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<title>Opportunistic TLS</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Opportunistic TLS</span></span>

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</style><div role="note" class="hatnote navigation-not-searchable">"STARTTLS" redirects here. For the certificate authority, see <a href="StartCom#StartSSL" title="StartCom">StartCom § StartSSL</a>.</div>

<p><b>Opportunistic TLS</b> (Transport Layer Security) refers to extensions in plain text communication protocols, which offer a way to upgrade a plain text connection to an encrypted (<a href="Transport_Layer_Security" title="Transport Layer Security">TLS</a> or <a href="Secure_Sockets_Layer" class="mw-redirect" title="Secure Sockets Layer">SSL</a>) connection instead of using a separate port for encrypted communication. Several protocols use a command named "<b>STARTTLS</b>" or "<b>Explicit TLS</b>" for this purpose. It is a form of <a href="Opportunistic_encryption" title="Opportunistic encryption">opportunistic encryption</a> and is primarily intended as a countermeasure to <a href="Passive_monitoring" title="Passive monitoring">passive monitoring</a>.

</p><p>The STARTTLS command for <a href="IMAP" class="mw-redirect" title="IMAP">IMAP</a> and <a href="POP3" class="mw-redirect" title="POP3">POP3</a> is defined in <style data-mw-deduplicate="TemplateStyles:r1238218222">

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</style><a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc2595">2595</a>, for <a href="SMTP" class="mw-redirect" title="SMTP">SMTP</a> in <a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc3207">3207</a>, for <a href="XMPP" title="XMPP">XMPP</a> in <a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc6120">6120</a> and for <a href="NNTP" class="mw-redirect" title="NNTP">NNTP</a> in <a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc4642">4642</a>. For <a href="IRC" title="IRC">IRC</a>, the IRCv3 Working Group defined a STARTTLS extension, though it was later deprecated.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> <a href="FTP" class="mw-redirect" title="FTP">FTP</a> uses the command "AUTH TLS" defined in <a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc4217">4217</a> and <a href="LDAP" class="mw-redirect" title="LDAP">LDAP</a> defines a protocol extension <a href="Object_identifier" title="Object identifier">OID</a> in <a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc2830">2830</a>. <a href="HTTP" title="HTTP">HTTP</a> uses an <a href="HTTP/1.1_Upgrade_header#Use_with_TLS" title="HTTP/1.1 Upgrade header">upgrade header</a>.

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<div class="mw-heading mw-heading2"><h2 id="Layering">Layering</h2></div>

<p>TLS is application-neutral; in the words of <a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc5246">5246</a>:

</p>

<dl><dd>One advantage of TLS is that it is application protocol independent. Higher-level protocols can layer on top of the TLS protocol transparently. The TLS standard, however, does not specify how protocols add security with TLS; the decisions on how to initiate TLS handshaking and how to interpret the authentication certificates exchanged are left to the judgment of the designers and implementors of protocols that run on top of TLS.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup></dd></dl>

<p>The style used to specify how to use TLS matches the same layer distinction that is also conveniently supported by several library implementations of TLS. E.g., the <a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc3207">3207</a> SMTP extension illustrates with the following dialog how a client and server can start a secure session:<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>

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C: <opens connection>

S: 220 mail.example.org ESMTP service ready

C: EHLO client.example.org

S: 250-mail.example.org offers a warm hug of welcome

S: 250 STARTTLS

C: STARTTLS

S: 220 Go ahead

C: <starts TLS negotiation>

C & S: <negotiate a TLS session>

C & S: <check result of negotiation>

C: EHLO client.example.org<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>

. . .

</pre>

<p>The last <i>EHLO</i> command above is issued over a secure channel. Note that authentication is optional in SMTP, and the omitted server reply may now safely advertise an <i>AUTH PLAIN</i> SMTP extension, which is not present in the plain-text reply.

