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My Windows 8.1 Pro laptop doesn't have TPM, so I can use bitlocker with either a USB key or a password.

If I use a password, it will be 20 almost random characters, it will contain no words in any language, it will have at least one small letter, one capital, one number and one special character.

Now, you will probably ask, in what environment you will use that laptop? It's a laptop, so I will take it with me everywhere. I am using bitlocker to protect it from:

a) thieves (most likely they will have no idea what encryption is)

b) friends (who know what encryption is better than me). We sit close to each other and I don't want them to see my personal projects.

Should I use a USB key or Password and why?

edit: In terms of brute force protection, how much better the usb key is?

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    Well I think the interesting point about this question isn't about brute-force protection. Concerning this the password can only lose (~130 bits against 512 bits+ possible with files). – SEJPM May 9 '15 at 20:10
  • It takes someone who knows a pretty decent amount about computers and crypto to bruteforce the password for FDE (and a truly massive cracking array to bruteforce the password you're describing). Anyone can steal a flash drive. Just something to keep in mind. – KnightOfNi May 9 '15 at 22:29
  • Some thieves do know, but you seem willing to accept some risk so here are some more: BitLocker's disk protection doesn't matter as much if the system is stolen "powered on" or in memory sleep; then it's the lockscreen and whether your files are network-accessible. You're back to password for the former (plus a lock delay timer) and EFS for the latter. – ǝɲǝɲbρɯͽ May 9 '15 at 23:08
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Why not both? If you enable BitLocker to without a TPM (group policy editor), then tell it to use "TPMAndPINAndStartupKey", it should require both the file-based startup key and the PIN (which, if you enable "Enhanced PINs" in group policy editor, can be actual passphrases).

Also, in either case, I wouldn't worry too much about brute forcing. BitLocker has reasonably strong protections against that, using a slow key derivation function (though I don't know the details) for the "PIN". It's even worse if you're going through the normal unlock process, where the UI itself will make brute-force attempts impractical, but your "friends" would probably extract the disk for offline attacks.

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The decision here is between something you know versus something you have.

Password, something you know:

  • Password cannot be stolen together with your laptop.
  • Humans are bad at choosing strong passwords.

USB key, something you have:

  • USB key will have some guaranteed cryptographic properties unlike most passwords.
  • If you keep the USB key with you, it might easily be stolen along with your laptop.

So for the two adversaries you mentioned. Thieves may be able to steal your USB key along with your laptop. Friends might also be able to get access to your USB key if you do not secure it. On the other hand, thieves cannot get your password if they only steal your device and friends can't casually get it if they fit my common-sense definition of friend.

However, if you have the scenario where someone has access to your device without you suspecting them, you are vulnerable to many attack vectors:

  • Someone installs a small keylogging device that intercepts your keyboard (only applicable if you use peripheral keyboard)
  • Someone with physical access to your device resets it and boots into a memory dumping utility. The memory dump can then be analyzed to retrieve the Bitlocker key (Elcomsoft Forensic Disk Decryptor can do this). This can be avoided with a TPM.
  • Someone shoulder surfs you and sees you typing in your password.
  • If you have a FireWire port, it's possible to dump the active memory by exploiting DMA. Dump gives them the key. I think it's possible to prevent this through Group Policy, disallowing FireWire devices from activating if the device is locked.

Other than that, it's worth remembering that if malware ever gets onto your machine (e.g. friend tricks you into running something) it can easily steal the encryption key (and do anything else).

In my opinion, a TPM is an important part of setting up BitLocker in a secure way.

Some additional information:

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It's worth consideration that the flash memory on a USB flash device can be attacked and read from when stolen. Many forensic acquisition tools include memory dumping, so if the USB flash memory drive is stolen along with the laptop, you're aware of the compromise but unable to do anything if the adversary achieves this. Adding a second USB device in addition to TPM and a secure password may provide a more secure option than a single USB device alone because the thief would need both to access your computer.

There are still attack vectors that can be used to attack your machine while the data is unencrypted, and they can simply scrape the password from the RAM of your computer. These include DMA memory access; keyboard loggers and USB sniffing malware; the frozen RAM attack; TEMPEST, van Eck, or other side channel attacks; a live network attack that compromises your computer and reads your data while you're legitimately using it; and there is always shoulder surfing and dongle theft. Being aware of shoulder surfing and people around you, turning off your computer completely when you leave it, running anti-virus and anti-malware software, and always securing the USB keys should become standard practices if you believe your friends are capable of these attacks.

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