<function>gen_salt()</function>
   are specifically designed for hashing passwords.
   <function>crypt()</function> does the hashing and <function>gen_salt()</function>
   prepares algorithm parameters for it.
  </para>

  <para>
   The algorithms in <function>crypt()</function> differ from the usual
   MD5 or SHA-1 hashing algorithms in the following respects:
  </para>

  <orderedlist>
   <listitem>
    <para>
     They are slow.  As the amount of data is so small, this is the only
     way to make brute-forcing passwords hard.
    </para>
   </listitem>
   <listitem>
    <para>
     They use a random value, called the <firstterm>salt</firstterm>, so that users
     having the same password will have different encrypted passwords.
     This is also an additional defense against reversing the algorithm.
    </para>
   </listitem>
   <listitem>
    <para>
     They include the algorithm type in the result, so passwords hashed with
     different algorithms can co-exist.
    </para>
   </listitem>
   <listitem>
    <para>
     Some of them are adaptive &mdash; that means when computers get
     faster, you can tune the algorithm to be slower, without
     introducing incompatibility with existing passwords.
    </para>
   </listitem>
  </orderedlist>

  <para>
   <xref linkend="pgcrypto-crypt-algorithms"/> lists the algorithms
   supported by the <function>crypt()</function> function.
  </para>

  <table id="pgcrypto-crypt-algorithms">
   <title>Supported Algorithms for <function>crypt()</function></title>
   <tgroup cols="6">
    <colspec colname="col1" colwidth="1.5*"/>
    <colspec colname="col2" colwidth="1.25*"/>
    <colspec colname="col3" colwidth="1*"/>
    <colspec colname="col4" colwidth="1*"/>
    <colspec colname="col5" colwidth="1*"/>
    <colspec colname="col6" colwidth="2.5*"/>
    <thead>
     <row>
      <entry>Algorithm</entry>
      <entry>Max Password Length</entry>
      <entry>Adaptive?</entry>
      <entry>Salt Bits</entry>
      <entry>Output Length</entry>
      <entry>Description</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry><literal>bf</literal></entry>
      <entry>72</entry>
      <entry>yes</entry>
      <entry>128</entry>
      <entry>60</entry>
      <entry>Blowfish-based, variant 2a</entry>
     </row>
     <row>
      <entry><literal>md5</literal></entry>
      <entry>unlimited</entry>
      <entry>no</entry>
      <entry>48</entry>
      <entry>34</entry>
      <entry>MD5-based crypt</entry>
     </row>
     <row>
      <entry><literal>xdes</literal></entry>
      <entry>8</entry>
      <entry>yes</entry>
      <entry>24</entry>
      <entry>20</entry>
      <entry>Extended DES</entry>
     </row>
     <row>
      <entry><literal>des</literal></entry>
      <entry>8</entry>
      <entry>no</entry>
      <entry>12</entry>
      <entry>13</entry>
      <entry>Original UNIX crypt</entry>
     </row>
     <row>
      <entry><literal>sha256crypt</literal></entry>
      <entry>unlimited</entry>
      <entry>yes</entry>
      <entry>up to 32</entry>
      <entry>80</entry>
      <entry>Adapted from publicly available reference implementation
       <ulink url="https://www.akkadia.org/drepper/SHA-crypt.txt">Unix crypt using SHA-256 and SHA-512
       </ulink>
      </entry>
     </row>
     <row>
      <entry><literal>sha512crypt</literal></entry>
      <entry>unlimited</entry>
      <entry>yes</entry>
      <entry>up to 32</entry>
      <entry>123</entry>
      <entry>Adapted from publicly available reference implementation
       <ulink url="https://www.akkadia.org/drepper/SHA-crypt.txt">Unix crypt using SHA-256 and SHA-512
       </ulink>
      </entry>
     </row>

    </tbody>
   </tgroup>
  </table>

  <sect3 id="pgcrypto-password-hashing-funcs-crypt">
   <title><function>crypt()</function></title>

   <indexterm>
    <primary>crypt</primary>
   </indexterm>

<synopsis>
crypt(password text, salt text) returns text
</synopsis>

   <para>
    Calculates a crypt(3)-style hash