Indeed, DH can be difficult to get right if you are not paying attention.

I wish I could go into more specifics for our implementation and what it is we do.

Either way the public/private key stuff is done using various "high" level libraries that make reasonable choices and are audited, so it is not code that we are implementing ourselves, nor is the HMAC. So long as there are no issues in the libraries I should be fairly good.

Thanks for the discussion :-).