184 lines
8.2 KiB
HTML
Executable File
184 lines
8.2 KiB
HTML
Executable File
<?xml version="1.0" ?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<title>SSL_read</title>
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<meta http-equiv="content-type" content="text/html; charset=utf-8" />
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<link rev="made" href="mailto:root@localhost" />
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</head>
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<body style="background-color: white">
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<!-- INDEX BEGIN -->
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<div name="index">
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<p><a name="__index__"></a></p>
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<ul>
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<li><a href="#name">NAME</a></li>
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<li><a href="#synopsis">SYNOPSIS</a></li>
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<li><a href="#description">DESCRIPTION</a></li>
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<li><a href="#notes">NOTES</a></li>
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<li><a href="#return_values">RETURN VALUES</a></li>
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<li><a href="#see_also">SEE ALSO</a></li>
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<li><a href="#history">HISTORY</a></li>
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<li><a href="#copyright">COPYRIGHT</a></li>
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</ul>
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<hr name="index" />
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</div>
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<!-- INDEX END -->
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<p>
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</p>
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<hr />
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<h1><a name="name">NAME</a></h1>
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<p>SSL_read_ex, SSL_read, SSL_peek_ex, SSL_peek
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- read bytes from a TLS/SSL connection</p>
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<p>
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</p>
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<hr />
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<h1><a name="synopsis">SYNOPSIS</a></h1>
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<pre>
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#include <openssl/ssl.h></pre>
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<pre>
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int SSL_read_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
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int SSL_read(SSL *ssl, void *buf, int num);</pre>
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<pre>
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int SSL_peek_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
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int SSL_peek(SSL *ssl, void *buf, int num);</pre>
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<p>
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</p>
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<hr />
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<h1><a name="description">DESCRIPTION</a></h1>
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<p><code>SSL_read_ex()</code> and <code>SSL_read()</code> try to read <strong>num</strong> bytes from the specified <strong>ssl</strong>
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into the buffer <strong>buf</strong>. On success <code>SSL_read_ex()</code> will store the number of bytes
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actually read in <strong>*readbytes</strong>.</p>
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<p><code>SSL_peek_ex()</code> and <code>SSL_peek()</code> are identical to <code>SSL_read_ex()</code> and <code>SSL_read()</code>
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respectively except no bytes are actually removed from the underlying BIO during
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the read, so that a subsequent call to <code>SSL_read_ex()</code> or <code>SSL_read()</code> will yield
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at least the same bytes.</p>
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<p>
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</p>
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<hr />
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<h1><a name="notes">NOTES</a></h1>
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<p>In the paragraphs below a "read function" is defined as one of <code>SSL_read_ex()</code>,
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<code>SSL_read()</code>, <code>SSL_peek_ex()</code> or <code>SSL_peek()</code>.</p>
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<p>If necessary, a read function will negotiate a TLS/SSL session, if not already
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explicitly performed by <em>SSL_connect(3)</em> or <em>SSL_accept(3)</em>. If the
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peer requests a re-negotiation, it will be performed transparently during
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the read function operation. The behaviour of the read functions depends on the
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underlying BIO.</p>
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<p>For the transparent negotiation to succeed, the <strong>ssl</strong> must have been
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initialized to client or server mode. This is being done by calling
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<em>SSL_set_connect_state(3)</em> or <code>SSL_set_accept_state()</code> before the first
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invocation of a read function.</p>
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<p>The read functions work based on the SSL/TLS records. The data are received in
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records (with a maximum record size of 16kB). Only when a record has been
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completely received, can it be processed (decryption and check of integrity).
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Therefore data that was not retrieved at the last read call can still be
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buffered inside the SSL layer and will be retrieved on the next read
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call. If <strong>num</strong> is higher than the number of bytes buffered then the read
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functions will return with the bytes buffered. If no more bytes are in the
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buffer, the read functions will trigger the processing of the next record.
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Only when the record has been received and processed completely will the read
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functions return reporting success. At most the contents of one record will
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be returned. As the size of an SSL/TLS record may exceed the maximum packet size
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of the underlying transport (e.g. TCP), it may be necessary to read several
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packets from the transport layer before the record is complete and the read call
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can succeed.</p>
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<p>If <strong>SSL_MODE_AUTO_RETRY</strong> has been switched off and a non-application data
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record has been processed, the read function can return and set the error to
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<strong>SSL_ERROR_WANT_READ</strong>.
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In this case there might still be unprocessed data available in the <strong>BIO</strong>.
