Randomizing the rows of a matrix and reversing

Asked by Mohsin Shah on 1 Aug 2017

Latest activity Commented on by Mohsin Shah on 2 Aug 2017

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Hello, I have a 988 x 3 matrix (say the vertices of 3D object) and I want to randomize the rows of this matrix and then do the reverse process to get the original matrix. I am using the following code to do the job:

The above code doesn't produce the original matrix. Can someone highlight my mistake and give me a solution?

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Answer by José-Luis on 1 Aug 2017

Edited by José-Luis on 1 Aug 2017

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If you want to return to the original matrix, you need to keep it. There is no way to undo a random permutation, unless your data was ordered/structured in some way (was it?). If it was ordered, how so? The key word is random.

Just to be psychotic about it. randperm() is not random but pseudo-random. If you knew the state of the random number generator, it could be possible to go back to the original. I am guessing this is not what you had in mind.

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Mohsin Shah on 1 Aug 2017

Actually, I am a student of reversible data hiding using 3D mesh models. I embed confidential data into a 3D model at the transmitter side, then transmit the marked model. At the receiver side, we have to decode the embedded data and recover the original 3D model. In such application scenarios, we don't keep the original model. All we can share is a secret key such rng data for recovery.

José-Luis on 1 Aug 2017

Then you need to pass the two indexes or the state of your random number generator to recreate them.

That being said, this does not feel like the best way of hiding data, but what do I know :)

Mohsin Shah on 2 Aug 2017

Thank you so much. This is only to permute the locations for achieving more security. The hiding itself is a complex algorithm and has nothing to do with permutation.

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