path: root/lib/ssl/doc/src/ssl.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1999</year><year>2018</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>
    <title>ssl</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>ssl.xml</file>
  </header>
  <module since="">ssl</module>
  <modulesummary>Interface Functions for Secure Socket Layer</modulesummary>
  <description>
    <p>
      This module contains interface functions for the SSL/TLS/DTLS protocol.
      For detailed information about the supported standards see 
      <seealso marker="ssl_app">ssl(6)</seealso>.
    </p>
  </description>

  <!--
      ================================================================
      =  Data types                                                  =
      ================================================================
  -->

  <datatypes>
    <datatype_title>Types used in SSL/TLS/DTLS</datatype_title>

    
    <datatype>
      <name name="socket"/>
    </datatype>
    
    <datatype>
      <name name="sslsocket"/>
      <desc>
	<p>An opaque reference to the TLS/DTLS connection.</p>
	</desc>
    </datatype>

    <datatype>
      <name name="tls_option"/>
    </datatype>
        
    <datatype>
      <name name="tls_client_option"/>
    </datatype>
    
    <datatype>
      <name name="tls_server_option"/>
    </datatype>
   
    
    <datatype>
      <name name="socket_option"/>
      <desc>
	<p>The default socket options are
	<c>[{mode,list},{packet, 0},{header, 0},{active, true}]</c>.</p>
	<p>For valid options, see the
	<seealso marker="kernel:inet">inet(3)</seealso>,
	<seealso marker="kernel:gen_tcp">gen_tcp(3)</seealso> and
	<seealso marker="kernel:gen_tcp">gen_udp(3)</seealso> 
	manual pages in Kernel. Note that stream oriented options such as packet
	are only relevant for SSL/TLS and not DTLS</p>
      </desc>
    </datatype>

    <datatype>
      <name name="socket_connect_option"/>
    </datatype>
    
     <datatype>
      <name name="socket_listen_option"/>
    </datatype>

    <datatype>
      <name name="active_msgs"/>
      <desc>
      <p>When an TLS/DTLS socket is in active mode (the default), data from the
      socket is delivered to the owner of the socket in the form of
      messages as described above.</p>
      </desc>
    </datatype>
      
    <datatype>
      <name name="transport_option"/>
      <desc>
	<p>Defaults to <c>{gen_tcp, tcp, tcp_closed, tcp_error}</c>
	for TLS and <c>{gen_udp, udp, udp_closed, udp_error}</c> for
	DTLS. Can be used to customize the transport layer. The tag
	values should be the values used by the underlying transport
	in its active mode messages. For TLS the callback module must implement a
	reliable transport protocol, behave as <c>gen_tcp</c>, and have functions
	corresponding to <c>inet:setopts/2</c>, <c>inet:getopts/2</c>,
	<c>inet:peername/1</c>, <c>inet:sockname/1</c>, and <c>inet:port/1</c>.
	The callback <c>gen_tcp</c> is treated specially and calls <c>inet</c>
	directly. For DTLS this feature must be considered exprimental.
	</p>
      </desc>
    </datatype>
   
      <datatype>
      <name name="path"/>
     </datatype>

     <datatype>
      <name name="host"/>
     </datatype>

     <datatype>
      <name name="hostname"/>
     </datatype>

       <datatype>
      <name name="ip_address"/>
     </datatype>

     <datatype>
       <name name="protocol_version"/>
     </datatype>

       <datatype>
       <name name="tls_version"/>
     </datatype>
     
     <datatype>
       <name name="dtls_version"/>
     </datatype>


   <datatype>
       <name name="legacy_version"/>
     </datatype>
     
     
      <datatype>
       <name name="verify_type"/>
     </datatype>
       
     <datatype>
       <name name="ciphers"/>
     </datatype>
     
      <datatype>
       <name name="erl_cipher_suite"/>
     </datatype>
     
     <datatype>
       <name name="cipher"/>
     </datatype>
    
     <datatype>
       <name name="legacy_cipher"/>
     </datatype>
    
     <datatype>
       <name name="cipher_filters"/>
     </datatype>
 
       <datatype>
       <name name="hash"/>
     </datatype>

     <datatype>
       <name name="sha2"/>
     </datatype>

     <datatype>
       <name name="legacy_hash"/>
     </datatype>

     <datatype>
      <name name="signature_algs"/>
     </datatype>

     <datatype>
      <name name="sign_algo"/>
     </datatype>
     
     <datatype>
       <name name="sign_scheme"/>
     </datatype>
     
     <datatype>
      <name name="key_algo"/>
     </datatype>

     <datatype>
      <name name="algo_filter"/>
     </datatype>

     <datatype>
      <name name="eccs"/>
     </datatype>

     <datatype>
      <name name="named_curve"/>
     </datatype>

     <datatype>
       <name name="psk_identity"/>
     </datatype>

     <datatype>
       <name name="srp_identity"/>
     </datatype>

     <datatype>
      <name name="srp_param_type"/>
     </datatype>
     
     <datatype>
       <name name="app_level_protocol"/>
     </datatype>

     <datatype>
      <name name="error_alert"/>
     </datatype>

     <datatype>
      <name name="tls_alert"/>
     </datatype>

    <datatype_title>TLS/DTLS OPTION DESCRIPTIONS - COMMON for SERVER and CLIENT</datatype_title>
    
    <datatype>
      <name name="common_option"/>
    </datatype>
    
    <datatype>
      <name since="OTP 20" name="protocol"/>
      <desc>
	<p>Choose TLS or DTLS protocol for the transport layer security.
	Defaults to <c>tls</c>. For DTLS other transports than UDP are not yet supported.</p>
      </desc>
    </datatype>
    
    <datatype>
      <name name="handshake_completion"/>
      <desc>
	<p>Defaults to <c>full</c>. If hello is specified the handshake will
	pause after the hello message and give the user a possibility make decisions
	based on hello extensions before continuing or aborting the handshake by calling
	<seealso marker="#handshake_continue-3"> handshake_continue/3</seealso> or
	<seealso marker="#handshake_cancel-1"> handshake_cancel/1</seealso></p>
      </desc>
    </datatype>
    
    <datatype>
      <name name="cert"/>
      <desc>
	<p>The DER-encoded users certificate. If this option
	is supplied, it overrides option <c>certfile</c>.</p>
      </desc>
    </datatype>
    
    <datatype>
      <name name="cert_pem"/>
      <desc>
	<p>Path to a file containing the user certificate on PEM format.</p>
      </desc>
    </datatype>
    
    <datatype>
      <name name="key"/>
      <desc>
	<p>The DER-encoded user's private key or a map refering to a crypto
	engine and its key reference that optionally can be password protected,
	seealso <seealso marker="crypto:crypto#engine_load-4"> crypto:engine_load/4
	</seealso> and  <seealso marker="crypto:engine_load"> Crypto's Users Guide</seealso>. If this option 
	is supplied, it overrides option <c>keyfile</c>.</p>
      </desc>
    </datatype>
    
     <datatype>
       <name name="key_pem"/>
       <desc>
	<p>Path to the file containing the user's
	private PEM-encoded key. As PEM-files can contain several
	entries, this option defaults to the same file as given by
	option <c>certfile</c>.</p>
       </desc>
     </datatype>
     
    <datatype>
      <name name="key_password"/>
      <desc>
	<p>String containing the user's password. Only used if the 
	private keyfile is password-protected.</p>
      </desc>
    </datatype>
    
    <datatype>
      <name name="cipher_suites"/>
      <desc>
	<p>Supported cipher suites. The function
	<c>cipher_suites/2</c> can be used to find all ciphers that
	are supported by default. <c>cipher_suites(all, 'tlsv1.2')</c> can be
	called to find all available cipher suites. Pre-Shared Key
	(<url href="http://www.ietf.org/rfc/rfc4279.txt">RFC
	4279</url> and <url
	href="http://www.ietf.org/rfc/rfc5487.txt">RFC 5487</url>),
	Secure Remote Password (<url
	href="http://www.ietf.org/rfc/rfc5054.txt">RFC 5054</url>),
	RC4, 3DES, DES cipher suites, and anonymous cipher suites only work if
	explicitly enabled by this option; they are supported/enabled
	by the peer also.  Anonymous cipher suites are supported for
	testing purposes only and are not be used when security
	matters.</p>
      </desc>
    </datatype>
    
    <datatype>
      <name name="eccs"/>
      <desc><p> Allows to specify the order of preference for named curves
      and to restrict their usage when using a cipher suite supporting them.</p>
      </desc>
    </datatype>

