import java.util.Scanner;
public class Exer5
{
public static void main(String[] args)
{
    Scanner sc=new Scanner(System.in);
    String str;
    String key;
    int keyLength;

    System.out.println("Enter message:");
    str=sc.nextLine();
    System.out.println("Enter encryption key:");
    key=sc.next();
    keyLength=key.length();
    //This for loop is repeated use of 'Enrypt' and 'Decrypt' options
    for(;;)
    {
        System.out.println("1.Encrypt\n2.Decrypt\n3.Exit...");
        int choice=sc.nextInt();
        switch(choice)
        {
            case 1:
            /*send input string keyLength to encrypt() method to encrypt it returns 'Encrypted' string*/
                System.out.println("Encrypted message..."+encrypt(str,keyLength));
                break;
            case 2:
                //send retrived string from encrypt() method and keyLength to decrypt() method it returns 'Decrypted' string
                System.out.println("Decryptedmessage..."+decrypt(encrypt(str,keyLength),keyLength));
                break;
            case 3:
                //exit from the program
                System.exit(0);
                break;
            default:
            System.out.println("Invalid option..");
        }
    }
}
public static String encrypt(String str,int keyLength)
{
    String encrypted="";
    for(int i=0;i<str.length();i++)
    {
        //stores ascii value of character in the string at index 'i'
        int c=str.charAt(i);
        //encryption logic for uppercase letters
        if(Character.isUpperCase(c))
        {
            c=c+(keyLength%26);
            //if c value exceeds the ascii value of 'Z' reduce it by subtracting 26(no.of alphabets) to keep in boundaries of ascii values of 'A' and 'Z'
            if(c>'Z')
                c=c-26;
        }
        //encryption logic for lowercase letters
        else if(Character.isLowerCase(c))
        {
            c=c+(keyLength%26);
            //if c value exceeds the ascii value of 'z' reduce it by subtracting 26(no.of alphabets) to keep in boundaries of ascii values of 'a' and 'z'
            if(c>'z')
                c=c-26;
        }
        //concatinate the encrypted characters/strings
        encrypted=encrypted+(char) c;
    }
    return encrypted;
}
public static String decrypt(String str,int keyLength)
{
    String decrypted="";
    for(int i=0;i<str.length();i++)
    {
        //stores ascii value of character in the string at index 'i'
        int c=str.charAt(i);
        //decryption logic for uppercase letters
        if(Character.isUpperCase(c))
        {
            c=c-(keyLength%26);
            //if c value deceed the ascii value of 'A' increase it by adding 26(no.of alphabets) to keep in boundaries of ascii values of 'A' and 'Z'
            if(c<'A')
                c=c+26;
        }
        //decryption logic for uppercase letters
        else if(Character.isLowerCase(c))
        {
            c=c-(keyLength%26);
            //if c value deceed the ascii value of 'A' increase it by adding 26(no.of alphabets) to keep in boundaries of ascii values of 'A' and 'Z'
            if(c<'a')
                c=c+26;
        }
        //concatinate the decrypted characters/strings
        decrypted=decrypted+(char) c;
    }
    return decrypted;
}
}