ssl(3)

NAME
ssl - Interface Functions for Secure Socket Layer

DESCRIPTION

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

DATA TYPES

  Types used in TLS/DTLS
socket() = gen_tcp:socket()
sslsocket() = any()

    An opaque reference to the TLS/DTLS connection, may be used for equality matching.
tls_option() = tls_client_option() | tls_server_option()
tls_client_option() =
    client_option() |
    common_option() |
    socket_option() |
    transport_option()
tls_server_option() =
    server_option() |
    common_option() |
    socket_option() |
    transport_option()
socket_option() =
    gen_tcp:connect_option() |
    gen_tcp:listen_option() |
    gen_udp: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 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() =
    {cb_info,
     {CallbackModule :: atom(),
      DataTag :: atom(),
      ClosedTag :: atom(),
      ErrTag :: atom()}} |
    {cb_info,
     {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 experimental.
host() = inet:hostname() | 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'
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_256_gcm | aes_128_gcm | aes_256_ccm | aes_128_ccm |
    chacha20_poly1305 | aes_256_ccm_8 | aes_128_ccm_8 |
    aes_128_cbc | aes_256_cbc |
    legacy_cipher()
legacy_cipher() = '3des_ede_cbc' | des_cbc | rc4_128
cipher_filters() =
    [{key_exchange | cipher | mac | prf, algo_filter()}]
hash() = sha2() | legacy_hash()
sha2() = sha512 | sha384 | sha256
legacy_hash() = sha224 | sha | md5
old_cipher_suite() =
    {kex_algo(), cipher(), hash()} |
    {kex_algo(), cipher(), hash() | aead, hash()}
sign_algo() = eddsa | ecdsa | rsa | dsa
sign_scheme() =
    eddsa_ed25519 | eddsa_ed448 | ecdsa_secp384r1_sha384 |
    ecdsa_secp521r1_sha512 | ecdsa_secp256r1_sha256 |
    rsassa_pss_scheme() |
    sign_scheme_legacy()
rsassa_pss_scheme() =
    rsa_pss_rsae_sha512 | rsa_pss_rsae_sha384 |
    rsa_pss_rsae_sha256 | rsa_pss_pss_sha512 |
    rsa_pss_pss_sha384 | rsa_pss_pss_sha256
sign_scheme_legacy() =
    rsa_pkcs1_sha512 | rsa_pkcs1_sha384 | rsa_pkcs1_sha256 |
    ecdsa_sha1 | rsa_pkcs1_sha1
group() =
    x25519 | x448 | secp256r1 | secp384r1 | secp521r1 |
    ffdhe2048 | ffdhe3072 | ffdhe4096 | ffdhe6144 | ffdhe8192
kex_algo() =
    ecdhe_ecdsa | ecdh_ecdsa | ecdh_rsa | rsa | dhe_rsa |
    dhe_dss | srp_rsa | srp_dss | dhe_psk | rsa_psk | psk |
    ecdh_anon | dh_anon | srp_anon | any
algo_filter() =
    fun((kex_algo() | cipher() | hash() | aead | default_prf) ->
            true | false)
named_curve() =
    x25519 | x448 | secp521r1 | brainpoolP512r1 |
    brainpoolP384r1 | secp384r1 | brainpoolP256r1 | secp256r1 |
    legacy_named_curve()
legacy_named_curve() =
    sect571r1 | sect571k1 | sect409k1 | sect409r1 | sect283k1 |
    sect283r1 | secp256k1 | sect239k1 | sect233k1 | sect233r1 |
    secp224k1 | secp224r1 | sect193r1 | sect193r2 | secp192k1 |
    secp192r1 | sect163k1 | sect163r1 | sect163r2 | secp160k1 |
    secp160r1 | secp160r2
psk_identity() = string()
srp_identity() = {Username :: string(), Password :: string()}
srp_param_type() =
    srp_8192 | srp_6144 | srp_4096 | srp_3072 | srp_2048 |
    srp_1536 | srp_1024
app_level_protocol() = binary()
protocol_extensions() =
    #{renegotiation_info => binary(),
      signature_algs => signature_algs(),
      alpn => app_level_protocol(),
      srp => binary(),
      next_protocol => app_level_protocol(),
      max_frag_enum => 1..4,
      ec_point_formats => [0..2],
      elliptic_curves => [public_key:oid()],
      sni => inet: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 |
    no_application_protocol
reason() = any()
bloom_filter_window_size() = integer()
bloom_filter_hash_functions() = integer()
bloom_filter_bits() = integer()
client_session_tickets() = disabled | manual | auto
server_session_tickets() =
    disabled | stateful | stateless | stateful_with_cert |
    stateless_with_cert

DATA TYPES

  TLS/DTLS OPTION DESCRIPTIONS - COMMON for SERVER and CLIENT
common_option() =
    {protocol, protocol()} |
    {handshake, handshake_completion()} |
    {cert, cert() | [cert()]} |
    {certfile, cert_pem()} |
    {key, key()} |
    {keyfile, key_pem()} |
    {password, key_pem_password()} |
    {certs_keys, certs_keys()} |
    {ciphers, cipher_suites()} |
    {eccs, [named_curve()]} |
    {signature_algs, signature_algs()} |
    {signature_algs_cert, sign_schemes()} |
    {supported_groups, supported_groups()} |
    {secure_renegotiate, secure_renegotiation()} |
    {keep_secrets, keep_secrets()} |
    {depth, allowed_cert_chain_length()} |
    {verify_fun, custom_verify()} |
    {allow_any_ca_purpose, allow_any_ca_purpose()} |
    {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()} |
    {session_tickets, session_tickets()} |
    {key_update_at, key_update_at()} |
    {receiver_spawn_opts, spawn_opts()} |
    {sender_spawn_opts, spawn_opts()}
protocol() = tls | dtls

