ssl

19992015 Ericsson AB. All Rights Reserved. The contents of this file are subject to the Erlang Public License, Version 1.1, (the "License"); you may not use this file except in compliance with the License. You should have received a copy of the Erlang Public License along with this software. If not, it can be retrieved online at http://www.erlang.org/. Software distributed under the License is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License for the specific language governing rights and limitations under the License. ssl ssl.xml

ssl Interface Functions for Secure Socket Layer

This module contains interface functions for the SSL.

SSL ssl requires the crypto and public_key applications. Supported SSL/TLS-versions are SSL-3.0, TLS-1.0, TLS-1.1, and TLS-1.2. For security reasons SSL-2.0 is not supported. For security reasons SSL-3.0 is no longer supported by default, but can be configured. Ephemeral Diffie-Hellman cipher suites are supported, but not Diffie Hellman Certificates cipher suites. Elliptic Curve cipher suites are supported if the crypto application supports it and named curves are used. Export cipher suites are not supported as the U.S. lifted its export restrictions in early 2000. IDEA cipher suites are not supported as they have become deprecated by the latest TLS specification so it is not motivated to implement them. CRL validation is supported. Policy certificate extensions are not supported. 'Server Name Indication' extension client side (RFC 6066, Section 3) is supported.

DATA TYPES

The following data types are used in the functions for ssl:

boolean()

= true | false

option()

= socketoption() | ssloption() | transportoption()

socketoption()

= proplists:property()

The default socket options are [{mode,list},{packet, 0},{header, 0},{active, true}].

For valid options, see the inet(3) and gen_tcp(3) manual pages in kernel.

ssloption()

= {verify, verify_type()}

| {verify_fun, {fun(), term()}}

| {fail_if_no_peer_cert, boolean()} {depth, integer()}

| {cert, public_key:der_encoded()}

| {certfile, path()}

| {key, {'RSAPrivateKey'| 'DSAPrivateKey' | 'ECPrivateKey' | 'PrivateKeyInfo', public_key:der_encoded()}}

| {keyfile, path()}

| {password, string()}

| {cacerts, [public_key:der_encoded()]}

| {cacertfile, path()}

| {dh, public_key:der_encoded()}

| {dhfile, path()}

| {ciphers, ciphers()}

| {user_lookup_fun, {fun(), term()}}, {psk_identity, string()}, {srp_identity, {string(), string()}}

| {reuse_sessions, boolean()}

| {reuse_session, fun()} {next_protocols_advertised, [binary()]}

| {client_preferred_next_protocols, {client | server, [binary()]} | {client | server, [binary()], binary()}}

| {log_alert, boolean()}

| {server_name_indication, hostname() | disable}

transportoption()

= {cb_info, {CallbackModule::atom(), DataTag::atom(), ClosedTag::atom(), ErrTag:atom()}}

Defaults to {gen_tcp, tcp, tcp_closed, tcp_error}. Can be used to customize the transport layer. The callback module must implement a reliable transport protocol, behave as gen_tcp, and have functions corresponding to inet:setopts/2, inet:getopts/2, inet:peername/1, inet:sockname/1, and inet:port/1. The callback gen_tcp is treated specially and calls inet directly.

CallbackModule

= atom()

DataTag

= atom()

Used in socket data message.

ClosedTag

= atom()

Used in socket close message.

verify_type()

= verify_none | verify_peer

path()

= string()

Represents a file path.

public_key:der_encoded()

= binary()

ASN.1 DER-encoded entity as an Erlang binary.

host()

= hostname() | ipaddress()

hostname()

= string()

ip_address()

= {N1,N2,N3,N4} % IPv4 | {K1,K2,K3,K4,K5,K6,K7,K8} % IPv6

sslsocket()

Opaque to the user.

protocol()

= sslv3 | tlsv1 | 'tlsv1.1' | 'tlsv1.2'

ciphers()

= [ciphersuite()] | string()

According to old API.

ciphersuite()

= {key_exchange(), cipher(), hash()}

key_exchange()

cipher()

hash()

= md5 | sha

prf_random()

= client_random | server_random

srp_param_type()

= srp_1024 | srp_1536 | srp_2048 | srp_3072 | srp_4096 | srp_6144 | srp_8192

SSL OPTION DESCRIPTIONS - COMMON for SERVER and CLIENT

The following options have the same meaning in the client and the server:

{cert, public_key:der_encoded()}

The DER-encoded users certificate. If this option is supplied, it overrides option certfile.

