Cesar Cipher Decription using java

Question

I am trying to decrypt this Cesar cipher using Java but cannot seem to get the code to decipher the code correctly using the key. I have used a for loop to iterate through all possible keys and print out the decrypted version. The decrypted text should have only capital letters in it.

/**
 * Write a description of class Main here.
 *
 * @author (your name)
 * @version (a version number or a date)
 */
public class Main
{
    // instance variables - replace the example below with your own
    private String message;
    private int key;
    private int c;


    /**
     * Constructor for objects of class Main
     */
    public Main()
    {
        // initialise instance variables
        // y is 76 ascii
        message = "YMUMNINYMNNBYZYUMCVCFCNSIZNBYCLWIOHMYFNBCMCMNBYWBCYZNBCHAVYHINJYLNOLVYXMUCXNBYJUAUHGILUFCMNNBUNQUMDOMNWFULYMIQHIJCHCIHVONBYQUMJYLNOLVYXFYNHINSIOLBYULNVYNLIOVFYXHYCNBYLFYNCNVYUZLUCXMUCXNBYHUTULYHYWFULYWBCGYXCHWILXCUFFSVONBCMBYULNQUMNLIOVFYXUFFNBYMUGYBIQBYQIOFXBUPYFCEYXNIWIHZLIHNNBIMYNQIALYUNNBCHEYLMUHXYULHYMNFSUJJYUFNINBYGUMZYFFIQGUHNIZYFFIQGYHUHXUMENBYGNINYFFBCGNBYCLGYNBIXBCMGIIXNLUHMGONYXCNMYFZCHNIUXIAAYXCHXCZZYLYHWYNCFFUNFYHANBBYZUHWCYXBYQUMFIIECHAIHBCMIQHYRCMNYHWYQCNBNBYJUMMCPYCHNYLYMNIZUHIONMCXYLBYQUMYGVCNNYLYXVSNBYWIHPCWNCIHNBUNUFFNBCMXYMIFUNCIHBUXVYYHVLIOABNUVIONVSNBYUWWCXYHNIZBYLVYCHAUXOLVYLPCFFYQBYHBYZIOHXNBUNNYMMWUGYIZNBUNYRBUOMNYXUHWCYHNFCHYUHXQUMHINIZNBYHYQNLCVYMZLIGVYFIQUMBYBUXZIHXFSXLYUGYXQBSBUXBYHINMNICWUFFSUVUHXIHYXBYLCHZCXYFCNSNIBCMJLCHWCJFYMNBCMQUMQBUNBYBUXAINVSUJIMNUMSUHXBCMJOHCMBGYHNQUMXYMYLPYXNBYHBYVYWUGYQYUL";
        for(int a = 0; a<27; a++)
        {
            key = a;

            for(int i=0; i<message.length(); i++)
            {

                char ch;
                c = message.charAt(i)-key%23;
                ch =(char) (c);
                System.out.print(ch); 


            }
            System.out.println("");
        }

    System.out.println("");

    }  
}

Any help would be much appreciated.

Answer 1

Basically you have to try out 26 keys and every time you leave the range of capital letters(<65) you add 26. Obviously the key you are looking for to decipher your message is 20.

    String message = "YMUMNINYMNNBYZYUMCVCFCNSIZNBYCLWIOHMYFNBCMCMNBYWBCYZNBCHAVYHINJYLNOLVYXMUCXNBYJUAUHGILUFCMNNBUNQUMDOMNWFULYMIQHIJCHCIHVONBYQUMJYLNOLVYXFYNHINSIOLBYULNVYNLIOVFYXHYCNBYLFYNCNVYUZLUCXMUCXNBYHUTULYHYWFULYWBCGYXCHWILXCUFFSVONBCMBYULNQUMNLIOVFYXUFFNBYMUGYBIQBYQIOFXBUPYFCEYXNIWIHZLIHNNBIMYNQIALYUNNBCHEYLMUHXYULHYMNFSUJJYUFNINBYGUMZYFFIQGUHNIZYFFIQGYHUHXUMENBYGNINYFFBCGNBYCLGYNBIXBCMGIIXNLUHMGONYXCNMYFZCHNIUXIAAYXCHXCZZYLYHWYNCFFUNFYHANBBYZUHWCYXBYQUMFIIECHAIHBCMIQHYRCMNYHWYQCNBNBYJUMMCPYCHNYLYMNIZUHIONMCXYLBYQUMYGVCNNYLYXVSNBYWIHPCWNCIHNBUNUFFNBCMXYMIFUNCIHBUXVYYHVLIOABNUVIONVSNBYUWWCXYHNIZBYLVYCHAUXOLVYLPCFFYQBYHBYZIOHXNBUNNYMMWUGYIZNBUNYRBUOMNYXUHWCYHNFCHYUHXQUMHINIZNBYHYQNLCVYMZLIGVYFIQUMBYBUXZIHXFSXLYUGYXQBSBUXBYHINMNICWUFFSUVUHXIHYXBYLCHZCXYFCNSNIBCMJLCHWCJFYMNBCMQUMQBUNBYBUXAINVSUJIMNUMSUHXBCMJOHCMBGYHNQUMXYMYLPYXNBYHBYVYWUGYQYUL";
    for(int a = -1; a<26; a++)
        {
            StringBuilder sb = new StringBuilder();
            for(int i=0; i<message.length(); i++)
            {
                char ch = message.charAt(i);
                int chAscii = (int) ch;
                chAscii -= a;
                if (chAscii < 65) {
                    chAscii = 26 + chAscii;
                }
                char newChar = (char) (chAscii);
                sb.append(newChar);
            }
            System.out.println("For key = "+a+" the message is "+sb.toString());
        }
    }