The ssl application provides secure communication over sockets.


The ssl application is an implementation of the SSL/TLS protocol in Erlang.

  • 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.


The SSL application uses the public_key and Crypto application to handle public keys and encryption, hence these applications must be loaded for the SSL application to work. In an embedded environment this means they must be started with application:start/[1,2] before the SSL application is started.


The application environment configuration parameters in this section are defined for the SSL application. For more information about configuration parameters, see the application(3) manual page in Kernel.

The environment parameters can be set on the command line, for example:

erl -ssl protocol_version "['tlsv1.2', 'tlsv1.1']"

protocol_version = ssl:protocol()<optional>

Protocol supported by started clients and servers. If this option is not set, it defaults to all protocols currently supported by the SSL application. This option can be overridden by the version option to ssl:connect/[2,3] and ssl:listen/2.

session_lifetime = integer() <optional>

Maximum lifetime of the session data in seconds.

session_cb = atom() <optional>

Name of the session cache callback module that implements the ssl_session_cache_api behavior. Defaults to ssl_session_cache.

session_cb_init_args = proplist:proplist() <optional>

List of extra user-defined arguments to the init function in the session cache callback module. Defaults to [].

session_cache_client_max = integer() <optional>
session_cache_server_max = integer() <optional>

Limits the growth of the clients/servers session cache, if the maximum number of sessions is reached, the current cache entries will be invalidated regardless of their remaining lifetime. Defaults to 1000.

ssl_pem_cache_clean = integer() <optional>

Number of milliseconds between PEM cache validations.

alert_timeout = integer() <optional>

Number of milliseconds between sending of a fatal alert and closing the connection. Waiting a little while improves the peers chances to properly receiving the alert so it may shutdown gracefully. Defaults to 5000 milliseconds.


The SSL application uses the default OTP error logger to log unexpected errors and TLS alerts. The logging of TLS alerts may be turned off with the log_alert option.