Steganalysis: Detecting Hidden Messages

The most basic steganalysis technique is simply looking closely at a suspicious image. Zooming in to the pixel level and examining regions of uniform colour can reveal block artefacts or noise patterns inconsistent with the image content. Converting to greyscale, adjusting contrast, and examining colour histograms can expose anomalies that suggest steganographic embedding has occurred in specific regions.

The chi-square test is a classical statistical test for LSB steganography. Natural images have characteristic distributions of colour values where adjacent even-odd value pairs occur with similar frequency. LSB embedding changes odd values to even or vice versa, creating detectable imbalances in these pairs. The chi-square statistic measures the deviation from expected pair frequencies and rises significantly when embedding is detected.