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To use the free trial of Nessus, you need an email address to receive an activation key.

There are two modes to activate a Nessus server:

  1. Online mode registration

If the computer running Nessus server has internet, you can activate the software by entering the activation key. It will authorize the use only if the right key is entered.

  1. Offline mode registration

If you don't have internet on the computer running Nessus server, you need to activate the software with a license:

  • On the Nessus server, check the box "Register offline". It will display a challenge.
  • With another computer having internet access, go to this page to generate a license to use the software. It will require the challenge generated by the offline Nessus server and the activation key received by mail during registration.
  • Copy the license generated by the website to the offline Nessus server to authorize the use of the software.

To avoid users bypassing the registration step through offline mode, it requires the activation key received by email before receiving the license needed to activate the software.

How can an "offline" challenge authentication be secure?

If challenges and responses are already generated before downloading the software, users can gather all the challenges and the responses in a list. At some point, all the challenges will exist in this list and no one will need to register by mail to get an activation key to have the response of the challenge.

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    I suggest you attempt to answer you own question by trying exactly what you propose, and then see what is stopping you from succeeding.
    – user163495
    Commented Aug 16, 2021 at 13:10
  • I need at least the same number of unique email addresses as the number of challenge they put in their software.
    – molik
    Commented Aug 16, 2021 at 14:08
  • What makes you think the challenges are already generated before downloading? I don't see a challenge-response protocol where the challenges are known in advance by the responder on Wikipedia: en.m.wikipedia.org/wiki/… Commented Aug 18, 2021 at 5:09

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