Interface Functions for Secure Socket Layer

This module contains interface functions for the SSL/TLS/DTLS protocol. For detailed information about the supported standards see ssl(6).


Types used in SSL/TLS/DTLS

socket() = gen_tcp:socket()

sslsocket() = any()

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

tls_client_option() =
            client_option() |
            common_option() |
            socket_option() |

tls_server_option() =
            server_option() |
            common_option() |
            socket_option() |

socket_option() =
            gen_tcp:connect_option() |
            gen_tcp:listen_option() |

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 SSL/TLS and not DTLS

active_msgs() =
            {ssl, sslsocket(), Data :: binary() | list()} |
            {ssl_closed, sslsocket()} |
            {ssl_error, sslsocket(), Reason :: any()} |
            {ssl_passive, sslsocket()}

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.

transport_option() =
             {CallbackModule :: atom(),
              DataTag :: atom(),
              ClosedTag :: atom(),
              ErrTag :: atom()}} |
             {CallbackModule :: atom(),
              DataTag :: atom(),
              ClosedTag :: atom(),
              ErrTag :: atom(),
              PassiveTag :: atom()}}

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.

hostname() = string()

ip_address() = inet:ip_address()

protocol_version() = tls_version() | dtls_version()

tls_version() = 'tlsv1.2' | 'tlsv1.3' | tls_legacy_version()

dtls_version() = 'dtlsv1.2' | dtls_legacy_version()

tls_legacy_version() = tlsv1 | 'tlsv1.1' | sslv3

dtls_legacy_version() = dtlsv1

prf_random() = client_random | server_random

verify_type() = verify_none | verify_peer

ciphers() = [erl_cipher_suite()] | string()

erl_cipher_suite() =
            #{key_exchange := kex_algo(),
              cipher := cipher(),
              mac := hash() | aead,
              prf := hash() | default_prf}

cipher() =
            aes_128_cbc | aes_256_cbc | aes_128_gcm | aes_256_gcm |
            aes_128_ccm | aes_256_ccm | aes_128_ccm_8 | aes_256_ccm_8 |
            chacha20_poly1305 |

legacy_cipher() = rc4_128 | des_cbc | '3des_ede_cbc'

cipher_filters() =
            [{key_exchange | cipher | mac | prf, algo_filter()}]

hash() = sha | sha2() | legacy_hash()

sha2() = sha224 | sha256 | sha384 | sha512

legacy_hash() = md5

old_cipher_suite() =
            {kex_algo(), cipher(), hash()} |
            {kex_algo(), cipher(), hash() | aead, hash()}

signature_algs() = [{hash(), sign_algo()}]

sign_algo() = rsa | dsa | ecdsa

sign_scheme() =
            rsa_pkcs1_sha256 | rsa_pkcs1_sha384 | rsa_pkcs1_sha512 |
            ecdsa_secp256r1_sha256 | ecdsa_secp384r1_sha384 |
            ecdsa_secp521r1_sha512 | rsa_pss_rsae_sha256 |
            rsa_pss_rsae_sha384 | rsa_pss_rsae_sha512 |
            rsa_pss_pss_sha256 | rsa_pss_pss_sha384 | rsa_pss_pss_sha512 |
            rsa_pkcs1_sha1 | ecdsa_sha1

kex_algo() =
            rsa | dhe_rsa | dhe_dss | ecdhe_ecdsa | ecdh_ecdsa |
            ecdh_rsa | srp_rsa | srp_dss | psk | dhe_psk | rsa_psk |
            dh_anon | ecdh_anon | srp_anon | any

algo_filter() =
            fun((kex_algo() | cipher() | hash() | aead | default_prf) ->
                    true | false)

named_curve() =
            sect571r1 | sect571k1 | secp521r1 | brainpoolP512r1 |
            sect409k1 | sect409r1 | brainpoolP384r1 | secp384r1 |
            sect283k1 | sect283r1 | brainpoolP256r1 | secp256k1 |
            secp256r1 | sect239k1 | sect233k1 | sect233r1 | secp224k1 |
            secp224r1 | sect193r1 | sect193r2 | secp192k1 | secp192r1 |
            sect163k1 | sect163r1 | sect163r2 | secp160k1 | secp160r1 |

psk_identity() = string()

