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>The most common usage of TPMs is to protect disk encryption keys - Microsoft Bitlocker defaults to storing its encryption key in the TPM, automatically unlocki

>The most common usage of TPMs is to protect disk encryption keys - Microsoft Bitlocker defaults to storing its encryption key in the TPM, automatically unlocking the drive if the boot process is unmodified. I had to go look this up. Apparently there is mode of full disk encryption that automatically decrypts the disk every time you boot up if some stuff has not been modified. That strikes me as quite pointless. * https://en.wikipedia.org/wiki/BitLocker#Encryption_modes https://en.wikipedia.org/wiki/BitLocker#Encryption_modes

elehack 9y ago
If the machine (1) has soldered-on RAM (preventing cold boot attacks) and (2) the portions of the OS that run prior to user authentication are sufficiently secure, then it really doesn't seem to be a problem. Last I knew, Windows does not like to let you enable this mode in a machine with removable RAM that don't have compensating security features.
- cryptonector 9y ago
  And also no Thunderbolt/Firewire, and/or has an IOMMU and the OS uses it.
- rootsudo 9y ago
  I'm sorry, what? Windows 100% allows you to use TPM + bitlocker and secure the keys on AD on any sort of computer, regardless of removable ram or not.
xmodem 9y ago
in what sense is it pointless? If the attacker can’t log into the system, and can’t modify the boot process without invalidating the TPM, then so long as the TPM is reasonably tamper proof it’s perfectly sufficient for the 99% use case for full disk encryption - a stolen laptop
lemoncucumber 9y ago
There are basically three options for how to do disk encryption unlocking: * The way that e.g. macOS FileVault works, where you enter your password in EFI, before the machine boots and before the OS is loaded. * The way that e.g. iOS works, where there is an unencrypted partition containing the OS (mounted read-only), and an encrypted partition containing the data. The OS boots off the unencrypted partition, then you have to enter your password to unlock the data partition. * The mode you're describing, where the unlocking happens automatically if and only if nothing has been tampered with (the encrypted hard drive hasn't been swapped into another machine, etc). With this mode you're relying on the fact that when the OS boots it won't let you do anything without entering a password, even though the disk is already unlocked. Assuming there are no bugs in the login screen, no way to physically read the contents of RAM (where the encryption keys are), and no way to access the contents of the TPM, this approach is just as secure.
- TehCorwiz 9y ago
  > "Assuming there are no bugs in the login screen, [assuming] no way to physically read the contents of RAM (where the encryption keys are), and [assuming] no way to access the contents of the TPM, this approach is just as secure. That's a lot of assumptions. Or, as Jayne Cobb might say, "I'm smelling a lot of 'if' coming off this plan."
  - marcoperaza 9y ago
    You're hitting a pet peeve of mine, which is the "all or nothing" misunderstanding of security. You should evaluate security against a threat model. Those assumptions that you criticize hold true for the realistic threat faced by the vast vast majority of people: opportunistic taking of personal data by device thieves, intruders, and others with physical access to the machine. If you are worried about more sophisticated attacks, then you can easily ratchet up BitLocker's security by requiring a boot-time PIN, dongle, or password in addition to the TPM/SecureBoot measurements. You will also want to be more selective about the hardware that you use.
    - 9y ago
marcoperaza 9y ago
This isn't just one of many possible modes, it is automatically enabled on pretty much every new Windows laptop. I used to work on BitLocker so I'm biased, but it's not pointless at all. If the hardware configuration, firmware, bootloader, boot settings, etc. are unchanged, as measured by Secure Boot and then each subsequent boot component, the TPM releases the key and automatically unlocks the drive. From that point forward, security depends on OS-level authentication and authorization. Even assuming poor hardware-level security, this will still protect against the only reasonable threat faced by most people: access to private data by device thieves or physical intruders. If you are in the tiny minority that actually wants to protect yourself against sophisticated actors who have exploits for the OS-level security, or the ability to extract keys from a running system or with a cold boot attack, you will want to be selective about your hardware and you might want to additionally rely on a PIN, password, or dongle in conjunction with the TPM/SecureBoot protection.
- upofadown 9y ago
  The interesting thing about whole disk encryption is that it is not really breakable if a reasonably long password/phrase is used. Using a TPM adds a huge attack surface for no practical reason. A large enough attack surface in this case that it is almost for sure going to be broken. The user only is going to be entering the password/phrase once per boot. Things are different in the case of something like a phone. In that case you are pretty much forced to use some low entropy key that could otherwise be brute forced. Then a secure hardware device makes sense, particularly if the phone does not use the device for everything but the kitchen sink as in this case.
  - mjg59 9y ago
    There's a huge practical benefit - the user gains disk encryption without having to remember a secure passphrase. That's a big deal.
    - marcoperaza 9y ago
      Exactly and to elaborate, it means that the only secret a user needs to remember is a short PIN for OS-level authentication. The TPM's protection against modified boot and OS components make a brute force very difficult. Further, Windows will actually feed that PIN to the TPM for purposes of OS-level authentication, which leverages the TPM's enforcement of rate limits and lockouts in addition to OS-level implementations of those protections. A nice improvement would be to use a PIN entered at boot for purposes of drive decryption and then automatically pass it through to the OS-level login.
gruez 9y ago
>That strikes me as quite pointless. evil maid attacks
tedunangst 9y ago
Also: https://docs.microsoft.com/en-us/windows/device-security/bitlocker/bitlocker-countermeasures https://docs.microsoft.com/en-us/windows/device-security/bit... More also: https://technet.microsoft.com/en-us/library/dn632182(v=ws.11).aspx https://technet.microsoft.com/en-us/library/dn632182(v=ws.11...
Neil44 9y ago
It prevents someone with physical access just taking the hard drive out and mounting it on another machine to read the data.
tinus_hn 9y ago
It doesn’t decrypt the whole disk every time because that takes forever. It releases the key so you can use the disk while it’s encrypted. It is kind of useless as it’s practically impossible to keep secrets from a local attacker on the Intel platform, but against an attacker that’s not very determined it helps. At least you can’t just boot a Linux cd or put the disk in another machine to get the data.

