An opaque reference to the TLS/DTLS connection, may be used for equality matching.

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

For valid options, see the inet(3), gen_tcp(3) and gen_udp(3) manual pages in Kernel. Note that stream oriented options such as packet are only relevant for TLS and not DTLS

When a TLS/DTLS socket is in active mode (the default), data from the socket is delivered to the owner of the socket in the form of messages as described above.

The ssl_passive message is sent only when the socket is in {active, N} mode and the counter dropped to 0. It indicates that the socket has transitioned to passive ({active, false}) mode.

Defaults to {gen_tcp, tcp, tcp_closed, tcp_error, tcp_passive} for TLS (for backward compatibility a four tuple will be converted to a five tuple with the last element "second_element"_passive) and {gen_udp, udp, udp_closed, udp_error} for DTLS (might also be changed to five tuple in the future). Can be used to customize the transport layer. The tag values should be the values used by the underlying transport in its active mode messages. For TLS 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. For DTLS this feature must be considered exprimental.

Choose TLS or DTLS protocol for the transport layer security. Defaults to tls. For DTLS other transports than UDP are not yet supported.

Defaults to full. If hello is specified the handshake will pause after the hello message and give the user a possibility make decisions based on hello extensions before continuing or aborting the handshake by calling handshake_continue/3 or handshake_cancel/1

The DER-encoded users certificate. Note that the cert option may also be a list of DER-encoded certificates where the first one is the users certificate and the rest of the certificates constitutes the certificate chain. For maximum interoperability the certificates in the chain should be in the correct order, the chain will be sent as is to the peer. If chain certificates are not provided, certificates from client_cacerts(), server_cacerts(), or client_cafile(), server_cafile() are used to construct the chain. If this option is supplied, it overrides option certfile.

Path to a file containing the user certificate on PEM format or possible several certificates where the first one is the users certificate and the rest of the certificates constitutes the certificate chain. For more details see cert(),

The DER-encoded user's private key or a map refering to a crypto engine and its key reference that optionally can be password protected, seealso crypto:engine_load/4 and Crypto's Users Guide. If this option is supplied, it overrides option keyfile.

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.

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

A list of cipher suites that should be supported

The function ssl:cipher_suites/2 can be used to find all cipher suites that are supported by default and all cipher suites that may be configured.

If you compose your own cipher_suites() make sure they are filtered for cryptolib support ssl:filter_cipher_suites/2 Additionaly the functions ssl:append_cipher_suites/2 , ssl:prepend_cipher_suites/2, ssl:suite_to_str/1, ssl:str_to_suite/1, and ssl:suite_to_openssl_str/1 also exist to help creating customized cipher suite lists.

Non-default cipher suites including anonymous cipher suites (PRE TLS-1.3) are supported for interop/testing purposes and may be used by adding them to your cipher suite list. Note that they must also be supported/enabled by the peer to actually be used.

In addition to the signature_algorithms extension from TLS 1.2, TLS 1.3 (RFC 5246 Section 4.2.3)adds the signature_algorithms_cert extension which enables having special requirements on the signatures used in the certificates that differs from the requirements on digital signatures as a whole. If this is not required this extension is not needed.

The client will send a signature_algorithms_cert extension (ClientHello), if TLS version 1.3 or later is used, and the signature_algs_cert option is explicitly specified. By default, only the signature_algs extension is sent.

The signature schemes shall be ordered according to the client's preference (favorite choice first).

TLS 1.3 introduces the "supported_groups" extension that is used for negotiating the Diffie-Hellman parameters in a TLS 1.3 handshake. Both client and server can specify a list of parameters that they are willing to use.

If it is not specified it will use a default list ([x25519, x448, secp256r1, secp384r1]) that is filtered based on the installed crypto library version.

Specifies if to reject renegotiation attempt that does not live up to RFC 5746. By default secure_renegotiate is set to true, that is, secure renegotiation is enforced. If set to false secure renegotiation will still be used if possible, but it falls back to insecure renegotiation if the peer does not support RFC 5746.

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 10.

The verification fun is to be defined as follows:

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

  Note that if the fun returns unknown for an extension marked as critical, validation will fail.

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, #'Extension'{critical = true}}, 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 intermediate 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

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 cannot 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).

Specify how to perform lookup and caching of certificate revocation lists. Module defaults to ssl_crl_cache with DbHandle being internal and an empty argument list.

