cm/kem/sntrup761-x25519-hkdf-blake2b

    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}
            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