Make sure your keyword doesn't fall into enemy hands.

Always send your keyword separately from your message. If possible, encode it as well to be extra careful. For example, use the newspaper code to give your partner the keyword used in your letter exchange code.

The most successful spies will only give their partner a clue about the keyword, rather than the word itself. They can then use this to work out the actual word. So you might say 'The name of my pet dog'. If your partner knows the name of your dog, then they have the keyword. It's a good idea to change your keyword quite often as well, just in case it's ever discovered. Just remember to let your secret agent partner know!