@Override
  public int write(ByteBuffer src) throws NonWritableChannelException {
    throw new NonWritableChannelException();
  }
Returns the expected size of the plaintext.
Note that this implementation does not perform an integrity check on the size.
I.e. if the file has been truncated then size() will return the wrong
result. Reading the last block of the ciphertext will verify whether size()
is correct.
java_src/src/main/java/com/google/crypto/tink/subtle/StreamingAeadSeekableDecryptingChannel.java