static X25519HpkeKemPrivateKey fromBytes(byte[] privateKey) throws GeneralSecurityException {
    return new X25519HpkeKemPrivateKey(privateKey, X25519.publicFromPrivate(privateKey));
  }
Private keys used in Diffie-Hellman-based X25519-HKDF HPKE KEM variant.
java_src/src/main/java/com/google/crypto/tink/hybrid/internal/X25519HpkeKemPrivateKey.java