kem-with-encap {
{field . {map}}
{field a {str} >0} {# sntrup761-x25519-hkdf-blake2b}
{# mceliece6960119-x25519-hkdf-shake256}
{field cek {bin} >0} {# wrapped CEK}
{field encap {bin} >0}
{field to {with fpr} optional} {# recipient's public key}
{field from {with fpr} optional} {# sender's public key}
}