Marc Lehmann > AnyEvent-7.05 > AnyEvent::TLS

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NAME ^

AnyEvent::TLS - SSLv2/SSLv3/TLSv1 contexts for use in AnyEvent::Handle

SYNOPSIS ^

   # via AnyEvent::Handle

   use AnyEvent;
   use AnyEvent::Handle;
   use AnyEvent::Socket;

   # simple https-client
   my $handle = new AnyEvent::Handle
      connect  => [$host, $port],
      tls      => "connect",
      tls_ctx  => { verify => 1, verify_peername => "https" },
      ...

   # simple ssl-server
   tcp_server undef, $port, sub {
      my ($fh) = @_;

      my $handle = new AnyEvent::Handle
         fh       => $fh,
         tls      => "accept",
         tls_ctx  => { cert_file => "my-server-keycert.pem" },
         ...

   # directly

   my $tls = new AnyEvent::TLS
      verify => 1,
      verify_peername => "ldaps",
      ca_file => "/etc/cacertificates.pem";

DESCRIPTION ^

This module is a helper module that implements TLS/SSL (Transport Layer Security/Secure Sockets Layer) contexts. A TLS context is a common set of configuration values for use in establishing TLS connections.

For some quick facts about SSL/TLS, see the section of the same name near the end of the document.

A single TLS context can be used for any number of TLS connections that wish to use the same certificates, policies etc.

Note that this module is inherently tied to Net::SSLeay, as this library is used to implement it. Since that perl module is rather ugly, and OpenSSL has a rather ugly license, AnyEvent might switch TLS providers at some future point, at which this API will change dramatically, at least in the Net::SSLeay-specific parts (most constructor arguments should still work, though).

Although this module does not require a specific version of Net::SSLeay, many features will gradually stop working, or bugs will be introduced with old versions (verification might succeed when it shouldn't - this is a real security issue). Version 1.35 is recommended, 1.33 should work, 1.32 might, and older versions are yours to keep.

USAGE EXAMPLES ^

See the AnyEvent::Handle manpage, NONFREQUENTLY ASKED QUESTIONS, for some actual usage examples.

PUBLIC METHODS AND FUNCTIONS ^

$tls = new AnyEvent::TLS key => value...

The constructor supports these arguments (all as key => value pairs).

method => "SSLv2" | "SSLv3" | "TLSv1" | "any"

The protocol parser to use. SSLv2, SSLv3 and TLSv1 will use a parser for those protocols only (so will not accept or create connections with/to other protocol versions), while any (the default) uses a parser capable of all three protocols.

The default is to use "any" but disable SSLv2. This has the effect of sending a SSLv2 hello, indicating the support for SSLv3 and TLSv1, but not actually negotiating an (insecure) SSLv2 connection.

Specifying a specific version is almost always wrong to use for a server speaking to a wide variety of clients (e.g. web browsers), and often wrong for a client. If you only want to allow a specific protocol version, use the sslv2, sslv3 or tlsv1 arguments instead.

For new services it is usually a good idea to enforce a TLSv1 method from the beginning.

sslv2 => $enabled

Enable or disable SSLv2 (normally disabled).

sslv3 => $enabled

Enable or disable SSLv3 (normally enabled).

tlsv1 => $enabled

Enable or disable TLSv1 (normally enabled).

verify => $enable

Enable or disable peer certificate checking (default is disabled, which is not recommended).

This is the "master switch" for all verify-related parameters and functions.

If it is disabled, then no peer certificate verification will be done - the connection will be encrypted, but the peer certificate won't be verified against any known CAs, or whether it is still valid or not. No peername verification or custom verification will be done either.

If enabled, then the peer certificate (required in client mode, optional in server mode, see verify_require_client_cert) will be checked against its CA certificate chain - that means there must be a signing chain from the peer certificate to any of the CA certificates you trust locally, as specified by the ca_file and/or ca_path and/or ca_cert parameters (or the system default CA repository, if all of those parameters are missing - see also the AnyEvent manpage for the description of PERL_ANYEVENT_CA_FILE).

