Replace the current Yubikey PBA implementation with the previous one.

Rationale:
  * The main reason for choosing to implement the PBA in accordance
    with the Yubico documentation was to prevent a MITM-USB-attack
    successfully recovering the new LUKS key.
  * However, a MITM-USB-attacker can read user id and password when
    they were entered for PBA, which allows him to recover the new
    challenge after the PBA is complete, with which he can challenge
    the Yubikey, decrypt the new AES blob and recover the LUKS key.
  * Additionally, since the Yubikey shared secret is stored in the
    same AES blob, after such an attack not only is the LUKS device
    compromised, the Yubikey is as well, since the shared secret
    has also been recovered by the attacker.
  * Furthermore, with this method an attacker could also bruteforce
    the AES blob, if he has access to the unencrypted device, which
    would again compromise the Yubikey, should he be successful.
  * Finally, with this method, once the LUKS key has been recovered
    once, the encryption is permanently broken, while with the previous
    system, the LUKS key itself it changed at every successful boot,
    so recovering it once will not necessarily result in a permanent
    breakage and will also not compromise the Yubikey itself (since
    its secret is never stored anywhere but on the Yubikey itself).

Summary:
The current implementation opens up up vulnerability to brute-forcing
the AES blob, while retaining the current MITM-USB attack, additionally
making the consequences of this attack permanent and extending it to
the Yubikey itself.
Update to the Yubikey PBA

Security-relevant changes:
 * No (salted) passphrase hash send to the yubikey, only hash of the salt (as it was in the original implementation).
 * Derive $k_luks with PBKDF2 from the yubikey $response (as the PBKDF2 salt) and the passphrase $k_user
   (as the PBKDF2 password), so that if two-factor authentication is enabled
   (a) a USB-MITM attack on the yubikey itself is not enough to break the system
   (b) the potentially low-entropy $k_user is better protected against brute-force attacks
 * Instead of using uuidgen, gather the salt (previously random uuid / uuid_r) directly from /dev/random.
 * Length of the new salt in byte added as the parameter "saltLength", defaults to 16 byte.
   Note: Length of the challenge is 64 byte, so saltLength > 64 may have no benefit over saltLengh = 64.
 * Length of $k_luks derived with PBKDF2 in byte added as the parameter "keyLength", defaults to 64 byte.
   Example: For a luks device with a 512-bit key, keyLength should be 64.
 * Increase of the PBKDF2 iteration count per successful authentication added as the
   parameter "iterationStep", defaults to 0.

Other changes:
 * Add optional grace period before trying to find the yubikey, defaults to 2 seconds.

Full overview of the yubikey authentication process:

  (1) Read $salt and $iterations from unencrypted device (UD).
  (2) Calculate the $challenge from the $salt with a hash function.
      Chosen instantiation: SHA-512($salt).
  (3) Challenge the yubikey with the $challenge and receive the $response.
  (4) Repeat three times:
    (a) Prompt for the passphrase $k_user.
    (b) Derive the key $k_luks for the luks device with a key derivation function from $k_user and $response.
        Chosen instantiation: PBKDF2(HMAC-SHA-512, $k_user, $response, $iterations, keyLength).
    (c) Try to open the luks device with $k_luks and escape loop (4) only on success.
  (5) Proceed only if luks device was opened successfully, fail otherwise.

  (6) Gather $new_salt from a cryptographically secure pseudorandom number generator
      Chosen instantiation: /dev/random
  (7) Calculate the $new_challenge from the $new_salt with the same hash function as (2).
  (8) Challenge the yubikey with the $new_challenge and receive the $new_response.
  (9) Derive the new key $new_k_luks for the luks device in the same manner as in (4) (b),
      but with more iterations as given by iterationStep.
 (10) Try to change the luks device's key $k_luks to $new_k_luks.
 (11) If (10) was successful, write the $new_salt and the $new_iterations to the UD.
      Note: $new_iterations = $iterations + iterationStep

Known (software) attack vectors:

 * A MITM attack on the keyboard can recover $k_user. This, combined with a USB-MITM
   attack on the yubikey for the $response (1) or the $new_response (2) will result in
   (1) $k_luks being recovered,
   (2) $new_k_luks being recovered.
 * Any attacker with access to the RAM state of stage-1 at mid- or post-authentication
   can recover $k_user, $k_luks, and  $new_k_luks
 * If an attacker has recovered $response or $new_response, he can perform a brute-force
   attack on $k_user with it without the Yubikey needing to be present (using cryptsetup's
   "luksOpen --verify-passphrase" oracle. He could even make a copy of the luks device's
   luks header and run the brute-force attack without further access to the system.
 * A USB-MITM attack on the yubikey will allow an attacker to attempt to brute-force
   the yubikey's internal key ("shared secret") without it needing to be present anymore.

Credits:

 * Florian Klien,
   for the original concept and the reference implementation over at
   https://github.com/flowolf/initramfs_ykfde
 * Anthony Thysse,
   for the reference implementation of accessing OpenSSL's PBKDF2 over at
   http://www.ict.griffith.edu.au/anthony/software/pbkdf2.c
Merge branch 'yubikey' of git://github.com/Calrama/nixpkgs