CherryPy's SSL problem

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saaj

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Apr 23, 2015, 9:34:08 AM4/23/15
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Hi Joseph, Sylvain, folks,

Rendered version is published here.

SSL
===
As a part of discussion of design decisions made in the test suite [6]_, Joseph Tate, another 
CherryPy contributor expressed opinion that SSL support in CherryPy should be deprecated. 
Basically the message is about how can we guarantee security of the implementation if we can't
even guarantee its functionality.

Later in discussion with Joseph when I stood on fact that CherryPy's SSL adapters are a wrapper on
wrapper, Python ``ssl`` or pyOpenSSL on OpenSSL, and we don't need a professional security expert
in the community or special maintenance, he relaxed his opinion.

  Distros have patched OpenSSL to remove insecure stuff, so maybe this is not as big a concern 
  as I'm making it out to be, but the lack of an expert on this code is hindering its viability.      

Also I want to inform Joseph, that his knowledge of CherryPy support for intermediate certificates
isn't up to date. ``cherrypy.server.ssl_certificate_chain`` is documented [14]_ and was there at 
least since 2011. I've verified it works in default branch.   

Anyway, I took Joseph's questions seriously and I'm sure **we should not in any way give our users 
false sense of security**. Security here is in theoretical sense as the state of art. I'm neither
about zero-day breaches in algorithms and implementations, nor about fundamental problem of CA 
trust, no-such-agencies and generally post-Snowden world we live in.     

On the other hand I believe that having pure Python functional (possibly insecure in theoretical
sense) HTTPS server is beneficial for many practical use cases. There're various development tools
that are HTTPS-only unless you fight them. There're environments where you have Python but can't
install other software. There're use cases then any SSL, as an obfuscation, is fairly enough 
secure.      

CherryPy is described and known as a complete web server and such reduction would be a big 
loss. And as I will show you below, although there're several issues it is possible to 
workaround them and configure CherryPy to serve an application over HTTPS securely even today. 

Experiment
----------
Recently there was a CherryPy StackOverflow question right about the topic [15]_. The guy asked
*how to block SSL protocols in favour of TLS*. It led me to coming to the following 
quick-and-dirty solution. 

.. sourcecode:: python

    #!/usr/bin/env python
    # -*- coding: utf-8 -*-
    
    
    import os
    import sys
    import ssl
    
    import cherrypy
    from cherrypy.wsgiserver.ssl_builtin import BuiltinSSLAdapter
    from cherrypy.wsgiserver.ssl_pyopenssl import pyOpenSSLAdapter
    
    from cherrypy import wsgiserver
    if sys.version_info < (3, 0):
      from cherrypy.wsgiserver.wsgiserver2 import ssl_adapters  
    else:
      from cherrypy.wsgiserver.wsgiserver3 import ssl_adapters
    
    try:
      from OpenSSL import SSL
    except ImportError:
      pass
    
    
    ciphers = (
      'ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:ECDH+HIGH:'
      'DH+HIGH:ECDH+3DES:DH+3DES:RSA+AESGCM:RSA+AES:RSA+HIGH:RSA+3DES:!aNULL:'
      '!eNULL:!MD5:!DSS:!RC4:!SSLv2'
    )
    
    bundle = os.path.join(os.path.dirname(cherrypy.__file__), 'test', 'test.pem')
    
    config = {
      'global' : {
        'server.socket_host' : '127.0.0.1',
        'server.socket_port' : 8443,
        'server.thread_pool' : 8,
    
