doc/pyOpenSSL.txt

                            Python OpenSSL Manual
     __________________________________________________________________

                            Python OpenSSL Manual

                               Martin Sjögren

                              martin@strakt.com

  Abstract:

   This module is a rather thin wrapper around (a subset of) the OpenSSL
   library. With thin wrapper I mean that a lot of the object methods do
   nothing more than calling a corresponding function in the OpenSSL
   library.

Contents

     * 1 Introduction
     * 2 Building and Installing
          + 2.1 Building the Module on a Unix System
          + 2.2 Building the Module on a Windows System
     * 3 OpenSSL -- Python interface to OpenSSL
          + 3.1 crypto -- Generic cryptographic module
          + 3.2 rand -- An interface to the OpenSSL pseudo random number
            generator
          + 3.3 SSL -- An interface to the SSL-specific parts of OpenSSL
     * 4 Internals
          + 4.1 Exceptions
          + 4.2 Callbacks
          + 4.3 Acessing Socket Methods


                                 1 Introduction

   The reason pyOpenSSL was created is that the SSL support in the socket
   module in Python 2.1 (the contemporary version of Python when the
   pyOpenSSL project was begun) was severely limited. Other OpenSSL
   wrappers for Python at the time were also limited, though in different
   ways. Unfortunately, Python's standard library SSL support has remained
   weak, although other packages (such as M2Crypto^1) have made great
   advances and now equal or exceed pyOpenSSL's functionality.

   The reason pyOpenSSL continues to be maintained is that there is a
   significant user community around it, as well as a large amount of
   software which depends on it. It is a great benefit to many people for
   pyOpenSSL to continue to exist and advance.


                           2 Building and Installing

   These instructions can also be found in the file INSTALL.

   I have tested this on Debian Linux systems (woody and sid), Solaris 2.6
   and 2.7. Others have successfully compiled it on Windows and NT.


2.1 Building the Module on a Unix System

   pyOpenSSL uses distutils, so there really shouldn't be any problems. To
   build the library:

python setup.py build

   If your OpenSSL header files aren't in /usr/include, you may need to
   supply the -I flag to let the setup script know where to look. The same
   goes for the libraries of course, use the -L flag. Note that build
   won't accept these flags, so you have to run first build_ext and then
   build! Example:

python setup.py build_ext -I/usr/local/ssl/include -L/usr/local/ssl/lib
python setup.py build

   Now you should have a directory called OpenSSL that contains e.g.
   SSL.so and __init__.py somewhere in the build dicrectory, so just:

python setup.py install

   If you, for some arcane reason, don't want the module to appear in the
   site-packages directory, use the --prefix option.

   You can, of course, do

python setup.py --help

   to find out more about how to use the script.


2.2 Building the Module on a Windows System

   Big thanks to Itamar Shtull-Trauring and Oleg Orlov for their help with
   Windows build instructions. Same as for Unix systems, we have to
   separate the build_ext and the build.

   Building the library:

setup.py build_ext -I ...\openssl\inc32 -L ...\openssl\out32dll
setup.py build

   Where ...\openssl is of course the location of your OpenSSL
   installation.

   Installation is the same as for Unix systems:

setup.py install

   And similarily, you can do

setup.py --help

   to get more information.


                    3 OpenSSL -- Python interface to OpenSSL

   This package provides a high-level interface to the functions in the
   OpenSSL library. The following modules are defined:

   crypto
          Generic cryptographic module. Note that if anything is
          incomplete, this module is!

   rand
          An interface to the OpenSSL pseudo random number generator.

   SSL
          An interface to the SSL-specific parts of OpenSSL.


3.1 crypto -- Generic cryptographic module

   X509Type
          A Python type object representing the X509 object type.

   X509()
          Factory function that creates an X509 object.

   X509NameType
          A Python type object representing the X509Name object type.

   X509Name(x509name)
          Factory function that creates a copy of x509name.

   X509ReqType
          A Python type object representing the X509Req object type.

   X509Req()
          Factory function that creates an X509Req object.

   X509StoreType
          A Python type object representing the X509Store object type.

   PKeyType
          A Python type object representing the PKey object type.

