Java Android Error “too much data for RSA block” [

Question

This question already has an answer here:

• Too much data for RSA block fail. What is PKCS#7? 7 answers

A have an error in my Android project (RSA encryption/decryption). The encryption passes OK, but when I trying to decrypt encrypted text, yhere are an error: "too much data for RSA block".

How to solve this problem?

code:

public String Decrypt(String text) throws Exception
{
    try{
        Log.i("Crypto.java:Decrypt", text);
        RSAPrivateKey privateKey = (RSAPrivateKey)kp.getPrivate();
        Cipher cipher = Cipher.getInstance("RSA/ECB/NoPadding");
            cipher.init(Cipher.DECRYPT_MODE, privateKey);
            byte[] cipherData = cipher.doFinal(text.getBytes());// <----ERROR: too much data for RSA block
            byte[] decryptedBytes = cipher.doFinal(cipherData);
            String decrypted = new String(decryptedBytes);

            Log.i("Decrypted", decrypted);

        return decrypted;
    }catch(Exception e){
        System.out.println(e.getMessage());
    }
    return null;
}

Answer 1

Your issue is that you need to encode/decode the ciphertext (just text in your code) if you want to transport it using a textual representation (String in your case).

Try and look up base 64 encoding on this site, there should be a lot of information about it. Encode after encryption and decode before decryption. You should also specify a specific character encoding for your plaintext.

Finally, you should probably encrypt with a symmetric cipher, and encrypt the symmetric key using RSA. Otherwise you may run out of space within the RSA calculation, because a public key cannot encrypt data larger than its modulus (key size).