Streamlined NTRU Prime 761 + X25519 + HKDF-BLAKE2b KEM.
=> Streamlined NTRU Prime KEM algorithm
=> X25519
=> RFC 5869, HKDF
=> RFC 7693, BLAKE2b

    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}
    }

"/kem/*/a" equals to "sntrup761-x25519-hkdf-blake2b".
Recipient public key with [cm/pub/sntrup761-x25519]
algorithm must be used. It should have "kem" key usage set.

Recipient's map "/kem/*/encap" field is a concatenation of 1047
bytes Streamlined NTRU Prime 761's ciphertext, containing
ephemeral key, with 32 bytes ephemeral X25519 public key.

Recipient performs X25519 and SNTRUP computations to derive/decapsulate
two 32-byte shared keys. Then it combines them to get the KEK decryption
key of the CEK.

            ================================================
                                WARNING
            ================================================
            Sender authentication uses only *NON*-PQ crypto!
            ================================================

    H = BLAKE2b
    PRK = HKDF-Extract(H, salt="", ikm=
        sntrup761-shared-key || es-x25519-shared-key ||
        H(sntrup761-sender-ciphertext || e-x25519-sender-public-key) ||
        H(sntrup761-recipient-public-key || s-x25519-recipient-public-key))
    if {specified sender}
        ss-x25519-shared-key = X25519(s-x25519-sender-private-key,
                                      s-x25519-recipient-public-key)
        PRK = HKDF-Expand(H, prk=PRK,
            info="cm/encrypted/sntrup761-x25519-hkdf-blake2b/auth")
        PRK = HKDF-Extract(H, salt=PRK, ikm= ss-x25519-shared-key ||
            s-x25519-sender-public-key || s-x25519-recipient-public-key)
    KEK = HKDF-Expand(H, prk=PRK,
        info="cm/encrypted/sntrup761-x25519-hkdf-blake2b" || /id)

"/kem/*/cek" is wrapped with [cm/keywrap/xchapoly] mechanism.

KEM combiner nearly fully resembles:
=> Chempat