Interface Functions for Secure Socket Layer

This module contains interface functions to the Secure Socket Layer.


  • ssl requires the crypto an public_key applications.
  • Supported SSL/TLS-versions are SSL-3.0 and TLS-1.0
  • For security reasons sslv2 is not supported.
  • Ephemeral Diffie-Hellman cipher suites are supported but not Diffie Hellman Certificates cipher suites.
  • Export cipher suites are not supported as the U.S. lifted its export restrictions in early 2000.
  • CRL and policy certificate extensions are not supported yet.


The following data types are used in the functions below:

boolean() = true | false

property() = atom()

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

socketoption() = [{property(), term()}] - defaults to [{mode,list},{packet, 0},{header, 0},{active, true}].

For valid options see inet(3) and gen_tcp(3) .

ssloption() = {verify, verify_type()} | {verify_fun, {fun(), term()}} | {fail_if_no_peer_cert, boolean()} {depth, integer()} | {cert, der_bin()}| {certfile, path()} | {key, der_bin()} | {keyfile, path()} | {password, string()} | {cacerts, [der_bin()]} | {cacertfile, path()} | |{dh, der_bin()} | {dhfile, path()} | {ciphers, ciphers()} | {ssl_imp, ssl_imp()} | {reuse_sessions, boolean()} | {reuse_session, fun()}

transportoption() = {CallbackModule, DataTag, ClosedTag} - defaults to {gen_tcp, tcp, tcp_closed}. Ssl may be run over any reliable transport protocol that has an equivalent API to gen_tcp's.

      CallbackModule = atom()

      DataTag = atom() - tag used in socket data message.

      ClosedTag = atom() - tag used in socket close message.

verify_type() = verify_none | verify_peer

path() = string() - representing a file path.

der_bin() = binary() -Asn1 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

ciphers() = [ciphersuite()] | string() (according to old API)

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

key_exchange() = rsa | dhe_dss | dhe_rsa | dh_anon

cipher() = rc4_128 | des_cbc | '3des_ede_cbc' | aes_128_cbc | aes_256_cbc

hash() = md5 | sha

ssl_imp() = new | old - default is new.


Options described here are options that are have the same meaning in the client and the server.

{cert, der_bin()}
The DER encoded users certificate. If this option is supplied it will override the certfile option.
{certfile, path()}
Path to a file containing the user's certificate.
{key, der_bin()}
The DER encoded users private key. If this option is supplied it will override the keyfile option.
{keyfile, path()}
Path to file containing user's private PEM encoded key. As PEM-files may contain several entries this option defaults to the same file as given by certfile option.
{password, string()}
String containing the user's password. Only used if the private keyfile is password protected.
{cacerts, [der_bin()]}
The DER encoded trusted certificates. If this option is supplied it will override the cacertfile option.
{cacertfile, path()}
Path to file containing PEM encoded CA certificates (trusted certificates used for verifying a peer certificate). May be omitted if you do not want to verify the peer.
{ciphers, ciphers()}
The cipher suites that should be supported. The function cipher_suites/0 can be used to find all available ciphers. Additionally some anonymous cipher suites ({dh_anon, rc4_128, md5}, {dh_anon, des_cbc, sha}, {dh_anon, '3des_ede_cbc', sha}, {dh_anon, aes_128_cbc, sha}, {dh_anon, aes_256_cbc, sha}) are supported for testing purposes and will only work if explicitly enabled by this option and they are supported/enabled by the peer also.
{ssl_imp, ssl_imp()}
Specify which ssl implementation you want to use. Defaults to new.
{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 i.e. secure renegotiation will be used if possible but it will fallback to unsecure renegotiation if the peer does not support RFC 5746.
{depth, integer()}
Specifies the maximum verification depth, i.e. how far in a chain of certificates the verification process can proceed before the verification is considered to fail. Peer certificate = 0, CA certificate = 1, higher level CA certificate = 2, etc. The value 2 thus means that a chain can at most contain peer cert, CA cert, next CA cert, and an additional CA cert. The default value is 1.
{verify_fun, {Verifyfun :: fun(), InitialUserState :: term()}}

