2 :mod:`ssl` --- SSL wrapper for socket objects
3 ====================================================================
6 :synopsis: SSL wrapper for socket objects
8 .. moduleauthor:: Bill Janssen <bill.janssen@gmail.com>
12 .. sectionauthor:: Bill Janssen <bill.janssen@gmail.com>
15 .. index:: single: OpenSSL; (use in module ssl)
17 .. index:: TLS, SSL, Transport Layer Security, Secure Sockets Layer
19 This module provides access to Transport Layer Security (often known
20 as "Secure Sockets Layer") encryption and peer authentication
21 facilities for network sockets, both client-side and server-side.
22 This module uses the OpenSSL library. It is available on all modern
23 Unix systems, Windows, Mac OS X, and probably additional
24 platforms, as long as OpenSSL is installed on that platform.
28 Some behavior may be platform dependent, since calls are made to the operating
29 system socket APIs. The installed version of OpenSSL may also cause
30 variations in behavior.
32 This section documents the objects and functions in the ``ssl`` module;
33 for more general information about TLS, SSL, and certificates, the
34 reader is referred to the documents in the "See Also" section at
37 This module provides a class, :class:`ssl.SSLSocket`, which is
38 derived from the :class:`socket.socket` type, and provides
39 a socket-like wrapper that also encrypts and decrypts the data
40 going over the socket with SSL. It supports additional
41 :meth:`read` and :meth:`write` methods, along with a method, :meth:`getpeercert`,
42 to retrieve the certificate of the other side of the connection, and
43 a method, :meth:`cipher`, to retrieve the cipher being used for the
46 Functions, Constants, and Exceptions
47 ------------------------------------
49 .. exception:: SSLError
51 Raised to signal an error from the underlying SSL implementation. This
52 signifies some problem in the higher-level
53 encryption and authentication layer that's superimposed on the underlying
54 network connection. This error is a subtype of :exc:`socket.error`, which
55 in turn is a subtype of :exc:`IOError`.
57 .. function:: wrap_socket (sock, keyfile=None, certfile=None, server_side=False, cert_reqs=CERT_NONE, ssl_version={see docs}, ca_certs=None)
59 Takes an instance ``sock`` of :class:`socket.socket`, and returns an instance of :class:`ssl.SSLSocket`, a subtype
60 of :class:`socket.socket`, which wraps the underlying socket in an SSL context.
61 For client-side sockets, the context construction is lazy; if the underlying socket isn't
62 connected yet, the context construction will be performed after :meth:`connect` is called
63 on the socket. For server-side sockets, if the socket has no remote peer, it is assumed
64 to be a listening socket, and the server-side SSL wrapping is automatically performed
65 on client connections accepted via the :meth:`accept` method. :func:`wrap_socket` may
66 raise :exc:`SSLError`.
68 The ``keyfile`` and ``certfile`` parameters specify optional files which contain a certificate
69 to be used to identify the local side of the connection. See the discussion of :ref:`ssl-certificates`
70 for more information on how the certificate is stored in the ``certfile``.
72 Often the private key is stored
73 in the same file as the certificate; in this case, only the ``certfile`` parameter need be
74 passed. If the private key is stored in a separate file, both parameters must be used.
75 If the private key is stored in the ``certfile``, it should come before the first certificate
76 in the certificate chain::
78 -----BEGIN RSA PRIVATE KEY-----
79 ... (private key in base64 encoding) ...
80 -----END RSA PRIVATE KEY-----
81 -----BEGIN CERTIFICATE-----
82 ... (certificate in base64 PEM encoding) ...
83 -----END CERTIFICATE-----
85 The parameter ``server_side`` is a boolean which identifies whether server-side or client-side
86 behavior is desired from this socket.
88 The parameter ``cert_reqs`` specifies whether a certificate is
89 required from the other side of the connection, and whether it will
90 be validated if provided. It must be one of the three values
91 :const:`CERT_NONE` (certificates ignored), :const:`CERT_OPTIONAL` (not required,
92 but validated if provided), or :const:`CERT_REQUIRED` (required and
93 validated). If the value of this parameter is not :const:`CERT_NONE`, then
94 the ``ca_certs`` parameter must point to a file of CA certificates.
96 The ``ca_certs`` file contains a set of concatenated "certification authority" certificates,
97 which are used to validate certificates passed from the other end of the connection.
