Node.js v24.7.0 documentation

Perfect forward secrecy#

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):

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

Perfect forward secrecy using ECDHE is enabled by default. The ecdhCurve option can be used when creating a TLS server to customize the list of supported ECDH curves to use. See tls.createServer() for more info.

DHE is disabled by default but can be enabled alongside ECDHE by setting the dhparam option to 'auto'. Custom DHE parameters are also supported but discouraged in favor of automatically selected, well-known parameters.

Perfect forward secrecy was optional up to TLSv1.2. As of TLSv1.3, (EC)DHE is always used (with the exception of PSK-only connections).

ALPN and SNI#

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 certificates.

Pre-shared keys#

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 the public key infrastructure (PKI) 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 and can be retrieved via openssl ciphers -v -s -tls1_3 -psk. On the client connection, a custom checkServerIdentity should be passed because the default one will fail in the absence of a certificate.

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.

To use TLS-PSK, client and server must specify the pskCallback option, a function that returns the PSK to use (which must be compatible with the selected cipher's digest).

It will be called first on the client:

• hint <string> optional message sent from the server to help the 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.

Then on the server:

• socket <tls.TLSSocket> the server socket instance, equivalent to this.
• identity <string> identity parameter sent from the client.
• Returns: <Buffer> | <TypedArray> | <DataView> the PSK (or null).

A return value of null stops the negotiation process and sends an unknown_psk_identity alert message 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.

Client-initiated renegotiation attack mitigation#

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).

openssl s_client -connect localhost:443 -reconnect copy

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

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

Setting security levels#

To adjust the security level in your Node.js application, you can include @SECLEVEL=X within a cipher string, where X is the desired security level. For example, to set the security level to 0 while using the default OpenSSL cipher list, you could use:

import { createServer, connect } from 'node:tls';
const port = 443;

createServer({ ciphers: 'DEFAULT@SECLEVEL=0', minVersion: 'TLSv1' }, function(socket) {
  console.log('Client connected with protocol:', socket.getProtocol());
  socket.end();
  this.close();
})
.listen(port, () => {
  connect(port, { ciphers: 'DEFAULT@SECLEVEL=0', maxVersion: 'TLSv1' });
});const { createServer, connect } = require('node:tls');
const port = 443;

createServer({ ciphers: 'DEFAULT@SECLEVEL=0', minVersion: 'TLSv1' }, function(socket) {
  console.log('Client connected with protocol:', socket.getProtocol());
  socket.end();
  this.close();
})
.listen(port, () => {
  connect(port, { ciphers: 'DEFAULT@SECLEVEL=0', maxVersion: 'TLSv1' });
});copy

This approach sets the security level to 0, allowing the use of legacy features while still leveraging the default OpenSSL ciphers.

Using --tls-cipher-list#

You can also set the security level and ciphers from the command line using the --tls-cipher-list=DEFAULT@SECLEVEL=X as described in Modifying the default TLS cipher suite. However, it is generally discouraged to use the command line option for setting ciphers and it is preferable to configure the ciphers for individual contexts within your application code, as this approach provides finer control and reduces the risk of globally downgrading the security level.

Event: 'connection'#

• 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 but will not receive events unlike the socket created from the net.Server 'connection' event. 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'#

• 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:

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);
}); copy

Event: 'newSession'#

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'#

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'#

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:

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);
}); copy

Event: 'secureConnection'#

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 because the client or the server did not send an ALPN extension, tlsSocket.alpnProtocol equals false.

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

Event: 'tlsClientError'#

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)#

• hostname <string> A SNI host name or wildcard (e.g. '*')
• context <Object> | <tls.SecureContext> An object containing any of the possible properties from the tls.createSecureContext() options arguments (e.g. key, cert, ca, etc), or a TLS context object created with tls.createSecureContext() itself.

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).

When there are multiple matching contexts, the most recently added one is used.

server.address()#

• 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])#

• 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.getTicketKeys()#

• 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)#

• 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)#

• keys <Buffer> | <TypedArray> | <DataView> 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.

new tls.TLSSocket(socket[, options])#

• 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