I wanted to have some knowledge about cryptography and steganography so I decided to write a steganography application with using LSB technique and for message encryption I wanted to combine RSA and AES. But I need some advice about my logic. After doing some research I decided to encrypt client message before embed it into picture so,I will generate 128-bit Private Key with RSA and with this key I will XOR some secure random number then I will use its output as a AES Key then I will encrypt the message with AES algorithm. My question is we cant create a key which has length of 128-bit in RSA so how people who used hybrid cryptosystems can combine those two algorithms which are AES and RSA.
I would say don't do it, it looks like someone else is already working on an openssl steganography implementation on github. If you are using pictures for your steganography they are sort of already a shared secret (if someone has the original picture they can see yours has modified LSBs) so you could pick an easier shared secret cryptosystem.