IO::Socket::Async::SSL

This module provides a secure sockets implementation with an API very much like that of the Raku built-in IO::Socket::Async class. For the client case, provided the standard certificate and host verification are sufficient, it is drop-in replacement. The server case only needs two extra arguments to listen, specifying the server key and certificate.

As with IO::Socket::Async, it is safe to have concurrent connections and to share them across threads.

Synopsis

Client:

use IO::Socket::Async::SSL;

my $conn = await IO::Socket::Async::SSL.connect('www.raku.org', 443);
$conn.print: "GET / HTTP/1.0\r\nHost: www.raku.org\r\n\r\n";
react {
    whenever $conn {
        .print
    }
}
$conn.close;

Server (assumes certificate and key files server-crt.pem and server-key.pem):

use IO::Socket::Async::SSL;

react {
    my %ssl-config =
        certificate-file => 'server-crt.pem',
        private-key-file => 'server-key.pem';
    whenever IO::Socket::Async::SSL.listen('localhost', 4433, |%ssl-config) -> $conn {
        my $req = '';
        whenever $conn {
            $req ~= $_;
            if $req.contains("\r\n\r\n") {
                say $req.lines[0];
                await $conn.print(
                    "HTTP/1.0 200 OK\r\nContent-type: text/html\r\n\r\n" ~
                    "<strong>Hello from a Raku HTTP server</strong>\n");
                $conn.close;
            }
        }
    }
}

Client

The connect method on IO::Socket::Async::SSL is used to establish a SSL connection to a server. It requies two positional arguments, which specify the host and port to connect to. It returns a Promise, which will be kept with an IO::Socket::Async::SSL instance when the connection is established and the SSL handshake completed.

my $conn = await IO::Socket::Async::SSL.connect($host, $port);

By default, the SSL certificate will be verified, using the default set of accepted Certificate Authorities. The Promise return by conenct will be broken if verification fails.

Sometimes it is convenient to create a CA and use it to sign certificates for internal use, for example to secure communications between a set of services on an internal network. In this case, the ca-file named argument can be passed to specify the certificate authority certificate file:

my $ca-file = '/config/ca-crt.pem';
my $conn = await IO::Socket::Async::SSL.connect('user-service', 443, :$ca-file);

Alternatively, a ca-path argument can be specified, indicating a directory where one or more certificates may be found.

It is possible to disable certificate verification by passing the insecure named argument a true value. As the name suggests, this is not a secure configuration, since there is no way for the client to be sure that it is communicating with the intended server. Therefore, it is vulnerable to man-in-the-middle attacks.

For control over the ciphers that may be used, pass the ciphers argument to connect. It should be a string in OpenSSL cipher list format.

If wishing to view encrypted traffic with a tool such as Wireshark for debugging purposes, pass a filename to ssl-key-log-file. Session keys will be written to this log file, and Wireshark can then be configured to introspect the encrypted traffic (Preferences -> Protocols -> TLS -> (Pre-)-Master-Secret log filename). Note that this key exposure compromises the security of the session!

Server

The listen method returns a Supply that, when tapped, will start an SSL server. The server can be shut down by closing the tap. Whenever a connection is made to the server, the Supply will emit an IO::Socket::Async::SSL instance. The listen method requires two positional arguments, specifying the host and port to listen on. Two named arguments are also required, providing the certificate-file and private-key-file.

my %ssl-config =
    certificate-file => 'server-crt.pem',
    private-key-file => 'server-key.pem';
my $connections = IO::Socket::Async::SSL.listen('localhost', 4433, |%ssl-config);
react {
    my $listener = do whenever $connections -> $conn {
        say "Got a connection!";
        $conn.close;
    }

    whenever signal(SIGINT) {
        say "Shutting down...";
        $listener.close;
        exit;
    }
}

For control over the ciphers that may be used, pass the ciphers argument to connect. It should be a string in OpenSSL cipher list format. The following boolean options are also accepted:

prefer-server-ciphers - indicates that the order of the ciphers list as configured on the server should be preferred over that of the one presented by the client
no-compression - disables compression
no-session-resumption-on-renegotiation

Common client and server functionality

Both the connect and listen methods take the following optional named arguments:

enc, which specifies the encoding to use when the socket is used in character mode. Defaults to utf-8.
scheduler, which specifies the scheduler to use for processing events from the underlying IO::Socket::Async instance. The default is $*SCHEDULER. There is rarely a need to change this.

The Supply, print, write, and close methods have the same semantics as in IO::Socket::Async.

Upgrading connections

Some protocols use opportunistic TLS, where the decision to use transport layer security is first negotiated using a non-encrypted protocol - provided negotiation is successful - a TLS handshake is then performed. This functionality is provided by the upgrade-server and upgrade-client methods. Note that the socket to upgrade must be an instance of IO::Socket::Async. Further, it is important to stop reading from the socket before initiating the upgrade, which will typically entail working with the Tap directly, something not normally needed in react/whenever blocks.

Here is an example of using upgrade-server.

react whenever IO::Socket::Async.listen('localhost', TEST_PORT) -> $plain-conn {
    my $plain-tap = do whenever $plain-conn.Supply -> $start {
        if $start eq "Psst, let's talk securely!\n" {
            # Must stop reading...
            $plain-tap.close;
            # ...so the module can take over the socket.
            my $enc-conn-handshake = IO::Socket::Async::SSL.upgrade-server(
                $plain-conn,
                private-key-file => 't/certs-and-keys/server.key',
                certificate-file => 't/certs-and-keys/server-bundle.crt'
            );
            whenever $enc-conn-handshake -> $enc-conn {
                uc-service($enc-conn);
            }
            $plain-conn.print("OK, let's talk securely!\n");
        }
        else {
            $plain-conn.print("OK, let's talk insecurely\n");
            uc-service($plain-conn);
        }
    }

    sub uc-service($conn) {
        whenever $conn -> $crypt-text {
            whenever $conn.print($crypt-text.uc) {
                $conn.close;
            }
        }
    }
}

Here's an example using upgrade-client; again, take note of the careful handling of the Tap.

my $plain-conn = await IO::Socket::Async.connect('localhost', TEST_PORT);
await $plain-conn.print("Psst, let's talk securely!\n");
react {
    my $plain-tap = do whenever $plain-conn -> $msg {
        $plain-tap.close;
        my $enc-conn-handshake = IO::Socket::Async::SSL.upgrade-client(
            $plain-conn,
            host => 'localhost',
            ca-file => 't/certs-and-keys/ca.crt');
        whenever $enc-conn-handshake -> $enc-conn {
            await $enc-conn.print("hello!\n");
            whenever $enc-conn.head -> $got {
                print $got; # HELLO!
                done;
            }
        }
    }
}

Bugs, feature requests, and contributions

Please use GitHub Issues to file bug reports and feature requests. If you wish to contribute to this module, please open a GitHub Pull Request, or email a Git patch (produced using format-patch) to jnthn@jnthn.net. Please also use this email address to report security vulnerabilities.