<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!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>53.3. 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="53.2. Message Flow" /><link rel="next" href="protocol-replication.html" title="53.4. Streaming Replication Protocol" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">53.3. SASL Authentication</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="protocol-flow.html" title="53.2. Message Flow">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="protocol.html" title="Chapter 53. Frontend/Backend Protocol">Up</a></td><th width="60%" align="center">Chapter 53. Frontend/Backend Protocol</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="protocol-replication.html" title="53.4. Streaming Replication Protocol">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="SASL-AUTHENTICATION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">53.3. SASL Authentication</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="sasl-authentication.html#SASL-SCRAM-SHA-256">53.3.1. 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-1.10.5.8.3"><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 an 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">53.3.1. SCRAM-SHA-256 authentication</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 RFC 7677 and RFC 5802. </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-1.10.5.8.5.8"><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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="protocol.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="protocol-replication.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">53.2. 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