Steganography in Thai text

Steganography, or communication through covert channels, is desirable when the mere existence of an encrypted message might provide useful information to eavesdroppers. Text is ideal for steganography due to its ubiquity. However, text communication channels do not necessarily provide sufficient redundancy for covert communication. We propose a steganographic scheme for Thai plain text documents that exploits redundancies in the way particular vowel, diacritical, and tonal symbols are composed in TIS-620, the standard Thai character set. The scheme is blind in that the original carrier text is not required for decoding. In an experimental evaluation, we find that the message embedding scheme allows 2.2 bytes of embedded covert text per kilobyte of carrier text on average, and that the document modifications are unnoticeable by casual observers. The method is thus a practical and effective method for covert communication over Thai plain text channels.