    <datatype>
      <name name="signature_schemes"/>
      <desc>
	  <p>
	    In addition to the signature_algorithms extension from TLS 1.2,
	    <url href="http://www.ietf.org/rfc/rfc8446.txt#section-4.2.3">TLS 1.3
	    (RFC 5246 Section 4.2.3)</url>adds the signature_algorithms_cert extension
	    which enables having special requirements on the signatures used in the
	    certificates that differs from the requirements on digital signatures as a whole.
	    If this is not required this extension is not needed.
	  </p>
	  <p>
	    The client will send a signature_algorithms_cert extension (ClientHello),
	    if TLS version 1.3 or later is used, and the signature_algs_cert option is
	    explicitly specified. By default, only the signature_algs extension is sent.
	  </p>
	  <p>
	    The signature schemes shall be ordered according to the client's preference
	    (favorite choice first).
	  </p>
      </desc>
     </datatype>
     
    <datatype>
      <name name="secure_renegotiation"/>
      <desc><p>Specifies if to reject renegotiation attempt that does
      not live up to <url
      href="http://www.ietf.org/rfc/rfc5746.txt">RFC 5746</url>.  By
      default <c>secure_renegotiate</c> is set to <c>true</c>, that
      is, secure renegotiation is enforced. If set to <c>false</c>
      secure renegotiation will still be used if possible, but it
      falls back to insecure renegotiation if the peer does not
      support <url href="http://www.ietf.org/rfc/rfc5746.txt">RFC
      5746</url>.</p>
      </desc>
    </datatype>
    
    <datatype>
      <name name="allowed_cert_chain_length"/>
      <desc><p>Maximum number of non-self-issued
      intermediate certificates that can follow the peer certificate 
      in a valid certification path. So, if depth is 0 the PEER must 
      be signed by the trusted ROOT-CA directly; if 1 the path can 
      be PEER, CA, ROOT-CA; if 2 the path can be PEER, CA, CA, 
      ROOT-CA, and so on. The default value is 1.</p>
      </desc>
    </datatype>
    
    <datatype>
      <name name="custom_verify"/>
      <desc>
	<p>The verification fun is to be defined as follows:</p>

	<code>
fun(OtpCert :: #'OTPCertificate'{}, Event :: {bad_cert, Reason :: atom() | {revoked,
atom()}} |
	     {extension, #'Extension'{}}, InitialUserState :: term()) ->
	{valid, UserState :: term()} | {valid_peer, UserState :: term()} |
	{fail, Reason :: term()} | {unknown, UserState :: term()}.
	</code>

	<p>The verification fun is called during the X509-path
	validation when an error or an extension unknown to the SSL
	application is encountered. It is also called when a
	certificate is considered valid by the path validation to
	allow access to each certificate in the path to the user
	application. It differentiates between the peer certificate
	and the CA certificates by using <c>valid_peer</c> or
	<c>valid</c> as second argument to the verification fun. See
	the <seealso marker="public_key:public_key_records">public_key
	User's Guide</seealso> for definition of
	<c>#'OTPCertificate'{}</c> and <c>#'Extension'{}</c>.</p>

	<list type="bulleted">
	  <item><p>If the verify callback fun returns <c>{fail,
	  Reason}</c>, the verification process is immediately
	  stopped, an alert is sent to the peer, and the TLS/DTLS
	  handshake terminates.</p></item>
	  <item><p>If the verify callback fun returns <c>{valid, UserState}</c>,
	  the verification process continues.</p></item> 
	  <item><p>If the verify callback fun always returns
	  <c>{valid, UserState}</c>, the TLS/DTLS handshake does not
	  terminate regarding verification failures and the connection is
	  established.</p></item>
	  <item><p>If called with an extension unknown to the user application,
	  return value <c>{unknown, UserState}</c> is to be used.</p>

	  <p>Note that if the fun returns <c>unknown</c> for an extension marked
	  as critical, validation will fail.</p>
	  </item>
	</list>

	<p>Default option <c>verify_fun</c> in <c>verify_peer mode</c>:</p>

      <code>
{fun(_,{bad_cert, _} = Reason, _) ->
	 {fail, Reason};
    (_,{extension, _}, UserState) ->
	 {unknown, UserState};
    (_, valid, UserState) ->
	 {valid, UserState};
    (_, valid_peer, UserState) ->
         {valid, UserState}
 end, []}
      </code>

      <p>Default option <c>verify_fun</c> in mode <c>verify_none</c>:</p>

       <code>
{fun(_,{bad_cert, _}, UserState) ->
	 {valid, UserState};
    (_,{extension, #'Extension'{critical = true}}, UserState) ->
	 {valid, UserState};
    (_,{extension, _}, UserState) ->
	 {unknown, UserState};
    (_, valid, UserState) ->
	 {valid, UserState};
    (_, valid_peer, UserState) ->
         {valid, UserState}
 end, []}
      </code>

      <p>The possible path validation errors are given on form
      <c>{bad_cert, Reason}</c> where <c>Reason</c> is:</p>

      <taglist>
	<tag><c>unknown_ca</c></tag>
	<item><p>No trusted CA was found in the trusted store. The
	trusted CA is normally a so called ROOT CA, which is a
	self-signed certificate. Trust can be claimed for an
	intermediate CA (trusted anchor does not have to be
	self-signed according to X-509) by using option
	<c>partial_chain</c>.</p>
	</item>

	<tag><c>selfsigned_peer</c></tag>
	<item><p>The chain consisted only of one self-signed certificate.</p></item>

	<tag><c>PKIX X-509-path validation error</c></tag>
	<item><p>For possible reasons, see <seealso
marker="public_key:public_key#pkix_path_validation-3">public_key:pkix_path_validation/3</seealso>
	</p></item>
      </taglist>
      </desc>
    </datatype>
    
    <datatype>
      <name name="crl_check"/>
      <desc>
	<p>Perform CRL (Certificate Revocation List) verification
	<seealso marker="public_key:public_key#pkix_crls_validate-3">
	(public_key:pkix_crls_validate/3)</seealso> on all the
	certificates during the path validation <seealso
	marker="public_key:public_key#pkix_path_validation-3">(public_key:pkix_path_validation/3)
	</seealso>
	of the certificate chain. Defaults to <c>false</c>.</p>
	
	<taglist>
	  <tag><c>peer</c></tag>
	  <item>check is only performed on the peer certificate.</item>

	  <tag><c>best_effort</c></tag>
	  <item>if certificate revocation status cannot be determined
	  it will be accepted as valid.</item>
	</taglist>
	
	<p>The CA certificates specified for the connection will be used to 
	construct the certificate chain validating the CRLs.</p>
 	
	<p>The CRLs will be fetched from a local or external cache. See
	<seealso marker="ssl:ssl_crl_cache_api">ssl_crl_cache_api(3)</seealso>.</p>
      </desc>
    </datatype>

    <datatype>
      <name name="crl_cache_opts"/>
      <desc>
    	<p>Specify how to perform lookup and caching of certificate revocation lists.
    	<c>Module</c> defaults to <seealso marker="ssl:ssl_crl_cache">ssl_crl_cache</seealso>
    	with <c> DbHandle </c> being <c>internal</c> and an
    	empty argument list.</p>
	
    	<p>There are two implementations available:</p>
	
    	<taglist>
    	  <tag><c>ssl_crl_cache</c></tag>
    	  <item>
    	    <p>This module maintains a cache of CRLs.  CRLs can be
    	    added to the cache using the function <seealso
    	    marker="ssl:ssl_crl_cache#insert-1">ssl_crl_cache:insert/1</seealso>,
    	    and optionally automatically fetched through HTTP if the
    	    following argument is specified:</p>
	    
    	    <taglist>
    	      <tag><c>{http, timeout()}</c></tag>
    	      <item><p>
    		Enables fetching of CRLs specified as http URIs in<seealso
    		marker="public_key:public_key_records">X509 certificate extensions</seealso>.
    		Requires the OTP inets application.</p>
    	      </item>
    	    </taglist>
    	  </item>
	  
    	  <tag><c>ssl_crl_hash_dir</c></tag>
    	  <item>
    	    <p>This module makes use of a directory where CRLs are
    	    stored in files named by the hash of the issuer name.</p>

    	    <p>The file names consist of eight hexadecimal digits
    	    followed by <c>.rN</c>, where <c>N</c> is an integer,
    	    e.g. <c>1a2b3c4d.r0</c>.  For the first version of the
    	    CRL, <c>N</c> starts at zero, and for each new version,
    	    <c>N</c> is incremented by one.  The OpenSSL utility
    	    <c>c_rehash</c> creates symlinks according to this
    	    pattern.</p>