{certfile, path()}

Path to a file containing the user certificate.

{key, {'RSAPrivateKey'| 'DSAPrivateKey' | 'ECPrivateKey' |'PrivateKeyInfo', public_key:der_encoded()}}

The DER-encoded user's private key. If this option is supplied, it overrides option keyfile.

{keyfile, path()}

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 certfile.

{password, string()}

String containing the user's password. Only used if the private keyfile is password-protected.

{ciphers, ciphers()}

Supported cipher suites. The function cipher_suites/0 can be used to find all ciphers that are supported by default. cipher_suites(all) can be called to find all available cipher suites. Pre-Shared Key (RFC 4279 and RFC 5487), Secure Remote Password (RFC 5054), 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.

{secure_renegotiate, boolean()}

Specifies if to reject renegotiation attempt that does not live up to RFC 5746. By default secure_renegotiate is set to false, that is, secure renegotiation is used if possible, but it fallback to unsecure renegotiation if the peer does not support RFC 5746.

{depth, integer()}

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.

{verify_fun, {Verifyfun :: fun(), InitialUserState :: term()}}

The verification fun is to be defined as follows:


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()}.

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 valid_peer or valid as second argument to the verification fun. See the public_key User's Guide for definition of #'OTPCertificate'{} and #'Extension'{}.

If the verify callback fun returns {fail, Reason}, the verification process is immediately stopped, an alert is sent to the peer, and the TLS/SSL handshake terminates.

If the verify callback fun returns {valid, UserState}, the verification process continues.

If the verify callback fun always returns {valid, UserState}, the TLS/SSL handshake does not terminate regarding verification failures and the connection is established.

If called with an extension unknown to the user application, return value {unknown, UserState} is to be used.

Default option verify_fun in verify_peer mode:


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

Default option verify_fun in mode verify_none:


{fun(_,{bad_cert, _}, UserState) ->
	 {valid, UserState};
    (_,{extension, _}, UserState) ->
	 {unknown, UserState};
    (_, valid, UserState) ->
	 {valid, UserState};
    (_, valid_peer, UserState) ->
         {valid, UserState}
 end, []}

The possible path validation errors are given on form {bad_cert, Reason} where Reason is:

unknown_ca

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 intermediat CA (trusted anchor does not have to be self-signed according to X-509) by using option partial_chain.

selfsigned_peer

The chain consisted only of one self-signed certificate.

PKIX X-509-path validation error

For possible reasons, see public_key:pkix_path_validation/3

{crl_check, boolean() | peer | best_effort } Perform CRL (Certificate Revocation List) verification (public_key:pkix_crls_validate/3) on all the certificates during the path validation (public_key:pkix_path_validation/3) of the certificate chain. Defaults to false.

peer - check is only performed on the peer certificate.

best_effort - if certificate revocation status can not be determined it will be accepted as valid.

The CA certificates specified for the connection will be used to construct the certificate chain validating the CRLs.

The CRLs will be fetched from a local or external cache see ssl_crl_cache_api(3).

{crl_cache, {Module :: atom(), {DbHandle :: internal | term(), Args :: list()}}}

Module defaults to ssl_crl_cache with DbHandle internal and an empty argument list. The following arguments may be specified for the internal cache.

{http, timeout()} Enables fetching of CRLs specified as http URIs in X509 cerificate extensions. Requires the OTP inets application. {partial_chain, fun(Chain::[DerCert]) -> {trusted_ca, DerCert} | unknown_ca }

Claim an intermediate CA in the chain as trusted. TLS then performs public_key:pkix_path_validation/3 with the selected CA as trusted anchor and the rest of the chain.

{versions, [protocol()]}

TLS protocol versions supported by started clients and servers. This option overrides the application environment option protocol_version. If the environment option is not set, it defaults to all versions, except SSL-3.0, supported by the ssl application. See also ssl(6).

{hibernate_after, integer()|undefined}

When an integer-value is specified, ssl_connection goes into hibernation after the specified number of milliseconds of inactivity, thus reducing its memory footprint. When undefined is specified (this is the default), the process never goes into hibernation.

{user_lookup_fun, {Lookupfun :: fun(), UserState :: term()}}

The lookup fun is to defined as follows:


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.