        'server.ssl_module'      : 'custom-pyopenssl',
        'server.ssl_certificate' : bundle,
        'server.ssl_private_key' : bundle,
      }
    }
    
    
    class BuiltinSsl(BuiltinSSLAdapter):
      '''Vulnerable, on py2 < 2.7.9, py3 < 3.3:
        * POODLE (SSLv3), adding ``!SSLv3`` to cipher list makes it very incompatible
        * can't disable TLS compression (CRIME)
        * supports Secure Client-Initiated Renegotiation (DOS)
        * no Forward Secrecy
      Also session caching doesn't work. Some tweaks are posslbe, but don't really 
      change much. For example, it's possible to use ssl.PROTOCOL_TLSv1 instead of 
      ssl.PROTOCOL_SSLv23 with little worse compatiblity.
      '''
    
      def wrap(self, sock):
        """Wrap and return the given socket, plus WSGI environ entries."""
        try:
          s = ssl.wrap_socket(
            sock, 
            ciphers = ciphers, # the override is for this line
            do_handshake_on_connect = True,
            server_side = True, 
            certfile = self.certificate,
            keyfile = self.private_key,
            ssl_version = ssl.PROTOCOL_SSLv23
          )
        except ssl.SSLError:
          e = sys.exc_info()[1]
          if e.errno == ssl.SSL_ERROR_EOF:
            # This is almost certainly due to the cherrypy engine
            # 'pinging' the socket to assert it's connectable;
            # the 'ping' isn't SSL.
            return None, {}
          elif e.errno == ssl.SSL_ERROR_SSL:
            if e.args[1].endswith('http request'):
              # The client is speaking HTTP to an HTTPS server.
              raise wsgiserver.NoSSLError
            elif e.args[1].endswith('unknown protocol'):
              # The client is speaking some non-HTTP protocol.
              # Drop the conn.
              return None, {}
          raise
    
        return s, self.get_environ(s)
    
    ssl_adapters['custom-ssl'] = BuiltinSsl
    
    
    class Pyopenssl(pyOpenSSLAdapter):
      '''Mostly fine, except:
        * Secure Client-Initiated Renegotiation
        * no Forward Secrecy, SSL.OP_SINGLE_DH_USE could have helped but it didn't
      '''
    
      def get_context(self):
        """Return an SSL.Context from self attributes."""
        c = SSL.Context(SSL.SSLv23_METHOD)
    
        # override:
        c.set_options(SSL.OP_NO_COMPRESSION | SSL.OP_SINGLE_DH_USE | SSL.OP_NO_SSLv2 | SSL.OP_NO_SSLv3)
        c.set_cipher_list(ciphers)
    
        c.use_privatekey_file(self.private_key)
        if self.certificate_chain:
            c.load_verify_locations(self.certificate_chain)
        c.use_certificate_file(self.certificate)
        return c
    
    ssl_adapters['custom-pyopenssl'] = Pyopenssl
    
    
    class App:
    
      @cherrypy.expose
      def index(self):
        return '<em>Is this secure?</em>'
    
    
    if __name__ == '__main__':
      cherrypy.quickstart(App(), '/', config)

After I regenerated our test certificate to:

* use longer than 1024-bit key
* not use MD5 and SHA1 (also deprecated [26]_) for signature
* use valid domain name for *common name* 

Using the following versions, we have **A-** for the customised pyOpenSSL adapter
on Qualys SSL Server Test [34]_:

* Debian Wheezy (stable)
* Python 2.7.3-4+deb7u1, ``ssl.OPENSSL_VERSION == 'OpenSSL 1.0.1e 11 Feb 2013'``
* OpenSSL 1.0.1e-2+deb7u16
* pyOpenSSL 0.14
* CherryPy from default branch (pre 3.7)
 


There's no error, but the warnings:

* Secure Client-Initiated Renegotiation -- Supported, DoS DANGER [16]_
* Downgrade attack prevention -- No, TLS_FALLBACK_SCSV not supported [17]_
* Forward Secrecy -- No, WEAK [18]_

Here's what Wikipedia says [25]_ about *forward secrecy*, to inform you that it is not
something critical or required for immediate operation.

  In cryptography, forward secrecy is a property of key-agreement protocols ensuring that a 
  session key derived from a set of long-term keys cannot be compromised if one of the 
  long-term keys is compromised in the future.

And here's what can be squeezed out of built-in ``ssl``, which is grievous.

