Encrypt/Decrypt a stream

Question

Could someone explain to me how I can encrypt a stream? It is like this: I have a encrypted text file, I would like to put the text in the memory then, decrypt it and leave it in the memory. So the file remains encrypted, only the text in the memory is decrypted and readable for us. When I write extra text to the memory, then save it, the text in the memory should be encrypted and overwrite the encrypted text file. I'm not sure if this works... Could anyone help me out with this?