srp_identity() = {Username :: string(), Password :: string()}

srp_param_type() =
            srp_1024 | srp_1536 | srp_2048 | srp_3072 | srp_4096 |
            srp_6144 | srp_8192

app_level_protocol() = binary()

protocol_extensions() =
            #{renegotiation_info => binary(),
              signature_algs => signature_algs(),
              alpn => app_level_protocol(),
              srp => binary(),
              next_protocol => app_level_protocol(),
              ec_point_formats => [0..2],
              elliptic_curves => [public_key:oid()],
              sni => hostname()}

error_alert() =
            {tls_alert, {tls_alert(), Description :: string()}}

tls_alert() =
            close_notify | unexpected_message | bad_record_mac |
            record_overflow | handshake_failure | bad_certificate |
            unsupported_certificate | certificate_revoked |
            certificate_expired | certificate_unknown |
            illegal_parameter | unknown_ca | access_denied |
            decode_error | decrypt_error | export_restriction |
            protocol_version | insufficient_security | internal_error |
            inappropriate_fallback | user_canceled | no_renegotiation |
            unsupported_extension | certificate_unobtainable |
            unrecognized_name | bad_certificate_status_response |
            bad_certificate_hash_value | unknown_psk_identity |

reason() = any()


common_option() =
            {protocol, protocol()} |
            {handshake, handshake_completion()} |
            {cert, cert()} |
            {certfile, cert_pem()} |
            {key, key()} |
            {keyfile, key_pem()} |
            {password, key_password()} |
            {ciphers, cipher_suites()} |
            {eccs, [named_curve()]} |
            {signature_algs_cert, signature_schemes()} |
            {secure_renegotiate, secure_renegotiation()} |
            {depth, allowed_cert_chain_length()} |
            {verify_fun, custom_verify()} |
            {crl_check, crl_check()} |
            {crl_cache, crl_cache_opts()} |
            {max_handshake_size, handshake_size()} |
            {partial_chain, root_fun()} |
            {versions, protocol_versions()} |
            {user_lookup_fun, custom_user_lookup()} |
            {log_level, logging_level()} |
            {log_alert, log_alert()} |
            {hibernate_after, hibernate_after()} |
            {padding_check, padding_check()} |
            {beast_mitigation, beast_mitigation()} |
            {ssl_imp, ssl_imp()}

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

handshake_completion() = hello | full

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

cert() = public_key:der_encoded()

The DER-encoded users certificate. If this option is supplied, it overrides option certfile.

cert_pem() = file:filename()

Path to a file containing the user certificate on PEM format.

key() =
            {'RSAPrivateKey' | 'DSAPrivateKey' | 'ECPrivateKey' |
             public_key:der_encoded()} |
            #{algorithm := rsa | dss | ecdsa,
              engine := crypto:engine_ref(),
              key_id := crypto:key_id(),
              password => crypto:password()}

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.

key_pem() = file:filename()

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.

key_password() = string()

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

cipher_suites() = ciphers()

Supported cipher suites. The function cipher_suites/2 can be used to find all ciphers that are supported by default. cipher_suites(all, 'tlsv1.2') can be called to find all available cipher suites. Pre-Shared Key (RFC 4279 and RFC 5487), Secure Remote Password (RFC 5054), RC4, 3DES, DES cipher suites, and anonymous cipher suites only work if explicitly enabled by this option; they are supported/enabled by the peer also. Anonymous cipher suites are supported for testing purposes only and are not be used when security matters.

signature_schemes() = [sign_scheme()]

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).

secure_renegotiation() = boolean()

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.

allowed_cert_chain_length() = integer()

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 1.

custom_verify() =
            {Verifyfun :: function(), InitialUserState :: any()}

The verification fun is to be defined as follows:

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


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.


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

crl_check() = boolean() | peer | best_effort

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.

check is only performed on the peer certificate.
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).

crl_cache_opts() = [any()]

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:


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.


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.

root_fun() = function()

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.

protocol_versions() = [protocol_version()]

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, except SSL-3.0, supported by the SSL application. See also ssl(6).

custom_user_lookup() =
            {Lookupfun :: function(), UserState :: any()}

The lookup fun is to defined as follows:

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

session_id() = binary()

Identifies a TLS session.

log_alert() = boolean()

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

logging_level() = logger:level()

Specifies the log level for TLS/DTLS. At verbosity level notice and above error reports are displayed in TLS/DTLS. The level debug triggers verbose logging of TLS/DTLS protocol messages.

hibernate_after() = timeout()

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.

handshake_size() = integer()

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

padding_check() = boolean()

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.


