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<title>GnuTLS 3.3.7: Authentication using SRP</title>
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<a name="Authentication-using-SRP"></a>
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<p>
Next: <a href="srptool-Invocation.html#srptool-Invocation" accesskey="n" rel="next">srptool Invocation</a>, Up: <a href="SRP-authentication.html#SRP-authentication" accesskey="u" rel="up">SRP authentication</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Authentication-using-SRP-1"></a>
<h4 class="subsubsection">4.3.1.1 Authentication using <acronym>SRP</acronym></h4>
<a name="index-SRP-authentication"></a>
<p><acronym>GnuTLS</acronym> supports authentication via the Secure Remote Password
or <acronym>SRP</acronym> protocol (see [<em>RFC2945,TOMSRP</em>] for a description).
The <acronym>SRP</acronym> key exchange is an extension to the
<acronym>TLS</acronym> protocol, and it provides an authenticated with a
password key exchange. The peers can be identified using a single password,
or there can be combinations where the client is authenticated using <acronym>SRP</acronym>
and the server using a certificate.
</p>
<p>The advantage of <acronym>SRP</acronym> authentication, over other proposed
secure password authentication schemes, is that <acronym>SRP</acronym> is not
susceptible to off-line dictionary attacks.
Moreover, SRP does not require the server to hold the user’s password.
This kind of protection is similar to the one used traditionally in the <acronym>UNIX</acronym>
<samp>/etc/passwd</samp> file, where the contents of this file did not cause
harm to the system security if they were revealed. The <acronym>SRP</acronym>
needs instead of the plain password something called a verifier, which
is calculated using the user’s password, and if stolen cannot be used
to impersonate the user.
</p>
<p>Typical conventions in SRP are a password file, called <samp>tpasswd</samp> that
holds the SRP verifiers (encoded passwords) and another file, <samp>tpasswd.conf</samp>,
which holds the allowed SRP parameters. The included in GnuTLS helper
follow those conventions. The srptool program, discussed in the next section
is a tool to manipulate the SRP parameters.
</p>
<p>The implementation in <acronym>GnuTLS</acronym> is based on [<em>TLSSRP</em>]. The
supported key exchange methods are shown below.
</p>
<dl compact="compact">
<dt><code>SRP:</code></dt>
<dd><p>Authentication using the <acronym>SRP</acronym> protocol.
</p>
</dd>
<dt><code>SRP_DSS:</code></dt>
<dd><p>Client authentication using the <acronym>SRP</acronym> protocol. Server is
authenticated using a certificate with DSA parameters.
</p>
</dd>
<dt><code>SRP_RSA:</code></dt>
<dd><p>Client authentication using the <acronym>SRP</acronym> protocol. Server is
authenticated using a certificate with RSA parameters.
</p>
</dd>
</dl>
<dl>
<dt><a name="index-gnutls_005fsrp_005fverifier"></a>Function: <em>int</em> <strong>gnutls_srp_verifier</strong> <em>(const char * <var>username</var>, const char * <var>password</var>, const gnutls_datum_t * <var>salt</var>, const gnutls_datum_t * <var>generator</var>, const gnutls_datum_t * <var>prime</var>, gnutls_datum_t * <var>res</var>)</em></dt>
<dd><p><var>username</var>: is the user’s name
</p>
<p><var>password</var>: is the user’s password
</p>
<p><var>salt</var>: should be some randomly generated bytes
</p>
<p><var>generator</var>: is the generator of the group
</p>
<p><var>prime</var>: is the group’s prime
</p>
<p><var>res</var>: where the verifier will be stored.
</p>
<p>This function will create an SRP verifier, as specified in
RFC2945. The <code>prime</code> and <code>generator</code> should be one of the static
parameters defined in gnutls/gnutls.h or may be generated.
</p>
<p>The verifier will be allocated with <code>gnutls_malloc</code> () and will be stored in
<code>res</code> using binary format.
</p>
<p><strong>Returns:</strong> On success, <code>GNUTLS_E_SUCCESS</code> (0) is returned, or an
error code.
</p></dd></dl>
<dl compact="compact">
<dt><code><var>int</var> <a href="Core-TLS-API.html#gnutls_005fsrp_005fbase64_005fencode_005falloc">gnutls_srp_base64_encode_alloc</a> (const gnutls_datum_t * <var>data</var>, gnutls_datum_t * <var>result</var>)</code></dt>
<dt><code><var>int</var> <a href="Core-TLS-API.html#gnutls_005fsrp_005fbase64_005fdecode_005falloc">gnutls_srp_base64_decode_alloc</a> (const gnutls_datum_t * <var>b64_data</var>, gnutls_datum_t * <var>result</var>)</code></dt>
</dl>
<hr>
<div class="header">
<p>
Next: <a href="srptool-Invocation.html#srptool-Invocation" accesskey="n" rel="next">srptool Invocation</a>, Up: <a href="SRP-authentication.html#SRP-authentication" accesskey="u" rel="up">SRP authentication</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
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