Hide a message inside a photo

A photo is made of millions of dots; each dot's color is stored as numbers. Changing the smallest digit of a number shifts the color too little for the eye to notice.

The letters of your message are written into these smallest digits, one by one. First it is locked with your password, then embedded. The same password is needed to take it back out.

Short answer: you can't tell by looking. A well-hidden message doesn't change the photo's colors by even a thousandth; put two photos side by side and you won't spot the difference. That's the whole point.

Signs that might still raise suspicion

• The photo is a PNG and its file size is oddly large for its kind.
• Someone keeps insisting on sending the photo “as a file, without compression.”
• The photo's name or place doesn't fit the subject — a plain landscape named something like “gizli.png.”

How to be sure

Drop the photo into the “Reveal” section above and try the password. If there is a message and the password is right, it opens; otherwise it says there is no hidden message. Without the password you can't read the content, because the message is encrypted.

What do experts do?

A method called steganalysis looks at statistical traces in the pixels. In a normal photo the smallest digits are spread randomly; when a message is hidden, that spread is disturbed. This brings you closer to “is something hidden here,” but without the password it still won't reveal the message.