problem059.java

package project_beuler;

import java.util.ArrayList;
import java.util.Scanner;

public class problem059 {
	
//	Each character on a computer is assigned a unique code and the
//	preferred standard is ASCII (American Standard Code for
//	Information Interchange). For example, uppercase A = 65, 
//	asterisk (*) = 42, and lowercase k = 107.
//
//	A modern encryption method is to take a text file, convert the bytes to ASCII,
//	then XOR each byte with a given value, taken from a secret key. The advantage
//	with the XOR function is that using the same encryption key on the cipher text,
//	restores the plain text; for example, 65 XOR 42 = 107, then 107 XOR 42 = 65.
//
//	For unbreakable encryption, the key is the same length as the plain text message,
//	and the key is made up of random bytes. The user would keep the encrypted message 
//	and the encryption key in different locations, and without both "halves", it is
//	impossible to decrypt the message.
//
//	Unfortunately, this method is impractical for most users, so the modified method
//	is to use a password as a key. If the password is shorter than the message,
//	which is likely, the key is repeated cyclically throughout the message. The
//	balance for this method is using a sufficiently long password key for security,
//	but short enough to be memorable.
//
//	Your task has been made easy, as the encryption key consists of three lower
//	case characters. Using p059_cipher.txt (right click and 'Save Link/Target As...'),
//	a file containing the encrypted ASCII codes, and the knowledge that the plain 
//	text must contain common English words, decrypt the message and find the sum 
//	of the ASCII values in the original text.

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		String[] input = Interface.parseArr("problem059.txt")[0].split(",");
		Scanner reader = new Scanner(System.in);
		
		for(char key1 = 'a';  key1 <= 'z'; key1++)
		for(char key2 = 'a'; key2 <= 'z'; key2++)
		for(char key3 = 'a'; key3 <= 'z'; key3++) {
			int cycle = 0;
			boolean english = true;
			String translated = "";
			int sum = 0;
			for(String i : input) {
				char n = decrypt(i, new char[] {key1, key2, key3}[cycle]);
				english &= isEnglish(n);
				translated += n;
				sum += n;
				cycle = (cycle+1)%3;
			}
			if( english && !quit(reader) ) {
				System.out.println(translated);
				System.out.println("KEYS: "+ key1 +" "+ key2 +" "+ key3);
				System.out.println("SUM: "+sum);
			}
		}
	}
	public static boolean isEnglish(char c) {
		return //32 <= c && c <= 127;
		 
		(
			('a' <= c && c <= 'z') ||
			('A' <= c && c <= 'Z') ||
			('0' <= c && c <= '9') ||
			 c == ' ' || c == '\t' || c == '\n' ||
			 c == '.' || c == ',' || c == '!' || c == '?' || c == '\"' || c == '\'' ||
			 c == '/' || c == '\\' || c == '-' || c == '_' || c == '(' || c == ')' || c == '+' || c == '=' ||
			 c == ';' || c == ':' || c == '#' || c == '$' || c == '%' || c == '^' || c == '&' || c == '*' || // c == '
			 c == '{' || c == '}' || c == '[' || c == ']' || c == '<' || c == '>' || c == '`'|| c == '~'
		);
	}
	
	public static char decrypt(String n, int key) {
		return (char) (Integer.parseInt(n) ^ key);
	}
	
	public static boolean quit(Scanner reader) {
		System.out.println("\nPress enter to continue, anything else to quit;");
		return reader.nextLine().length() > 0;
	}

}