    	    <p>For a given hash value, this module finds all
    	    consecutive <c>.r*</c> files starting from zero, and those
    	    files taken together make up the revocation list.  CRL
    	    files whose <c>nextUpdate</c> fields are in the past, or
    	    that are issued by a different CA that happens to have the
    	    same name hash, are excluded.</p>
	    
    	    <p>The following argument is required:</p>

    	    <taglist>
    	      <tag><c>{dir, string()}</c></tag>
    	      <item><p>Specifies the directory in which the CRLs can be found.</p></item>
    	    </taglist>
    	  </item>
	  </taglist>
	</desc>
    </datatype>

    <datatype>
      <name name="root_fun"/>
      <desc>
	<code>
fun(Chain::[public_key:der_encoded()]) ->
	{trusted_ca, DerCert::public_key:der_encoded()} | unknown_ca}
	</code>
	<p>Claim an intermediate CA in the chain as trusted. TLS then
	performs <seealso
	marker="public_key:public_key#pkix_path_validation-3">public_key:pkix_path_validation/3</seealso>
	with the selected CA as trusted anchor and the rest of the chain.</p>
      </desc>
    </datatype>


    <datatype>
      <name name="protocol_versions"/>
      <desc><p>TLS protocol versions supported by started clients and servers.
      This option overrides the application environment option
      <c>protocol_version</c> and  <c>dtls_protocol_version</c>. If the environment option is not set, it defaults
      to all versions, except SSL-3.0, supported by the SSL application.
      See also <seealso marker="ssl:ssl_app">ssl(6).</seealso></p>
      </desc>
    </datatype>


      <datatype>
      <name name="custom_user_lookup"/>
      <desc><p>The lookup fun is to defined as follows:</p>

	<code>
fun(psk, PSKIdentity ::string(), UserState :: term()) ->
	{ok, SharedSecret :: binary()} | error;
fun(srp, Username :: string(), UserState :: term()) ->
	{ok, {SRPParams :: srp_param_type(), Salt :: binary(), DerivedKey :: binary()}} | error.
	</code>

	<p>For Pre-Shared Key (PSK) cipher suites, the lookup fun is
	called by the client and server to determine the shared
	secret. When called by the client, <c>PSKIdentity</c> is set to the
	hint presented by the server or to undefined. When called by the
	server, <c>PSKIdentity</c> is the identity presented by the client.</p>

	<p>For Secure Remote Password (SRP), the fun is only used by the server to
	obtain parameters that it uses to generate its session keys.
	<c>DerivedKey</c> is to be derived according to
	<url href="http://tools.ietf.org/html/rfc2945#section-3"> RFC 2945</url> and
	<url href="http://tools.ietf.org/html/rfc5054#section-2.4"> RFC 5054</url>:
	<c>crypto:sha([Salt, crypto:sha([Username, &lt;&lt;$:&gt;&gt;, Password])])</c>
	</p>
      </desc>
    </datatype>

    <datatype>
      <name name="session_id"/>
      <desc>
	<p>Identifies a TLS session.</p>
      </desc>
    </datatype>
    
    <datatype>    
      <name name="log_alert"/> 
      <desc><p>If set to <c>false</c>, error reports are not displayed.
      Deprecated in OTP 22, use {log_level, <seealso marker="#type-logging_level">logging_level()</seealso>} instead.</p> 
      </desc> 
    </datatype> 

    <datatype>    
      <name name="logging_level"/> 
      <desc><p>Specifies the log level for TLS/DTLS. At verbosity level <c>notice</c> and above error reports are
      displayed in TLS. The level <c>debug</c> triggers verbose logging of TLS protocol
      messages and logging of ignored alerts in DTLS.</p>
      </desc> 
    </datatype> 
    
    <datatype>    
    <name name="hibernate_after"/> 
    <desc><p>When an integer-value is specified, <c>TLS/DTLS-connection</c> 
    goes into hibernation after the specified number of milliseconds 
    of inactivity, thus reducing its memory footprint. When
    <c>undefined</c> is specified (this is the default), the process 
    never goes into hibernation.</p> 
    </desc> 
    </datatype> 

    <datatype>
      <name name="handshake_size"/>
      <desc>
    	<p>Integer (24 bits unsigned). Used to limit the size of
    	valid TLS handshake packets to avoid DoS attacks.
    	Defaults to 256*1024.</p>
      </desc>
    </datatype>
    
    <datatype>
      <name name="padding_check"/>
      <desc><p>Affects TLS-1.0 connections only.
      If set to <c>false</c>, it disables the block cipher padding check
      to be able to interoperate with legacy software.</p>
      <warning><p>Using <c>{padding_check, boolean()}</c> makes TLS
	vulnerable to the Poodle attack.</p></warning>
      </desc>
    </datatype>

    <datatype>
      <name name="beast_mitigation"/>
      <desc><p>Affects SSL-3.0 and TLS-1.0 connections only. Used to change the BEAST
       mitigation strategy to interoperate with legacy software.
       Defaults to <c>one_n_minus_one</c>.</p>

      <p><c>one_n_minus_one</c> - Perform 1/n-1 BEAST mitigation.</p>

      <p><c>zero_n</c> - Perform 0/n BEAST mitigation.</p>

      <p><c>disabled</c> - Disable BEAST mitigation.</p>

      <warning><p>Using <c>{beast_mitigation, disabled}</c> makes SSL-3.0 or TLS-1.0
        vulnerable to the BEAST attack.</p></warning>
      </desc>
    </datatype>
    

    <datatype_title>TLS/DTLS OPTION DESCRIPTIONS - CLIENT</datatype_title>
    
    <datatype>
      <name name="client_option"/>
    </datatype>
   
     <datatype>
      <name name="client_verify_type"/>
      <desc><p>In mode <c>verify_none</c> the default behavior is to allow
      all x509-path validation errors. See also option  <seealso marker="#type-custom_verify">verify_fun</seealso>.</p>
      </desc>
    </datatype>

    <datatype>
      <name name="client_reuse_session"/>
      <desc>
	<p>Reuses a specific session earlier saved with the option
	<c>{reuse_sessions, save} since OTP-21.3 </c>
      </p>
      </desc>
    </datatype>

    <datatype>
      <name name="client_reuse_sessions"/>
      <desc>
	<p>When <c>save</c> is specified a new connection will be negotiated
      and saved for later reuse. The session ID can be fetched with 
      <seealso marker="#connection_information-2">connection_information/2</seealso>
      and used with the client option <seealso marker="#type-client_reuse_session">reuse_session</seealso>
      The boolean value true specifies that if possible, automatized session reuse will
      be performed. If a new session is created, and is unique in regard 
      to previous stored sessions, it will be saved for possible later reuse. Since OTP-21.3</p>
      </desc>
    </datatype>

    <datatype>
      <name name="client_cacerts"/>
      <desc>
	<p>The DER-encoded trusted certificates. If this option
      is supplied it overrides option <c>cacertfile</c>.</p>
      </desc>
    </datatype>
    
    <datatype>
      <name name="client_cafile"/>
      <desc>
	<p>Path to a file containing PEM-encoded CA certificates. The CA
      certificates are used during server authentication and when building the
      client certificate chain.</p>
      </desc>
    </datatype>
    
    <datatype>
      <name name="client_alpn"/>
      <desc>
	<p>The list of protocols supported by the client to be sent to the
	server to be used for an Application-Layer Protocol Negotiation (ALPN).
	If the server supports ALPN then it will choose a protocol from this
	list; otherwise it will fail the connection with a "no_application_protocol"
	alert. A server that does not support ALPN will ignore this value.</p>
	
	<p>The list of protocols must not contain an empty binary.</p>
	
	<p>The negotiated protocol can be retrieved using the <c>negotiated_protocol/1</c> function.</p>
      </desc>
    </datatype>
    
   <datatype>
      <name name="client_preferred_next_protocols"/>
      <desc>
	<p>Indicates that the client is to try to perform Next Protocol
	Negotiation.</p>
	
	<p>If precedence is server, the negotiated protocol is the
	first protocol to be shown on the server advertised list, which is
	also on the client preference list.</p>
	
	<p>If precedence is client, the negotiated protocol is the
	first protocol to be shown on the client preference list, which is
	also on the server advertised list.</p>
	
	<p>If the client does not support any of the server advertised
	protocols or the server does not advertise any protocols, the
	client falls back to the first protocol in its list or to the
	default protocol (if a default is supplied). If the
	server does not support Next Protocol Negotiation, the
	connection terminates if no default protocol is supplied.</p>
      </desc>
   </datatype>
   
