private int getSegmentNr(long plaintextPosition) {
    return (int) ((plaintextPosition + ciphertextOffset) / plaintextSegmentSize);
  }
Tries to read the header of the ciphertext and derive the key used for the ciphertext from the
information in the header.

@return true if the header was fully read and has a correct format. Returns false if the header
    could not be read.
@throws IOException if the header was incorrectly formatted or if there was an exception during
    the key derivation.
java_src/src/main/java/com/google/crypto/tink/subtle/StreamingAeadSeekableDecryptingChannel.java