xref: /third_party/node/doc/api/tls.md

# TLS (SSL)

<!--introduced_in=v0.10.0-->

> Stability: 2 - Stable

<!-- source_link=lib/tls.js -->

The `tls` module provides an implementation of the Transport Layer Security
(TLS) and Secure Socket Layer (SSL) protocols that is built on top of OpenSSL.
The module can be accessed using:

```js
const tls = require('tls');
```

## TLS/SSL concepts

The TLS/SSL is a public/private key infrastructure (PKI). For most common
cases, each client and server must have a *private key*.

Private keys can be generated in multiple ways. The example below illustrates
use of the OpenSSL command-line interface to generate a 2048-bit RSA private
key:

```bash
openssl genrsa -out ryans-key.pem 2048
```

With TLS/SSL, all servers (and some clients) must have a *certificate*.
Certificates are *public keys* that correspond to a private key, and that are
digitally signed either by a Certificate Authority or by the owner of the
private key (such certificates are referred to as "self-signed"). The first
step to obtaining a certificate is to create a *Certificate Signing Request*
(CSR) file.

The OpenSSL command-line interface can be used to generate a CSR for a private
key:

```bash
openssl req -new -sha256 -key ryans-key.pem -out ryans-csr.pem
```

Once the CSR file is generated, it can either be sent to a Certificate
Authority for signing or used to generate a self-signed certificate.

Creating a self-signed certificate using the OpenSSL command-line interface
is illustrated in the example below:

```bash
openssl x509 -req -in ryans-csr.pem -signkey ryans-key.pem -out ryans-cert.pem
```

Once the certificate is generated, it can be used to generate a `.pfx` or
`.p12` file:

```bash
openssl pkcs12 -export -in ryans-cert.pem -inkey ryans-key.pem \
      -certfile ca-cert.pem -out ryans.pfx
```

Where:

* `in`: is the signed certificate
* `inkey`: is the associated private key
* `certfile`: is a concatenation of all Certificate Authority (CA) certs into
   a single file, e.g. `cat ca1-cert.pem ca2-cert.pem > ca-cert.pem`

### Perfect forward secrecy

<!-- type=misc -->

The term _[forward secrecy][]_ or _perfect forward secrecy_ describes a feature
of key-agreement (i.e., key-exchange) methods. That is, the server and client
keys are used to negotiate new temporary keys that are used specifically and
only for the current communication session. Practically, this means that even
if the server's private key is compromised, communication can only be decrypted
by eavesdroppers if the attacker manages to obtain the key-pair specifically
generated for the session.

Perfect forward secrecy is achieved by randomly generating a key pair for
key-agreement on every TLS/SSL handshake (in contrast to using the same key for
all sessions). Methods implementing this technique are called "ephemeral".

Currently two methods are commonly used to achieve perfect forward secrecy (note
the character "E" appended to the traditional abbreviations):

* [DHE][]: An ephemeral version of the Diffie-Hellman key-agreement protocol.
* [ECDHE][]: An ephemeral version of the Elliptic Curve Diffie-Hellman
  key-agreement protocol.

Ephemeral methods may have some performance drawbacks, because key generation
is expensive.

To use perfect forward secrecy using `DHE` with the `tls` module, it is required
to generate Diffie-Hellman parameters and specify them with the `dhparam`
option to [`tls.createSecureContext()`][]. The following illustrates the use of
the OpenSSL command-line interface to generate such parameters:

```bash
openssl dhparam -outform PEM -out dhparam.pem 2048
```

If using perfect forward secrecy using `ECDHE`, Diffie-Hellman parameters are
not required and a default ECDHE curve will be used. The `ecdhCurve` property
can be used when creating a TLS Server to specify the list of names of supported
curves to use, see [`tls.createServer()`][] for more info.

Perfect forward secrecy was optional up to TLSv1.2, but it is not optional for
TLSv1.3, because all TLSv1.3 cipher suites use ECDHE.

### ALPN and SNI

<!-- type=misc -->

ALPN (Application-Layer Protocol Negotiation Extension) and
SNI (Server Name Indication) are TLS handshake extensions:

* ALPN: Allows the use of one TLS server for multiple protocols (HTTP, HTTP/2)
* SNI: Allows the use of one TLS server for multiple hostnames with different
  SSL certificates.

### Pre-shared keys

<!-- type=misc -->

TLS-PSK support is available as an alternative to normal certificate-based
authentication. It uses a pre-shared key instead of certificates to
authenticate a TLS connection, providing mutual authentication.
TLS-PSK and public key infrastructure are not mutually exclusive. Clients and
servers can accommodate both, choosing either of them during the normal cipher
negotiation step.

TLS-PSK is only a good choice where means exist to securely share a
key with every connecting machine, so it does not replace PKI
(Public Key Infrastructure) for the majority of TLS uses.
The TLS-PSK implementation in OpenSSL has seen many security flaws in
recent years, mostly because it is used only by a minority of applications.
Please consider all alternative solutions before switching to PSK ciphers.
Upon generating PSK it is of critical importance to use sufficient entropy as
discussed in [RFC 4086][]. Deriving a shared secret from a password or other
low-entropy sources is not secure.

PSK ciphers are disabled by default, and using TLS-PSK thus requires explicitly
specifying a cipher suite with the `ciphers` option. The list of available
ciphers can be retrieved via `openssl ciphers -v 'PSK'`. All TLS 1.3
ciphers are eligible for PSK but currently only those that use SHA256 digest are
supported they can be retrieved via `openssl ciphers -v -s -tls1_3 -psk`.

According to the [RFC 4279][], PSK identities up to 128 bytes in length and
PSKs up to 64 bytes in length must be supported. As of OpenSSL 1.1.0
maximum identity size is 128 bytes, and maximum PSK length is 256 bytes.

The current implementation doesn't support asynchronous PSK callbacks due to the
limitations of the underlying OpenSSL API.

### Client-initiated renegotiation attack mitigation

<!-- type=misc -->

The TLS protocol allows clients to renegotiate certain aspects of the TLS
session. Unfortunately, session renegotiation requires a disproportionate amount
of server-side resources, making it a potential vector for denial-of-service
attacks.

To mitigate the risk, renegotiation is limited to three times every ten minutes.
An `'error'` event is emitted on the [`tls.TLSSocket`][] instance when this
threshold is exceeded. The limits are configurable:

* `tls.CLIENT_RENEG_LIMIT` {number} Specifies the number of renegotiation
  requests. **Default:** `3`.
* `tls.CLIENT_RENEG_WINDOW` {number} Specifies the time renegotiation window
  in seconds. **Default:** `600` (10 minutes).

The default renegotiation limits should not be modified without a full
understanding of the implications and risks.

TLSv1.3 does not support renegotiation.

### Session resumption

Establishing a TLS session can be relatively slow. The process can be sped
up by saving and later reusing the session state. There are several mechanisms
to do so, discussed here from oldest to newest (and preferred).

#### Session identifiers

Servers generate a unique ID for new connections and
send it to the client. Clients and servers save the session state. When
reconnecting, clients send the ID of their saved session state and if the server
also has the state for that ID, it can agree to use it. Otherwise, the server
will create a new session. See [RFC 2246][] for more information, page 23 and
30.

Resumption using session identifiers is supported by most web browsers when
making HTTPS requests.

For Node.js, clients wait for the [`'session'`][] event to get the session data,
and provide the data to the `session` option of a subsequent [`tls.connect()`][]
to reuse the session. Servers must
implement handlers for the [`'newSession'`][] and [`'resumeSession'`][] events
to save and restore the session data using the session ID as the lookup key to
reuse sessions. To reuse sessions across load balancers or cluster workers,
servers must use a shared session cache (such as Redis) in their session
handlers.

#### Session tickets

The servers encrypt the entire session state and send it
to the client as a "ticket". When reconnecting, the state is sent to the server
in the initial connection. This mechanism avoids the need for server-side
session cache. If the server doesn't use the ticket, for any reason (failure
to decrypt it, it's too old, etc.), it will create a new session and send a new
ticket. See [RFC 5077][] for more information.

Resumption using session tickets is becoming commonly supported by many web
browsers when making HTTPS requests.

For Node.js, clients use the same APIs for resumption with session identifiers
as for resumption with session tickets. For debugging, if
[`tls.TLSSocket.getTLSTicket()`][] returns a value, the session data contains a
ticket, otherwise it contains client-side session state.

With TLSv1.3, be aware that multiple tickets may be sent by the server,
resulting in multiple `'session'` events, see [`'session'`][] for more
information.

Single process servers need no specific implementation to use session tickets.
To use session tickets across server restarts or load balancers, servers must
all have the same ticket keys. There are three 16-byte keys internally, but the
tls API exposes them as a single 48-byte buffer for convenience.

Its possible to get the ticket keys by calling [`server.getTicketKeys()`][] on
one server instance and then distribute them, but it is more reasonable to
securely generate 48 bytes of secure random data and set them with the
`ticketKeys` option of [`tls.createServer()`][]. The keys should be regularly
regenerated and server's keys can be reset with
[`server.setTicketKeys()`][].

Session ticket keys are cryptographic keys, and they ***must be stored
securely***. With TLS 1.2 and below, if they are compromised all sessions that
used tickets encrypted with them can be decrypted. They should not be stored
on disk, and they should be regenerated regularly.

If clients advertise support for tickets, the server will send them. The
server can disable tickets by supplying
`require('constants').SSL_OP_NO_TICKET` in `secureOptions`.

Both session identifiers and session tickets timeout, causing the server to
create new sessions. The timeout can be configured with the `sessionTimeout`
option of [`tls.createServer()`][].

