XChaCha20-Poly1305 key wrapping mechanism.
Key is encrypted using XChaCha20-Poly1305 algorithm.
Random 192-bit nonce is prepended to the ciphertext.
KEK has 256-bit length.

    NONCE = random(24 bytes)
    NONCE || XChaCha20-Poly1305(key=KEK, ad="", nonce=NONCE, data=CEK)