Java simple encryption, slightly different output

Question

I coded this simple encryption program:

public static void main(String[] args) {
    int password = 123456789;
    int firstLayer = password / 58;
    int secondLayer = firstLayer * 23;
    int thirdLayer = secondLayer + 85;
    System.out.println(thirdLayer);

    int encryptedPassword = thirdLayer;
    int thirdLayerD = encryptedPassword - 85;
    int secondLayerD = thirdLayerD / 23;
    int firstLayerD = secondLayerD * 58;


    System.out.println(firstLayerD);

}

but I have a weird problem. After decrypting the password I get: 123456770

I was expecting to get the same password as the original, but I don't know what the problem is.

If you understand data types in Java, it seems like you could've easily figured this out with debugging. If you don't understand data types, you should probably read a textbook. [so] is not really a good substitute for either of those. — Bernhard Barker, Aug 24 '17 at 17:06
[What does your step debugger tell you?](http://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems) — , Aug 24 '17 at 17:20

score 1 · Answer 1 · answered Aug 24 '17 at 17:01

1

This is because 123456789 / 58 = 2128565,32759, this gets truncated to an int.

answered Aug 24 '17 at 17:01

Dorian Gray

2,781
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so is it better to use double for all the variables? – Trap Town Aug 24 '17 at 17:04
Yes, otherwise the result gets truncated. – Dorian Gray Aug 24 '17 at 17:05
I replaced all the variables with double, I got this: 1.2345678900000001E8 – Trap Town Aug 24 '17 at 17:07
@TrapTown [Is floating point math broken?](https://stackoverflow.com/questions/588004/is-floating-point-math-broken) – Bernhard Barker Aug 24 '17 at 17:08
So there is no way to fix it? – Trap Town Aug 24 '17 at 17:09
Yes, do not create your own "encryption" algorithms. Use existing ones. – Guillaume F. Aug 24 '17 at 17:11
Uhm, okay? I made this one just for fun... – Trap Town Aug 24 '17 at 17:13

Bohemian · Answer 2 · 2017-08-24T18:05:12.200

0

Integer division arithmetic results in truncation of any remainder.

Use double, then call Math.round() to produce an integer number again:

public static void main(String[] args) {
    int password = 123456789;
    double firstLayer = password / 58d;
    double secondLayer = firstLayer * 23;
    double thirdLayer = secondLayer + 85;
    double encryptedPassword = thirdLayer;

    double thirdLayerD = encryptedPassword - 85;
    double secondLayerD = thirdLayerD / 23;
    int firstLayerD = (int)Math.round(secondLayerD * 58);

    System.out.println(firstLayerD);
}

Output:

123456789

edited Aug 24 '17 at 18:05

answered Aug 24 '17 at 17:15

Bohemian

389,931
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What happened to the nine? – Trap Town Aug 24 '17 at 18:01
@Trap I didn't even notice that. it's back now: see edited code. – Bohemian Aug 24 '17 at 18:06

Java simple encryption, slightly different output

2 Answers2