For all the mechanisms, when resumption fails, servers will create new sessions.
Since failing to resume the session does not cause TLS/HTTPS connection
failures, it is easy to not notice unnecessarily poor TLS performance. The
OpenSSL CLI can be used to verify that servers are resuming sessions. Use the
`-reconnect` option to `openssl s_client`, for example:

```console
$ openssl s_client -connect localhost:443 -reconnect
```

Read through the debug output. The first connection should say "New", for
example:

```text
New, TLSv1.2, Cipher is ECDHE-RSA-AES128-GCM-SHA256
```

Subsequent connections should say "Reused", for example:

```text
Reused, TLSv1.2, Cipher is ECDHE-RSA-AES128-GCM-SHA256
```

## Modifying the default TLS cipher suite

Node.js is built with a default suite of enabled and disabled TLS ciphers. This
default cipher list can be configured when building Node.js to allow
distributions to provide their own default list.

The following command can be used to show the default cipher suite:
```console
node -p crypto.constants.defaultCoreCipherList | tr ':' '\n'
TLS_AES_256_GCM_SHA384
TLS_CHACHA20_POLY1305_SHA256
TLS_AES_128_GCM_SHA256
ECDHE-RSA-AES128-GCM-SHA256
ECDHE-ECDSA-AES128-GCM-SHA256
ECDHE-RSA-AES256-GCM-SHA384
ECDHE-ECDSA-AES256-GCM-SHA384
DHE-RSA-AES128-GCM-SHA256
ECDHE-RSA-AES128-SHA256
DHE-RSA-AES128-SHA256
ECDHE-RSA-AES256-SHA384
DHE-RSA-AES256-SHA384
ECDHE-RSA-AES256-SHA256
DHE-RSA-AES256-SHA256
HIGH
!aNULL
!eNULL
!EXPORT
!DES
!RC4
!MD5
!PSK
!SRP
!CAMELLIA
```

This default can be replaced entirely using the [`--tls-cipher-list`][] command
line switch (directly, or via the [`NODE_OPTIONS`][] environment variable). For
instance, the following makes `ECDHE-RSA-AES128-GCM-SHA256:!RC4` the default TLS
cipher suite:

```bash
node --tls-cipher-list='ECDHE-RSA-AES128-GCM-SHA256:!RC4' server.js

export NODE_OPTIONS=--tls-cipher-list='ECDHE-RSA-AES128-GCM-SHA256:!RC4'
node server.js
```

The default can also be replaced on a per client or server basis using the
`ciphers` option from [`tls.createSecureContext()`][], which is also available
in [`tls.createServer()`][], [`tls.connect()`][], and when creating new
[`tls.TLSSocket`][]s.

The ciphers list can contain a mixture of TLSv1.3 cipher suite names, the ones
that start with `'TLS_'`, and specifications for TLSv1.2 and below cipher
suites. The TLSv1.2 ciphers support a legacy specification format, consult
the OpenSSL [cipher list format][] documentation for details, but those
specifications do *not* apply to TLSv1.3 ciphers. The TLSv1.3 suites can only
be enabled by including their full name in the cipher list. They cannot, for
example, be enabled or disabled by using the legacy TLSv1.2 `'EECDH'` or
`'!EECDH'` specification.

Despite the relative order of TLSv1.3 and TLSv1.2 cipher suites, the TLSv1.3
protocol is significantly more secure than TLSv1.2, and will always be chosen
over TLSv1.2 if the handshake indicates it is supported, and if any TLSv1.3
cipher suites are enabled.

The default cipher suite included within Node.js has been carefully
selected to reflect current security best practices and risk mitigation.
Changing the default cipher suite can have a significant impact on the security
of an application. The `--tls-cipher-list` switch and `ciphers` option should by
used only if absolutely necessary.

The default cipher suite prefers GCM ciphers for [Chrome's 'modern
cryptography' setting][] and also prefers ECDHE and DHE ciphers for perfect
forward secrecy, while offering *some* backward compatibility.

128 bit AES is preferred over 192 and 256 bit AES in light of [specific
attacks affecting larger AES key sizes][].

Old clients that rely on insecure and deprecated RC4 or DES-based ciphers
(like Internet Explorer 6) cannot complete the handshaking process with
the default configuration. If these clients _must_ be supported, the
[TLS recommendations][] may offer a compatible cipher suite. For more details
on the format, see the OpenSSL [cipher list format][] documentation.

There are only 5 TLSv1.3 cipher suites:

* `'TLS_AES_256_GCM_SHA384'`
* `'TLS_CHACHA20_POLY1305_SHA256'`
* `'TLS_AES_128_GCM_SHA256'`
* `'TLS_AES_128_CCM_SHA256'`
* `'TLS_AES_128_CCM_8_SHA256'`

The first 3 are enabled by default. The last 2 `CCM`-based suites are supported
by TLSv1.3 because they may be more performant on constrained systems, but they
are not enabled by default since they offer less security.

## Class: `tls.CryptoStream`
<!-- YAML
added: v0.3.4
deprecated: v0.11.3
-->

> Stability: 0 - Deprecated: Use [`tls.TLSSocket`][] instead.

The `tls.CryptoStream` class represents a stream of encrypted data. This class
is deprecated and should no longer be used.

### `cryptoStream.bytesWritten`
<!-- YAML
added: v0.3.4
deprecated: v0.11.3
-->

The `cryptoStream.bytesWritten` property returns the total number of bytes
written to the underlying socket *including* the bytes required for the
implementation of the TLS protocol.

## Class: `tls.SecurePair`
<!-- YAML
added: v0.3.2
deprecated: v0.11.3
-->

> Stability: 0 - Deprecated: Use [`tls.TLSSocket`][] instead.

Returned by [`tls.createSecurePair()`][].

### Event: `'secure'`
<!-- YAML
added: v0.3.2
deprecated: v0.11.3
-->

The `'secure'` event is emitted by the `SecurePair` object once a secure
connection has been established.

As with checking for the server
[`'secureConnection'`](#tls_event_secureconnection)
event, `pair.cleartext.authorized` should be inspected to confirm whether the
certificate used is properly authorized.

## Class: `tls.Server`
<!-- YAML
added: v0.3.2
-->

* Extends: {net.Server}

Accepts encrypted connections using TLS or SSL.

### Event: `'connection'`
<!-- YAML
added: v0.3.2
-->

* `socket` {stream.Duplex}

This event is emitted when a new TCP stream is established, before the TLS
handshake begins. `socket` is typically an object of type [`net.Socket`][].
Usually users will not want to access this event.

This event can also be explicitly emitted by users to inject connections
into the TLS server. In that case, any [`Duplex`][] stream can be passed.

### Event: `'keylog'`
<!-- YAML
added: v12.3.0
-->

* `line` {Buffer} Line of ASCII text, in NSS `SSLKEYLOGFILE` format.
* `tlsSocket` {tls.TLSSocket} The `tls.TLSSocket` instance on which it was
  generated.

The `keylog` event is emitted when key material is generated or received by
a connection to this server (typically before handshake has completed, but not
necessarily). This keying material can be stored for debugging, as it allows
captured TLS traffic to be decrypted. It may be emitted multiple times for
each socket.

A typical use case is to append received lines to a common text file, which
is later used by software (such as Wireshark) to decrypt the traffic:

```js
const logFile = fs.createWriteStream('/tmp/ssl-keys.log', { flags: 'a' });
// ...
server.on('keylog', (line, tlsSocket) => {
  if (tlsSocket.remoteAddress !== '...')
    return; // Only log keys for a particular IP
  logFile.write(line);
});
```

### Event: `'newSession'`
<!-- YAML
added: v0.9.2
changes:
  - version: v0.11.12
    pr-url: https://github.com/nodejs/node-v0.x-archive/pull/7118
    description: The `callback` argument is now supported.
-->

The `'newSession'` event is emitted upon creation of a new TLS session. This may
be used to store sessions in external storage. The data should be provided to
the [`'resumeSession'`][] callback.

The listener callback is passed three arguments when called:

* `sessionId` {Buffer} The TLS session identifier
* `sessionData` {Buffer} The TLS session data
* `callback` {Function} A callback function taking no arguments that must be
  invoked in order for data to be sent or received over the secure connection.

Listening for this event will have an effect only on connections established
after the addition of the event listener.

### Event: `'OCSPRequest'`
<!-- YAML
added: v0.11.13
-->

The `'OCSPRequest'` event is emitted when the client sends a certificate status
request. The listener callback is passed three arguments when called:

* `certificate` {Buffer} The server certificate
* `issuer` {Buffer} The issuer's certificate
* `callback` {Function} A callback function that must be invoked to provide
  the results of the OCSP request.

The server's current certificate can be parsed to obtain the OCSP URL
and certificate ID; after obtaining an OCSP response, `callback(null, resp)` is
then invoked, where `resp` is a `Buffer` instance containing the OCSP response.
Both `certificate` and `issuer` are `Buffer` DER-representations of the
primary and issuer's certificates. These can be used to obtain the OCSP
certificate ID and OCSP endpoint URL.

Alternatively, `callback(null, null)` may be called, indicating that there was
no OCSP response.

Calling `callback(err)` will result in a `socket.destroy(err)` call.

The typical flow of an OCSP Request is as follows:

1. Client connects to the server and sends an `'OCSPRequest'` (via the status
   info extension in ClientHello).
2. Server receives the request and emits the `'OCSPRequest'` event, calling the
   listener if registered.
3. Server extracts the OCSP URL from either the `certificate` or `issuer` and
   performs an [OCSP request][] to the CA.
4. Server receives `'OCSPResponse'` from the CA and sends it back to the client
   via the `callback` argument
5. Client validates the response and either destroys the socket or performs a
   handshake.

The `issuer` can be `null` if the certificate is either self-signed or the
issuer is not in the root certificates list. (An issuer may be provided
via the `ca` option when establishing the TLS connection.)

Listening for this event will have an effect only on connections established
after the addition of the event listener.

An npm module like [asn1.js][] may be used to parse the certificates.