elehack 9y ago

If the machine (1) has soldered-on RAM (preventing cold boot attacks) and (2) the portions of the OS that run prior to user authentication are sufficiently secure, then it really doesn't seem to be a problem. Last I knew, Windows does not like to let you enable this mode in a machine with removable RAM that don't have compensating security features.

cryptonector 9y ago
And also no Thunderbolt/Firewire, and/or has an IOMMU and the OS uses it.
rootsudo 9y ago
I'm sorry, what? Windows 100% allows you to use TPM + bitlocker and secure the keys on AD on any sort of computer, regardless of removable ram or not.

xmodem 9y ago

in what sense is it pointless? If the attacker can’t log into the system, and can’t modify the boot process without invalidating the TPM, then so long as the TPM is reasonably tamper proof it’s perfectly sufficient for the 99% use case for full disk encryption - a stolen laptop

lemoncucumber 9y ago

There are basically three options for how to do disk encryption unlocking: * The way that e.g. macOS FileVault works, where you enter your password in EFI, before the machine boots and before the OS is loaded. * The way that e.g. iOS works, where there is an unencrypted partition containing the OS (mounted read-only), and an encrypted partition containing the data. The OS boots off the unencrypted partition, then you have to enter your password to unlock the data partition. * The mode you're describing, where the unlocking happens automatically if and only if nothing has been tampered with (the encrypted hard drive hasn't been swapped into another machine, etc). With this mode you're relying on the fact that when the OS boots it won't let you do anything without entering a password, even though the disk is already unlocked. Assuming there are no bugs in the login screen, no way to physically read the contents of RAM (where the encryption keys are), and no way to access the contents of the TPM, this approach is just as secure.

