                              Python OpenSSL Manual

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

                             Python OpenSSL Manual

                              Jean-Paul Calderone

                           exarkun@twistedmatrix.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 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[13]^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
           See X509.

   class X509()
           A class representing X.509 certificates.

   X509NameType
           See X509Name.

   class X509Name(x509name)
           A class representing X.509 Distinguished Names.

           This constructor creates a copy of x509name which should be an
           instance of X509Name.

   X509ReqType
           See X509Req.

   class X509Req()
           A class representing X.509 certificate requests.

   X509StoreType
           A Python type object representing the X509Store object type.

   PKeyType
           See PKey.

   class PKey()
           A class representing DSA or RSA keys.

   PKCS7Type
           A Python type object representing the PKCS7 object type.

   PKCS12Type
           A Python type object representing the PKCS12 object type.

   X509ExtensionType
           See X509Extension.

   class X509Extension(typename, critical, value[, subject][, issuer])
           A class representing an X.509 v3 certificate extensions. See
           [14]http://openssl.org/docs/apps/x509v3_config.html#STANDARD_EXTENSIONSfor
           typename strings and their options. Optional parameters subject
           and issuer must be X509 objects.

   NetscapeSPKIType
           See NetscapeSPKI.

   class NetscapeSPKI([enc])
           A class representing Netscape SPKI objects.

           If the enc argument is present, it should be a base64-encoded
           string representing a NetscapeSPKI object, as returned by the
           b64_encode method.

   class CRL()
           A class representing Certifcate Revocation List objects.

   class Revoked()
           A class representing Revocation objects of CRL.

   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_crl(type, buffer)
           Load Certificate Revocation List (CRL) data from a string buffer.
           buffer encoded with the type type. The type type must either
           FILETYPE_PEM or FILETYPE_ASN1).

   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. The MAC is always
           checked and thus required.

           See also the man page for the C function PKCS12_parse.

   sign(key, data, digest)
           Sign a data string using the given key and message digest.

           key is a PKey instance. data is a str instance. digest is a str
           naming a supported message digest type, for example ``sha1''. New
           in version 0.11.

   verify(certificate, signature, data, digest)
           Verify the signature for a data string.

           certificate is a X509 instance corresponding to the private key
           which generated the signature. signature is a str instance giving
           the signature itself. data is a str instance giving the data to
           which the signature applies. digest is a str instance naming the
           message digest type of the signature, for example ``sha1''. New in
           version 0.11.


  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_signature_algorithm()
           Return the signature algorithm used in the certificate. If the
           algorithm is undefined, raise ValueError.

   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.
           digest_name must be a string describing a digest algorithm
           supported by OpenSSL (by EVP_get_digestbyname, specifically). For
           example, "md5" or "sha1".

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

   get_extension_count()
           Return the number of extensions on this certificate. New in
           version 0.12.

   get_extension(index)
           Retrieve the extension on this certificate at the given index.

           Extensions on a certificate are kept in order. The index parameter
           selects which extension will be returned. The returned object will
           be an X509Extension instance. New in version 0.12.


  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.

   set_version(version)
           Set the version (RFC 2459, 4.1.2.1) of the certificate request to
           version.

   get_version()
           Get the version (RFC 2459, 4.1.2.1) of the certificate request.


  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.

   check()
           Check the consistency of this key, returning True if it is
           consistent and raising an exception otherwise. This is only valid
           for RSA keys. See the OpenSSL RSA_check_key man page for further
           limitations.


  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:

   export([passphrase=None][, iter=2048][, maciter=1])
           Returns a PKCS12 object as a string.

           The optional passphrase must be a string not a callback.

           See also the man page for the C function PKCS12_create.

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

   get_certificate()
           Return certificate portion of the PKCS12 structure.

   get_friendlyname()
           Return friendlyName portion of the PKCS12 structure.

   get_privatekey()
           Return private key portion of the PKCS12 structure

   set_ca_certificates(cacerts)
           Replace or set the CA certificates within the PKCS12 object with
           the sequence cacerts.

           Set cacerts to None to remove all CA certificates.

   set_certificate(cert)
           Replace or set the certificate portion of the PKCS12 structure.

   set_friendlyname(name)
           Replace or set the friendlyName portion of the PKCS12 structure.

   set_privatekey(pkey)
           Replace or set private key portion of the PKCS12 structure


  3.1.8 X509Extension objects

   X509Extension objects have several methods:

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

   get_short_name()
           Retrieve the short descriptive name for this extension.