Here goes the what-is-wrong list:

* POODLE (SSLv3) -- Vulnerable, INSECURE [19]_
* TLS compression -- Yes, INSECURE [20]_

There's also half-implemented *session resumption* -- *No (IDs assigned but not accepted)*. And
the same three warnings as for PyOpenSSL apply. ``ssl_builtin`` doesn't handle underlying ``ssl`` 
module exceptions correctly, as the log was flooded by exceptions originating mostly from 
``do_handshake()``. PyOpenSSL's log was the way more quiet with just a couple of page accesses.

::

  [27/Mar/2015:14:07:49] ENGINE Error in HTTPServer.tick
  Traceback (most recent call last):
    File "venv/local/lib/python2.7/site-packages/cherrypy/wsgiserver/wsgiserver2.py", line 1968, in start
      self.tick()
    File "venv/local/lib/python2.7/site-packages/cherrypy/wsgiserver/wsgiserver2.py", line 2035, in tick
      s, ssl_env = self.ssl_adapter.wrap(s)
    File "./test.py", line 53, in wrap
      ssl_version = ssl.PROTOCOL_SSLv23
    File "/usr/lib/python2.7/ssl.py", line 381, in wrap_socket
      ciphers=ciphers)
    File "/usr/lib/python2.7/ssl.py", line 143, in __init__
      self.do_handshake()
    File "/usr/lib/python2.7/ssl.py", line 305, in do_handshake
      self._sslobj.do_handshake()
      
  SSLError: [Errno 1] _ssl.c:504: error:1408A0C1:SSL routines:SSL3_GET_CLIENT_HELLO:no shared cipher
  SSLError: [Errno 1] _ssl.c:504: error:14076102:SSL routines:SSL23_GET_CLIENT_HELLO:unsupported protocol
  SSLError: [Errno 1] _ssl.c:504: error:1408F119:SSL routines:SSL3_GET_RECORD:decryption failed or bad record mac
  SSLError: [Errno 1] _ssl.c:504: error:14094085:SSL routines:SSL3_READ_BYTES:ccs received early
  
But nothing is really that bad even with ``ssl_builtin``. Let's relax stability policy, make sure
you understand the consequences if you're going to do it on a production server, and add 
Debain Jessie's (testing) source to ``/etc/apt/sources.list`` of the server (Debian Wheezy) and 
see what next Debain has in its briefcase:

* Python 2.7.9-1, ``ssl.OPENSSL_VERSION == 'OpenSSL 1.0.1k 8 Jan 2015'``
* Python 3.4.2-2, ``ssl.OPENSSL_VERSION == 'OpenSSL 1.0.1k 8 Jan 2015'``
* OpenSSL 1.0.1k-3
* pyOpenSSL 0.15.1 (installed via ``pip``, probably has been updated since)

For you information, related excerpt from Python 2.7.9 release change log [21]_. It was in fact 
a huge security update, not very well represented by the change of a patch version.

  * The entirety of Python 3.4's ssl module has been backported for Python 2.7.9. See 
    PEP 466 [22]_ for justification.
  * HTTPS certificate validation using the system's certificate store is now enabled by 
    default. See PEP 476 [23]_ for details.
  * SSLv3 has been disabled by default in httplib and its reverse dependencies due to the 
    POODLE attack.

All errors have gone, and in spite of the warning it supports *forward secrecy* for most 
clients. Here are just couple of warnings left:

* Secure Client-Initiated Renegotiation -- Supported, DoS DANGER [16]_
* Session resumption (caching) -- No (IDs assigned but not accepted)

The result is pretty good. If you run it on plain ``ssl_builtin``, is has **B**, because RC4 
ciphers [24]_ aren't excluded by default. Python 3.4.2 test on the customised ``ssl_builtin`` 
is obviously the same **A-** because both underlying libraries are the same. Customised 
``ssl_pyopenssl`` on Python 2.7.9 gives same **A-** -- again it has functional *session 
resumption* but doesn't have *forward secrecy* for any client. 

