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>2015</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>ssl</module>
  <modulesummary>Interface Functions for Secure Socket Layer</modulesummary>
  <description>
    <p>
      This module contains interface functions for the SSL/TLS protocol.
      For detailed information about the supported standards see 
      <seealso marker="ssl_app">ssl(6)</seealso>.
    </p>
  </description>
    
  <section>
    <title>DATA TYPES</title>
    <p>The following data types are used in the functions for SSL:</p>

    <taglist>

      <tag><c>boolean() =</c></tag>
      <item><p><c>true | false</c></p></item>

      <tag><c>option() =</c></tag>
      <item><p><c>socketoption() | ssloption() | transportoption()</c></p>
      </item>

      <tag><c>socketoption() =</c></tag>
      <item><p><c>proplists:property()</c></p>
      <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> and
      <seealso marker="kernel:gen_tcp">gen_tcp(3)</seealso> manual pages
      in Kernel.</p></item>

      <tag><marker id="type-ssloption"/><c>ssloption() =</c></tag>
      <item>
	<p><c>{verify, verify_type()}</c></p>
	<p><c>| {verify_fun, {fun(), term()}}</c></p>
	<p><c>| {fail_if_no_peer_cert, boolean()}</c></p>
	<p><c>| {depth, integer()}</c></p>
	<p><c>| {cert, public_key:der_encoded()}</c></p>
	<p><c>| {certfile, path()}</c></p>
	<p><c>| {key, {'RSAPrivateKey'| 'DSAPrivateKey' | 'ECPrivateKey' 
	| 'PrivateKeyInfo', public_key:der_encoded()}}</c></p>
	<p><c>| {keyfile, path()}</c></p>
	<p><c>| {password, string()}</c></p>
	<p><c>| {cacerts, [public_key:der_encoded()]}</c></p>
	<p><c>| {cacertfile, path()}</c></p>
	<p><c>| {dh, public_key:der_encoded()}</c></p>
	<p><c>| {dhfile, path()}</c></p>
	<p><c>| {ciphers, ciphers()}</c></p>
	<p><c>| {user_lookup_fun, {fun(), term()}}, {psk_identity, string()},
	{srp_identity, {string(), string()}}</c></p>
	<p><c>| {reuse_sessions, boolean()}</c></p>
	<p><c>| {reuse_session, fun()} {next_protocols_advertised, [binary()]}</c></p>
	<p><c>| {client_preferred_next_protocols, {client | server,
	[binary()]} | {client | server, [binary()], binary()}}</c></p>
	<p><c>| {log_alert, boolean()}</c></p>
	<p><c>| {server_name_indication, hostname() | disable}</c></p>
	<p><c>| {sni_hosts, [{hostname(), ssloptions()}]}</c></p>
	<p><c>| {sni_fun, SNIfun::fun()}</c></p>
      </item>
      
      <tag><c>transportoption() =</c></tag>
      <item><p><c>{cb_info, {CallbackModule::atom(), DataTag::atom(),

      ClosedTag::atom(), ErrTag:atom()}}</c></p>
      <p>Defaults to <c>{gen_tcp, tcp, tcp_closed, tcp_error}</c>. Can be used
      to customize the transport layer. 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.</p>
      <taglist>
	<tag><c>CallbackModule =</c></tag>
	<item><p><c>atom()</c></p></item>
	<tag><c>DataTag =</c></tag>
	<item><p><c>atom()</c></p>
	<p>Used in socket data message.</p></item>
	<tag><c>ClosedTag =</c></tag>
	<item><p><c>atom()</c></p>
	<p>Used in socket close message.</p></item>
      </taglist>
      </item>

      <tag><c>verify_type() =</c></tag>
      <item><p><c>verify_none | verify_peer</c></p></item>

      <tag><c>path() =</c></tag>
      <item><p><c>string()</c></p>
      <p>Represents a file path.</p></item>

      <tag><c>public_key:der_encoded() =</c></tag>
      <item><p><c>binary()</c></p>
      <p>ASN.1 DER-encoded entity as an Erlang binary.</p></item>

      <tag><c>host() =</c></tag>
      <item><p><c>hostname() | ipaddress()</c></p></item>