Other basic checks, such as checking the validity period, will also be done, as well as optional peername/hostname/common name verification verify_peername.

An optional verify_cb callback can also be set, which will be invoked with the verification results, and which can override the decision.

verify_require_client_cert => $enable

Enable or disable mandatory client certificates (default is disabled). When this mode is enabled, then a client certificate will be required in server mode (a server certificate is mandatory, so in client mode, this switch has no effect).

verify_peername => $scheme | $callback->($tls, $cert, $peername)

TLS only protects the data that is sent - it cannot automatically verify that you are really talking to the right peer. The reason is that certificates contain a "common name" (and a set of possible alternative "names") that need to be checked against the peername (usually, but not always, the DNS name of the server) in a protocol-dependent way.

This can be implemented by specifying a callback that has to verify that the actual $peername matches the given certificate in $cert.

Since this can be rather hard to implement, AnyEvent::TLS offers a variety of predefined "schemes" (lifted from IO::Socket::SSL) that are named like the protocols that use them:

ldap (rfc4513), pop3,imap,acap (rfc2995), nntp (rfc4642)

Simple wildcards in subjectAltNames are possible, e.g. *.example.org matches www.example.org but not lala.www.example.org. If nothing from subjectAltNames matches, it checks against the common name, but there are no wildcards allowed.

http (rfc2818)

Extended wildcards in subjectAltNames are possible, e.g. *.example.org or even www*.example.org. Wildcards in the common name are not allowed. The common name will be only checked if no host names are given in subjectAltNames.

smtp (rfc3207)

This RFC isn't very useful in determining how to do verification so it just assumes that subjectAltNames are possible, but no wildcards are possible anywhere.

[$check_cn, $wildcards_in_alt, $wildcards_in_cn]

You can also specify a scheme yourself by using an array reference with three integers.

$check_cn specifies if and how the common name field is used: 0 means it will be completely ignored, 1 means it will only be used if no host names have been found in the subjectAltNames, and 2 means the common name will always be checked against the peername.

$wildcards_in_alt and $wildcards_in_cn specify whether and where wildcards (*) are allowed in subjectAltNames and the common name, respectively. 0 means no wildcards are allowed, 1 means they are allowed only as the first component (*.example.org), and 2 means they can be used anywhere (www*.example.org), except that very dangerous matches will not be allowed (*.org or *).

You can specify either the name of the parent protocol (recommended, e.g. http, ldap), the protocol name as usually used in URIs (e.g. https, ldaps) or the RFC (not recommended, e.g. rfc2995, rfc3920).

This verification will only be done when verification is enabled (verify => 1).

verify_cb => $callback->($tls, $ref, $cn, $depth, $preverify_ok, $x509_store_ctx, $cert)

Provide a custom peer verification callback used by TLS sessions, which is called with the result of any other verification (verify, verify_peername).

This callback will only be called when verification is enabled (verify => 1).

$tls is the AnyEvent::TLS object associated with the session, while $ref is whatever the user associated with the session (usually an AnyEvent::Handle object when used by AnyEvent::Handle).

$depth is the current verification depth - $depth = 0 means the certificate to verify is the peer certificate, higher levels are its CA certificate and so on. In most cases, you can just return $preverify_ok if the $depth is non-zero:

   verify_cb => sub {
      my ($tls, $ref, $cn, $depth, $preverify_ok, $x509_store_ctx, $cert) = @_;

      return $preverify_ok
         if $depth;

      # more verification
   },

$preverify_ok is true iff the basic verification of the certificates was successful (a valid CA chain must exist, the certificate has passed basic validity checks, peername verification succeeded).

$x509_store_ctx is the Net::SSLeay::X509_CTX> object.

$cert is the Net::SSLeay::X509 object representing the peer certificate, or zero if there was an error. You can call AnyEvent::TLS::certname $cert to get a nice user-readable string to identify the certificate.

