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I am planning on designing a system that needs a password to be operated, using a rolling key.

Now I am facing two options, as far as I am concerned:

  1. Having an index saved, which will be incremented on each iteration, send to e.g. my phone, encrypted via a private key, send back to the secured system, decrypted via the public key, compared to the actual index
  2. Having a set of pre-generated keys on both systems, each time sending a new key of that set.

Although the first solution would be more memory-efficient in my eyes (talking about a uC), i have a fear that using the same keypair with a numerically increasing predictable index would give quite some attack surface to crack the key.

  • Is there an asymmetrical encryption algorithm I could use like that?
  • Or is there an approach I didn't consider at all?
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