    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 user certificate. Note that the cert option may also be a list of DER-encoded certificates where the first one is the user 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.
cert_pem() = file:filename()

    Path to a file containing the user certificate on PEM format or possible several certificates where the first one is the user certificate and the rest of the certificates constitutes the certificate chain. For more details see cert(),
key() =
    {'RSAPrivateKey' | 'DSAPrivateKey' | 'ECPrivateKey' |
     'PrivateKeyInfo',
     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 referring to a crypto engine and its key reference that optionally can be password protected, see also crypto:engine_load/3 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_pem_password() = iodata() | fun(() -> iodata())

    String containing the user's password or a function returning same type. Only used if the private keyfile is password-protected.
certs_keys() = [cert_key_conf()]

    A list of a certificate (or possible a certificate and its chain) and the associated key of the certificate, that may be used to authenticate the client or the server. The certificate key pair that is considered best and matches negotiated parameters for the connection will be selected. Different signature algorithms are prioritized in the order eddsa, ecdsa, rsa_pss_pss, rsa and dsa . If more than one key is supplied for the same signing algorithm (which is probably an unusual use case) they will prioritized by strength unless it is a so called engine key that will be favoured over other keys. As engine keys cannot be inspected, supplying more than one engine key will make no sense. This offers flexibility to for instance configure a newer certificate that is expected to be used in most cases and an older but acceptable certificate that will only be used to communicate with legacy systems. Note that there is a trade off between the induced overhead and the flexibility so alternatives should be chosen for good reasons. If the certs_keys option is specified it overrides all single certificate and key options. For examples see the Users Guide

        Note:
    eddsa certificates are only supported by TLS-1.3 that does not support dsa certificates. rsa_pss_pss (RSA certificates using Probabilistic Signature Scheme) are supported in TLS-1.2 and TLS-1.3, but some TLS-1.2 implementations may not support rsa_pss_pss.
cert_key_conf() =
    #{cert => cert(),
      key => key(),
      certfile => cert_pem(),
      keyfile => key_pem(),
      password => key_pem_password()}

    A certificate (or possibly a certificate and its chain) and its associated key on one of the possible formats. For the PEM file format there may also be a password associated with the file containg the key.
cipher_suites() = ciphers()

    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 Additionally 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.

        Note:
    Note that TLS-1.3 and TLS-1.2 cipher suites are not overlapping sets of cipher suites so to support both these versions cipher suites from both versions need to be included. Also if the supplied list does not comply with the configured versions or cryptolib so that the list becomes empty, this option will fallback on its appropriate default value for the configured versions.

    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.
signature_algs() = [{hash(), sign_algo()} | sign_scheme()]

    Explicitly list acceptable signature algorithms for certificates and handshake messages in the preferred order. The client will send its list as the client hello signature_algorithm extension introduced in TLS-1.2, see Section 7.4.1.4.1 in RFC 5246. Previously these algorithms where implicitly chosen and partly derived from the cipher suite.

    In TLS-1.2 a somewhat more explicit negotiation is made possible using a list of {hash(), sign_algo()} pairs.

    In TLS-1.3 these algorithm pairs are replaced by so called signature schemes sign_scheme() and completely decoupled from the cipher suite.

    Signature algorithms used for certificates may be overridden by the signature schemes (algorithms) supplied by the signature_algs_cert option.

    TLS-1.2 default is Default_TLS_12_Alg_Pairs interleaved with rsa_pss_schemes since ssl-11.0 (OTP-25) pss_pss is prefered over pss_rsae that is prefered over rsa

    Default_TLS_12_Alg_Pairs =

    [
    %% SHA2
    {sha512, ecdsa},
    {sha512, rsa},
    {sha384, ecdsa},
    {sha384, rsa},
    {sha256, ecdsa},
    {sha256, rsa}
    ]

    Support for {md5, rsa} was removed from the the TLS-1.2 default in ssl-8.0 (OTP-22) and support for SHA1 {sha, _} and SHA224 {sha224, _} was removed in ssl-11.0 (OTP-26)

    rsa_pss_schemes =

    [rsa_pss_pss_sha512,
    rsa_pss_pss_sha384,
    rsa_pss_pss_sha256,
    rsa_pss_rsae_sha512,
    rsa_pss_rsae_sha384,
    rsa_pss_rsae_sha256]

    TLS_13_Legacy_Schemes =

     [
     %% Legacy algorithms only applicable to certificate signatures
    rsa_pkcs1_sha512, %% Corresponds to {sha512, rsa}
    rsa_pkcs1_sha384, %% Corresponds to {sha384, rsa}
    rsa_pkcs1_sha256, %% Corresponds to {sha256, rsa}
    ]

    Default_TLS_13_Schemes =

     [
     %% EDDSA
    eddsa_ed25519,
    eddsa_ed448
    %% ECDSA
    ecdsa_secp521r1_sha512,
    ecdsa_secp384r1_sha384,
    ecdsa_secp256r1_sha256] ++
    %% RSASSA-PSS
    rsa_pss_schemes()

    EDDSA was made highest priority in ssl-10.8 (OTP-25)

    TLS-1.3 default is

    Default_TLS_13_Schemes

    If both TLS-1.3 and TLS-1.2 are supported the default will be

    Default_TLS_13_Schemes ++ TLS_13_Legacy_Schemes ++ Default_TLS_12_Alg_Pairs (not represented in TLS_13_Legacy_Schemes)

    so appropriate algorithms can be chosen for the negotiated version.

