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<h1><a href="../index.html">MIT Kerberos Documentation</a></h1>
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<div class="section" id="encryption-types">
<span id="enctypes"></span><h1>Encryption types<a class="headerlink" href="#encryption-types" title="Permalink to this headline">¶</a></h1>
<p>Kerberos can use a variety of cipher algorithms to protect data. A
Kerberos <strong>encryption type</strong> (also known as an <strong>enctype</strong>) is a
specific combination of a cipher algorithm with an integrity algorithm
to provide both confidentiality and integrity to data.</p>
<div class="section" id="enctypes-in-requests">
<h2>Enctypes in requests<a class="headerlink" href="#enctypes-in-requests" title="Permalink to this headline">¶</a></h2>
<p>Clients make two types of requests (KDC-REQ) to the KDC: AS-REQs and
TGS-REQs. The client uses the AS-REQ to obtain initial tickets
(typically a Ticket-Granting Ticket (TGT)), and uses the TGS-REQ to
obtain service tickets.</p>
<p>The KDC uses three different keys when issuing a ticket to a client:</p>
<ul class="simple">
<li>The long-term key of the service: the KDC uses this to encrypt the
actual service ticket. The KDC only uses the first long-term key in
the most recent kvno for this purpose.</li>
<li>The session key: the KDC randomly chooses this key and places one
copy inside the ticket and the other copy inside the encrypted part
of the reply.</li>
<li>The reply-encrypting key: the KDC uses this to encrypt the reply it
sends to the client. For AS replies, this is a long-term key of the
client principal. For TGS replies, this is either the session key of the
authenticating ticket, or a subsession key.</li>
</ul>
<p>Each of these keys is of a specific enctype.</p>
<p>Each request type allows the client to submit a list of enctypes that
it is willing to accept. For the AS-REQ, this list affects both the
session key selection and the reply-encrypting key selection. For the
TGS-REQ, this list only affects the session key selection.</p>
</div>
<div class="section" id="session-key-selection">
<span id="id1"></span><h2>Session key selection<a class="headerlink" href="#session-key-selection" title="Permalink to this headline">¶</a></h2>
<p>The KDC chooses the session key enctype by taking the intersection of
its <strong>permitted_enctypes</strong> list, the list of long-term keys for the
most recent kvno of the service, and the client’s requested list of
enctypes. If <strong>allow_weak_crypto</strong> is true, all services are assumed
to support des-cbc-crc.</p>
<p>Starting in krb5-1.11, <strong>des_crc_session_supported</strong> in
<a class="reference internal" href="conf_files/kdc_conf.html#kdc-conf-5"><em>kdc.conf</em></a> allows additional control over whether the KDC
issues des-cbc-crc session keys.</p>
<p>Also starting in krb5-1.11, it is possible to set a string attribute
on a service principal to control what session key enctypes the KDC
may issue for service tickets for that principal. See
<a class="reference internal" href="admin_commands/kadmin_local.html#set-string"><em>set_string</em></a> in <a class="reference internal" href="admin_commands/kadmin_local.html#kadmin-1"><em>kadmin</em></a> for details.</p>
</div>
<div class="section" id="choosing-enctypes-for-a-service">
<h2>Choosing enctypes for a service<a class="headerlink" href="#choosing-enctypes-for-a-service" title="Permalink to this headline">¶</a></h2>
<p>Generally, a service should have a key of the strongest
enctype that both it and the KDC support. If the KDC is running a
release earlier than krb5-1.11, it is also useful to generate an
additional key for each enctype that the service can support. The KDC
will only use the first key in the list of long-term keys for encrypting
the service ticket, but the additional long-term keys indicate the
other enctypes that the service supports.</p>
<p>As noted above, starting with release krb5-1.11, there are additional
configuration settings that control session key enctype selection
independently of the set of long-term keys that the KDC has stored for
a service principal.</p>
</div>
<div class="section" id="configuration-variables">
<h2>Configuration variables<a class="headerlink" href="#configuration-variables" title="Permalink to this headline">¶</a></h2>
<p>The following <tt class="docutils literal"><span class="pre">[libdefaults]</span></tt> settings in <a class="reference internal" href="conf_files/krb5_conf.html#krb5-conf-5"><em>krb5.conf</em></a> will