Problem
-------
After the experiment has taken place, it is time to generalise the results. Here's what is
wrong with CherryPy's SSL support now:

#. giving false sense of security
#. not flexible configuration: protocol, ciphers, options
#. not up to date with builit-in ``ssl`` (no SSL Context [27]_ support)
#. no support for pyOpenSSL for *py3*
#. no security assessment

Plan
----
I think that the problem is solvable with a reasonable effort. What has been said above may
already have convinced you that no special cryptography knowledge is required. There's a scalar
letter grade for A to F and several design decisions aimed at flexibility and compatibility.  

Outright state of affairs
~~~~~~~~~~~~~~~~~~~~~~~~~
Now we have the following note in *SSL support* section [14]_ in *Deploy* documentation page.

  You may want to test your server for SSL using the services from Qualys, Inc.

To me after doing this research it doesn't even look like a half-true thing expressed modestly. 
It looks we lie to users, giving them false sense of security. It may look like we're so sure
that it's okay that we let them do a voluntary task of checking it once again, just in case. 
But in fact it is to deploy their *py2* application and see it's vulnerable (I assume 
Python 2.7.9 isn't going to updated to soon for various reasons). This is absolutely 
redundant and we can tell that Python < 2.7.9 is a insecure platform right away. There should be
a warning in documentation, like "Please update to Python 2.7.9, install pyOpenSSL or start off 
with Python 3.4+. Otherwise it will only obfuscate your channel". 

Python < 2.7.9 should be considered an insecure platform with appropriate Python warning, see 
below.

Configuration
~~~~~~~~~~~~~
``cherrypy.server`` in addition to ``ssl_certificate``, ``ssl_certificate_chain``, 
``ssl_private_key`` should support:

* ``ssl_cipher_list`` available for all Python versions and both adapters. Mostly needed for
  Python < 2.7.9. 
* ``ssl_context`` available for Python 2.7.9+ and 3.3+. pyOpenSSL is almost there. SSL 
  Context [27]_ allows user to set: protocol, ciphers, options.
  
SSL Context [27]_ when not provided from user should be created with 
``ssl.create_default_context`` which takes care of security defaults. It is available for
Python 2.7.9+ and 3.4+. For Python 3.3 context should be configured at CherryPy side 
(the function is two dozen of lines [29]_).   

``ssl_adapters`` should be available at ``cherrypy.wsgiserver`` for user be able to set 
custom adapter (see the code above). Already implemented in my fork. 

Built-in SSL adapter update
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The adapter should make use of PEP 466 [22]_. This way, inside it may be two adapters. One for
Python 2.7.9+ with SSL Context [27]_ support and one for Python < 2.7.9. The latter should be
explicitly documented as vulnerable and emit warning when used. Documentation and warning text
can be taken from ``urllib3``'s ``InsecurePlatformWarning`` [30]_.   

The adapter should handle SSL exceptions according to the protocol, not flooding the log with 
them. 

Python 3 support for pyOpenSSL adapter
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Purpose of pyOpenSSL adapter should be rethought as Python < 2.6 is no longer supported
and Python 2.6+ has ``ssl`` module built in. Thus the purpose is not to provide a fallback.
The purpose is to provide more flexibility for users to manage their own security. Python 
binary that comes from a disto's repository have their own version of OpenSSL. When an OpenSSL
breach is discovered it is reasonable to expect patched OpenSSL package to be shipped first.
This gives an user ability to rebuild pyOpenSSL with latest OpenSSL as soon as possible in matter
of doing:
 
.. sourcecode:: bash

    pip install -U pyOpenSSL
    
Embracing this purpose Python 3 support should be provided. Currently 
``pyOpenSSLAdapter.makefile`` uses ``wsgiserver.CP_fileobject`` directly and uses 
``wsgiserver.ssl_pyopenssl.SSL_fileobject`` which inherits from ``wsgiserver.CP_fileobject``. 
``wsgiserver.CP_fileobject`` is available only in ``wsgiserver.wsgiserver2``. Therefore 
``pyOpenSSLAdapter.makefile`` should use ``wsgiserver.CP_makefile`` in *py3* just like 
``BuiltinSSLAdapter`` does. Here an advice or involvement of original authors is likely to
be needed because it's unclear in what circumstances ``pyOpenSSLAdapter.makefile`` should return
non-SSL socket. 
    
