reference/openssl/functions/openssl-dh-compute-key.xml
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<?xml version="1.0" encoding="utf-8"?>
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<!-- $Revision$ -->
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<refentry xml:id="function.openssl-dh-compute-key" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
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<refnamediv>
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<refname>openssl_dh_compute_key</refname>
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<refpurpose>Computes shared secret for public value of remote DH key and local DH key</refpurpose>
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<refpurpose>Computes shared secret for public value of remote DH public key and local DH key</refpurpose>
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</refnamediv>
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<refsect1 role="description">
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&reftitle.description;
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<methodsynopsis>
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<type>string</type><methodname>openssl_dh_compute_key</methodname>
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<methodparam><type>string</type><parameter>pub_key</parameter></methodparam>
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<methodparam><type>resource</type><parameter>dh_key</parameter></methodparam>
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<type class="union"><type>string</type><type>false</type></type><methodname>openssl_dh_compute_key</methodname>
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<methodparam><type>string</type><parameter>public_key</parameter></methodparam>
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<methodparam><type>OpenSSLAsymmetricKey</type><parameter>private_key</parameter></methodparam>
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</methodsynopsis>
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&warn.undocumented.func;
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<para>
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The shared secret returned by <function>openssl_dh_compute_key</function> is
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often used as an encryption key to secretly communicate with a remote party.
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This is known as the Diffie-Hellman key exchange.
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</para>
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<note>
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<simpara>
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ECDH is only supported as of PHP 8.1.0 <emphasis>and</emphasis> OpenSSL 3.0.0.
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</simpara>
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</note>
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</refsect1>
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<refsect1 role="parameters">
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<para>
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<variablelist>
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<varlistentry>
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<term><parameter>pub_key</parameter></term>
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<term><parameter>public_key</parameter></term>
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<listitem>
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<para>
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Public key
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DH Public key of the remote party.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>dh_key</parameter></term>
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<term><parameter>private_key</parameter></term>
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<listitem>
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<para>
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DH key
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A local DH private key, corresponding to the public key to be shared with the remote party.
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</para>
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</listitem>
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</varlistentry>
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<refsect1 role="returnvalues">
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&reftitle.returnvalues;
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<para>
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Returns computed key on success&return.falseforfailure;.
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Returns shared secret on success&return.falseforfailure;.
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</para>
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</refsect1>
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</refentry>
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<refsect1 role="changelog">
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&reftitle.changelog;
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<informaltable>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>&Version;</entry>
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<entry>&Description;</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>8.0.0</entry>
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<entry>
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<parameter>private_key</parameter> accepts an <classname>OpenSSLAsymmetricKey</classname> now;
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previously, a &resource; of type <literal>OpenSSL key</literal> was accepted.
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</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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</refsect1>
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<refsect1 role="examples">
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&reftitle.examples;
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<para>
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<example>
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<title>Compute a shared secret</title>
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<simpara>First generate a public/private DH keypair locally, and have
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the remote party do the same. We need to use the <literal>openssl</literal>
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command-line utility.</simpara>
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<programlisting role="shell">
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<![CDATA[
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# generate private/public key keypair
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openssl dhparam -out dhparam.pem 2048
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openssl genpkey -paramfile dhparam.pem -out privatekey.pem
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# extract public key only
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openssl pkey -in privatekey.pem -pubout -out publickey.pem
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]]>
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</programlisting>
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<simpara>
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Next, send your public key to the remote party. Use the <literal>openssl
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pkey</literal> command to view the public key you will be sent from
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the remote party.</simpara>
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<programlisting role="shell">
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<![CDATA[
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openssl pkey -pubin -in remotepublickey.pem -text -noout
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]]>
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</programlisting>
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&example.outputs.similar;
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<screen>
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<![CDATA[
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PKCS#3 DH Public-Key: (2048 bit)
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public-key:
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67:e5:e5:fa:e0:7b:0f:96:2c:dc:96:44:5f:50:02:
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9e:8d:c2:6c:04:68:b0:d1:1d:75:66:fc:63:f5:e3:
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[...abbreviated...]
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]]>
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</screen>
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<simpara>Use this public key as a parameter to <function>openssl_dh_compute_key</function>
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in order to compute the shared secret.</simpara>
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<programlisting role="php">
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<![CDATA[
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<?php
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$remote_public_key = '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';
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$local_priv_key = openssl_pkey_get_private('file://privatekey.pem');
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$shared_secret = openssl_dh_compute_key(hex2bin($remote_public_key), $local_priv_key);
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echo bin2hex($shared_secret)."\n";
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?>
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]]>
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</programlisting>
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</example>
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</para>
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<para>
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<example>
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<title>Generate a DH public/private keypair in php</title>
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<simpara>First, generate the DH prime number</simpara>
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<programlisting role="shell">
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<![CDATA[
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openssl dhparam -out dhparam.pem 2048
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openssl dh -in dhparam.pem -noout -text
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]]>
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</programlisting>
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&example.outputs.similar;
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<screen>
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<![CDATA[
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PKCS#3 DH Parameters: (2048 bit)
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prime:
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00:a3:25:1e:73:3f:44:b9:2b:ee:f4:9d:9f:37:6a:
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generator: 2 (0x2)
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]]>
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</screen>
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<simpara>Prime and generator values ares passed as p and g into <function>openssl_pkey_new</function></simpara>
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<programlisting role="php">
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<![CDATA[
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<?php
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$configargs = array();
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$configargs['p'] = hex2bin('00a3251e733f44b92beef49d9f376a4bfd1dbdf4afdac810775941c65f73d2882939cd1c5fc39f0f22d29c20c1e4c01803b8b6d8daad3b39a6da8efe1230e9035d22baef18d27b69f95bcb78c60c8c6bf24992c249e0457772b3553630f2401789185003fa2d547a7f344c7332b688145114be805795e6a3f651ff17474f15d60e6c4753722c2a4c21cb7df34997c9475e40337b99527e7af3522780de1b266b40bb14110bfbe6d82fcfa0062f96b91c0bb4cbd3a6629c4867f681f2c6ff45030a9d679dce27d96b485dcafbc25d849b8bcb40c7a40c8a6ef4abbab610c3b8254dcf6096f4dbe8001c58477afb5186d122d74e94317ad5da3d53dedabb648d626b');
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$configargs['g'] = hex2bin('02');
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$private_key = openssl_pkey_new(array('dh' => $configargs));
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openssl_pkey_export_to_file($private_key,'privatekey.pem',$passphrase='y0urp@s5phr@se');
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$details = openssl_pkey_get_details($private_key);
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$local_pub_key = $details['dh']['pub_key'];
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echo bin2hex($local_pub_key)."\n";//you can send your public key to the remote party
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$details = openssl_pkey_get_details(openssl_pkey_get_public("file://remotepublickey.pem"));
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$remote_public_key = $details['dh']['pub_key'];
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$shared_secret = openssl_dh_compute_key($remote_public_key, $private_key);
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echo bin2hex($shared_secret)."\n";
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?>
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]]>
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</programlisting>
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</example>
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</para>
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</refsect1>
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<refsect1 role="seealso">
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&reftitle.seealso;
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<para>
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<simplelist>
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<member><function>openssl_pkey_new</function></member>
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<member><function>openssl_pkey_get_details</function></member>
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<member><function>openssl_pkey_get_private</function></member>
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<member><function>openssl_pkey_get_public</function></member>
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</simplelist>
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</para>
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</refsect1>
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</refentry>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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