Thread: [HELP] Encryption - Decryption alphabet

I want to create a java program that can be encrypt-decrypt alphabet from characters . Which is the shifting of the alphabet is starting from the the first character to increase 4 and 5 increased in the second character, increasing 6 at the third character, and so on..
Then when one of the characters increasement is more than the ascii code of 'z', that character will decrease from the variable i from the loop. (Sorry my english isn't good enough)
This is the output of the program that i want:
Encryption:
Plain text: "abcz"
Cipher text: "egiz"

Decryption:
Cipher text: "egiz"
Plain text: "abcz"

This is my code for now:

 
package -;
 
public class - {
 
    public static void main(String[] args) {
 
        String enkripsi = "abcz".toLowerCase();
        String dekripsi = "egiz".toLowerCase();
        System.out.println("============================");
        System.out.println("\tENKRIPSI");
        System.out.println("============================");
        System.out.println("Plain text: ");
        System.out.println(enkripsi);
        System.out.println("");
        System.out.println("Cipher text:");
        enkrip(enkripsi);
        System.out.println("");
        System.out.println("============================");
        System.out.println("");
        System.out.println("============================");
        System.out.println("\t DEKRIPSI");
        System.out.println("============================");
        System.out.println("Cipher text:");
        System.out.println(dekripsi);
        System.out.println("");
        System.out.println("Plain text:");
        dekrip(dekripsi);
        System.out.println("");
        System.out.println("============================");
        System.out.println("");
        System.out.println("============================");
 
    }
 
    public static void enkrip(String enkripsi) {
        int a[] = new int[enkripsi.length()];
        for (int i = 0; i < a.length; i++) {
            a[i] = enkripsi.charAt(i) + 4 + i;
            if (a[i] >= 0 && a[i] <= 96 || a[i] > 122 && a[i] <= 255 ) {
                a[i] = enkripsi.charAt(i) - 1;
                a[i] = enkripsi.charAt(i) - (i % 26);
                if (a[i] < (byte) 'a') {
                    a[i] = enkripsi.charAt(i) + (i % 26);
                } else if (a[i] > (byte) 'z') {
                    a[i] = enkripsi.charAt(i) + (i % 26);
                }
 
            }
 
            System.out.print((char) a[i]);
        }
    }
 
    public static void dekrip(String dekripsi) {
        int b[] = new int[dekripsi.length()];
        for (int i = 0; i < b.length; i++) {
 
            b[i] = dekripsi.charAt(i) - 4 - i;
            if (b[i] >= 0 && b[i] <= 96 || b[i] > 122 && b[i] <= 255) {
            b[i]= dekripsi.charAt(i)-1;    
            }
            if (b[i] < (byte) 'z') {
 
                b[i] = dekripsi.charAt(i) + (i%26) ;
                if (b[i] < (byte) 'a') {
                    b[i] = (i % 26) - dekripsi.charAt(i);
                } else if (b[i] > (byte) 'z') {
                    b[i] = (i % 26) - dekripsi.charAt(i);
                }
 
       }
 
            System.out.print((char) b[i]);
        }
    }
}

What's wrong with my code? It won't work. Specially in the decryption . Please help me...

This is the example when my program executed, and it's wrong in the decryption.

============================
	ENKRIPSI
============================
Plain text: 
lamb
 
Cipher text:
pfsi
============================
 
============================
	 DEKRIPSI
============================
Cipher text:
pfsi
 
Plain text:
pgul
============================
 
============================

============================
	ENKRIPSI
============================
Plain text: 
friedchicken
 
Cipher text:
jwolllrtoxsy
============================
 
============================
	 DEKRIPSI
============================
Cipher text:
jwolllrtoxsy
 
Plain text:
jxqopqxﾓwﾑﾗﾒ
============================
 
============================

Anyone can't help me?

It's interesting that the incorrect decryption results vary from the encrypted input in a simple way. For example:

Encrypted: pfsi
Decrypted: pgul

Can you see that

p + 0 = p
f + 1 = g
s + 2 = u
i + 3 = l

when what you want is:

p - 4 = l
f - 5 = a
s - 6 = m
i - 7 = b

Can you see the problem in the results? If you can see the problem in the results, then you should be able to inspect the code and determine the fix.

Originally Posted by GregBrannon

It's interesting that the incorrect decryption results vary from the encrypted input in a simple way. For example:

Encrypted: pfsi
Decrypted: pgul

Can you see that

p + 0 = p
f + 1 = g
s + 2 = u
i + 3 = l

when what you want is:

p - 4 = l
f - 5 = a
s - 6 = m
i - 7 = b

Can you see the problem in the results? If you can see the problem in the results, then you should be able to inspect the code and determine the fix.