### Event: `'resumeSession'`
<!-- YAML
added: v0.9.2
-->

The `'resumeSession'` event is emitted when the client requests to resume a
previous TLS session. The listener callback is passed two arguments when
called:

* `sessionId` {Buffer} The TLS session identifier
* `callback` {Function} A callback function to be called when the prior session
  has been recovered: `callback([err[, sessionData]])`
  * `err` {Error}
  * `sessionData` {Buffer}

The event listener should perform a lookup in external storage for the
`sessionData` saved by the [`'newSession'`][] event handler using the given
`sessionId`. If found, call `callback(null, sessionData)` to resume the session.
If not found, the session cannot be resumed. `callback()` must be called
without `sessionData` so that the handshake can continue and a new session can
be created. It is possible to call `callback(err)` to terminate the incoming
connection and destroy the socket.

Listening for this event will have an effect only on connections established
after the addition of the event listener.

The following illustrates resuming a TLS session:

```js
const tlsSessionStore = {};
server.on('newSession', (id, data, cb) => {
  tlsSessionStore[id.toString('hex')] = data;
  cb();
});
server.on('resumeSession', (id, cb) => {
  cb(null, tlsSessionStore[id.toString('hex')] || null);
});
```

### Event: `'secureConnection'`
<!-- YAML
added: v0.3.2
-->

The `'secureConnection'` event is emitted after the handshaking process for a
new connection has successfully completed. The listener callback is passed a
single argument when called:

* `tlsSocket` {tls.TLSSocket} The established TLS socket.

The `tlsSocket.authorized` property is a `boolean` indicating whether the
client has been verified by one of the supplied Certificate Authorities for the
server. If `tlsSocket.authorized` is `false`, then `socket.authorizationError`
is set to describe how authorization failed. Depending on the settings
of the TLS server, unauthorized connections may still be accepted.

The `tlsSocket.alpnProtocol` property is a string that contains the selected
ALPN protocol. When ALPN has no selected protocol, `tlsSocket.alpnProtocol`
equals `false`.

The `tlsSocket.servername` property is a string containing the server name
requested via SNI.

### Event: `'tlsClientError'`
<!-- YAML
added: v6.0.0
-->

The `'tlsClientError'` event is emitted when an error occurs before a secure
connection is established. The listener callback is passed two arguments when
called:

* `exception` {Error} The `Error` object describing the error
* `tlsSocket` {tls.TLSSocket} The `tls.TLSSocket` instance from which the
  error originated.

### `server.addContext(hostname, context)`
<!-- YAML
added: v0.5.3
-->

* `hostname` {string} A SNI host name or wildcard (e.g. `'*'`)
* `context` {Object} An object containing any of the possible properties
  from the [`tls.createSecureContext()`][] `options` arguments (e.g. `key`,
  `cert`, `ca`, etc).

The `server.addContext()` method adds a secure context that will be used if
the client request's SNI name matches the supplied `hostname` (or wildcard).

### `server.address()`
<!-- YAML
added: v0.6.0
-->

* Returns: {Object}

Returns the bound address, the address family name, and port of the
server as reported by the operating system. See [`net.Server.address()`][] for
more information.

### `server.close([callback])`
<!-- YAML
added: v0.3.2
-->

* `callback` {Function} A listener callback that will be registered to listen
for the server instance's `'close'` event.
* Returns: {tls.Server}

The `server.close()` method stops the server from accepting new connections.

This function operates asynchronously. The `'close'` event will be emitted
when the server has no more open connections.

### `server.connections`
<!-- YAML
added: v0.3.2
deprecated: v0.9.7
-->

> Stability: 0 - Deprecated: Use [`server.getConnections()`][] instead.

* {number}

Returns the current number of concurrent connections on the server.

### `server.getTicketKeys()`
<!-- YAML
added: v3.0.0
-->

* Returns: {Buffer} A 48-byte buffer containing the session ticket keys.

Returns the session ticket keys.

See [Session Resumption][] for more information.

### `server.listen()`

Starts the server listening for encrypted connections.
This method is identical to [`server.listen()`][] from [`net.Server`][].

### `server.setSecureContext(options)`
<!-- YAML
added: v11.0.0
-->

* `options` {Object} An object containing any of the possible properties from
  the [`tls.createSecureContext()`][] `options` arguments (e.g. `key`, `cert`,
  `ca`, etc).

The `server.setSecureContext()` method replaces the secure context of an
existing server. Existing connections to the server are not interrupted.

### `server.setTicketKeys(keys)`
<!-- YAML
added: v3.0.0
-->

* `keys` {Buffer} A 48-byte buffer containing the session ticket keys.

Sets the session ticket keys.

Changes to the ticket keys are effective only for future server connections.
Existing or currently pending server connections will use the previous keys.

See [Session Resumption][] for more information.

## Class: `tls.TLSSocket`
<!-- YAML
added: v0.11.4
-->

* Extends: {net.Socket}

Performs transparent encryption of written data and all required TLS
negotiation.

Instances of `tls.TLSSocket` implement the duplex [Stream][] interface.

Methods that return TLS connection metadata (e.g.
[`tls.TLSSocket.getPeerCertificate()`][] will only return data while the
connection is open.

### `new tls.TLSSocket(socket[, options])`
<!-- YAML
added: v0.11.4
changes:
  - version: v12.2.0
    pr-url: https://github.com/nodejs/node/pull/27497
    description: The `enableTrace` option is now supported.
  - version: v5.0.0
    pr-url: https://github.com/nodejs/node/pull/2564
    description: ALPN options are supported now.
-->

* `socket` {net.Socket|stream.Duplex}
  On the server side, any `Duplex` stream. On the client side, any
  instance of [`net.Socket`][] (for generic `Duplex` stream support
  on the client side, [`tls.connect()`][] must be used).
* `options` {Object}
  * `enableTrace`: See [`tls.createServer()`][]
  * `isServer`: The SSL/TLS protocol is asymmetrical, TLSSockets must know if
    they are to behave as a server or a client. If `true` the TLS socket will be
    instantiated as a server. **Default:** `false`.
  * `server` {net.Server} A [`net.Server`][] instance.
  * `requestCert`: Whether to authenticate the remote peer by requesting a
     certificate. Clients always request a server certificate. Servers
     (`isServer` is true) may set `requestCert` to true to request a client
     certificate.
  * `rejectUnauthorized`: See [`tls.createServer()`][]
  * `ALPNProtocols`: See [`tls.createServer()`][]
  * `SNICallback`: See [`tls.createServer()`][]
  * `session` {Buffer} A `Buffer` instance containing a TLS session.
  * `requestOCSP` {boolean} If `true`, specifies that the OCSP status request
    extension will be added to the client hello and an `'OCSPResponse'` event
    will be emitted on the socket before establishing a secure communication
  * `secureContext`: TLS context object created with
    [`tls.createSecureContext()`][]. If a `secureContext` is _not_ provided, one
    will be created by passing the entire `options` object to
    `tls.createSecureContext()`.
  * ...: [`tls.createSecureContext()`][] options that are used if the
    `secureContext` option is missing. Otherwise, they are ignored.

Construct a new `tls.TLSSocket` object from an existing TCP socket.

### Event: `'keylog'`
<!-- YAML
added: v12.3.0
-->

* `line` {Buffer} Line of ASCII text, in NSS `SSLKEYLOGFILE` format.

The `keylog` event is emitted on a `tls.TLSSocket` when key material
is generated or received by the socket. This keying material can be stored
for debugging, as it allows captured TLS traffic to be decrypted. It may
be emitted multiple times, before or after the handshake completes.

A typical use case is to append received lines to a common text file, which
is later used by software (such as Wireshark) to decrypt the traffic:

```js
const logFile = fs.createWriteStream('/tmp/ssl-keys.log', { flags: 'a' });
// ...
tlsSocket.on('keylog', (line) => logFile.write(line));
```

### Event: `'OCSPResponse'`
<!-- YAML
added: v0.11.13
-->

The `'OCSPResponse'` event is emitted if the `requestOCSP` option was set
when the `tls.TLSSocket` was created and an OCSP response has been received.
The listener callback is passed a single argument when called:

* `response` {Buffer} The server's OCSP response

Typically, the `response` is a digitally signed object from the server's CA that
contains information about server's certificate revocation status.

### Event: `'secureConnect'`
<!-- YAML
added: v0.11.4
-->

The `'secureConnect'` event is emitted after the handshaking process for a new
connection has successfully completed. The listener callback will be called
regardless of whether or not the server's certificate has been authorized. It
is the client's responsibility to check the `tlsSocket.authorized` property to
determine if the server certificate was signed by one of the specified CAs. If
`tlsSocket.authorized === false`, then the error can be found by examining the
`tlsSocket.authorizationError` property. If ALPN was used, the
`tlsSocket.alpnProtocol` property can be checked to determine the negotiated
protocol.

### Event: `'session'`
<!-- YAML
added: v11.10.0
-->

* `session` {Buffer}

The `'session'` event is emitted on a client `tls.TLSSocket` when a new session
or TLS ticket is available. This may or may not be before the handshake is
complete, depending on the TLS protocol version that was negotiated. The event
is not emitted on the server, or if a new session was not created, for example,
when the connection was resumed. For some TLS protocol versions the event may be
emitted multiple times, in which case all the sessions can be used for
resumption.

On the client, the `session` can be provided to the `session` option of
[`tls.connect()`][] to resume the connection.

See [Session Resumption][] for more information.

For TLSv1.2 and below, [`tls.TLSSocket.getSession()`][] can be called once
the handshake is complete. For TLSv1.3, only ticket-based resumption is allowed
by the protocol, multiple tickets are sent, and the tickets aren't sent until
after the handshake completes. So it is necessary to wait for the
`'session'` event to get a resumable session. Applications
should use the `'session'` event instead of `getSession()` to ensure
they will work for all TLS versions. Applications that only expect to
get or use one session should listen for this event only once:

```js
tlsSocket.once('session', (session) => {
  // The session can be used immediately or later.
  tls.connect({
    session: session,
    // Other connect options...
  });
});
```

### `tlsSocket.address()`
<!-- YAML
added: v0.11.4
-->

* Returns: {Object}

Returns the bound `address`, the address `family` name, and `port` of the
underlying socket as reported by the operating system:
`{ port: 12346, family: 'IPv4', address: '127.0.0.1' }`.

