## TLS (SSL)

Use `require('tls')` to access this module.

The `tls` module uses OpenSSL to provide Transport Layer Security and/or
Secure Socket Layer: encrypted stream communication.

TLS/SSL is a public/private key infrastructure. Each client and each
server must have a private key. A private key is created like this

    openssl genrsa -out ryans-key.pem 1024

All severs and some clients need to have a certificate. Certificates are public
keys signed by a Certificate Authority or self-signed. The first step to
getting a certificate is to create a "Certificate Signing Request" (CSR)
file. This is done with:

    openssl req -new -key ryans-key.pem -out ryans-csr.pem

To create a self-signed certificate with the CSR, do this:

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

Alternatively you can send the CSR to a Certificate Authority for signing.

(TODO: docs on creating a CA, for now interested users should just look at
`test/fixtures/keys/Makefile` in the Node source code)

### Client-initiated renegotiation attack mitigation

The TLS protocol lets the client renegotiate certain aspects of the TLS session.
Unfortunately, session renegotiation requires a disproportional amount of
server-side resources, which makes it a potential vector for denial-of-service
attacks.

To mitigate this, renegotiations are limited to three times every 10 minutes. An
error is emitted on the [CleartextStream](#tls.CleartextStream) instance when
the threshold is exceeded. The limits are configurable:

  - `tls.CLIENT_RENEG_LIMIT`: renegotiation limit, default is 3.

  - `tls.CLIENT_RENEG_WINDOW`: renegotiation window in seconds, default is
                               10 minutes.

Don't change the defaults unless you know what you are doing.

To test your server, connect to it with `openssl s_client -connect address:port`
and tap `R<CR>` (that's the letter `R` followed by a carriage return) a few
times.

#### tls.createServer(options, [secureConnectionListener])

Creates a new [tls.Server](#tls.Server).
The `connectionListener` argument is automatically set as a listener for the
[secureConnection](#event_secureConnection_) event.
The `options` object has these possibilities:

  - `key`: A string or `Buffer` containing the private key of the server in
    PEM format. (Required)

  - `passphrase`: A string of passphrase for the private key.

  - `cert`: A string or `Buffer` containing the certificate key of the server in
    PEM format. (Required)

  - `ca`: An array of strings or `Buffer`s of trusted certificates. If this is
    omitted several well known "root" CAs will be used, like VeriSign.
    These are used to authorize connections.

  - `ciphers`: A string describing the ciphers to use or exclude. Consult
    <http://www.openssl.org/docs/apps/ciphers.html#CIPHER_LIST_FORMAT> for
    details on the format.

  - `requestCert`: If `true` the server will request a certificate from
    clients that connect and attempt to verify that certificate. Default:
    `false`.

  - `rejectUnauthorized`: If `true` 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: `false`.

  - `NPNProtocols`: An array or `Buffer` of possible NPN protocols. (Protocols
    should be ordered by their priority).

  - `SNICallback`: A function that will be called if client supports SNI TLS
    extension. Only one argument will be passed to it: `servername`. And
    `SNICallback` should return SecureContext instance.
    (You can use `crypto.createCredentials(...).context` to get proper
    SecureContext). If `SNICallback` wasn't provided - default callback with
    high-level API will be used (see below).

  - `sessionIdContext`: A string containing a opaque identifier for session
    resumption. If `requestCert` is `true`, the default is MD5 hash value
    generated from command-line. Otherwise, the default is not provided.

Here is a simple example echo server:

    var tls = require('tls');
    var fs = require('fs');

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

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

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

    var server = tls.createServer(options, function(cleartextStream) {
      console.log('server connected',
                  cleartextStream.authorized ? 'authorized' : 'unauthorized');
      cleartextStream.write("welcome!\n");
      cleartextStream.setEncoding('utf8');
      cleartextStream.pipe(cleartextStream);
    });
    server.listen(8000, function() {
      console.log('server bound');
    });


You can test this server by connecting to it with `openssl s_client`:


    openssl s_client -connect 127.0.0.1:8000


#### tls.connect(port, [host], [options], [secureConnectListener])

Creates a new client connection to the given `port` and `host`. (If `host`
defaults to `localhost`.) `options` should be an object which specifies

  - `key`: A string or `Buffer` containing the private key of the client in
    PEM format.

  - `passphrase`: A string of passphrase for the private key.

  - `cert`: A string or `Buffer` containing the certificate key of the client in
    PEM format.

  - `ca`: An array of strings or `Buffer`s of trusted certificates. If this is
    omitted several well known "root" CAs will be used, like VeriSign.
    These are used to authorize connections.

  - `NPNProtocols`: An array of string or `Buffer` containing supported NPN
    protocols. `Buffer` should have following format: `0x05hello0x05world`,
    where first byte is next protocol name's length. (Passing array should
    usually be much simpler: `['hello', 'world']`.)

  - `servername`: Servername for SNI (Server Name Indication) TLS extension.

  - `socket`: Establish secure connection on a given socket rather than
    creating a new socket. If this option is specified, `host` and `port`
    are ignored.  This is intended FOR INTERNAL USE ONLY.  As with all
    undocumented APIs in Node, they should not be used.

The `secureConnectListener` parameter will be added as a listener for the
['secureConnect'](#event_secureConnect_) event.

