Vigenère cipher in Java for all UTF-8 characters

Question

I have this simple function for encrypting strings via Vigenère in Java. I omitted the decryption as this is just a "-" instead of the "+" in the line where the new value is calculated.

But this function works only for the normal alphabet A-Z. How can I change the function so that it supports lowercase letters as well as uppercase letters and all other UTF-8 chars?

public static String vigenere_encrypt(String plaintext, String key) {
    String encryptedText = "";
    for (int i = 0, j = 0; i < plaintext.length(); i++, j++) {
        if (j == key.length()) { j = 0; } // use key again if end reached
        encryptedText += (char) ((plaintext.charAt(i)+key.charAt(j)-130)%26 + 65);
    }
    return encryptedText;
}

Thank you very much for your help!

Answer 1

Well, you asked for it and I felt like puzzling, but print out the cipher text and you will know what you just asked for...

public static String vigenereUNICODE(String plaintext, String key, boolean encrypt) {

    final int textSize = plaintext.length();
    final int keySize = key.length();

    final StringBuilder encryptedText = new StringBuilder(textSize);
    for (int i = 0; i < textSize; i++) {
        final int plainNR = plaintext.codePointAt(i);
        final int keyNR = key.codePointAt(i % keySize);

        final long cipherNR;
        if (encrypt) {
            cipherNR = ((long) plainNR + (long) keyNR) & 0xFFFFFFFFL;
        } else {
            cipherNR = ((long) plainNR - (long) keyNR) & 0xFFFFFFFFL;
        }

        encryptedText.appendCodePoint((int) cipherNR);
    }

    return encryptedText.toString();
}

EDIT: Please don't ever use in production code, as I haven't got a clue if the code points can indeed be encoded/decoded. Not all points have been defined, as far as I know, and the standard is a moving target.