java basic encryption program

Question

I'm trying to make a basic encryption program that used random numbers to encrypt the whole alphabet, encrypt a user submitted phrase and then decrypt it back to the original phrase but I am finding it very hard. Can anyone help point out my mistakes please! It shouldn't code two letters to the same letter, ie a and b shouldn't ever be both matched to c.

public class MainClass {
public static final int ALPHASIZE = 26;
public static final char[] Lalpha =
    { 'a','b','c','d','e','f','g','h','i','j','k','l',
    'm','n','o','p','q','r','s','t','u','v','w','x','y','z'
    };
public static final char[] Ualpha =
    {'A','B','C','D','E','F','G','H','I','J','K','L',
    'M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
    };
protected static char[] encryptU = new char[ALPHASIZE];
protected static int[] decrypt = new int[ALPHASIZE];
protected static char[] encryptL = new char[ALPHASIZE];



Random rgenerator = new Random();

public MainClass(){
    int randNum = rgenerator.nextInt(ALPHASIZE);


    for(int i=0; i<ALPHASIZE ; i++)
    {
        //makes sure that it won't assign a letter to itself or to one that has already been assigned

        do {
            randNum = rgenerator.nextInt(26);

         } while (randNum%26==0 &&Arrays.asList(encryptU).contains(Ualpha[randNum]));       

        encryptU[i] = Ualpha[randNum]; 
        encryptL[i] = Lalpha[randNum];
        decrypt[i] = randNum;

    }
}

public String encrypt(String secret)
{
    System.out.println(Arrays.toString(encryptU));
        int position = 0;
    char[] mess = secret.toCharArray();
    for(int i = 0 ; i<mess.length;i++)
    {
        if(Character.isUpperCase(mess[i]))
        {
    for(int j = 0; j < encryptU.length; j++) {
        if(mess[i]==Ualpha[j]) {
            position = j;
    }
    mess[i] = encryptU[position];

        }

        }

        if(Character.isLowerCase(mess[i]))
        {
    for(int j = 0; j < encryptU.length; j++) {
        if(mess[i]==Lalpha[j]) {
            position = j;
    }
    mess[i] = encryptL[position];

        }

        }

    }
    return new String(mess);
}

public String decrypt(String secret)
{
    char[] mess = secret.toCharArray();
    for(int i = 0 ; i<mess.length;i++)
    {
        if(Character.isUpperCase(mess[i]))
                {
                for(int j = 0; j<ALPHASIZE; j++){
                    if(mess[i]==encryptU[j]){
                        mess[i] = Ualpha[j];

                        }
                    }
                }           

        if(Character.isLowerCase(mess[i]))
        {                   
            for(int j = 0; j<ALPHASIZE; j++){
                if(mess[i]==encryptL[j]){
                    mess[i] = Lalpha[j];

                    }
                }
            }
    }

    return new String(mess);
}
}

Answer 1

You should really consider using a Map to store character/encoding pairs. Oh, and to create these random pairs you can add your characters to a List and make use of Collections.shuffle instead of reinventing the wheel yourself.

---

Let me demonstrate using only Lalpha (only lowercase letters). You want something along these lines:

List<Character> l = new ArrayList<Character>(Lalpha.length); 

for (char c : Lalpha)
    l.add(c);

Collections.shuffle(l);

Map<Character, Character> encoding = new HashMap<Character, Character>(Lalpha.length);
Map<Character, Character> decoding = new HashMap<Character, Character>(Lalpha.length);

for (int i = 0 ; i < Lalpha.length ; i++) {
    encoding.put(Lalpha[i], l.get(i));
    decoding.put(l.get(i), Lalpha[i]);
}

Now lets say we wanted to encode / decode the string helloworld, we would do this:

String s = "helloworld";    

// Encode:
String enc = "";
for (char c : s.toCharArray())
    enc += encoding.get(c);

System.out.println(enc);

// Decode:
String dec = "";
for (char c : enc.toCharArray())
    dec += decoding.get(c);

System.out.println(dec);

Output (one of many possible):

vjwwmtmcwz
helloworld

Of course, you can incorporate uppercase letters and what-not using the same idea.