No luck with RSA and existing cert

Question

I want to encrypt data in my software, data which will be sent to me by the user, in such a way that only I can decrypt it. This seems to call for asymmetric encryption (only the public key would be embedded in the software), so I am trying to use RSA.
Specifically I am trying to encrypt and decrypt data using the key pairs found in a cert that we bought from a cert authority. The cert says that key is a "Sun RSA public key, 1024 bits". In the following test, I encrypt using the cert's public key and decrypt using the same, for want of a method to return the private key but the results are the same if I initialize the cipher for decryption with the cert itself (which presumably contains the private key).
        Key key = cert.getPublicKey();
        Cipher cipher = Cipher.getInstance("RSA");
        cipher.init(Cipher.ENCRYPT_MODE, key);
        byte[] enc = cipher.doFinal(test.getBytes());
        cipher.init(Cipher.DECRYPT_MODE, key);
        byte[] dec = cipher.doFinal(enc);but at the decyrption stage I get the following error:
Exception in thread "main" javax.crypto.BadPaddingException: Data must start with zero.which I don't know what to make of. It seems to me that I am following the (rather scant) instructions to the letter. If I specify "RSA/ECB/NoPadding" as the transformation I don't get the above error but the roundtrip fails to recreate the original string.
Furthermore, as I said before, I wanted to use public key encryption because I must include the encryption key in the software and I do not want it to be sufficient to decrypt the cipher. I was hoping that with RSA you'd encrypt using the public key but that you'd need either the secret key or the whole cert to decrypt. However the Javadocs do not say so explicitely and I am left unsure as to how this works exactly. Can anyone shed some light?

Answer 1

I agree, the documentation is inadequate. Have you also looked at the JCE reference (http://java.sun.com/j2se/1.5.0/docs/guide/security/jce/JCERefGuide.html)? This expands a lot on the javadocs for the classes. It might also help to learn more about cryptography; one book that others recommend is "Practical Cryptography" by Ferguson and Schneier.
I think the one key misunderstanding you have is what is in a certificate. A certificate contains only the public key, some information about the identity of the owner of the private key, and a digital signature over this public key and identifying information. The private key is not in the certificate! Nor should it be. If it were, it would no longer be private and the security of the system would fall apart.
The location of the private key depends entirely on the application that created the key pair. java's keytool, for example, stores the private key in a password protected file.
The error you are seeing makes sense once you understand that , for an RSA cipher, the type of key, public or private, as well as the mode Cipher.ENCRYPT_MODE or Cipher.DECRYPT_MODE, determine the interpretation of the subsequent update or doFinal method calls.
Thus in your example, your first call to cipher.doFinal gives the RSA encryption of the data, which is what you wanted. Your second, however, attempts to decrypt this encrypted data with the public key, which makes no sense in this context. It checks to see if the result is has the proper padding, which it does not. If you tell it to assume no padding, you won't get an exception but the result still won't make any sense. You need to init the cipher with the private key for the second part.