affect how enctypes are chosen.</p>
<dl class="docutils">
<dt><strong>allow_weak_crypto</strong></dt>
<dd>defaults to <em>false</em> starting with krb5-1.8. When <em>false</em>, removes
single-DES enctypes (and other weak enctypes) from
<strong>permitted_enctypes</strong>, <strong>default_tkt_enctypes</strong>, and
<strong>default_tgs_enctypes</strong>. Do not set this to <em>true</em> unless the
use of weak enctypes is an acceptable risk for your environment
and the weak enctypes are required for backward compatibility.</dd>
<dt><strong>permitted_enctypes</strong></dt>
<dd>controls the set of enctypes that a service will accept as session
keys.</dd>
<dt><strong>default_tkt_enctypes</strong></dt>
<dd>controls the default set of enctypes that the Kerberos client
library requests when making an AS-REQ. Do not set this unless
required for specific backward compatibility purposes; stale
values of this setting can prevent clients from taking advantage
of new stronger enctypes when the libraries are upgraded.</dd>
<dt><strong>default_tgs_enctypes</strong></dt>
<dd>controls the default set of enctypes that the Kerberos client
library requests when making a TGS-REQ. Do not set this unless
required for specific backward compatibility purposes; stale
values of this setting can prevent clients from taking advantage
of new stronger enctypes when the libraries are upgraded.</dd>
</dl>
<p>The following per-realm setting in <a class="reference internal" href="conf_files/kdc_conf.html#kdc-conf-5"><em>kdc.conf</em></a> affects the
generation of long-term keys.</p>
<dl class="docutils">
<dt><strong>supported_enctypes</strong></dt>
<dd>controls the default set of enctype-salttype pairs that <a class="reference internal" href="admin_commands/kadmind.html#kadmind-8"><em>kadmind</em></a>
will use for generating long-term keys, either randomly or from
passwords</dd>
</dl>
</div>
<div class="section" id="enctype-compatibility">
<h2>Enctype compatibility<a class="headerlink" href="#enctype-compatibility" title="Permalink to this headline">¶</a></h2>
<p>See <a class="reference internal" href="conf_files/kdc_conf.html#encryption-types"><em>Encryption types</em></a> for additional information about enctypes.</p>
<table border="1" class="docutils">
<colgroup>
<col width="53%" />
<col width="12%" />
<col width="19%" />
<col width="16%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">enctype</th>
<th class="head">weak?</th>
<th class="head">krb5</th>
<th class="head">Windows</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>des-cbc-crc</td>
<td>weak</td>
<td>all</td>
<td>>=2000</td>
</tr>
<tr class="row-odd"><td>des-cbc-md4</td>
<td>weak</td>
<td>all</td>
<td>?</td>
</tr>
<tr class="row-even"><td>des-cbc-md5</td>
<td>weak</td>
<td>all</td>
<td>>=2000</td>
</tr>
<tr class="row-odd"><td>des3-cbc-sha1</td>
<td> </td>
<td>>=1.1</td>
<td>none</td>
</tr>
<tr class="row-even"><td>arcfour-hmac</td>
<td> </td>
<td>>=1.3</td>
<td>>=2000</td>
</tr>
<tr class="row-odd"><td>arcfour-hmac-exp</td>
<td>weak</td>
<td>>=1.3</td>
<td>>=2000</td>
</tr>
<tr class="row-even"><td>aes128-cts-hmac-sha1-96</td>
<td> </td>
<td>>=1.3</td>
<td>>=Vista</td>
</tr>
<tr class="row-odd"><td>aes256-cts-hmac-sha1-96</td>
<td> </td>
<td>>=1.3</td>
<td>>=Vista</td>
</tr>
<tr class="row-even"><td>camellia128-cts-cmac</td>
<td> </td>
<td>>=1.9</td>
<td>none</td>
</tr>
<tr class="row-odd"><td>camellia256-cts-cmac</td>
<td> </td>
<td>>=1.9</td>
<td>none</td>
</tr>
</tbody>
</table>
<p>krb5 releases 1.8 and later disable the single-DES enctypes by
default. Microsoft Windows releases Windows 7 and later disable
single-DES enctypes by default.</p>
</div>
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<h2>On this page</h2>
<ul>
<li><a class="reference internal" href="#">Encryption types</a><ul>
<li><a class="reference internal" href="#enctypes-in-requests">Enctypes in requests</a></li>
<li><a class="reference internal" href="#session-key-selection">Session key selection</a></li>
<li><a class="reference internal" href="#choosing-enctypes-for-a-service">Choosing enctypes for a service</a></li>
<li><a class="reference internal" href="#configuration-variables">Configuration variables</a></li>
<li><a class="reference internal" href="#enctype-compatibility">Enctype compatibility</a></li>
</ul>
</li>
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