           The result is a byte string like ``basicConstraints''. New in
           version 0.12.

   get_data()
           Retrieve the data for this extension.

           The result is the ASN.1 encoded form of the extension data as a
           byte string. New in version 0.12.


  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.
           digest_name must be a string describing a digest algorithm
           supported by OpenSSL (by EVP_get_digestbyname, specifically). For
           example, "md5" or "sha1".

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


  3.1.10 CRL objects

   CRL objects have the following methods:

   add_revoked(revoked)
           Add a Revoked object to the CRL, by value not reference.

   export(cert, key[, type=FILETYPE_PEM][, days=100])
           Use cert and key to sign the CRL and return the CRL as a string.
           days is the number of days before the next CRL is due.

   get_revoked()
           Return a tuple of Revoked objects, by value not reference.


  3.1.11 Revoked objects

   Revoked objects have the following methods:

   all_reasons()
           Return a list of all supported reasons.

   get_reason()
           Return the revocation reason as a str. Can be None, which differs
           from "Unspecified".

   get_rev_date()
           Return the revocation date as a str. The string is formatted as an
           ASN1 GENERALIZEDTIME.

   get_serial()
           Return a str containing a hex number of the serial of the revoked
           certificate.

   set_reason(reason)
           Set the revocation reason. reason must be None or a string, but
           the values are limited. Spaces and case are ignored. See
           all_reasons.

   set_rev_date(date)
           Set the revocation date. The string is formatted as an ASN1
           GENERALIZEDTIME.

   set_serial(serial)
           serial is a string containing a hex number of the serial of the
           revoked certificate.


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.

   bytes(num_bytes)
           Get some random bytes from the PRNG as a string.

           This is a wrapper for the C function RAND_bytes.

   cleanup()
           Erase the memory used by the PRNG.

           This is a wrapper for the C function RAND_cleanup.

   egd(path[, bytes])
           Query the Entropy Gathering Daemon[16]^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.

   exception Error
           If the current RAND method supports any errors, this is raised
           when needed. The default method does not raise this when the
           entropy pool is depleted.

           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.


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.

   SSLEAY_VERSION

   SSLEAY_CFLAGS

   SSLEAY_BUILT_ON

   SSLEAY_PLATFORM

   SSLEAY_DIR
           Constants used with SSLeay_version to specify what OpenSSL version
           information to retrieve. See the man page for the SSLeay_version C
           API for details.

   OPENSSL_VERSION_NUMBER
           An integer giving the version number of the OpenSSL library used
           to build this version of pyOpenSSL. See the man page for the
           SSLeay_version C API for details.

   SSLeay_version(type)
           Retrieve a string describing some aspect of the underlying OpenSSL
           version. The type passed in should be one of the SSLEAY_*
           constants defined in this module.

   ContextType
           See Context.

   class Context(method)
           A class representing SSL contexts. Contexts define the parameters
           of one or more SSL connections.

           method should be SSLv2_METHOD, SSLv3_METHOD, SSLv23_METHOD or
           TLSv1_METHOD.

   ConnectionType
           See Connection.

   class Connection(context, socket)
           A class representing SSL connections.

           context should be an instance of Context and socket should be a
           socket [17]^3 object. socket may be None; in this case, the
           Connection is created with a memory BIO: see the bio_read,
           bio_write, and bio_shutdown methods.

   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.

           The wanted read is for dirty data sent over the network, not the
           clean data inside the tunnel. For a socket based SSL connection,
           read means data coming at us over the network. Until that read
           succeeds, the attempted OpenSSL.SSL.Connection.recv,
           OpenSSL.SSL.Connection.send, or
           OpenSSL.SSL.Connection.do_handshake is prevented or incomplete.
           You probably want to select() on the socket before trying again.

   exception WantWriteError
           See WantReadError. The socket send buffer may be too full to write
           more data.

   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.

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

   set_client_ca_list(certificate_authorities)
           Replace the current list of preferred certificate signers that
           would be sent to the client when requesting a client certificate
           with the certificate_authorities sequence of
           OpenSSL.crypto.X509Names.

           New in version 0.10.

   add_client_ca(certificate_authority)
           Extract a OpenSSL.crypto.X509Name from the certificate_authority
           OpenSSL.crypto.X509 certificate and add it to the list of
           preferred certificate signers sent to the client when requesting a
           client certificate.