</p>

<div class="mw-heading mw-heading2"><h2 id="SSL_ports">SSL ports</h2></div>

<p>Besides the use of opportunistic TLS, a number of TCP ports were defined for SSL-secured versions of well-known protocols. These establish secure communications and then present a communication stream identical to the old un-encrypted protocol. Separate SSL ports have the advantage of fewer <a href="Round-trip_delay_time" class="mw-redirect" title="Round-trip delay time">round-trips</a>; also less meta-data is transmitted in unencrypted form.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> Some examples include:

</p>

<th>Protocol</th>

<th>Purpose</th>

<th>Normal port</th>

<th>SSL variant</th>

<th>SSL port

</th></tr>

<tr>

<td>Send email</td>

<td><a href="SMTPS" title="SMTPS">SMTPS</a></td>

<td>465

</td></tr>

<tr>

<td>Retrieve email</td>

<td>995

</td></tr>

<tr>

<td>Read email</td>

<td><a href="IMAPS" class="mw-redirect" title="IMAPS">IMAPS</a></td>

<td>993

</td></tr>

<tr>

<td>News reader</td>

<td><a href="NNTPS" class="mw-redirect" title="NNTPS">NNTPS</a></td>

<td>563

</td></tr>

<tr>

<td>Directory Access</td>

<td><a href="LDAPS" class="mw-redirect" title="LDAPS">LDAPS</a></td>

<td>636

</td></tr>

<tr>

<td>File transfer</td>

<td>990

</td></tr>

<tr>

<td>Instant messaging</td>

<td>5223

</td></tr></tbody></table>

<p>At least for the email related protocols, <a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc8314">8314</a> favors <i>Implicit TLS</i> (using separate SSL ports) instead of STARTTLS.

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<div class="mw-heading mw-heading2"><h2 id="Weaknesses_and_mitigations">Weaknesses and mitigations</h2></div>

<p>Opportunistic TLS is an <a href="Opportunistic_encryption" title="Opportunistic encryption">opportunistic encryption</a> mechanism. Because the initial handshake takes place in plain text, an attacker in control of the network can modify the server messages via a <a href="Man-in-the-middle_attack" title="Man-in-the-middle attack">man-in-the-middle attack</a> to make it appear that TLS is unavailable (called a <b>STRIPTLS attack</b>). Most SMTP clients will then send the email and possibly passwords in plain text, often with no notification to the user. In particular, many SMTP connections occur between mail servers, where user notification is not practical.

</p><p>In September 2014, two ISPs in <a href="Thailand" title="Thailand">Thailand</a> were found to be doing this to their own customers.<sup id="cite_ref-eff-isp-removing-encryption_6-0" class="reference"><a href="#cite_note-eff-isp-removing-encryption-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> In October 2014, <a href="Cricket_Wireless" title="Cricket Wireless">Cricket Wireless</a>, a subsidiary of <a href="AT%26T" title="AT&T">AT&T</a>, was revealed to be doing this to their customers. This behavior started as early as September 2013 by <a href="Aio_Wireless" title="Aio Wireless">Aio Wireless</a>, who later merged with Cricket where the practice continued.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-eff-isp-removing-encryption_6-1" class="reference"><a href="#cite_note-eff-isp-removing-encryption-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup>

</p><p>STRIPTLS attacks can be blocked by configuring SMTP clients to require TLS for outgoing connections (for example, the <a href="Exim" title="Exim">Exim</a> <a href="Message_transfer_agent" title="Message transfer agent">Message transfer agent</a> can require TLS via the directive "hosts_require_tls"<sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup>). However, since not every mail server supports TLS, it is not practical to simply require TLS for all connections.

</p><p>An example of a STRIPTLS attack of the type used in Thai <a href="Mass_surveillance" title="Mass surveillance">mass surveillance</a> technology:<sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup>

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<pre> 220 smtp.gmail.com ESMTP mail.redacted.com - gsmtp

ehlo a

250-smtp.gmail.com at your service, [REDACTED SERVICE]

