RSA Encryption / Decryption using Java

Question

I am doing a simple program to encrypt/decrypt using RSA algorithm in Java. I create a cipher object as follows:

//Create a Cipher object
Cipher rsaCipher =          


        

Answer 1

Your problem is indeed with the padding. Some kind of padding, either PKCS#1 1.5 or OAEP padding in practice, is required for secure RSA functionality. Furthermore, it is required to find the start and end of the encrypted plain text.

The modular exponentiation of RSA is performed using large integers. The results of these operations are then represented as octet strings. These octet strings are basically big endian, unsigned, fixed length representation of an integer. These integers are left padded with 00 valued bytes (this is called the I2OS primitive in the RSA standard). So what you are seeing is the result of the modular exponentiation, with the 00 padding still in place.

Long story short, always use a padding scheme. Nowadays, OAEP would be preferable. Use it together with hybrid encryption scheme, or use a higher level container format such as CMS or PGP.