How to safeguard our data integrity?
How to identify who we are talking to?
Defensive Cryptography is defined as cryptographic means and actions taken to protect one's interest in the realm of communication, data integrity, and data critical operations.
What is Crypto? Cryptography is means, tools, methods, and actions to create a data-oriented advantage based on knowledge discrimination -- allowing the ones with certain knowledge to distinguish themselves from those who lack that knoweldge (The discriminating knowledge is called: key).
These broadly stated definitions often boil down to the simple question: How to communicate a secret?
The other two main topics are:
Defensive cryptography begins with threat analysis. Don't confuse 'threat analysis' with 'vulnerability analysis'.