Hidden Writing and National Security.

The article focuses on the use of encryption techniques such as steganography and cryptography. It comments on historical uses of steganography using a tattooed message on a person's scalp, and more modern applications such as watermarks, microdots, and invisible inks. It mentions Fabien A. Petitcolas book "Information Hiding, Techniques for Steganography and Digital Watermarking." It talks about the use of steganography in digital photographs.