@Nullable
  static Cipher getThreadLocalCipherOrNull() {
    return localCipher.get();
  }
Returns a thread-local instance of the ChaCha20Poly1305, or null if ChaCha20Poly1305 is
not supported.
java_src/src/main/java/com/google/crypto/tink/aead/internal/ChaCha20Poly1305Jce.java