
.. _pbkdf:

Password Based Key Derivation
========================================

Often one needs to convert a human readable password into a cryptographic
key. It is useful to slow down the computation of these computations in order to
reduce the speed of brute force search, thus they are parameterized in some
way which allows their required computation to be tuned.

PBKDF
---------

:cpp:class:`PBKDF` is the older API for this functionality, presented in header
``pbkdf.h``. It does not support Scrypt, nor will it be able to support other
future hashes (such as Argon2) that may be added in the future. In addition,
this API requires the passphrase be entered as a ``std::string``, which means
the secret will be stored in memory that will not be zeroed.

.. cpp:class:: PBKDF

   .. cpp:function:: void pbkdf_iterations(uint8_t out[], size_t out_len, \
                            const std::string& passphrase, \
                            const uint8_t salt[], size_t salt_len, \
                            size_t iterations) const

      Run the PBKDF algorithm for the specified number of iterations,
      with the given salt, and write output to the buffer.

   .. cpp:function:: void pbkdf_timed(uint8_t out[], size_t out_len, \
                         const std::string& passphrase, \
                         const uint8_t salt[], size_t salt_len, \
                         std::chrono::milliseconds msec, \
                         size_t& iterations) const

      Choose (via short run-time benchmark) how many iterations to perform
      in order to run for roughly msec milliseconds. Writes the number
      of iterations used to reference argument.

   .. cpp:function:: OctetString derive_key( \
               size_t output_len, const std::string& passphrase, \
               const uint8_t* salt, size_t salt_len, \
               size_t iterations) const

   Computes a key from *passphrase* and the *salt* (of length
   *salt_len* bytes) using an algorithm-specific interpretation of
   *iterations*, producing a key of length *output_len*.

   Use an iteration count of at least 10000. The salt should be
   randomly chosen by a good random number generator (see
   :ref:`random_number_generators` for how), or at the very least
   unique to this usage of the passphrase.

   If you call this function again with the same parameters, you will
   get the same key.

PasswordHash
--------------

.. versionadded:: 2.8.0

This API has two classes, one representing the algorithm (such as
"PBKDF2(SHA-256)", or "Scrypt") and the other representing a specific instance
of the problem which is fully specified (say "Scrypt" with N=8192,r=64,p=8).

.. cpp:class:: PasswordHash

   .. cpp:function:: void derive_key(uint8_t out[], size_t out_len, \
                     const char* password, const size_t password_len, \
                     const uint8_t salt[], size_t salt_len) const

      Derive a key, placing it into output

   .. cpp:function:: std::string to_string() const

      Return a descriptive string including the parameters (iteration count, etc)

The ``PasswordHashFamily`` creates specific instances of ``PasswordHash``:

.. cpp:class:: PasswordHashFamily

   .. cpp:function:: static std::unique_ptr<PasswordHashFamily> create(const std::string& what)

      For example "PBKDF2(SHA-256)", "Scrypt", "OpenPGP-S2K(SHA-384)". Returns
      null if not available.

   .. cpp:function:: std::unique_ptr<PasswordHash> default_params() const

      Create a default instance of the password hashing algorithm. Be warned the
      value returned here may change from release to release.

   .. cpp:function:: std::unique_ptr<PasswordHash> tune(size_t output_len, std::chrono::milliseconds msec) const

      Return a password hash instance tuned to run for approximately ``msec``
      milliseconds when producing an output of length ``output_len``. (Accuracy
      may vary, use the command line utility ``botan pbkdf_tune`` to check.)

   .. cpp:function:: std::unique_ptr<PasswordHash> from_configuration( \
         size_t i1, size_t i2 = 0, size_t i3 = 0, size_t i4 = 0, const char* cfg_str = nullptr) const

      Return a new password hash instance based on some number of integer and
      string parameters. Any values not used by a particular scheme should be
      set to zero/null.

Available Schemes
----------------------

PBKDF2
^^^^^^^^^^^^

PBKDF2 is the "standard" password derivation scheme, widely implemented in many
different libraries. It uses HMAC internally.

Scrypt
^^^^^^^^^^

Scrypt is a relatively newer design which is "memory hard" - in
addition to requiring large amounts of CPU power it uses a large block
of memory to compute the hash. This makes brute force attacks using
ASICs substantially more expensive.

Scrypt is not supported through :cpp:class:`PBKDF`, only :cpp:class:`PasswordHash`,
starting in 2.8.0. In addition, starting in version 2.7.0, scrypt is available
with this function:

.. cpp:function:: void scrypt(uint8_t output[], size_t output_len, \
                              const std::string& password, \
                              const uint8_t salt[], size_t salt_len, \
                              size_t N, size_t r, size_t p)

   Computes the Scrypt using the password and salt, and produces an output
   of arbitrary length.

   The N, r, p parameters control how much work and memory Scrypt
   uses.  N is the primary control of the workfactor, and must be a
   power of 2. For interactive logins use 32768, for protection of
   secret keys or backups use 1048576.

   The r parameter controls how 'wide' the internal hashing operation
   is. It also increases the amount of memory that is used. Values
   from 1 to 8 are reasonable.

   Setting p parameter to greater than one splits up the work in a way
   that up to p processors can work in parallel.

   As a general recommendation, use N=32768, r=8, p=1

OpenPGP S2K
^^^^^^^^^^^^

.. warning::

   The OpenPGP algorithm is weak and strange, and should be avoided unless
   implementing OpenPGP.

There are some oddities about OpenPGP's S2K algorithms that are documented
here. For one thing, it uses the iteration count in a strange manner; instead of
specifying how many times to iterate the hash, it tells how many *bytes* should
be hashed in total (including the salt). So the exact iteration count will
depend on the size of the salt (which is fixed at 8 bytes by the OpenPGP
standard, though the implementation will allow any salt size) and the size of
the passphrase.

To get what OpenPGP calls "Simple S2K", set iterations to 0, and do not specify
a salt. To get "Salted S2K", again leave the iteration count at 0, but give an
8-byte salt. "Salted and Iterated S2K" requires an 8-byte salt and some
iteration count (this should be significantly larger than the size of the
longest passphrase that might reasonably be used; somewhere from 1024 to 65536
would probably be about right). Using both a reasonably sized salt and a large
iteration count is highly recommended to prevent password guessing attempts.

PBKDF1
^^^^^^^^^^^^

PBKDF1 is an old scheme that can only produce an output length at most
as long as the hash function. It is deprecated and will be removed in
a future release. It is not supported through :cpp:class:`PasswordHash`.