   <datatype>
      <name name="client_psk_identity"/>
      <desc>
	<p>Specifies the identity the client presents to the server.
	The matching secret is found by calling <c>user_lookup_fun</c></p>
      </desc>
      </datatype>

      <datatype>
	<name name="client_srp_identity"/>
	<desc>
	  <p>Specifies the username and password to use to authenticate
	  to the server.</p>
	</desc>
      </datatype>

      <datatype>
      <name name="sni"/>
      <desc>
	<p>Specify the hostname to be used in TLS Server Name Indication extension.
	If not specified it will default to the <c>Host</c> argument of <seealso marker="#connect-3">connect/[3,4]</seealso>
	unless it is of type inet:ipaddress().</p>
	<p>
	  The <c>HostName</c> will also be used in the hostname verification of the peer certificate using
	  <seealso marker="public_key:public_key#pkix_verify_hostname-2">public_key:pkix_verify_hostname/2</seealso>.
	</p>
	<p> The special value <c>disable</c> prevents the Server Name Indication extension from being sent and
	disables the hostname verification check
	<seealso marker="public_key:public_key#pkix_verify_hostname-2">public_key:pkix_verify_hostname/2</seealso> </p>
      </desc>
      </datatype>
         
      <datatype>
	<name name="customize_hostname_check"/>
	<desc>
	  <p> Customizes the hostname verification of the peer certificate, as different protocols that use
	  TLS such as HTTP or LDAP may want to do it differently, for possible options see
	  <seealso marker="public_key:public_key#pkix_verify_hostname-3">public_key:pkix_verify_hostname/3</seealso> </p>
	</desc>
      </datatype>
      
      <datatype>
	<name name="fallback"/>
	<desc>
	  <p> Send special cipher suite TLS_FALLBACK_SCSV to avoid undesired TLS version downgrade.
	  Defaults to false</p>
	  <warning><p>Note this option is not needed in normal TLS usage and should not be used
	  to implement new clients. But legacy clients that retries connections in the following manner</p>
	  
	  <p><c> ssl:connect(Host, Port, [...{versions, ['tlsv2', 'tlsv1.1', 'tlsv1', 'sslv3']}])</c></p>
	  <p><c>  ssl:connect(Host, Port, [...{versions, [tlsv1.1', 'tlsv1', 'sslv3']}, {fallback, true}])</c></p>
	  <p><c>  ssl:connect(Host, Port, [...{versions, ['tlsv1', 'sslv3']}, {fallback, true}]) </c></p>
	  <p><c>  ssl:connect(Host, Port, [...{versions, ['sslv3']}, {fallback, true}]) </c></p>
	  
	  <p>may use it to avoid undesired TLS version downgrade. Note that TLS_FALLBACK_SCSV must also
	  be supported by the server for the prevention to work.
	  </p></warning>
	</desc>
      </datatype>
      
      <datatype>
	<name name="client_signature_algs"/>
	<desc>
	  <p>In addition to the algorithms negotiated by the cipher
	suite used for key exchange, payload encryption, message
	authentication and pseudo random calculation, the TLS signature
	algorithm extension <url
	href="http://www.ietf.org/rfc/rfc5246.txt">Section 7.4.1.4.1 in RFC 5246</url> may be
	used, from TLS 1.2, to negotiate which signature algorithm to use during the
	TLS handshake. If no lower TLS versions than 1.2 are supported,
	the client will send a TLS signature algorithm extension
	with the algorithms specified by this option.
	Defaults to</p>
	
	<code>[
%% SHA2
{sha512, ecdsa},
{sha512, rsa},
{sha384, ecdsa},
{sha384, rsa},
{sha256, ecdsa},
{sha256, rsa},
{sha224, ecdsa},
{sha224, rsa},
%% SHA
{sha, ecdsa},
{sha, rsa},
{sha, dsa},
]</code>
<p>
	The algorithms should be in the preferred order.
	Selected signature algorithm can restrict which hash functions
	that may be selected. Default support for {md5, rsa} removed in ssl-8.0
	</p>
	</desc>
      </datatype>
      
      <datatype_title>TLS/DTLS OPTION DESCRIPTIONS - SERVER </datatype_title>


      <datatype>
	<name name="server_option"/>
      </datatype>
     
      <datatype>
	<name name="server_cacerts"/>
	<desc><p>The DER-encoded trusted certificates. If this option
      is supplied it overrides option <c>cacertfile</c>.</p>
	</desc>
      </datatype>
            
      <datatype>
	<name name="server_cafile"/>
	<desc><p>Path to a file containing PEM-encoded CA
	certificates. The CA certificates are used to build the server
	certificate chain and for client authentication. The CAs are
	also used in the list of acceptable client CAs passed to the
	client when a certificate is requested. Can be omitted if
	there is no need to verify the client and if there are no
	intermediate CAs for the server certificate.</p>
	</desc>
      </datatype>

      <datatype>
	<name name="dh_der"/>
	<desc><p>The DER-encoded Diffie-Hellman parameters. If
	specified, it overrides option <c>dhfile</c>.</p>
	</desc>
      </datatype>
    
      <datatype>
	<name name="dh_file"/>
	<desc><p>Path to a file containing PEM-encoded Diffie Hellman
	parameters to be used by the server if a cipher suite using
	Diffie Hellman key exchange is negotiated. If not specified,
	default parameters are used.</p>
	</desc>
      </datatype>

    <datatype>
	<name name="server_verify_type"/>
	<desc><p>A server only does x509-path validation in mode
	<c>verify_peer</c>, as it then sends a certificate request to
	the client (this message is not sent if the verify option is
	<c>verify_none</c>).  You can then also want to specify option
	<c>fail_if_no_peer_cert</c>. </p>
	</desc>
      </datatype>

      <datatype>
	<name name="fail_if_no_peer_cert"/>
	<desc><p>Used together with <c>{verify, verify_peer}</c> by an
	TLS/DTLS server.  If set to <c>true</c>, the server fails if
	the client does not have a certificate to send, that is, sends
	an empty certificate. If set to <c>false</c>, it fails only if
	the client sends an invalid certificate (an empty certificate
	is considered valid). Defaults to false.</p>
	</desc>
      </datatype>
      
      <datatype>
	<name name="server_reuse_sessions"/>
	<desc><p>The boolean value true specifies that the server will
	agree to reuse sessions. Setting it to false will result in an empty
	session table, that is no sessions will be reused. 
	See also option <seealso marker="#type-server_reuse_session">reuse_session</seealso>
      </p>
	</desc>
      </datatype>
      
      <datatype>
	<name name="server_reuse_session"/>
	<desc><p>Enables the TLS/DTLS server to have a local policy
	for deciding if a session is to be reused or not.  Meaningful
	only if <c>reuse_sessions</c> is set to <c>true</c>.
	<c>SuggestedSessionId</c> is a <c>binary()</c>,
	<c>PeerCert</c> is a DER-encoded certificate,
	<c>Compression</c> is an enumeration integer, and
	<c>CipherSuite</c> is of type <c>ciphersuite()</c>.</p>
	</desc>
      </datatype>

      <datatype>
	<name name="server_alpn"/>
	<desc>  
	  <p>Indicates the server will try to perform
	  Application-Layer Protocol Negotiation (ALPN).</p>
	  
	  <p>The list of protocols is in order of preference. The
	  protocol negotiated will be the first in the list that
	  matches one of the protocols advertised by the client. If no
	  protocol matches, the server will fail the connection with a
	  "no_application_protocol" alert.</p>
	  
	  <p>The negotiated protocol can be retrieved using the
	  <c>negotiated_protocol/1</c> function.</p>
	</desc>
      </datatype>
      
      <datatype>
	<name name="server_next_protocol"/>
	<desc><p>List of protocols to send to the client if the client
	indicates that it supports the Next Protocol extension. The
	client can select a protocol that is not on this list. The
	list of protocols must not contain an empty binary. If the
	server negotiates a Next Protocol, it can be accessed using
	the <c>negotiated_next_protocol/1</c> method.</p>
	</desc>
      </datatype>

      <datatype>
	<name name="server_psk_identity"/>
	<desc>
	  <p>Specifies the server identity hint, which the server presents to
	  the client.</p>
	</desc>
      </datatype>

      <datatype>
	<name name="honor_cipher_order"/>
	<desc>
	  <p>If set to <c>true</c>, use the server preference for cipher
	  selection. If set to <c>false</c> (the default), use the client
	  preference.</p>
	</desc>
      </datatype>

      <datatype>
	<name name="sni_hosts"/>
	<desc><p>If the server receives a SNI (Server Name Indication) from the client
	matching a host listed in the <c>sni_hosts</c> option, the specific options for
	that host will override previously specified options.
	