Automated security assessment
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Debian package of OpenSSL is described as:  

  [When] you need it to perform certain cryptographic actions like... 
  SSL/TLS client and server tests.

There are also things like sslyze [31]_, which is described as *fast and full-featured SSL 
scanner*. I think at some extent this approach will give automated security test, but it's an 
rough road. Basically, the quality of results will depend on our proficiency and maintenance of 
the chosen tool. Much easier way is to you a service.

Luckily, in January Qualys announced SSL Labs API beta [32]_. At the dedicated page [33]_ 
there're links to official command like client and protocol manual. It makes sense to expect
the same behaviour from the API as the normal tester [34]_ has. If so it won't work on bare IP
address and on non 443 port. 

To externalise this idea we have some prerequisite. If there's real CherryPy instance running 
behind Nginx on cherrypy.org at WebFaction, then do we have access to it? Can we run several 
additional instances there? if not, can we run them somewhere else?

If think of instances for the following environments:

* *py2-ssl-insecure.ssl.cherrypy.org* with Python < 2.7.9 that will probably be stable version
  in distos' repositories for quite some time 
* *py2-ssl-secure.ssl.cherrypy.org* with Python 2.7.9+
* *py2-pyopenssl-secure.ssl.cherrypy.org* with any *py2* with pyOpenSSL
* *py3-ssl-secure.ssl.cherrypy.org* with Python 3.4+
* *py3-pyopenssl-secure.ssl.cherrypy.org* with any *py3* with pyOpenSSL once it's available

Then we need to multiplex them to outer port 443 because likely it'll be running on single
server. Because Nginx doesn't seem to be able passthrough upstream SSL connections [35]_
we need to employ something else, maybe HAProxy [36]_ [37]_. 

What's left to do thereafter is to implement SSL Labs API, run in some schedule and we're done. 
It is able to give us detailed and actual reports on every environment that can be exposed to 
users to make their decisions.

Questions
---------
Sylvain, can you update the documentation to explicitly express the state of our SSL support? I 
think it is needed as soon as possible.

____

Sylvain Hellegouarch

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Apr 23, 2015, 3:50:29 PM4/23/15
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Hey saaj,

I know it's poor etiquette to respond on top but your message is quite rich :)

Thank you for your effotr once again, much appreciated. I would like to preface that, I agree with Joseph's concerns regarding a false sense of security just because we can enable ssl in CherryPy.
However, your work here does demonstrate that we could reach a point where our security is, by default, "good enough". I do realsie that this kind of attitude may not be the most appropriate from a server maintainer. Afterall, one would expect us to be much more strict with what we release.

My beliefs is that security is as much technology as it is human understanding of what they do. There are various ways to address the topic:

1/ we simply drop SSL/TLS support altogether and we ask folks to use a proxy if they want support for it
2/ we don't do anything which is misleading as we've stated already
3/ we improve the situation but we clearly state that, if people are looking for security, they ought to seriously review the whole stack and perhaps use a proxy with a better security background

From what I gather, 3/ is doable if we follow what you described here so I'm +1 on it.

* Document the vulnerabilities when using <2.7.9 and add a warning as you suggested for older releases
* Extend the ssl_* attributes to allow users greater control over their SSL/TLS configuration.
* Should we also document a common, good default configuration?
* Should we drop pyopenssl altogether?

As for the automated security assesment, I don't quite understand what they would give to the project really. Maybe, I'm missing something.

- Sylvain




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saaj

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Apr 24, 2015, 9:21:56 AM4/24/15
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I know it's poor etiquette to respond on top but your message is quite rich :)
;-) 

 
Thank you for your effotr once again, much appreciated. I would like to preface that, I agree with Joseph's concerns regarding a false sense of security just because we can enable ssl in CherryPy.
However, your work here does demonstrate that we could reach a point where our security is, by default, "good enough". I do realsie that this kind of attitude may not be the most appropriate from a server maintainer. Afterall, one would expect us to be much more strict with what we release.