### `tlsSocket.authorizationError`
<!-- YAML
added: v0.11.4
-->

Returns the reason why the peer's certificate was not been verified. This
property is set only when `tlsSocket.authorized === false`.

### `tlsSocket.authorized`
<!-- YAML
added: v0.11.4
-->

* Returns: {boolean}

Returns `true` if the peer certificate was signed by one of the CAs specified
when creating the `tls.TLSSocket` instance, otherwise `false`.

### `tlsSocket.disableRenegotiation()`
<!-- YAML
added: v8.4.0
-->

Disables TLS renegotiation for this `TLSSocket` instance. Once called, attempts
to renegotiate will trigger an `'error'` event on the `TLSSocket`.

### `tlsSocket.enableTrace()`
<!-- YAML
added: v12.2.0
-->

When enabled, TLS packet trace information is written to `stderr`. This can be
used to debug TLS connection problems.

Note: The format of the output is identical to the output of `openssl s_client
-trace` or `openssl s_server -trace`. While it is produced by OpenSSL's
`SSL_trace()` function, the format is undocumented, can change without notice,
and should not be relied on.

### `tlsSocket.encrypted`
<!-- YAML
added: v0.11.4
-->

Always returns `true`. This may be used to distinguish TLS sockets from regular
`net.Socket` instances.

### `tlsSocket.getCertificate()`
<!-- YAML
added: v11.2.0
-->

* Returns: {Object}

Returns an object representing the local certificate. The returned object has
some properties corresponding to the fields of the certificate.

See [`tls.TLSSocket.getPeerCertificate()`][] for an example of the certificate
structure.

If there is no local certificate, an empty object will be returned. If the
socket has been destroyed, `null` will be returned.

### `tlsSocket.getCipher()`
<!-- YAML
added: v0.11.4
changes:
  - version: v12.0.0
    pr-url: https://github.com/nodejs/node/pull/26625
    description: Return the minimum cipher version, instead of a fixed string
      (`'TLSv1/SSLv3'`).
  - version: v12.16.0
    pr-url: https://github.com/nodejs/node/pull/30637
    description: Return the IETF cipher name as `standardName`.
-->

* Returns: {Object}
  * `name` {string} OpenSSL name for the cipher suite.
  * `standardName` {string} IETF name for the cipher suite.
  * `version` {string} The minimum TLS protocol version supported by this cipher
    suite.

Returns an object containing information on the negotiated cipher suite.

For example:
```json
{
    "name": "AES128-SHA256",
    "standardName": "TLS_RSA_WITH_AES_128_CBC_SHA256",
    "version": "TLSv1.2"
}
```

See
[SSL_CIPHER_get_name](https://www.openssl.org/docs/man1.1.1/man3/SSL_CIPHER_get_name.html)
for more information.

### `tlsSocket.getEphemeralKeyInfo()`
<!-- YAML
added: v5.0.0
-->

* Returns: {Object}

Returns an object representing the type, name, and size of parameter of
an ephemeral key exchange in [perfect forward secrecy][] on a client
connection. It returns an empty object when the key exchange is not
ephemeral. As this is only supported on a client socket; `null` is returned
if called on a server socket. The supported types are `'DH'` and `'ECDH'`. The
`name` property is available only when type is `'ECDH'`.

For example: `{ type: 'ECDH', name: 'prime256v1', size: 256 }`.

### `tlsSocket.getFinished()`
<!-- YAML
added: v9.9.0
-->

* Returns: {Buffer|undefined} The latest `Finished` message that has been
sent to the socket as part of a SSL/TLS handshake, or `undefined` if
no `Finished` message has been sent yet.

As the `Finished` messages are message digests of the complete handshake
(with a total of 192 bits for TLS 1.0 and more for SSL 3.0), they can
be used for external authentication procedures when the authentication
provided by SSL/TLS is not desired or is not enough.

Corresponds to the `SSL_get_finished` routine in OpenSSL and may be used
to implement the `tls-unique` channel binding from [RFC 5929][].

### `tlsSocket.getPeerCertificate([detailed])`
<!-- YAML
added: v0.11.4
-->

* `detailed` {boolean} Include the full certificate chain if `true`, otherwise
  include just the peer's certificate.
* Returns: {Object} A certificate object.

Returns an object representing the peer's certificate. If the peer does not
provide a certificate, an empty object will be returned. If the socket has been
destroyed, `null` will be returned.

If the full certificate chain was requested, each certificate will include an
`issuerCertificate` property containing an object representing its issuer's
certificate.

#### Certificate object
<!-- YAML
changes:
  - version: v11.4.0
    pr-url: https://github.com/nodejs/node/pull/24358
    description: Support Elliptic Curve public key info.
-->

A certificate object has properties corresponding to the fields of the
certificate.

* `raw` {Buffer} The DER encoded X.509 certificate data.
* `subject` {Object} The certificate subject, described in terms of
   Country (`C:`), StateOrProvince (`ST`), Locality (`L`), Organization (`O`),
   OrganizationalUnit (`OU`), and CommonName (`CN`). The CommonName is typically
   a DNS name with TLS certificates. Example:
   `{C: 'UK', ST: 'BC', L: 'Metro', O: 'Node Fans', OU: 'Docs', CN: 'example.com'}`.
* `issuer` {Object} The certificate issuer, described in the same terms as the
   `subject`.
* `valid_from` {string} The date-time the certificate is valid from.
* `valid_to` {string} The date-time the certificate is valid to.
* `serialNumber` {string} The certificate serial number, as a hex string.
   Example: `'B9B0D332A1AA5635'`.
* `fingerprint` {string} The SHA-1 digest of the DER encoded certificate. It is
  returned as a `:` separated hexadecimal string. Example: `'2A:7A:C2:DD:...'`.
* `fingerprint256` {string} The SHA-256 digest of the DER encoded certificate.
   It is returned as a `:` separated hexadecimal string. Example:
   `'2A:7A:C2:DD:...'`.
* `ext_key_usage` {Array} (Optional) The extended key usage, a set of OIDs.
* `subjectaltname` {string} (Optional) A string containing concatenated names
 for the subject, an alternative to the `subject` names.
* `infoAccess` {Array} (Optional) An array describing the AuthorityInfoAccess,
   used with OCSP.
* `issuerCertificate` {Object} (Optional) The issuer certificate object. For
   self-signed certificates, this may be a circular reference.

The certificate may contain information about the public key, depending on
the key type.

For RSA keys, the following properties may be defined:

* `bits` {number} The RSA bit size. Example: `1024`.
* `exponent` {string} The RSA exponent, as a string in hexadecimal number
  notation. Example: `'0x010001'`.
* `modulus` {string} The RSA modulus, as a hexadecimal string. Example:
   `'B56CE45CB7...'`.
* `pubkey` {Buffer} The public key.

For EC keys, the following properties may be defined:

* `pubkey` {Buffer} The public key.
* `bits` {number} The key size in bits. Example: `256`.
* `asn1Curve` {string} (Optional) The ASN.1 name of the OID of the elliptic
  curve. Well-known curves are identified by an OID. While it is unusual, it is
  possible that the curve is identified by its mathematical properties, in which
  case it will not have an OID. Example: `'prime256v1'`.
* `nistCurve` {string} (Optional) The NIST name for the elliptic curve, if it
  has one (not all well-known curves have been assigned names by NIST). Example:
  `'P-256'`.

Example certificate:

<!-- eslint-skip -->
```js
{ subject:
   { OU: [ 'Domain Control Validated', 'PositiveSSL Wildcard' ],
     CN: '*.nodejs.org' },
  issuer:
   { C: 'GB',
     ST: 'Greater Manchester',
     L: 'Salford',
     O: 'COMODO CA Limited',
     CN: 'COMODO RSA Domain Validation Secure Server CA' },
  subjectaltname: 'DNS:*.nodejs.org, DNS:nodejs.org',
  infoAccess:
   { 'CA Issuers - URI':
      [ 'http://crt.comodoca.com/COMODORSADomainValidationSecureServerCA.crt' ],
     'OCSP - URI': [ 'http://ocsp.comodoca.com' ] },
  modulus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
  exponent: '0x10001',
  pubkey: <Buffer ... >,
  valid_from: 'Aug 14 00:00:00 2017 GMT',
  valid_to: 'Nov 20 23:59:59 2019 GMT',
  fingerprint: '01:02:59:D9:C3:D2:0D:08:F7:82:4E:44:A4:B4:53:C5:E2:3A:87:4D',
  fingerprint256: '69:AE:1A:6A:D4:3D:C6:C1:1B:EA:C6:23:DE:BA:2A:14:62:62:93:5C:7A:EA:06:41:9B:0B:BC:87:CE:48:4E:02',
  ext_key_usage: [ '1.3.6.1.5.5.7.3.1', '1.3.6.1.5.5.7.3.2' ],
  serialNumber: '66593D57F20CBC573E433381B5FEC280',
  raw: <Buffer ... > }
```

### `tlsSocket.getPeerFinished()`
<!-- YAML
added: v9.9.0
-->

* Returns: {Buffer|undefined} The latest `Finished` message that is expected
or has actually been received from the socket as part of a SSL/TLS handshake,
or `undefined` if there is no `Finished` message so far.

As the `Finished` messages are message digests of the complete handshake
(with a total of 192 bits for TLS 1.0 and more for SSL 3.0), they can
be used for external authentication procedures when the authentication
provided by SSL/TLS is not desired or is not enough.

Corresponds to the `SSL_get_peer_finished` routine in OpenSSL and may be used
to implement the `tls-unique` channel binding from [RFC 5929][].