98 See the discussion of :ref:`ssl-certificates` for more information about how to arrange
99 the certificates in this file.
101 The parameter ``ssl_version`` specifies which version of the SSL protocol to use.
102 Typically, the server chooses a particular protocol version, and the client
103 must adapt to the server's choice. Most of the versions are not interoperable
104 with the other versions. If not specified, for client-side operation, the
105 default SSL version is SSLv3; for server-side operation, SSLv23. These
106 version selections provide the most compatibility with other versions.
108 Here's a table showing which versions in a client (down the side)
109 can connect to which versions in a server (along the top):
113 ======================== ========= ========= ========== =========
114 *client* / **server** **SSLv2** **SSLv3** **SSLv23** **TLSv1**
115 ------------------------ --------- --------- ---------- ---------
116 *SSLv2* yes no yes* no
117 *SSLv3* yes yes yes no
118 *SSLv23* yes no yes no
119 *TLSv1* no no yes yes
120 ======================== ========= ========= ========== =========
122 In some older versions of OpenSSL (for instance, 0.9.7l on OS X 10.4),
123 an SSLv2 client could not connect to an SSLv23 server.
125 .. function:: RAND_status()
127 Returns True if the SSL pseudo-random number generator has been
128 seeded with 'enough' randomness, and False otherwise. You can use
129 :func:`ssl.RAND_egd` and :func:`ssl.RAND_add` to increase the randomness
130 of the pseudo-random number generator.
132 .. function:: RAND_egd(path)
134 If you are running an entropy-gathering daemon (EGD) somewhere, and ``path``
135 is the pathname of a socket connection open to it, this will read
136 256 bytes of randomness from the socket, and add it to the SSL pseudo-random number generator
137 to increase the security of generated secret keys. This is typically only
138 necessary on systems without better sources of randomness.
140 See http://egd.sourceforge.net/ or http://prngd.sourceforge.net/ for
141 sources of entropy-gathering daemons.
143 .. function:: RAND_add(bytes, entropy)
145 Mixes the given ``bytes`` into the SSL pseudo-random number generator.
146 The parameter ``entropy`` (a float) is a lower bound on the entropy
147 contained in string (so you can always use :const:`0.0`).
148 See :rfc:`1750` for more information on sources of entropy.
150 .. function:: cert_time_to_seconds(timestring)
152 Returns a floating-point value containing a normal seconds-after-the-epoch time
153 value, given the time-string representing the "notBefore" or "notAfter" date
159 >>> ssl.cert_time_to_seconds("May 9 00:00:00 2007 GMT")
162 >>> time.ctime(ssl.cert_time_to_seconds("May 9 00:00:00 2007 GMT"))
163 'Wed May 9 00:00:00 2007'
166 .. function:: get_server_certificate (addr, ssl_version=PROTOCOL_SSLv3, ca_certs=None)
168 Given the address ``addr`` of an SSL-protected server, as a
169 (*hostname*, *port-number*) pair, fetches the server's certificate,
170 and returns it as a PEM-encoded string. If ``ssl_version`` is
171 specified, uses that version of the SSL protocol to attempt to
172 connect to the server. If ``ca_certs`` is specified, it should be
173 a file containing a list of root certificates, the same format as
174 used for the same parameter in :func:`wrap_socket`. The call will
175 attempt to validate the server certificate against that set of root
176 certificates, and will fail if the validation attempt fails.
178 .. function:: DER_cert_to_PEM_cert (DER_cert_bytes)
180 Given a certificate as a DER-encoded blob of bytes, returns a PEM-encoded
181 string version of the same certificate.
183 .. function:: PEM_cert_to_DER_cert (PEM_cert_string)
185 Given a certificate as an ASCII PEM string, returns a DER-encoded
186 sequence of bytes for that same certificate.
190 Value to pass to the ``cert_reqs`` parameter to :func:`sslobject`
191 when no certificates will be required or validated from the other
192 side of the socket connection.
194 .. data:: CERT_OPTIONAL
196 Value to pass to the ``cert_reqs`` parameter to :func:`sslobject`
197 when no certificates will be required from the other side of the
198 socket connection, but if they are provided, will be validated.
