in := io.Reader(hashReader)
			if size > encryptBufferThreshold {
				// The encryption reads in blocks of 64KB.
				// We add a buffer on bigger files to reduce the number of syscalls upstream.
				in = bufio.NewReaderSize(hashReader, encryptBufferSize)
			}

			var nonce [12]byte
			tmp := sha256.Sum256(fmt.Append(nil, uploadID, partID))
			copy(nonce[:], tmp[:12])