RSA encryption and decryption of a long message in java

I want to use java standard library, and as much as I know, its functions inputs are limited. So I implemented two methods to this purpose. Here they are:

private byte[] RSAenc(String in) throws Exception {
    Cipher c = Cipher.getInstance("RSA");
    c.init(Cipher.ENCRYPT_MODE, privKey);
    int l = in.length();

    byte[] part;
    byte[] result = new byte[(int)(64*java.lang.Math.ceil(l/20.0))];
    int i = 0;
    while(i*20+20<l) {
        part = c.doFinal(in.substring(i*20,i*20+19).getBytes("UTF-8"));
        System.arraycopy(part, 0, result, i*64, part.length);
        i = i+1;
    }
    part = c.doFinal(in.substring(i*20,l-1).getBytes("UTF-8"));
    System.arraycopy(part, 0, result, i*64, part.length);
    return result;

}

private String RSAdec(byte [] in) throws Exception {
    Cipher c = Cipher.getInstance("RSA");
    c.init(Cipher.DECRYPT_MODE, privK开发者_如何学Cey);

    String result = "";
    byte[] part = new byte[64];
    int l = in.length;
    int i = 0;
    while(i+64<=l) {
        System.arraycopy(in, i, part, 0, part.length);
        result = result + new String(c.doFinal(part), "UTF-8");
        i= i+64;
    }
    return result;
}

They work in this manner: for encryption I break the string to at most, 20 size substrings, and then use the Cipher to encrypt them. For decryption, I break byte array to 64 byte blocks, and apply Cipher decryption to them. Is there already a function which do this? Or at least is there a neater solution? How safe is my approach? Is encryption result length (result.length) always 64 on all distributions of JRE?

Thanx,

RSA is suited to key encipherment, not bulk data encryption.

Instead of encrypting messages with RSA, most protocols generate a key for a symmetric cipher, like AES, and encrypt the message with that. Then, RSA is used to encrypt that symmetric key so that only the message recipient can recover it. The RSA-encrypted symmetric key is sent with the AES-encrypted message to the recipient.