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>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>
    
  <section>
    <title>DATA TYPES</title>
    <p>The following data types are used in the functions for SSL/TLS/DTLS:</p>

    <taglist>

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

      <tag><c>option() =</c></tag>
      <item><p><c>socketoption() | ssl_option() | transport_option()</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>,
      <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></item>

      <tag><marker id="type-ssloption"/><c>ssl_option() =</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()} |
	#{algorithm := rsa | dss | ecdsa,
	engine := crypto:engine_ref(), key_id := crypto:key_id(), password => crypto:password()}</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>| {customize_hostname_check, list()}</c></p>
	<p><c>| {sni_hosts, [{hostname(), [ssl_option()]}]}</c></p>
	<p><c>| {sni_fun, SNIfun::fun()}</c></p>
      </item>
      
      <tag><c>transport_option() =</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> for TLS
      and <c>{gen_udp, udp, udp_closed, udp_error}</c> for DTLS. Can be used
      to customize the transport layer. 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>
      <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() - DNS hostname</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_version() =</c></tag>
      <item><p><c> ssl_tls_protocol() |  dtls_protocol() </c></p></item>
      
      <item><p><c>sslv3 | tlsv1 | 'tlsv1.1' | 'tlsv1.2'</c></p></item>

      <tag><marker id="type-protocol"/><c> dtls_protocol() =</c></tag>
      <item><p><c>'dtlsv1' | 'dtlsv1.2'</c></p></item>

      <tag><c>ciphers() =</c></tag>
      <item><p><c>= [ciphersuite()]</c></p>
      <p>Tuples and string formats accepted by versions
      before ssl-8.2.4 will be converted for backwards compatibility</p></item>

      <tag><c>ciphersuite() =</c></tag>
      <item><p><c>
      #{key_exchange := key_exchange(),
        cipher := cipher(),
        mac := MAC::hash() | aead,
        prf := PRF::hash() | default_prf} </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 | chacha20_poly1305</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>cipher_filters() =</c></tag>
      <item><p><c> [{key_exchange | cipher | mac | prf, algo_filter()}])</c></p></item>

      <tag><c>algo_filter() =</c></tag>
      <item><p>fun(key_exchange() | cipher() | hash() | aead | default_prf) -> true | false </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()) -> [ssl_option()]</c></p></item>

      <tag><c>named_curve() =</c></tag>
      <item><p><c>sect571r1 | sect571k1 | secp521r1 | brainpoolP512r1
       | sect409k1 | sect409r1 | brainpoolP384r1 | secp384r1
       | sect283k1 | sect283r1 | brainpoolP256r1 | secp256k1 | secp256r1
       | sect239k1 | sect233k1 | sect233r1 | secp224k1 | secp224r1
       | sect193r1 | sect193r2 | secp192k1 | secp192r1 | sect163k1
       | sect163r1 | sect163r2 | secp160k1 | secp160r1 | secp160r2</c></p></item>

       <tag><c>hello_extensions() =</c></tag>
       <item><p><c>#{renegotiation_info => 
                   signature_algs => [{hash(), ecsda| rsa| dsa}] | undefined
		   alpn => binary() | undefined,
		   next_protocol_negotiation,
		   srp => string() | undefined,
		   ec_point_formats ,
		   elliptic_curves = [oid] | undefined
		   sni = string()} 
       }</c></p></item>
      

    </taglist>
  </section>

  <section>
    <title>TLS/DTLS 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>{protocol, tls | dtls}</c></tag>
      <item><p>Choose TLS or DTLS protocol for the transport layer security.
      Defaults to <c>tls</c> Introduced in OTP 20, DTLS support is considered
      experimental in this release. Other transports than UDP are not yet supported.</p></item>

      <tag><c>{handshake, hello | full}</c></tag>
      <item><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></item>
      
      <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>
	<marker id="key_option_def"/>
	<c>{key, {'RSAPrivateKey'| 'DSAPrivateKey' | 'ECPrivateKey'
      |'PrivateKeyInfo', public_key:der_encoded()} |  #{algorithm := rsa | dss | ecdsa,
	engine := crypto:engine_ref(), key_id :=  crypto:key_id(), password => crypto:password()}</c></tag>
      <item><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></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>{eccs, [named_curve()]}</c></tag>
      <item><p> Allows to specify the order of preference for named curves
      and to restrict their usage when using a cipher suite supporting them.
      </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>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>
      </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><marker id="verify_fun"/><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/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>
      </item>

      <tag><c>{crl_check, boolean() | peer | best_effort }</c></tag>
      <item>
	<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 can not 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>
      </item>

      <tag><c>{crl_cache, {Module :: atom(), {DbHandle :: internal | term(), Args :: list()}}}</c></tag>
      <item>
	<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>

	  <tag><c>max_handshake_size</c></tag>
	  <item>
	    <p>Integer (24 bits unsigned). Used to limit the size of
	    valid TLS handshake packets to avoid DoS attacks.
	    Defaults to 256*1024.</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_version()]}</c></tag>