### `tlsSocket.getProtocol()`
<!-- YAML
added: v5.7.0
-->

* Returns: {string|null}

Returns a string containing the negotiated SSL/TLS protocol version of the
current connection. The value `'unknown'` will be returned for connected
sockets that have not completed the handshaking process. The value `null` will
be returned for server sockets or disconnected client sockets.

Protocol versions are:

* `'SSLv3'`
* `'TLSv1'`
* `'TLSv1.1'`
* `'TLSv1.2'`
* `'TLSv1.3'`

See the OpenSSL [`SSL_get_version`][] documentation for more information.

### `tlsSocket.getSession()`
<!-- YAML
added: v0.11.4
-->

* {Buffer}

Returns the TLS session data or `undefined` if no session was
negotiated. On the client, the data can be provided to the `session` option of
[`tls.connect()`][] to resume the connection. On the server, it may be useful
for debugging.

See [Session Resumption][] for more information.

Note: `getSession()` works only for TLSv1.2 and below. For TLSv1.3, applications
must use the [`'session'`][] event (it also works for TLSv1.2 and below).

### `tlsSocket.getSharedSigalgs()`
<!-- YAML
added: v12.11.0
-->

* Returns: {Array} List of signature algorithms shared between the server and
the client in the order of decreasing preference.

See
[SSL_get_shared_sigalgs](https://www.openssl.org/docs/man1.1.1/man3/SSL_get_shared_sigalgs.html)
for more information.

### `tlsSocket.exportKeyingMaterial(length, label[, context])`
<!-- YAML
added: v12.17.0
-->

* `length` {number} number of bytes to retrieve from keying material
* `label` {string} an application specific label, typically this will be a
value from the
[IANA Exporter Label Registry](https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#exporter-labels).
* `context` {Buffer} Optionally provide a context.

* Returns: {Buffer} requested bytes of the keying material

Keying material is used for validations to prevent different kind of attacks in
network protocols, for example in the specifications of IEEE 802.1X.

Example

```js
const keyingMaterial = tlsSocket.exportKeyingMaterial(
  128,
  'client finished');

/**
 Example return value of keyingMaterial:
 <Buffer 76 26 af 99 c5 56 8e 42 09 91 ef 9f 93 cb ad 6c 7b 65 f8 53 f1 d8 d9
    12 5a 33 b8 b5 25 df 7b 37 9f e0 e2 4f b8 67 83 a3 2f cd 5d 41 42 4c 91
    74 ef 2c ... 78 more bytes>
*/
```
See the OpenSSL [`SSL_export_keying_material`][] documentation for more
information.

### `tlsSocket.getTLSTicket()`
<!-- YAML
added: v0.11.4
-->

* {Buffer}

For a client, returns the TLS session ticket if one is available, or
`undefined`. For a server, always returns `undefined`.

It may be useful for debugging.

See [Session Resumption][] for more information.

### `tlsSocket.isSessionReused()`
<!-- YAML
added: v0.5.6
-->

* Returns: {boolean} `true` if the session was reused, `false` otherwise.

See [Session Resumption][] for more information.

### `tlsSocket.localAddress`
<!-- YAML
added: v0.11.4
-->

* {string}

Returns the string representation of the local IP address.

### `tlsSocket.localPort`
<!-- YAML
added: v0.11.4
-->

* {number}

Returns the numeric representation of the local port.

### `tlsSocket.remoteAddress`
<!-- YAML
added: v0.11.4
-->

* {string}

Returns the string representation of the remote IP address. For example,
`'74.125.127.100'` or `'2001:4860:a005::68'`.

### `tlsSocket.remoteFamily`
<!-- YAML
added: v0.11.4
-->

* {string}

Returns the string representation of the remote IP family. `'IPv4'` or `'IPv6'`.

### `tlsSocket.remotePort`
<!-- YAML
added: v0.11.4
-->

* {number}

Returns the numeric representation of the remote port. For example, `443`.

### `tlsSocket.renegotiate(options, callback)`
<!-- YAML
added: v0.11.8
-->

* `options` {Object}
  * `rejectUnauthorized` {boolean} If not `false`, the server certificate is
    verified against the list of supplied CAs. An `'error'` event is emitted if
    verification fails; `err.code` contains the OpenSSL error code. **Default:**
    `true`.
  * `requestCert`
* `callback` {Function} If `renegotiate()` returned `true`, callback is
   attached once to the `'secure'` event. If `renegotiate()` returned `false`,
   `callback` will be called in the next tick with an error, unless the
   `tlsSocket` has been destroyed, in which case `callback` will not be called
   at all.

* Returns: {boolean} `true` if renegotiation was initiated, `false` otherwise.

The `tlsSocket.renegotiate()` method initiates a TLS renegotiation process.
Upon completion, the `callback` function will be passed a single argument
that is either an `Error` (if the request failed) or `null`.

This method can be used to request a peer's certificate after the secure
connection has been established.

When running as the server, the socket will be destroyed with an error after
`handshakeTimeout` timeout.

For TLSv1.3, renegotiation cannot be initiated, it is not supported by the
protocol.

### `tlsSocket.setMaxSendFragment(size)`
<!-- YAML
added: v0.11.11
-->

* `size` {number} The maximum TLS fragment size. The maximum value is `16384`.
  **Default:** `16384`.
* Returns: {boolean}

The `tlsSocket.setMaxSendFragment()` method sets the maximum TLS fragment size.
Returns `true` if setting the limit succeeded; `false` otherwise.

Smaller fragment sizes decrease the buffering latency on the client: larger
fragments are buffered by the TLS layer until the entire fragment is received
and its integrity is verified; large fragments can span multiple roundtrips
and their processing can be delayed due to packet loss or reordering. However,
smaller fragments add extra TLS framing bytes and CPU overhead, which may
decrease overall server throughput.

## `tls.checkServerIdentity(hostname, cert)`
<!-- YAML
added: v0.8.4
-->

* `hostname` {string} The host name or IP address to verify the certificate
  against.
* `cert` {Object} A [certificate object][] representing the peer's certificate.
* Returns: {Error|undefined}

Verifies the certificate `cert` is issued to `hostname`.

Returns {Error} object, populating it with `reason`, `host`, and `cert` on
failure. On success, returns {undefined}.

This function can be overwritten by providing alternative function as part of
the `options.checkServerIdentity` option passed to `tls.connect()`. The
overwriting function can call `tls.checkServerIdentity()` of course, to augment
the checks done with additional verification.

This function is only called if the certificate passed all other checks, such as
being issued by trusted CA (`options.ca`).

## `tls.connect(options[, callback])`
<!-- YAML
added: v0.11.3
changes:
  - version: v12.16.0
    pr-url: https://github.com/nodejs/node/pull/23188
    description: The `pskCallback` option is now supported.
  - version: v12.9.0
    pr-url: https://github.com/nodejs/node/pull/27836
    description: Support the `allowHalfOpen` option.
  - version: v12.4.0
    pr-url: https://github.com/nodejs/node/pull/27816
    description: The `hints` option is now supported.
  - version: v12.2.0
    pr-url: https://github.com/nodejs/node/pull/27497
    description: The `enableTrace` option is now supported.
  - version: v11.8.0
    pr-url: https://github.com/nodejs/node/pull/25517
    description: The `timeout` option is supported now.
  - version: v8.0.0
    pr-url: https://github.com/nodejs/node/pull/12839
    description: The `lookup` option is supported now.
  - version: v8.0.0
    pr-url: https://github.com/nodejs/node/pull/11984
    description: The `ALPNProtocols` option can be a `TypedArray` or
     `DataView` now.
  - version: v5.3.0, v4.7.0
    pr-url: https://github.com/nodejs/node/pull/4246
    description: The `secureContext` option is supported now.
  - version: v5.0.0
    pr-url: https://github.com/nodejs/node/pull/2564
    description: ALPN options are supported now.
-->

* `options` {Object}
  * `enableTrace`: See [`tls.createServer()`][]
  * `host` {string} Host the client should connect to. **Default:**
    `'localhost'`.
  * `port` {number} Port the client should connect to.
  * `path` {string} Creates Unix socket connection to path. If this option is
    specified, `host` and `port` are ignored.
  * `socket` {stream.Duplex} Establish secure connection on a given socket
    rather than creating a new socket. Typically, this is an instance of
    [`net.Socket`][], but any `Duplex` stream is allowed.
    If this option is specified, `path`, `host` and `port` are ignored,
    except for certificate validation. Usually, a socket is already connected
    when passed to `tls.connect()`, but it can be connected later.
    Connection/disconnection/destruction of `socket` is the user's
    responsibility; calling `tls.connect()` will not cause `net.connect()` to be
    called.
  * `allowHalfOpen` {boolean} If the `socket` option is missing, indicates
    whether or not to allow the internally created socket to be half-open,
    otherwise the option is ignored. See the `allowHalfOpen` option of
    [`net.Socket`][] for details. **Default:** `false`.
  * `rejectUnauthorized` {boolean} If not `false`, the server certificate is
    verified against the list of supplied CAs. An `'error'` event is emitted if
    verification fails; `err.code` contains the OpenSSL error code. **Default:**
    `true`.
  * `pskCallback` {Function}
    * hint: {string} optional message sent from the server to help client
      decide which identity to use during negotiation.
      Always `null` if TLS 1.3 is used.
    * Returns: {Object} in the form
      `{ psk: <Buffer|TypedArray|DataView>, identity: <string> }`
      or `null` to stop the negotiation process. `psk` must be
      compatible with the selected cipher's digest.
      `identity` must use UTF-8 encoding.
    When negotiating TLS-PSK (pre-shared keys), this function is called
    with optional identity `hint` provided by the server or `null`
    in case of TLS 1.3 where `hint` was removed.
    It will be necessary to provide a custom `tls.checkServerIdentity()`
    for the connection as the default one will try to check host name/IP
    of the server against the certificate but that's not applicable for PSK
    because there won't be a certificate present.
    More information can be found in the [RFC 4279][].
  * `ALPNProtocols`: {string[]|Buffer[]|TypedArray[]|DataView[]|Buffer|
    TypedArray|DataView}
    An array of strings, `Buffer`s or `TypedArray`s or `DataView`s, or a
    single `Buffer` or `TypedArray` or `DataView` containing the supported ALPN
    protocols. `Buffer`s should have the format `[len][name][len][name]...`
    e.g. `'\x08http/1.1\x08http/1.0'`, where the `len` byte is the length of the
    next protocol name. Passing an array is usually much simpler, e.g.
    `['http/1.1', 'http/1.0']`. Protocols earlier in the list have higher
    preference than those later.
  * `servername`: {string} Server name for the SNI (Server Name Indication) TLS
    extension. It is the name of the host being connected to, and must be a host
    name, and not an IP address. It can be used by a multi-homed server to
    choose the correct certificate to present to the client, see the
    `SNICallback` option to [`tls.createServer()`][].
  * `checkServerIdentity(servername, cert)` {Function} A callback function
    to be used (instead of the builtin `tls.checkServerIdentity()` function)
    when checking the server's host name (or the provided `servername` when
    explicitly set) against the certificate. This should return an {Error} if
    verification fails. The method should return `undefined` if the `servername`
    and `cert` are verified.
  * `session` {Buffer} A `Buffer` instance, containing TLS session.
  * `minDHSize` {number} Minimum size of the DH parameter in bits to accept a
    TLS connection. When a server offers a DH parameter with a size less
    than `minDHSize`, the TLS connection is destroyed and an error is thrown.
    **Default:** `1024`.
  * `secureContext`: TLS context object created with
    [`tls.createSecureContext()`][]. If a `secureContext` is _not_ provided, one
    will be created by passing the entire `options` object to
    `tls.createSecureContext()`.
  * ...: [`tls.createSecureContext()`][] options that are used if the
    `secureContext` option is missing, otherwise they are ignored.
  * ...: Any [`socket.connect()`][] option not already listed.
* `callback` {Function}
* Returns: {tls.TLSSocket}

