Running head: STEGANOGRAPHY IN THAI TEXT

Steganography, or communication through covert channels, is desirable when the mere existence of an encrypted message might cause suspicion or provide useful information to eavesdroppers. Text is effective for steganography due to its ubiquity. However, text communication channels do not necessarily provide sufficient redundancy for covert communication. We propose a novel steganographic embedding 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. We provide a Thai text stegosystem following a provably secure construction that guarantees covertness, privacy, and integrity of the hiddentext message under meaningful attacks against computational adversaries. In an experimental evaluation, we find that the message embedding scheme allows 203 bytes of embedded hiddentext message per 100KB of covertext on average, and that the document modifications are not readily noticed by observers. The stegosystem is thus a practical and effective secure system for covert communication over Thai plain text channels.