   PKey()
          Factory function that creates a PKey object.

   PKCS7Type
          A Python type object representing the PKCS7 object type.

   PKCS12Type
          A Python type object representing the PKCS12 object type.

   X509ExtensionType
          A Python type object representing the X509Extension object type.

   X509Extension(typename, critical, value)
          Factory function that creates a X509Extension object.

   NetscapeSPKIType
          A Python type object representing the NetscapeSPKI object type.

   NetscapeSPKI([enc])
          Factory function that creates a NetscapeSPKI object. If the enc
          argument is present, it should be a base64-encoded string
          representing a NetscapeSPKI object, as returned by the
          b64_encode method.

   FILETYPE_PEM

   FILETYPE_ASN1
          File type constants.

   TYPE_RSA

   TYPE_DSA
          Key type constants.

   exception Error
          Generic exception used in the crypto module.

   dump_certificate(type, cert)
          Dump the certificate cert into a buffer string encoded with the
          type type.

   dump_certificate_request(type, req)
          Dump the certificate request req into a buffer string encoded
          with the type type.

   dump_privatekey(type, pkey[, cipher, passphrase])
          Dump the private key pkey into a buffer string encoded with the
          type type, optionally (if type is FILETYPE_PEM) encrypting it
          using cipher and passphrase.

          passphrase must be either a string or a callback for providing
          the pass phrase.

   load_certificate(type, buffer)
          Load a certificate (X509) from the string buffer encoded with
          the type type.

   load_certificate_request(type, buffer)
          Load a certificate request (X509Req) from the string buffer
          encoded with the type type.

   load_privatekey(type, buffer[, passphrase])
          Load a private key (PKey) from the string buffer encoded with
          the type type (must be one of FILETYPE_PEM and FILETYPE_ASN1).

          passphrase must be either a string or a callback for providing
          the pass phrase.

   load_pkcs7_data(type, buffer)
          Load pkcs7 data from the string buffer encoded with the type
          type.

   load_pkcs12(buffer[, passphrase])
          Load pkcs12 data from the string buffer. If the pkcs12 structure
          is encrypted, a passphrase must be included.


  3.1.1 X509 objects

   X509 objects have the following methods:

   get_issuer()
          Return an X509Name object representing the issuer of the
          certificate.

   get_pubkey()
          Return a PKey object representing the public key of the
          certificate.

   get_serial_number()
          Return the certificate serial number.

   get_subject()
          Return an X509Name object representing the subject of the
          certificate.

   get_version()
          Return the certificate version.

   get_notBefore()
          Return a string giving the time before which the certificate is
          not valid. The string is formatted as an ASN1 GENERALIZEDTIME:

                 YYYYMMDDhhmmssZ
                 YYYYMMDDhhmmss+hhmm
                 YYYYMMDDhhmmss-hhmm

          If no value exists for this field, None is returned.

   get_notAfter()
          Return a string giving the time after which the certificate is
          not valid. The string is formatted as an ASN1 GENERALIZEDTIME:

                 YYYYMMDDhhmmssZ
                 YYYYMMDDhhmmss+hhmm
                 YYYYMMDDhhmmss-hhmm

          If no value exists for this field, None is returned.

   set_notBefore(when)
          Change the time before which the certificate is not valid. when
          is a string formatted as an ASN1 GENERALIZEDTIME:

                 YYYYMMDDhhmmssZ
                 YYYYMMDDhhmmss+hhmm
                 YYYYMMDDhhmmss-hhmm

   set_notAfter(when)
          Change the time after which the certificate is not valid. when
          is a string formatted as an ASN1 GENERALIZEDTIME:

                 YYYYMMDDhhmmssZ
                 YYYYMMDDhhmmss+hhmm
                 YYYYMMDDhhmmss-hhmm

   gmtime_adj_notBefore(time)
          Adjust the timestamp (in GMT) when the certificate starts being
          valid.

   gmtime_adj_notAfter(time)
          Adjust the timestamp (in GMT) when the certificate stops being
          valid.

   has_expired()
          Checks the certificate's time stamp against current time.
          Returns true if the certificate has expired and false otherwise.

