Finding what's hidden in a picture

Steganography hides a message so that nobody knows it’s there: in the least noticeable bits of a picture, in characters that don’t show, or in the frequencies of a sound. It turns up in puzzles and capture-the-flag challenges, and in watermarks. These tools look in the usual places. Your files stay in your browser.

1. Look at the file itself

Drop the file on the Hex Viewer. It names the file type from its first bytes, so a “picture” that is really a ZIP shows up at once. Page through to the end too: text or a second file is sometimes simply added after the image’s data, where viewers ignore it.

Then open it in Photo Metadata. Comments, descriptions, artist and copyright fields, XMP notes and PNG text chunks are easy places to leave a clue, and the GPS location or the date taken may be the answer itself.

2. Step through the bit planes

Open the picture in the Bit-Plane Inspector and step through each bit of red, green, blue and alpha. A message hidden in the lowest bits is invisible in the picture but often shows as noise, a band or a shape in bit 0 or 1. A second copy of the image, if you have one, can be compared pixel by pixel.

To read what’s there, Extract the lowest bit of RGB, row by row. Text that starts TBX1 was hidden with this site’s Image Steganography tool: choose the picture under Reveal there to get the message, typing the password if it has one.

3. Check any text that came with it

A caption, file name or message may carry text of its own. Paste it into Hidden Text: zero-width characters, Unicode tag characters and variation selectors are invisible, but can spell out a message or mark who a copy was sent to. The tool shows what’s hidden and gives you a clean copy.

4. If it’s a sound

Pictures can hide in sound too, drawn in the frequencies so they appear in a spectrogram. Open the audio file in the Spectrogram tool and look for letters or shapes; choose a higher Up to frequency if the picture is cut off.