copy(objectEncryptionKey[:], key)

			partEncryptionKey := objectEncryptionKey.DerivePartKey(uint32(partID))
			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