           New in version 0.10.

   load_verify_locations(pemfile, capath)
           Specify where CA certificates for verification purposes are
           located. These are trusted certificates. Note that the
           certificates have to be in PEM format. If capath is passed, it
           must be a directory prepared using the c_rehash tool included with
           OpenSSL. Either, but not both, of pemfile or capath may be None.

   set_default_verify_paths()
           Specify that the platform provided CA certificates are to be used
           for verification purposes. This method may not work properly on OS
           X.

   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.

   set_tlsext_servername_callback(callback)
           Specify a one-argument callable to use as the TLS extension server
           name callback. When a connection using the server name extension
           is made using this context, the callback will be invoked with the
           Connection instance. New in version 0.13.


  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_client_ca_list()
           Retrieve the list of preferred client certificate issuers sent by
           the server as OpenSSL.crypto.X509Name objects.

           If this is a client Connection, the list will be empty until the
           connection with the server is established.

           If this is a server Connection, return the list of certificate
           authorities that will be sent or has been sent to the client, as
           controlled by this Connection's Context.

           New in version 0.10.

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

   set_context(context)
           Specify a replacement Context object for this Connection.

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

   get_peer_cert_chain()
           Retrieve the tuple of the other side's certificate chain (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.

   bio_write(bytes)
           If the Connection was created with a memory BIO, this method can
           be used to add bytes to the read end of that memory BIO. The
           Connection can then read the bytes (for example, in response to a
           call to recv).

   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.

   bio_read(bufsize)
           If the Connection was created with a memory BIO, this method can
           be used to read bytes from the write end of that memory BIO. Many
           Connection methods will add bytes which must be read in this
           manner or the buffer will eventually fill up and the Connection
           will be able to take no further actions.

   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.

   bio_shutdown()
           If the Connection was created with a memory BIO, this method can
           be used to indicate that ``end of file'' has been reached on the
           read end of that memory BIO.

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

   client_random()
           Retrieve the random value used with the client hello message.

   server_random()
           Retrieve the random value used with the server hello message.

   master_key()
           Retrieve the value of the master key for this session.

   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.

   set_tlsext_host_name(name)
           Specify the byte string to send as the server name in the client
           hello message. New in version 0.13.

   get_servername()
           Get the value of the server name received in the client hello
           message. New in version 0.13.


                                  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 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?

                            About this document ...

   Python OpenSSL Manual

   This document was generated using the [19]LaTeX2HTML translator.

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

   The application of [23]LaTeX2HTML to the Python documentation has been
   heavily tailored by Fred L. Drake, Jr. Original navigation icons were
   contributed by Christopher Petrilli.

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

    Footnotes

   ... M2Crypto[24]^1
           See [25]http://chandlerproject.org/Projects/MeTooCrypto

   ... Daemon[26]^2
           See [27]http://www.lothar.com/tech/crypto/

   ... socket[28]^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!

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

                              Python OpenSSL Manual

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

   Release 0.13.

References

   Visible links
   1. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#SECTION000200000000000000000
   2. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#SECTION000300000000000000000
   3. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#SECTION000310000000000000000
   4. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#SECTION000320000000000000000
   5. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#SECTION000400000000000000000
   6. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#SECTION000410000000000000000
   7. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#SECTION000420000000000000000
   8. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#SECTION000430000000000000000
   9. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#SECTION000500000000000000000
  10. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#SECTION000510000000000000000
  11. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#SECTION000520000000000000000
  12. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#SECTION000530000000000000000
  13. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/index.html#foot1246
  14. http://openssl.org/docs/apps/x509v3_config.html#STANDARD_EXTENSIONS
  15. http://www.ietf.org/rfc/rfc1750.txt
  16. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/index.html#foot1272
  17. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/index.html#foot1276
  18. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/index.html#openssl-ssl
  19. http://saftsack.fs.uni-bayreuth.de/~latex2ht/
  20. http://saftsack.fs.uni-bayreuth.de/~latex2ht/
  21. http://cbl.leeds.ac.uk/nikos/personal.html
  22. http://www.maths.mq.edu.au/~ross/
  23. http://saftsack.fs.uni-bayreuth.de/~latex2ht/
  24. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#tex2html1
  25. http://chandlerproject.org/Projects/MeTooCrypto
  26. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#tex2html2
  27. http://www.lothar.com/tech/crypto/
  28. file:///builddir/build/BUILD/pyOpenSSL-0.13.1/doc/pyOpenSSL-temp-html/pyOpenSSL.html#tex2html4