250-SIZE 35882577

250-8BITMIME

# The STARTTLS command is stripped here

250-ENHANCEDSTATUSCODES

250-PIPELINING

250 SMTPUTF8

</pre>

</td>

<pre> 220 smtp.gmail.com ESMTP - gsmtp

ehlo a

250-smtp.gmail.com at your service

250-SIZE 35882577

250-8BITMIME

250-STARTTLS

250-ENHANCEDSTATUSCODES

250-PIPELINING

250 SMTPUTF8

</pre>

<p>

</p>

</td></tr></tbody></table></div>

<p>Supposing the client side supports it (name resolution of the client and upstream DNS server of the client), this problem can be addressed by <a href="DNS-based_Authentication_of_Named_Entities" title="DNS-based Authentication of Named Entities">DNS-based Authentication of Named Entities</a> (DANE), a part of <a href="DNSSEC" class="mw-redirect" title="DNSSEC">DNSSEC</a>, and in particular by <a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc7672">7672</a> for SMTP. DANE allows to advertise support for secure SMTP via a TLSA record. This tells connecting clients they should require TLS, thus preventing STRIPTLS attacks. The STARTTLS Everywhere project from the <a href="Electronic_Frontier_Foundation" title="Electronic Frontier Foundation">Electronic Frontier Foundation</a> works in a similar way. However, DNSSEC, due to deployment complexities and peculiar criticism,<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> faced a low adoption rate and a new protocol called SMTP MTA Strict Transport Security or MTA-STS has been drafted<sup id="cite_ref-12" class="reference"><a href="#cite_note-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> by a group of major email service providers including Microsoft, Google and Yahoo. MTA-STS does not require the use of DNSSEC to authenticate DANE TLSA records but relies on the <a href="Certificate_authority" title="Certificate authority">certificate authority</a> (CA) system and a trust-on-first-use (TOFU) approach to avoid interceptions. The TOFU model reduces complexity but without the guarantees on first use offered by DNSSEC. In addition, MTA-STS introduces a mechanism for failure reporting and a report-only mode, enabling progressive roll-out and auditing for compliance.

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<div class="mw-heading mw-heading2"><h2 id="Popularity">Popularity</h2></div>

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<p>Following the revelations made by <a href="Edward_Snowden" title="Edward Snowden">Edward Snowden</a> in light of the <a href="Global_surveillance_disclosures_(2013%E2%80%93present)" class="mw-redirect" title="Global surveillance disclosures (2013–present)">global mass surveillance scandal</a>, popular email providers have bettered their email security by enabling STARTTLS.<sup id="cite_ref-13" class="reference"><a href="#cite_note-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> <a href="Facebook" title="Facebook">Facebook</a> reported that after enabling STARTTLS and encouraging other providers to do the same, until Facebook discontinued its email service in February 2014, 95% of outbound email was encrypted with both <a href="Forward_secrecy" title="Forward secrecy">perfect forward secrecy</a> and strict certificate validation.<sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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<li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://ircv3.net/specs/deprecated/tls">"tls Extension"</a>. IRCv3 Working Group. 2012<span class="reference-accessdate">. Retrieved <span class="nowrap">6 April</span> 2024</span>.</cite></span>

</li>

<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite id="CITEREFTim_DierksEric_Rescorla2008" class="citation web cs1">Tim Dierks; Eric Rescorla (August 2008). <a rel="nofollow" class="external text" href="http://tools.ietf.org/html/rfc5246">"The Transport Layer Security (TLS) Protocol"</a>. <a href="RFC_Editor" class="mw-redirect" title="RFC Editor">RFC Editor</a><span class="reference-accessdate">. Retrieved <span class="nowrap">8 October</span> 2009</span>.</cite></span>

</li>

<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><cite id="CITEREFPaul_Hoffman2002" class="citation web cs1">Paul Hoffman (February 2002). <a rel="nofollow" class="external text" href="http://tools.ietf.org/html/rfc3207">"SMTP Service Extension for Secure SMTP over Transport Layer Security"</a>. <a href="RFC_Editor" class="mw-redirect" title="RFC Editor">RFC Editor</a><span class="reference-accessdate">. Retrieved <span class="nowrap">8 October</span> 2009</span>.</cite></span>

</li>

<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text">The last line in the example added for clarity. See e.g. the thread started by <cite id="CITEREFPaul_Smith2009" class="citation web cs1">Paul Smith (26 January 2009). <a rel="nofollow" class="external text" href="http://www.ietf.org/mail-archive/web/ietf-smtp/current/msg01818.html">"STARTTLS & EHLO"</a>. <i>ietf-smtp mailing list</i>. <a href="Internet_Mail_Consortium" title="Internet Mail Consortium">Internet Mail Consortium</a><span class="reference-accessdate">. Retrieved <span class="nowrap">16 September</span> 2015</span>.</cite></span>

</li>

<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><a href="Dovecot_(software)" title="Dovecot (software)">Dovecot</a> SSL documentation: <a rel="nofollow" class="external free" href="http://wiki2.dovecot.org/SSL">http://wiki2.dovecot.org/SSL</a></span>

</li>

<li id="cite_note-eff-isp-removing-encryption-6"><span class="mw-cite-backlink">^ <a href="#cite_ref-eff-isp-removing-encryption_6-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-eff-isp-removing-encryption_6-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><cite id="CITEREFHoffman-Andrews2014" class="citation news cs1">Hoffman-Andrews, Jacob (11 November 2014). <a rel="nofollow" class="external text" href="https://www.eff.org/deeplinks/2014/11/starttls-downgrade-attacks">"ISPs Removing Their Customers' Email Encryption"</a>. <i><a href="Electronic_Frontier_Foundation" title="Electronic Frontier Foundation">Electronic Frontier Foundation</a></i><span class="reference-accessdate">. Retrieved <span class="nowrap">19 January</span> 2019</span>.</cite></span>