      <tag><c>hostname() =</c></tag>
      <item><p><c>string()</c></p></item>

      <tag><c>ip_address() =</c></tag>
      <item><p><c>{N1,N2,N3,N4} % IPv4 | {K1,K2,K3,K4,K5,K6,K7,K8} % IPv6
      </c></p></item>

      <tag><c>sslsocket() =</c></tag>
      <item><p>opaque()</p></item>

      <tag><marker id="type-protocol"/><c>protocol() =</c></tag>
      <item><p><c>sslv3 | tlsv1 | 'tlsv1.1' | 'tlsv1.2'</c></p></item>

      <tag><c>ciphers() =</c></tag>
      <item><p><c>= [ciphersuite()] | string()</c></p>
      <p>According to old API.</p></item>

      <tag><c>ciphersuite() =</c></tag>

      <item><p><c>{key_exchange(), cipher(), MAC::hash()} |
      {key_exchange(), cipher(), MAC::hash(), PRF::hash()}</c></p></item>

      <tag><c>key_exchange()=</c></tag>
      <item><p><c>rsa | dhe_dss | dhe_rsa | dh_anon | psk | dhe_psk
      | rsa_psk | srp_anon | srp_dss | srp_rsa | ecdh_anon | ecdh_ecdsa
      | ecdhe_ecdsa | ecdh_rsa | ecdhe_rsa</c></p></item>

      <tag><c>cipher() =</c></tag>
      <item><p><c>rc4_128 | des_cbc | '3des_ede_cbc'
      | aes_128_cbc | aes_256_cbc | aes_128_gcm | aes_256_gcm</c></p></item>

      <tag><c>hash() =</c></tag>
      <item><p><c>md5 | sha | sha224 | sha256 | sha348 | sha512</c></p></item>

      <tag><c>prf_random() =</c></tag>
      <item><p><c>client_random | server_random</c></p></item>

      <tag><c>srp_param_type() =</c></tag>
      <item><p><c>srp_1024 | srp_1536 | srp_2048 | srp_3072
      | srp_4096 | srp_6144 | srp_8192</c></p></item>

      <tag><c>SNIfun::fun()</c></tag>
      <item><p><c>= fun(ServerName :: string()) -> ssloptions()</c></p></item>

    </taglist>
  </section>

  <section>
    <title>SSL OPTION DESCRIPTIONS - COMMON for SERVER and CLIENT</title>

    <p>The following options have the same meaning in the client and 
    the server:</p>
    
    <taglist>

      <tag><c>{cert, public_key:der_encoded()}</c></tag>
      <item><p>The DER-encoded users certificate. If this option
      is supplied, it overrides option <c>certfile</c>.</p></item>
      
      <tag><c>{certfile, path()}</c></tag>
      <item><p>Path to a file containing the user certificate.</p></item>
      
      <tag><c>{key, {'RSAPrivateKey'| 'DSAPrivateKey' | 'ECPrivateKey'
      |'PrivateKeyInfo', public_key:der_encoded()}}</c></tag>
      <item><p>The DER-encoded user's private key. If this option 
      is supplied, it overrides option <c>keyfile</c>.</p></item>
      
      <tag><c>{keyfile, path()}</c></tag>
      <item><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></item>

      <tag><c>{password, string()}</c></tag>
      <item><p>String containing the user's password. Only used if the 
      private keyfile is password-protected.</p></item>

      <tag><c>{ciphers, ciphers()}</c></tag>
      <item><p>Supported cipher suites. The function
      <c>cipher_suites/0</c> can be used to find all ciphers that are
      supported by default. <c>cipher_suites(all)</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 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></item>

      <tag><c>{secure_renegotiate, boolean()}</c></tag>
      <item><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>false</c>, 
      that is, secure renegotiation is 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>
      </item>

      <tag><c>{depth, integer()}</c></tag>
      <item><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></item>

      <tag><c>{verify_fun, {Verifyfun :: fun(), InitialUserState ::
      term()}}</c></tag>
      <item><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/SSL 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/SSL 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></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, _}, 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>
      </item>

      <tag><c>{crl_check, boolean() | peer | best_effort }</c></tag>
      <item>
	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 false.
	
