I humorously propose a cryptography between human beings based on
   shared experience alone. The “private key” is the impact of that shared
   experience on the human heart.
   In pair with an AI capable of simulating the recipient(s) based on
   information known about them, one considers a unique experience shared
   with the recipient(s). Implications, details, and feelings regarding
   the experience are enumerated until the AI discerns strong enough
   likelihood that the recipient(s) would find a cryptographically
   sufficient set of similar ones.
   The enumerations are compressed into ideas, trigger words, a short
   stimulating phrase, etc. These are the public key material. They must
   depend on both parties’ personalities, and also the moment of shared
   experience.
   The details considered in the mind of the sender(s) or recipient(s) are
   the private key material, which are privately extracted by the AI and
   used as a symmetric key.
   On receipt, the recipient(s) engage the ideas, trigger words, etc,
   gently and slowly, calmly thinking of them and feeling around them. An
   AI watches their mind, looking for private key material. Once
   sufficient material is found, the message is decrypted.
   The system may work with a stranger if something important is known
   about the stranger indicating a need to contact them — and fails if
   this important thing turns out to be false.
   There are vulnerabilities that can be addressed by building a
   respectful society and government if you have one.
   The AI can be replaced with very extensive practice on the part of the
   sender, and patient work on the part of the recipient.
   All early daydream guesses for fun! I believe it is quite
   implementable. I did not review for vulnerabilities much.