</li>

<li id="cite_note-7"><span class="mw-cite-backlink"><b><a href="#cite_ref-7">^</a></b></span> <span class="reference-text"><cite class="citation news cs1"><a rel="nofollow" class="external text" href="http://www.telecomasia.net/content/google-yahoo-smtp-email-severs-hit-thailand">"Google, Yahoo SMTP email servers hit in Thailand"</a>. 12 September 2014<span class="reference-accessdate">. Retrieved <span class="nowrap">31 July</span> 2015</span>.</cite></span>

</li>

<li id="cite_note-8"><span class="mw-cite-backlink"><b><a href="#cite_ref-8">^</a></b></span> <span class="reference-text"><cite class="citation news cs1"><a rel="nofollow" class="external text" href="http://www.goldenfrog.com/blog/fcc-must-prevent-isps-blocking-encryption">"The FCC Must Prevent ISPs From Blocking Encryption"</a>. 4 November 2014<span class="reference-accessdate">. Retrieved <span class="nowrap">31 July</span> 2015</span>.</cite></span>

</li>

<li id="cite_note-9"><span class="mw-cite-backlink"><b><a href="#cite_ref-9">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.exim.org/exim-html-current/doc/html/spec_html/ch-the_smtp_transport.html">"Exim Internet Mailer - The smtp transport"</a>. <i>exim.org</i>. <q>hosts_require_tls - Exim will insist on using a TLS session when delivering to any host that matches this list.</q></cite></span>

</li>

<li id="cite_note-10"><span class="mw-cite-backlink"><b><a href="#cite_ref-10">^</a></b></span> <span class="reference-text"><cite class="citation journal cs1"><a rel="nofollow" class="external text" href="https://privacyinternational.org/sites/default/files/2017-10/thailand_2017_0.pdf">"Who's That Knocking at my door? Understanding Surveillance in Thailand"</a> <span class="cs1-format">(PDF)</span>. <i>Privacy International</i>: 21. January 2017<span class="reference-accessdate">. Retrieved <span class="nowrap">7 February</span> 2020</span>.</cite></span>

</li>

<li id="cite_note-11"><span class="mw-cite-backlink"><b><a href="#cite_ref-11">^</a></b></span> <span class="reference-text"><cite id="CITEREFThomas_Ptacek2016" class="citation news cs1">Thomas Ptacek (18 March 2016). <a rel="nofollow" class="external text" href="http://sockpuppet.org/blog/2015/01/15/against-dnssec/">"Against DNSSEC"</a>.</cite></span>

</li>

<li id="cite_note-12"><span class="mw-cite-backlink"><b><a href="#cite_ref-12">^</a></b></span> <span class="reference-text"><cite id="CITEREFRamakrishnanBrotmanJonesMargolis" class="citation web cs1">Ramakrishnan, Binu; Brotman, Alexander; Jones, Janet; Margolis, Daniel; Risher, Mark. <a rel="nofollow" class="external text" href="https://tools.ietf.org/html/rfc8461.html">"SMTP MTA Strict Transport Security (MTA-STS)"</a>. <i>tools.ietf.org</i><span class="reference-accessdate">. Retrieved <span class="nowrap">22 February</span> 2019</span>.</cite></span>

</li>

<li id="cite_note-13"><span class="mw-cite-backlink"><b><a href="#cite_ref-13">^</a></b></span> <span class="reference-text"><cite id="CITEREFPeterson2014" class="citation news cs1">Peterson, Andrea (12 August 2014). <a rel="nofollow" class="external text" href="https://www.washingtonpost.com/blogs/the-switch/wp/2014/08/12/facebooks-security-chief-on-the-snowden-effect-the-messenger-app-backlash-and-staying-optimistic/">"Facebook's security chief on the Snowden effect, the Messenger app backlash and staying optimistic"</a>. <i><a href="The_Washington_Post" title="The Washington Post">The Washington Post</a></i><span class="reference-accessdate">. Retrieved <span class="nowrap">2 November</span> 2014</span>.</cite></span>

