Hardware-based full disk encryption
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Hardware-based full disk encryption (FDE) is available from many hard disk drive (HDD) vendors, including: Seagate Technology, Hitachi, Western Digital, Samsung, Toshiba and also solid-state drive vendors such as OCZ, SanDisk, Samsung, Micron and Integral Memory. The symmetric encryption key is maintained independently from the CPU, thus removing computer memory as a potential attack vector. In relation to hard disk drives, the term Self-encrypting drive (SED) is in more common usage.
Hardware-FDE has two major components: the hardware encryptor and the data store. There are currently three varieties of hardware-FDE in common use:
- Hard disk drive (HDD) FDE (usually referred to as SED)
- Enclosed hard disk drive FDE
- Bridge and Chipset (BC) FDE
Hard disk drive FDE
HDD FDE is made by HDD vendors using the OPAL and Enterprise standards developed by the Trusted Computing Group. Key management takes place within the hard disk controller and encryption keys are 128 or 256 bit Advanced Encryption Standard (AES) keys. Authentication on power up of the drive must still take place within the CPU via either a software pre-boot authentication environment (i.e., with a software-based full disk encryption component - hybrid full disk encryption) or with a BIOS password.
Hitachi, Micron, Seagate, Samsung, and Toshiba are the disk drive manufacturers offering TCG OPAL SATA drives. Older technologies include the proprietary Seagate DriveTrust, and the older, and less secure, PATA Security command standard shipped by all drive makers including Western Digital. Enterprise SAS versions of the TCG standard are called "TCG Enterprise" drives.
Enclosed hard disk drive FDE
Within a standard hard drive form factor case both the encryptor (BC) and a smaller form factor, commercially available, hard disk drive is enclosed.
- The enclosed hard disk drive's case can be tamper-evident, so when retrieved the user can be assured that the data has not been compromised.
- The encryptors electronics and integral hard drive, if it is solid-state, can be protected by other tamper respondent measures.
- Tampering is not an issue for SEDs as they cannot be read without the decryption key, regardless of access to the internal electronics [clarification needed].
The encryptor bridge and chipset (BC) is placed between the computer and the standard hard disk drive, encrypting every sector written to it.
Hardware-based encryption when built into the drive or within the drive enclosure is notably transparent to the user. The drive except for bootup authentication operates just like any drive with no degradation in performance. There is no complication or performance overhead, unlike disk encryption software, since all the encryption is invisible to the operating system and the host computers processor.
The two main use cases are Data at Rest protection, and Cryptographic Disk Erasure.
In Data at Rest protection a laptop is simply powered off. The disk now self-protects all the data on it. The data is safe because all of it, even the OS, is now encrypted, with a secure mode of AES, and locked from reading and writing. The drive requires an authentication code which can be as strong as 32 bytes (2^256) to unlock.
Crypto-shredding is the practice of 'deleting' data by (only) deleting or overwriting the encryption keys. When a cryptographic disk erasure (or crypto erase) command is given (with proper authentication credentials), the drive self-generates a new media encryption key and goes into a 'new drive' state. Without the old key, the old data becomes irretrievable and therefore an efficient means of providing disk sanitization which can be a lengthy (and costly) process. For example, an unencrypted and unclassified computer hard drive that requires sanitizing to conform with Department of Defense Standards must be overwritten 3+ times; a one Terabyte Enterprise SATA3 disk would take many hours to complete this process. Although the use of faster solid-state drives (SSD) technologies improves this situation, the take up by enterprise has so far been slow. The problem will worsen as disk sizes increase every year. With encrypted drives a complete and secure data erasure action takes just a few milliseconds with a simple key change, so a drive can be safely repurposed very quickly. This sanitization activity is protected in SEDs by the drive's own key management system built into the firmware in order to prevent accidental data erasure with confirmation passwords and secure authentications related to the original key required. There is no way to retrieve data once erased in this way - the keys are self generated randomly so there is no record of them anywhere. Protecting this data from accidental loss or theft is achieved through a consistent and comprehensive data backup policy.
Protection from alternative boot methods
Recent hardware models circumvents booting from other devices and allowing access by using a dual Master Boot Record (MBR) system whereby the MBR for the operating system and data files is all encrypted along with a special MBR which is required to boot the operating system. All data requests are intercepted in the SED firmware and will not allow decryption to take place unless the system has been booted from the special SED operating system which will then load the MBR of the encrypted part of the drive. This works by having a separate partition, hidden from view, which contains the proprietary operating system for the encryption management system. This means no other boot methods will allow access to the drive.
Typical self-encrypting drives, once unlocked, will remain unlocked as long as power is provided. Researchers at Universität Erlangen-Nürnberg have demonstrated a number of attacks based on moving the drive to another computer without cutting power. Additionally, it may be possible to reboot the computer into an attacker-controlled operating system without cutting power to the drive.
When a computer with a self-encrypting drive is put into sleep mode, the drive is powered down, but the encryption password is retained in memory so that the drive can be quickly resumed without requesting the password. An attacker can take advantage of this to gain easier physical access to the drive, for instance, by inserting extension cables.
The firmware of the drive may be compromised and so any data that is sent to may be at risk. Even if the data is encrypted on the physical medium of the drive, the fact that firmware is controlled by a malicious third-party means that it can be decrypted by that third-party. If data is encrypted by the operating system, and it is sent in a scrambled form to the drive, then it would not matter if the firmware is malicious or not.
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