Encrypting with large numbers, using a Caesar Cipher

Question

Right now I have the following code for a Caesar Cipher, but I run into an issue when I try encrypting text with a very large number. For example 1000.

static String encrypt(String plaintext) {
    StringBuilder ciphertext = new StringBuilder(plaintext);
    for (int i = 0; i < ciphertext.length(); i++) {
        ciphertext.setCharAt(i, encrypt(ciphertext.charAt(i)));
    }
    return ciphertext.toString();
}

static String decrypt(String plaintext) {
    StringBuilder ciphertext = new StringBuilder(plaintext);
    for (int i = 0; i < ciphertext.length(); i++) {
        ciphertext.setCharAt(i, decrypt(ciphertext.charAt(i)));
    }
    return ciphertext.toString();
}

static char encrypt(char c) {
    return (char) ('!' + (c - '!' + 1000) % ('~' - '!' + 1));
}

static char decrypt(char c) {
    return (char) ('!' + (c - '!' - 1000) % ('~' - '!' + 1));
}

Lets say I input "abc123" into the encryption, using 1000 as my key, I get a bunch of unknown characters. Keep in mind I don't want it to just cycle through a-z but symbols too, using ASCII codes.

Any help would be great!

Since you only want to map on the visible ascii characters, 126-33+1=94 possible values. Any key above 93 will have a corresponding smaller key that will do the same thing. 95 will encrypt the same as 1, 0 as 94 etc so a large value like 1000 is unnecessary. — ryanpattison, Nov 05 '14 at 23:40
@rpattiso Even if I use 93 in place of 1000 I still get unknown characters, or nonvisible characters. — Arman, Nov 05 '14 at 23:44

score 1 · Accepted Answer · edited May 23 '17 at 12:05

1

In Java, the result of modulo is the same sign as the dividend. So when you compute c - '!' - 1000 you will get a negative value and after the modulo it will still be negative. When you add '!', you will have computed a values less than '!', which will be invisible or underflow for a char.

static char decrypt(char c) {
    char d = '~' - '!' + 1;
    int x = (c - '!' - 1000) % d;
    if (x < 0) x += d;
    return (char)('!' + x);
}

Here is a discussion for the problem your running into.

How does java do modulus calculations with negative numbers?

edited May 23 '17 at 12:05

Community

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answered Nov 06 '14 at 00:05

ryanpattison

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Thank you very much! By the way x needs to be an int and the return statement needs to be casted to a char. But it works great, thanks again! – Arman Nov 06 '14 at 00:23
Good catch, please hit the check to close the question. – ryanpattison Nov 06 '14 at 00:24

Encrypting with large numbers, using a Caesar Cipher

1 Answers1