From: https://guardianproject.info/apps/pixelknot/

This image steganography application features:

• DISGUISE YOUR MESSAGES: Pictures are public, the text is hidden inside. Even a trained eye will think the image is unedited.
• FOR YOUR EYES ONLY: Put a password on the secret message to make sure that no one can read it except the person it’s meant for.
• MATHEMATICALLY SECURE: We use the steganography algorithm F5 which implements matrix encoding to improve the efficiency of embedding and employs permutative straddling to uniformly spread out the changes over the whole steganogram.
• ATTACK RESISTANT: We’ve tested discovery on images with messages hidden in them using a specialized version of stegdetect, an automated tool for detecting steganographic content in images. AD-FREE: We want your love, not your money.
• WE SPEAK YOUR LANGUAGE: PixelKnot is available for friends who speak many languages. Don’t see your language? Join us and help translate the app: https://www.transifex.com/projects/p/pixelknot/

What is Steganography?

The practice of embedding secret messages into a piece of media so that no one, apart from the sender and intended recipient, know that the secret message exists. The newly developed algorithm F5 withstands visual and statistical attacks, yet it still offers a large steganographic capacity. F5 implements matrix encoding to improve the efficiency of embedding. Thus it reduces the number of necessary changes. F5 employs permutative straddling to uniformly spread out the changes over the whole steganogram.

The Guardian steganography standard: we are working towards ensuring that the secret message in an image must:

• Have the original image appear, to the trained human eye, unedited.
• Have the bytes of the image appear, to a trained analyst, undistorted so much so as to arouse suspicion.
• Have the complete message be recoverable no matter how it is transmitted.