The verification fun should be defined as:

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

The verify fun will be called during the X509-path validation when an error or an extension unknown to the ssl application is encountered. Additionally it will be called when a certificate is considered valid by the path validation to allow access to each certificate in the path to the user application. Note that the it will differentiate between the peer certificate and CA certificates by using valid_peer or valid as the second argument to the verify 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 and an alert is sent to the peer and the TLS/SSL handshake is terminated. If the verify callback fun returns {valid, UserState}, the verification process is continued. If the verify callback fun always returns {valid, UserState}, the TLS/SSL handshake will not be terminated with respect to verification failures and the connection will be established. If called with an extension unknown to the user application the return value {unknown, UserState} should be used.

The default verify_fun option in verify_peer mode:

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

The default verify_fun option in verify_none mode:

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

Possible path validation errors:

{bad_cert, cert_expired}, {bad_cert, invalid_issuer}, {bad_cert, invalid_signature}, {bad_cert, unknown_ca}, {bad_cert, name_not_permitted}, {bad_cert, missing_basic_constraint}, {bad_cert, invalid_key_usage}


Options described here are client specific or has a slightly different meaning in the client than in the server.

{verify, verify_type()}
In verify_none mode the default behavior will be to allow all x509-path validation errors. See also the verify_fun option.
{reuse_sessions, boolean()}
Specifies if client should try to reuse sessions when possible.


Options described here are server specific or has a slightly different meaning in the server than in the client.

{dh, der_bin()}
The DER encoded Diffie Hellman parameters. If this option is supplied it will override the dhfile option.
{dhfile, path()}
Path to file containing PEM encoded Diffie Hellman parameters, for the server to use if a cipher suite using Diffie Hellman key exchange is negotiated. If not specified default parameters will be used.
{verify, verify_type()}
Servers only do the x509-path validation in verify_peer mode, as it then will send a certificate request to the client (this message is not sent if the verify option is verify_none) and you may then also want to specify the option fail_if_no_peer_cert.
{fail_if_no_peer_cert, boolean()}
Used together with {verify, verify_peer} by a ssl server. If set to true, the server will fail if the client does not have a certificate to send, i.e. sends a empty certificate, if set to false it will only fail if the client sends a invalid certificate (an empty certificate is considered valid).
{reuse_sessions, boolean()}
Specifies if the server should agree to reuse sessions when the clients request to do so. See also the reuse_session option.
{reuse_session, fun(SuggestedSessionId, PeerCert, Compression, CipherSuite) -> boolean()}
Enables the ssl server to have a local policy for deciding if a session should be reused or not, only meaning full if reuse_sessions is set to true. SuggestedSessionId is a binary(), PeerCert is a DER encoded certificate, Compression is an enumeration integer and CipherSuite of type ciphersuite().


When a 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 timeout in milliseconds. The default value for a Timeout argument is infinity.


cipher_suites() ->

cipher_suites(Type) -> ciphers()

  • Type = erlang | openssl

Returns a list of supported cipher suites. cipher_suites() is equivalent to cipher_suites(erlang). Type openssl is provided for backwards compatibility with old ssl that used openssl.

connect(Socket, SslOptions) ->

connect(Socket, SslOptions, Timeout) -> {ok, SslSocket} | {error, Reason}

  • Socket = socket()
  • SslOptions = [ssloption()]
  • Timeout = integer() | infinity
  • SslSocket = sslsocket()
  • Reason = term()

Upgrades a gen_tcp, or equivalent, connected socket to a ssl socket i.e. performs the client-side ssl handshake.

connect(Host, Port, Options) ->

connect(Host, Port, Options, Timeout) -> {ok, SslSocket} | {error, Reason}

  • 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}

  • SslSocket = sslsocket()
  • Reason = term()

Close a ssl connection.

controlling_process(SslSocket, NewOwner) -> ok | {error, Reason}

  • SslSocket = sslsocket()
  • NewOwner = pid()
  • Reason = term()

Assigns a new controlling process to the ssl-socket. A controlling process is the owner of a ssl-socket, and receives all messages from the socket.

