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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>55.3. SASL Authentication</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="protocol-flow.html" title="55.2. Message Flow" /><link rel="next" href="protocol-replication.html" title="55.4. Streaming Replication Protocol" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">55.3. SASL Authentication</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="protocol-flow.html" title="55.2. Message Flow">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="protocol.html" title="Chapter 55. Frontend/Backend Protocol">Up</a></td><th width="60%" align="center">Chapter 55. Frontend/Backend Protocol</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="protocol-replication.html" title="55.4. Streaming Replication Protocol">Next</a></td></tr></table><hr /></div><div class="sect1" id="SASL-AUTHENTICATION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">55.3. SASL Authentication <a href="#SASL-AUTHENTICATION" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="sasl-authentication.html#SASL-SCRAM-SHA-256">55.3.1. SCRAM-SHA-256 Authentication</a></span></dt></dl></div><p>
<em class="firstterm">SASL</em> is a framework for authentication in connection-oriented
protocols. At the moment, <span class="productname">PostgreSQL</span> implements two SASL
authentication mechanisms, SCRAM-SHA-256 and SCRAM-SHA-256-PLUS. More
might be added in the future. The below steps illustrate how SASL
authentication is performed in general, while the next subsection gives
more details on SCRAM-SHA-256 and SCRAM-SHA-256-PLUS.
</p><div class="procedure" id="id-1.10.6.8.3"><p class="title"><strong>SASL Authentication Message Flow</strong></p><ol class="procedure" type="1"><li class="step" id="SASL-AUTH-BEGIN"><p>
To begin a SASL authentication exchange, the server sends an
AuthenticationSASL message. It includes a list of SASL authentication
mechanisms that the server can accept, in the server's preferred order.
</p></li><li class="step" id="SASL-AUTH-INITIAL-RESPONSE"><p>
The client selects one of the supported mechanisms from the list, and sends
a SASLInitialResponse message to the server. The message includes the name
of the selected mechanism, and an optional Initial Client Response, if the
selected mechanism uses that.
</p></li><li class="step" id="SASL-AUTH-CONTINUE"><p>
One or more server-challenge and client-response message will follow. Each
server-challenge is sent in an AuthenticationSASLContinue message, followed
by a response from client in a SASLResponse message. The particulars of
the messages are mechanism specific.
</p></li><li class="step" id="SASL-AUTH-END"><p>
Finally, when the authentication exchange is completed successfully, the
server sends an AuthenticationSASLFinal message, followed
immediately by an AuthenticationOk message. The AuthenticationSASLFinal
contains additional server-to-client data, whose content is particular to the
selected authentication mechanism. If the authentication mechanism doesn't
use additional data that's sent at completion, the AuthenticationSASLFinal
message is not sent.
</p></li></ol></div><p>
On error, the server can abort the authentication at any stage, and send an
ErrorMessage.
</p><div class="sect2" id="SASL-SCRAM-SHA-256"><div class="titlepage"><div><div><h3 class="title">55.3.1. SCRAM-SHA-256 Authentication <a href="#SASL-SCRAM-SHA-256" class="id_link">#</a></h3></div></div></div><p>
The implemented SASL mechanisms at the moment
are <code class="literal">SCRAM-SHA-256</code> and its variant with channel
binding <code class="literal">SCRAM-SHA-256-PLUS</code>. They are described in
detail in <a class="ulink" href="https://datatracker.ietf.org/doc/html/rfc7677" target="_top">RFC 7677</a>
and <a class="ulink" href="https://datatracker.ietf.org/doc/html/rfc5802" target="_top">RFC 5802</a>.
</p><p>
When SCRAM-SHA-256 is used in PostgreSQL, the server will ignore the user name
that the client sends in the <code class="structname">client-first-message</code>. The user name
that was already sent in the startup message is used instead.
<span class="productname">PostgreSQL</span> supports multiple character encodings, while SCRAM
dictates UTF-8 to be used for the user name, so it might be impossible to
represent the PostgreSQL user name in UTF-8.