        Note:
    TLS-1.2 algorithms will not be negotiated for TLS-1.3, but TLS-1.3 RSASSA-PSS rsassa_pss_scheme() signature schemes may be negotiated also for TLS-1.2 from 24.1 (fully working from 24.1.3). However if TLS-1.3 is negotiated when both TLS-1.3 and TLS-1.2 is supported using defaults, the corresponding TLS-1.2 algorithms to the TLS-1.3 legacy signature schemes will be considered as the legacy schemes and applied only to certificate signatures.
sign_schemes() = [sign_scheme()]

    Explicitly list acceptable signature schemes (algorithms) in the preferred order. Overrides the algorithms supplied in signature_algs option for certificates.

    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 need.

    The client will send a `signature_algorithms_cert` extension (in the client hello message), if TLS version 1.2 (back-ported to TLS 1.2 in 24.1) or later is used, and the signature_algs_cert option is explicitly specified. By default, only the extension signature_algs is sent with the exception of when signature_algs option is not explicitly specified, in which case it will append the rsa_pkcs1_sha1 algorithm to the default value of signature_algs and use it as value for signature_algs_cert to allow certificates to have this signature but still disallow sha1 use in the TLS protocol, since OTP-26.2.5.2

        Note:
    Note that supported signature schemes for TLS-1.2 are sign_scheme_legacy() and rsassa_pss_scheme()
supported_groups() = [group()]

    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.
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 10.
custom_verify() =
    {Verifyfun :: function(), InitialUserState :: any()}

    The verification fun is to be defined as follows:

    fun(OtpCert :: #'OTPCertificate'{},
        Event, InitialUserState :: term()) ->
    	{valid, UserState :: term()} |
    	{fail, Reason :: term()} | {unknown, UserState :: term()}.
    fun(OtpCert :: #'OTPCertificate'{}, DerCert :: public_key:der_encoded(),
        Event, InitialUserState :: term()) ->
    	{valid, UserState :: term()} |
    	{fail, Reason :: term()} | {unknown, UserState :: term()}.
    Types:
          Event = {bad_cert, Reason :: atom() |
                  {revoked, atom()}} |
    	      {extension, #'Extension'{}} |
                  valid |
                  valid_peer

    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 Event 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.

        {invalid_ext_keyusage, [public_key:oid()]}:
            If the peer certificate specifies the extended keyusage extension and does not include the purpose for either being a TLS server (id-kp-ServerAuth) or TLS client (id-kp-ClientAuth) depending on the peers role.

        {ca_invalid_ext_keyusage, [public_key:oid()]} :
            If a CA certificate specifies the extended keyusage extension and does not include the purpose for either being a TLS server (id-kp-ServerAuth) or TLS client (id-kp-ClientAuth) depending on the role of the peer chained with this CA, or the option allow_any_ca_purpose is set to `true` but the special any-value (anyExtendedKeyUsage) is not included in the CA cert purposes.

        PKIX X-509-path validation error:
            For possible reasons, see public_key:pkix_path_validation/3
allow_any_ca_purpose() = boolean()

    If a CA certificate has an extended key usage extension but it does not want to restrict the usages of the key it can include a special `anyExtendedKeyUsage` purpose. If this is option is set to `true` all key usage purposes is automatically accepted for the CA that includes this purpose, the option default to false.
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.

        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).
crl_cache_opts() =
    {Module :: atom(),
     {DbHandle :: internal | term(), Args :: list()}}

    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.
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, 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 :: 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])])
session_id() = binary()

    Identifies a TLS session.
log_alert() = boolean()

    If set to false, TLS/DTLS Alert reports are not displayed. Deprecated in OTP 22, use {log_level, logging_level()} instead.
logging_level() = logger:level() | none | all

    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)
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.

        Warning:
    Using {padding_check, boolean()} makes TLS vulnerable to the Poodle attack.
beast_mitigation() = one_n_minus_one | zero_n | disabled

    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.

        Warning:
    Using {beast_mitigation, disabled} makes TLS-1.0 vulnerable to the BEAST attack.
ssl_imp() = new | old

    Deprecated since OTP-17, has no effect.
session_tickets() =
    client_session_tickets() | server_session_tickets()

    Configures the session ticket functionality in TLS 1.3 client and server.
key_update_at() = integer() >= 1

    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.

        Warning:
    The default value of this option shall provide the above mentioned security guarantees and it shall be reasonable for most applications (~353 TB).
middlebox_comp_mode() = boolean()

    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.
spawn_opts() = [erlang:spawn_opt_option()]

    Configures spawn options of TLS sender and receiver processes.

    Setting up garbage collection options can be helpful for trade-offs between CPU usage and Memory usage. See erlang:spawn_opt/2.

