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<h2 class="chapter">8 Key Derivation</h2>
<p><acronym>Libgcypt</acronym> provides a general purpose function to derive keys
from strings.
<div class="defun">
— Function: gpg_error_t <b>gcry_kdf_derive</b> (<var> const void *passphrase<!-- /@w -->, size_t passphraselen<!-- /@w -->, int algo<!-- /@w -->, int subalgo<!-- /@w -->, const void *salt<!-- /@w -->, size_t saltlen<!-- /@w -->, unsigned long iterations<!-- /@w -->, size_t keysize<!-- /@w -->, void *keybuffer<!-- /@w --> </var>)<var><a name="index-gcry_005fkdf_005fderive-204"></a></var><br>
<blockquote>
<p>Derive a key from a passphrase. <var>keysize</var> gives the requested
size of the keys in octets. <var>keybuffer</var> is a caller provided
buffer filled on success with the derived key. The input passphrase
is taken from <var>passphrase</var> which is an arbitrary memory buffer of
<var>passphraselen</var> octets. <var>algo</var> specifies the KDF algorithm to
use; see below. <var>subalgo</var> specifies an algorithm used internally
by the KDF algorithms; this is usually a hash algorithm but certain
KDF algorithms may use it differently. <var>salt</var> is a salt of length
<var>saltlen</var> octets, as needed by most KDF algorithms.
<var>iterations</var> is a positive integer parameter to most KDFs.
<p class="noindent">On success 0 is returned; on failure an error code.
<p class="noindent">Currently supported KDFs (parameter <var>algo</var>):
<dl>
<dt><code>GCRY_KDF_SIMPLE_S2K</code><dd>The OpenPGP simple S2K algorithm (cf. RFC4880). Its use is strongly
deprecated. <var>salt</var> and <var>iterations</var> are not needed and may be
passed as <code>NULL</code>/<code>0</code>.
<br><dt><code>GCRY_KDF_SALTED_S2K</code><dd>The OpenPGP salted S2K algorithm (cf. RFC4880). Usually not used.
<var>iterations</var> is not needed and may be passed as <code>0</code>. <var>saltlen</var>
must be given as 8.
<br><dt><code>GCRY_KDF_ITERSALTED_S2K</code><dd>The OpenPGP iterated+salted S2K algorithm (cf. RFC4880). This is the
default for most OpenPGP applications. <var>saltlen</var> must be given as
8. Note that OpenPGP defines a special encoding of the
<var>iterations</var>; however this function takes the plain decoded
iteration count.
<br><dt><code>GCRY_KDF_PBKDF2</code><dd>The PKCS#5 Passphrase Based Key Derivation Function number 2.
</dl>
</p></blockquote></div>
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