New to JCE

Question

Hi,
I am new to JCE. I have a RSA certificate. I dont know how can I use that to dncrypt the data that I send and decrypt the data that I receive. I need some code help please.

Answer 1

Certificates are not used to decrypt. A Certificate is a wrapper for a Public Key.. Public Key's are used for encryption. You need the associated private key for decryption...
Now those are reversed for Signatures...
To use pub/priv key encryption you will be dealing with the Cipher object. You do this, in your case, with the RSA algorithm. So using Cipher you need to get an Cipher object implementing RSA algorithm with an appropriate mode and padding.. Now RSA does not really support modes so in reality it is just an algorithm and a padding...
Typically it would look like...
Cipher c = Cipher.getInstance("RSA//PKCS1Padding");
But the exact transformation string specifying algorithm/mode/padding that your provider uses is specific to the provider. Note: the SunJCE does not supply an RSA algorithm implementation for Cipher. You will need to download a third party provider and install it properly.. Something like BouncyCastle as an example...
nce you have done that and you have figured out what transformation string to use in the above getInstance() call, you then need to init it.
For encryption you use the PublicKey which in your case is the Certificate.
c.init(Cipher.ENCRYPT_MODE, cert);
For decryption you need to use the private key.. Private and public keys are typically stored in a keystore.
KeyStore ks = KeyStore.getInstance("JCEKS");
You will need to load the keystore...
ks.load(new FileInputStream(keystorefile), passwd.toCharArray());
Then you will need to locate your keys.
Key privKey = ks.getKey(myAlias, keyPasswd.toCharArray());
Likewise, you will need to get your certificate.
Once you have both your Cert and Key loaded you can do your crypto magic.. But it sounds like you have a bunch to learn so.. Well this will give you a start...