</li>

<li id="cite_note-14"><span class="mw-cite-backlink"><b><a href="#cite_ref-14">^</a></b></span> <span class="reference-text"><cite id="CITEREFCohen2014" class="citation web cs1">Cohen, David (19 August 2014). <a rel="nofollow" class="external text" href="http://allfacebook.com/facebook-95-notification-emails-encrypted-thanks-providers-starttls-deployment_b134023">"Facebook: 95% of Notification Emails Encrypted Thanks to Providers' STARTTLS Deployment"</a>. <i>allfacebook.com</i>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20140922222720/http://allfacebook.com/facebook-95-notification-emails-encrypted-thanks-providers-starttls-deployment_b134023">Archived</a> from the original on 22 September 2014.</cite></span>

</li>

</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a rel="nofollow" class="external text" href="https://www.checktls.com/TestReceiver">Secure Email Tests and Tools</a> verify STARTTLS in real-time dialog like example above</li>

<li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20140610053143/https://starttls.info/">Verify if a receiving domain has STARTTLS enabled for email and with which security level</a></li>

<li><cite id="CITEREFMargolisRisherRamakrishnanBrotman" class="citation web cs1">Margolis, Daniel; Risher, Mark; Ramakrishnan, Binu; Brotman, Alexander; Jones, Janet. <a rel="nofollow" class="external text" href="https://tools.ietf.org/html/draft-ietf-uta-mta-sts">"SMTP MTA Strict Transport Security (MTA-STS)"</a>. IETF.</cite> A mechanism enabling mail service providers to declare their ability to receive Transport Layer Security (TLS) secure SMTP connections.</li></ul>

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</style><div id="TLS_and_SSL182" style="font-size:114%;margin:0 4em"><a href="Transport_Layer_Security" title="Transport Layer Security">TLS and SSL</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%">Protocols and technologies</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Transport_Layer_Security" title="Transport Layer Security">Transport Layer Security / Secure Sockets Layer</a> (TLS/SSL)</li>

<li><a href="Datagram_Transport_Layer_Security" title="Datagram Transport Layer Security">Datagram Transport Layer Security</a> (DTLS)</li>

<li><a href="Server_Name_Indication" title="Server Name Indication">Server Name Indication</a> (SNI)</li>

<li><a href="Application-Layer_Protocol_Negotiation" title="Application-Layer Protocol Negotiation">Application-Layer Protocol Negotiation</a> (ALPN)</li>

<li><a href="DNS-based_Authentication_of_Named_Entities" title="DNS-based Authentication of Named Entities">DNS-based Authentication of Named Entities</a> (DANE)</li>

<li><a href="DNS_Certification_Authority_Authorization" title="DNS Certification Authority Authorization">DNS Certification Authority Authorization</a> (CAA)</li>

<li><a href="HTTPS" title="HTTPS">HTTPS</a></li>

<li><a href="HTTP_Strict_Transport_Security" title="HTTP Strict Transport Security">HTTP Strict Transport Security</a> (HSTS)</li>

<li><a href="HTTP_Public_Key_Pinning" title="HTTP Public Key Pinning">HTTP Public Key Pinning</a> (HPKP)</li>

<li><a href="OCSP_stapling" title="OCSP stapling">OCSP stapling</a></li>

<li><a href="Forward_secrecy" title="Forward secrecy">Perfect forward secrecy</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Public-key infrastructure</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Automated_Certificate_Management_Environment" class="mw-redirect" title="Automated Certificate Management Environment">Automated Certificate Management Environment</a> (ACME)</li>

<li><a href="Certificate_authority" title="Certificate authority">Certificate authority</a> (CA)</li>

<li><a href="CA/Browser_Forum" title="CA/Browser Forum">CA/Browser Forum</a></li>

<li><a href="Certificate_policy" title="Certificate policy">Certificate policy</a></li>

<li><a href="Certificate_revocation" title="Certificate revocation">Certificate revocation</a>

<ul><li><a href="Certificate_revocation_list" title="Certificate revocation list">Certificate revocation list</a> (CRL)</li>

<li><a href="Online_Certificate_Status_Protocol" title="Online Certificate Status Protocol">Online Certificate Status Protocol</a> (OCSP)</li>

<li><a href="OCSP_stapling" title="OCSP stapling">OCSP stapling</a></li></ul></li>

<li><a href="Domain-validated_certificate" title="Domain-validated certificate">Domain-validated certificate</a> (DV)</li>

<li><a href="Extended_Validation_Certificate" title="Extended Validation Certificate">Extended Validation Certificate</a> (EV)</li>

<li><a href="Public_key_certificate" title="Public key certificate">Public key certificate</a></li>