	The option <c>sni_fun</c>, and <c>sni_hosts</c> are mutually exclusive.</p>
	</desc>
      </datatype>

      <datatype>
	<name name="sni_fun"/>
	<desc>
	  <p>If the server receives a SNI (Server Name Indication)
	  from the client, the given function will be called to
	  retrieve <seealso marker="#type-server_option">[server_option()] </seealso> for the indicated server.
	  These options will be merged into predefined
	  <seealso marker="#type-server_option">[server_option()] </seealso> list.
	  
	  The function should be defined as:
          fun(ServerName :: string()) -> <seealso marker="#type-server_option">[server_option()] </seealso>
	  and can be specified as a fun or as named <c>fun module:function/1</c>
	  
	  The option <c>sni_fun</c>, and <c>sni_hosts</c> are mutually exclusive.</p>
	</desc>
      </datatype>

      <datatype>
	<name name="client_renegotiation"/>
	<desc><p>In protocols that support client-initiated
	renegotiation, the cost of resources of such an operation is
	higher for the server than the client. This can act as a
	vector for denial of service attacks. The SSL application
	already takes measures to counter-act such attempts, but
	client-initiated renegotiation can be strictly disabled by
	setting this option to <c>false</c>. The default value is
	<c>true</c>.  Note that disabling renegotiation can result in
	long-lived connections becoming unusable due to limits on the
	number of messages the underlying cipher suite can
	encipher.</p>
	</desc>
      </datatype>
      
      <datatype>
	<name name="honor_cipher_order"/>
	<desc><p>If true, use the server's preference for cipher
	selection. If false (the default), use the client's
	preference.</p>
	</desc>
      </datatype>

      <datatype>
	<name name="honor_ecc_order"/>
	<desc><p>If true, use the server's preference for ECC curve
	selection. If false (the default), use the client's
	preference.</p>
	</desc>
      </datatype>

      <datatype>
	<name name="server_signature_algs"/>
	<desc><p> The algorithms specified by this option will be the
	ones accepted by the server in a signature algorithm
	negotiation, introduced in TLS-1.2. The algorithms will also
	be offered to the client if a client certificate is
	requested. For more details see the <seealso
	marker="#type-client_signature_algs">corresponding client
	option</seealso>.
      </p>
	</desc>
      </datatype>
    </datatypes>

<!--
    ================================================================
    =  Function definitions                                        =
    ================================================================
-->

  <funcs>

    <func>
      <name since="OTP 20.3">append_cipher_suites(Deferred, Suites) -> ciphers() </name>
      <fsummary></fsummary>
      <type>
        <v>Deferred = <seealso marker="#type-ciphers">ciphers()</seealso> |
	<seealso marker="#type-cipher_filters">cipher_filters()</seealso></v>
	<v>Suites  =  <seealso marker="#type-ciphers">ciphers()</seealso></v>
      </type>
      <desc><p>Make <c>Deferred</c> suites become the least preferred
      suites, that is put them at the end of the cipher suite list
      <c>Suites</c> after removing them from <c>Suites</c> if
      present. <c>Deferred</c> may be a list of cipher suits or a
      list of filters in which case the filters are use on  <c>Suites</c> to
      extract the Deferred cipher list.</p> 
      </desc>
    </func>
    
    <func>
      <name since="OTP R14B">cipher_suites() -></name>
      <name since="OTP R14B">cipher_suites(Type) -> old_ciphers()</name>
      <fsummary>Returns a list of supported cipher suites.</fsummary>
      <type>
        <v>Type = erlang | openssl | all</v>
      </type>
      <desc>
	<p>Deprecated in OTP 21, use <seealso marker="#cipher_suites-2">cipher_suites/2</seealso> instead.</p>
      </desc>
    </func>
    
   <func>
      <name since="OTP 20.3">cipher_suites(Supported, Version) ->  ciphers()</name>
      <fsummary>Returns a list of all default or
      all supported cipher suites.</fsummary>
      <type>
        <v> Supported = default | all | anonymous </v>
	<v> Version = <seealso marker="#type-protocol_version">protocol_version() </seealso></v>
      </type>
      <desc><p>Returns all default or all supported (except anonymous),
      or all anonymous cipher suites for a
      TLS version</p>
    </desc>
    </func>

    <func>
      <name since="OTP 19.2">eccs() -></name>
      <name since="OTP 19.2">eccs(Version) -> NamedCurves</name>
      <fsummary>Returns a list of supported ECCs.</fsummary>

      <type>
	<v> Version = <seealso marker="#type-protocol_version">protocol_version() </seealso></v>
	<v> NamedCurves = <seealso marker="#type-named_curve">[named_curve()] </seealso></v>
	
      </type>
      
      <desc><p>Returns a list of supported ECCs. <c>eccs()</c>
      is equivalent to calling <c>eccs(Protocol)</c> with all
      supported protocols and then deduplicating the output.</p>
      </desc>
    </func>
    
    <func>
      <name since="OTP 17.5">clear_pem_cache() -> ok </name>
      <fsummary> Clears the pem cache</fsummary>

      <desc><p>PEM files, used by ssl API-functions, are cached. The
      cache is regularly checked to see if any cache entries should be
      invalidated, however this function provides a way to
      unconditionally clear the whole cache.
      </p>
      </desc>
    </func>
   
    <func>
      <name since="OTP R14B">connect(Socket, Options) -> </name>
      <name since="">connect(Socket, Options, Timeout) -> {ok, SslSocket} | {ok, SslSocket, Ext}
	| {error, Reason}</name>
      <fsummary>Upgrades a <c>gen_tcp</c>, or
	equivalent, connected socket to an TLS socket.</fsummary>
      <type>
	<v>Socket = <seealso marker="#type-socket"> socket() </seealso></v>
	<v>Options = <seealso marker="#type-client_option"> [client_option()] </seealso></v>
	<v>Timeout = timeout()</v>
	<v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
	<v>Ext = hello_extensions()</v>
	<v>Reason = closed | timeout | <seealso marker="#type-error_alert"> error_alert() </seealso></v>
      </type>
      <desc><p>Upgrades a <c>gen_tcp</c>, or equivalent,
	  connected socket to an TLS socket, that is, performs the
	  client-side TLS handshake.</p>
	  
	  <note><p>If the option <c>verify</c> is set to
	  <c>verify_peer</c> the option <c>server_name_indication</c>
	  shall also be specified, if it is not no Server Name
	  Indication extension will be sent, and <seealso
	  marker="public_key:public_key#pkix_verify_hostname-2">public_key:pkix_verify_hostname/2</seealso>
	  will be called with the IP-address of the connection as
	  <c>ReferenceID</c>, which is proably not what you want.</p>
	  </note>

	  <p> If the option <c>{handshake, hello}</c> is used the
	  handshake is paused after receiving the server hello message
	  and the success response is <c>{ok, SslSocket, Ext}</c>
	  instead of <c>{ok, SslSocket}</c>. Thereafter the handshake
	  is continued or canceled by calling <seealso
	  marker="#handshake_continue-3">
	  <c>handshake_continue/3</c></seealso> or <seealso
	  marker="#handshake_cancel-1"><c>handshake_cancel/1</c></seealso>.
	  </p>
	  
	  <p> If the option <c>active</c> is set to <c>once</c> or <c>true</c> the
	  process owning the sslsocket will receive messages of type 
	  <seealso marker="#type-active_msgs"> active_msgs() </seealso>
	  </p>
    </desc>
    </func>

    <func>
      <name since="">connect(Host, Port, Options) -></name>
      <name since="">connect(Host, Port, Options, Timeout) ->
	  {ok, SslSocket}| {ok, SslSocket, Ext} | {error, Reason}</name>
      <fsummary>Opens an TLS/DTLS connection to <c>Host</c>, <c>Port</c>.</fsummary>
      <type>
	  <v>Host =<seealso marker="#type-host"> host() </seealso> </v>
	  <v>Port = <seealso marker="kernel:inet#type-port_number">inet:port_number()</seealso></v>
	  <v>Options = <seealso marker="#type-client_option"> [client_option()]</seealso></v>
	  <v>Timeout = timeout()</v>
	  <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
	  <v>Reason = closed | timeout | <seealso marker="#type-error_alert"> error_alert() </seealso></v>
      </type>
      <desc><p>Opens an TLS/DTLS connection to <c>Host</c>, <c>Port</c>.</p>
      
      <p> When the option <c>verify</c> is set to <c>verify_peer</c> the check 
      <seealso marker="public_key:public_key#pkix_verify_hostname-2">public_key:pkix_verify_hostname/2</seealso> 
      will be performed in addition to the usual x509-path validation checks. If the check fails the error {bad_cert, hostname_check_failed} will
      be propagated to the path validation fun <seealso marker="#type-custom_verify">verify_fun</seealso>, where it is possible to do customized
      checks by using the full possibilities of the <seealso marker="public_key:public_key#pkix_verify_hostname-3">public_key:pkix_verify_hostname/3</seealso> API.
      