   set_issuer(issuer)
          Set the issuer of the certificate to issuer.

   set_pubkey(pkey)
          Set the public key of the certificate to pkey.

   set_serial_number(serialno)
          Set the serial number of the certificate to serialno.

   set_subject(subject)
          Set the subject of the certificate to subject.

   set_version(version)
          Set the certificate version to version.

   sign(pkey, digest)
          Sign the certificate, using the key pkey and the message digest
          algorithm identified by the string digest.

   subject_name_hash()
          Return the hash of the certificate subject.

   digest(digest_name)
          Return a digest of the certificate, using the digest_name
          method.

   add_extensions(extensions)
          Add the extensions in the sequence extensions to the
          certificate.


  3.1.2 X509Name objects

   X509Name objects have the following methods:

   hash()
          Return an integer giving the first four bytes of the MD5 digest
          of the DER representation of the name.

   der()
          Return a string giving the DER representation of the name.

   get_components()
          Return a list of two-tuples of strings giving the components of
          the name.

   X509Name objects have the following members:

   countryName
          The country of the entity. C may be used as an alias for
          countryName.

   stateOrProvinceName
          The state or province of the entity. ST may be used as an alias
          for stateOrProvinceName·

   localityName
          The locality of the entity. L may be used as an alias for
          localityName.

   organizationName
          The organization name of the entity. O may be used as an alias
          for organizationName.

   organizationalUnitName
          The organizational unit of the entity. OU may be used as an
          alias for organizationalUnitName.

   commonName
          The common name of the entity. CN may be used as an alias for
          commonName.

   emailAddress
          The e-mail address of the entity.


  3.1.3 X509Req objects

   X509Req objects have the following methods:

   get_pubkey()
          Return a PKey object representing the public key of the
          certificate request.

   get_subject()
          Return an X509Name object representing the subject of the
          certificate.

   set_pubkey(pkey)
          Set the public key of the certificate request to pkey.

   sign(pkey, digest)
          Sign the certificate request, using the key pkey and the message
          digest algorithm identified by the string digest.

   verify(pkey)
          Verify a certificate request using the public key pkey.


  3.1.4 X509Store objects

   The X509Store object has currently just one method:

   add_cert(cert)
          Add the certificate cert to the certificate store.


  3.1.5 PKey objects

   The PKey object has the following methods:

   bits()
          Return the number of bits of the key.

   generate_key(type, bits)
          Generate a public/private key pair of the type type (one of
          TYPE_RSA and TYPE_DSA) with the size bits.

   type()
          Return the type of the key.


  3.1.6 PKCS7 objects

   PKCS7 objects have the following methods:

   type_is_signed()
          FIXME

   type_is_enveloped()
          FIXME

   type_is_signedAndEnveloped()
          FIXME

   type_is_data()
          FIXME

   get_type_name()
          Get the type name of the PKCS7.


  3.1.7 PKCS12 objects

   PKCS12 objects have the following methods:

   get_certificate()
          Return certificate portion of the PKCS12 structure.

   get_privatekey()
          Return private key portion of the PKCS12 structure

   get_ca_certificates()
          Return CA certificates within the PKCS12 object as a tuple.
          Returns None if no CA certificates are present.


  3.1.8 X509Extension objects

   X509Extension objects currently only have one method:

   get_critical()
          Return the critical field of the extension object.


  3.1.9 NetscapeSPKI objects

   NetscapeSPKI objects have the following methods:

   b64_encode()
          Return a base64-encoded string representation of the object.

   get_pubkey()
          Return the public key of object.

   set_pubkey(key)
          Set the public key of the object to key.

   sign(key, digest_name)
          Sign the NetscapeSPKI object using the given key and
          digest_name.

   verify(key)
          Verify the NetscapeSPKI object using the given key.


