Enum Mode 

pub enum Mode<'a> {
    Base,
    PreSharedKey {
        pre_shared_key: &'a [u8],
        pre_shared_key_id: &'a [u8],
    },
}

The HPKE session mode, shared by the sender and recipient roles.

The mode selects whether the key schedule is bound to a pre-shared key (Mode::PreSharedKey) or not (Mode::Base).

Pre-shared key bytes and their identifier are carried together, so the inconsistent inputs rejected by the specification cannot be expressed.

draft-ietf-hpke-hpke-05 removed the asymmetric authenticated modes of RFC 9180 (mode_auth and mode_auth_psk). For sender identity authentication, sign the (encapped_key, ciphertext) tuple (see the crate documentation); a pre-shared key only proves possession of the key.

Variants

Base

No sender authentication.

PreSharedKey

Pre-shared key mode: the shared key is mixed into the key schedule, proving possession of it. This does not authenticate a sender identity.

Fields

pre_shared_key: &'a [u8]

The pre-shared key. MUST contain at least 32 bytes of entropy.

pre_shared_key_id: &'a [u8]

A public identifier for the pre-shared key.

It’s purpose is let a recipient pick which PSK to use out of several it holds, and to be bound into the key schedule so a wrong id yields a different key.

As per the spec, pre_shared_key_id MUST not be empty in pre-shared key mode. We recommend the Nil UUID (00000000-0000-0000-0000-000000000000) if you don’t have a relevant identifier for the pre-shared key.