Steganography by Neil F. Johnson

With the improvement of technology and the ease as to the decoding of these invisible inks, more sophisticated inks were developed which react to various chemicals. Some messages had to be "developed" much as photographs are developed with a number of chemicals in processing labs.

Null ciphers (unencrypted messages) were also used. The real message is "camouflaged" in an innocent sounding message. Due to the "sound" of many open coded messages, the suspect communications were detected by mail filters. However "innocent" messages were allowed to flow through. An example of a message containing such a null cipher from [JDJ01] is:

Fishing freshwater bends and saltwater coasts rewards anyone feeling stressed. Resourceful anglers usually find masterful leapers fun and admit swordfish rank overwhelming anyday.