      When the option <c>server_name_indication</c> is provided, its value (the DNS name) will be used as <c>ReferenceID</c>
      to <seealso marker="public_key:public_key#pkix_verify_hostname-2">public_key:pkix_verify_hostname/2</seealso>.
      When no <c>server_name_indication</c> option is given, the <c>Host</c> argument will be used as
      Server Name Indication extension. The <c>Host</c> argument will also be used for the
      <seealso marker="public_key:public_key#pkix_verify_hostname-2">public_key:pkix_verify_hostname/2</seealso> check and if the <c>Host</c>
      argument is an <c>inet:ip_address()</c> the <c>ReferenceID</c> used for the check will be <c>{ip, Host}</c> otherwise
      <c>dns_id</c> will be assumed with a fallback to <c>ip</c> if that fails. </p>
      <note><p>According to good practices certificates should not use IP-addresses as "server names". It would
      be very surprising if this happen outside a closed network. </p></note> 


      <p> If the option <c>{handshake, hello}</c> is used the
	  handshake is paused after receiving the server hello message
	  and the success response is <c>{ok, SslSocket, Ext}</c>
	  instead of <c>{ok, SslSocket}</c>. Thereafter the handshake is continued or
	  canceled by calling <seealso marker="#handshake_continue-3">
	  <c>handshake_continue/3</c></seealso> or <seealso
	  marker="#handshake_cancel-1"><c>handshake_cancel/1</c></seealso>.
	  </p>

	  <p> If the option <c>active</c> is set to <c>once</c> or <c>true</c> the
	  process owning the sslsocket will receive messages of type 
	  <seealso marker="#type-active_msgs"> active_msgs() </seealso>
	  </p>
      </desc>
    </func>

    <func>
      <name since="">close(SslSocket) -> ok | {error, Reason}</name>
      <fsummary>Closes an TLS/DTLS connection.</fsummary>
      <type>
	  <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
	  <v>Reason = term()</v>
      </type>
      <desc><p>Closes an TLS/DTLS connection.</p>
      </desc>
    </func>

    <func>
      <name since="OTP 18.1">close(SslSocket, How) -> ok | {ok, port()} | {error, Reason}</name>
      <fsummary>Closes an TLS connection.</fsummary>
      <type>
	  <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
	  <v>How =  timeout() | {NewController::pid(), timeout()} </v>
	  <v>Reason = term()</v>
      </type>
      <desc><p>Closes or downgrades an TLS connection. In the latter case the transport
      connection will be handed over to the <c>NewController</c> process after receiving
      the TLS close alert from the peer. The returned transport socket will have
      the following options set: <c>[{active, false}, {packet, 0}, {mode, binary}]</c></p>
      </desc>
    </func>
    
    <func>
      <name since="">controlling_process(SslSocket, NewOwner) ->
	ok | {error, Reason}</name>
	<fsummary>Assigns a new controlling process to the
	TLS/DTLS socket.</fsummary>
	<type>
	  <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
	  <v>NewOwner = pid()</v>
	  <v>Reason = term()</v>
	</type>
	<desc><p>Assigns a new controlling process to the SSL socket. A
	controlling process is the owner of an SSL socket, and receives
	all messages from the socket.</p>
      </desc>
    </func>

    <func>
      <name since="OTP 18.0">connection_information(SslSocket) ->
        {ok, Result} |  {error, Reason} </name>
      <fsummary>Returns all the connection information.
      </fsummary>
      <type>
	<v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
        <v>Item = protocol | selected_cipher_suite | sni_hostname | ecc | session_id | atom()</v>
	<d>Meaningful atoms, not specified above, are the ssl option names.</d>
	<v>Result = [{Item::atom(), Value::term()}]</v>
        <v>Reason = term()</v>
      </type>
      <desc><p>Returns the most relevant information about the connection, ssl options that
      are undefined will be filtered out. Note that values that affect the security of the
      connection will only be returned if explicitly requested by connection_information/2.</p>
      <note><p>The legacy <c>Item = cipher_suite</c> is still supported
        and returns the cipher suite on its (undocumented) legacy format.
        It should be replaced by <c>selected_cipher_suite</c>.</p></note>
      </desc>
    </func>

    <func>
      <name since="OTP 18.0">connection_information(SslSocket, Items) ->
        {ok, Result} |  {error, Reason} </name>
      <fsummary>Returns the requested connection information.
      </fsummary>
      <type>
	<v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
	<v>Items = [Item]</v>
	<v>Item = protocol | cipher_suite | sni_hostname | ecc | session_id | client_random 
          | server_random  | master_secret | atom()</v>
	<d>Note that client_random, server_random  and master_secret are values
        that affect the security of connection. Meaningful atoms, not specified above, are the ssl option names.</d>
	<v>Result = [{Item::atom(), Value::term()}]</v>
        <v>Reason = term()</v>
      </type>
      <desc><p>Returns the requested information items about the connection,
      if they are defined.</p>
      <note><p>If only undefined options are requested the
      resulting list can be empty.</p></note>
      </desc>
    </func>

      <func>
      <name since="OTP 20.3">filter_cipher_suites(Suites, Filters) -> ciphers()</name>
      <fsummary></fsummary>
      <type>
	<v> Suites =  <seealso marker="#type-ciphers"> ciphers() </seealso></v>
        <v> Filters =  <seealso marker="#type-cipher_filters"> cipher_filters() </seealso></v>
      </type>
      <desc><p>Removes cipher suites if any of the filter functions
      returns false for any part of the cipher suite. This function
      also calls default filter functions to make sure the cipher
      suites are supported by crypto. If no filter function is supplied for some
      part the default behaviour is fun(Algorithm) -> true.</p>
    </desc>
    </func>
    
    <func>
      <name since="">format_error(Reason) -> string()</name>
      <fsummary>Returns an error string.</fsummary>
      <type>
        <v>Reason = term()</v>
      </type>
      <desc>
        <p>Presents the error returned by an SSL function as a printable string.</p>
      </desc>
    </func>
   
    <func>
      <name since="">getopts(SslSocket, OptionNames) ->
	{ok, [socketoption()]} | {error, Reason}</name>
      <fsummary>Gets the values of the specified options.</fsummary>
      <type>
	<v>Socket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
	<v>OptionNames = [atom()]</v>
      </type>
      <desc>
	<p>Gets the values of the specified socket options.
	</p>
      </desc>
    </func>

    <func>
      <name since="OTP 19.0">getstat(SslSocket) ->
        {ok, OptionValues} | {error, inet:posix()}</name>
      <name since="OTP 19.0">getstat(SslSocket, OptionNames) ->
        {ok, OptionValues} | {error, inet:posix()}</name>
      <fsummary>Get one or more statistic options for a socket</fsummary>
      <type>
    <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
    <v>OptionNames = [atom()]</v>
        <v>OptionValues = [{inet:stat_option(), integer()}]</v>
      </type>
      <desc>
        <p>Gets one or more statistic options for the underlying TCP socket.</p>
        <p>See inet:getstat/2 for statistic options description.</p>
      </desc>
    </func>

    <func>
      <name since="OTP 21.0">handshake(HsSocket) -> </name>
      <name since="OTP 21.0">handshake(HsSocket, Timeout) -> {ok, SslSocket}  | {error, Reason}</name>
      <fsummary>Performs server-side SSL/TLS handshake.</fsummary>
      <type>
        <v>HsSocket = SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
        <v>Timeout = timeout()</v>
        <v>Reason = closed | timeout | <seealso marker="#type-error_alert"> error_alert() </seealso></v>
      </type>
      <desc>
        <p>Performs the SSL/TLS/DTLS server-side handshake.</p>
	<p>Returns a new TLS/DTLS socket if the handshake is successful.</p>

	<p> If the option <c>active</c> is set to <c>once</c> or <c>true</c> the
	process owning the sslsocket will receive messages of type 
	<seealso marker="#type-active_msgs"> active_msgs() </seealso>
	</p>	
      </desc>
    </func>

