Is there a cryptographic function that generates a reconizable visual pattern?

Question

I have to build a system that will ultimately print a pass allowing access to a building. The validation of this ticket will be done manually (it can also be automatic but that's not likely at this stage). There is the chance that the person checking does not have access to a computer to validate it.

The idea is to print some recognizable pattern for the day.

I started thinking about an hash, but that wouldn't work. So I thought of some symmetric cryptography where the key would generate different patterns. But that doesn't make much sense either.

The only request is that the person that checks the pattern can recognize it. This pattern would change in a daily or weekly basis.

I know these would be subject of replay attacks, but for now this would be enough.

Answer 1

I'm going to ignore the horrible usability issue in this answer and just focus on the title question here.

--

One idea is to generate a visual fingerprint like OpenSSH visual fingerprint. OpenSSH Visual fingerprints produce blotch of ASCII characters that are supposedly somewhat memorable can be quickly compared visually.

If you want to be slightly fancy, there's the Aestheticode. Aestheticodes like Penguin Barcode can be scanned, is distinctive and aesthetically pleasing, and is fairly easy to visually compare without a machine. Problem is that aestheticode are short and most aestheticode are generated manually; you'll need to be able to procedurally generate these images if you're going to do this everyday.

Answer 2

There is a trivial answer: decide of a daily password, print it on a pass and you have exactly the system you're describing: a "pattern" that is printed on a physical medium, that can be check by a human being without any extra device or hardware.

If you're willing to rfelax requirement a bit, there is a technique called "visual cryptography" that does something a bit similar but that also requires the person performing the validation to have access to a specific piece of hardware (in this case, a transparent mask which - which can be printed whenever needed).

The Wikipedia article linked has a good, simple example of such an algorythm:

There is a simple algorithm for binary (black and white) visual cryptography that creates 2 encrypted images from an original unencrypted image. The algorithm is as follows: First create an image of random pixels the same size and shape as the original image. Next, create a second image the same size and shape as the first, but where a pixel of the original image is the same as the corresponding pixel in the first encrypted image, set the same pixel of the second encrypted image to the opposite color. Where a pixel of the original image is different than the corresponding pixel in the first encrypted image, set the same pixel of the second encrypted image to the same color as the corresponding pixel of the first encrypted image. The two apparently random images can now be combined using an exclusive-or (XOR) to re-create the original image.

This simple technique has several disadvantages, however: first, anyone having access to a valid access pass can reproduce it very easily, even without understanding the code. second, you need to roll out the validation mask to everyone each time you generate a new access password. Third, all access are equivalent.

There are more advances algorythm that are more flexible (for instance, that will use a single mask to generate a different output depending on th input which, in turn could be used for discriminating between different access rights or simple making each access pass unique).