    For dist connections, default sender option is [...{priority, max}], this priority option cannot be changed. For all connections, ...link is added to receiver and cannot be changed.
keep_secrets() = boolean()

    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

DATA TYPES

  TLS/DTLS OPTION DESCRIPTIONS - CLIENT
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,
     client_preferred_next_protocols()} |
    {psk_identity, client_psk_identity()} |
    {srp_identity, client_srp_identity()} |
    {server_name_indication, sni()} |
    {max_fragment_length, max_fragment_length()} |
    {customize_hostname_check, customize_hostname_check()} |
    {fallback, fallback()} |
    {middlebox_comp_mode, middlebox_comp_mode()} |
    {certificate_authorities, client_certificate_authorities()} |
    {session_tickets, client_session_tickets()} |
    {use_ticket, use_ticket()} |
    {early_data, client_early_data()} |
    {use_srtp, use_srtp()}
client_verify_type() = verify_type()

    Defaults to verify_peer, since OTP-26, which means the option cacerts or cacertfile is also required to perform the certificate verification unless verify_none is explicitly configured. For example an HTTPS client would normally use the option {cacerts, public_key:cacerts_get()} (available since OTP-25) to access the CA certificates provided by the OS. Using verify_none means that all x509-certificate path validation errors will be ignored. See also option verify_fun.
client_reuse_session() =
    session_id() | {session_id(), SessionData :: binary()}

    Reuses a specific session. The session should be referred 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.
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, automated 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_certificate_authorities() = boolean()

    If set to true, sends the certificate authorities extension in TLS-1.3 client hello. The default is false. Note that setting it to true may result in a big overhead if you have many trusted CA certificates. Since OTP-24.3.
client_cacerts() =
    [public_key:der_encoded()] | [public_key:combined_cert()]

    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.

        Note:
    When PEM caching is enabled, files provided with this option will be checked for updates at fixed time intervals specified by the ssl_pem_cache_clean environment parameter.

        Note:
    Alternatively, CA certificates can be provided as a DER-encoded binary with client_cacerts option.
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.
max_fragment_length() = undefined | 512 | 1024 | 2048 | 4096

    Specifies the maximum fragment length the client is prepared to accept from the server. See RFC 6066
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() = inet: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 example the get standard HTTPS handling provide the already implememnted fun from the public_key application for HTTPS. {customize_hostname_check, [{match_fun, public_key:pkix_verify_hostname_match_fun(https)}]} For futher description of customize 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

        Warning:
    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']}])

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

    ssl:connect(Host, Port, [...{versions, ['tlsv1']}, {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_session_tickets() = disabled | manual | auto

    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).

        Note:
    This option is supported by TLS 1.3 and above. See also SSL's Users Guide, Session Tickets and Session Resumption in TLS 1.3
use_ticket() = [binary()]

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

        Note:
    Session tickets are only sent to user if option session_tickets is set to manual

    This option is supported by TLS 1.3 and above. See also SSL's Users Guide, Session Tickets and Session Resumption in TLS 1.3
client_early_data() = binary()

    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.

        Warning:
    It is the responsibility of the user to handle a rejected Early Data and to resend when it is appropriate.
use_srtp() =
    #{protection_profiles := [binary()], mki => binary()}

    Configures the use_srtp DTLS hello extension.

    In order to negotiate the use of SRTP data protection, clients include an extension of type "use_srtp" in the DTLS extended client hello. This extension MUST only be used when the data being transported is RTP or RTCP.

    The value is a map with a mandatory protection_profiles and an optional mki parameters.

    protection_profiles configures the list of the client's acceptable SRTP Protection Profiles. Each profile is a 2-byte binary. Example: #{protection_profiles => [<<0,2>>, <<0,5>>]}

    mki configures the SRTP Master Key Identifier chosen by the client.

    The srtp_mki field contains the value of the SRTP MKI which is associated with the SRTP master keys derived from this handshake. Each SRTP session MUST have exactly one master key that is used to protect packets at any given time. The client MUST choose the MKI value so that it is distinct from the last MKI value that was used, and it SHOULD make these values unique for the duration of the TLS session.

        Note:
    This extension MUST only be used with DTLS, and not with TLS.

        Note:
    OTP does not handle SRTP, so an external implementations of SRTP encoder/decoder and a packet demultiplexer are needed to make use of the use_srtp extension. See also cb_info option.

DATA TYPES

  TLS/DTLS OPTION DESCRIPTIONS - SERVER
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()} |
    {certificate_authorities, server_certificate_authorities()} |
    {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()} |
    {sni_hosts, sni_hosts()} |
    {sni_fun, sni_fun()} |
    {honor_cipher_order, honor_cipher_order()} |
    {honor_ecc_order, honor_ecc_order()} |
    {client_renegotiation, client_renegotiation()} |
    {session_tickets, server_session_tickets()} |
    {stateless_tickets_seed, stateless_tickets_seed()} |
    {anti_replay, anti_replay()} |
    {cookie, cookie()} |
    {early_data, server_early_data()} |
    {use_srtp, use_srtp()}
server_cacerts() =
    [public_key:der_encoded()] | [public_key:combined_cert()]

    The DER-encoded trusted certificates. If this option is supplied it overrides option cacertfile.
server_certificate_authorities() = boolean()

    Determines if a TLS-1.3 server should include the authorities extension in its certificate request message that will be sent if the option verify is set to verify_peer. Defaults to true.

    If set to false for older TLS versions its corresponding certificate authorities definition in its certificate request will be set to the empty list instead of including the appropriate certificate authorities. This has the same affect as excluding the TLS-1.3 extension.

    A reason to exclude the certificate authorities would be if the server wants to communicate with clients incapable of sending complete certificate chains that adhere to the certificate authorities, but the server still has the capability to recreate a chain that it can verify.
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.

        Note:
    When PEM caching is enabled, files provided with this option will be checked for updates at fixed time intervals specified by the ssl_pem_cache_clean environment parameter.

        Note:
    Alternatively, CA certificates can be provided as a DER-encoded binary with server_cacerts option.
dh_der() = binary()

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

        Warning:
    The dh_der option is not supported by TLS 1.3. Use the supported_groups option instead.
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.