    <func>
      <name since="OTP 21.0">handshake(Socket, Options) -> </name>
      <name since="OTP 21.0">handshake(Socket, Options, Timeout) -> {ok, SslSocket} | {ok, SslSocket, Ext} | {error, Reason}</name>
      <fsummary>Performs server-side SSL/TLS/DTLS handshake.</fsummary>
      <type>
        <v>Socket = socket() |  <seealso marker="#type-sslsocket"> socket() </seealso> </v>
	<v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso> </v>
	<v>Ext = hello_extensions()</v>
	<v>Options = <seealso marker="#type-server_option"> [server_option()] </seealso>  </v>
        <v>Timeout = timeout()</v>
        <v>Reason = closed | timeout | <seealso marker="#type-error_alert"> error_alert() </seealso></v>
      </type>
      <desc>
        <p>If <c>Socket</c> is a ordinary <c>socket()</c>: upgrades a <c>gen_tcp</c>,
	or equivalent, socket to an SSL socket, that is, performs
        the SSL/TLS server-side handshake and returns a TLS socket.</p>

	<warning><p>The <c>Socket</c> shall be in passive mode ({active,
	false}) before calling this function or else the behavior of this function
	is undefined.
	</p></warning>
	
	<p>If <c>Socket</c> is an 
	<seealso marker="#type-sslsocket"> sslsocket() </seealso>: provides extra SSL/TLS/DTLS
	options to those specified in
	<seealso marker="#listen-2">listen/2 </seealso> and then performs
	the SSL/TLS/DTLS handshake. Returns a new TLS/DTLS socket if the handshake is successful.</p>
		
	<p>
	  If option <c>{handshake, hello}</c> is specified the handshake is
	  paused after receiving the client hello message and the
	  success response is <c>{ok, SslSocket, Ext}</c>  instead of <c>{ok,
	  SslSocket}</c>. Thereafter the handshake is continued or
	  canceled by calling <seealso marker="#handshake_continue-3">
	  <c>handshake_continue/3</c></seealso> or <seealso
	  marker="#handshake_cancel-1"><c>handshake_cancel/1</c></seealso>.
	</p>
	
	<p> If the option <c>active</c> is set to <c>once</c> or <c>true</c> the
	process owning the sslsocket will receive messages of type 
	<seealso marker="#type-active_msgs"> active_msgs() </seealso>
	</p>
	
      </desc>
    </func>

    <func>
      <name since="OTP 21.0">handshake_cancel(SslSocket) -> ok </name>
      <fsummary>Cancel handshake with a fatal alert</fsummary>
      <type>
        <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
      </type>
      <desc>
        <p>Cancel the handshake with a fatal <c>USER_CANCELED</c> alert.</p>
      </desc>
    </func>

    <func>
      <name since="OTP 21.0">handshake_continue(HsSocket, Options) -> {ok, SslSocket} | {error, Reason}</name>
      <name since="OTP 21.0">handshake_continue(HsSocket, Options, Timeout) -> {ok, SslSocket} | {error, Reason}</name>
      <fsummary>Continue the SSL/TLS handshake.</fsummary>
      <type>
	<v>HsSocket = SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
	<v>Options = <seealso marker="#type-tls_option"> tls_option() </seealso> </v>
	<v>Timeout = timeout()</v>
	<v>Reason = closed | timeout | <seealso marker="#type-error_alert"> error_alert() </seealso></v>
      </type>
      <desc>
        <p>Continue the SSL/TLS handshake possiby with new, additional or changed options.</p>
      </desc>
    </func>
    
    <func>
      <name since="">listen(Port, Options) ->
	{ok, ListenSocket} | {error, Reason}</name>
      <fsummary>Creates an SSL listen socket.</fsummary>
      <type>
	<v>Port = <seealso marker="kernel:inet#type-port_number">inet:port_number()</seealso></v>
	<v>Options = <seealso marker="#type-server_option"> [server_option()] </seealso></v>
	<v>ListenSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
      </type>
      <desc>
	<p>Creates an SSL listen socket.</p>
      </desc>
    </func>

    <func>
      <name since="OTP 18.0">negotiated_protocol(SslSocket) -> {ok, Protocol} | {error, protocol_not_negotiated}</name>
      <fsummary>Returns the protocol negotiated through ALPN or NPN extensions.</fsummary>
      <type>
        <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
        <v>Protocol = binary()</v>
      </type>
      <desc>
        <p>
          Returns the protocol negotiated through ALPN or NPN extensions.
        </p>
      </desc>
    </func>
    
    <func>
      <name since="">peercert(SslSocket) -> {ok, Cert} | {error, Reason}</name>
      <fsummary>Returns the peer certificate.</fsummary>
     <type>
        <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
        <v>Cert = binary()</v>
      </type>
      <desc>
        <p>The peer certificate is returned as a DER-encoded binary.
	  The certificate can be decoded with
	  <seealso marker="public_key:public_key#pkix_decode_cert-2">public_key:pkix_decode_cert/2</seealso>
	</p>
      </desc>
    </func>

    <func>
      <name since="">peername(SslSocket) -> {ok, {Address, Port}} |
	{error, Reason}</name>
      <fsummary>Returns the peer address and port.</fsummary>
      <type>
        <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
        <v>Address = ipaddress()</v>
	<v>Port = <seealso marker="kernel:inet#type-port_number">inet:port_number()</seealso></v>
      </type>
      <desc>
        <p>Returns the address and port number of the peer.</p>
      </desc>
    </func>
    
      <func>
      <name since="OTP 20.3">prepend_cipher_suites(Preferred, Suites) -> ciphers()</name>
      <fsummary></fsummary>
      <type>
	<v>Preferred = <seealso marker="#type-ciphers">ciphers()</seealso> |
	<seealso marker="#type-cipher_filters">cipher_filters()</seealso></v>
	<v>Suites  =  <seealso marker="#type-ciphers">ciphers()</seealso></v>
      </type>
      <desc><p>Make <c>Preferred</c> suites become the most preferred
      suites that is put them at the head of the cipher suite list
      <c>Suites</c> after removing them from <c>Suites</c> if
      present. <c>Preferred</c> may be a list of cipher suits or a
      list of filters in which case the filters are use on <c>Suites</c> to
      extract the preferred cipher list. </p>
      </desc>
    </func>

    <func>
      <name since="OTP R15B01">prf(Socket, Secret, Label, Seed, WantedLength) -> {ok, binary()} | {error, reason()}</name>
      <fsummary>Uses a session Pseudo-Random Function to generate key material.</fsummary>
      <type>
	<v>Socket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
	<v>Secret = binary() | master_secret</v>
	<v>Label = binary()</v>
	<v>Seed = [binary() | prf_random()]</v>
	<v>WantedLength = non_neg_integer()</v>
      </type>
      <desc>
        <p>Uses the Pseudo-Random Function (PRF) of a TLS session to generate
	  extra key material. It either takes user-generated values for
	  <c>Secret</c> and <c>Seed</c> or atoms directing it to use a specific
	  value from the session security parameters.</p>
        <p>Can only be used with TLS/DTLS connections; <c>{error, undefined}</c>
	  is returned for SSLv3 connections.</p>
      </desc>
    </func>
    
    <func>
      <name since="">recv(SslSocket, Length) -> </name>
      <name since="">recv(SslSocket, Length, Timeout) -> {ok, Data} | {error,
	Reason}</name>
      <fsummary>Receives data on a socket.</fsummary>
      <type>
        <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
        <v>Length = integer()</v>
        <v>Timeout = timeout()</v>
        <v>Data = [char()] | binary()</v>
      </type>
      <desc>
        <p>Receives a packet from a socket in passive
          mode. A closed socket is indicated by return value
          <c>{error, closed}</c>.</p>
        <p>Argument <c>Length</c> is meaningful only when
          the socket is in mode <c>raw</c> and denotes the number of
          bytes to read. If <c>Length</c> = 0, all available bytes are
          returned. If <c>Length</c> &gt; 0, exactly <c>Length</c>
          bytes are returned, or an error; possibly discarding less
          than <c>Length</c> bytes of data when the socket gets closed
          from the other side.</p>
        <p>Optional argument <c>Timeout</c> specifies a time-out in
          milliseconds. The default value is <c>infinity</c>.</p>
      </desc>
    </func>
    
    <func>
      <name since="OTP R14B">renegotiate(SslSocket) -> ok | {error, Reason}</name>
      <fsummary>Initiates a new handshake.</fsummary>
      <type>
	<v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
      </type>
      <desc><p>Initiates a new handshake. A notable return value is
      <c>{error, renegotiation_rejected}</c> indicating that the peer
      refused to go through with the renegotiation, but the connection
      is still active using the previously negotiated session.</p>
      </desc>
    </func>
    
