AnyEvent::TLS(3pm) User Contributed Perl Documentation AnyEvent::TLS(3pm)
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" | "TLSv1_1" | "TLSv1_2" |
"any"
The protocol parser to use. "SSLv2", "SSLv3", "TLSv1",
"TLSv1_1" and "TLSv1_2" 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",
"tlsv1", "tlsv1_1" or "tlsv1_2" arguments instead.
For new services it is usually a good idea to enforce a "TLSv1"
method from the beginning.
"TLSv1_1" and "TLSv1_2" require Net::SSLeay >= 1.55 and OpenSSL
>= 1.0.1. Check the Net::SSLeay and OpenSSL documentations for
more details.
sslv2 => $enabled
Enable or disable SSLv2 (normally disabled).
sslv3 => $enabled
Enable or disable SSLv3 (normally enabled).
tlsv1 => $enabled
Enable or disable TLSv1 (normally enabled).
tlsv1_1 => $enabled
Enable or disable TLSv1_1 (normally enabled).
This requires Net::SSLeay >= 1.55 and OpenSSL >= 1.0.1. Check
the Net::SSLeay and OpenSSL documentations for more details.
tlsv1_2 => $enabled
Enable or disable TLSv1_2 (normally enabled).
This requires Net::SSLeay >= 1.55 and OpenSSL >= 1.0.1. Check
the Net::SSLeay and OpenSSL documentations for more details.
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 $cert.
Since this can be rather hard to implement, AnyEvent::TLS
offers a variety of predefined "schemes" (lifted from
IO::Socket::SSL) that are named like the protocols that use
them:
ldap (rfc4513), pop3,imap,acap (rfc2995), nntp (rfc4642)
Simple wildcards in subjectAltNames are possible, e.g.
*.example.org matches www.example.org but not
lala.www.example.org. If nothing from subjectAltNames
matches, it checks against the common name, but there are
no wildcards allowed.
http (rfc2818)
Extended wildcards in subjectAltNames are possible, e.g.
*.example.org or even www*.example.org. Wildcards in the
common name are not allowed. The common name will be only
checked if no host names are given in subjectAltNames.
smtp (rfc3207)
This RFC isn't very useful in determining how to do
verification so it just assumes that subjectAltNames are
possible, but no wildcards are possible anywhere.
[$wildcards_in_alt, $wildcards_in_cn, $check_cn]
You can also specify a scheme yourself by using an array
reference with three integers.
$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 "*").
$check_cn specifies if and how the common name field is
checked: 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.
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 certificate
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 "ffdhe3072" unless
"dh_file" was specified.
AnyEvent::TLS supports supports a number of precomputed DH
parameters, since computing them is expensive. They are:
# from RFC 7919 - recommended
ffdhe2048, ffdhe3072, ffdhe4096, ffdhe6144, ffdhe8192
# from "Assigned Number for SKIP Protocols"
skip512, skip1024, skip2048, skip4096
# from schmorp
schmorp1024, schmorp1539, schmorp2048, schmorp4096, schmorp8192
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:
• A certificate is the public key part, a key is the private key
part.
While not strictly true, certificates are the things you can hand
around publicly as a kind of identity, while keys should really be
kept private, as proving that you have the private key is usually
interpreted as being the entity behind the certificate.
• A certificate is signed by a CA (Certificate Authority).
By signing, the CA basically claims that the certificate it signs
really belongs to the identity named in it, verified according to
the CA policies. For e.g. HTTPS, the CA usually makes some checks
that the hostname mentioned in the certificate really belongs to
the company/person that requested the signing and owns the domain.
• CAs can be certified by other CAs.
Or by themselves - a certificate that is signed by a CA that is
itself is called a self-signed certificate, a trust chain of length
zero. When you find a certificate signed by another CA, which is in
turn signed by another CA you trust, you have a trust chain of
depth two.
• "Trusting" a CA means trusting all certificates it has signed.
If you "trust" a CA certificate, then all certificates signed by it
are automatically considered trusted as well.
• A successfully verified certificate means that you can be
reasonably sure that whoever you are talking with really is who he
claims he is.
By verifying certificates against a number of CAs that you trust
(meaning it is signed directly or indirectly by such a CA), you can
find out that the other side really is whoever he claims, according
to the CA policies, and your belief in the integrity of the CA.
• Verifying the certificate signature is not everything.
Even when the certificate is correct, it might belong to somebody
else: if www.attacker.com can make your computer believe that it is
really called www.mybank.com (by making your DNS server believe
this for example), then it could send you the certificate for
www.attacker.com that your software trusts because it is signed by
a CA you trust, and intercept all your traffic that you think goes
to www.mybank.com. This works because your software sees that the
certificate is correctly signed (for www.attacker.com) and you
think you are talking to your bank.
To thwart this attack vector, peername verification should be used,
which basically checks that the certificate (for www.attacker.com)
really belongs to the host you are trying to talk to
(www.mybank.com), which in this example is not the case, as
www.attacker.com (from the certificate) doesn't match
www.mybank.com (the hostname used to create the connection).
So peername verification is almost as important as checking the CA
signing. Unfortunately, every protocol implements this differently,
if at all...
• Switching off verification is sometimes reasonable.
You can switch off verification. You still get an encrypted
connection that is protected against eavesdropping and injection -
you just lose protection against man in the middle attacks, i.e.
somebody else with enough abilities to intercept all traffic can
masquerade herself as the other side.
For many applications, switching off verification is entirely
reasonable. Downloading random stuff from websites using HTTPS for
no reason is such an application. Talking to your bank and entering
TANs is not such an application.
• A SSL/TLS server always needs a certificate/key pair to operate,
for clients this is optional.
Apart from (usually disabled) anonymous cipher suites, a server
always needs a certificate/key pair to operate.
Clients almost never use certificates, but if they do, they can be
used to authenticate the client, just as server certificates can be
used to authenticate the server.
• SSL version 2 is very insecure.
SSL version 2 is old and not only has it some security issues,
SSLv2-only implementations are usually buggy, too, due to their
age.
• Sometimes, even losing your "private" key might not expose all your
data.
With Diffie-Hellman ephemeral key exchange, you can lose the DH
parameters (the "keys"), but all your connections are still
protected. Diffie-Hellman needs special set-up (done by default by
AnyEvent::TLS).
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
Due 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.
perl v5.36.0 2022-10-20 AnyEvent::TLS(3pm)
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