There are two implementations available:

ssl_crl_cache

This module maintains a cache of CRLs. CRLs can be added to the cache using the function ssl_crl_cache:insert/1, and optionally automatically fetched through HTTP if the following argument is specified:

{http, timeout()}

Enables fetching of CRLs specified as http URIs inX509 certificate extensions. Requires the OTP inets application.

ssl_crl_hash_dir

This module makes use of a directory where CRLs are stored in files named by the hash of the issuer name.

The file names consist of eight hexadecimal digits followed by .rN, where N is an integer, e.g. 1a2b3c4d.r0. For the first version of the CRL, N starts at zero, and for each new version, N is incremented by one. The OpenSSL utility c_rehash creates symlinks according to this pattern.

For a given hash value, this module finds all consecutive .r* files starting from zero, and those files taken together make up the revocation list. CRL files whose nextUpdate fields are in the past, or that are issued by a different CA that happens to have the same name hash, are excluded.

The following argument is required:

{dir, string()}

Specifies the directory in which the CRLs can be found.

fun(Chain::[public_key:der_encoded()]) ->
	{trusted_ca, DerCert::public_key:der_encoded()} | 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.

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

The lookup fun is to defined as follows:

fun(psk, PSKIdentity :: binary(), UserState :: term()) ->
	{ok, SharedSecret :: binary()} | error;
fun(srp, Username :: binary(), 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])])

Identifies a TLS session.

If set to false, TLS/DTLS Alert reports are not displayed. Deprecated in OTP 22, use {log_level, logging_level()} instead.

Specifies the log level for a TLS/DTLS connection. Alerts are logged on notice level, which is the default level. The level debug triggers verbose logging of TLS/DTLS protocol messages. See also ssl(6)

When an integer-value is specified, TLS/DTLS-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.

Integer (24 bits unsigned). Used to limit the size of valid TLS handshake packets to avoid DoS attacks. Defaults to 256*1024.

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.

Affects TLS-1.0 connections only. Used to change the BEAST mitigation strategy to interoperate with legacy software. Defaults to one_n_minus_one.

one_n_minus_one - Perform 1/n-1 BEAST mitigation.

zero_n - Perform 0/n BEAST mitigation.

disabled - Disable BEAST mitigation.

Deprecated since OTP-17, has no affect.

Configures the session ticket functionalty in TLS 1.3 client and server.

Configures the maximum amount of bytes that can be sent on a TLS 1.3 connection before an automatic key update is performed.

There are cryptographic limits on the amount of plaintext which can be safely encrypted under a given set of keys. The current default ensures that data integrity will not be breached with probability greater than 1/2^57. For more information see Limits on Authenticated Encryption Use in TLS.

Configures the middlebox compatibility mode on a TLS 1.3 connection.

A significant number of middleboxes misbehave when a TLS 1.3 connection is negotiated. Implementations can increase the chance of making connections through those middleboxes by making the TLS 1.3 handshake more like a TLS 1.2 handshake.

The middlebox compatibility mode is enabled (true) by default.

Configures a TLS 1.3 connection for keylogging

In order to retrieve keylog information on a TLS 1.3 connection, it must be configured in advance to keep the client_random and various handshake secrets.

The keep_secrets functionality is disabled (false) by default.

Added in OTP 23.2

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

Reuses a specific session. The session should be refered by its session id if it is earlier saved with the option {reuse_sessions, save} since OTP-21.3 or explicitly specified by its session id and associated data since OTP-22.3. See also SSL's Users Guide, Session Reuse pre TLS 1.3

When save is specified a new connection will be negotiated and saved for later reuse. The session ID can be fetched with connection_information/2 and used with the client option reuse_session The boolean value true specifies that if possible, automatized session reuse will be performed. If a new session is created, and is unique in regard to previous stored sessions, it will be saved for possible later reuse. Since OTP-21.3

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

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

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.

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.

Specifies the maximum fragment length the client is prepared to accept from the server. See RFC 6066

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

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

Specify the hostname to be used in TLS Server Name Indication extension. If not specified it will default to the Host argument of connect/[3,4] unless it is of type inet:ipaddress().

The HostName will also be used in the hostname verification of the peer certificate using public_key:pkix_verify_hostname/2.