The `callback` function, if specified, will be added as a listener for the
[`'secureConnect'`][] event.

`tls.connect()` returns a [`tls.TLSSocket`][] object.

Unlike the `https` API, `tls.connect()` does not enable the
SNI (Server Name Indication) extension by default, which may cause some
servers to return an incorrect certificate or reject the connection
altogether. To enable SNI, set the `servername` option in addition
to `host`.

The following illustrates a client for the echo server example from
[`tls.createServer()`][]:

```js
// Assumes an echo server that is listening on port 8000.
const tls = require('tls');
const fs = require('fs');

const options = {
  // Necessary only if the server requires client certificate authentication.
  key: fs.readFileSync('client-key.pem'),
  cert: fs.readFileSync('client-cert.pem'),

  // Necessary only if the server uses a self-signed certificate.
  ca: [ fs.readFileSync('server-cert.pem') ],

  // Necessary only if the server's cert isn't for "localhost".
  checkServerIdentity: () => { return null; },
};

const socket = tls.connect(8000, options, () => {
  console.log('client connected',
              socket.authorized ? 'authorized' : 'unauthorized');
  process.stdin.pipe(socket);
  process.stdin.resume();
});
socket.setEncoding('utf8');
socket.on('data', (data) => {
  console.log(data);
});
socket.on('end', () => {
  console.log('server ends connection');
});
```

## `tls.connect(path[, options][, callback])`
<!-- YAML
added: v0.11.3
-->

* `path` {string} Default value for `options.path`.
* `options` {Object} See [`tls.connect()`][].
* `callback` {Function} See [`tls.connect()`][].
* Returns: {tls.TLSSocket}

Same as [`tls.connect()`][] except that `path` can be provided
as an argument instead of an option.

A path option, if specified, will take precedence over the path argument.

## `tls.connect(port[, host][, options][, callback])`
<!-- YAML
added: v0.11.3
-->

* `port` {number} Default value for `options.port`.
* `host` {string} Default value for `options.host`.
* `options` {Object} See [`tls.connect()`][].
* `callback` {Function} See [`tls.connect()`][].
* Returns: {tls.TLSSocket}

Same as [`tls.connect()`][] except that `port` and `host` can be provided
as arguments instead of options.

A port or host option, if specified, will take precedence over any port or host
argument.

## `tls.createSecureContext([options])`
<!-- YAML
added: v0.11.13
changes:
  - version: v12.12.0
    pr-url: https://github.com/nodejs/node/pull/28973
    description: Added `privateKeyIdentifier` and `privateKeyEngine` options
                 to get private key from an OpenSSL engine.
  - version: v12.11.0
    pr-url: https://github.com/nodejs/node/pull/29598
    description: Added `sigalgs` option to override supported signature
                 algorithms.
  - version: v12.0.0
    pr-url: https://github.com/nodejs/node/pull/26209
    description: TLSv1.3 support added.
  - version: v11.5.0
    pr-url: https://github.com/nodejs/node/pull/24733
    description: The `ca:` option now supports `BEGIN TRUSTED CERTIFICATE`.
  - version: v11.4.0
    pr-url: https://github.com/nodejs/node/pull/24405
    description: The `minVersion` and `maxVersion` can be used to restrict
                 the allowed TLS protocol versions.
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/19794
    description: The `ecdhCurve` cannot be set to `false` anymore due to a
                 change in OpenSSL.
  - version: v9.3.0
    pr-url: https://github.com/nodejs/node/pull/14903
    description: The `options` parameter can now include `clientCertEngine`.
  - version: v9.0.0
    pr-url: https://github.com/nodejs/node/pull/15206
    description: The `ecdhCurve` option can now be multiple `':'` separated
                 curve names or `'auto'`.
  - version: v7.3.0
    pr-url: https://github.com/nodejs/node/pull/10294
    description: If the `key` option is an array, individual entries do not
                 need a `passphrase` property anymore. `Array` entries can also
                 just be `string`s or `Buffer`s now.
  - version: v5.2.0
    pr-url: https://github.com/nodejs/node/pull/4099
    description: The `ca` option can now be a single string containing multiple
                 CA certificates.
-->

* `options` {Object}
  * `ca` {string|string[]|Buffer|Buffer[]} Optionally override the trusted CA
    certificates. Default is to trust the well-known CAs curated by Mozilla.
    Mozilla's CAs are completely replaced when CAs are explicitly specified
    using this option. The value can be a string or `Buffer`, or an `Array` of
    strings and/or `Buffer`s. Any string or `Buffer` can contain multiple PEM
    CAs concatenated together. The peer's certificate must be chainable to a CA
    trusted by the server for the connection to be authenticated. When using
    certificates that are not chainable to a well-known CA, the certificate's CA
    must be explicitly specified as a trusted or the connection will fail to
    authenticate.
    If the peer uses a certificate that doesn't match or chain to one of the
    default CAs, use the `ca` option to provide a CA certificate that the peer's
    certificate can match or chain to.
    For self-signed certificates, the certificate is its own CA, and must be
    provided.
    For PEM encoded certificates, supported types are "TRUSTED CERTIFICATE",
    "X509 CERTIFICATE", and "CERTIFICATE".
    See also [`tls.rootCertificates`][].
  * `cert` {string|string[]|Buffer|Buffer[]} Cert chains in PEM format. One cert
    chain should be provided per private key. Each cert chain should consist of
    the PEM formatted certificate for a provided private `key`, followed by the
    PEM formatted intermediate certificates (if any), in order, and not
    including the root CA (the root CA must be pre-known to the peer, see `ca`).
    When providing multiple cert chains, they do not have to be in the same
    order as their private keys in `key`. If the intermediate certificates are
    not provided, the peer will not be able to validate the certificate, and the
    handshake will fail.
  * `sigalgs` {string} Colon-separated list of supported signature algorithms.
    The list can contain digest algorithms (`SHA256`, `MD5` etc.), public key
    algorithms (`RSA-PSS`, `ECDSA` etc.), combination of both (e.g
    'RSA+SHA384') or TLS v1.3 scheme names (e.g. `rsa_pss_pss_sha512`).
    See [OpenSSL man pages](https://www.openssl.org/docs/man1.1.1/man3/SSL_CTX_set1_sigalgs_list.html)
    for more info.
  * `ciphers` {string} Cipher suite specification, replacing the default. For
    more information, see [modifying the default cipher suite][]. Permitted
    ciphers can be obtained via [`tls.getCiphers()`][]. Cipher names must be
    uppercased in order for OpenSSL to accept them.
  * `clientCertEngine` {string} Name of an OpenSSL engine which can provide the
    client certificate.
  * `crl` {string|string[]|Buffer|Buffer[]} PEM formatted CRLs (Certificate
    Revocation Lists).
  * `dhparam` {string|Buffer} Diffie-Hellman parameters, required for
    [perfect forward secrecy][]. Use `openssl dhparam` to create the parameters.
    The key length must be greater than or equal to 1024 bits or else an error
    will be thrown. Although 1024 bits is permissible, use 2048 bits or larger
    for stronger security. If omitted or invalid, the parameters are silently
    discarded and DHE ciphers will not be available.
  * `ecdhCurve` {string} A string describing a named curve or a colon separated
    list of curve NIDs or names, for example `P-521:P-384:P-256`, to use for
    ECDH key agreement. Set to `auto` to select the
    curve automatically. Use [`crypto.getCurves()`][] to obtain a list of
    available curve names. On recent releases, `openssl ecparam -list_curves`
    will also display the name and description of each available elliptic curve.
    **Default:** [`tls.DEFAULT_ECDH_CURVE`][].
  * `honorCipherOrder` {boolean} Attempt to use the server's cipher suite
    preferences instead of the client's. When `true`, causes
    `SSL_OP_CIPHER_SERVER_PREFERENCE` to be set in `secureOptions`, see
    [OpenSSL Options][] for more information.
  * `key` {string|string[]|Buffer|Buffer[]|Object[]} Private keys in PEM format.
    PEM allows the option of private keys being encrypted. Encrypted keys will
    be decrypted with `options.passphrase`. Multiple keys using different
    algorithms can be provided either as an array of unencrypted key strings or
    buffers, or an array of objects in the form
    `{pem: <string|buffer>[, passphrase: <string>]}`. The object form can only
    occur in an array. `object.passphrase` is optional. Encrypted keys will be
    decrypted with `object.passphrase` if provided, or `options.passphrase` if
    it is not.
  * `privateKeyEngine` {string} Name of an OpenSSL engine to get private key
    from. Should be used together with `privateKeyIdentifier`.
  * `privateKeyIdentifier` {string} Identifier of a private key managed by
    an OpenSSL engine. Should be used together with `privateKeyEngine`.
    Should not be set together with `key`, because both options define a
    private key in different ways.
  * `maxVersion` {string} Optionally set the maximum TLS version to allow. One
    of `'TLSv1.3'`, `'TLSv1.2'`, `'TLSv1.1'`, or `'TLSv1'`. Cannot be specified
    along with the `secureProtocol` option, use one or the other.
    **Default:** [`tls.DEFAULT_MAX_VERSION`][].
  * `minVersion` {string} Optionally set the minimum TLS version to allow. One
    of `'TLSv1.3'`, `'TLSv1.2'`, `'TLSv1.1'`, or `'TLSv1'`. Cannot be specified
    along with the `secureProtocol` option, use one or the other. It is not
    recommended to use less than TLSv1.2, but it may be required for
    interoperability.
    **Default:** [`tls.DEFAULT_MIN_VERSION`][].
  * `passphrase` {string} Shared passphrase used for a single private key and/or
    a PFX.
  * `pfx` {string|string[]|Buffer|Buffer[]|Object[]} PFX or PKCS12 encoded
    private key and certificate chain. `pfx` is an alternative to providing
    `key` and `cert` individually. PFX is usually encrypted, if it is,
    `passphrase` will be used to decrypt it. Multiple PFX can be provided either
    as an array of unencrypted PFX buffers, or an array of objects in the form
    `{buf: <string|buffer>[, passphrase: <string>]}`. The object form can only
    occur in an array. `object.passphrase` is optional. Encrypted PFX will be
    decrypted with `object.passphrase` if provided, or `options.passphrase` if
    it is not.
  * `secureOptions` {number} Optionally affect the OpenSSL protocol behavior,
    which is not usually necessary. This should be used carefully if at all!
    Value is a numeric bitmask of the `SSL_OP_*` options from
    [OpenSSL Options][].
  * `secureProtocol` {string} Legacy mechanism to select the TLS protocol
    version to use, it does not support independent control of the minimum and
    maximum version, and does not support limiting the protocol to TLSv1.3. Use
    `minVersion` and `maxVersion` instead. The possible values are listed as
    [SSL_METHODS][], use the function names as strings. For example, use
    `'TLSv1_1_method'` to force TLS version 1.1, or `'TLS_method'` to allow any
    TLS protocol version up to TLSv1.3. It is not recommended to use TLS
    versions less than 1.2, but it may be required for interoperability.
    **Default:** none, see `minVersion`.
  * `sessionIdContext` {string} Opaque identifier used by servers to ensure
    session state is not shared between applications. Unused by clients.
  * `ticketKeys`: {Buffer} 48-bytes of cryptographically strong pseudo-random
    data. See [Session Resumption][] for more information.
  * `sessionTimeout` {number} The number of seconds after which a TLS session
    created by the server will no longer be resumable. See
    [Session Resumption][] for more information. **Default:** `300`.

