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Browsing through the site I encountered this Is there a way to negotiate a secret between 2 parties with no prior knowledge?

What is an active attack then?

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up vote 2 down vote accepted

An active attack is one where the attacker modifies network packets while they are in transit, or sends forged network packets.

A passive attack is one where the attacker merely eavesdrops on packets that others are sending, without injecting any new packets and without modifying any of the packets others have sent.

This should be covered in any cryptography textbook.

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