#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>

char xlat[] = {
	0x64, 0x73, 0x66, 0x64, 0x3b, 0x6b, 0x66, 0x6f,
	0x41, 0x2c, 0x2e, 0x69, 0x79, 0x65, 0x77, 0x72,
	0x6b, 0x6c, 0x64, 0x4a, 0x4b, 0x44, 0x48, 0x53,
	0x55, 0x42, 0x73, 0x67, 0x76, 0x63, 0x61, 0x36,
	0x39, 0x38, 0x33, 0x34, 0x6e, 0x63, 0x78, 0x76,
	0x39, 0x38, 0x37, 0x33, 0x32, 0x35, 0x34, 0x6b,
	0x3b, 0x66, 0x67, 0x38, 0x37, 0x64, 0x73, 0x66,
	0x64, 0x3b, 0x6b, 0x66, 0x6f, 0x41, 0x2c, 0x2e,
	0x69, 0x79, 0x65, 0x77, 0x72, 0x6b, 0x6c, 0x64,
	0x4a, 0x4b, 0x44, 0x48, 0x53, 0x55, 0x42, 0x73,
	0x67, 0x76, 0x63, 0x61, 0x36, 0x39, 0x38, 0x33,
	0x34, 0x6e, 0x63, 0x78, 0x76, 0x39, 0x38, 0x37,
	0x33, 0x32, 0x35, 0x34, 0x6b, 0x3b, 0x66, 0x67,
	0x38, 0x37
};

char *pname;

int decrypt(unsigned char *enc_pw, unsigned char *dec_pw)
{
	unsigned int seed, i, val = 0;

	if (strlen(enc_pw) & 1)
		return -1;

	seed = (enc_pw[0] - '0') * 10 + enc_pw[1] - '0';

	if (seed > 15 || !isdigit(enc_pw[0]) || !isdigit(enc_pw[1]))
		return -2;

	for (i = 2 ; i <= strlen(enc_pw); i++) 
	{
		if (i !=2 && !(i & 1)) {
			dec_pw[i/2 - 2] = val ^ xlat[seed++];
			val = 0;
		}

		val *= 16;

		if (isdigit(enc_pw[i] = toupper(enc_pw[i]))) {
			val += enc_pw[i] - '0';
			continue;
		}

		if (enc_pw[i] >= 'A' && enc_pw[i] <= 'F') {
			val += enc_pw[i] - 'A' + 10;
			continue;
		}

		if (strlen(enc_pw) != i)
			return -4;
	}
	dec_pw[++i / 2] = 0;
	return 0;
}

void usage()
{
	fprintf(stdout, "Usage: %s <encrypted password>\n", pname);
	exit (1);
}

int main(int argc, char *argv[])
{
	char *passwd;
	pname = argv[0];
	int i;

	if (argc != 2)
		usage();

	if ((strlen(argv[1])%2)!=0) {
		fprintf(stderr, "Invalid password length - must be even!\n\n");
		usage();
	}

	for (i=0; i<strlen(argv[1]); i++)
	{
		*argv[1]=toupper(*argv[1]);
		if( ! ((*argv[1]>='A' && *argv[1]<='F') || (*argv[1]>='0' && *argv[1]<='9')) ) {
			fprintf(stderr, "Invalid character in password found!\n");
			usage();
		}
	}

	if (!(passwd = calloc(1,(sizeof(argv[1]))%2+2)))
		exit (1);

	if (decrypt(argv[1], passwd)) {
	fprintf(stderr, "String decryption error!\n");
	free(passwd);
	exit (1);
	}
	fprintf(stdout, "Passwd: %s\n", passwd);
	free(passwd);
	return 0;
}