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.