</p><p>
The SCRAM specification dictates that the password is also in UTF-8, and is
processed with the <em class="firstterm">SASLprep</em> algorithm.
<span class="productname">PostgreSQL</span>, however, does not require UTF-8 to be used for
the password. When a user's password is set, it is processed with SASLprep
as if it was in UTF-8, regardless of the actual encoding used. However, if
it is not a legal UTF-8 byte sequence, or it contains UTF-8 byte sequences
that are prohibited by the SASLprep algorithm, the raw password will be used
without SASLprep processing, instead of throwing an error. This allows the
password to be normalized when it is in UTF-8, but still allows a non-UTF-8
password to be used, and doesn't require the system to know which encoding
the password is in.
</p><p>
<em class="firstterm">Channel binding</em> is supported in PostgreSQL builds with
SSL support. The SASL mechanism name for SCRAM with channel binding is
<code class="literal">SCRAM-SHA-256-PLUS</code>. The channel binding type used by
PostgreSQL is <code class="literal">tls-server-end-point</code>.
</p><p>
In <acronym class="acronym">SCRAM</acronym> without channel binding, the server chooses
a random number that is transmitted to the client to be mixed with the
user-supplied password in the transmitted password hash. While this
prevents the password hash from being successfully retransmitted in
a later session, it does not prevent a fake server between the real
server and client from passing through the server's random value
and successfully authenticating.
</p><p>
<acronym class="acronym">SCRAM</acronym> with channel binding prevents such
man-in-the-middle attacks by mixing the signature of the server's
certificate into the transmitted password hash. While a fake server can
retransmit the real server's certificate, it doesn't have access to the
private key matching that certificate, and therefore cannot prove it is
the owner, causing SSL connection failure.
</p><div class="procedure" id="id-1.10.6.8.5.8"><p class="title"><strong>Example</strong></p><ol class="procedure" type="1"><li class="step" id="SCRAM-BEGIN"><p>
The server sends an AuthenticationSASL message. It includes a list of
SASL authentication mechanisms that the server can accept.
This will be <code class="literal">SCRAM-SHA-256-PLUS</code>
and <code class="literal">SCRAM-SHA-256</code> if the server is built with SSL
support, or else just the latter.
</p></li><li class="step" id="SCRAM-CLIENT-FIRST"><p>
The client responds by sending a SASLInitialResponse message, which
indicates the chosen mechanism, <code class="literal">SCRAM-SHA-256</code> or
<code class="literal">SCRAM-SHA-256-PLUS</code>. (A client is free to choose either
mechanism, but for better security it should choose the channel-binding
variant if it can support it.) In the Initial Client response field, the
message contains the SCRAM <code class="structname">client-first-message</code>.
The <code class="structname">client-first-message</code> also contains the channel
binding type chosen by the client.
</p></li><li class="step" id="SCRAM-SERVER-FIRST"><p>
Server sends an AuthenticationSASLContinue message, with a SCRAM
<code class="structname">server-first-message</code> as the content.
</p></li><li class="step" id="SCRAM-CLIENT-FINAL"><p>
Client sends a SASLResponse message, with SCRAM
<code class="structname">client-final-message</code> as the content.
</p></li><li class="step" id="SCRAM-SERVER-FINAL"><p>
Server sends an AuthenticationSASLFinal message, with the SCRAM
<code class="structname">server-final-message</code>, followed immediately by
an AuthenticationOk message.
</p></li></ol></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="protocol-flow.html" title="55.2. Message Flow">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="protocol.html" title="Chapter 55. Frontend/Backend Protocol">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="protocol-replication.html" title="55.4. Streaming Replication Protocol">Next</a></td></tr><tr><td width="40%" align="left" valign="top">55.2. Message Flow </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 55.4. Streaming Replication Protocol</td></tr></table></div></body></html>
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