3.2 rand -- An interface to the OpenSSL pseudo random number generator

   This module handles the OpenSSL pseudo random number generator (PRNG)
   and declares the following:

   add(string, entropy)
          Mix bytes from string into the PRNG state. The entropy argument
          is (the lower bound of) an estimate of how much randomness is
          contained in string, measured in bytes. For more information,
          see e.g. RFC 1750.

   egd(path[, bytes])
          Query the Entropy Gathering Daemon^2 on socket path for bytes
          bytes of random data and and uses add to seed the PRNG. The
          default value of bytes is 255.

   load_file(path[, bytes])
          Read bytes bytes (or all of it, if bytes is negative) of data
          from the file path to seed the PRNG. The default value of bytes
          is -1.

   screen()
          Add the current contents of the screen to the PRNG state.
          Availability: Windows.

   seed(string)
          This is equivalent to calling add with entropy as the length of
          the string.

   status()
          Returns true if the PRNG has been seeded with enough data, and
          false otherwise.

   write_file(path)
          Write a number of random bytes (currently 1024) to the file
          path. This file can then be used with load_file to seed the PRNG
          again.


3.3 SSL -- An interface to the SSL-specific parts of OpenSSL

   This module handles things specific to SSL. There are two objects
   defined: Context, Connection.

   SSLv2_METHOD

   SSLv3_METHOD

   SSLv23_METHOD

   TLSv1_METHOD
          These constants represent the different SSL methods to use when
          creating a context object.

   VERIFY_NONE

   VERIFY_PEER

   VERIFY_FAIL_IF_NO_PEER_CERT
          These constants represent the verification mode used by the
          Context object's set_verify method.

   FILETYPE_PEM

   FILETYPE_ASN1
          File type constants used with the use_certificate_file and
          use_privatekey_file methods of Context objects.

   OP_SINGLE_DH_USE

   OP_EPHEMERAL_RSA

   OP_NO_SSLv2

   OP_NO_SSLv3

   OP_NO_TLSv1
          Constants used with set_options of Context objects.
          OP_SINGLE_DH_USE means to always create a new key when using
          ephemeral Diffie-Hellman. OP_EPHEMERAL_RSA means to always use
          ephemeral RSA keys when doing RSA operations. OP_NO_SSLv2,
          OP_NO_SSLv3 and OP_NO_TLSv1 means to disable those specific
          protocols. This is interesting if you're using e.g.
          SSLv23_METHOD to get an SSLv2-compatible handshake, but don't
          want to use SSLv2.

   ContextType
          A Python type object representing the Context object type.

   Context(method)
          Factory function that creates a new Context object given an SSL
          method. The method should be SSLv2_METHOD, SSLv3_METHOD,
          SSLv23_METHOD or TLSv1_METHOD.

   ConnectionType
          A Python type object representing the Connection object type.

   Connection(context, socket)
          Factory fucnction that creates a new Connection object given an
          SSL context and a socket ^3 object.

   exception Error
          This exception is used as a base class for the other SSL-related
          exceptions, but may also be raised directly.

          Whenever this exception is raised directly, it has a list of
          error messages from the OpenSSL error queue, where each item is
          a tuple (lib, function, reason). Here lib, function and reason
          are all strings, describing where and what the problem is. See
          err(3) for more information.

   exception ZeroReturnError
          This exception matches the error return code
          SSL_ERROR_ZERO_RETURN, and is raised when the SSL Connection has
          been closed. In SSL 3.0 and TLS 1.0, this only occurs if a
          closure alert has occurred in the protocol, i.e. the connection
          has been closed cleanly. Note that this does not necessarily
          mean that the transport layer (e.g. a socket) has been closed.

          It may seem a little strange that this is an exception, but it
          does match an SSL_ERROR code, and is very convenient.

   exception WantReadError
          The operation did not complete; the same I/O method should be
          called again later, with the same arguments. Any I/O method can
          lead to this since new handshakes can occur at any time.

   exception WantWriteError
          See WantReadError.

   exception WantX509LookupError
          The operation did not complete because an application callback
          has asked to be called again. The I/O method should be called
          again later, with the same arguments. Note: This won't occur in
          this version, as there are no such callbacks in this version.

   exception SysCallError
          The SysCallError occurs when there's an I/O error and OpenSSL's
          error queue does not contain any information. This can mean two
          things: An error in the transport protocol, or an end of file
          that violates the protocol. The parameter to the exception is
          always a pair (errnum, errstr).


