TL;DR – Hybrid Public Key Encryption

Just from a quick scan of the RFC: this seems to generate the encrypted session key directly from the public key of the recipient somehow. This also seems to be an attempt to standardize generating the public/secret keypair from something like a passphrase.

Traditionally you generate a random session key and then encrypt it using the public key of the recipient. The public/secret keypair is generated standalone where you optionally encrypt the secret bit of the keypair separately with a passphrase.

Not yet sure how this scheme would work in practice and what the advantage is...

Yes, it's an effort to standardize and future proof.

Cloudflare's post [0] has some more background information.

[0] https://blog.cloudflare.com/hybrid-public-key-encryption/

I thought Alex and Sacha were forms of the same name? (Alexander, Xander and so on) seems a confusing choice.

Bob and Alice fell out and are no longer in touch.

In Soviet Russia, Alex and Sasha secure you.

(never mind the hostile climate today, headduck)

It doesn't look like this would even work for the offline stuff that PGP targets. The session key seems to be linked to the public key. So really only good for things where you throw away the keys after the session has closed. Forward secret TLS for example. Unless I am missing something, the reference to OpenPGP and S/MIME is gratuitous.

HPKE allows you to use private/public keypairs that are signed by a trusted authority, therefore enabling two parties that have never met prior to securely communicate. That is, as long as they trust the same authority.

Another major benefit of HPKE is a shared key schedule. Two parties can generate as many symmetric keys as they would like over a persistent conversation. This kind of provides ‘sub keys’, but within the context of a conversation, as they are not long lived.

GPG’s web of trust model is powerful but is limited in contexts where you have no prior trust. The world’s existing PKI structure, with a handful of root CAs, is already well established and trusted by nearly every device on the planet.