199 Note that use of this setting requires a valid certificate
200 validation file also be passed as a value of the ``ca_certs``
203 .. data:: CERT_REQUIRED
205 Value to pass to the ``cert_reqs`` parameter to :func:`sslobject`
206 when certificates will be required from the other side of the
207 socket connection. Note that use of this setting requires a valid certificate
208 validation file also be passed as a value of the ``ca_certs``
211 .. data:: PROTOCOL_SSLv2
213 Selects SSL version 2 as the channel encryption protocol.
215 .. data:: PROTOCOL_SSLv23
217 Selects SSL version 2 or 3 as the channel encryption protocol.
218 This is a setting to use with servers for maximum compatibility
219 with the other end of an SSL connection, but it may cause the
220 specific ciphers chosen for the encryption to be of fairly low
223 .. data:: PROTOCOL_SSLv3
225 Selects SSL version 3 as the channel encryption protocol.
226 For clients, this is the maximally compatible SSL variant.
228 .. data:: PROTOCOL_TLSv1
230 Selects TLS version 1 as the channel encryption protocol. This is
231 the most modern version, and probably the best choice for maximum
232 protection, if both sides can speak it.
238 .. method:: SSLSocket.read([nbytes=1024])
240 Reads up to ``nbytes`` bytes from the SSL-encrypted channel and returns them.
242 .. method:: SSLSocket.write(data)
244 Writes the ``data`` to the other side of the connection, using the
245 SSL channel to encrypt. Returns the number of bytes written.
247 .. method:: SSLSocket.getpeercert(binary_form=False)
249 If there is no certificate for the peer on the other end of the
250 connection, returns ``None``.
252 If the the parameter ``binary_form`` is :const:`False`, and a
253 certificate was received from the peer, this method returns a
254 :class:`dict` instance. If the certificate was not validated, the
255 dict is empty. If the certificate was validated, it returns a dict
256 with the keys ``subject`` (the principal for which the certificate
257 was issued), and ``notAfter`` (the time after which the certificate
258 should not be trusted). The certificate was already validated, so
259 the ``notBefore`` and ``issuer`` fields are not returned. If a
260 certificate contains an instance of the *Subject Alternative Name*
261 extension (see :rfc:`3280`), there will also be a
262 ``subjectAltName`` key in the dictionary.
264 The "subject" field is a tuple containing the sequence of relative
265 distinguished names (RDNs) given in the certificate's data
266 structure for the principal, and each RDN is a sequence of
269 {'notAfter': 'Feb 16 16:54:50 2013 GMT',
270 'subject': ((('countryName', u'US'),),
271 (('stateOrProvinceName', u'Delaware'),),
272 (('localityName', u'Wilmington'),),
273 (('organizationName', u'Python Software Foundation'),),
274 (('organizationalUnitName', u'SSL'),),
275 (('commonName', u'somemachine.python.org'),))}
277 If the ``binary_form`` parameter is :const:`True`, and a
278 certificate was provided, this method returns the DER-encoded form
279 of the entire certificate as a sequence of bytes, or :const:`None` if the
280 peer did not provide a certificate. This return
281 value is independent of validation; if validation was required
282 (:const:`CERT_OPTIONAL` or :const:`CERT_REQUIRED`), it will have
283 been validated, but if :const:`CERT_NONE` was used to establish the
284 connection, the certificate, if present, will not have been validated.
286 .. method:: SSLSocket.cipher()
288 Returns a three-value tuple containing the name of the cipher being
289 used, the version of the SSL protocol that defines its use, and the
290 number of secret bits being used. If no connection has been
291 established, returns ``None``.
294 .. index:: single: certificates
296 .. index:: single: X509 certificate
298 .. _ssl-certificates:
303 Certificates in general are part of a public-key / private-key system. In this system, each *principal*,
304 (which may be a machine, or a person, or an organization) is assigned a unique two-part encryption key.
305 One part of the key is public, and is called the *public key*; the other part is kept secret, and is called
306 the *private key*. The two parts are related, in that if you encrypt a message with one of the parts, you can
307 decrypt it with the other part, and **only** with the other part.
309 A certificate contains information about two principals. It contains
310 the name of a *subject*, and the subject's public key. It also
311 contains a statement by a second principal, the *issuer*, that the
312 subject is who he claims to be, and that this is indeed the subject's
313 public key. The issuer's statement is signed with the issuer's
314 private key, which only the issuer knows. However, anyone can verify
315 the issuer's statement by finding the issuer's public key, decrypting
316 the statement with it, and comparing it to the other information in
317 the certificate. The certificate also contains information about the
318 time period over which it is valid. This is expressed as two fields,
319 called "notBefore" and "notAfter".