Using {padding_check, boolean()} makes TLS vulnerable to the Poodle attack.

beast_mitigation() = one_n_minus_one | zero_n | disabled

Affects SSL-3.0 and 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.


Using {beast_mitigation, disabled} makes SSL-3.0 or TLS-1.0 vulnerable to the BEAST attack.

ssl_imp() = new | old

Deprecated since OTP-17, has no affect.


client_option() =
            {verify, client_verify_type()} |
            {reuse_session, client_reuse_session()} |
            {reuse_sessions, client_reuse_sessions()} |
            {cacerts, client_cacerts()} |
            {cacertfile, client_cafile()} |
            {alpn_advertised_protocols, client_alpn()} |
             client_preferred_next_protocols()} |
            {psk_identity, client_psk_identity()} |
            {srp_identity, client_srp_identity()} |
            {server_name_indication, sni()} |
            {customize_hostname_check, customize_hostname_check()} |
            {signature_algs, client_signature_algs()} |
            {fallback, fallback()}

client_verify_type() = verify_type()

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

client_reuse_session() = session_id()

Reuses a specific session earlier saved with the option {reuse_sessions, save} since OTP-21.3

client_reuse_sessions() = boolean() | save

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

client_cacerts() = [public_key:der_encoded()]

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

client_cafile() = file:filename()

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

client_alpn() = [app_level_protocol()]

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.

client_preferred_next_protocols() =
            {Precedence :: server | client,
             ClientPrefs :: [app_level_protocol()]} |
            {Precedence :: server | client,
             ClientPrefs :: [app_level_protocol()],
             Default :: app_level_protocol()}

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.

client_psk_identity() = psk_identity()

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

client_srp_identity() = srp_identity()

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

sni() = hostname() | disable

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

customize_hostname_check() = list()

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

fallback() = boolean()

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


Note this option is not needed in normal TLS usage and should not be used to implement new clients. But legacy clients that retries connections in the following manner

ssl:connect(Host, Port, [...{versions, ['tlsv2', 'tlsv1.1', 'tlsv1', 'sslv3']}])

ssl:connect(Host, Port, [...{versions, [tlsv1.1', 'tlsv1', 'sslv3']}, {fallback, true}])

ssl:connect(Host, Port, [...{versions, ['tlsv1', 'sslv3']}, {fallback, true}])

ssl:connect(Host, Port, [...{versions, ['sslv3']}, {fallback, true}])

may use it to avoid undesired TLS version downgrade. Note that TLS_FALLBACK_SCSV must also be supported by the server for the prevention to work.

client_signature_algs() = signature_algs()

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 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


server_option() =
            {cacerts, server_cacerts()} |
            {cacertfile, server_cafile()} |
            {dh, dh_der()} |
            {dhfile, dh_file()} |
            {verify, server_verify_type()} |
            {fail_if_no_peer_cert, fail_if_no_peer_cert()} |
            {reuse_sessions, server_reuse_sessions()} |
            {reuse_session, server_reuse_session()} |
            {alpn_preferred_protocols, server_alpn()} |
            {next_protocols_advertised, server_next_protocol()} |
            {psk_identity, server_psk_identity()} |
            {honor_cipher_order, boolean()} |
            {sni_hosts, sni_hosts()} |
            {sni_fun, sni_fun()} |
            {honor_cipher_order, honor_cipher_order()} |
            {honor_ecc_order, honor_ecc_order()} |
            {client_renegotiation, client_renegotiation()} |
            {signature_algs, server_signature_algs()}

server_cacerts() = [public_key:der_encoded()]

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

server_cafile() = file:filename()

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.

dh_der() = binary()

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

dh_file() = file:filename()

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.

server_verify_type() = verify_type()

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.

fail_if_no_peer_cert() = boolean()

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.

server_reuse_sessions() = boolean()

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

server_reuse_session() = function()

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().

server_alpn() = [app_level_protocol()]

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.

server_next_protocol() = [app_level_protocol()]

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.

server_psk_identity() = psk_identity()

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

honor_cipher_order() = boolean()

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

sni_hosts() =
            [{hostname(), [server_option() | common_option()]}]

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.

sni_fun() = function()

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.

client_renegotiation() = boolean()