	<p><c>peer</c> - check is only performed on
	the peer certificate.</p>

	<p><c>best_effort</c> - if certificate revocation status can not be determined
	it will be accepted as valid.</p>

	<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>
      </item>

      <tag><c>{crl_cache, {Module :: atom(), {DbHandle :: internal | term(), Args :: list()}}}</c></tag>
      <item>
	<p>Module defaults to ssl_crl_cache with <c> DbHandle </c> internal and an
	empty argument list. The following arguments may be specified for the internal cache.</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>{partial_chain, fun(Chain::[DerCert]) -> {trusted_ca, DerCert} |
      unknown_ca }</c></tag>
      <item><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></item>

      <tag><c>{versions, [protocol()]}</c></tag>
      <item><p>TLS protocol versions supported by started clients and servers.
      This option overrides the application environment option
      <c>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></item>

      <tag><c>{hibernate_after, integer()|undefined}</c></tag>
      <item><p>When an integer-value is specified, <c>ssl_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></item>

      <tag><c>{user_lookup_fun, {Lookupfun :: fun(), UserState :: term()}}</c></tag>
      <item><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>
      </item>

      <tag><c>{padding_check, boolean()}</c></tag>
      <item><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></item>

    </taglist>

        <warning><p>Using <c>{padding_check, boolean()}</c> makes TLS
	vulnerable to the Poodle attack.</p></warning>

  </section>
  
  <section>
    <title>SSL OPTION DESCRIPTIONS - CLIENT SIDE</title>

    <p>The following options are client-specific or have a slightly different
    meaning in the client than in the server:</p>

    <taglist>

      <tag><c>{verify, verify_type()}</c></tag>
      <item><p>In mode <c>verify_none</c> the default behavior is to allow
      all x509-path validation errors. See also option <c>verify_fun</c>.</p>
      </item>

      <tag><c>{reuse_sessions, boolean()}</c></tag>
      <item><p>Specifies if the client is to try to reuse sessions
      when possible.</p></item>

      <tag><c>{cacerts, [public_key:der_encoded()]}</c></tag>
      <item><p>The DER-encoded trusted certificates. If this option
      is supplied it overrides option <c>cacertfile</c>.</p></item>
      
      <tag><c>{cacertfile, path()}</c></tag>
      <item><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>
    </item>

      <tag><c>{alpn_advertised_protocols, [binary()]}</c></tag>
      <item>
      <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>
      </item>

      <tag><c>{client_preferred_next_protocols, {Precedence :: server | client, ClientPrefs :: [binary()]}}</c><br/>
      <c>{client_preferred_next_protocols, {Precedence :: server | client, ClientPrefs :: [binary()], Default :: binary()}}</c></tag>
	   <item>
	   <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>
	   </item>

      <tag><c>{psk_identity, string()}</c></tag>
      <item><p>Specifies the identity the client presents to the server.
      The matching secret is found by calling <c>user_lookup_fun</c>.</p>
      </item>

      <tag><c>{srp_identity, {Username :: string(), Password :: string()}
      </c></tag>
      <item><p>Specifies the username and password to use to authenticate
      to the server.</p></item>

      <tag><c>{server_name_indication, hostname()}</c></tag>
      <item><p>Can be specified when upgrading a TCP socket to a TLS
        socket to use the TLS Server Name Indication extension.</p></item>

	<tag><c>{server_name_indication, disable}</c></tag>
      <item>
        <p>When starting a TLS connection without upgrade, the Server Name
        Indication extension is sent if possible. This option can be
        used to disable that behavior.</p>
      </item>
      <tag><c>{fallback, boolean()}</c></tag>
      <item>
	<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>
      </item>
    </taglist>
   </section>

  <section>
    <title>SSL OPTION DESCRIPTIONS - SERVER SIDE</title>

    <p>The following options are server-specific or have a slightly different
    meaning in the server than in the client:</p>

    <taglist>

      <tag><c>{cacerts, [public_key:der_encoded()]}</c></tag>
      <item><p>The DER-encoded trusted certificates. If this option
      is supplied it overrides option <c>cacertfile</c>.</p></item>
      
      <tag><c>{cacertfile, path()}</c></tag>
      <item><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></item>
  
      <tag><c>{dh, public_key:der_encoded()}</c></tag>
      <item><p>The DER-encoded Diffie-Hellman parameters. If specified,
      it overrides option <c>dhfile</c>.</p></item>

      <tag><c>{dhfile, path()}</c></tag>
      <item><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></item>

      <tag><c>{verify, verify_type()}</c></tag>
      <item><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></item>

