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<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<refentry xml:id="function.sodium-crypto-secretbox-open" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
<refnamediv>
<refname>sodium_crypto_secretbox_open</refname>
<refpurpose>Authenticated shared-key decryption</refpurpose>
</refnamediv>
<refsect1 role="description">
&reftitle.description;
<methodsynopsis>
<type class="union"><type>string</type><type>false</type></type><methodname>sodium_crypto_secretbox_open</methodname>
<methodparam><type>string</type><parameter>ciphertext</parameter></methodparam>
<methodparam><type>string</type><parameter>nonce</parameter></methodparam>
<methodparam><modifier role="attribute">#[\SensitiveParameter]</modifier><type>string</type><parameter>key</parameter></methodparam>
</methodsynopsis>
<para>
Decrypt an encrypted message with a symmetric (shared) key.
</para>
</refsect1>
<refsect1 role="parameters">
&reftitle.parameters;
<variablelist>
<varlistentry>
<term><parameter>ciphertext</parameter></term>
<listitem>
<para>
Must be in the format provided by <function>sodium_crypto_secretbox</function>
(ciphertext and tag, concatenated).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>nonce</parameter></term>
<listitem>
<para>
A number that must be only used once, per message. 24 bytes long.
This is a large enough bound to generate randomly (i.e. <function>random_bytes</function>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>key</parameter></term>
<listitem>
<para>
Encryption key (256-bit).
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 role="returnvalues">
&reftitle.returnvalues;
<para>
The decrypted string on success&return.falseforfailure;.
</para>
</refsect1>
<refsect1 role="errors">
&reftitle.errors;
<itemizedlist>
<listitem>
<simpara>
If <parameter>nonce</parameter> has a length of bytes different than
<link linkend="constant.sodium-crypto-secretbox-noncebytes"><constant>SODIUM_CRYPTO_SECRETBOX_NONCEBYTES</constant></link>
(24 bytes), a <classname>SodiumException</classname> will be thrown.
</simpara>
</listitem>
<listitem>
<simpara>
If <parameter>key</parameter> has a length of bytes different than
<link linkend="constant.sodium-crypto-secretbox-keybytes"><constant>SODIUM_CRYPTO_SECRETBOX_KEYBYTES</constant></link>
(32 bytes), a <classname>SodiumException</classname> will be thrown.
</simpara>
</listitem>
</itemizedlist>
</refsect1>
<refsect1 role="examples">
&reftitle.examples;
<para>
<example>
<title><function>sodium_crypto_secretbox_open</function> example</title>
<programlisting role="php">
<![CDATA[
<?php
// The $key must be kept confidential
$key = random_bytes(SODIUM_CRYPTO_SECRETBOX_KEYBYTES);
// Do not reuse $nonce with the same key
$nonce = random_bytes(SODIUM_CRYPTO_SECRETBOX_NONCEBYTES);
$ciphertext = sodium_crypto_secretbox('message to be encrypted', $nonce, $key);
// The same nonce and key are required to decrypt the $ciphertext
$plaintext = sodium_crypto_secretbox_open($ciphertext, $nonce, $key);
if ($plaintext !== false) {
echo $plaintext . PHP_EOL;
}
?>
]]>
</programlisting>
&example.outputs;
<screen>
<![CDATA[
message to be encrypted
]]>
</screen>
</example>
</para>
</refsect1>
<refsect1 role="seealso">
&reftitle.seealso;
<para>
<simplelist>
<member><function>sodium_crypto_secretbox</function></member>
<member><function>sodium_crypto_secretbox_keygen</function></member>
<member><function>random_bytes</function></member>
</simplelist>
</para>
</refsect1>
</refentry>
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