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<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<refentry xml:id="function.sodium-crypto-pwhash-scryptsalsa208sha256" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
<refnamediv>
<refname>sodium_crypto_pwhash_scryptsalsa208sha256</refname>
<refpurpose>Derives a key from a password, using scrypt</refpurpose>
</refnamediv>
<refsect1 role="description">
&reftitle.description;
<methodsynopsis>
<type>string</type><methodname>sodium_crypto_pwhash_scryptsalsa208sha256</methodname>
<methodparam><type>int</type><parameter>length</parameter></methodparam>
<methodparam><modifier role="attribute">#[\SensitiveParameter]</modifier><type>string</type><parameter>password</parameter></methodparam>
<methodparam><type>string</type><parameter>salt</parameter></methodparam>
<methodparam><type>int</type><parameter>opslimit</parameter></methodparam>
<methodparam><type>int</type><parameter>memlimit</parameter></methodparam>
</methodsynopsis>
<para>
This is the scrypt counterpart to <function>sodium_crypto_pwhash</function>.
</para>
<para>
A common reason to use this particular function is to derive the seeds for cryptographic keys from a password and salt,
and then use these seeds to generate the actual keys needed for some purpose (e.g. <function>sodium_crypto_sign_detached</function>).
</para>
</refsect1>
<refsect1 role="parameters">
&reftitle.parameters;
<variablelist>
<varlistentry>
<term><parameter>length</parameter></term>
<listitem>
<para>
The length of the password hash to generate, in bytes.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>password</parameter></term>
<listitem>
<para>
The password to generate a hash for.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>salt</parameter></term>
<listitem>
<para>
A salt to add to the password before hashing. The salt should be unpredictable, ideally generated from a good random number source such as <function>random_bytes</function>, and have a length of at least <constant>SODIUM_CRYPTO_PWHASH_SCRYPTSALSA208SHA256_SALTBYTES</constant> bytes.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>opslimit</parameter></term>
<listitem>
<para>
Represents a maximum amount of computations to perform. Raising this number will make the function require more CPU cycles to compute a key. There are some constants available to set the operations limit to appropriate values depending on intended use, in order of strength: <constant>SODIUM_CRYPTO_PWHASH_SCRYPTSALSA208SHA256_OPSLIMIT_INTERACTIVE</constant> and <constant>SODIUM_CRYPTO_PWHASH_SCRYPTSALSA208SHA256_OPSLIMIT_SENSITIVE</constant>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>memlimit</parameter></term>
<listitem>
<para>
The maximum amount of RAM that the function will use, in bytes. There are constants to help you choose an appropriate value, in order of size: <constant>SODIUM_CRYPTO_PWHASH_SCRYPTSALSA208SHA256_MEMLIMIT_INTERACTIVE</constant> and <constant>SODIUM_CRYPTO_PWHASH_SCRYPTSALSA208SHA256_MEMLIMIT_SENSITIVE</constant>. Typically these should be paired with the matching <parameter>opslimit</parameter> values.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 role="returnvalues">
&reftitle.returnvalues;
<para>
A string of bytes of the desired length.
</para>
</refsect1>
</refentry>
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