      <tag><c>{fail_if_no_peer_cert, boolean()}</c></tag>
      <item><p>Used together with <c>{verify, verify_peer}</c> by an SSL 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>
      </item>

      <tag><c>{reuse_sessions, boolean()}</c></tag>
      <item><p>Specifies if the server is to agree to reuse sessions
      when requested by the clients. See also option <c>reuse_session</c>.
      </p></item>

      <tag><c>{reuse_session, fun(SuggestedSessionId,
      PeerCert, Compression, CipherSuite) -> boolean()}</c></tag>
      <item><p>Enables the SSL 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></item>

      <tag><c>{alpn_preferred_protocols, [binary()]}</c></tag>
      <item>
      <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>
      </item>

      <tag><c>{next_protocols_advertised, Protocols :: [binary()]}</c></tag>
      <item><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></item>

      <tag><c>{psk_identity, string()}</c></tag>
      <item><p>Specifies the server identity hint, which the server presents to
      the client.</p></item>

      <tag><c>{log_alert, boolean()}</c></tag>
      <item><p>If set to <c>false</c>, error reports are not displayed.</p></item>

      <tag><c>{honor_cipher_order, boolean()}</c></tag>
      <item><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></item>

      <tag><c>{sni_hosts, [{hostname(), ssloptions()}]}</c></tag>
      <item><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></item>

      <tag><c>{sni_fun, SNIfun::fun()}</c></tag>
      <item><p>If the server receives a SNI (Server Name Indication) from the client,
      the given function will be called to retrieve <c>ssloptions()</c> for the indicated server.
      These options will be merged into predefined <c>ssloptions()</c>.

      The function should be defined as:
        <c>fun(ServerName :: string()) -> ssloptions()</c>
      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></item>

      <tag><c>{client_renegotiation, boolean()}</c></tag>
      <item>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.
      </item>

      <tag><c>{honor_cipher_order, boolean()}</c></tag>
      <item>If true, use the server's preference for cipher selection. If false
      (the default), use the client's preference.
      </item>
    </taglist>
  </section>
  
  <section>
    <title>General</title>
      
    <p>When an SSL socket is in active mode (the default), data from the
      socket is delivered to the owner of the socket in the form of
      messages:</p>

    <list type="bulleted">
      <item><p><c>{ssl, Socket, Data}</c></p></item>
      <item><p><c>{ssl_closed, Socket}</c></p></item>
      <item><p><c>{ssl_error, Socket, Reason}</c></p></item>
    </list>

    <p>A <c>Timeout</c> argument specifies a time-out in milliseconds. The
      default value for argument <c>Timeout</c> is <c>infinity</c>.</p>
  </section>
  
  <funcs>
    <func>
      <name>cipher_suites() -></name>
      <name>cipher_suites(Type) -> ciphers()</name>
      <fsummary>Returns a list of supported cipher suites.</fsummary>
      <type>
        <v>Type = erlang | openssl | all</v>
      </type>
      <desc><p>Returns a list of supported cipher suites.
	<c>cipher_suites()</c> is equivalent to <c>cipher_suites(erlang).</c>
	Type <c>openssl</c> is provided for backwards compatibility with the
	old SSL, which used OpenSSL. <c>cipher_suites(all)</c> returns
	all available cipher suites. The cipher suites not present
	in <c>cipher_suites(erlang)</c> but included in
	<c>cipher_suites(all)</c> are not used unless explicitly configured
	by the user.</p>
    </desc>
    </func>

    <func>
      <name>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>connect(Socket, SslOptions) -> </name>
      <name>connect(Socket, SslOptions, Timeout) -> {ok, SslSocket}
	| {error, Reason}</name>
      <fsummary>Upgrades a <c>gen_tcp</c>, or
	equivalent, connected socket to an SSL socket.</fsummary>
      <type>
	<v>Socket = socket()</v>
	<v>SslOptions = [ssloption()]</v>
	<v>Timeout = integer() | infinity</v>
	<v>SslSocket = sslsocket()</v>
	<v>Reason = term()</v>
      </type>
      <desc><p>Upgrades a <c>gen_tcp</c>, or equivalent,
	  connected socket to an SSL socket, that is, performs the
	  client-side ssl handshake.</p>
    </desc>
    </func>