321 In the Python use of certificates, a client or server
322 can use a certificate to prove who they are. The other
323 side of a network connection can also be required to produce a certificate,
324 and that certificate can be validated to the satisfaction
325 of the client or server that requires such validation.
326 The connection attempt can be set to raise an exception if
327 the validation fails. Validation is done
328 automatically, by the underlying OpenSSL framework; the
329 application need not concern itself with its mechanics.
330 But the application does usually need to provide
331 sets of certificates to allow this process to take place.
333 Python uses files to contain certificates. They should be formatted
334 as "PEM" (see :rfc:`1422`), which is a base-64 encoded form wrapped
335 with a header line and a footer line::
337 -----BEGIN CERTIFICATE-----
338 ... (certificate in base64 PEM encoding) ...
339 -----END CERTIFICATE-----
341 The Python files which contain certificates can contain a sequence
342 of certificates, sometimes called a *certificate chain*. This chain
343 should start with the specific certificate for the principal who "is"
344 the client or server, and then the certificate for the issuer of that
345 certificate, and then the certificate for the issuer of *that* certificate,
346 and so on up the chain till you get to a certificate which is *self-signed*,
347 that is, a certificate which has the same subject and issuer,
348 sometimes called a *root certificate*. The certificates should just
349 be concatenated together in the certificate file. For example, suppose
350 we had a three certificate chain, from our server certificate to the
351 certificate of the certification authority that signed our server certificate,
352 to the root certificate of the agency which issued the certification authority's
355 -----BEGIN CERTIFICATE-----
356 ... (certificate for your server)...
357 -----END CERTIFICATE-----
358 -----BEGIN CERTIFICATE-----
359 ... (the certificate for the CA)...
360 -----END CERTIFICATE-----
361 -----BEGIN CERTIFICATE-----
362 ... (the root certificate for the CA's issuer)...
363 -----END CERTIFICATE-----
365 If you are going to require validation of the other side of the connection's
366 certificate, you need to provide a "CA certs" file, filled with the certificate
367 chains for each issuer you are willing to trust. Again, this file just
368 contains these chains concatenated together. For validation, Python will
369 use the first chain it finds in the file which matches.
370 Some "standard" root certificates are available from various certification
372 `CACert.org <http://www.cacert.org/index.php?id=3>`_,
373 `Thawte <http://www.thawte.com/roots/>`_,
374 `Verisign <http://www.verisign.com/support/roots.html>`_,
375 `Positive SSL <http://www.PositiveSSL.com/ssl-certificate-support/cert_installation/UTN-USERFirst-Hardware.crt>`_ (used by python.org),
376 `Equifax and GeoTrust <http://www.geotrust.com/resources/root_certificates/index.asp>`_.
378 In general, if you are using
379 SSL3 or TLS1, you don't need to put the full chain in your "CA certs" file;
380 you only need the root certificates, and the remote peer is supposed to
381 furnish the other certificates necessary to chain from its certificate to
383 See :rfc:`4158` for more discussion of the way in which
384 certification chains can be built.
386 If you are going to create a server that provides SSL-encrypted
387 connection services, you will need to acquire a certificate for that
388 service. There are many ways of acquiring appropriate certificates,
389 such as buying one from a certification authority. Another common
390 practice is to generate a self-signed certificate. The simplest
391 way to do this is with the OpenSSL package, using something like
394 % openssl req -new -x509 -days 365 -nodes -out cert.pem -keyout cert.pem
395 Generating a 1024 bit RSA private key
397 .............................++++++
398 writing new private key to 'cert.pem'
400 You are about to be asked to enter information that will be incorporated
401 into your certificate request.
402 What you are about to enter is what is called a Distinguished Name or a DN.
403 There are quite a few fields but you can leave some blank
404 For some fields there will be a default value,
405 If you enter '.', the field will be left blank.
407 Country Name (2 letter code) [AU]:US
408 State or Province Name (full name) [Some-State]:MyState
409 Locality Name (eg, city) []:Some City
410 Organization Name (eg, company) [Internet Widgits Pty Ltd]:My Organization, Inc.
411 Organizational Unit Name (eg, section) []:My Group
412 Common Name (eg, YOUR name) []:myserver.mygroup.myorganization.com
413 Email Address []:ops@myserver.mygroup.myorganization.com
416 The disadvantage of a self-signed certificate is that it is its
417 own root certificate, and no one else will have it in their cache
418 of known (and trusted) root certificates.
