Question

I have a hex string after RSA encryption. When I convert it to a byte[], the RSA decryption gives javax.crypto.BadPaddingException: Blocktype mismatch: 0

I am using this method for conversion (got it on Stack overflow itself)

public static byte[] hexStringToByteArray(String data) {
    int k = 0;
    byte[] results = new byte[data.length() / 2];
    for (int i = 0; i < data.length();) {
        results[k] = (byte) (Character.digit(data.charAt(i++), 16) << 4);
        results[k] += (byte) (Character.digit(data.charAt(i++), 16));
        k++;
    }
    return results;
}

Any suggestions please.

Answer 1

The encryption method requires the input to be a fixed-length; you’re going to have to add padding to the required length in order to avoid this exception. This size will depend on the key size.

EDIT: There is also a potential bug in your iteration of data: If its length is not divisible by two then the second i++ will cause an IndexOutOfBoundsException. You are better off incrementing i by 2 in the for loop and using [i] and [i+1] when accessing the data:

for (int i = 0; i + 1 < data.length(); i += 2, k++)
{
    results[k] = (byte) (Character.digit(data.charAt(i), 16) << 4);
    results[k] += (byte) (Character.digit(data.charAt(i + 1), 16));
}