Image Steganography — Hide Text in an Image

Steganography hides a message so that nobody notices it is there. This tool writes your text into the least significant bits of an image’s red, green and blue values — changes of at most one or three levels out of 255, invisible to the eye — and saves the result as a PNG. The same page reads the message back, and an optional password encrypts it first with AES-256-GCM. Everything happens in your browser.

How to use this tool

1. Choose an image and type the message.
2. Optionally set a password, and pick 2 bits per channel if you need more room.
3. Download the PNG; the tool reads it back once to confirm the message survived.
4. To read a message, switch to Reveal, choose the PNG and enter the password if one was used.

Formula

Capacity ≈ opaque pixels × 3 channels × bits ÷ 8 bytes, minus a 13-byte header (marker, flags, length and CRC-32) and 44 bytes for encryption.

Each message bit replaces the lowest bit (or two bits) of one colour channel, in pixel order. Only fully opaque pixels are used, because browsers do not keep exact colours in semi-transparent ones. With a password, the key comes from PBKDF2-SHA256 (600,000 rounds) and the text is sealed with AES-256-GCM.

Example

A 1920 × 1080 photo has 2,073,600 pixels, so at 1 bit per channel it holds about 2,073,600 × 3 ÷ 8 ≈ 777,600 bytes — enough for a short novel. The 160 × 100 test image holds about 6,000 bytes.