crypt.xml

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<refentry xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://docbook.org/ns/docbook" xml:id="function.crypt">
 <refnamediv>
  <refname>crypt</refname>
  <refpurpose>One-way string hashing</refpurpose>
 </refnamediv>
 
 <refsynopsisdiv>
  &note.not-bin-safe;
 </refsynopsisdiv>
 
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>string</type><methodname>crypt</methodname>
   <methodparam><modifier role="attribute">#[\SensitiveParameter]</modifier><type>string</type><parameter>string</parameter></methodparam>
   <methodparam><type>string</type><parameter>salt</parameter></methodparam>
  </methodsynopsis>
  <para>
   <function>crypt</function> will return a hashed string using the
   standard Unix <abbrev>DES</abbrev>-based algorithm or
   alternative algorithms. <function>password_verify</function> is
   compatible with <function>crypt</function>. Therefore, password
   hashes created by <function>crypt</function> can be used with
   <function>password_verify</function>.
  </para>
  <para>
   Prior to PHP 8.0.0, the <parameter>salt</parameter> parameter was optional. However, <function>crypt</function> creates a weak hash without the <parameter>salt</parameter>, and raises an <constant>E_NOTICE</constant> error without it. Make sure to specify a strong enough salt for better security. 
  </para>
  <para>
   <function>password_hash</function> uses a strong hash, generates a strong salt, and applies proper rounds automatically. <function>password_hash</function> is a simple <function>crypt</function> wrapper and compatible with existing password hashes. Use of <function>password_hash</function> is encouraged.
  </para>
  <para>
   The hash type is triggered by the salt argument.
   If no salt is provided, PHP will
   auto-generate either a standard two character (DES) salt, or a twelve
   character (MD5), depending on the availability of MD5 crypt().  PHP sets a
   constant named <constant>CRYPT_SALT_LENGTH</constant> which indicates the
   longest valid salt allowed by the available hashes.
  </para>
  <para>
   The standard DES-based <function>crypt</function> returns the
   salt as the first two characters of the output. It also only uses the
   first eight characters of <parameter>string</parameter>, so longer strings
   that start with the same eight characters will generate the same result
   (when the same salt is used).
  </para>
  <simpara>
   The following hash types are supported:
  </simpara>
  <itemizedlist>
   <listitem>
    <simpara>
     <constant>CRYPT_STD_DES</constant> - Standard DES-based hash with a two character salt
       from the alphabet "./0-9A-Za-z". Using invalid characters in the salt will cause
       crypt() to fail.
    </simpara>
   </listitem>
   <listitem>
    <simpara>
     <constant>CRYPT_EXT_DES</constant> - Extended DES-based hash. The "salt" is a
     9-character string consisting of an underscore followed by 4 characters of iteration count
     and 4 characters of salt. Each of these 4-character strings encode 24 bits, least significant
     character first. The values <literal>0</literal> to <literal>63</literal> are encoded as
     <literal>./0-9A-Za-z</literal>. Using invalid characters in the salt will cause crypt() to fail.
    </simpara>
   </listitem>
   <listitem>
    <simpara>
     <constant>CRYPT_MD5</constant> - MD5 hashing with a twelve character salt starting with
     $1$
    </simpara>
   </listitem>
   <listitem>
    <simpara>
     <constant>CRYPT_BLOWFISH</constant> - Blowfish hashing with a salt as
     follows: "$2a$", "$2x$" or "$2y$", a two digit cost parameter, "$", and
     22 characters from the alphabet "./0-9A-Za-z". Using characters outside of
     this range in the salt will cause crypt() to return a zero-length string.
     The two digit cost parameter is the base-2 logarithm of the iteration
     count for the underlying Blowfish-based hashing algorithm and must be
     in range 04-31, values outside this range will cause crypt() to fail.
     "$2x$" hashes are potentially weak; "$2a$" hashes are compatible and
     mitigate this weakness. For new hashes, "$2y$" should be used.
    </simpara>
   </listitem>
   <listitem>
    <simpara>
     <constant>CRYPT_SHA256</constant> - SHA-256 hash with a sixteen character salt
     prefixed with $5$. If the salt string starts with 'rounds=&lt;N&gt;$', the numeric value of N
     is used to indicate how many times the hashing loop should be executed, much like the cost
     parameter on Blowfish. The default number of rounds is 5000, there is a minimum of
     1000 and a maximum of 999,999,999. Any selection of N outside this range will be truncated to
     the nearest limit.
    </simpara>
   </listitem>
   <listitem>
    <simpara>
     <constant>CRYPT_SHA512</constant> - SHA-512 hash with a sixteen character salt
     prefixed with $6$. If the salt string starts with 'rounds=&lt;N&gt;$', the numeric value of N
     is used to indicate how many times the hashing loop should be executed, much like the cost
     parameter on Blowfish. The default number of rounds is 5000, there is a minimum of
     1000 and a maximum of 999,999,999. Any selection of N outside this range will be truncated to
     the nearest limit.
    </simpara>
   </listitem>
  </itemizedlist>
 </refsect1>
 
 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>string</parameter></term>
     <listitem>
      <para>
       The string to be hashed.
      </para>
      <caution>
       <para>
        Using the <constant>CRYPT_BLOWFISH</constant> algorithm, will result
        in the <parameter>string</parameter> parameter being truncated to a
        maximum length of 72 bytes.
       </para>
      </caution>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>salt</parameter></term>
     <listitem>
      <para>
       A salt string to base the hashing on. If not provided, the
       behaviour is defined by the algorithm implementation and can lead to
       unexpected results.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>
 
 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   Returns the hashed string or a string that is shorter than 13 characters
   and is guaranteed to differ from the salt on failure.
  </para>
  <warning>
   <simpara>
    When validating passwords, a string comparison function that isn't
    vulnerable to timing attacks should be used to compare the output of
    <function>crypt</function> to the previously known hash. PHP
    provides <function>hash_equals</function> for this purpose.
   </simpara>
  </warning>
 </refsect1>
 
 <refsect1 role="changelog">
  &reftitle.changelog;
  <informaltable>
   <tgroup cols="2">
    <thead>
     <row>
      <entry>&Version;</entry>
      <entry>&Description;</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry>8.0.0</entry>
      <entry>
       The <parameter>salt</parameter> is no longer optional.
      </entry>
     </row>
    </tbody>
   </tgroup>
  </informaltable>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title><function>crypt</function> examples</title>
    <programlisting role="php">
<![CDATA[
<?php
$user_input = 'rasmuslerdorf';
$hashed_password = '$6$rounds=1000000$NJy4rIPjpOaU$0ACEYGg/aKCY3v8O8AfyiO7CTfZQ8/W231Qfh2tRLmfdvFD6XfHk12u6hMr9cYIA4hnpjLNSTRtUwYr9km9Ij/';

// Validate an existing crypt() hash in a way that is compatible with non-PHP software.
if (hash_equals($hashed_password, crypt($user_input, $hashed_password))) {
   echo "Password verified!";
}
?>
]]>
    </programlisting>
   </example>
  </para>
 </refsect1>
 
 <refsect1 role="notes">
  &reftitle.notes;
  <note>
   <simpara>
    There is no decrypt function, since <function>crypt</function> uses a
    one-way algorithm.
   </simpara>
  </note>
 </refsect1>
 
 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>hash_equals</function></member>
    <member><function>password_hash</function></member>
    <member>The Unix man page for your crypt function for more information</member>
   </simplelist>
  </para>
 </refsect1>
 
</refentry>
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