connection_info(SslSocket) -> {ok, {ProtocolVersion, CipherSuite}} | {error, Reason}

  • CipherSuite = ciphersuite()
  • ProtocolVersion = protocol()

Returns the negotiated protocol version and cipher suite.

format_error(Reason) -> string()

  • Reason = term()

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

getopts(Socket) ->

getopts(Socket, OptionNames) -> {ok, [socketoption()]} | {error, Reason}

  • Socket = sslsocket()
  • OptionNames = [property()]

Get the value of the specified socket options, if no options are specified all options are returned.

listen(Port, Options) -> {ok, ListenSocket} | {error, Reason}

  • Port = integer()
  • Options = options()
  • ListenSocket = sslsocket()

Creates a ssl listen socket.

peercert(Socket) -> {ok, Cert} | {error, Reason}

  • 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}

  • Socket = sslsocket()
  • Address = ipaddress()
  • Port = integer()

Returns the address and port number of the peer.

recv(Socket, Length) ->

recv(Socket, Length, Timeout) -> {ok, Data} | {error, Reason}

  • Socket = sslsocket()
  • Length = integer()
  • Timeout = integer()
  • Data = [char()] | binary()

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

The Length argument is only meaningful when the socket is in raw mode 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.

The optional Timeout parameter specifies a timeout in milliseconds. The default value is infinity.

renegotiate(Socket) -> ok | {error, Reason}

  • 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}

  • Socket = sslsocket()
  • Data = iolist() | binary()

Writes Data to Socket.

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

setopts(Socket, Options) -> ok | {error, Reason}

  • Socket = sslsocket()
  • Options = [socketoption]()

Sets options according to Options for the socket Socket.

shutdown(Socket, How) -> ok | {error, Reason}

  • Socket = sslsocket()
  • How = read | write | read_write
  • Reason = reason()

Immediately close 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, the {exit_on_close, false} option is useful.

ssl_accept(ListenSocket) ->

ssl_accept(ListenSocket, Timeout) -> ok | {error, Reason}

  • ListenSocket = sslsocket()
  • Timeout = integer()
  • Reason = term()

The ssl_accept function establish the SSL connection on the server side. It should be called directly after transport_accept, in the spawned server-loop.

ssl_accept(ListenSocket, SslOptions) ->

ssl_accept(ListenSocket, SslOptions, Timeout) -> {ok, Socket} | {error, Reason}

  • ListenSocket = socket()
  • SslOptions = ssloptions()
  • Timeout = integer()
  • Reason = term()

Upgrades a gen_tcp, or equivalent, socket to a ssl socket i.e. performs the ssl server-side handshake.


Note that the listen socket should be in {active, false} mode before telling the client that the server is ready to upgrade and calling this function, otherwise the upgrade may or may not succeed depending on timing.

sockname(Socket) -> {ok, {Address, Port}} | {error, Reason}

  • Socket = sslsocket()
  • Address = ipaddress()
  • Port = integer()

Returns the local address and port number of the socket Socket.

start() ->

start(Type) -> ok | {error, Reason}

  • Type = permanent | transient | temporary

Starts the Ssl application. Default type is temporary. application(3)

stop() -> ok

Stops the Ssl application. application(3)

transport_accept(Socket) ->

transport_accept(Socket, Timeout) -> {ok, NewSocket} | {error, Reason}

  • Socket = NewSocket = sslsocket()
  • Timeout = integer()
  • Reason = reason()

Accepts an incoming connection request on a listen socket. ListenSocket must be a socket returned from listen/2. The socket returned should be passed to ssl_accept to complete ssl handshaking and establishing the connection.


The socket returned can only be used with ssl_accept, no traffic can be sent or received before that call.

The accepted socket inherits the options set for ListenSocket in 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() -> [{SslAppVer, SupportedSslVer, AvailableSslVsn}]

  • SslAppVer = string()
  • SupportedSslVer = [protocol()]
  • AvailableSslVsn = [protocol()]

Returns version information relevant for the ssl application.


inet(3) and gen_tcp(3)

View Functions