HILL (High-pass, Low-pass, and Low-pass)

HILL is an advanced spatial-domain steganography algorithm designed to resist modern steganalysis.

How it works

Unlike simple LSB substitution, HILL uses a cost-based framework to decide where to embed data. It employs a combination of high-pass and low-pass filters to calculate a "cost" for modifying each pixel.

• The high-pass filter identifies complex, highly textured regions of the image (like edges or noise) where changes are harder to detect.
• The low-pass filters are then used to spread the embedding costs, clustering modifications into these textured areas rather than scattering them randomly across smooth, flat regions (where they would be obvious to steganalysis).

Trade-offs

• Speed: Slower than LSB due to the computational overhead of applying multiple filters and calculating embedding costs.
• Capacity: Lower capacity than LSB, as it actively avoids embedding in smooth regions to maintain security.
• Stealth: High security. It performs exceptionally well against modern steganalysis tools.

Use cases

Use HILL when you need a balanced approach that provides strong security without the extreme computational overhead of the most intensive algorithms.