How to communicate AES initialization Vector to client for hybrid cryptosystem

Question

I need to implemented security for client-server communication. I have implemented the following hybrid cryptosystem

To encrypt a message addressed to Alice in a hyb

Answer 1

You don't encrypt the IV. Bundle it with the encrypted key and send it (in the clear) to the recipient.

Standards for this do exist. This scheme is called "KeyTransRecipientInfo" in CMS (upon which S/MIME is based), and PGP offers a similar mode. TLS also includes the initialization vector as a parameter in the key encryption algorithm identifier, using the same ASN.1 syntax as CMS. A robust, open-source library to perform this operation is available for many, many platforms.

At the very least, studying the CMS specification might help avoid some of the many pitfalls in a home-brew implementation. See §6.1 and §6.2.1 of RFC 3369.