      <item><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></item>

      <tag><c>{hibernate_after, integer()|undefined}</c></tag>
      <item><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></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>
      <warning><p>Using <c>{padding_check, boolean()}</c> makes TLS
	vulnerable to the Poodle attack.</p></warning>
      </item>

      

      <tag><c>{beast_mitigation, one_n_minus_one | zero_n | disabled}</c></tag>
      <item><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 or TLS
        vulnerable to the BEAST attack.</p></warning>
      </item>
      </taglist>

  </section>
  
  <section>
    <title>TLS/DTLS 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 :: hostname()}</c></tag>
      <item><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>
      </item>
      <tag><c>{server_name_indication, disable}</c></tag>
	<item>
          <p> 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>
	</item>

	<tag><c>{customize_hostname_check, Options::list()}</c></tag>
	<item>
          <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>
	</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>
      <tag><marker id="client_signature_algs"/><c>{signature_algs, [{hash(), ecdsa | rsa | dsa}]}</c></tag>
	<item>
	<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>
	</item>
      </taglist>
   </section>
   
  <section>
    <title>TLS/DTLS 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 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>
      </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 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></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(), [ssl_option()]}]}</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>[ssl_option()]</c> for the indicated server.
      These options will be merged into predefined <c>[ssl_option()]</c>.

      The function should be defined as:
        <c>fun(ServerName :: string()) -> [ssl_option()]</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>
      
      <tag><c>{honor_ecc_order, boolean()}</c></tag>
      <item>If true, use the server's preference for ECC curve selection. If false
      (the default), use the client's preference.
      </item>

      <tag><c>{signature_algs, [{hash(), ecdsa | rsa | dsa}]}</c></tag>
      <item><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="#client_signature_algs">corresponding client option</seealso>.
      </p> </item>

    </taglist>
  </section>
  
  <section>
    <title>General</title>
      
    <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:</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>append_cipher_suites(Deferred, Suites) -> ciphers() </name>
      <fsummary></fsummary>
      <type>
           <v>Deferred = ciphers() | cipher_filters() </v>
	   <v>Suites  = ciphers() </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>cipher_suites() -></name>
      <name>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>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 = protocol_version() </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>eccs() -></name>
      <name>eccs(protocol_version()) -> [named_curve()]</name>
      <fsummary>Returns a list of supported ECCs.</fsummary>

      <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>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} | {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 = socket()</v>
	<v>SslOptions = [{handshake, hello| full} | ssl_option()]</v>
	<v>Timeout = integer() | infinity</v>
	<v>SslSocket = sslsocket()</v>
	<v>Ext = hello_extensions()</v>
	<v>Reason = term()</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>
	  
    </desc>
    </func>

    <func>
      <name>connect(Host, Port, Options) -></name>
      <name>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 = 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 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="#verify_fun">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>
      </desc>
    </func>

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

    <func>
      <name>close(SslSocket, How) -> ok | {ok, port()} | {error, Reason}</name>
      <fsummary>Closes an TLS 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 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>controlling_process(SslSocket, NewOwner) ->
	ok | {error, Reason}</name>
	<fsummary>Assigns a new controlling process to the
	TLS/DTLS 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>SslSocket = sslsocket()</v>
        <v>Item = protocol | 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>
      </desc>
    </func>

    <func>
      <name>connection_information(SslSocket, Items) ->
        {ok, Result} |  {error, Reason} </name>
      <fsummary>Returns the requested connection information.
      </fsummary>
      <type>
	<v>SslSocket = sslsocket()</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>filter_cipher_suites(Suites, Filters) -> ciphers()</name>
      <fsummary></fsummary>
      <type>
	<v> Suites = ciphers()</v>
        <v> Filters = cipher_filters()</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>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(SslSocket, 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>getstat(SslSocket) ->
        {ok, OptionValues} | {error, inet:posix()}</name>
      <name>getstat(SslSocket, OptionNames) ->
        {ok, OptionValues} | {error, inet:posix()}</name>
      <fsummary>Get one or more statistic options for a socket</fsummary>
      <type>
    <v>SslSocket = sslsocket()</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>handshake(HsSocket) -> </name>
      <name>handshake(HsSocket, Timeout) -> {ok, SslSocket}  | {error, Reason}</name>
      <fsummary>Performs server-side SSL/TLS handshake.</fsummary>
      <type>
        <v>HsSocket = SslSocket = sslsocket()</v>
        <v>Timeout = integer()</v>
        <v>Reason = term()</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>
      </desc>
    </func>