    <func>
      <name>connect(Host, Port, Options) -></name>
      <name>connect(Host, Port, Options, Timeout) ->
	  {ok, SslSocket} | {error, Reason}</name>
      <fsummary>Opens an SSL connection to <c>Host</c>, <c>Port</c>.</fsummary>
      <type>
	  <v>Host = host()</v>
	  <v>Port = integer()</v>
	  <v>Options = [option()]</v>
	  <v>Timeout = integer() | infinity</v>
	  <v>SslSocket = sslsocket()</v>
	  <v>Reason = term()</v>
      </type>
      <desc><p>Opens an SSL connection to <c>Host</c>, <c>Port</c>.</p></desc>
    </func>

    <func>
      <name>close(SslSocket) -> ok | {error, Reason}</name>
      <fsummary>Closes an SSL connection.</fsummary>
      <type>
	  <v>SslSocket = sslsocket()</v>
	  <v>Reason = term()</v>
      </type>
      <desc><p>Closes an SSL connection.</p>
      </desc>
    </func>

    <func>
      <name>close(SslSocket, How) -> ok | {ok, port()} | {error, Reason}</name>
      <fsummary>Closes an SSL connection.</fsummary>
      <type>
	  <v>SslSocket = sslsocket()</v>
	  <v>How =  timeout() | {NewController::pid(), timeout()} </v>
	  <v>Reason = term()</v>
      </type>
      <desc><p>Closes or downgrades an SSL 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>controlling_process(SslSocket, NewOwner) ->
	ok | {error, Reason}</name>
	<fsummary>Assigns a new controlling process to the
	SSL socket.</fsummary>
	<type>
	  <v>SslSocket = sslsocket()</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>connection_information(SslSocket) ->
        {ok, Result} |  {error, Reason} </name>
      <fsummary>Returns all the connection information.
      </fsummary>
      <type>
        <v>Item = protocol | cipher_suite | sni_hostname | 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 all relevant information about the connection, ssl options that
      are undefined will be filtered out.</p>
      </desc>
    </func>

    <func>
      <name>connection_information(SslSocket, Items) ->
        {ok, Result} |  {error, Reason} </name>
      <fsummary>Returns the requested connection information.
      </fsummary>
      <type>
	<v>Items = [Item]</v>
	<v>Item = protocol | cipher_suite | sni_hostname | 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 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>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>getopts(Socket, OptionNames) ->
	{ok, [socketoption()]} | {error, Reason}</name>
      <fsummary>Gets the values of the specified options.</fsummary>
      <type>
	<v>Socket = sslsocket()</v>
	<v>OptionNames = [atom()]</v>
      </type>
      <desc>
	<p>Gets the values of the specified socket options.
	</p>
      </desc>
    </func>

    <func>
      <name>listen(Port, Options) ->
	{ok, ListenSocket} | {error, Reason}</name>
      <fsummary>Creates an SSL listen socket.</fsummary>
      <type>
	<v>Port = integer()</v>
	<v>Options = options()</v>
	<v>ListenSocket = sslsocket()</v>
      </type>
      <desc>
	<p>Creates an SSL listen socket.</p>
      </desc>
    </func>

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

    <func>
      <name>peername(Socket) -> {ok, {Address, Port}} |
	{error, Reason}</name>
      <fsummary>Returns the peer address and port.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
        <v>Address = ipaddress()</v>
        <v>Port = integer()</v>
      </type>
      <desc>
        <p>Returns the address and port number of the peer.</p>
      </desc>
    </func>

    <func>
      <name>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 = sslsocket()</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 connections; <c>{error, undefined}</c>
	  is returned for SSLv3 connections.</p>
      </desc>
    </func>
    
    <func>
      <name>recv(Socket, Length) -> </name>
      <name>recv(Socket, Length, Timeout) -> {ok, Data} | {error,
	Reason}</name>
      <fsummary>Receives data on a socket.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
        <v>Length = integer()</v>
        <v>Timeout = integer()</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>renegotiate(Socket) -> ok | {error, Reason}</name>
      <fsummary>Initiates a new handshake.</fsummary>
      <type>
	<v>Socket = sslsocket()</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>send(Socket, Data) -> ok | {error, Reason}</name>
      <fsummary>Writes data to a socket.</fsummary>
      <type>
        <v>Socket = sslsocket()</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>setopts(Socket, Options) -> ok | {error, Reason}</name>
      <fsummary>Sets socket options.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
        <v>Options = [socketoption]()</v>
      </type>
      <desc>
        <p>Sets options according to <c>Options</c> for socket
          <c>Socket</c>.</p>
      </desc>
    </func>