        Warning:
    The dh_file option is not supported by TLS 1.3. Use the supported_groups option instead.
server_verify_type() = verify_type()

    Client certificates are an optional part of the TLS protocol. A server only does x509-certificate path validation in mode verify_peer. By default the server is in verify_none mode an hence will not send an certificate request to the client. When using verify_peer you may also want to specify the options fail_if_no_peer_cert and certificate_authorities.
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 `true`, the default value was changed in OTP-26.0.
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() =
    [{inet: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() = fun((string()) -> [] | undefined)

    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_session_tickets() =
    disabled | stateful | stateless | stateful_with_cert |
    stateless_with_cert

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

    If it is not set to disabled, session resumption with pre-shared keys is enabled and the server will send stateful or stateless session tickets to the client after successful connections.

        Note:
    Pre-shared key session ticket resumption does not include any certificate exchange, hence the function ssl:peercert/1 will not be able to return the peer certificate as it is only communicated in the initial handshake. The server options stateful_with_cert or stateless_with_cert may be used to make a server associate the client certificate from the original handshake with the tickets it issues.

    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.

        Warning:
    If it is set to stateful_with_cert the client certificate is stored with the internal state information, increasing memory consumption. If it is set to stateless_with_cert the client certificate is encoded in the self-encrypted binary that is sent to the client, increasing the payload size.

        Note:
    This option is supported by TLS 1.3 and above. See also SSL's Users Guide, Session Tickets and Session Resumption in TLS 1.3
stateless_tickets_seed() = binary()

    Configures the seed used for the encryption of stateless session tickets. Allowed values are any randomly generated binary(). If this option is not configured, an encryption seed will be randomly generated.

        Warning:
    Reusing the ticket encryption seed between multiple server instances enables stateless session tickets to work across multiple server instances, but it breaks anti-replay protection across instances.

    Inaccurate time synchronization between server instances can also affect session ticket freshness checks, potentially causing false negatives as well as false positives.

        Note:
    This option is supported by TLS 1.3 and above and only with stateless session tickets.
anti_replay() =
    '10k' | '100k' |
    {bloom_filter_window_size(),
     bloom_filter_hash_functions(),
     bloom_filter_bits()}

    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 of ClientHello. 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).

        Note:
    This option is supported by TLS 1.3 and above and only with stateless session tickets. Ticket lifetime, the number of tickets sent by the server and the maximum number of tickets stored by the server in stateful mode are configured by application variables. See also SSL's Users Guide, Anti-Replay Protection in TLS 1.3
cookie() = boolean()

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

        Note:
    The cookie extension has two main purposes. It allows the server to force the client to demonstrate reachability at their apparent network address (thus providing a measure of DoS protection). This is primarily useful for non-connection-oriented transports. It also allows to offload the server's state to the client. The cookie extension is enabled by default as it is a mandatory extension in RFC8446.
server_early_data() = disabled | enabled

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

        Warning:
    This option is a placeholder, early data is not yet implemented on the server side.
use_srtp() =
    #{protection_profiles := [binary()], mki => binary()}

    Configures the use_srtp DTLS hello extension.

    Servers that receive an extended hello containing a "use_srtp" extension can agree to use SRTP by including an extension of type "use_srtp", with the chosen protection profile in the extended server hello. This extension MUST only be used when the data being transported is RTP or RTCP.

    The value is a map with a mandatory protection_profiles and an optional mki parameters.

        *   protection_profiles configures the list of the server's chosen SRTP Protection Profile as a list of a single 2-byte binary. Example: #{protection_profiles => [<<0,5>>]}

        *   mki configures the server's SRTP Master Key Identifier.

            Upon receipt of a "use_srtp" extension containing a "srtp_mki" field, the server MUST either (assuming it accepts the extension at all):

            *   include a matching "srtp_mki" value in its "use_srtp" extension to indicate that it will make use of the MKI, or

            *   return an empty "srtp_mki" value to indicate that it cannot make use of the MKI (default).

        Note:
    This extension MUST only be used with DTLS, and not with TLS.

        Note:
    OTP does not handle SRTP, so an external implementations of SRTP encoder/decoder and a packet demultiplexer are needed to make use of the use_srtp extension. See also cb_info option.
connection_info() =
    [common_info() |
     curve_info() |
     ssl_options_info() |
     security_info()]
common_info() =
    {protocol, protocol_version()} |
    {session_id, session_id()} |
    {session_resumption, boolean()} |
    {selected_cipher_suite, erl_cipher_suite()} |
    {sni_hostname, term()} |
    {srp_username, term()}
curve_info() = {ecc, {named_curve, term()}}
ssl_options_info() = tls_option()
security_info() =
    {client_random, binary()} |
    {server_random, binary()} |
    {master_secret, binary()}
connection_info_items() = [connection_info_item()]
connection_info_item() =
    protocol | session_id | session_resumption |
    selected_cipher_suite | sni_hostname | srp_username | ecc |
    client_random | server_random | master_secret | keylog |
    tls_options_name()
tls_options_name() = atom()

EXPORTS

append_cipher_suites(Deferred, Suites) -> ciphers()

    Types:
        Deferred = ciphers() | cipher_filters()
        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 suites or a list of filters in which case the filters are use on Suites to extract the Deferred cipher list.

cipher_suites(Description, Version) -> ciphers()

    Types:
        Description =
            default | all | exclusive | anonymous | exclusive_anonymous
        Version = protocol_version() | ssl_record:ssl_version()

    Lists all possible cipher suites corresponding to Description that are available. The exclusive and exclusive_anonymous option will exclusively list cipher suites first supported in Version whereas the other options are inclusive from the lowest possible version to Version. The all options includes all suites except the anonymous and no anonymous suites are supported by default.