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, PSKIdentity is set to the hint presented by the server or to undefined. When called by the server, PSKIdentity is the identity presented by the client.

For Secure Remote Password (SRP), the fun is only used by the server to obtain parameters that it uses to generate its session keys. DerivedKey is to be derived according to RFC 2945 and RFC 5054: crypto:sha([Salt, crypto:sha([Username, <<$:>>, Password])])

{padding_check, boolean()}

Affects TLS-1.0 connections only. If set to false, it disables the block cipher padding check to be able to interoperate with legacy software.

Using {padding_check, boolean()} makes TLS vulnerable to the Poodle attack.

SSL OPTION DESCRIPTIONS - CLIENT SIDE

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

{verify, verify_type()}

In mode verify_none the default behavior is to allow all x509-path validation errors. See also option verify_fun.

{reuse_sessions, boolean()}

Specifies if the client is to try to reuse sessions when possible.

{cacerts, [public_key:der_encoded()]}

The DER-encoded trusted certificates. If this option is supplied it overrides option cacertfile.

{cacertfile, path()}

Path to a file containing PEM-encoded CA certificates. The CA certificates are used during server authentication and when building the client certificate chain.

{alpn_advertised_protocols, [binary()]}

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.

The list of protocols must not contain an empty binary.

The negotiated protocol can be retrieved using the negotiated_protocol/1 function.

{client_preferred_next_protocols, {Precedence :: server | client, ClientPrefs :: [binary()]}} {client_preferred_next_protocols, {Precedence :: server | client, ClientPrefs :: [binary()], Default :: binary()}}

Indicates that the client is to try to perform Next Protocol Negotiation.

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.

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.

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.

{psk_identity, string()}

Specifies the identity the client presents to the server. The matching secret is found by calling user_lookup_fun.

{srp_identity, {Username :: string(), Password :: string()}

Specifies the username and password to use to authenticate to the server.

{server_name_indication, hostname()}

Can be specified when upgrading a TCP socket to a TLS socket to use the TLS Server Name Indication extension.

{server_name_indication, disable}

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.

{fallback, boolean()}

Send special cipher suite TLS_FALLBACK_SCSV to avoid undesired TLS version downgrade. Defaults to false

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

ssl:connect(Host, Port, [...{versions, ['tlsv2', 'tlsv1.1', 'tlsv1', 'sslv3']}])

ssl:connect(Host, Port, [...{versions, [tlsv1.1', 'tlsv1', 'sslv3']}, {fallback, true}])

ssl:connect(Host, Port, [...{versions, ['tlsv1', 'sslv3']}, {fallback, true}])

ssl:connect(Host, Port, [...{versions, ['sslv3']}, {fallback, true}])

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.

SSL OPTION DESCRIPTIONS - SERVER SIDE

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

{cacerts, [public_key:der_encoded()]}

The DER-encoded trusted certificates. If this option is supplied it overrides option cacertfile.

{cacertfile, path()}

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.

{dh, public_key:der_encoded()}

The DER-encoded Diffie-Hellman parameters. If specified, it overrides option dhfile.

{dhfile, path()}

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.

{verify, verify_type()}

A server only does x509-path validation in mode verify_peer, as it then sends a certificate request to the client (this message is not sent if the verify option is verify_none). You can then also want to specify option fail_if_no_peer_cert.

{fail_if_no_peer_cert, boolean()}

Used together with {verify, verify_peer} by an SSL server. If set to true, the server fails if the client does not have a certificate to send, that is, sends an empty certificate. If set to false, it fails only if the client sends an invalid certificate (an empty certificate is considered valid). Defaults to false.

{reuse_sessions, boolean()}

Specifies if the server is to agree to reuse sessions when requested by the clients. See also option reuse_session.

{reuse_session, fun(SuggestedSessionId, PeerCert, Compression, CipherSuite) -> boolean()}

Enables the SSL server to have a local policy for deciding if a session is to be reused or not. Meaningful only if reuse_sessions is set to true. SuggestedSessionId is a binary(), PeerCert is a DER-encoded certificate, Compression is an enumeration integer, and CipherSuite is of type ciphersuite().

{alpn_preferred_protocols, [binary()]}

Indicates the server will try to perform Application-Layer Protocol Negotiation (ALPN).

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.

The negotiated protocol can be retrieved using the negotiated_protocol/1 function.

{next_protocols_advertised, Protocols :: [binary()]}

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 negotiated_next_protocol/1 method.