    <func>
      <name>shutdown(Socket, How) -> ok | {error, Reason}</name>
      <fsummary>Immediately closes a socket.</fsummary>
      <type>
        <v>Socket = sslsocket()</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>ssl_accept(Socket) -> </name>
      <name>ssl_accept(Socket, Timeout) -> ok | {error, Reason}</name>
      <fsummary>Performs server-side SSL/TLS handshake.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
        <v>Timeout = integer()</v>
        <v>Reason = term()</v>
      </type>
      <desc>
        <p>Performs the SSL/TLS server-side handshake.</p>
	<p><c>Socket</c> is a socket as returned by
	<seealso marker="#transport_accept-2">ssl:transport_accept/[1,2]</seealso>
	</p>
      </desc>
    </func>

    <func>
      <name>ssl_accept(Socket, SslOptions) -> </name>
      <name>ssl_accept(Socket, SslOptions, Timeout) -> {ok, Socket} | ok | {error, Reason}</name>
      <fsummary>Performs server-side SSL/TLS handshake.</fsummary>
      <type>
        <v>Socket = socket() | sslsocket() </v>
	<v>SslOptions = ssloptions()</v>
        <v>Timeout = integer()</v>
        <v>Reason = term()</v>
      </type>
      <desc>
        <p>If <c>Socket</c> is a <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 the SSL socket.</p>
	
	<warning><p>The listen socket is to be in mode <c>{active, false}</c>
	before telling the client that the server is ready to upgrade
	by calling this function, else the upgrade succeeds or does not
	succeed depending on timing.</p></warning>
	
	<p>If <c>Socket</c> is an <c>sslsocket()</c>: provides extra SSL/TLS
	options to those specified in
	<seealso marker="#listen-2">ssl:listen/2 </seealso> and then performs
	the SSL/TLS handshake.
	</p>
      </desc>
    </func>
    
    <func>
      <name>sockname(Socket) -> {ok, {Address, Port}} |
	{error, Reason}</name>
      <fsummary>Returns the local address and port.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
        <v>Address = ipaddress()</v>
        <v>Port = integer()</v>
      </type>
      <desc>
        <p>Returns the local address and port number of socket
          <c>Socket</c>.</p>
      </desc>
    </func>
    
    <func>
      <name>start() -> </name>
      <name>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>stop() -> ok </name>
      <fsummary>Stops the SSL application.</fsummary>
      <desc>
        <p>Stops the SSL application.</p>
      </desc>
    </func>

    <func>
      <name>transport_accept(ListenSocket) -></name>
      <name>transport_accept(ListenSocket, Timeout) ->
	{ok, NewSocket} | {error, Reason}</name>
      <fsummary>Accepts an incoming connection and
	prepares for <c>ssl_accept</c>.</fsummary>
      <type>
        <v>ListenSocket = NewSocket = sslsocket()</v>
        <v>Timeout = integer()</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"> ssl:listen/2</seealso>.
	The socket returned is to be passed to
	<seealso marker="#ssl_accept-2"> ssl:ssl_accept[2,3]</seealso>
	to complete handshaking, that is,
	establishing the SSL/TLS connection.</p>
        <warning>
          <p>The socket returned can only be used with
	  <seealso marker="#ssl_accept-2"> ssl:ssl_accept[2,3]</seealso>.
	  No traffic can be sent or received before that call.</p>
        </warning>
        <p>The accepted socket inherits the options set for
	<c>ListenSocket</c> in
	<seealso marker="#listen-2"> ssl: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>versions() -> [versions_info()]</name>
      <fsummary>Returns version information relevant for the
	SSL application.</fsummary>
      <type>
	<v>versions_info() = {app_vsn, string()} | {supported | available, [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>TLS/SSL 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 TLS/SSL version, see <seealso
	  marker="#connection_information-1">ssl:connection_information/1
	  </seealso>.</item>
  
	  <tag><c>available</c></tag>
	  <item>All TLS/SSL versions supported by the SSL application.
	  TLS 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>
    </p>
  </section>

</erlref>