資訊科技小貼士 安全清除—預防硬磁碟中的文件被復原; Information Security Tips Series Secure Erasure: File Recovery Prevention in Hard Disks (Offbeat 964)

有否察覺到已經「刪除」的數據還留在硬磁碟內?因為硬磁碟內的數據還是可以從「已刪除」的文件或「可用儲存空間」被擷取出來。 雖然很多人都知道數據保安和在棄置硬磁碟前要「刪除」數據或把硬磁碟「格式化」,但這些方法只會移除文件的「索引」或「指標」,真實的文件或數據可能仍留 在硬磁碟內。

安全清除
覆寫、消磁和物理破壞是用來防止任何「已刪除」的數據從硬磁碟內被擷取的方法,但它們都有其利弊。

覆 寫
不論是付費或是免費的安全清除軟件,都是利用覆寫技術來清除檔案在硬磁碟中的實際數據區域,一旦被覆寫,那些檔案或其內容便不能輕易地被擷取出來。因應所 需要的安全程度,不同的覆寫技術或標準如一次性覆寫方法和美國國防部的多次覆寫方法,會使用不同的覆寫次數和各種覆寫數據模組來清除硬磁碟內的數據。雖然 覆寫技術成本低且功效大,但它是很費時的。另外,覆寫並不是一個完美的解決方法,如果硬磁碟包含損壞磁區或故障,軟件可能無法讀取損壞磁區或硬磁碟而導致 覆寫失敗。

消磁和物理破壞
消磁是使用強大和不斷變化的磁場來破壞硬磁碟內的磁像,因而同時清除碟內的數據,物理破壞則是把硬磁碟弄成碎片,兩者都可以解決損壞磁區或硬磁碟故障的問題。不過,這兩種方法也有其缺點。首先,消磁和物理破壞只適用於清除整個硬磁碟的數據,而不能處理個別文件或磁碟分區。其次,處理過的硬磁碟不能再使用及 失去儲存功能。最後,這些方法都需要專用設備,而這些設備對於一般用家來說都是很昂貴的,或者用家只能尋求服務供應商提供數據清除服務。

上述的方法未必適用於其他硬磁碟如固態硬盤或USB手指驅動器。因為有別於傳統硬磁碟的儲存方式和數據管理規則,某些覆寫技術可能無法有效地清除一些先進儲存設備如固態硬盤和USB手指驅動器內的數據,而消磁亦無法清除這種儲存設備內的數據。

簡而言之,安全清除是把雙刃劍。一方面,它可以防止他人擷取數據;另一方面,你也不能還原被意外刪除的文件。在時下的資訊科技世界中要處理棄置的儲存設備 並不容易,同時新的儲存技術也為安全清除帶來了許多新的挑戰。在選擇安全清除方法時,用戶要非常小心及確保所選擇的方法是可兼容和能夠清除特定硬磁碟或儲 存設備內的數據。

Are you aware your data may still be inside the hard disks even if you have “deleted" them? Many recent incidents have showed disposed hard disks are treasures for “interested parties" as data could still be retrieved from the “deleted" files or “free space" inside these hard disks. Although many people are aware of data security and “delete" all data or “format" hard disks before disposal, these methods only remove the “index" or “pointer" of the files but the actual files or data may still remain untouched in hard disks. These methods are not secure enough.

Secure erasure is one of the ways to prevent any “deleted"data from being retrieved from hard disks. In general, there are three methods, (1) overwriting, (2) degaussing and (3) physical destruction. Three methods have pros and cons.

Overwriting


Secure erasure software, whether paid or free, makes use of overwriting technique to erase the actual data areas of the files in hard disks. Once overwritten, the files or their content can no longer be retrieved easily. Depending on the security level required, different overwriting techniques or standards, like one-pass overwriting and US DoD method, use different overwriting passes and various data patterns. Although overwriting is cost effective and powerful, it is time-consuming. It is not always a perfect solution if hard disks contain bad sectors or malfunction. Bad sectors or malfunctioning may make the data areas inaccessible or unable to be overwritten by software.

Degaussing and physical destruction


Degaussing uses strong and changing magnetic fields to destroy the magnetic patterns and hence the data in hard disks. Physical destruction means breaking a hard disk into pieces. Both can overcome the problems of bad sectors or malfunctioning of a hard disk. Nevertheless, they have drawbacks. Firstly, degaussing and physical destruction can be used on a whole hard disk only but not individual file or partition. Secondly, hard disk cannot be re-used or functional afterwards. Lastly, these methods require proprietary equipment that is expensive for general users. Users can only approach service providers for erasure service.

The above methods may not be used in other hard disks like Solid-State Drives (SSD) or USB thumb drives. Owing to the different ways storing data and data management algorithm from traditional magnetic hard disks, some advanced storage devices like SSD and USB thumb drives may make certain overwriting techniques ineffective. Degaussing cannot erase the data of such drives.

On one hand, secure erasure prevents data from being retrieved by others, and on the other hand, you cannot rescue any files that have been deleted accidentally. Disposal of storage device is not an easy task in the modern IT world and new storage technologies are also bringing new challenges of secure erasure. Users should make sure the method chosen is compatible and can erase the data in such hard disk or storage device.

 

 

 

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