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hash-hkdf.xml
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hash-hkdf.xml
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<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<refentry xml:id="function.hash-hkdf" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
<refnamediv>
<refname>hash_hkdf</refname>
<refpurpose>Generate a HKDF key derivation of a supplied key input</refpurpose>
</refnamediv>
<refsect1 role="description">
&reftitle.description;
<methodsynopsis>
<type>string</type><methodname>hash_hkdf</methodname>
<methodparam><type>string</type><parameter>algo</parameter></methodparam>
<methodparam><modifier role="attribute">#[\SensitiveParameter]</modifier><type>string</type><parameter>key</parameter></methodparam>
<methodparam choice="opt"><type>int</type><parameter>length</parameter><initializer>0</initializer></methodparam>
<methodparam choice="opt"><type>string</type><parameter>info</parameter><initializer>""</initializer></methodparam>
<methodparam choice="opt"><type>string</type><parameter>salt</parameter><initializer>""</initializer></methodparam>
</methodsynopsis>
</refsect1>
<refsect1 role="parameters">
&reftitle.parameters;
<para>
<variablelist>
<varlistentry>
<term><parameter>algo</parameter></term>
<listitem>
<para>
Name of selected hashing algorithm (i.e. "sha256", "sha512", "haval160,4", etc..)
See <function>hash_algos</function> for a list of supported algorithms.
<note>
<para>
Non-cryptographic hash functions are not allowed.
</para>
</note>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>key</parameter></term>
<listitem>
<para>
Input keying material (raw binary). Cannot be empty.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>length</parameter></term>
<listitem>
<para>
Desired output length in bytes.
Cannot be greater than 255 times the chosen hash function size.
</para>
<para>
If <parameter>length</parameter> is <literal>0</literal>, the output length
will default to the chosen hash function size.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>info</parameter></term>
<listitem>
<para>
Application/context-specific info string.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>salt</parameter></term>
<listitem>
<para>
Salt to use during derivation.
</para>
<para>
While optional, adding random salt significantly improves the strength of HKDF.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect1>
<refsect1 role="returnvalues">
&reftitle.returnvalues;
<para>
Returns a string containing a raw binary representation of the derived key
(also known as output keying material - OKM).
</para>
</refsect1>
<refsect1 role="errors">
&reftitle.errors;
<para>
Throws a <classname>ValueError</classname> exception if <parameter>key</parameter>
is empty, <parameter>algo</parameter> is unknown/non-cryptographic,
<parameter>length</parameter> is less than <literal>0</literal> or too large
(greater than 255 times the size of the hash function).
</para>
</refsect1>
<refsect1 role="changelog">
&reftitle.changelog;
<para>
<informaltable>
<tgroup cols="2">
<thead>
<row>
<entry>&Version;</entry>
<entry>&Description;</entry>
</row>
</thead>
<tbody>
<row>
<entry>8.0.0</entry>
<entry>
Now throws a <classname>ValueError</classname> exception on error.
Previously, &false; was returned and an <constant>E_WARNING</constant>
message was emitted.
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</refsect1>
<refsect1 role="examples">
&reftitle.examples;
<para>
<example>
<title><function>hash_hkdf</function> example</title>
<programlisting role="php">
<![CDATA[
<?php
// Generate a random key, and salt to strengthen it during derivation.
$inputKey = random_bytes(32);
$salt = random_bytes(16);
// Derive a pair of separate keys, using the same input created above.
$encryptionKey = hash_hkdf('sha256', $inputKey, 32, 'aes-256-encryption', $salt);
$authenticationKey = hash_hkdf('sha256', $inputKey, 32, 'sha-256-authentication', $salt);
var_dump($encryptionKey !== $authenticationKey); // bool(true)
?>
]]>
</programlisting>
<para>
The above example produces a pair of separate keys, suitable for creation of an
encrypt-then-HMAC construct, using AES-256 and SHA-256 for encryption and
authentication respectively.
</para>
</example>
</para>
</refsect1>
<refsect1 role="seealso">
&reftitle.seealso;
<para>
<simplelist>
<member><function>hash_pbkdf2</function></member>
<member><link xlink:href="&url.rfc;5869">RFC 5869</link></member>
<member><link xlink:href="&url.git.hub;narfbg/hash_hkdf_compat">userland implementation</link></member>
</simplelist>
</para>
</refsect1>
</refentry>
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