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If a device uses full disk encryption such as LUKS, BitLocker (with a pin), Veracrypt and others and boots using a complex password, what is the need for complex passwords to log into the operating system thereafter and why?

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There are a few reasons why you'd still want complex passwords, though, they're all a little situational.

  • Local access or theft of your machine while it's on (or in sleep mode) but locked. A bigger problem for laptops or workstations in open offices/public areas, especially if you don't disable normal sleep and go straight to hibernate.
  • Remote access via password (RDP, SMB, various RPC / remoting systems on Windows; SSH or VNC / Screen Share or similar on others). Obviously not an issue if remote access is disabled, public-key only, or enabled only for specific accounts that do have strong passwords.
  • Combined with a TPM for defense-in-depth if your FDE key/password is compromised; the thief still wouldn't be able to mount offline attacks without the TPM, and still wouldn't be able to mount online attacks (aside from things like freezing the RAM to extract the master key) without running into the OS anti-brute-forcing protection.
  • Defense in depth (in the case the FDE is either bypassed via an online attack or similar, or totally compromised) for data encrypted with EFS or DPAPI (including Credential Vault and saved private keys) on Windows, for Keychain on Mac (assuming your Keychain password is different from your FileVault password; by default they're both the same as your login password), and of course for SSH, GPG, and KWallet/Gnome Wallet/etc. keys on Linux (which aren't tied to your login password but should still be secure).
  • (2) isn't an immediate issue as remote access is disabled although it would be of interest if there are any other RPC threats. (3) If TPM isn't available, what online attacks would it be open to? (4) Is the assumption that if full disk encryption is bypassed (do you mean via brute force) that subsequent passwords such as DPAPI or keychains are unique? – Ryan Jan 12 at 16:44
  • Online attacks don't depend in any way on the TPM being present or not, they just become the easier option if it's present since offline attacks will just outright not work. The question inherently assumes the login password (used for protecting DPAPI and EFS keys, and used by default for Keychain keys) is different from the FDE password (although on Mac that is, by default, synchronized with the login password). Also brute force isn't the only way to bypass FDE, and some methods - like freezing the RAM of a live system and pulling its secrets - won't even yield the actual password. – CBHacking Jan 13 at 10:40
  • I am curious as to the efficacy of online attacks since I am assuming that the attacker must be either physically present and/or on the same network (if remote access is disabled). – Ryan Jan 13 at 16:52
  • If yes, how do attackers for example avoid incrimination if authorities raid their premise since typically the devices are confiscated which suggests that the machines would have been shutdown (if a desktop), put to sleep or hibernation (if a laptop on the basis there is sufficient charge for further investigation). I would have thought that full disk encryption would have be defeated first to have any immediate value. – Ryan Jan 13 at 16:52
  • A TPM complicates online attacks on the FDE a lot - it can wipe its key after some number of failed attempts, and unlike a disk it cannot easily be cloned - but if the attacker already has the FDE password (maybe they shoulder surfed it, or keylogged it but somehow didn't get the OS password, or similar) then the TPM makes the difference whether the disk still can be attacked offline or not. With TPM, the attacker would then need to try brute-forcing or bypassing the OS password. Given enough time (steal the hardware) they can have unlimited tries, but it can still be prohibitively hard. – CBHacking Jan 14 at 9:41
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Yes all passwords should have a standard that enforces a high level of entropy. If not for any other reasons than these two things:

Contributing toward the randomness of the password hash via the source password, and secondly the principle of defense in depth. It is foolish to rely on single layers of defense against compromise. In fact I would argue that surrounding a security layer with weakly configured layers in fact weakens that layer as well. Thinking of a chain with weak links here, and the cumulative strength being compromised, where the strength of the chain is determined by the links, inversely the strength of each link is determined by the chain.

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I think you are confusing two independent concepts.

Full disk encryption protects a dead system, that is a non-running system.

Once the system is running, FDE is no longer relevant. The running system has the same password risks and requirements as any other system as FDE no longer protects it.

  • I understand full disk encryption protects a system that is not yet running an operating environment. Are you referring to brute force attacks to a running system? If yes, what other forms of attack is the running system to? If a host is protected against brute force attacks e.g. IDS, does this minimize the risk? – Ryan Jan 13 at 6:47

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