code: public static long ciphertextSize(long cleartextSize, Cryptor cryptor) {
  		checkArgument(cleartextSize >= 0, "expected cleartextSize to be positive, but was %s", cleartextSize);
  		long cleartextChunkSize = cryptor.fileContentCryptor().cleartextChunkSize();
  		long ciphertextChunkSize = cryptor.fileContentCryptor().ciphertextChunkSize();
  		long overheadPerChunk = ciphertextChunkSize - cleartextChunkSize;
  		long numFullChunks = cleartextSize / cleartextChunkSize; // floor by int-truncation
  		long additionalCleartextBytes = cleartextSize % cleartextChunkSize;
  		long additionalCiphertextBytes = (additionalCleartextBytes == 0) ? 0 : additionalCleartextBytes + overheadPerChunk;
  		assert additionalCiphertextBytes >= 0;
  		return ciphertextChunkSize * numFullChunks + additionalCiphertextBytes;
  	}
docs: Calculates the size of the ciphertext resulting from the given cleartext encrypted with the given cryptor.
  
  @param cleartextSize Length of a unencrypted payload.
  @param cryptor The cryptor which defines the cleartext/ciphertext ratio
  
  @return Ciphertext length of a <code>cleartextSize</code>-sized cleartext encrypted with <code>cryptor</code>.
  Not including the {@link FileHeader#getFilesize() length of the header}.