
.. _pbkdf:

PBKDF Algorithms
========================================

There are various procedures for turning a passphrase into a arbitrary
length key for use with a symmetric cipher. A general interface for
such algorithms is presented in ``pbkdf.h``. The main function is
``derive_key``, which takes a passphrase, a salt, an iteration count,
and the desired length of the output key, and returns a key of that
length, deterministically produced from the passphrase and salt. If an
algorithm can't produce a key of that size, it will throw an exception
(most notably, PKCS #5's PBKDF1 can only produce strings between 1 and
$n$ bytes, where $n$ is the output size of the underlying hash
function).

The purpose of the iteration count is to make the algorithm take
longer to compute the final key (reducing the speed of brute-force
attacks of various kinds). Most standards recommend an iteration count
of at least 10000. Currently defined PBKDF algorithms are
"PBKDF1(digest)", "PBKDF2(digest)", and "OpenPGP-S2K(digest)"; you can
retrieve any of these using the ``get_pbkdf``, found in
``lookup.h``. As of this writing, "PBKDF2(SHA-256)" with 10000
iterations and a 16 byte salt is recommend for new applications.

.. cpp:function:: OctetString PBKDF::derive_key( \
   size_t output_len, const std::string& passphrase, \
   const byte* 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.

::

   PBKDF* pbkdf = get_pbkdf("PBKDF2(SHA-256)");
   AutoSeeded_RNG rng;

   secure_vector<byte> salt = rng.random_vec(16);
   OctetString aes256_key = pbkdf->derive_key(32, "password",
                                              &salt[0], salt.size(),
                                              10000);


OpenPGP S2K
----------------------------------------

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.

