Interface Functions for Secure Socket Layer

This module contains interface functions to the Secure Socket Layer.


This manual page describes functions that are defined in the ssl module and represents the new ssl implementation that coexists with the old one, as the new implementation is not yet complete enough to replace the old one.

The new implementation can be accessed by providing the option {ssl_imp, new} to the ssl:connect and ssl:listen functions.

The new implementation is Erlang based and all logic is in Erlang and only payload encryption calculations are done in C via the crypto application. The main reason for making a new implementation is that the old solution was very crippled as the control of the ssl-socket was deep down in openssl making it hard if not impossible to support all inet options, ipv6 and upgrade of a tcp connection to a ssl connection. The alfa version has a few limitations that will be removed before the ssl-4.0 release. Main differences and limitations in the alfa are listed below.


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()} | {fail_if_no_peer_cert, boolean()} {depth, integer()} | {certfile, path()} | {keyfile, path()} | {password, string()} | {cacertfile, 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.

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()] | sting() (according to old API)

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

key_exchange() = rsa | dh_dss | dh_rsa | dh_anon | dhe_dss | dhe_rsa | krb5 | KeyExchange_export

cipher() = rc4_128 | idea_cbc | des_cbc | '3des_ede_cbc' des40_cbc | dh_dss | aes_128_cbc | aes_256_cbc | rc2_cbc_40 | rc4_40

hash() = md5 | sha

exportable() = export | no_export | ignore

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


{verify, verify_type()}
If verify_none is specified x509-certificate path validation errors at the client side will not automatically cause the connection to fail, as it will if the verify type is verify_peer. See also the option verify_fun. Servers only do the path validation if verify_peer is set to true, 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, e.i 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).
{verify_fun, fun(ErrorList) -> boolean()}
Used by the ssl client to determine if x509-certificate path validations errors are acceptable or if the connection should fail. Defaults to: fun(ErrorList) -> case lists:foldl(fun({bad_cert,unknown_ca}, Acc) -> Acc; (Other, Acc) -> [Other | Acc] end, [], ErrorList) of [] -> true; [_|_] -> false end end I.e. by default if the only error found was that the CA-certificate holder was unknown this will be accepted. Possible errors in the error list are: {bad_cert, cert_expired}, {bad_cert, invalid_issuer}, {bad_cert, invalid_signature}, {bad_cert, name_not_permitted}, {bad_cert, unknown_ca}, {bad_cert, cert_expired}, {bad_cert, invalid_issuer}, {bad_cert, invalid_signature}, {bad_cert, name_not_permitted}, {bad_cert, cert_revoked} (not implemented yet), {bad_cert, unknown_critical_extension} or {bad_cert, term()} (Will be relevant later when an option is added for the user to be able to verify application specific extensions.)
{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.
{certfile, path()}
Path to a file containing the user's certificate. Optional for clients but note that some servers requires that the client can certify itself.
{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.
{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 function ciphers_suites/0 can be used to find all available ciphers.
{ssl_imp, ssl_imp()}
Specify which ssl implementation you want to use.
{reuse_sessions, boolean()}
Specifies if ssl sessions should be reused when possible.
{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:

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 e.i 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.

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) ->

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

  • Socket = sslsocket()
  • Opts = [] | [otp] | [plain]
  • Cert = term()
  • Subject = term()

peercert(Cert) is equivalent to peercert(Cert, []).

The form of the returned certificate depends on the options.

If the options list is empty the certificate is returned as a DER encoded binary.

The option otp or plain implies that the certificate will be returned as a parsed ASN.1 structure in the form of an Erlang term. For detail see the public_key application. Currently only plain is officially supported see the public_key users guide.

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.

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 e.i performs the ssl server-side handshake.

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)