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If read ahead was set using <em>SSL_CTX_set_read_ahead(3)</em>, there might also still
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be unprocessed data available in the <strong>SSL</strong>.
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This behaviour can be controlled using the <em>SSL_CTX_set_mode(3)</em> call.</p>
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<p>If the underlying BIO is <strong>blocking</strong>, a read function will only return once the
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read operation has been finished or an error occurred, except when a
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non-application data record has been processed and <strong>SSL_MODE_AUTO_RETRY</strong> is
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not set.
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Note that if <strong>SSL_MODE_AUTO_RETRY</strong> is set and only non-application data is
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available the call will hang.</p>
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<p>If the underlying BIO is <strong>non-blocking</strong>, a read function will also return when
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the underlying BIO could not satisfy the needs of the function to continue the
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operation.
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In this case a call to <em>SSL_get_error(3)</em> with the
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return value of the read function will yield <strong>SSL_ERROR_WANT_READ</strong> or
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<strong>SSL_ERROR_WANT_WRITE</strong>.
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As at any time it's possible that non-application data needs to be sent,
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a read function can also cause write operations.
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The calling process then must repeat the call after taking appropriate action
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to satisfy the needs of the read function.
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The action depends on the underlying BIO.
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When using a non-blocking socket, nothing is to be done, but <code>select()</code> can be
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used to check for the required condition.
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When using a buffering BIO, like a BIO pair, data must be written into or
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retrieved out of the BIO before being able to continue.</p>
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<p><em>SSL_pending(3)</em> can be used to find out whether there
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are buffered bytes available for immediate retrieval.
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In this case the read function can be called without blocking or actually
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receiving new data from the underlying socket.</p>
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<p>
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</p>
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<hr />
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<h1><a name="return_values">RETURN VALUES</a></h1>
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<p><code>SSL_read_ex()</code> and <code>SSL_peek_ex()</code> will return 1 for success or 0 for failure.
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Success means that 1 or more application data bytes have been read from the SSL
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connection.
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Failure means that no bytes could be read from the SSL connection.
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Failures can be retryable (e.g. we are waiting for more bytes to
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be delivered by the network) or non-retryable (e.g. a fatal network error).
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In the event of a failure call <em>SSL_get_error(3)</em> to find out the reason which
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indicates whether the call is retryable or not.</p>
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<p>For <code>SSL_read()</code> and <code>SSL_peek()</code> the following return values can occur:</p>
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<dl>
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<dt><strong><a name="__0" class="item">> 0</a></strong></dt>
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<dd>
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<p>The read operation was successful.
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The return value is the number of bytes actually read from the TLS/SSL
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connection.</p>
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</dd>
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<dt><strong><a name="___0" class="item"><= 0</a></strong></dt>
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<dd>
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<p>The read operation was not successful, because either the connection was closed,
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an error occurred or action must be taken by the calling process.
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Call <em>SSL_get_error(3)</em> with the return value <strong>ret</strong> to find out the reason.</p>
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<p>Old documentation indicated a difference between 0 and -1, and that -1 was
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retryable.
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You should instead call <code>SSL_get_error()</code> to find out if it's retryable.</p>
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</dd>
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</dl>
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<p>
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</p>
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<hr />
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<h1><a name="see_also">SEE ALSO</a></h1>
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<p><em>SSL_get_error(3)</em>, <em>SSL_write_ex(3)</em>,
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<em>SSL_CTX_set_mode(3)</em>, <em>SSL_CTX_new(3)</em>,
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<em>SSL_connect(3)</em>, <em>SSL_accept(3)</em>
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<em>SSL_set_connect_state(3)</em>,
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<em>SSL_pending(3)</em>,
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<em>SSL_shutdown(3)</em>, <em>SSL_set_shutdown(3)</em>,
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<em>ssl(7)</em>, <em>bio(7)</em></p>
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<p>
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</p>
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<hr />
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<h1><a name="history">HISTORY</a></h1>
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<p>The <code>SSL_read_ex()</code> and <code>SSL_peek_ex()</code> functions were added in OpenSSL 1.1.1.</p>
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<p>
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</p>
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<hr />
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<h1><a name="copyright">COPYRIGHT</a></h1>
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<p>Copyright 2000-2019 The OpenSSL Project Authors. All Rights Reserved.</p>
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<p>Licensed under the Apache License 2.0 (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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<a href="https://www.openssl.org/source/license.html">https://www.openssl.org/source/license.html</a>.</p>
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</body>
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</html>
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