Simple Stateless Steganography

Steganography is the science of hiding the very presence of a secret message within a public com-munication channel. In Crypto 2002, Hopper, Langford, and von Ahn proposed the first complexity-theoretic definition and constructions of stegosystems. They later pointed out a flaw in their basic con-struction. Their proposed fix for this flaw dramatically reduces the efficiency of the construction, becauseit requires the use of strong error-correcting codes.Our first contribution is to demonstrate that the construction that was thought flawed is actually oftennot. By carefully analyzing the severity of the flaw in their original construction, we show that it is safe touse under proper conditions—thus eliminating the need for expensive error-correction. Moreover, whensuch conditions do not hold, we provide an alternative fix for the flaw, which is often more efficient.In addition, we demonstrate that for memoryless channels, the construction can be used to sendmultiple bits statelessly (maintaining synchronized state between the sender and the recipient, as wasproposed for the original construction, is particularly problematic in steganography). We provide tightbounds on the security of such an approach.