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DeviceEncryption

Limitations

  • On Replicant 6.0, the encryption scheme is specific to Android. Because of that if the device is broken, even if you have the passphrase, it's more complicated to recover the data.
  • Replicant doesn't have protections against Evil maid attacks

Full encryption

While that Android feature is called "Device encryption", it doesn't encrypt everything.

For instance, on a Galaxy SIII, enabling "Device encryption" only encrypts the USERDATA partition.

As the encrypted partitions have to be opened, and that the user need to type a password, code has to run to prompt user for the password and open the encrypted partition. That code cannot come from within the encrypted partition.

This is why "full disk encryption" or "device encryption" schemes often have parts that are unencrypted.

Setting a device encryption password separate from the lockscreen password

By default on Android, the encryption password is the same as the lockscreen password. As users tend to use a simple PIN, password or pattern for the lockscreen, the encryption can be easily circumvented with a brute-force attack.

Replicant allows to set an encryption password that is not tied to the lockscreen:
  1. Encrypt your device (In the settings: Security -> Encrypt phone)
  2. After the phone has rebooted and the encryption is set up, select Change encryption password in the Security menu of the settings
  3. Choose a strong passphrase. You will only have to enter this passphrase once when the device boots. There is a section below that elaborates more on how to choose a strong passphrase.
  4. Reboot the device and verify that the encryption works properly by entering the previously chosen passphrase

If a separate encryption password is in place and a PIN or password is set for the lockscreen, another security measure is active: After five unsuccessful attempts to unlock the screen, the device is rebooted and the attacker is faced with the much stronger encryption passphrase. This makes brute-force attacks on the lockscreen much harder.

Choosing a strong passphrase

As Android uses cryptsetup, most or all the Cryptsetup FAQ also apply to Replicant as well.

That FAQ has a Security Aspects section where it details the cost of breaking a passphrase in a table like this one:

Passphrase entropy Cost to break
50 bit EUR/USD 600k
55 bit EUR/USD 20M
70 bit EUR/USD 600B
75 bit EUR/USD 20T

Be sure to look at the FAQ for potentially more up to date figures and the details that goes with them.

As for calculating the passphrase entropy, tools like keepassxc (which is available in Parabola) have a password generator that is able to calculate the entropy. At the time of writing, in keepassxc, this can be found in Tools->Password generator.

Real example of a bad password

For instance if we use Replicant as a password is a very bad idea for several reasons:
  • Casual attackers knowing that the device is running Replicant and can simply try various variations on Replicant by hand. They'll most probably find it.
  • Attackers can easily copy the encrypted partition and try entries from dictionaries, it will probably find it very fast too.
  • Slightly more sophisticated attackers probably have optimized dictionaries that try variations on common words with accurate statistical models. It will probably find it very fast too.
  • Even with brute-force it's way too easy to find: It's too short and not enough random. Using Keepasxc to calculate the entropy gives us 15.32 bits of entropy. That's about 20452 tries in average (1/2 * 2^(bits of entropy in passphrase, according to the cryptsetup FAQ). So even with a slow computer that only does 1 try per second, we only spend 5h40 to find it.

Other tips

  • Do not set the default keyboard (LatinIME) as a non-system app if you use encryption: it will prevent you from entering the password to open the encrypted storage.

Updated by Denis 'GNUtoo' Carikli 2 months ago · 2 revisions