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|
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 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^1 is developed using
SWIG^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. RFC 1750.
egd(path[, bytes])
Query the Entropy Gathering Daemon^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 ^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 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^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?
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Python OpenSSL Manual
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Footnotes
... M2Crypto^1
See http://www.post1.com/home/ngps/m2/
... SWIG^2
See http://swig.sourceforge.net/
... Daemon^3
See http://www.lothar.com/tech/crypto/
... socket^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^5
I'm not sure why this is necessary, but otherwise I get a
segmentation violation on PyEval_CallObject
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Python OpenSSL Manual
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Release 0.6.
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