public synchronized long verifiedSize() throws IOException {
    if (tryLoadSegment(numberOfSegments - 1)) {
      return plaintextSize;
    } else {
      throw new IOException("could not verify the size");
    }
  }
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