By embedding messages in heavily quantized DCT frequency components at base JPEG quality 30, TRIST achieves near-zero bit error rates when images are transcoded to higher quality levels and back. The quantization mapping is many-to-one, so noise introduced by re-encoding tends to be stabilized on output, making the message robust against commodity transcoding proxies that re-encode images in-flight.
From 2014-connolly-trist — TRIST: Circumventing Censorship with Transcoding-Resistant Image Steganography
· §4–§5
· 2014
· Free and Open Communications on the Internet
Implications
Embed steganographic payloads at the lowest practical JPEG quality (30 or below) using heavily quantized low-frequency DCT components — this exploits quantization stabilization to survive re-encoding by censor-deployed transcoding proxies.
Avoid high-frequency DCT components for embedding; they are more aggressively quantized and lose information under transcoding, whereas low-frequency coefficients are stabilized.