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.

honor_cipher_order() = boolean()

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

honor_ecc_order() = boolean()

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

server_signature_algs() = signature_algs()

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.


append_cipher_suites(Deferred, Suites) -> ciphers()

Make Deferred suites become the least preferred suites, that is put them at the end of the cipher suite list Suites after removing them from Suites if present. Deferred may be a list of cipher suits or a list of filters in which case the filters are use on Suites to extract the Deferred cipher list.

cipher_suites() -> [old_cipher_suite()] | [string()]

cipher_suites(Type) -> [old_cipher_suite() | string()]

  • Type = erlang | openssl | all

Deprecated in OTP 21, use cipher_suites/2 instead.

cipher_suites(Supported, Version) -> ciphers()

Returns all default or all supported (except anonymous), or all anonymous cipher suites for a TLS version

cipher_suites(Supported, Version, StringType :: rfc | openssl) ->

Same as cipher_suites/2 but lists RFC or OpenSSL string names instead of erl_cipher_suite()

eccs() -> NamedCurves

eccs(Version) -> NamedCurves

Returns a list of supported ECCs. eccs() is equivalent to calling eccs(Protocol) with all supported protocols and then deduplicating the output.

clear_pem_cache() -> ok

PEM files, used by ssl API-functions, are cached. The cache is regularly checked to see if any cache entries should be invalidated, however this function provides a way to unconditionally clear the whole cache.

connect(TCPSocket, TLSOptions) ->
           {ok, sslsocket()} |
           {error, reason()} |
           {option_not_a_key_value_tuple, any()}

Upgrades a gen_tcp, or equivalent, connected socket to an TLS socket, that is, performs the client-side TLS handshake.


If the option verify is set to verify_peer the option server_name_indication shall also be specified, if it is not no Server Name Indication extension will be sent, and public_key:pkix_verify_hostname/2 will be called with the IP-address of the connection as ReferenceID, which is proably not what you want.

If the option {handshake, hello} is used the handshake is paused after receiving the server hello message and the success response is {ok, SslSocket, Ext} instead of {ok, SslSocket}. Thereafter the handshake is continued or canceled by calling handshake_continue/3 or handshake_cancel/1.

If the option active is set to once, true or an integer value, the process owning the sslsocket will receive messages of type active_msgs()

Opens an TLS/DTLS connection to Host, Port.

When the option verify is set to verify_peer the check public_key:pkix_verify_hostname/2 will be performed in addition to the usual x509-path validation checks. If the check fails the error {bad_cert, hostname_check_failed} will be propagated to the path validation fun verify_fun, where it is possible to do customized checks by using the full possibilities of the public_key:pkix_verify_hostname/3 API. When the option server_name_indication is provided, its value (the DNS name) will be used as ReferenceID to public_key:pkix_verify_hostname/2. When no server_name_indication option is given, the Host argument will be used as Server Name Indication extension. The Host argument will also be used for the public_key:pkix_verify_hostname/2 check and if the Host argument is an inet:ip_address() the ReferenceID used for the check will be {ip, Host} otherwise dns_id will be assumed with a fallback to ip if that fails.


According to good practices certificates should not use IP-addresses as "server names". It would be very surprising if this happen outside a closed network.

If the option {handshake, hello} is used the handshake is paused after receiving the server hello message and the success response is {ok, SslSocket, Ext} instead of {ok, SslSocket}. Thereafter the handshake is continued or canceled by calling handshake_continue/3 or handshake_cancel/1.

If the option active is set to once, true or an integer value, the process owning the sslsocket will receive messages of type active_msgs()

Closes an TLS/DTLS connection.

Closes or downgrades an TLS connection. In the latter case the transport connection will be handed over to the NewController process after receiving the TLS close alert from the peer. The returned transport socket will have the following options set: [{active, false}, {packet, 0}, {mode, binary}]

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

Returns the most relevant information about the connection, ssl options that are undefined will be filtered out. Note that values that affect the security of the connection will only be returned if explicitly requested by connection_information/2.


The legacy Item = cipher_suite is still supported and returns the cipher suite on its (undocumented) legacy format. It should be replaced by selected_cipher_suite.

Returns the requested information items about the connection, if they are defined.

Note that client_random, server_random and master_secret are values that affect the security of connection. Meaningful atoms, not specified above, are the ssl option names.


If only undefined options are requested the resulting list can be empty.

