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<refentry xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="function.sodium-crypto-stream-xchacha20-xor-ic">
 <refnamediv>
  <refname>sodium_crypto_stream_xchacha20_xor_ic</refname>
  <refpurpose>Encrypts a message using a nonce and a secret key (no authentication)</refpurpose>
 </refnamediv>

 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>string</type><methodname>sodium_crypto_stream_xchacha20_xor_ic</methodname>
   <methodparam><type>string</type><parameter>message</parameter></methodparam>
   <methodparam><type>string</type><parameter>nonce</parameter></methodparam>
   <methodparam><type>int</type><parameter>counter</parameter></methodparam>
   <methodparam><type>string</type><parameter>key</parameter></methodparam>
  </methodsynopsis>
  <para>
   The function is similar to <function>sodium_crypto_stream_xchacha20_xor</function>
   but adds the ability to set the initial value of the block counter to a non-zero value.
   This permits direct access to any block without having to compute the previous ones.
  </para>

  <caution>
   <para>
    This encryption is unauthenticated, and does not prevent chosen-ciphertext attacks.
    Make sure to combine the ciphertext with a Message Authentication Code,
    for example with <function>sodium_crypto_aead_xchacha20poly1305_ietf_encrypt</function> function,
    or <function>sodium_crypto_auth</function>.
   </para>
  </caution>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <variablelist>
   <varlistentry>
    <term><parameter>message</parameter></term>
    <listitem>
     <para>
      The message to encrypt.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><parameter>nonce</parameter></term>
    <listitem>
     <para>
      24-byte nonce.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><parameter>counter</parameter></term>
    <listitem>
     <para>
      The initial value of the block counter.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><parameter>key</parameter></term>
    <listitem>
     <para>
      Key, possibly generated from <function>sodium_crypto_stream_xchacha20_keygen</function>.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   Encrypted message, &return.falseforfailure;.
  </para>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <example>
   <title><function>sodium_crypto_stream_xchacha20_xor_ic</function> example</title>
   <programlisting role="php">
<![CDATA[
<?php
$n2 = random_bytes(SODIUM_CRYPTO_STREAM_XCHACHA20_NONCEBYTES);
$left  = str_repeat("\x01", 64);
$right = str_repeat("\xfe", 64);

// All at once:
$stream7_unified = sodium_crypto_stream_xchacha20_xor($left . $right, $n2, $key);

// Piecewise, with initial counter:
$stream7_left  = sodium_crypto_stream_xchacha20_xor_ic($left, $n2, 0, $key);
$stream7_right = sodium_crypto_stream_xchacha20_xor_ic($right, $n2, 1, $key);
$stream7_concat = $stream7_left . $stream7_right;

var_dump(strlen($stream7_concat));
var_dump($stream7_unified === $stream7_concat);
?>
]]>
   </programlisting>
   &example.outputs.similar;
   <screen>
<![CDATA[
int(128)
bool(true)
]]>
   </screen>
  </example>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>sodium_crypto_stream_xchacha20_xor</function></member>
   </simplelist>
  </para>
 </refsect1>

</refentry>
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