Predictable rain?

Abstract

Symmetric steganographic communication requires a secret stego-key pre-shared between the communicating parties. Public-key steganography (PKS) overcomes this inconvenience. In this case, the steganographic security is based solely on the underlying asymmetric encryption function. This implies that the embedding positions are either public or hidden by clever coding, for instance using Wet Paper Codes (WPC), but with public code parameters. We show that using WPC with efficient encoding algorithms may leak information which can facilitate an attack. The public parameters allow an attacker to predict among the possible embedding positions the ones most likely used for embedding. This approach is independent of the embedding operation. We demonstrate it for the case of least significant bit (LSB) replacement and present two new variants of Weighted Stego-Image (WS) steganalysis specifically tailored to detect PKS using efficient WPC. Experiments show that our WS variants can detect PKS with higher accuracy than known methods, especially for low embedding rates. The attack is applicable even if a hybrid stegosystem is constructed and public-key cryptography is only used to encapsulate a secret stego-key.