Which padding is used by javax.crypto.Cipher for RSA

Question

I need to decrypt messages via RSA in order to send it over an unsecured channel, but I\'m afraid of the Padding Oracle Attack. Therefore I already have asked the follwoing

Answer 1

That's not a good advice given in the first link to the cryptography site. You should never rely on the defaults of cryptographic libraries cryptographic algorithms. There are quite a few reasons for this:

1. Different implementations, different defaults (there are no requirements for cryptography providers concerning defaults, although most will copy the Oracle/Sun defaults);
2. What's secure now may not be considered secure tomorrow, and because for backwards compatibility, you can never change the default;
3. It's unclear to anybody reading your software what the default is (you could document it, but in that case you might as well write it out).

The SunJCEProvider provided by Oracle defaults to PKCS#1 padding ("PKCS1Padding") for historical reasons (see reason #2 above). This is not well documented.

At that time that default was set you basically had just the insecure textbook RSA ("NoPadding") and the PKCS#1 v1.5 version ("PKCS1Padding" or RSAES-PKCS1-v1_5 in the PKCS#1 v2.1 standard). At that time RSAES-PKCS1-v1_5 was definitely the more secure choice. Changing the default now to OAEP would break every RSA implementation out there that uses the default.

The advice of otus (in the first link within this answer) is be better suited to protocol implementations in libraries than to cryptographic algorithms. In the end you should be able to defend the security of the choices made, whatever you choose.