When creating a multi-revealer, 3 or more revealers will be created. The secret will be then encrypted with the combination of all those revealers. That means that you will need all of those revealers and the encrypted secret to decrypt them.
The secret can only be decrypted if you have all the revealers (and the encrypted secret of course). If any of them are missing, then it is impossible to decrypt the secret.
The revealer combining works by overlapping them, and for each overlapping square counting how many of them are white. If their count is an even number, then in the combined revealer the corresponding square will be black. If it is odd, then that square will be white. Combining the revealers create a brand new revealer, which then can be used to encrypt your secret. Decrypting works the same way: combining all the revealers and the encrypted secret will result in an image of the decrypted secret.
Note: this does not work with physical revealers, since you cannot overlap them this way. Also note: this is not an official method of generating revealers, so currently there are no other implementations for decoding multi-revealers. You can save this page for later, it is a single html file (just press CTRL-S and save it). Alternatively, you can use an image editing software (e.g. Photoshop) to overlay the images, and set the blending mode to Difference for each layer. All images together will reveal the decrypted secret.
Note: some of the features below may not work in all browsers
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Preview:
Create this many revealers:
(must be odd)
Or enter a list of already existing revealers (one per line): If you enter less revealers than the number you selected, then the remaining ones will be generated automatically.