Hi,
There is a program for the iPhone that I have been using and is quite good, and is called SMS Encryption (which funnily enough, made it easy to find)
It allows you store frequently used codes,
http://appshopper.com/utilities/sms-encryption
so you can either store them, then tell the person which encryption key you have used, by what you have saved previously, then you can tell them in the message, which isn't telling anyone else what the actual encryption key is.
The cut and copy can be annoying, so I just email most of the time as it just puts it into the message for you.