mirror of
https://github.com/moparisthebest/mailiverse
synced 2024-11-30 04:02:14 -05:00
77 lines
3.2 KiB
Plaintext
77 lines
3.2 KiB
Plaintext
|
|
.. _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;
|
|
|
|
SecureVector<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.
|
|
|