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Is it possible to create a public key out of a private key, then create another public key from the first public key. And treat the first public key as a private key (So you have two levels of private keys). Is the complexity still there? Is it possible to calculate back from the second public key to the first one? or do these keys behave the same as a private/public keypair?

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    $\begingroup$ This sounds suspiciously like an XY problem. What exactly are you trying to achieve? $\endgroup$ – Maeher Dec 20 '18 at 14:54
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By definition a public key can't be(come) a secret key. So a cryptosystem such as the one you describe can't be realised. But you can have a look at broadcast encryption, it may help you !

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    $\begingroup$ We'll, it is possible to realize what's in the question. It is just wholly insecure for the reason pointed in the answer. a public key is not secret, and a private key should be. $\endgroup$ – fgrieu Dec 20 '18 at 14:48
  • $\begingroup$ Also note that, in RSA case the public key is small and fixed as 3, 7, 39 etc. Using this as private key will be insecure. $\endgroup$ – kelalaka Dec 21 '18 at 9:04

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