{psk_identity, string()}

Specifies the server identity hint, which the server presents to the client.

{log_alert, boolean()}

If set to false, error reports are not displayed.

{honor_cipher_order, boolean()}

If set to true, use the server preference for cipher selection. If set to false (the default), use the client preference.

General

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:

{ssl, Socket, Data}

{ssl_closed, Socket}

{ssl_error, Socket, Reason}

A Timeout argument specifies a time-out in milliseconds. The default value for argument Timeout is infinity.

cipher_suites() -> cipher_suites(Type) -> ciphers() Returns a list of supported cipher suites. Type = erlang | openssl | all

Returns a list of supported cipher suites. cipher_suites() is equivalent to cipher_suites(erlang). Type openssl is provided for backwards compatibility with the old SSL, which used OpenSSL. cipher_suites(all) returns all available cipher suites. The cipher suites not present in cipher_suites(erlang) but included in cipher_suites(all) are not used unless explicitly configured by the user.

clear_pem_cache() -> ok Clears the pem cache

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.

connect(Socket, SslOptions) -> connect(Socket, SslOptions, Timeout) -> {ok, SslSocket} | {error, Reason} Upgrades a gen_tcp, or equivalent, connected socket to an SSL socket. Socket = socket() SslOptions = [ssloption()] Timeout = integer() | infinity SslSocket = sslsocket() Reason = term()

Upgrades a gen_tcp, or equivalent, connected socket to an SSL socket, that is, performs the client-side ssl handshake.

connect(Host, Port, Options) -> connect(Host, Port, Options, Timeout) -> {ok, SslSocket} | {error, Reason} Opens an SSL connection to Host, Port. Host = host() Port = integer() Options = [option()] Timeout = integer() | infinity SslSocket = sslsocket() Reason = term()

Opens an SSL connection to Host, Port.

close(SslSocket) -> ok | {error, Reason} Closes an SSL connection. SslSocket = sslsocket() Reason = term()

Closes an SSL connection.

connection_info(SslSocket) -> {ok, {ProtocolVersion, CipherSuite}} | {error, Reason} Returns the Negotiated Protocol version and cipher suite. CipherSuite = ciphersuite() ProtocolVersion = protocol()

Returns the Negotiated Protocol version and cipher suite.

controlling_process(SslSocket, NewOwner) -> ok | {error, Reason} Assigns a new controlling process to the SSL socket. SslSocket = sslsocket() NewOwner = pid() Reason = term()

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.

format_error(Reason) -> string() Returns an error string. Reason = term()

Presents the error returned by an SSL function as a printable string.

getopts(Socket, OptionNames) -> {ok, [socketoption()]} | {error, Reason} Gets the values of the specified options. Socket = sslsocket() OptionNames = [atom()]

Gets the values of the specified socket options.

listen(Port, Options) -> {ok, ListenSocket} | {error, Reason} Creates an SSL listen socket. Port = integer() Options = options() ListenSocket = sslsocket()

Creates an SSL listen socket.

negotiated_protocol(Socket) -> {ok, Protocol} | {error, protocol_not_negotiated} Returns the protocol negotiated through ALPN or NPN extensions. Socket = sslsocket() Protocol = binary()

Returns the protocol negotiated through ALPN or NPN extensions.

peercert(Socket) -> {ok, Cert} | {error, Reason} Returns the peer certificate. Socket = sslsocket() Cert = binary()

The peer certificate is returned as a DER-encoded binary. The certificate can be decoded with public_key:pkix_decode_cert/2.

peername(Socket) -> {ok, {Address, Port}} | {error, Reason} Returns the peer address and port. Socket = sslsocket() Address = ipaddress() Port = integer()

Returns the address and port number of the peer.

prf(Socket, Secret, Label, Seed, WantedLength) -> {ok, binary()} | {error, reason()} Uses a session Pseudo-Random Function to generate key material. Socket = sslsocket() Secret = binary() | master_secret Label = binary() Seed = [binary() | prf_random()] WantedLength = non_neg_integer()

Uses the Pseudo-Random Function (PRF) of a TLS session to generate extra key material. It either takes user-generated values for Secret and Seed or atoms directing it to use a specific value from the session security parameters.

