Encrypting Block Algorithm

package com.company;

import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {
    public static String message;
public static String key;
public static int min,max;
public static StringBuilder encrypted = new StringBuilder();
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        getInput(sc);
        testForKeyLength();
        testForRepeatingNumbers();
        message = addIntervalsIfNeeded(message);
        min = getMin();
        max = getMax();
        int i = 0, j=8;
        String block;
        while(true){
            try {
                block = message.substring(i, j);
            } catch(IndexOutOfBoundsException e){
                break;
            }
            i += 8;
            j += 8;
        for(int y=0; y<block.length(); y++) {
            int index = getIndex((char) min);
            encrypted.append(block.charAt(index));
            min++;
            if(min>max){
                min= getMin();
            }
        }
        }
        System.out.println(encrypted.toString());


    }

    private static void getInput(Scanner sc) {
        System.out.println("Enter message:");
        message = sc.nextLine();
        System.out.println("Enter key:");
        key = sc.nextLine();
    }

    private static void testForKeyLength() {
        if(key.length() != 8){
            System.out.println("ERROR KEY MUST BE 8 SYMBOLS!");
            System.exit(0);
        }
    }

    private static void testForRepeatingNumbers() {
        LinkedHashMap<Character,Integer> map = new LinkedHashMap<Character,Integer>();
        for(char ch: key.toCharArray()){
            if(map.containsKey(ch)){
                map.put(ch,map.get(ch)+1);
                continue;
            }
            map.put(ch,1);
        }

        for(Map.Entry<Character,Integer> entry : map.entrySet()){
            if(entry.getValue()>=2){
                System.out.println("ERROR KEY MUST BE WITH UNIQUE NUMBERS!");
                System.exit(0);
            }
        }
}

    private static String addIntervalsIfNeeded(String message) {
        if(message.length()%8!=0) {
            StringBuilder temp = new StringBuilder(message);
            int numIntervals = message.length()%8;
            temp.append(" ".repeat(Math.max(0, numIntervals)));
            message = temp.toString();
        }
        return message;
    }

    public static int getMin(){
        int min = Integer.MAX_VALUE;
    for(int i=0; i<key.length(); i++){
        if(key.charAt(i) - '0' < min){
            min = key.charAt(i) - '0';
        }
    }
    return min;
}



    public static int getMax(){
        int min = Integer.MIN_VALUE;
        for(int i=0; i<key.length(); i++){
            if(key.charAt(i) - '0' > max){
                max = key.charAt(i) - '0';
            }
        }
        return max;
    }
    public static int getIndex(char ch){
        for(int i=0; i<key.length(); i++){
            if(key.charAt(i)-48==ch){
                return i;
            }
        }
        return -1;
    }
}