Yup, i know my mistake and I fixed it. So this is my code for now:

 public static void main(String[] args) {
        String enkripsi = "lamb".toLowerCase();
        String dekripsi = "pfsi".toLowerCase();
        System.out.println("============================");
        System.out.println("\tENKRIPSI");
        System.out.println("============================");
        System.out.println("Plain text: ");
        System.out.println(enkripsi);
        System.out.println("");
        System.out.println("Cipher text:");
        enkrip(enkripsi);
        System.out.println("");
        System.out.println("============================");
        System.out.println("");
        System.out.println("============================");
        System.out.println("\t DEKRIPSI");
        System.out.println("============================");
        System.out.println("Cipher text:");
        System.out.println(dekripsi);
        System.out.println("");
        System.out.println("Plain text:");
        dekrip(dekripsi);
        System.out.println("");
        System.out.println("============================");
        System.out.println("");
        System.out.println("============================");
 
    }
 
    public static void enkrip(String enkripsi) {
        int a[] = new int[enkripsi.length()];
        for (int i = 0; i < a.length; i++) {
            a[i] = enkripsi.charAt(i) + 4 + i;
            if (a[i] > (byte) 'z') {
                a[i] = enkripsi.charAt(i) - (i % 26);
            } else if (a[i] < (byte) 'a') {
                a[i] = enkripsi.charAt(i) + (i % 26);
            }
            System.out.print((char) a[i]);
        }
 
    }
 
    public static void dekrip(String dekripsi) {
        int b[] = new int[dekripsi.length()];
        for (int i = 0; i < b.length; i++) {
 
            b[i] = dekripsi.charAt(i) - 4 - i;
 
            System.out.print((char) b[i]);
        }
    }
}

And the ouput is:

============================
	ENKRIPSI
============================
Plain text: 
lamb
 
Cipher text:
pfsi
============================
 
============================
	 DEKRIPSI
============================
Cipher text:
pfsi
 
Plain text:
lamb
============================
 
============================

BUT when I change the plain text into "abczwi" the encryption is correct, the cipher text is: "egiwsr"
But in the decryption the cipher text not same with the plain text. Like this:

============================
	 DEKRIPSI
============================
Cipher text:
egiwsr
 
Plain text:
abcpki
============================
 
============================

How is it?

You should be able to look at the ciphered text, egiwsr, the resulting decrypted text, abcpki, (which should be abczwi) and then inspect your code to determine why the resulting decryption is incorrect, just as I did for you above.

Okay, I'll try here.
And when I'm stuck I'll ask again if you don't mind

I'm stuck and I've found this methodology to complete my codes. But, what and where I should type my code here? :/

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package cobacoba;
 
/**
 *
 * @author Hengky
 */
public class Cobacoba {
 
    // prints out instead
    // while this is fundmentally useless
    // it appears to be an assignment requirement
    public static void encode(String s, int shift) {
        for (char c : s.toCharArray()) {
            encode(c, shift);
        }
        System.out.println();
    }
 
    // prints out the character
    public static void encode(char c, int shift) {
        // your code here
 
    }
 
    public static void enkrip(String s) {
        encode(s, 4);
    }
 
    public static void dekrip(String s) {
        encode(s, -4);
    }
 
    public static void main(String[] args) {
        System.out.println("Encryption:");
        String s = "abcz";
        System.out.print("Plain text: \"");
        System.out.print(s);
        System.out.println("\"");
        System.out.print("Cipher text: \"");
        enkrip(s);
        System.out.println("\"");
 
        System.out.println("Decryption:");
        s = "efgd";
        System.out.print("Cipher text: \"");
        System.out.print(s);
        System.out.println("\"");
        System.out.print("Plain text: \"");
        dekrip(s);
        System.out.println("\"");
 
    }
}

This is my code for now

package cobacoba;
 
/**
 *
 * @author Hengky
 */
public class Cobacoba {
 
    public static void main(String[] args) {
        String enkrip = "abczzzzzz".toLowerCase();
        String dekrip = "egidddddd".toLowerCase();
        int inc = 4;
        for (int i = 0; i < enkrip.length(); i++) {
            char abjad = enkrip.charAt(i);
            inc %= 26;
 
            if ((abjad >= 'a') && (abjad <= 'z')) {
                abjad += inc + i;
                if (abjad > 'z') {
 
                    abjad = (char) ((abjad - 'z' + 'a') - 1 - i);
            }
            System.out.print(abjad);
        }
        System.out.println("");
        for (int i = 0; i < dekrip.length(); i++) {
            char huruf= dekrip.charAt(i);
            inc %=26;
            if (huruf >= 'a' && huruf <= 'z' ) {
                huruf-=inc +i;
                if (huruf < 'a') {
                    huruf= (char)((huruf+ 'z'-'a')+1+i );
                }
 
            }
            System.out.print(huruf);
        }
    }
}

The output will be:
The "abczzzzzzz" will be encrypted to egidddddd, then "egiddddd" will be decrypted into abczzzz.
BUT when I input "qwerty" it will encrypt "uaky^c". When I input "uak^y" it'll decrypt "qwer^y".
Why the non alpahabetical character still pop out? And what going on with my decryption?
any suggest? I'm trully stuck here -_-

The encrypt/decrypt algorithm must correctly handle the ends of the alphabet. When a character less than 'a' is needed, the algorithm must wrap to 'z' and then continue backwards from there. When a letter greater than 'z' is needed, the algorithm must wrap to 'a' and continue from there. You can accomplish this using a combination of if statements and the % operator.

Originally Posted by GregBrannon

The encrypt/decrypt algorithm must correctly handle the ends of the alphabet. When a character less than 'a' is needed, the algorithm must wrap to 'z' and then continue backwards from there. When a letter greater than 'z' is needed, the algorithm must wrap to 'a' and continue from there. You can accomplish this using a combination of if statements and the % operator.

I solved this Thanks for your responses guys!