        Note:
    TLS-1.3 has no overlapping cipher suites with previous TLS versions, that is the result of cipher_suites(all, 'tlsv1.3'). contains a separate set of suites that can be used with TLS-1.3 an other set that can be used if a lower version is negotiated. PRE TLS-1.3 so called PSK and SRP suites need extra configuration to work see user lookup function. No anonymous suites are supported by TLS-1.3.

    Also note that the cipher suites returned by this function are the cipher suites that the OTP ssl application can support provided that they are supported by the cryptolib linked with the OTP crypto application. Use ssl:filter_cipher_suites(Suites, []). to filter the list for the current cryptolib. Note that cipher suites may be filtered out because they are too old or too new depending on the cryptolib

cipher_suites(Description, Version, StringType :: rfc | openssl) ->
                 [string()]

    Types:
        Description = default | all | exclusive | anonymous
        Version = protocol_version() | ssl_record:ssl_version()

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

eccs() -> NamedCurves

    Types:
        NamedCurves = [named_curve()]

    Returns a list of all supported elliptic curves, including legacy curves, for all TLS/DTLS versions pre TLS-1.3.

eccs(Version) -> NamedCurves

    Types:
        Version = 'tlsv1.2' | 'tlsv1.1' | tlsv1 | 'dtlsv1.2' | dtlsv1
        NamedCurves = [named_curve()]

    Returns the by default supported elliptic curves for Version, which is a subset of what eccs/0 returns.

clear_pem_cache() -> ok

    PEM files, used by ssl API-functions, are cached for performance reasons. The cache is automatically checked at regular intervals to see if any cache entries should be invalidated.

    This function provides a way to unconditionally clear the entire cache, thereby forcing a reload of previously cached PEM files.

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

    Types:
        TCPSocket = socket()
        TLSOptions = [tls_client_option()]
        Timeout = timeout()

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

        Note:
    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 probably 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()

connect(Host, Port, TLSOptions) ->
           {ok, sslsocket()} |
           {ok, sslsocket(), Ext :: protocol_extensions()} |
           {error, reason()} |
           {option_not_a_key_value_tuple, any()}
connect(Host, Port, TLSOptions, Timeout) ->
           {ok, sslsocket()} |
           {ok, sslsocket(), Ext :: protocol_extensions()} |
           {error, reason()} |
           {option_not_a_key_value_tuple, any()}

    Types:
        Host = host()
        Port = inet:port_number()
        TLSOptions = [tls_client_option()]
        Timeout = timeout()

    Opens a 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.

        Note:
    According to good practices certificates should not use IP-addresses as "server names". It would be very surprising if this happened 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()

close(SslSocket) -> ok | {error, Reason}

    Types:
        SslSocket = sslsocket()
        Reason = any()

    Closes a TLS/DTLS connection.

close(SslSocket, How) ->
         ok | {ok, port()} | {ok, port(), Data} | {error, Reason}

    Types:
        SslSocket = sslsocket()
        How = timeout() | {NewController :: pid(), timeout()}
        Data = binary()
        Reason = any()

    Closes or downgrades a 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}].

    In case of downgrade, the close function might return some binary data that should be treated by the user as the first bytes received on the downgraded connection.

controlling_process(SslSocket, NewOwner) -> ok | {error, Reason}

    Types:
        SslSocket = sslsocket()
        NewOwner = pid()
        Reason = any()

    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.

connection_information(SslSocket) ->
                          {ok, Result} | {error, reason()}

    Types:
        SslSocket = sslsocket()
        Result = connection_info()

    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.

        Note:
    The legacy Item = cipher_suite was removed in OTP-23. Previously it returned the cipher suite on its (undocumented) legacy format. It is replaced by selected_cipher_suite.

connection_information(SslSocket, Items) ->
                          {ok, Result} | {error, reason()}

    Types:
        SslSocket = sslsocket()
        Items = connection_info_items()
        Result = connection_info()

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

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

    In order to retrieve keylog and other secret information from a TLS 1.3 connection, keep_secrets must be configured in advance and set to true.

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

filter_cipher_suites(Suites, Filters) -> Ciphers

    Types:
        Suites = ciphers()
        Filters = cipher_filters()
        Ciphers = ciphers()

    Removes cipher suites if any of the filter functions returns false for any part of the cipher suite. If no filter function is supplied for some part the default behaviour regards it as if there was a filter function that returned true. For examples see Customizing cipher suites Additionally, this function also filters the cipher suites to exclude cipher suites not supported by the cryptolib used by the OTP crypto application. That is calling ssl:filter_cipher_suites(Suites, []) will be equivalent to only applying the filters for cryptolib support.

format_error(Reason :: Reason | {error, Reason}) -> string()

    Types:
        Reason = any()

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

getopts(SslSocket, OptionNames) ->
           {ok, [gen_tcp:option()]} | {error, reason()}

    Types:
        SslSocket = sslsocket()
        OptionNames = [gen_tcp:option_name()]

    Gets the values of the specified socket options.

getstat(SslSocket) -> {ok, OptionValues} | {error, inet:posix()}
getstat(SslSocket, Options) ->
           {ok, OptionValues} | {error, inet:posix()}

    Types:
        SslSocket = sslsocket()
        Options = [inet:stat_option()]
        OptionValues = [{inet:stat_option(), integer()}]

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

    See inet:getstat/2 for statistic options description.