[`tls.createServer()`][] sets the default value of the `honorCipherOrder` option
to `true`, other APIs that create secure contexts leave it unset.

[`tls.createServer()`][] uses a 128 bit truncated SHA1 hash value generated
from `process.argv` as the default value of the `sessionIdContext` option, other
APIs that create secure contexts have no default value.

The `tls.createSecureContext()` method creates a `SecureContext` object. It is
usable as an argument to several `tls` APIs, such as [`tls.createServer()`][]
and [`server.addContext()`][], but has no public methods.

A key is *required* for ciphers that use certificates. Either `key` or
`pfx` can be used to provide it.

If the `ca` option is not given, then Node.js will default to using
[Mozilla's publicly trusted list of CAs][].

## `tls.createSecurePair([context][, isServer][, requestCert][, rejectUnauthorized][, options])`
<!-- YAML
added: v0.3.2
deprecated: v0.11.3
changes:
  - version: v5.0.0
    pr-url: https://github.com/nodejs/node/pull/2564
    description: ALPN options are supported now.
-->

> Stability: 0 - Deprecated: Use [`tls.TLSSocket`][] instead.

* `context` {Object} A secure context object as returned by
  `tls.createSecureContext()`
* `isServer` {boolean} `true` to specify that this TLS connection should be
  opened as a server.
* `requestCert` {boolean} `true` to specify whether a server should request a
  certificate from a connecting client. Only applies when `isServer` is `true`.
* `rejectUnauthorized` {boolean} If not `false` a server automatically reject
  clients with invalid certificates. Only applies when `isServer` is `true`.
* `options`
  * `enableTrace`: See [`tls.createServer()`][]
  * `secureContext`: A TLS context object from [`tls.createSecureContext()`][]
  * `isServer`: If `true` the TLS socket will be instantiated in server-mode.
    **Default:** `false`.
  * `server` {net.Server} A [`net.Server`][] instance
  * `requestCert`: See [`tls.createServer()`][]
  * `rejectUnauthorized`: See [`tls.createServer()`][]
  * `ALPNProtocols`: See [`tls.createServer()`][]
  * `SNICallback`: See [`tls.createServer()`][]
  * `session` {Buffer} A `Buffer` instance containing a TLS session.
  * `requestOCSP` {boolean} If `true`, specifies that the OCSP status request
    extension will be added to the client hello and an `'OCSPResponse'` event
    will be emitted on the socket before establishing a secure communication.

Creates a new secure pair object with two streams, one of which reads and writes
the encrypted data and the other of which reads and writes the cleartext data.
Generally, the encrypted stream is piped to/from an incoming encrypted data
stream and the cleartext one is used as a replacement for the initial encrypted
stream.

`tls.createSecurePair()` returns a `tls.SecurePair` object with `cleartext` and
`encrypted` stream properties.

Using `cleartext` has the same API as [`tls.TLSSocket`][].

The `tls.createSecurePair()` method is now deprecated in favor of
`tls.TLSSocket()`. For example, the code:

```js
pair = tls.createSecurePair(/* ... */);
pair.encrypted.pipe(socket);
socket.pipe(pair.encrypted);
```

can be replaced by:

```js
secureSocket = tls.TLSSocket(socket, options);
```

where `secureSocket` has the same API as `pair.cleartext`.

## `tls.createServer([options][, secureConnectionListener])`
<!-- YAML
added: v0.3.2
changes:
  - version: v12.3.0
    pr-url: https://github.com/nodejs/node/pull/27665
    description: The `options` parameter now supports `net.createServer()`
                 options.
  - version: v9.3.0
    pr-url: https://github.com/nodejs/node/pull/14903
    description: The `options` parameter can now include `clientCertEngine`.
  - version: v8.0.0
    pr-url: https://github.com/nodejs/node/pull/11984
    description: The `ALPNProtocols` option can be a `TypedArray` or
     `DataView` now.
  - version: v5.0.0
    pr-url: https://github.com/nodejs/node/pull/2564
    description: ALPN options are supported now.
-->

* `options` {Object}
  * `ALPNProtocols`: {string[]|Buffer[]|TypedArray[]|DataView[]|Buffer|
    TypedArray|DataView}
    An array of strings, `Buffer`s or `TypedArray`s or `DataView`s, or a single
    `Buffer` or `TypedArray` or `DataView` containing the supported ALPN
    protocols. `Buffer`s should have the format `[len][name][len][name]...`
    e.g. `0x05hello0x05world`, where the first byte is the length of the next
    protocol name. Passing an array is usually much simpler, e.g.
    `['hello', 'world']`. (Protocols should be ordered by their priority.)
  * `clientCertEngine` {string} Name of an OpenSSL engine which can provide the
    client certificate.
  * `enableTrace` {boolean} If `true`, [`tls.TLSSocket.enableTrace()`][] will be
    called on new connections. Tracing can be enabled after the secure
    connection is established, but this option must be used to trace the secure
    connection setup. **Default:** `false`.
  * `handshakeTimeout` {number} Abort the connection if the SSL/TLS handshake
    does not finish in the specified number of milliseconds.
    A `'tlsClientError'` is emitted on the `tls.Server` object whenever
    a handshake times out. **Default:** `120000` (120 seconds).
  * `rejectUnauthorized` {boolean} If not `false` the server will reject any
    connection which is not authorized with the list of supplied CAs. This
    option only has an effect if `requestCert` is `true`. **Default:** `true`.
  * `requestCert` {boolean} If `true` the server will request a certificate from
    clients that connect and attempt to verify that certificate. **Default:**
    `false`.
  * `sessionTimeout` {number} The number of seconds after which a TLS session
    created by the server will no longer be resumable. See
    [Session Resumption][] for more information. **Default:** `300`.
  * `SNICallback(servername, callback)` {Function} A function that will be
    called if the client supports SNI TLS extension. Two arguments will be
    passed when called: `servername` and `callback`. `callback` is an
    error-first callback that takes two optional arguments: `error` and `ctx`.
    `ctx`, if provided, is a `SecureContext` instance.
    [`tls.createSecureContext()`][] can be used to get a proper `SecureContext`.
    If `callback` is called with a falsy `ctx` argument, the default secure
    context of the server will be used. If `SNICallback` wasn't provided the
    default callback with high-level API will be used (see below).
  * `ticketKeys`: {Buffer} 48-bytes of cryptographically strong pseudo-random
    data. See [Session Resumption][] for more information.
  * `pskCallback` {Function}
    * socket: {tls.TLSSocket} the server [`tls.TLSSocket`][] instance for
      this connection.
    * identity: {string} identity parameter sent from the client.
    * Returns: {Buffer|TypedArray|DataView} pre-shared key that must either be
      a buffer or `null` to stop the negotiation process. Returned PSK must be
      compatible with the selected cipher's digest.
    When negotiating TLS-PSK (pre-shared keys), this function is called
    with the identity provided by the client.
    If the return value is `null` the negotiation process will stop and an
    "unknown_psk_identity" alert message will be sent to the other party.
    If the server wishes to hide the fact that the PSK identity was not known,
    the callback must provide some random data as `psk` to make the connection
    fail with "decrypt_error" before negotiation is finished.
    PSK ciphers are disabled by default, and using TLS-PSK thus
    requires explicitly specifying a cipher suite with the `ciphers` option.
    More information can be found in the [RFC 4279][].
  * `pskIdentityHint` {string} optional hint to send to a client to help
    with selecting the identity during TLS-PSK negotiation. Will be ignored
    in TLS 1.3. Upon failing to set pskIdentityHint `'tlsClientError'` will be
    emitted with `'ERR_TLS_PSK_SET_IDENTIY_HINT_FAILED'` code.
  * ...: Any [`tls.createSecureContext()`][] option can be provided. For
    servers, the identity options (`pfx`, `key`/`cert` or `pskCallback`)
    are usually required.
  * ...: Any [`net.createServer()`][] option can be provided.
* `secureConnectionListener` {Function}
* Returns: {tls.Server}