424 Testing for SSL support
425 ^^^^^^^^^^^^^^^^^^^^^^^
427 To test for the presence of SSL support in a Python installation, user code should use the following idiom::
434 [ do something that requires SSL support ]
436 Client-side operation
437 ^^^^^^^^^^^^^^^^^^^^^
439 This example connects to an SSL server, prints the server's address and certificate,
440 sends some bytes, and reads part of the response::
442 import socket, ssl, pprint
444 s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
446 # require a certificate from the server
447 ssl_sock = ssl.wrap_socket(s,
448 ca_certs="/etc/ca_certs_file",
449 cert_reqs=ssl.CERT_REQUIRED)
451 ssl_sock.connect(('www.verisign.com', 443))
453 print repr(ssl_sock.getpeername())
454 print ssl_sock.cipher()
455 print pprint.pformat(ssl_sock.getpeercert())
457 # Set a simple HTTP request -- use httplib in actual code.
458 ssl_sock.write("""GET / HTTP/1.0\r
459 Host: www.verisign.com\r\n\r\n""")
461 # Read a chunk of data. Will not necessarily
462 # read all the data returned by the server.
463 data = ssl_sock.read()
465 # note that closing the SSLSocket will also close the underlying socket
468 As of September 6, 2007, the certificate printed by this program
471 {'notAfter': 'May 8 23:59:59 2009 GMT',
472 'subject': ((('serialNumber', u'2497886'),),
473 (('1.3.6.1.4.1.311.60.2.1.3', u'US'),),
474 (('1.3.6.1.4.1.311.60.2.1.2', u'Delaware'),),
475 (('countryName', u'US'),),
476 (('postalCode', u'94043'),),
477 (('stateOrProvinceName', u'California'),),
478 (('localityName', u'Mountain View'),),
479 (('streetAddress', u'487 East Middlefield Road'),),
480 (('organizationName', u'VeriSign, Inc.'),),
481 (('organizationalUnitName',
482 u'Production Security Services'),),
483 (('organizationalUnitName',
484 u'Terms of use at www.verisign.com/rpa (c)06'),),
485 (('commonName', u'www.verisign.com'),))}
487 which is a fairly poorly-formed ``subject`` field.
489 Server-side operation
490 ^^^^^^^^^^^^^^^^^^^^^
492 For server operation, typically you'd need to have a server certificate, and private key, each in a file.
493 You'd open a socket, bind it to a port, call :meth:`listen` on it, then start waiting for clients
498 bindsocket = socket.socket()
499 bindsocket.bind(('myaddr.mydomain.com', 10023))
502 When one did, you'd call :meth:`accept` on the socket to get the new socket from the other
503 end, and use :func:`wrap_socket` to create a server-side SSL context for it::
506 newsocket, fromaddr = bindsocket.accept()
507 connstream = ssl.wrap_socket(newsocket,
509 certfile="mycertfile",
511 ssl_protocol=ssl.PROTOCOL_TLSv1)
512 deal_with_client(connstream)
514 Then you'd read data from the ``connstream`` and do something with it till you are finished with the client (or the client is finished with you)::
516 def deal_with_client(connstream):
518 data = connstream.read()
519 # null data means the client is finished with us
521 if not do_something(connstream, data):
522 # we'll assume do_something returns False
523 # when we're finished with client
525 data = connstream.read()
526 # finished with client
529 And go back to listening for new client connections.
534 Class :class:`socket.socket`
535 Documentation of underlying :mod:`socket` class
537 `Introducing SSL and Certificates using OpenSSL <http://old.pseudonym.org/ssl/wwwj-index.html>`_
540 `RFC 1422: Privacy Enhancement for Internet Electronic Mail: Part II: Certificate-Based Key Management <http://www.ietf.org/rfc/rfc1422>`_
543 `RFC 1750: Randomness Recommendations for Security <http://www.ietf.org/rfc/rfc1750>`_
546 `RFC 3280: Internet X.509 Public Key Infrastructure Certificate and CRL Profile <http://www.ietf.org/rfc/rfc3280>`_