handshake(HsSocket) ->
             {ok, SslSocket} |
             {ok, SslSocket, Ext} |
             {error, Reason}
handshake(HsSocket, Timeout) ->
             {ok, SslSocket} |
             {ok, SslSocket, Ext} |
             {error, Reason}

    Types:
        HsSocket = sslsocket()
        Timeout = timeout()
        SslSocket = sslsocket()
        Ext = protocol_extensions()
        Reason = closed | timeout | error_alert()

    Performs the 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()

        Warning:
    Not setting the timeout makes the server more vulnerable to DoS attacks.

handshake(Socket, Options) ->
             {ok, SslSocket} |
             {ok, SslSocket, Ext} |
             {error, Reason}
handshake(Socket, Options, Timeout) ->
             {ok, SslSocket} |
             {ok, SslSocket, Ext} |
             {error, Reason}

    Types:
        Socket = socket() | sslsocket()
        SslSocket = sslsocket()
        Options = [server_option()]
        Timeout = timeout()
        Ext = protocol_extensions()
        Reason = closed | timeout | {options, any()} | error_alert()

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

        Warning:
    The ordinary Socket shall be in passive mode ({active, false}) before calling this function, and before the client tries to connect with TLS, or else the behavior of this function is undefined. The best way to ensure this is to create the ordinary listen socket in passive mode.

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

        Warning:
    Not setting the timeout makes the server more vulnerable to DoS attacks.

    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()

handshake_cancel(Sslsocket :: #sslsocket{}) -> any()

    Cancel the handshake with a fatal USER_CANCELED alert.

handshake_continue(HsSocket, Options) ->
                      {ok, SslSocket} | {error, Reason}
handshake_continue(HsSocket, Options, Timeout) ->
                      {ok, SslSocket} | {error, Reason}

    Types:
        HsSocket = sslsocket()
        Options = [tls_client_option() | tls_server_option()]
        Timeout = timeout()
        SslSocket = sslsocket()
        Reason = closed | timeout | error_alert()

    Continue the TLS handshake, possibly with new, additional or changed options.

listen(Port, Options) -> {ok, ListenSocket} | {error, reason()}

    Types:
        Port = inet:port_number()
        Options = [tls_server_option()]
        ListenSocket = sslsocket()

    Creates an SSL listen socket.

negotiated_protocol(SslSocket) -> {ok, Protocol} | {error, Reason}

    Types:
        SslSocket = sslsocket()
        Protocol = binary()
        Reason = protocol_not_negotiated | closed

    Returns the protocol negotiated through ALPN or NPN extensions.

peercert(SslSocket) -> {ok, Cert} | {error, reason()}

    Types:
        SslSocket = sslsocket()
        Cert = public_key:der_encoded()

    The peer certificate is returned as a DER-encoded binary. The certificate can be decoded with public_key:pkix_decode_cert/2 Suggested further reading about certificates is public_key User's Guide and ssl User's Guide

peername(SslSocket) -> {ok, {Address, Port}} | {error, reason()}

    Types:
        SslSocket = sslsocket()
        Address = inet:ip_address()
        Port = inet:port_number()

    Returns the address and port number of the peer.

prepend_cipher_suites(Preferred, Suites) -> ciphers()

    Types:
        Preferred = ciphers() | cipher_filters()
        Suites = ciphers()

    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 suites or a list of filters in which case the filters are use on Suites to extract the preferred cipher list.

prf(SslSocket, Secret, Label, Seed, WantedLength) ->
       {ok, binary()} | {error, reason()}

    Types:
        SslSocket = sslsocket()
        Secret = binary() | master_secret
        Label = binary()
        Seed = [binary() | prf_random()]
        WantedLength = integer() >= 0

    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.

        Note:
    This function will be replaced by a new function export_key_materials/4 in OTP-27, which is equivalent to prf(TLSSocket, master_secret, Label, [client_random, server_random, Context], WantedLength) pre TLS-1.3 and also will behave correctly for TLS-1.3, although the API is not really logical in the TLS-1.3 context. Other ways of calling this function was for testing purposes only and has no use case.

recv(SslSocket, Length) -> {ok, Data} | {error, reason()}
recv(SslSocket, Length, Timeout) -> {ok, Data} | {error, reason()}

    Types:
        SslSocket = sslsocket()
        Length = integer() >= 0
        Data = binary() | list() | HttpPacket
        Timeout = timeout()
        HttpPacket = any()
            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.

renegotiate(SslSocket) -> ok | {error, reason()}

    Types:
        SslSocket = sslsocket()

    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.

    TLS-1.3 has removed the renegotiate feature of earlier TLS versions and instead adds a new feature called key update that replaces the most important part of renegotiate, that is the refreshing of session keys. This is triggered automatically after reaching a plaintext limit and can be configured by option key_update_at.

update_keys(SslSocket, Type) -> ok | {error, reason()}

    Types:
        SslSocket = sslsocket()
        Type = write | read_write

    There are cryptographic limits on the amount of plaintext which can be safely encrypted under a given set of keys. If the amount of data surpasses those limits, a key update is triggered and a new set of keys are installed. See also the option key_update_at.

    This function can be used to explicitly start a key update on a TLS 1.3 connection. There are two types of the key update: if Type is set to write, only the writing key is updated; if Type is set to read_write, both the reading and writing keys are updated.

send(SslSocket, Data) -> ok | {error, reason()}

    Types:
        SslSocket = sslsocket()
        Data = iodata()

    Writes Data to SslSocket.

