One time pad, encryption and decryption

Question

I am trying to pick up cryptography and had been trying this exercise

Write a program (preferably Java) to generate a one-time pad, which is a relatively large file

Answer 1

https://en.wikipedia.org/wiki/One-time_pad

public static String crypt(String string, String keyString) {
    // convert secret text to byte array
    byte[] bytes = string != null ? string.getBytes() : new byte[0];
    int size = bytes != null ? bytes.length : 0;
    final byte[] encoded = new byte[size];
    final byte[] key = keyString != null ? keyString.getBytes() : new byte[0];
    // loop on input  bytes
    for (int i = 0; i < size; i++) {
        // shift key index 
        // (we assume key can be smaller or equal if larger then adjust)
        int keyi = i >= keySize ? size % (keySize-1) : i;
        // pad
        encoded[i] = (byte) (bytes[i] ^ key[keyi]);
    }
    return new String(encoded);
}

public static void test(String string, String keyString) {
    String encrypt = crypt(string, keyString);
    String decrypt = crypt(encrypt, keyString);
    assert(string.equals(decrypt));
}

• test("test","1234");
• test("test","123");

ps. you can refactor method by pull strings up and replace with bytes

public static byte[] crypt(byte[] bytes, byte[] key) {
    int size = bytes != null ? bytes.length : 0;
    final byte[] encoded = new byte[size];
    int keySize = key != null ? key.length : 0;
    // loop on input bytes 
    for (int i = 0; i < size; i++) {
        // shift key index (assuming key <= bytes)
        int keyi = i >= keySize ? size % (keySize-1) : i;
        // pad 
        encoded[i] = (byte) (bytes[i] ^ key[keyi]);
    }
    return encoded;
}