TehCorwiz 9y ago
> "Assuming there are no bugs in the login screen, [assuming] no way to physically read the contents of RAM (where the encryption keys are), and [assuming] no way to access the contents of the TPM, this approach is just as secure. That's a lot of assumptions. Or, as Jayne Cobb might say, "I'm smelling a lot of 'if' coming off this plan."
- marcoperaza 9y ago
  You're hitting a pet peeve of mine, which is the "all or nothing" misunderstanding of security. You should evaluate security against a threat model. Those assumptions that you criticize hold true for the realistic threat faced by the vast vast majority of people: opportunistic taking of personal data by device thieves, intruders, and others with physical access to the machine. If you are worried about more sophisticated attacks, then you can easily ratchet up BitLocker's security by requiring a boot-time PIN, dongle, or password in addition to the TPM/SecureBoot measurements. You will also want to be more selective about the hardware that you use.
  - 9y ago

marcoperaza 9y ago

This isn't just one of many possible modes, it is automatically enabled on pretty much every new Windows laptop. I used to work on BitLocker so I'm biased, but it's not pointless at all. If the hardware configuration, firmware, bootloader, boot settings, etc. are unchanged, as measured by Secure Boot and then each subsequent boot component, the TPM releases the key and automatically unlocks the drive. From that point forward, security depends on OS-level authentication and authorization. Even assuming poor hardware-level security, this will still protect against the only reasonable threat faced by most people: access to private data by device thieves or physical intruders. If you are in the tiny minority that actually wants to protect yourself against sophisticated actors who have exploits for the OS-level security, or the ability to extract keys from a running system or with a cold boot attack, you will want to be selective about your hardware and you might want to additionally rely on a PIN, password, or dongle in conjunction with the TPM/SecureBoot protection.

upofadown 9y ago
The interesting thing about whole disk encryption is that it is not really breakable if a reasonably long password/phrase is used. Using a TPM adds a huge attack surface for no practical reason. A large enough attack surface in this case that it is almost for sure going to be broken. The user only is going to be entering the password/phrase once per boot. Things are different in the case of something like a phone. In that case you are pretty much forced to use some low entropy key that could otherwise be brute forced. Then a secure hardware device makes sense, particularly if the phone does not use the device for everything but the kitchen sink as in this case.
- mjg59 9y ago
  There's a huge practical benefit - the user gains disk encryption without having to remember a secure passphrase. That's a big deal.
  - marcoperaza 9y ago
    Exactly and to elaborate, it means that the only secret a user needs to remember is a short PIN for OS-level authentication. The TPM's protection against modified boot and OS components make a brute force very difficult. Further, Windows will actually feed that PIN to the TPM for purposes of OS-level authentication, which leverages the TPM's enforcement of rate limits and lockouts in addition to OS-level implementations of those protections. A nice improvement would be to use a PIN entered at boot for purposes of drive decryption and then automatically pass it through to the OS-level login.

gruez 9y ago

>That strikes me as quite pointless. evil maid attacks

tedunangst 9y ago

Also: https://docs.microsoft.com/en-us/windows/device-security/bitlocker/bitlocker-countermeasures https://docs.microsoft.com/en-us/windows/device-security/bit... More also: https://technet.microsoft.com/en-us/library/dn632182(v=ws.11).aspx https://technet.microsoft.com/en-us/library/dn632182(v=ws.11...

Neil44 9y ago

It prevents someone with physical access just taking the hard drive out and mounting it on another machine to read the data.

tinus_hn 9y ago

It doesn’t decrypt the whole disk every time because that takes forever. It releases the key so you can use the disk while it’s encrypted. It is kind of useless as it’s practically impossible to keep secrets from a local attacker on the Intel platform, but against an attacker that’s not very determined it helps. At least you can’t just boot a Linux cd or put the disk in another machine to get the data.