    <func>
      <name>handshake(Socket, SslOptions) -> </name>
      <name>handshake(Socket, SslOptions, Timeout) -> {ok, SslSocket} | {ok, SslSocket, Ext} | {error, Reason}</name>
      <fsummary>Performs server-side SSL/TLS/DTLS handshake.</fsummary>
      <type>
        <v>Socket = socket() | sslsocket() </v>
	<v>SslSocket = sslsocket() </v>
	<v>Ext = hello_extensions()</v>
	<v>SslOptions = [{handshake, hello| full} | ssl_option()]</v>
        <v>Timeout = integer()</v>
        <v>Reason = term()</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 <c>sslsocket()</c>: 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>
      </desc>
    </func>

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

    <func>
      <name>handshake_continue(HsSocket, SSLOptions) -> {ok, SslSocket} | {error, Reason}</name>
      <name>handshake_continue(HsSocket, SSLOptions, Timeout) -> {ok, SslSocket} | {error, Reason}</name>
      <fsummary>Continue the SSL/TLS handshake.</fsummary>
      <type>
	<v>HsSocket = SslSocket = sslsocket()</v>
	<v>SslOptions = [ssl_option()]</v>
	<v>Timeout = integer()</v>
	<v>Reason = term()</v>
      </type>
      <desc>
        <p>Continue the SSL/TLS handshake possiby with new, additional or changed 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(SslSocket) -> {ok, Protocol} | {error, protocol_not_negotiated}</name>
      <fsummary>Returns the protocol negotiated through ALPN or NPN extensions.</fsummary>
      <type>
        <v>SslSocket = sslsocket()</v>
        <v>Protocol = binary()</v>
      </type>
      <desc>
        <p>
          Returns the protocol negotiated through ALPN or NPN extensions.
        </p>
      </desc>
    </func>
    
    <func>
      <name>peercert(SslSocket) -> {ok, Cert} | {error, Reason}</name>
      <fsummary>Returns the peer certificate.</fsummary>
     <type>
        <v>SslSocket = 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
	  <seealso marker="public_key:public_key#pkix_decode_cert-2">public_key:pkix_decode_cert/2</seealso>
	</p>
      </desc>
    </func>

    <func>
      <name>peername(SslSocket) -> {ok, {Address, Port}} |
	{error, Reason}</name>
      <fsummary>Returns the peer address and port.</fsummary>
      <type>
        <v>SslSocket = 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>prepend_cipher_suites(Preferred, Suites) -> ciphers()</name>
      <fsummary></fsummary>
      <type>
        <v>Preferred = ciphers() | cipher_filters() </v>
	<v>Suites = ciphers() </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>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/DTLS connections; <c>{error, undefined}</c>
	  is returned for SSLv3 connections.</p>
      </desc>
    </func>
    
    <func>
      <name>recv(SslSocket, Length) -> </name>
      <name>recv(SslSocket, Length, Timeout) -> {ok, Data} | {error,
	Reason}</name>
      <fsummary>Receives data on a socket.</fsummary>
      <type>
        <v>SslSocket = 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(SslSocket) -> ok | {error, Reason}</name>
      <fsummary>Initiates a new handshake.</fsummary>
      <type>
	<v>SslSocket = 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(SslSocket, Data) -> ok | {error, Reason}</name>
      <fsummary>Writes data to a socket.</fsummary>
      <type>
        <v>SslSocket = 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(SslSocket, Options) -> ok | {error, Reason}</name>
      <fsummary>Sets socket options.</fsummary>
      <type>
        <v>SslSocket = 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(SslSocket, How) -> ok | {error, Reason}</name>
      <fsummary>Immediately closes a socket.</fsummary>
      <type>
        <v>SslSocket = 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(SslSocket) -> </name>
      <name>ssl_accept(SslSocket, Timeout) -> ok | {error, Reason}</name>
      <fsummary>Performs server-side SSL/TLS handshake.</fsummary>
      <type>
        <v>SslSocket = sslsocket()</v>
        <v>Timeout = integer()</v>
        <v>Reason = term()</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>ssl_accept(Socket, SslOptions) -> </name>
      <name>ssl_accept(Socket, SslOptions, Timeout) -> {ok, Socket} | ok | {error, Reason}</name>
      <fsummary>Performs server-side SSL/TLS/DTLS handshake.</fsummary>
      <type>
        <v>Socket = socket() | sslsocket() </v>
	<v>SslOptions = [ssl_option()]</v>
        <v>Timeout = integer()</v>
        <v>Reason = term()</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>sockname(SslSocket) -> {ok, {Address, Port}} |
	{error, Reason}</name>
      <fsummary>Returns the local address and port.</fsummary>
      <type>
        <v>SslSocket = 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>suite_to_str(CipherSuite) -> String</name>
      <fsummary>Returns the string representation of a cipher suite.</fsummary>
      <type>
        <v>CipherSuite = erl_cipher_suite()</v>
	<v>String = string()</v>
      </type>
      <desc>
        <p>Returns the string representation of a cipher suite.</p>
      </desc>
    </func>

    <func>
      <name>transport_accept(ListenSocket) -></name>
      <name>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 = 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"> 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>The socket returned can only be used with
	  <seealso marker="#handshake-2"> handshake/[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"> 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, [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>