr, 8)==0;
}

int main() {
	int infile;
	char *str="Dearest ", encrypted[8];
	char passwd[8] = "abcxy";

	/* Open the file, read first 8 characters */
	infile = open(ENCRYPTED_FILE, O_RDONLY);
	if (infile == -1) {
		fprintf(stderr, "%s: %s\n", strerror(errno), ENCRYPTED_FILE);
		return -1;
	}
	read(infile, encrypted, 8);
	close(infile);

	/* Try to decrypt for all characters */
	for (passwd[5] = ' '; passwd[5]<='~'; ++passwd[5])
		for (passwd[6]=' '; passwd[6]<='~'; ++passwd[6])
			for (passwd[7]=' '; passwd[7]<='~'; ++passwd[7])
				if (rightPassword(str, passwd, encrypted)) {
					printf("The password is %.8s\n",
					       passwd);
					return 0;
				}
	return 1;
}