Removes cipher suites if any of the filter functions returns false for any part of the cipher suite. This function also calls default filter functions to make sure the cipher suites are supported by crypto. If no filter function is supplied for some part the default behaviour is fun(Algorithm) -> true.

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

Gets the values of the specified socket options.

Gets one or more statistic options for the underlying TCP socket.

See inet:getstat/2 for statistic options description.

Performs the SSL/TLS/DTLS server-side handshake.

Returns a new TLS/DTLS socket if the handshake is successful.

If the option active is set to once, true or an integer value, the process owning the sslsocket will receive messages of type active_msgs()

If Socket is a ordinary socket(): upgrades a gen_tcp, or equivalent, socket to an SSL socket, that is, performs the SSL/TLS server-side handshake and returns a TLS socket.


The Socket shall be in passive mode ({active, false}) before calling this function or else the behavior of this function is undefined.

If Socket is an sslsocket() : provides extra SSL/TLS/DTLS options to those specified in listen/2 and then performs the SSL/TLS/DTLS handshake. Returns a new TLS/DTLS socket if the handshake is successful.

If option {handshake, hello} is specified the handshake is paused after receiving the client hello message and the success response is {ok, SslSocket, Ext} instead of {ok, SslSocket}. Thereafter the handshake is continued or canceled by calling handshake_continue/3 or handshake_cancel/1.

If the option active is set to once, true or an integer value, the process owning the sslsocket will receive messages of type active_msgs()

Cancel the handshake with a fatal USER_CANCELED alert.

Continue the SSL/TLS handshake possiby with new, additional or changed options.

Creates an SSL listen socket.

Returns the protocol negotiated through ALPN or NPN extensions.

The peer certificate is returned as a DER-encoded binary. The certificate can be decoded with public_key:pkix_decode_cert/2

Returns the address and port number of the peer.

Make Preferred suites become the most preferred suites that is put them at the head of the cipher suite list Suites after removing them from Suites if present. Preferred may be a list of cipher suits or a list of filters in which case the filters are use on Suites to extract the preferred cipher list.

Uses the Pseudo-Random Function (PRF) of a TLS session to generate extra key material. It either takes user-generated values for Secret and Seed or atoms directing it to use a specific value from the session security parameters.

Can only be used with TLS/DTLS connections; {error, undefined} is returned for SSLv3 connections.

See the description of HttpPacket in erlang:decode_packet/3 in ERTS.

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

Argument Length is meaningful only when the socket is in mode raw 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.

Optional argument Timeout specifies a time-out in milliseconds. The default value is infinity.

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.

Writes Data to SslSocket.

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

Sets options according to Options for socket SslSocket.

Immediately closes 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, option {exit_on_close, false} is useful.

Deprecated in OTP 21, use handshake/[1,2] instead.


handshake/[1,2] always returns a new socket.

Deprecated in OTP 21, use handshake/[2,3] instead.


handshake/[2,3] always returns a new socket.

Returns the local address and port number of socket SslSocket.

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

Starts the SSL application. Default type is temporary.

Stops the SSL application.

Converts an RFC or OpenSSL name string to an erl_cipher_suite() Returns an error if the cipher suite is not supported or the name is not a valid cipher suite name.

Converts erl_cipher_suite() to OpenSSL name string.

PRE TLS-1.3 these names differ for RFC names

Converts erl_cipher_suite() to RFC name string.

Accepts an incoming connection request on a listen socket. ListenSocket must be a socket returned from listen/2. The socket returned is to be passed to handshake/[2,3] to complete handshaking, that is, establishing the SSL/TLS/DTLS connection.


Most API functions require that the TLS/DTLS connection is established to work as expected.

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.

Returns version information relevant for the SSL application.

The application version of the SSL application.
SSL/TLS versions supported by default. Overridden by a version option on connect/[2,3,4], listen/2, and ssl_accept/[1,2,3]. For the negotiated SSL/TLS version, see connection_information/1 .
DTLS versions supported by default. Overridden by a version option on connect/[2,3,4], listen/2, and ssl_accept/[1,2,3]. For the negotiated DTLS version, see connection_information/1 .
All SSL/TLS versions supported by the SSL application. TLS 1.2 requires sufficient support from the Crypto application.
All DTLS versions supported by the SSL application. DTLS 1.2 requires sufficient support from the Crypto application.