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<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>SSL</title>
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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="asio.overview.ssl"></a><a class="link" href="ssl.html" title="SSL">SSL</a>
</h3></div></div></div>
<p>
Asio contains classes and class templates for basic SSL support. These classes
allow encrypted communication to be layered on top of an existing stream,
such as a TCP socket.
</p>
<p>
Before creating an encrypted stream, an application must construct an SSL
context object. This object is used to set SSL options such as verification
mode, certificate files, and so on. As an illustration, client-side initialisation
may look something like:
</p>
<pre class="programlisting">ssl::context ctx(ssl::context::sslv23);
ctx.set_verify_mode(ssl::verify_peer);
ctx.load_verify_file("ca.pem");
</pre>
<p>
To use SSL with a TCP socket, one may write:
</p>
<pre class="programlisting">ssl::stream<ip::tcp::socket> ssl_sock(my_io_context, ctx);
</pre>
<p>
To perform socket-specific operations, such as establishing an outbound connection
or accepting an incoming one, the underlying socket must first be obtained
using the <code class="computeroutput">ssl::stream</code> template's <a class="link" href="../reference/ssl__stream/lowest_layer.html" title="ssl::stream::lowest_layer"><code class="computeroutput">lowest_layer()</code></a>
member function:
</p>
<pre class="programlisting">ip::tcp::socket::lowest_layer_type& sock = ssl_sock.lowest_layer();
sock.connect(my_endpoint);
</pre>
<p>
In some use cases the underlying stream object will need to have a longer
lifetime than the SSL stream, in which case the template parameter should
be a reference to the stream type:
</p>
<pre class="programlisting">ip::tcp::socket sock(my_io_context);
ssl::stream<ip::tcp::socket&> ssl_sock(sock, ctx);
</pre>
<p>
SSL handshaking must be performed prior to transmitting or receiving data
over an encrypted connection. This is accomplished using the <code class="computeroutput">ssl::stream</code>
template's <a class="link" href="../reference/ssl__stream/handshake.html" title="ssl::stream::handshake">handshake()</a>
or <a class="link" href="../reference/ssl__stream/async_handshake.html" title="ssl::stream::async_handshake">async_handshake()</a>
member functions.
</p>
<p>
Once connected, SSL stream objects are used as synchronous or asynchronous
read and write streams. This means the objects can be used with any of the
<a class="link" href="../reference/read.html" title="read">read()</a>, <a class="link" href="../reference/async_read.html" title="async_read">async_read()</a>,
<a class="link" href="../reference/write.html" title="write">write()</a>, <a class="link" href="../reference/async_write.html" title="async_write">async_write()</a>,
<a class="link" href="../reference/read_until.html" title="read_until">read_until()</a> or <a class="link" href="../reference/async_read_until.html" title="async_read_until">async_read_until()</a>
free functions.
</p>
<h5>
<a name="asio.overview.ssl.h0"></a>
<span><a name="asio.overview.ssl.certificate_verification"></a></span><a class="link" href="ssl.html#asio.overview.ssl.certificate_verification">Certificate
Verification</a>
</h5>
<p>
Asio provides various methods for configuring the way SSL certificates are
verified:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<a class="link" href="../reference/ssl__context/set_default_verify_paths.html" title="ssl::context::set_default_verify_paths">ssl::context::set_default_verify_paths()</a>
</li>
<li class="listitem">
<a class="link" href="../reference/ssl__context/set_verify_mode.html" title="ssl::context::set_verify_mode">ssl::context::set_verify_mode()</a>
</li>
<li class="listitem">
<a class="link" href="../reference/ssl__context/set_verify_callback.html" title="ssl::context::set_verify_callback">ssl::context::set_verify_callback()</a>
</li>
<li class="listitem">
<a class="link" href="../reference/ssl__context/load_verify_file.html" title="ssl::context::load_verify_file">ssl::context::load_verify_file()</a>
</li>
<li class="listitem">
<a class="link" href="../reference/ssl__stream/set_verify_mode.html" title="ssl::stream::set_verify_mode">ssl::stream::set_verify_mode()</a>
</li>
<li class="listitem">
<a class="link" href="../reference/ssl__stream/set_verify_callback.html" title="ssl::stream::set_verify_callback">ssl::stream::set_verify_callback()</a>
</li>
</ul></div>
<p>
To simplify use cases where certificates are verified according to the rules
in RFC 6125 (identity verification in the context of Transport Layer Security),
Asio provides a reusable verification callback as a function object:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
<a class="link" href="../reference/ssl__host_name_verification.html" title="ssl::host_name_verification">ssl::host_name_verification</a>
</li></ul></div>
<p>
The following example shows verification of a remote host's certificate according
to the rules used by HTTPS:
</p>
<pre class="programlisting">using asio::ip::tcp;
namespace ssl = asio::ssl;
typedef ssl::stream<tcp::socket> ssl_socket;
// Create a context that uses the default paths for
// finding CA certificates.
