ssl - The ssl application provides secure communication over
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
- For security reasons SSL-2.0 is not supported.
- For security reasons SSL-3.0 is no longer supported by default, but can be
- For security reasons DES cipher suites are no longer supported by default,
but can be configured.
Renegotiation Indication Extension RFC 5746 is supported
- 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.
- Compression is not supported.
- CRL validation is supported.
- Policy certificate extensions are not supported.
- 'Server Name Indication' extension (RFC 6066) is supported.
- Application Layer Protocol Negotiation (ALPN) and its successor Next
Protocol Negotiation (NPN) are supported.
- It is possible to use Pre-Shared Key (PSK) and Secure Remote Password
(SRP) cipher suites, but they are not enabled by default.
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
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 =
- 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. Defaults to 24 hours
which is the maximum recommended lifetime by RFC 5246. However sessions
may be invalidated earlier due to the maximum limitation of the session
- session_cb = atom() <optional>:
- Name of the session cache callback module that implements the
ssl_session_cache_api behavior. Defaults to
- session_cb_init_args = proplist:proplist()
- List of extra user-defined arguments to the init function in the
session cache callback module. Defaults to .
- session_cache_client_max = integer() <optional>
: Limits the growth of the clients session cache, that is how many sessions
towards servers that are cached to be used by new client connections. If
the maximum number of sessions is reached, the current cache entries will
be invalidated regardless of their remaining lifetime. Defaults to
- session_cache_server_max = integer() <optional>:
- Limits the growth of the servers session cache, that is how many client
sessions are cached by the server. 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. Defaults to 2
- bypass_pem_cache = boolean() <optional>:
- Introduced in ssl-8.0.2. Disables the PEM-cache. The PEM cache has proven
to be a bottleneck, until the implementation has been improved this can be
used as a workaround. Defaults to false.
- 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
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