Securely Delete files - Ubuntu

So recently there was a lot of talk at work about keeping our customer data secure. Each of us was fully responsible for the customer data that we had on disk.

I use Ubuntu 12.04 with a ton of Virtual machines. Here's what I ended up doing to do my bit to keep all our customer data safe.

a) Set a BIOS password - If your laptop gets stolen and someone wants to boot off a USB, this makes it harder. Obviously though, they can just take your hard disk out and plug it into another laptop..

b) Full Disk Encryption - Sure ..they can plug your disk into another machine. If all your data is encrypted (Ubuntu allows you to encrypt data while installing it) and you have a reasonably strong passphrase (Greater than 10 characters + Capital letters, small letters, digits and special characters) it's going to be really hard to try and crack.

c) Do not store any customer data on your laptop - It's hard to do this, but really it's the best way. Let customer data be stored on secure servers inside a server room or datacenter, where it can't be stolen that easily. Some customer data storage though might be unavoidable...

d) Use Truecrypt if you must store Customer data - Whatever data there is on your laptop, encrypt that again using Truecrypt and a strong passphrase. So even if someone cracks your full disk encryption passphrase, all they will find is a Truecrypt file.

e) Securely delete content all the time - Using rm -rf or Shift + Delete is no good, as forensics tools will be able to recover data. Use the secure-delete suite of tools to delete data securely. I added an alias to my rm command so I don't ever accidentally only use 'rm' instead of 'srm'.

alias rm = 'srm -rv'

This overwrites files 38 times before deleting them by default. Each file :D. It's probably overkill. So I'd recommend doing something like srm -rfvl filename (The l does just 2 passes instead of 38) and doing an rm filename at the end of every project.

f) I also plan to read up on the other tools that the secure-delete suite offers and run those to clean up my RAM (Run sdmem -v as root)and fill (Run sfill -v MountPoint as root. You can identify your mount points either by running the mount command, or by running df -kh and looking at the Mounted On column) up all my unused space with random data. This is needed because I've been deleting insecurely for a long time now. As of now, I also plan to never delete from Nautilus because adding commands to the context menus using various guides is proving to be an utter pain.

g) Formatted all my flash drives and created a Truecrypt volume on the only flash drive that I plan to use to store customer data. So even if the flash drive gets lost, the data is still reasonably hard to get at.

Salt your passwords = Existing accounts password reset

This one is just a simple conclusion that I drew when a customer had MD5 hashed passwords in his database and we recommended them to salt them instead and use PBKDF as the algorithm of choice.

Now the customer added code in, but left the old code in as well which did the simple hash comparisons. So I asked them why and they said..for old users. I started thinking about it and it made sense. Here's why:

Think of it. I have a MD5 password in the database. Say I add a new column for 'salt' , generate 1000 random hashes..one for each user..and add a new column. I then change the code to query the DB for the 'salt' as well, when a person logs in and run the user's password through the PBKDF function before comparing it with the hash. Looks good?

Sure. Except that no one will be able to login. Why? It's super secure now :). No one can find any more vulnerabilities inside. Jokes apart, what's happening here? Lets say the user's original password was abc123 and it had a 32 bit MD5 hash XYZ. Now when the user logs in something like MD5(user entered password) runs..hashes what he entered and compares it to the stored hash. Match? Login successful.

Now there's the salt. So when the user logs in, the new 'salt+pbkdf' code is triggered. So PBKDF(userpwd + salt) is calculated and compared to ..what? The stored MD5 hash. Is there ever going to be a match? Largely no. Because..there is no way you can predict the salt for a particular hash..BEFORE you store it in the DB. Remember those 1000 salts you generated? All random? Did you check if they'd give you the user's password after "decryption"? Nope.

So since there's no way to get the "right salt" for each user ID, the easiest solution is to force a password reset across users. The next time they login everything will automatically fall into place.