public byte[] open(byte[] ciphertext, byte[] associatedData) throws GeneralSecurityException {
    byte[] nonce = computeNonceAndIncrementSequenceNumber();
    return aead.open(key, nonce, ciphertext, associatedData);
  }
Performs AEAD decryption of {@code ciphertext} with {@code associatedData} according to
ContextR.Open() defined in https://www.rfc-editor.org/rfc/rfc9180.html#section-5.2-10.

@return plaintext
java_src/src/main/java/com/google/crypto/tink/hybrid/internal/HpkeContext.java