My beliefs is that security is as much technology as it is human understanding of what they do. There are various ways to address the topic:

1/ we simply drop SSL/TLS support altogether and we ask folks to use a proxy if they want support for it
2/ we don't do anything which is misleading as we've stated already
3/ we improve the situation but we clearly state that, if people are looking for security, they ought to seriously review the whole stack and perhaps use a proxy with a better security background

From what I gather, 3/ is doable if we follow what you described here so I'm +1 on it.
:-)

 
* Document the vulnerabilities when using <2.7.9 and add a warning as you suggested for older releases
Warning should show up only for built-in adapter on < 2.7.9.  

* Should we also document a common, good default configuration?
It's inherently volatile, and without automated assessment we just don't know what's good. We could go a lazy path and
say -- "Look, Python's ``ssl.create_default_context`` takes care of all reasonable defaults". But it limits room for manoeuvres
for us. There's also 3.3 and < 2.7.9 without it.
 
* Should we drop pyopenssl altogether?

That is what the penalty for doing lengthy writings is -- people get you the other way about. Or were you reading between the lines? ;-)

There's dedicated section on why should we not only keep it, but also support for py3. Basically, pyOpenSSL, it gives fastest
and simplest ways of to have application patched once a vulnerability has been discovered. Just imagine the flow: breach discovered,
documented, then there's a patch, the OpenSSL package with fix is shipped, then you have Nginx, ..., and only then Python patch update.
Building Python is much harder than updating OpenSSL and typing ``pip install -U pyOpenSSL``. Joseph also mentioned that having a 
separate OpenSSL binding is correct way of dealing with the problem.
 
As for the automated security assesment, I don't quite understand what they would give to the project really. Maybe, I'm missing something.

We don't have empirical knowledge whatsoever. We don't know in a timely manner when a breach is discovered, what ciphers, protocols, or any
other aspect it affects. We don't know whether any default SSL settings are correct. We can't recommend anything to our users unless we ask an
expect to examine it. There's Qualys. Security is their business. They investigate vulnerabilities, track them, maintain their SSL tester which gives
comprehensive reports. It's free and has API. Why don't we delegate it to them so we know what settings are secure and what aren't?


Joseph S. Tate

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Apr 25, 2015, 10:29:50 AM4/25/15
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Can you say more about Qualys and how we can use it?
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saaj

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Apr 25, 2015, 1:42:55 PM4/25/15
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Surely. I see I still have important part of the plan in my head only, and the section doesn't answer all important questions. 

Objective: 
  * for maintainers to know what settings are secure 
  * for users to be able see what security latest release provides (like this report)
  * automation
  * notification (plan B only)

Prerequisite:
  * Qualys SSL tester is a web-service, thus we need public reference CherryPy instances (simple app that serves hello-world on SSL)
  * it only accepts hosts with symbolic names (not a raw IP) 
  * it only tests connection on port 443

Plan A (if there weren't API or we can't use it by some reason):
  * it can be a cron schedule (per week, per month, etc) or release-based schedule
  * write SSL section in documentation or on the site and provide links to the reports (they are cached by Qualys)

Plan B (with API):
  * create separate repo on Bitbucket, e.g. cherrypy/ssl-test
  * it needs only contain contain a simple test case with several tests that will call Qualys API
  * in there's no error, no warning, rate is not less than A, or similar condition test cases, fails otherwise
  * run the test case on drone.io CI for this repo
  * it gives notification, status badge for SSL we can put on cherrypy/cherrypy repo, and probably the same report URLs 

Both ways need a way to update reference instances' code (repo push hook). Also I think Sylvain's experience with Docker can 
come in very handy. These several instances as I told in the OP, should represent our expectation of most popular Python 
version people run CherryPy apps on. E.g. latest stable Debain has Python 2.7.3, so we can show a report for insecure built-in adapter,
and report for pyOpenSSL adapter as the only way to make it secure on CherryPy side. 

So with Docker we can relatively easily build images with say, latest stable Debain, Python, OpenSSL for 2.7.3, or some Ubuntu LTS 
for latest Python 2.7.9 and 3.4. And as easily rebuild them on schedule.
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