SSD – Why you should not upgrade to a SSD on privacy grounds
The most dangerous assumption is that magnetic drive erasure techniques will work on Flash based SSD drives, so lets discuss what’s different between these two very different technologies.
Assumption 1 – Magnetic Erasure tactics work on SSD’s
“Flash based solid state drives (SSD’s) differ from hard drives in both the technology they use to store data (flash chips vs magnetic disks) and the algorithms they use to manage and access that data.
There are no agreed erasure standards for SSD’s – every state authority issues different guidelines.
Assumption 2 – Manufacturers built in commands will work
Manufacturer implementations of secure erase commands were found to be faulty or have catastrophic bugs; this has resulted in all data remaining intact on the disk.
Wei et al (2010) tested 12 drives. Only four drives executed the “ERASE UNIT” command reliably. One drive reported erasure as successful, when all the data remained intact. Two more drives suffered coding flaws the prevented the “ERASE UNIT” command working, unless a firmware reset had taken place.
Wei et al (2010) stated that “The wide variance among the drives leads us to conclude that each implementation of the security commands must be individually tested before it can be trusted to properly sanitise the drive”.
This is an important point – Manufacturer claims are not verified to be true, yet consumers rely on manufacturers claims when purchasing hardware. Clearly there exists a SIGNIFICANT number of issues, enough that perhaps the EU ought to verify the erasure ability of drive manufacturers. This issue appears to be the ability to implement “Secure Erasure”. It’s so badly implemented, that in a huge percentage it simply doesn’t work at all, let alone as advertised. The laws to fine both manufacturers and retailers exist under current Trading Standards laws. Failure to work as advertised also impacts on EU Data Protection laws; where a user has taken steps to erase sensitive data such as encryption keys, passwords, banking and financial data, the erasure then does not take place (worse still the drive may alert the user that the erasure HAS occurred). The user may sell or donate the drive to third parties, trusting in the manufacturers coding of secure erasure commands.
Assumption 3 – All erasure is safe
We use the mnemonic “LAD” to the three levels of erasure; Logical, Analogue and Digital.
Logical is the LEAST safe. The data can be forensically recovered. Where users overwrite parts of the drive, equivalent to CLEARING in NIST 800-80.
Analogue is the most SAFE. Analogue makes reconstructing the signal effectively impossible, it is equivalent to PURGING in NIST 800-80.
Digital means disk overwriting and then deletion, but may not erase bad blocks (these often contain data).
Assumption 4 – Cryptographic erasure is safe
Firstly the drive stores the encryption key. An analogy for this is to fit the world’s strongest burglar alarm, and then hide the door key under the front door mat.
AES is a symmetric cipher, which means a single key. The key to encrypt is the same key that decrypts the data. If this single key can be recovered – you’re in BIG trouble. EVERYTHING depends on the strength of the encryption… now here we enter the realm of the cryptologists, and they have a lot to say about AES 128 and AES 256.
Amongst cryptologists, AES 128 & 256 have been openly criticised.
Schneier attempted to add extra rounds to make AES robust – but the cipher became too slow to use. The reduced rounds used in AES are it’s Achilles heel. Too read more on AES, look for Schneiers work from 2000.
That “cryptographic erasure” doesn’t look so great now, does it?
Assumption 5 – Pages vs Blocks.
Flash memory is broken into pages and blocks. An analogy is pages and books. If we wish to erase a single page, SSD’s make us erase the entire book. The program operations that apply to pages can only change 1’s to 0’s.
Erasure operations only apply to Blocks (or the entire book), and set all the bits in a block to 1.
So SSD’s are not equipped for erasure. Yet if we can write to pages, why doesn’t the coding to ERASE pages exist? Surely the need to erase data was considered?
Assumption 6 – We can overwrite single files
Since we can only erase entire blocks, file level erasure is not possible. Overwriting a file only provides logical erasure.
Assumption 7 – Data Remnants – what’s this?
Digital remnants on SSD’s can range between 6 and 25% of the entire drive.. yes, a quarter of the SSD drive may hold “remnants of data”, which has massive implications for privacy.
Assumption 8 – The Capacity of the drive is as advertised.
In SSD’s the drives are often larger than they advertise as their logical capacity.
Assumption 9 – Only 1 copy of a file exists.
In SSD’s up to 16 stale copies of a file may exist. Even if you overwrite one file, there are another 15 copies that can be forensically recovered. Again, this single point has massive implications for privacy. How are you going to locate these floating around copies?
The differences between hard drives and SSD’s potentially lead to a dangerous disconnect between user expectations and the drives actual behaviour”, (Wei et al, 2010).
I would suggest that the EU test and verifies built in drive erasure commands to ensure compliance with Trading Standards. If the onus is placed back on the manufacturer, the manufacturer will test and verify that erasure works. It’s simply a case of “follow the money”. Consumers are not able to carry out such testing, as they do not have technical ability to recover data to ensure the code works. Therefore we need to force manufacturers to carry out “due diligence”.