Table of Contents

• TLS (SSL)
  • TLS/SSL Concepts
    • Perfect Forward Secrecy
    • ALPN, NPN and SNI
    • Client-initiated renegotiation attack mitigation
  • Modifying the Default TLS Cipher suite
  • Class: tls.Server
    • Event: 'tlsClientError'
    • Event: 'newSession'
    • Event: 'OCSPRequest'
    • Event: 'resumeSession'
    • Event: 'secureConnection'
    • server.addContext(hostname, context)
    • server.address()
    • server.close([callback])
    • server.connections
    • server.getTicketKeys()
    • server.listen(port[, hostname][, callback])
    • server.setTicketKeys(keys)
  • Class: tls.TLSSocket
    • new tls.TLSSocket(socket[, options])
    • Event: 'OCSPResponse'
    • Event: 'secureConnect'
    • tlsSocket.address()
    • tlsSocket.authorized
    • tlsSocket.authorizationError
    • tlsSocket.encrypted
    • tlsSocket.getCipher()
    • tlsSocket.getEphemeralKeyInfo()
    • tlsSocket.getPeerCertificate([ detailed ])
    • tlsSocket.getProtocol()
    • tlsSocket.getSession()
    • tlsSocket.getTLSTicket()
    • tlsSocket.localAddress
    • tlsSocket.localPort
    • tlsSocket.remoteAddress
    • tlsSocket.remoteFamily
    • tlsSocket.remotePort
    • tlsSocket.renegotiate(options, callback)
    • tlsSocket.setMaxSendFragment(size)
  • tls.connect(port[, host][, options][, callback])
  • tls.connect(path[, options][, callback])
  • tls.connect(options[, callback])
  • tls.createSecureContext(options)
  • tls.createServer([options][, secureConnectionListener])
  • tls.getCiphers()
  • tls.DEFAULT_ECDH_CURVE
  • Deprecated APIs
    • Class: CryptoStream
      • cryptoStream.bytesWritten
    • Class: SecurePair
      • Event: 'secure'
    • tls.createSecurePair([context][, isServer][, requestCert][, rejectUnauthorized][, options])

TLS (SSL)#

Stability: 2 - Stable

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:

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:

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:

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:

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:

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#

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:

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 name of an alternative curve to use, see tls.createServer() for more info.

ALPN, NPN and SNI#

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

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

Note: Use of ALPN is recommended over NPN. The NPN extension has never been formally defined or documented and generally not recommended for use.

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. Defaults to 3.
• tls.CLIENT_RENEG_WINDOW <number> Specifies the time renegotiation window in seconds. Defaults to 600 (10 minutes).

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

To test the renegotiation limits on a server, connect to it using the OpenSSL command-line client (openssl s_client -connect address:port) then input R<CR> (i.e., the letter R followed by a carriage return) multiple times.

Modifying the Default TLS Cipher suite#

Node.js is built with a default suite of enabled and disabled TLS ciphers. Currently, the default cipher suite is:

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. For instance, the following makes ECDHE-RSA-AES128-GCM-SHA256:!RC4 the default TLS cipher suite:

node --tls-cipher-list="ECDHE-RSA-AES128-GCM-SHA256:!RC4"

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.TLSSockets.

Consult OpenSSL cipher list format documentation for details on the format.

Note: 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.

Class: tls.Server#

The tls.Server class is a subclass of net.Server that accepts encrypted connections using TLS or SSL.

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.

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 listener callback is passed three arguments when called:

• sessionId - The TLS session identifier
• sessionData - 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.

Note: 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.

Note: 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.)

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

Note: 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 - The TLS/SSL session identifier
• callback <Function> A callback function to be called when the prior session has been recovered.

When called, the event listener may perform a lookup in external storage using the given sessionId and invoke callback(null, sessionData) once finished. If the session cannot be resumed (i.e., doesn't exist in storage) the callback may be invoked as callback(null, null). Calling callback(err) will terminate the incoming connection and destroy the socket.

Note: 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);
});

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. Note that depending on the settings of the TLS server, unauthorized connections may still be accepted.

The tlsSocket.npnProtocol and tlsSocket.alpnProtocol properties are strings that contain the selected NPN and ALPN protocols, respectively. When both NPN and ALPN extensions are received, ALPN takes precedence over NPN and the next protocol is selected by ALPN.

When ALPN has no selected protocol, tlsSocket.alpnProtocol returns false.

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

server.addContext(hostname, context)#

• hostname <string> A SNI hostname 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 hostname matches the supplied hostname (or wildcard).

server.address()#

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> An optional listener callback that will be registered to listen for the server instance's 'close' event.

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#

Returns the current number of concurrent connections on the server.

server.getTicketKeys()#

Returns a Buffer instance holding the keys currently used for encryption/decryption of the TLS Session Tickets

server.listen(port[, hostname][, callback])#

• port <number> The TCP/IP port on which to begin listening for connections. A value of 0 (zero) will assign a random port.
• hostname <string> The hostname, IPv4, or IPv6 address on which to begin listening for connections. If undefined, the server will accept connections on any IPv6 address (::) when IPv6 is available, or any IPv4 address (0.0.0.0) otherwise.
• callback <Function> A callback function to be invoked when the server has begun listening on the port and hostname.

The server.listen() methods instructs the server to begin accepting connections on the specified port and hostname.

This function operates asynchronously. If the callback is given, it will be called when the server has started listening.

See net.Server for more information.

server.setTicketKeys(keys)#

• keys <Buffer> The keys used for encryption/decryption of the TLS Session Tickets.

Updates the keys for encryption/decryption of the TLS Session Tickets.

Note: The key's Buffer should be 48 bytes long. See ticketKeys option in tls.createServer for more information on how it is used.