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

setopts(SslSocket, Options) -> ok | {error, reason()}

    Types:
        SslSocket = sslsocket()
        Options = [gen_tcp:option()]

    Sets options according to Options for socket SslSocket.

shutdown(SslSocket, How) -> ok | {error, reason()}

    Types:
        SslSocket = sslsocket()
        How = read | write | read_write

    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.

signature_algs(Description, Version) -> signature_algs()

    Types:
        Description = default | all | exclusive
        Version = protocol_version()

    Lists all possible signature algorithms corresponding to Description that are available. The exclusive option will exclusively list algorithms/schemes for that protocol version, whereas the default and all options lists the combined list to support the range of protocols from (D)TLS-1.2, the first version to support configuration of the signature algorithms, to Version.

    Example:

          1> ssl:signature_algs(default, 'tlsv1.3').
          [eddsa_ed25519,eddsa_ed448,ecdsa_secp521r1_sha512,
          ecdsa_secp384r1_sha384,ecdsa_secp256r1_sha256,
          rsa_pss_pss_sha512,rsa_pss_pss_sha384,rsa_pss_pss_sha256,
          rsa_pss_rsae_sha512,rsa_pss_rsae_sha384,rsa_pss_rsae_sha256,
          rsa_pkcs1_sha512,rsa_pkcs1_sha384,rsa_pkcs1_sha256,
          {sha512,ecdsa},
          {sha384,ecdsa},
          {sha256,ecdsa}]
          2>ssl:signature_algs(all, 'tlsv1.3').
          [eddsa_ed25519,eddsa_ed448,ecdsa_secp521r1_sha512,
          ecdsa_secp384r1_sha384,ecdsa_secp256r1_sha256,
          rsa_pss_pss_sha512,rsa_pss_pss_sha384,rsa_pss_pss_sha256,
          rsa_pss_rsae_sha512,rsa_pss_rsae_sha384,rsa_pss_rsae_sha256,
          rsa_pkcs1_sha512,rsa_pkcs1_sha384,rsa_pkcs1_sha256,
          {sha512,ecdsa},
          {sha384,ecdsa},
          {sha256,ecdsa},
          {sha224,ecdsa},
          {sha224,rsa},
          {sha,rsa},
          {sha,dsa}]
          3> ssl:signature_algs(exclusive, 'tlsv1.3').
          [eddsa_ed25519,eddsa_ed448,ecdsa_secp521r1_sha512,
          ecdsa_secp384r1_sha384,ecdsa_secp256r1_sha256,
          rsa_pss_pss_sha512,rsa_pss_pss_sha384,rsa_pss_pss_sha256,
          rsa_pss_rsae_sha512,rsa_pss_rsae_sha384,rsa_pss_rsae_sha256]

        Note:
    Some TLS-1-3 scheme names overlap with TLS-1.2 algorithm-tuple-pair-names and then TLS-1.3 names will be used, for example rsa_pkcs1_sha256 instead of {sha256, rsa} these are legacy algorithms in TLS-1.3 that apply only to certificate signatures in this version of the protocol.

sockname(SslSocket) -> {ok, {Address, Port}} | {error, reason()}

    Types:
        SslSocket = sslsocket()
        Address = inet:ip_address()
        Port = inet:port_number()

    Returns the local address and port number of socket SslSocket.

start() -> ok | {error, reason()}
start(Type :: permanent | transient | temporary) ->
         ok | {error, reason()}

    Starts the SSL application. Default type is temporary.

stop() -> ok

    Stops the SSL application.

str_to_suite(CipherSuiteName) ->
                erl_cipher_suite() |
                {error, {not_recognized, CipherSuiteName}}

    Types:
        CipherSuiteName = string()

    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.

suite_to_openssl_str(CipherSuite) -> string()

    Types:
        CipherSuite = erl_cipher_suite()

    Converts erl_cipher_suite() to OpenSSL name string.

    PRE TLS-1.3 these names differ for RFC names

suite_to_str(CipherSuite) -> string()

    Types:
        CipherSuite = erl_cipher_suite()

    Converts erl_cipher_suite() to RFC name string.

transport_accept(ListenSocket) ->
                    {ok, SslSocket} | {error, reason()}
transport_accept(ListenSocket, Timeout) ->
                    {ok, SslSocket} | {error, reason()}

    Types:
        ListenSocket = sslsocket()
        Timeout = timeout()
        SslSocket = sslsocket()

    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 TLS/DTLS connection.

        Warning:
    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.

versions() -> [VersionInfo]

    Types:
        VersionInfo =
            {ssl_app, string()} |
            {supported | available | implemented, [tls_version()]} |
            {supported_dtls | available_dtls | implemented_dtls,
             [dtls_version()]}

    Lists information, mainly concerning TLS/DTLS versions, in runtime for debugging and testing purposes.

        app_vsn:
            The application version of the SSL application.

        supported:
            TLS versions supported with current application environment and crypto library configuration. Overridden by a version option on connect/[2,3,4], listen/2, and handshake/[2,3]. For the negotiated TLS version, see connection_information/1 .

        supported_dtls:
            DTLS versions supported with current application environment and crypto library configuration. Overridden by a version option on connect/[2,3,4], listen/2, and handshake/[2,3]. For the negotiated DTLS version, see connection_information/1 .

        available:
            All TLS versions supported with the linked crypto library.

        available_dtls:
            All DTLS versions supported with the linked crypto library.

        implemented:
            All TLS versions supported by the SSL application if linked with a crypto library with the necessary support.

        implemented_dtls:
            All DTLS versions supported by the SSL application if linked with a crypto library with the necessary support.

SEE ALSO

inet(3) and gen_tcp(3) gen_udp(3)