                              Python OpenSSL Manual                           

   ---------------------------------------------------------------------------

                             Python OpenSSL Manual

                                Martin Sjoegren

                               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]1 Introduction
     * [2]2 Building and Installing

          * [3]2.1 Building the Module on a Unix System
          * [4]2.2 Building the Module on a Windows System

     * [5]3 OpenSSL -- Python interface to OpenSSL

          * [6]3.1 crypto -- Generic cryptographic module
          * [7]3.2 rand -- An interface to the OpenSSL pseudo random number
            generator
          * [8]3.3 SSL -- An interface to the SSL-specific parts of OpenSSL

     * [9]4 Internals

          * [10]4.1 Exceptions
          * [11]4.2 Callbacks
          * [12]4.3 Acessing Socket Methods

                                        
                                 1 Introduction

   The reason this module exists at all is that the SSL support in the socket
   module in the Python 2.1 distribution (which is what we used, of course I
   cannot speak for later versions) is severely limited.

   When asking about SSL on the comp.lang.python newsgroup (or on
   python-list@python.org) people usually pointed you to the M2Crypto
   package. The M2Crypto.SSL module does implement a lot of OpenSSL's
   functionality but unfortunately its error handling system does not seem to
   be finished, especially for non-blocking I/O. I think that much of the
   reason for this is that M2Crypto^[13]1 is developed using SWIG^[14]2. This
   makes it awkward to create functions that e.g. can return both an integer
   and NULL since (as far as I know) you basically write C functions and SWIG
   makes wrapper functions that parses the Python argument list and calls
   your C function, and finally transforms your return value to a Python
   object.

                                        
                           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 a borrowed reference to a X509Name object representing the
           issuer of the certificate. When the corresponding X509 or X509Req
           object is destroyed, this object will be invalid!

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

   get_serial_number()
           Return the certificate serial number.

   get_subject()
           Return a borrowed reference to a X509Name object representing the
           subject of the certificate. When the corresponding X509 or X509Req
           object is destroyed, this object will be invalid!

   get_version()
           Return the certificate version.

   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 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 a borrowed reference to a X509Name object representing the
           subject of the certificate. When the corresponding X509 or X509Req
           object is destroyed, this object will be invalid!

   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. [15]RFC 1750.

   egd(path[, bytes])
           Query the Entropy Gathering Daemon^[16]3 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 ^[17]4 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 true if they match, false 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 should take a boolean argument repeat and an arbitrary
           argument data and return the passphrase entered by the user. If
           repeat is true then callback should ask for the passphrase twice
           and make sure that the two entries are equal. The data argument is
           the userdata variable passed to the set_passwd_cb method. 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.

   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.

   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.

   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 [18]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 thread lock to do a potentially blocking
   operation, and the operation calls a callback. Then we must take the
   thread lock back^[19]5.

   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 also partially solved by app_data. Since we're
   associating our wrapper objects with the ''real'' objects, we can easily
   access data from the Connection object. The solution then is to simply
   include a PyThreadState variable in the Connection declaration, and write
   macros similar to Py_BEGIN_ALLOW_THREADS and Py_END_ALLOW_THREADS that
   allows specifying of the PyThreadState variable to use. Now we can simply
   ''begin allow threads'' before a potentially blocking operation, and ''end
   allow threads'' before calling a callback.

 
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?

                            About this document ...

   Python OpenSSL Manual

   This document was generated using the [20]LaTeX2HTML translator.

   [21]LaTeX2HTML is Copyright (c) 1993, 1994, 1995, 1996, 1997, [22]Nikos
   Drakos, Computer Based Learning Unit, University of Leeds, and Copyright
   (c) 1997, 1998, [23]Ross Moore, Mathematics Department, Macquarie
   University, Sydney.

   The application of [24]LaTeX2HTML to the Python documentation has been
   heavily tailored by Fred L. Drake, Jr. Original navigation icons were
   contributed by Christopher Petrilli.

   ---------------------------------------------------------------------------

    Footnotes

   ... M2Crypto^[25]1
           See [26]http://www.post1.com/home/ngps/m2/

   ... SWIG^[27]2
           See [28]http://swig.sourceforge.net/

   ... Daemon^[29]3
           See [30]http://www.lothar.com/tech/crypto/

   ... socket^[31]4
           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!

   ... back^[32]5
           I'm not sure why this is necessary, but otherwise I get a
           segmentation violation on PyEval_CallObject

   ---------------------------------------------------------------------------

                              Python OpenSSL Manual                           

   ---------------------------------------------------------------------------

   Release 0.6.

References

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  15. http://www.ietf.org/rfc/rfc1750.txt
  16. file:///usr/src/build/437388-i386/BUILD/pyOpenSSL-0.6/doc/pyOpenSSL-temp-html/index.html#foot898
  17. file:///usr/src/build/437388-i386/BUILD/pyOpenSSL-0.6/doc/pyOpenSSL-temp-html/index.html#foot901
  18. file:///usr/src/build/437388-i386/BUILD/pyOpenSSL-0.6/doc/pyOpenSSL-temp-html/index.html#openssl-ssl
  19. file:///usr/src/build/437388-i386/BUILD/pyOpenSSL-0.6/doc/pyOpenSSL-temp-html/index.html#foot916
  20. http://saftsack.fs.uni-bayreuth.de/~latex2ht/
  21. http://saftsack.fs.uni-bayreuth.de/~latex2ht/
  22. http://cbl.leeds.ac.uk/nikos/personal.html
  23. http://www.maths.mq.edu.au/~ross/
  24. http://saftsack.fs.uni-bayreuth.de/~latex2ht/
  25. file:///usr/src/build/437388-i386/BUILD/pyOpenSSL-0.6/doc/pyOpenSSL-temp-html/pyOpenSSL.html#tex2html1
  26. http://www.post1.com/home/ngps/m2/
  27. file:///usr/src/build/437388-i386/BUILD/pyOpenSSL-0.6/doc/pyOpenSSL-temp-html/pyOpenSSL.html#tex2html2
  28. http://swig.sourceforge.net/
  29. file:///usr/src/build/437388-i386/BUILD/pyOpenSSL-0.6/doc/pyOpenSSL-temp-html/pyOpenSSL.html#tex2html3
  30. http://www.lothar.com/tech/crypto/
  31. file:///usr/src/build/437388-i386/BUILD/pyOpenSSL-0.6/doc/pyOpenSSL-temp-html/pyOpenSSL.html#tex2html5
  32. file:///usr/src/build/437388-i386/BUILD/pyOpenSSL-0.6/doc/pyOpenSSL-temp-html/pyOpenSSL.html#tex2html6