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

Class: tls.TLSSocket#

The tls.TLSSocket is a subclass of net.Socket that performs transparent encryption of written data and all required TLS negotiation.

Instances of tls.TLSSocket implement the duplex Stream interface.

Note: 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])#

• socket <net.Socket> An instance of net.Socket
• options <Object>
  • isServer: The SSL/TLS protocol is asymetrical, 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. Defaults to false.
  • server <net.Server> An optional 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 optionally set requestCert to true to request a client certificate.
  • rejectUnauthorized: Optional, see tls.createServer()
  • NPNProtocols: Optional, see tls.createServer()
  • ALPNProtocols: Optional, see tls.createServer()
  • SNICallback: Optional, see tls.createServer()
  • session <Buffer> An optional 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: Optional TLS context object created with tls.createSecureContext(). If a secureContext is not provided, one will be created by calling tls.createSecureContext() with no options.

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

Event: 'OCSPResponse'#

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'#

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 either ALPN or NPN was used, the tlsSocket.alpnProtocol or tlsSocket.npnProtocol properties can be checked to determine the negotiated protocol.

tlsSocket.address()#

Returns the bound address, the address family name, and port of the underlying socket as reported by the operating system. Returns an object with three properties, e.g., { port: 12346, family: 'IPv4', address: '127.0.0.1' }

tlsSocket.authorized#

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

tlsSocket.authorizationError#

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

tlsSocket.encrypted#

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

tlsSocket.getCipher()#

Returns an object representing the cipher name and the SSL/TLS protocol version that first defined the cipher.

For example: { name: 'AES256-SHA', version: 'TLSv1/SSLv3' }

See SSL_CIPHER_get_name() and SSL_CIPHER_get_version() in https://www.openssl.org/docs/man1.0.2/ssl/SSL_CIPHER_get_name.html for more information.

tlsSocket.getEphemeralKeyInfo()#

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.getPeerCertificate([ detailed ])#

• detailed <boolean> Include the full certificate chain if true, otherwise include just the peer's certificate.

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

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

For example:

{ subject:
   { C: 'UK',
     ST: 'Acknack Ltd',
     L: 'Rhys Jones',
     O: 'node.js',
     OU: 'Test TLS Certificate',
     CN: 'localhost' },
  issuer:
   { C: 'UK',
     ST: 'Acknack Ltd',
     L: 'Rhys Jones',
     O: 'node.js',
     OU: 'Test TLS Certificate',
     CN: 'localhost' },
  issuerCertificate:
   { ... another certificate, possibly with a .issuerCertificate ... },
  raw: < RAW DER buffer >,
  valid_from: 'Nov 11 09:52:22 2009 GMT',
  valid_to: 'Nov  6 09:52:22 2029 GMT',
  fingerprint: '2A:7A:C2:DD:E5:F9:CC:53:72:35:99:7A:02:5A:71:38:52:EC:8A:DF',
  serialNumber: 'B9B0D332A1AA5635' }

If the peer does not provide a certificate, an empty object will be returned.

tlsSocket.getProtocol()#

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.

Example responses include:

• SSLv3
• TLSv1
• TLSv1.1
• TLSv1.2
• unknown

See https://www.openssl.org/docs/man1.0.2/ssl/SSL_get_version.html for more information.

tlsSocket.getSession()#

Returns the ASN.1 encoded TLS session or undefined if no session was negotiated. Can be used to speed up handshake establishment when reconnecting to the server.

tlsSocket.getTLSTicket()#

Returns the TLS session ticket or undefined if no session was negotiated.

Note: This only works with client TLS sockets. Useful only for debugging, for session reuse provide session option to tls.connect().

tlsSocket.localAddress#

Returns the string representation of the local IP address.

tlsSocket.localPort#

Returns the numeric representation of the local port.

tlsSocket.remoteAddress#

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

tlsSocket.remoteFamily#

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

tlsSocket.remotePort#

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

tlsSocket.renegotiate(options, callback)#

• options <Object>
  • rejectUnauthorized <boolean>
  • requestCert
• callback <Function> A function that will be called when the renegotiation request has been completed.

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.

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

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

tlsSocket.setMaxSendFragment(size)#

• size <number> The maximum TLS fragment size. Defaults to 16384. The maximum value is 16384.

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.connect(port[, host][, options][, callback])#

• port <number> Default value for options.port.
• host <string> Optional default value for options.host.
• options <Object> See tls.connect().
• callback <Function> See tls.connect().

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

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

tls.connect(path[, options][, callback])#

• path <string> Default value for options.path.
• options <Object> See tls.connect().
• callback <Function> See tls.connect().

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

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

tls.connect(options[, callback])#

• options <Object>
  • host <string> Host the client should connect to, defaults to '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 <net.Socket> Establish secure connection on a given socket rather than creating a new socket. If this option is specified, path, host and port are ignored. Usually, a socket is already connected when passed to tls.connect(), but it can be connected later. Note that connection/disconnection/destruction of socket is the user's responsibility, calling tls.connect() will not cause net.connect() to be called.
  • rejectUnauthorized <boolean> If true, 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. Defaults to true.
  • NPNProtocols <string[]> | <Buffer[]> An array of strings or Buffers containing supported NPN protocols. Buffers 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'].
  • ALPNProtocols: <string[]> | <Buffer[]> An array of strings or Buffers containing the supported ALPN protocols. Buffers 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: ['hello', 'world'].)
  • servername: <string> Server name for the SNI (Server Name Indication) TLS extension.
  • checkServerIdentity(servername, cert) <Function> A callback function to be used when checking the server's hostname against the certificate. This should throw an error if verification fails. The method should return undefined if the servername and cert are verified.