  3.3.1 Context objects

   Context objects have the following methods:

   check_privatekey()
          Check if the private key (loaded with use_privatekey[_file])
          matches the certificate (loaded with use_certificate[_file]).
          Returns None if they match, raises Error otherwise.

   get_app_data()
          Retrieve application data as set by set_app_data.

   get_cert_store()
          Retrieve the certificate store (a X509Store object) that the
          context uses. This can be used to add "trusted" certificates
          without using the. load_verify_locations() method.

   get_timeout()
          Retrieve session timeout, as set by set_timeout. The default is
          300 seconds.

   get_verify_depth()
          Retrieve the Context object's verify depth, as set by
          set_verify_depth.

   get_verify_mode()
          Retrieve the Context object's verify mode, as set by
          set_verify_mode.

   load_client_ca(pemfile)
          Read a file with PEM-formatted certificates that will be sent to
          the client when requesting a client certificate.

   load_verify_locations(pemfile)
          Specify where CA certificates for verification purposes are
          located. These are trusted certificates. Note that the
          certificates have to be in PEM format.

   load_tmp_dh(dhfile)
          Load parameters for Ephemeral Diffie-Hellman from dhfile.

   set_app_data(data)
          Associate data with this Context object. data can be retrieved
          later using the get_app_data method.

   set_cipher_list(ciphers)
          Set the list of ciphers to be used in this context. See the
          OpenSSL manual for more information (e.g. ciphers(1))

   set_info_callback(callback)
          Set the information callback to callback. This function will be
          called from time to time during SSL handshakes. callback should
          take three arguments: a Connection object and two integers. The
          first integer specifies where in the SSL handshake the function
          was called, and the other the return code from a (possibly
          failed) internal function call.

   set_options(options)
          Add SSL options. Options you have set before are not cleared!
          This method should be used with the OP_* constants.

   set_passwd_cb(callback[, userdata])
          Set the passphrase callback to callback. This function will be
          called when a private key with a passphrase is loaded. callback
          must accept three positional arguments. First, an integer giving
          the maximum length of the passphrase it may return. If the
          returned passphrase is longer than this, it will be truncated.
          Second, a boolean value which will be true if the user should be
          prompted for the passphrase twice and the callback should verify
          that the two values supplied are equal. Third, the value given
          as the userdata parameter to set_passwd_cb. If an error occurs,
          callback should return a false value (e.g. an empty string).

   set_session_id(name)
          Set the context name within which a session can be reused for
          this Context object. This is needed when doing session
          resumption, because there is no way for a stored session to know
          which Context object it is associated with. name may be any
          binary data.

   set_timeout(timeout)
          Set the timeout for newly created sessions for this Context
          object to timeout. timeout must be given in (whole) seconds. The
          default value is 300 seconds. See the OpenSSL manual for more
          information (e.g. SSL_CTX_set_timeout(3)).

   set_verify(mode, callback)
          Set the verification flags for this Context object to mode and
          specify that callback should be used for verification callbacks.
          mode should be one of VERIFY_NONE and VERIFY_PEER. If
          VERIFY_PEER is used, mode can be OR:ed with
          VERIFY_FAIL_IF_NO_PEER_CERT and VERIFY_CLIENT_ONCE to further
          control the behaviour. callback should take five arguments: A
          Connection object, an X509 object, and three integer variables,
          which are in turn potential error number, error depth and return
          code. callback should return true if verification passes and
          false otherwise.

   set_verify_depth(depth)
          Set the maximum depth for the certificate chain verification
          that shall be allowed for this Context object.

   use_certificate(cert)
          Use the certificate cert which has to be a X509 object.

   add_extra_chain_cert(cert)
          Adds the certificate cert, which has to be a X509 object, to the
          certificate chain presented together with the certificate.

   use_certificate_chain_file(file)
          Load a certificate chain from file which must be PEM encoded.

   use_privatekey(pkey)
          Use the private key pkey which has to be a PKey object.

   use_certificate_file(file[, format])
          Load the first certificate found in file. The certificate must
          be in the format specified by format, which is either
          FILETYPE_PEM or FILETYPE_ASN1. The default is FILETYPE_PEM.

   use_privatekey_file(file[, format])
          Load the first private key found in file. The private key must
          be in the format specified by format, which is either
          FILETYPE_PEM or FILETYPE_ASN1. The default is FILETYPE_PEM.