    <func>
      <name since="">send(SslSocket, Data) -> ok | {error, Reason}</name>
      <fsummary>Writes data to a socket.</fsummary>
      <type>
        <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
        <v>Data = iodata()</v>
      </type>
      <desc>
        <p>Writes <c>Data</c> to <c>Socket</c>.</p>
        <p>A notable return value is <c>{error, closed}</c> indicating that
          the socket is closed.</p>
      </desc>
    </func>

    <func>
      <name since="">setopts(SslSocket, Options) -> ok | {error, Reason}</name>
      <fsummary>Sets socket options.</fsummary>
      <type>
        <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
        <v>Options = <seealso marker="#type-socket_option"> [socket_option()] </seealso></v>
      </type>
      <desc>
        <p>Sets options according to <c>Options</c> for socket
          <c>Socket</c>.</p>
      </desc>
    </func>

    <func>
      <name since="OTP 22.0">set_log_level(Level) -> ok | {error, Reason}</name>
      <fsummary>Sets log level for the SSL application.</fsummary>
      <type>
        <v>Level = atom()</v>
      </type>
      <desc>
        <p>Sets log level for the SSL application.</p>
      </desc>
    </func>

    <func>
      <name since="OTP R14B">shutdown(SslSocket, How) -> ok | {error, Reason}</name>
      <fsummary>Immediately closes a socket.</fsummary>
      <type>
        <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
        <v>How = read | write | read_write</v>
        <v>Reason = reason()</v>
      </type>
      <desc>
        <p>Immediately closes a socket in one or two directions.</p>
        <p><c>How == write</c> means closing the socket for writing,
          reading from it is still possible.</p>
        <p>To be able to handle that the peer has done a shutdown on
          the write side, option <c>{exit_on_close, false}</c>
          is useful.</p>
      </desc>
    </func>
    
    <func>
      <name since="">ssl_accept(SslSocket) -> </name>
      <name since="">ssl_accept(SslSocket, Timeout) -> ok | {error, Reason}</name>
      <fsummary>Performs server-side SSL/TLS handshake.</fsummary>
      <type>
        <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
        <v>Timeout = timeout()</v>
        <v>Reason = closed | timeout | <seealso marker="#type-error_alert"> error_alert() </seealso></v>
      </type>
      <desc>
	<p>Deprecated in OTP 21, use <seealso marker="#handshake-1">handshake/[1,2]</seealso> instead.</p>
	<note><p>handshake/[1,2] always returns a new socket.</p></note>
      </desc>
    </func>

    <func>
      <name since="">ssl_accept(Socket, Options) -> </name>
      <name since="OTP R14B">ssl_accept(Socket, Options, Timeout) -> {ok, Socket} | ok | {error, Reason}</name>
      <fsummary>Performs server-side SSL/TLS/DTLS handshake.</fsummary>
      <type>
        <v>Socket = socket() |  <seealso marker="#type-sslsocket"> sslsocket() </seealso> </v>
	<v>Options =  <seealso marker="#type-server_option"> [server_option()] </seealso> </v>
        <v>Timeout = timeout()</v>
        <v>Reason = closed | timeout | <seealso marker="#type-error_alert"> error_alert() </seealso></v>
      </type>
      <desc>
	<p>Deprecated in OTP 21, use  <seealso marker="#handshake-3">handshake/[2,3]</seealso> instead.</p>
	<note><p>handshake/[2,3] always returns a new socket.</p></note>
      </desc>
    </func>
    
    <func>
      <name since="">sockname(SslSocket) -> {ok, {Address, Port}} |
	{error, Reason}</name>
      <fsummary>Returns the local address and port.</fsummary>
      <type>
        <v>SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
	<v>Address = <seealso marker="#type-ip_address">ip_address()</seealso></v>
	<v>Port = <seealso marker="kernel:inet#type-port_number">inet:port_number()</seealso></v>
      </type>
      <desc>
        <p>Returns the local address and port number of socket
          <c>Socket</c>.</p>
      </desc>
    </func>
    
    <func>
      <name since="OTP R14B">start() -> </name>
      <name since="OTP R14B">start(Type) -> ok | {error, Reason}</name>
      <fsummary>Starts the SSL application.</fsummary>
      <type>
        <v>Type = permanent | transient | temporary</v>
      </type>
      <desc>
        <p>Starts the SSL application. Default type
          is <c>temporary</c>.</p>
      </desc>
    </func>

    <func>
      <name since="OTP R14B">stop() -> ok </name>
      <fsummary>Stops the SSL application.</fsummary>
      <desc>
        <p>Stops the SSL application.</p>
      </desc>
    </func>

    <func>
      <name since="OTP 21.0">suite_to_str(CipherSuite) -> String</name>
      <fsummary>Returns the string representation of a cipher suite.</fsummary>
      <type>
        <v>CipherSuite =  <seealso marker="#type-erl_cipher_suite"> erl_cipher_suite() </seealso></v>
	<v>String = string()</v>
      </type>
      <desc>
        <p>Returns the string representation of a cipher suite.</p>
      </desc>
    </func>

    <func>
      <name since="">transport_accept(ListenSocket) -></name>
      <name since="">transport_accept(ListenSocket, Timeout) ->
	{ok, SslSocket} | {error, Reason}</name>
      <fsummary>Accepts an incoming connection and
	prepares for <c>ssl_accept</c>.</fsummary>
      <type>
        <v>ListenSocket = SslSocket =  <seealso marker="#type-sslsocket"> sslsocket() </seealso></v>
        <v>Timeout = timeout()</v>
        <v>Reason = reason()</v>
      </type>
      <desc>
        <p>Accepts an incoming connection request on a listen socket.
	<c>ListenSocket</c> must be a socket returned from
	<seealso marker="#listen-2"> listen/2</seealso>.
	The socket returned is to be passed to
	<seealso marker="#handshake-2"> handshake/[2,3]</seealso>
	to complete handshaking, that is,
	establishing the SSL/TLS/DTLS connection.</p>
        <warning>
          <p>Most API functions require that the TLS/DTLS 
          connection is established to work as expected.
	  </p>
        </warning>
        <p>The accepted socket inherits the options set for
	<c>ListenSocket</c> in
	<seealso marker="#listen-2"> listen/2</seealso>.</p>
	<p>The default
	value for <c>Timeout</c> is <c>infinity</c>. If
	<c>Timeout</c> is specified and no connection is accepted
	within the given time, <c>{error, timeout}</c> is
	returned.</p>
      </desc>
    </func>
    
    <func>
      <name since="OTP R14B">versions() -> [versions_info()]</name>
      <fsummary>Returns version information relevant for the
	SSL application.</fsummary>
      <type>
	<v>versions_info() = {app_vsn, string()} | {supported | available, [ssl_tls_protocol()]} |
	{supported_dtls | available_dtls, [dtls_protocol()]}	</v>
      </type>
      <desc>
	<p>Returns version information relevant for the SSL
	application.</p>
	<taglist>
	  <tag><c>app_vsn</c></tag>
	  <item>The application version of the SSL application.</item>

	  <tag><c>supported</c></tag>
	  <item>SSL/TLS versions supported by default.
	  Overridden by a version option on
	  <seealso marker="#connect-2"> connect/[2,3,4]</seealso>,
	  <seealso marker="#listen-2"> listen/2</seealso>, and <seealso
	  marker="#ssl_accept-2">ssl_accept/[1,2,3]</seealso>.
	  For the negotiated SSL/TLS version, see <seealso
	  marker="#connection_information-1">connection_information/1
	  </seealso>.</item>
	  
	  <tag><c>supported_dtls</c></tag>
	  <item>DTLS versions supported by default.
	  Overridden by a version option on
	  <seealso marker="#connect-2"> connect/[2,3,4]</seealso>,
	  <seealso marker="#listen-2"> listen/2</seealso>, and <seealso
	  marker="#ssl_accept-2">ssl_accept/[1,2,3]</seealso>.
	  For the negotiated DTLS version, see <seealso
	  marker="#connection_information-1">connection_information/1
	  </seealso>.</item>

	  <tag><c>available</c></tag>
	  <item>All SSL/TLS versions supported by the SSL application.
	  TLS 1.2 requires sufficient support from the Crypto
	  application.</item>

	  <tag><c>available_dtls</c></tag>
	  <item>All DTLS versions supported by the SSL application.
	  DTLS 1.2 requires sufficient support from the Crypto
	  application.</item>
	  
	</taglist>
      </desc>
    </func>
   
  </funcs> 

  <section>
    <title>SEE ALSO</title>
    <p><seealso marker="kernel:inet">inet(3)</seealso> and
      <seealso marker="kernel:gen_tcp">gen_tcp(3)</seealso>
      <seealso marker="kernel:gen_udp">gen_udp(3)</seealso>
    </p>
  </section>

</erlref>