How to implement Double Encryption? (Urgent)

Question

I can encrypt and decrypt a message but I can't encrypt a message twice by using 2 different public keys.
//encrypt is a method I created by myself
// 10001 is plaintext and work at all
encrypt cipherObj_T = new encrypt();
byte[] S1 = cipherObj_T.to_encrypt_T("10001");
//but it has error from here
encrypt cipherObj = new encrypt();
byte[] cipher = cipherObj.to_encrypt(S1);
It shows error:
javax.crypto.IllegalBlockSizeException: Data must not be longer than 117 bytes
How to fix it? Please help me! Thanks!!!

Answer 1

I bet you are using RSA! Depending on the implementation, the data to be encrypted must be shorter than the key length.
RSA is normally used to encrypt a session key, not the data. Typically, to send with RSA, you use
1) Create a random session key for your chosen symetric encryption (AES, DES etc).
2) Encrypt the session key with the recipient's public key and ship the encrypted session key to the recipient.
3) Encrypt your data with the session key and ship that to the recipient.
and to receive you use
1) Get the encrpted session key.
2) Decrypt the session key using your private key.
3) Decrypt the data using the session key.