  3.3.2 Connection objects

   Connection objects have the following methods:

   accept()
          Call the accept method of the underlying socket and set up SSL
          on the returned socket, using the Context object supplied to
          this Connection object at creation. Returns a pair (conn,
          address). where conn is the new Connection object created, and
          address is as returned by the socket's accept.

   bind(address)
          Call the bind method of the underlying socket.

   close()
          Call the close method of the underlying socket. Note: If you
          want correct SSL closure, you need to call the shutdown method
          first.

   connect(address)
          Call the connect method of the underlying socket and set up SSL
          on the socket, using the Context object supplied to this
          Connection object at creation.

   connect_ex(address)
          Call the connect_ex method of the underlying socket and set up
          SSL on the socket, using the Context object supplied to this
          Connection object at creation. Note that if the connect_ex
          method of the socket doesn't return 0, SSL won't be initialized.

   do_handshake()
          Perform an SSL handshake (usually called after renegotiate or
          one of set_accept_state or set_accept_state). This can raise the
          same exceptions as send and recv.

   fileno()
          Retrieve the file descriptor number for the underlying socket.

   listen(backlog)
          Call the listen method of the underlying socket.

   get_app_data()
          Retrieve application data as set by set_app_data.

   get_cipher_list()
          Retrieve the list of ciphers used by the Connection object.
          WARNING: This API has changed. It used to take an optional
          parameter and just return a string, but not it returns the
          entire list in one go.

   get_context()
          Retrieve the Context object associated with this Connection.

   get_peer_certificate()
          Retrieve the other side's certificate (if any)

   getpeername()
          Call the getpeername method of the underlying socket.

   getsockname()
          Call the getsockname method of the underlying socket.

   getsockopt(level, optname[, buflen])
          Call the getsockopt method of the underlying socket.

   pending()
          Retrieve the number of bytes that can be safely read from the
          SSL buffer (not the underlying transport buffer).

   recv(bufsize)
          Receive data from the Connection. The return value is a string
          representing the data received. The maximum amount of data to be
          received at once, is specified by bufsize.

   renegotiate()
          Renegotiate the SSL session. Call this if you wish to change
          cipher suites or anything like that.

   send(string)
          Send the string data to the Connection.

   sendall(string)
          Send all of the string data to the Connection. This calls send
          repeatedly until all data is sent. If an error occurs, it's
          impossible to tell how much data has been sent.

   set_accept_state()
          Set the connection to work in server mode. The handshake will be
          handled automatically by read/write.

   set_app_data(data)
          Associate data with this Connection object. data can be
          retrieved later using the get_app_data method.

   set_connect_state()
          Set the connection to work in client mode. The handshake will be
          handled automatically by read/write.

   setblocking(flag)
          Call the setblocking method of the underlying socket.

   setsockopt(level, optname, value)
          Call the setsockopt method of the underlying socket.

   shutdown()
          Send the shutdown message to the Connection. Returns true if the
          shutdown message exchange is completed and false otherwise (in
          which case you call recv() or send() when the connection becomes
          readable/writeable.

   get_shutdown()
          Get the shutdown state of the Connection. Returns a bitvector of
          either or both of SENT_SHUTDOWN and RECEIVED_SHUTDOWN.

   set_shutdown(state)
          Set the shutdown state of the Connection. state is a bitvector
          of either or both of SENT_SHUTDOWN and RECEIVED_SHUTDOWN.

   sock_shutdown(how)
          Call the shutdown method of the underlying socket.

   state_string()
          Retrieve a verbose string detailing the state of the Connection.

   want_read()
          Checks if more data has to be read from the transport layer to
          complete an operation.

   want_write()
          Checks if there is data to write to the transport layer to
          complete an operation.


                                  4 Internals

   We ran into three main problems developing this: Exceptions, callbacks
   and accessing socket methods. This is what this chapter is about.


