//
	// 2. the (decrypted) start offset `off` and (decrypted)
	//    length to read `length`
	//
	// These are the inputs to the rest of the algorithm below.

	// Locate the part containing the start of the required range
	var partEnd int
	var cumulativeSum, encCumulativeSum int64
	for i, size := range sizes {
		if off < cumulativeSum+size {
			partStart = i
			break
		}