The callback must return either 0 to indicate failure, or 1 to indicate success.

verify_client_once => $enable

Enable or disable skipping the client certificate verification on renegotiations (default is disabled, the certificate will always be checked). Only makes sense in server mode.

ca_file => $path

If this parameter is specified and non-empty, it will be the path to a file with (server) CA certificates in PEM format that will be loaded. Each certificate will look like:

   -----BEGIN CERTIFICATE-----
   ... (CA certificate in base64 encoding) ...
   -----END CERTIFICATE-----

You have to enable verify mode (verify => 1) for this parameter to have any effect.

ca_path => $path

If this parameter is specified and non-empty, it will be the path to a directory with hashed CA certificate files in PEM format. When the ca certificate is being verified, the certificate will be hashed and looked up in that directory (see http://www.openssl.org/docs/ssl/SSL_CTX_load_verify_locations.html for details)

The certificates specified via ca_file take precedence over the ones found in ca_path.

You have to enable verify mode (verify => 1) for this parameter to have any effect.

ca_cert => $string

In addition or instead of using ca_file and/or ca_path, you can also use ca_cert to directly specify the CA certificates (there can be multiple) in PEM format, in a string.

check_crl => $enable

Enable or disable certificate revocation list checking. If enabled, then peer certificates will be checked against a list of revoked certificates issued by the CA. The revocation lists will be expected in the ca_path directory.

certificate verification will fail if this is enabled but no revocation list was found.

This requires OpenSSL >= 0.9.7b. Check the OpenSSL documentation for more details.

key_file => $path

Path to the local private key file in PEM format (might be a combined certificate/private key file).

The local certificate is used to authenticate against the peer - servers mandatorily need a certificate and key, clients can use a certificate and key optionally to authenticate, e.g. for log-in purposes.

The key in the file should look similar this:

   -----BEGIN RSA PRIVATE KEY-----
   ...header data
   ... (key data in base64 encoding) ...
   -----END RSA PRIVATE KEY-----
key => $string

The private key string in PEM format (see key_file, only one of key_file or key can be specified).

The idea behind being able to specify a string is to avoid blocking in I/O. Unfortunately, Net::SSLeay fails to implement any interface to the needed OpenSSL functionality, this is currently implemented by writing to a temporary file.

cert_file => $path

The path to the local certificate file in PEM format (might be a combined certificate/private key file, including chained certificates).

The local certificate (and key) are used to authenticate against the peer - servers mandatorily need a certificate and key, clients can use certificate and key optionally to authenticate, e.g. for log-in purposes.

The certificate in the file should look like this:

   -----BEGIN CERTIFICATE-----
   ... (certificate in base64 encoding) ...
   -----END CERTIFICATE-----

If the certificate file or string contain both the certificate and private key, then there is no need to specify a separate key_file or key.

Additional signing certifiates to send to the peer (in SSLv3 and newer) can be specified by appending them to the certificate proper: the order must be from issuer certificate over any intermediate CA certificates to the root CA.

So the recommended ordering for a combined key/cert/chain file, specified via cert_file or cert looks like this:

  certificate private key
  client/server certificate
  ca 1, signing client/server certficate
  ca 2, signing ca 1
  ...
cert => $string

The local certificate in PEM format (might be a combined certificate/private key file). See cert_file.

The idea behind being able to specify a string is to avoid blocking in I/O. Unfortunately, Net::SSLeay fails to implement any interface to the needed OpenSSL functionality, this is currently implemented by writing to a temporary file.

cert_password => $string | $callback->($tls)

The certificate password - if the certificate is password-protected, then you can specify its password here.

Instead of providing a password directly (which is not so recommended), you can also provide a password-query callback. The callback will be called whenever a password is required to decode a local certificate, and is supposed to return the password.

dh_file => $path

Path to a file containing Diffie-Hellman parameters in PEM format, for use in servers. See also dh on how to specify them directly, or use a pre-generated set.

Diffie-Hellman key exchange generates temporary encryption keys that are not transferred over the connection, which means that even if the certificate key(s) are made public at a later time and a full dump of the connection exists, the key still cannot be deduced.