<li><a href="Public-key_cryptography" title="Public-key cryptography">Public-key cryptography</a></li>

<li><a href="Public_key_infrastructure" title="Public key infrastructure">Public key infrastructure</a> (PKI)</li>

<li><a href="Root_certificate" title="Root certificate">Root certificate</a></li>

<li><a href="Self-signed_certificate" title="Self-signed certificate">Self-signed certificate</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">See also</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Domain_Name_System_Security_Extensions" title="Domain Name System Security Extensions">Domain Name System Security Extensions</a> (DNSSEC)</li>

<li><a href="Internet_Protocol_Security" class="mw-redirect" title="Internet Protocol Security">Internet Protocol Security</a> (IPsec)</li>

<li><a href="Secure_Shell" title="Secure Shell">Secure Shell</a> (SSH)</li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">History</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Export_of_cryptography_from_the_United_States" title="Export of cryptography from the United States">Export of cryptography from the United States</a></li>

<li><a href="Server-Gated_Cryptography" title="Server-Gated Cryptography">Server-Gated Cryptography</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Comparison_of_TLS_implementations" title="Comparison of TLS implementations">Implementations</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Bouncy_Castle_(cryptography)" title="Bouncy Castle (cryptography)">Bouncy Castle</a></li>

<li><a href="BoringSSL" class="mw-redirect" title="BoringSSL">BoringSSL</a></li>

<li><a href="Botan_(programming_library)" title="Botan (programming library)">Botan</a></li>

<li><a href="BSAFE" title="BSAFE">BSAFE</a></li>

<li><a href="Cryptlib" title="Cryptlib">cryptlib</a></li>

<li><a href="GnuTLS" title="GnuTLS">GnuTLS</a></li>

<li><a href="LibreSSL" title="LibreSSL">LibreSSL</a></li>

<li><a href="MatrixSSL" title="MatrixSSL">MatrixSSL</a></li>

<li><a href="OpenSSL" title="OpenSSL">OpenSSL</a></li>

<li><a href="Rustls" title="Rustls">Rustls</a></li>

<li><a href="Security_Support_Provider_Interface" title="Security Support Provider Interface">SChannel</a></li>

<li><a href="SSLeay" title="SSLeay">SSLeay</a></li>

<li><a href="Stunnel" title="Stunnel">stunnel</a></li>

<li><a href="WolfSSL" title="WolfSSL">wolfSSL</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Notaries</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Certificate_Transparency" title="Certificate Transparency">Certificate Transparency</a></li>

<li><a href="Convergence_(SSL)" title="Convergence (SSL)">Convergence</a></li>

<li><a href="HTTPS_Everywhere" title="HTTPS Everywhere">HTTPS Everywhere</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Vulnerabilities</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:1%">Theory</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Man-in-the-middle_attack" title="Man-in-the-middle attack">Man-in-the-middle attack</a></li>

<li><a href="Padding_oracle_attack" title="Padding oracle attack">Padding oracle attack</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Cipher</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Bar_mitzvah_attack" title="Bar mitzvah attack">Bar mitzvah attack</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Protocol</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="BEAST_(security_exploit)" class="mw-redirect" title="BEAST (security exploit)">BEAST</a></li>

<li><a href="BREACH" title="BREACH">BREACH</a></li>

<li><a href="CRIME" title="CRIME">CRIME</a></li>

<li><a href="DROWN_attack" title="DROWN attack">DROWN</a></li>

<li><a href="Logjam_(computer_security)" title="Logjam (computer security)">Logjam</a></li>

<li><a href="POODLE" title="POODLE">POODLE</a> (in regards to SSL 3.0)</li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Implementation</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Certificate_authority_compromise" class="mw-redirect" title="Certificate authority compromise">Certificate authority compromise</a></li>

<li><a href="Random_number_generator_attack" title="Random number generator attack">Random number generator attacks</a></li>

<li><a href="FREAK" title="FREAK">FREAK</a></li>

<li><a href="Heartbleed" title="Heartbleed">Heartbleed</a></li>

<li><a href="Lucky_Thirteen_attack" title="Lucky Thirteen attack">Lucky Thirteen attack</a></li>

<li><a href="POODLE" title="POODLE">POODLE</a> (in regards to TLS 1.0)</li>

<li><a href="Kazakhstan_man-in-the-middle_attack" title="Kazakhstan man-in-the-middle attack">Kazakhstan MITM attack</a></li></ul>

</div></td></tr></tbody></table><div></div></td></tr></tbody></table></div></div><div><div class="zim-footer">

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