ssl::context ctx(ssl::context::sslv23);
ctx.set_default_verify_paths();
// Open a socket and connect it to the remote host.
asio::io_context io_context;
ssl_socket sock(io_context, ctx);
tcp::resolver resolver(io_context);
tcp::resolver::query query("host.name", "https");
asio::connect(sock.lowest_layer(), resolver.resolve(query));
sock.lowest_layer().set_option(tcp::no_delay(true));
// Perform SSL handshake and verify the remote host's
// certificate.
sock.set_verify_mode(ssl::verify_peer);
sock.set_verify_callback(ssl::host_name_verification("host.name"));
sock.handshake(ssl_socket::client);
// ... read and write as normal ...
</pre>
<h5>
<a name="asio.overview.ssl.h1"></a>
<span><a name="asio.overview.ssl.ssl_and_threads"></a></span><a class="link" href="ssl.html#asio.overview.ssl.ssl_and_threads">SSL
and Threads</a>
</h5>
<p>
SSL stream objects perform no locking of their own. Therefore, it is essential
that all asynchronous SSL operations are performed in an implicit or explicit
<a class="link" href="core/strands.html" title="Strands: Use Threads Without Explicit Locking">strand</a>. Note that this
means that no synchronisation is required (and so no locking overhead is
incurred) in single threaded programs.
</p>
<h5>
<a name="asio.overview.ssl.h2"></a>
<span><a name="asio.overview.ssl.see_also"></a></span><a class="link" href="ssl.html#asio.overview.ssl.see_also">See
Also</a>
</h5>
<p>
<a class="link" href="../reference/ssl__context.html" title="ssl::context">ssl::context</a>, <a class="link" href="../reference/ssl__host_name_verification.html" title="ssl::host_name_verification">ssl::host_name_verification</a>,
<a class="link" href="../reference/ssl__stream.html" title="ssl::stream">ssl::stream</a>, <a class="link" href="../examples/cpp03_examples.html#asio.examples.cpp03_examples.ssl">SSL
example (C++03)</a>, <a class="link" href="../examples/cpp11_examples.html#asio.examples.cpp11_examples.ssl">SSL
example (C++11)</a>.
</p>
<h5>
<a name="asio.overview.ssl.h3"></a>
<span><a name="asio.overview.ssl.notes"></a></span><a class="link" href="ssl.html#asio.overview.ssl.notes">Notes</a>
</h5>
<p>
<a href="http://www.openssl.org" target="_top">OpenSSL</a> is required to make use
of Asio's SSL support. When an application needs to use OpenSSL functionality
that is not wrapped by Asio, the underlying OpenSSL types may be obtained
by calling <a class="link" href="../reference/ssl__context/native_handle.html" title="ssl::context::native_handle"><code class="computeroutput">ssl::context::native_handle()</code></a>
or <a class="link" href="../reference/ssl__stream/native_handle.html" title="ssl::stream::native_handle"><code class="computeroutput">ssl::stream::native_handle()</code></a>.
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M.
Kohlhoff<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
</div></td>
</tr></table>
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