Here is a useful safety tip if you are constructing HMAC signatures for structured messages,things that you want to parse later: put a format version number intoeither the key or the message.

If you want messages in the old format to fail signature checks so thatthe rest of your code doesn't have to deal with them, put the versionnumber into the key. Put it into the message if you want your code togracefully deal with several generations of message formats; each willstill verify, but the