The special value disable prevents the Server Name Indication extension from being sent and disables the hostname verification check public_key:pkix_verify_hostname/2

Customizes the hostname verification of the peer certificate, as different protocols that use TLS such as HTTP or LDAP may want to do it differently, for possible options see public_key:pkix_verify_hostname/3

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

In addition to the algorithms negotiated by the cipher suite used for key exchange, payload encryption, message authentication and pseudo random calculation, the TLS signature algorithm extension Section 7.4.1.4.1 in RFC 5246 may be used, from TLS 1.2, to negotiate which signature algorithm to use during the TLS handshake. If no lower TLS versions than 1.2 are supported, the client will send a TLS signature algorithm extension with the algorithms specified by this option. Defaults to

[
%% SHA2
{sha512, ecdsa},
{sha512, rsa},
{sha384, ecdsa},
{sha384, rsa},
{sha256, ecdsa},
{sha256, rsa},
{sha224, ecdsa},
{sha224, rsa},
%% SHA
{sha, ecdsa},
{sha, rsa},
{sha, dsa},
]

The algorithms should be in the preferred order. Selected signature algorithm can restrict which hash functions that may be selected. Default support for {md5, rsa} removed in ssl-8.0

Configures the session ticket functionality. Allowed values are disabled, manual and auto. If it is set to manual the client will send the ticket information to user process in a 3-tuple:

{ssl, session_ticket, {SNI, TicketData}}

where SNI is the ServerNameIndication and TicketData is the extended ticket data that can be used in subsequent session resumptions.

If it is set to auto, the client automatically handles received tickets and tries to use them when making new TLS connections (session resumption with pre-shared keys).

Configures the session tickets to be used for session resumption. It is a mandatory option in manual mode (session_tickets = manual).

Configures the early data to be sent by the client.

In order to be able to verify that the server has the intention to process the early data, the following 3-tuple is sent to the user process:

{ssl, SslSocket, {early_data, Result}}

where Result is either accepted or rejected.

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

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.

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

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.

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.

Used together with {verify, verify_peer} by an TLS/DTLS 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.

The boolean value true specifies that the server will agree to reuse sessions. Setting it to false will result in an empty session table, that is no sessions will be reused. See also option reuse_session

Enables the TLS/DTLS 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().

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.

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.

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

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

If the server receives a SNI (Server Name Indication) from the client matching a host listed in the sni_hosts option, the specific options for that host will override previously specified options. The option sni_fun, and sni_hosts are mutually exclusive.

If the server receives a SNI (Server Name Indication) from the client, the given function will be called to retrieve [server_option()] for the indicated server. These options will be merged into predefined [server_option()] list. The function should be defined as: fun(ServerName :: string()) -> [server_option()] and can be specified as a fun or as named fun module:function/1 The option sni_fun, and sni_hosts are mutually exclusive.

In protocols that support client-initiated renegotiation, the cost of resources of such an operation is higher for the server than the client. This can act as a vector for denial of service attacks. The SSL application already takes measures to counter-act such attempts, but client-initiated renegotiation can be strictly disabled by setting this option to false. The default value is true. Note that disabling renegotiation can result in long-lived connections becoming unusable due to limits on the number of messages the underlying cipher suite can encipher.

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

If true, use the server's preference for ECC curve selection. If false (the default), use the client's preference.

The algorithms specified by this option will be the ones accepted by the server in a signature algorithm negotiation, introduced in TLS-1.2. The algorithms will also be offered to the client if a client certificate is requested. For more details see the corresponding client option.

Configures the session ticket functionality. Allowed values are disabled, stateful and stateless.

If it is set to stateful or stateless, session resumption with pre-shared keys is enabled and the server will send stateful or stateless session tickets to the client after successful connections.

A stateful session ticket is a database reference to internal state information. A stateless session ticket is a self-encrypted binary that contains both cryptographic keying material and state data.

Configures the server's built-in anti replay feature based on Bloom filters.

Allowed values are the pre-defined '10k', '100k' or a custom 3-tuple that defines the properties of the bloom filters: {WindowSize, HashFunctions, Bits}. WindowSize is the number of seconds after the current Bloom filter is rotated and also the window size used for freshness checks. HashFunctions is the number hash functions and Bits is the number of bits in the bit vector. '10k' and '100k' are simple defaults with the following properties:

• '10k': Bloom filters can hold 10000 elements with 3% probability of false positives. WindowSize: 10, HashFunctions: 5, Bits: 72985 (8.91 KiB).

• '100k': Bloom filters can hold 100000 elements with 3% probability of false positives. WindowSize: 10, HashFunctions: 5, Bits: 729845 (89.09 KiB).

If true (default), the server sends a cookie extension in its HelloRetryRequest messages.

Configures if the server accepts (enabled) or rejects (rejects) early data sent by a client. The default value is disabled.