Can only be used with TLS connections; {error, undefined} is returned for SSLv3 connections.

recv(Socket, Length) -> recv(Socket, Length, Timeout) -> {ok, Data} | {error, Reason} Receives data on a socket. Socket = sslsocket() Length = integer() Timeout = integer() Data = [char()] | binary()

Receives a packet from a socket in passive mode. A closed socket is indicated by return value {error, closed}.

Argument Length is meaningful only when the socket is in mode raw and denotes the number of bytes to read. If Length = 0, all available bytes are returned. If Length > 0, exactly Length bytes are returned, or an error; possibly discarding less than Length bytes of data when the socket gets closed from the other side.

Optional argument Timeout specifies a time-out in milliseconds. The default value is infinity.

renegotiate(Socket) -> ok | {error, Reason} Initiates a new handshake. Socket = sslsocket()

Initiates a new handshake. A notable return value is {error, renegotiation_rejected} indicating that the peer refused to go through with the renegotiation, but the connection is still active using the previously negotiated session.

send(Socket, Data) -> ok | {error, Reason} Writes data to a socket. Socket = sslsocket() Data = iodata()

Writes Data to Socket.

A notable return value is {error, closed} indicating that the socket is closed.

setopts(Socket, Options) -> ok | {error, Reason} Sets socket options. Socket = sslsocket() Options = [socketoption]()

Sets options according to Options for socket Socket.

shutdown(Socket, How) -> ok | {error, Reason} Immediately closes a socket. Socket = sslsocket() How = read | write | read_write Reason = reason()

Immediately closes a socket in one or two directions.

How == write means closing the socket for writing, reading from it is still possible.

To be able to handle that the peer has done a shutdown on the write side, option {exit_on_close, false} is useful.

ssl_accept(Socket) -> ssl_accept(Socket, Timeout) -> ok | {error, Reason} Performs server-side SSL/TLS handshake. Socket = sslsocket() Timeout = integer() Reason = term()

Performs the SSL/TLS server-side handshake.

Socket is a socket as returned by ssl:transport_accept/[1,2]

ssl_accept(Socket, SslOptions) -> ssl_accept(Socket, SslOptions, Timeout) -> {ok, Socket} | ok | {error, Reason} Performs server-side SSL/TLS handshake. Socket = socket() | sslsocket() SslOptions = ssloptions() Timeout = integer() Reason = term()

If Socket is a socket(): upgrades a gen_tcp, or equivalent, socket to an SSL socket, that is, performs the SSL/TLS server-side handshake and returns the SSL socket.

The listen socket is to be in mode {active, false} 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.

If Socket is an sslsocket(): provides extra SSL/TLS options to those specified in ssl:listen/2 and then performs the SSL/TLS handshake.

sockname(Socket) -> {ok, {Address, Port}} | {error, Reason} Returns the local address and port. Socket = sslsocket() Address = ipaddress() Port = integer()

Returns the local address and port number of socket Socket.

start() -> start(Type) -> ok | {error, Reason} Starts the sslapplication. Type = permanent | transient | temporary

Starts the ssl application. Default type is temporary.

stop() -> ok Stops the ssl application.

Stops the ssl application.

transport_accept(ListenSocket) -> transport_accept(ListenSocket, Timeout) -> {ok, NewSocket} | {error, Reason} Accepts an incoming connection and prepares for ssl_accept. ListenSocket = NewSocket = sslsocket() Timeout = integer() Reason = reason()

Accepts an incoming connection request on a listen socket. ListenSocket must be a socket returned from ssl:listen/2. The socket returned is to be passed to ssl:ssl_accept[2,3] to complete handshaking, that is, establishing the SSL/TLS connection.

The socket returned can only be used with ssl:ssl_accept[2,3]. No traffic can be sent or received before that call.

The accepted socket inherits the options set for ListenSocket in ssl:listen/2.

The default value for Timeout is infinity. If Timeout is specified and no connection is accepted within the given time, {error, timeout} is returned.

versions() -> [versions_info()] Returns version information relevant for the ssl application. versions_info() = {app_vsn, string()} | {supported | available, [protocol()]

Returns version information relevant for the ssl application.

app_vsn The application version of the ssl application. supported TLS/SSL versions supported by default. Overridden by a version option on connect/[2,3,4], listen/2, and ssl_accept/[1,2,3]. For the negotiated TLS/SSL version, see ssl:connection_info/1 . available All TLS/SSL versions supported by the ssl application. TLS 1.2 requires sufficient support from the crypto application.