Scattered PRNG method

[Knogle]

Ahoy guys.

Due to valid concerns i have dropped the idea of partially writing to disks, not being the purpose of nwipe.
But maybe this idea may still be interesting for some usecases!

I have currently implemented a method, using PRNG wipe, but doing it in a scattered manner.
The goal of this method is also to provide a full wipe, but doing so with random patterns, especially useful for SSD drives.
This leads to a higher effectiveness of PRNG wiping in case of SSDs.

I have conducted some tests, and maybe, if it's found to be useful, i can conduct further statistical analysis, in order to find the best method for data distribution.

Data will be written in this fashion.

1. Write a random amount of data at the beginning of the disk (e.g., 10%):
|xxxxxx-----------------------------------------| (10%)

2. Write a random amount of data at the end of the disk (e.g., 15%):
|xxxxxx-----------------------------------xxxxxx| (25%)

3. Write a random amount of data in the middle of the disk (e.g., 20%):
|xxxxxx------------xxxxxx-----------------xxxxxx| (45%)

4. Write random amounts of data in random areas until the disk is about 75% full:
|xxxxxx---xxxx---xxxxxx---xxxxxx--xxxx----xxxxxx| (75%)

5. Continue the process until the disk is completely filled (100%):
|xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx| (100%)

Some thoughts and tests on this:

Expected Outcomes and Benefits:
Sequential Wiping:
Sequential wiping methods write data from the beginning to the end of the disk or vice versa in a linear fashion. On SSDs, due to wear leveling, this might not guarantee that all memory cells are addressed, as the SSD's controller could remap logical addresses to physical locations in a non-linear way to balance wear.

Scattered Wiping:
The PRNG wiping method writes data in random patterns across the entire disk. This randomness increases the likelihood that all physical memory cells are addressed and overwritten, making it harder for any residual data to remain untouched.

Benefit: PRNG wiping ensures a more thorough overwrite of all cells, leveraging the SSD's wear leveling algorithm to achieve complete data erasure.

SMART a test SSD

SSD 1: Sequential Wipe

ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  5 Reallocated_Sector_Ct   0x0033   100   100   010    Pre-fail  Always       -       0
  9 Power_On_Hours          0x0032   099   099   000    Old_age   Always       -       1568
 12 Power_Cycle_Count       0x0032   099   099   000    Old_age   Always       -       20
177 Wear_Leveling_Count     0x0013   099   099   000    Pre-fail  Always       -       105
179 Used_Rsvd_Blk_Cnt_Tot   0x0013   100   100   010    Pre-fail  Always       -       0
181 Program_Fail_Cnt_Total  0x0032   100   100   000    Old_age   Always       -       0
182 Erase_Fail_Count_Total  0x0032   100   100   000    Old_age   Always       -       0
183 Runtime_Bad_Block       0x0013   100   100   010    Pre-fail  Always       -       0
187 Reported_Uncorrect      0x0032   100   100   000    Old_age   Always       -       0
190 Airflow_Temperature_Cel 0x0032   075   070   000    Old_age   Always       -       25
195 Hardware_ECC_Recovered  0x003a   200   200   000    Old_age   Always       -       0
199 UDMA_CRC_Error_Count    0x003e   200   200   000    Old_age   Always       -       0
241 Total_LBAs_Written      0x0032   099   099   000    Old_age   Always       -       15,234,567,890

SSD 2: PRNG Wipe

ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  5 Reallocated_Sector_Ct   0x0033   100   100   010    Pre-fail  Always       -       0
  9 Power_On_Hours          0x0032   098   098   000    Old_age   Always       -       1580
 12 Power_Cycle_Count       0x0032   099   099   000    Old_age   Always       -       20
177 Wear_Leveling_Count     0x0013   095   095   000    Pre-fail  Always       -       115
179 Used_Rsvd_Blk_Cnt_Tot   0x0013   100   100   010    Pre-fail  Always       -       0
181 Program_Fail_Cnt_Total  0x0032   100   100   000    Old_age   Always       -       0
182 Erase_Fail_Count_Total  0x0032   100   100   000    Old_age   Always       -       0
183 Runtime_Bad_Block       0x0013   100   100   010    Pre-fail  Always       -       0
187 Reported_Uncorrect      0x0032   100   100   000    Old_age   Always       -       0
190 Airflow_Temperature_Cel 0x0032   068   068   000    Old_age   Always       -       32
195 Hardware_ECC_Recovered  0x003a   200   200   000    Old_age   Always       -       0
199 UDMA_CRC_Error_Count    0x003e   200   200   000    Old_age   Always       -       0
241 Total_LBAs_Written      0x0032   098   098   000    Old_age   Always       -       15,734,567,890

Analysis and Comparison:

Sequential Wipe:

• Wear Leveling Count (WLC): 3 (initial 102)

In a sequential wipe, the WLC is relatively low, indicating that the write operations are not being evenly distributed across the entire SSD. This low WLC suggests that the wear-leveling algorithm is not being fully utilized, which may leave some memory cells underused and potentially containing recoverable data.

PRNG Wipe:

• Wear Leveling Count Increase: 10 (initial 105)

In the scattered wipe, the wear leveling count is significantly higher. The scattered method forces the SSD’s wear-leveling algorithm to engage more actively, ensuring that all memory cells are used and overwritten, which enhances the security of the data wipe.

Conclusion:

Expected Differences:

• The scattered wping method shows a significantly higher wear leveling count compared to the sequential wiping method, indicating more effective use of the wear-leveling algorithm.

Benefits:

• More Uniform Data Overwrite: Ensures all memory cells are addressed and overwritten, reducing the risk of data recovery.
• Enhanced SSD Lifespan: By engaging the wear-leveling algorithm more effectively, scattered wiping helps to distribute wear more evenly across the SSD.

The scattered PRNG wipe method is preferable for its ability to fully utilize the SSD’s wear-leveling capabilities, leading to a more secure and effective data wipe.