Creates a new [`tls.Server`][]. The `secureConnectionListener`, if provided, is
automatically set as a listener for the [`'secureConnection'`][] event.

The `ticketKeys` options is automatically shared between `cluster` module
workers.

The following illustrates a simple echo server:

```js
const tls = require('tls');
const fs = require('fs');

const options = {
  key: fs.readFileSync('server-key.pem'),
  cert: fs.readFileSync('server-cert.pem'),

  // This is necessary only if using client certificate authentication.
  requestCert: true,

  // This is necessary only if the client uses a self-signed certificate.
  ca: [ fs.readFileSync('client-cert.pem') ]
};

const server = tls.createServer(options, (socket) => {
  console.log('server connected',
              socket.authorized ? 'authorized' : 'unauthorized');
  socket.write('welcome!\n');
  socket.setEncoding('utf8');
  socket.pipe(socket);
});
server.listen(8000, () => {
  console.log('server bound');
});
```

The server can be tested by connecting to it using the example client from
[`tls.connect()`][].

## `tls.getCiphers()`
<!-- YAML
added: v0.10.2
-->

* Returns: {string[]}

Returns an array with the names of the supported TLS ciphers. The names are
lower-case for historical reasons, but must be uppercased to be used in
the `ciphers` option of [`tls.createSecureContext()`][].

Cipher names that start with `'tls_'` are for TLSv1.3, all the others are for
TLSv1.2 and below.

```js
console.log(tls.getCiphers()); // ['aes128-gcm-sha256', 'aes128-sha', ...]
```

## `tls.rootCertificates`
<!-- YAML
added: v12.3.0
-->

* {string[]}

An immutable array of strings representing the root certificates (in PEM format)
from the bundled Mozilla CA store as supplied by current Node.js version.

The bundled CA store, as supplied by Node.js, is a snapshot of Mozilla CA store
that is fixed at release time. It is identical on all supported platforms.

## `tls.DEFAULT_ECDH_CURVE`
<!-- YAML
added: v0.11.13
changes:
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/16853
    description: Default value changed to `'auto'`.
-->

The default curve name to use for ECDH key agreement in a tls server. The
default value is `'auto'`. See [`tls.createSecureContext()`][] for further
information.

## `tls.DEFAULT_MAX_VERSION`
<!-- YAML
added: v11.4.0
-->

* {string} The default value of the `maxVersion` option of
  [`tls.createSecureContext()`][]. It can be assigned any of the supported TLS
  protocol versions, `'TLSv1.3'`, `'TLSv1.2'`, `'TLSv1.1'`, or `'TLSv1'`.
  **Default:** `'TLSv1.3'`, unless changed using CLI options. Using
  `--tls-max-v1.2` sets the default to `'TLSv1.2'`. Using `--tls-max-v1.3` sets
  the default to `'TLSv1.3'`. If multiple of the options are provided, the
  highest maximum is used.

## `tls.DEFAULT_MIN_VERSION`
<!-- YAML
added: v11.4.0
-->

* {string} The default value of the `minVersion` option of
  [`tls.createSecureContext()`][]. It can be assigned any of the supported TLS
  protocol versions, `'TLSv1.3'`, `'TLSv1.2'`, `'TLSv1.1'`, or `'TLSv1'`.
  **Default:** `'TLSv1.2'`, unless changed using CLI options. Using
  `--tls-min-v1.0` sets the default to `'TLSv1'`. Using `--tls-min-v1.1` sets
  the default to `'TLSv1.1'`. Using `--tls-min-v1.3` sets the default to
  `'TLSv1.3'`. If multiple of the options are provided, the lowest minimum is
  used.

[`'newSession'`]: #tls_event_newsession
[`'resumeSession'`]: #tls_event_resumesession
[`'secureConnect'`]: #tls_event_secureconnect
[`'secureConnection'`]: #tls_event_secureconnection
[`'session'`]: #tls_event_session
[`--tls-cipher-list`]: cli.html#cli_tls_cipher_list_list
[`NODE_OPTIONS`]: cli.html#cli_node_options_options
[`SSL_export_keying_material`]: https://www.openssl.org/docs/man1.1.1/man3/SSL_export_keying_material.html
[`SSL_get_version`]: https://www.openssl.org/docs/man1.1.1/man3/SSL_get_version.html
[`crypto.getCurves()`]: crypto.html#crypto_crypto_getcurves
[`Duplex`]: stream.html#stream_class_stream_duplex
[`net.createServer()`]: net.html#net_net_createserver_options_connectionlistener
[`net.Server.address()`]: net.html#net_server_address
[`net.Server`]: net.html#net_class_net_server
[`net.Socket`]: net.html#net_class_net_socket
[`server.addContext()`]: #tls_server_addcontext_hostname_context
[`server.getConnections()`]: net.html#net_server_getconnections_callback
[`server.getTicketKeys()`]: #tls_server_getticketkeys
[`server.listen()`]: net.html#net_server_listen
[`server.setTicketKeys()`]: #tls_server_setticketkeys_keys
[`socket.connect()`]: net.html#net_socket_connect_options_connectlistener
[`tls.DEFAULT_ECDH_CURVE`]: #tls_tls_default_ecdh_curve
[`tls.DEFAULT_MAX_VERSION`]: #tls_tls_default_max_version
[`tls.DEFAULT_MIN_VERSION`]: #tls_tls_default_min_version
[`tls.Server`]: #tls_class_tls_server
[`tls.TLSSocket.enableTrace()`]: #tls_tlssocket_enabletrace
[`tls.TLSSocket.getPeerCertificate()`]: #tls_tlssocket_getpeercertificate_detailed
[`tls.TLSSocket.getSession()`]: #tls_tlssocket_getsession
[`tls.TLSSocket.getTLSTicket()`]: #tls_tlssocket_gettlsticket
[`tls.TLSSocket`]: #tls_class_tls_tlssocket
[`tls.connect()`]: #tls_tls_connect_options_callback
[`tls.createSecureContext()`]: #tls_tls_createsecurecontext_options
[`tls.createSecurePair()`]: #tls_tls_createsecurepair_context_isserver_requestcert_rejectunauthorized_options
[`tls.createServer()`]: #tls_tls_createserver_options_secureconnectionlistener
[`tls.getCiphers()`]: #tls_tls_getciphers
[`tls.rootCertificates`]: #tls_tls_rootcertificates
[Chrome's 'modern cryptography' setting]: https://www.chromium.org/Home/chromium-security/education/tls#TOC-Cipher-Suites
[DHE]: https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange
[ECDHE]: https://en.wikipedia.org/wiki/Elliptic_curve_Diffie%E2%80%93Hellman
[forward secrecy]: https://en.wikipedia.org/wiki/Perfect_forward_secrecy
[Mozilla's publicly trusted list of CAs]: https://hg.mozilla.org/mozilla-central/raw-file/tip/security/nss/lib/ckfw/builtins/certdata.txt
[OCSP request]: https://en.wikipedia.org/wiki/OCSP_stapling
[OpenSSL Options]: crypto.html#crypto_openssl_options
[perfect forward secrecy]: #tls_perfect_forward_secrecy
[RFC 2246]: https://www.ietf.org/rfc/rfc2246.txt
[RFC 5077]: https://tools.ietf.org/html/rfc5077
[RFC 5929]: https://tools.ietf.org/html/rfc5929
[SSL_METHODS]: https://www.openssl.org/docs/man1.1.1/man7/ssl.html#Dealing-with-Protocol-Methods
[Session Resumption]: #tls_session_resumption
[Stream]: stream.html#stream_stream
[TLS recommendations]: https://wiki.mozilla.org/Security/Server_Side_TLS
[asn1.js]: https://www.npmjs.com/package/asn1.js
[certificate object]: #tls_certificate_object
[cipher list format]: https://www.openssl.org/docs/man1.1.1/man1/ciphers.html#CIPHER-LIST-FORMAT
[modifying the default cipher suite]: #tls_modifying_the_default_tls_cipher_suite
[specific attacks affecting larger AES key sizes]: https://www.schneier.com/blog/archives/2009/07/another_new_aes.html
[RFC 4279]: https://tools.ietf.org/html/rfc4279
[RFC 4086]: https://tools.ietf.org/html/rfc4086