`tls.connect()` returns a [CleartextStream](#tls.CleartextStream) object.

Here is an example of a client of echo server as described previously:

    var tls = require('tls');
    var fs = require('fs');

    var options = {
      // These are necessary only if using the client certificate authentication
      key: fs.readFileSync('client-key.pem'),
      cert: fs.readFileSync('client-cert.pem'),
    
      // This is necessary only if the server uses the self-signed certificate
      ca: [ fs.readFileSync('server-cert.pem') ]
    };

    var cleartextStream = tls.connect(8000, options, function() {
      console.log('client connected',
                  cleartextStream.authorized ? 'authorized' : 'unauthorized');
      process.stdin.pipe(cleartextStream);
      process.stdin.resume();
    });
    cleartextStream.setEncoding('utf8');
    cleartextStream.on('data', function(data) {
      console.log(data);
    });
    cleartextStream.on('end', function() {
      server.close();
    });


### NPN and SNI

NPN (Next Protocol Negotiation) and SNI (Server Name Indication) are TLS
handshake extensions allowing you:

  * NPN - to use one TLS server for multiple protocols (HTTP, SPDY)
  * SNI - to use one TLS server for multiple hostnames with different SSL
    certificates.

### pair = tls.createSecurePair([credentials], [isServer], [requestCert], [rejectUnauthorized])

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

 - `credentials`: A credentials object from crypto.createCredentials( ... )

 - `isServer`: A boolean indicating whether this tls connection should be
   opened as a server or a client.

 - `requestCert`: A boolean indicating whether a server should request a
   certificate from a connecting client. Only applies to server connections.

 - `rejectUnauthorized`: A boolean indicating whether a server should
   automatically reject clients with invalid certificates. Only applies to
   servers with `requestCert` enabled.

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

#### Event: 'secure'

The event is emitted from the SecurePair once the pair has successfully
established a secure connection.

Similarly to the checking for the server 'secureConnection' event,
pair.cleartext.authorized should be checked to confirm whether the certificate
used properly authorized.

### tls.Server

This class is a subclass of `net.Server` and has the same methods on it.
Instead of accepting just raw TCP connections, this accepts encrypted
connections using TLS or SSL.

#### Event: 'secureConnection'

`function (cleartextStream) {}`

This event is emitted after a new connection has been successfully
handshaked. The argument is a instance of
[CleartextStream](#tls.CleartextStream). It has all the common stream methods
and events.

`cleartextStream.authorized` is a boolean value which indicates if the
client has verified by one of the supplied certificate authorities for the
server. If `cleartextStream.authorized` is false, then
`cleartextStream.authorizationError` is set to describe how authorization
failed. Implied but worth mentioning: depending on the settings of the TLS
server, you unauthorized connections may be accepted.
`cleartextStream.npnProtocol` is a string containing selected NPN protocol.
`cleartextStream.servername` is a string containing servername requested with
SNI.


#### Event: 'clientError'

`function (exception) { }`

When a client connection emits an 'error' event before secure connection is
established - it will be forwarded here.


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

Begin accepting connections on the specified `port` and `host`.  If the
`host` is omitted, the server will accept connections directed to any
IPv4 address (`INADDR_ANY`).

This function is asynchronous. The last parameter `callback` will be called
when the server has been bound.

See `net.Server` for more information.


#### server.close()

Stops the server from accepting new connections. This function is
asynchronous, the server is finally closed when the server emits a `'close'`
event.

#### server.address()

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

#### server.addContext(hostname, credentials)

Add secure context that will be used if client request's SNI hostname is
matching passed `hostname` (wildcards can be used). `credentials` can contain
`key`, `cert` and `ca`.

#### server.maxConnections

Set this property to reject connections when the server's connection count
gets high.

#### server.connections

The number of concurrent connections on the server.


### tls.CleartextStream

This is a stream on top of the *Encrypted* stream that makes it possible to
read/write an encrypted data as a cleartext data.

This instance implements a duplex [Stream](streams.html#streams) interfaces.
It has all the common stream methods and events.

#### Event: 'secureConnect'

`function () {}`

This event is emitted after a new connection has been successfully handshaked. 
The listener will be called no matter if the server's certificate was
authorized or not. It is up to the user to test `cleartextStream.authorized`
to see if the server certificate was signed by one of the specified CAs.
If `cleartextStream.authorized === false` then the error can be found in
`cleartextStream.authorizationError`. Also if NPN was used - you can check
`cleartextStream.npnProtocol` for negotiated protocol.

#### cleartextStream.authorized

A boolean that is `true` if the peer certificate was signed by one of the
specified CAs, otherwise `false`

#### cleartextStream.authorizationError

The reason why the peer's certificate has not been verified. This property
becomes available only when `cleartextStream.authorized === false`.

#### cleartextStream.getPeerCertificate()

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

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' },
      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' }

If the peer does not provide a certificate, it returns `null` or an empty
object.

#### cleartextStream.address()

Returns the bound address and port of the underlying socket as reported by the
operating system. Returns an object with two properties, e.g.
`{"address":"192.168.57.1", "port":62053}`

#### cleartextStream.remoteAddress

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

#### cleartextStream.remotePort

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