These ciphers are only available with SSLv3 and later (which is the default with AnyEvent::TLS), and are only used in server/accept mode. Anonymous DH protocols are usually disabled by default, and usually not even compiled into the underlying library, as they provide no direct protection against man-in-the-middle attacks. The same is true for the common practise of self-signed certificates that you have to accept first, of course.

dh => $string

Specify the Diffie-Hellman parameters in PEM format directly as a string (see dh_file), the default is schmorp1539 unless dh_file was specified.

AnyEvent::TLS supports supports a number of precomputed DH parameters, since computing them is expensive. They are:

   # from "Assigned Number for SKIP Protocols"
   skip512, skip1024, skip2048, skip4096

   # from schmorp
   schmorp1024, schmorp1539, schmorp2048, schmorp4096, schmorp8192

The default was chosen as a trade-off between security and speed, and should be secure for a few years. It is said that 2048 bit DH parameters are safe till 2030, and DH parameters shorter than 900 bits are totally insecure.

To disable DH protocols completely, specify undef as dh parameter.

dh_single_use => $enable

Enables or disables "use only once" mode when using Diffie-Hellman key exchange. When enabled (default), each time a new key is exchanged a new Diffie-Hellman key is generated, which improves security as each key is only used once. When disabled, the key will be created as soon as the AnyEvent::TLS object is created and will be reused.

All the DH parameters supplied with AnyEvent::TLS should be safe with dh_single_use switched off, but YMMV.

cipher_list => $string

The list of ciphers to use, as a string (example: AES:ALL:!aNULL:!eNULL:+RC4:@STRENGTH). The format of this string and its default value is documented at http://www.openssl.org/docs/apps/ciphers.html#CIPHER_STRINGS.

session_ticket => $enable

Enables or disables RC5077 support (Session Resumption without Server-Side State). The default is disabled for clients, as many (buggy) TLS/SSL servers choke on it, but enabled for servers.

When enabled and supported by the server, a session ticket will be provided to the client, which allows fast resuming of connections.

prepare => $coderef->($tls)

If this argument is present, then it will be called with the new AnyEvent::TLS object after any other initialisation has bee done, in case you wish to fine-tune something...

$tls = new_from_ssleay AnyEvent::TLS $ctx

This constructor takes an existing Net::SSLeay SSL_CTX object (which is just an integer) and converts it into an AnyEvent::TLS object. This only works because AnyEvent::TLS is currently implemented using Net::SSLeay. As this is such a horrible perl module and OpenSSL has such an annoying license, this might change in the future, in which case this method might vanish.

$ctx = $tls->ctx

Returns the actual Net::SSLeay::CTX object (just an integer).

AnyEvent::TLS::init

AnyEvent::TLS does on-demand initialisation, and normally there is no need to call an initialise function.

As initialisation might take some time (to read e.g. /dev/urandom), this could be annoying in some highly interactive programs. In that case, you can call AnyEvent::TLS::init to make sure there will be no costly initialisation later. It is harmless to call AnyEvent::TLS::init multiple times.

$certname = AnyEvent::TLS::certname $x509

Utility function that returns a user-readable string identifying the X509 certificate object.

SSL/TLS QUICK FACTS ^

Here are some quick facts about TLS/SSL that might help you:

SECURITY CONSIDERATIONS ^

When you use any of the options that pass in keys or certificates as strings (e.g. ca_cert), then, due to serious shortcomings in Net::SSLeay, this module creates a temporary file to store the string - see File::Temp and possibly its safe_level setting for more details on what to watch out for.

BUGS ^

To to the abysmal code quality of Net::SSLeay, this module will leak small amounts of memory per TLS connection (currently at least one perl scalar).

AUTHORS ^

Marc Lehmann <schmorp@schmorp.de>.

Some of the API, documentation and implementation (verify_hostname), and a lot of ideas/workarounds/knowledge have been taken from the IO::Socket::SSL module. Care has been taken to keep the API similar to that and other modules, to the extent possible while providing a sensible API for AnyEvent.

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