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Say I want to hide a PDF file "A". I have with me a picture of a cat "B". I want software which can take both files as input and "encrypt" them to form "C" in some way such that:

  • If someone asks me to supply the password for the file "C" (say under coercion) I can say password "1" which will cause the file to decrypt to give the cat picture "B".

  • If I want to access the file "A", I use a different password "2" to decrypt the same file "C" to "A" (and optionally "B").

Is this possible? And is it obvious from analysing the encrypted file that it's not just a encrypted cat picture?

For OS X and/or Windows.

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i assume you are talking about Plausible deniability or to be more concrete Deniable encryption?

well, read a bit further to https://www.privacytools.io/#encrypt and there is VeraCrypt recommended...
since VeraCrypt and CipherShed are based on TrueCrypt they both provide a feature called "Hidden Volumes":

Hidden Volume

all 3 of them are open source and therefore gratis and also cross-platform (at least for Windows, Mac & Linux) available.

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    accidentally came across Elettra: it has a similar goal, but a different passphrase per file must be chosen and it creates archives/files (which i guess are unfortunately only accessible with the same application). -- additionally i'm not sure if it is actively maintained and how secure it is... anyhow i just wanted to mention it.
    – DJCrashdummy
    May 15, 2017 at 6:38
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Not quite what you are looking for but you could use Steganography to hide your really sensitive data in the picture of the cat, (possibly after encrypting the text data itself), and then encrypt the cat picture with a second password.

You would then need the first password to extract the cat and the other key(s), plus the knowledge that the data was there, to extract the hidden data from within the picture.

There are some techniques that can lead an expert to believe that there is hidden data within the picture and the quantity of information that can be embedded in a single picture is limited. The main reason that an expert would look to see if there is additional information hidden in the picture of a cat is the question of why you would encrypt a picture of a cat. To avoid this question I would suggest that a picture of a subject which is at least mildly embarrassing but not in itself grossly illegal in the jurisdiction concerned would be much more likely to escape detection of the hidden content.

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    a good Quote on the site the questioner posted: "Arguing that you don't care about the right to privacy because you have nothing to hide is no different than saying you don't care about free speech because you have nothing to say." - Edward Snowden on reddit
    – DJCrashdummy
    Oct 6, 2016 at 12:08
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    @DJCrashdummy A reminder that some software might have legal issues in some places cannot hurt. But it shouldn't prevent us from recommending the software (if it has legal uses). We're not a "law experts" site, but "software experts": Leave a hint, let everyone decide him/herself – as you wrote. // Cleaning up those comments now as well, as this isn't the place to discuss this – feel free to raise it at Meta :)
    – Izzy ♦
    Oct 7, 2016 at 10:29
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    Just to clarify - it is not a good idea to recommend anything that might be legal at other locations but is not at the time & place that you are in when doing the recommending.
    – Steve Barnes
    Oct 7, 2016 at 10:58
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    I've meant Meta.SR, and to raise a question like "Can I recommend software that might be illegal to use in some places?" Outline the issue (including the software you're talking about is legal where you live), self-answer with your opinion (from the now deleted comments) and optionally include some "reference cases" (like this one; I vaguely remember we've had some more). // @SteveBarnes Would be fine with me if you did so – but might have "unwanted consequences" for you then, so I agree with that :)
    – Izzy ♦
    Oct 7, 2016 at 11:01
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    because a accidentally came across them, i just want to mention these two stenography-applications: OpenStego and Stegosuite
    – DJCrashdummy
    May 15, 2017 at 6:55

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