If you can share a secret then some hmac + e.g. aes128 will make communication virtually impenetrable and protected aganst all or almost all possible attacks. a

If you can share a secret then some hmac + e.g. aes128 will make communication virtually impenetrable and protected aganst all or almost all possible attacks. aes needed for hiding data inside, hmac for data integrity (protection against resending etc). If you don't have any crypto built into hardware it will be tough