4.1 Exceptions

   We realized early that most of the exceptions would be raised by the
   I/O functions of OpenSSL, so it felt natural to mimic OpenSSL's error
   code system, translating them into Python exceptions. This naturally
   gives us the exceptions SSL.ZeroReturnError, SSL.WantReadError,
   SSL.WantWriteError, SSL.WantX509LookupError and SSL.SysCallError.

   For more information about this, see section 3.3.


4.2 Callbacks

   There are a number of problems with callbacks. First of all, OpenSSL is
   written as a C library, it's not meant to have Python callbacks, so a
   way around that is needed. Another problem is thread support. A lot of
   the OpenSSL I/O functions can block if the socket is in blocking mode,
   and then you want other Python threads to be able to do other things.
   The real trouble is if you've released the global CPython interpreter
   lock to do a potentially blocking operation, and the operation calls a
   callback. Then we must take the GIL back, since calling Python APIs
   without holding it is not allowed.

   There are two solutions to the first problem, both of which are
   necessary. The first solution to use is if the C callback allows
   ''userdata'' to be passed to it (an arbitrary pointer normally). This
   is great! We can set our Python function object as the real userdata
   and emulate userdata for the Python function in another way. The other
   solution can be used if an object with an ''app_data'' system always is
   passed to the callback. For example, the SSL object in OpenSSL has
   app_data functions and in e.g. the verification callbacks, you can
   retrieve the related SSL object. What we do is to set our wrapper
   Connection object as app_data for the SSL object, and we can easily
   find the Python callback.

   The other problem is solved using thread local variables. Whenever the
   GIL is released before calling into an OpenSSL API, the PyThreadState
   pointer returned by PyEval_SaveState is stored in a global thread local
   variable (using Python's own TLS API, PyThread_set_key_value). When it
   is necessary to re-acquire the GIL, either after the OpenSSL API
   returns or in a C callback invoked by that OpenSSL API, the value of
   the thread local variable is retrieved (PyThread_get_key_value) and
   used to re-acquire the GIL. This allows Python threads to execute while
   OpenSSL APIs are running and allows use of any particular pyOpenSSL
   object from any Python thread, since there is no per-thread state
   associated with any of these objects and since OpenSSL is threadsafe
   (as long as properly initialized, as pyOpenSSL initializes it).


4.3 Acessing Socket Methods

   We quickly saw the benefit of wrapping socket methods in the
   SSL.Connection class, for an easy transition into using SSL. The
   problem here is that the socket module lacks a C API, and all the
   methods are declared static. One approach would be to have OpenSSL as a
   submodule to the socket module, placing all the code in socketmodule.c,
   but this is obviously not a good solution, since you might not want to
   import tonnes of extra stuff you're not going to use when importing the
   socket module. The other approach is to somehow get a pointer to the
   method to be called, either the C function, or a callable Python
   object. This is not really a good solution either, since there's a lot
   of lookups involved.

   The way it works is that you have to supply a ``socket-like'' transport
   object to the SSL.Connection. The only requirement of this object is
   that it has a fileno() method that returns a file descriptor that's
   valid at the C level (i.e. you can use the system calls read and
   write). If you want to use the connect() or accept() methods of the
   SSL.Connection object, the transport object has to supply such methods
   too. Apart from them, any method lookups in the SSL.Connection object
   that fail are passed on to the underlying transport object.

   Future changes might be to allow Python-level transport objects, that
   instead of having fileno() methods, have read() and write() methods, so
   more advanced features of Python can be used. This would probably
   entail some sort of OpenSSL ``BIOs'', but converting Python strings
   back and forth is expensive, so this shouldn't be used unless
   necessary. Other nice things would be to be able to pass in different
   transport objects for reading and writing, but then the fileno() method
   of SSL.Connection becomes virtually useless. Also, should the method
   resolution be used on the read-transport or the write-transport?

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   Python OpenSSL Manual

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    Footnotes

   ... M2Crypto^1
          See http://chandlerproject.org/Projects/MeTooCrypto

   ... Daemon^2
          See http://www.lothar.com/tech/crypto/

   ... socket^3
          Actually, all that is required is an object that behaves like a
          socket, you could even use files, even though it'd be tricky to
          get the handshakes right!
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                            Python OpenSSL Manual
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   Release 0.8.