:mod:`ssl` --- SSL wrapper for socket objects ==================================================================== .. module:: ssl :synopsis: SSL wrapper for socket objects .. moduleauthor:: Bill Janssen .. sectionauthor:: Bill Janssen This module provides access to Transport Layer Security (often known as "Secure Sockets Layer") encryption and peer authentication facilities for network sockets, both client-side and server-side. This module uses the OpenSSL library. It is available on all modern Unix systems, Windows, Mac OS X, and probably additional platforms, as long as OpenSSL is installed on that platform. .. note:: Some behavior may be platform dependent, since calls are made to the operating system socket APIs. This section documents the objects and functions in the ``ssl`` module; for more general information about TLS, SSL, and certificates, the reader is referred to the documents in the :ref:`ssl-references` section. This module defines a class, :class:`ssl.sslsocket`, which is derived from the :class:`socket.socket` type, and supports additional :meth:`read` and :meth:`write` methods, along with a method, :meth:`getpeercert`, to retrieve the certificate of the other side of the connection. This module defines the following functions, exceptions, and constants: .. function:: cert_time_to_seconds(timestring) Returns a floating-point value containing a normal seconds-after-the-epoch time value, given the time-string representing the "notBefore" or "notAfter" date from a certificate. Here's an example:: >>> import ssl >>> ssl.cert_time_to_seconds("May 9 00:00:00 2007 GMT") 1178694000.0 >>> import time >>> time.ctime(ssl.cert_time_to_seconds("May 9 00:00:00 2007 GMT")) 'Wed May 9 00:00:00 2007' >>> .. exception:: sslerror Raised to signal an error from the underlying SSL implementation. This signifies some problem in the higher-level encryption and authentication layer that's superimposed on the underlying network connection. .. data:: CERT_NONE Value to pass to the ``cert_reqs`` parameter to :func:`sslobject` when no certificates will be required or validated from the other side of the socket connection. .. data:: CERT_OPTIONAL Value to pass to the ``cert_reqs`` parameter to :func:`sslobject` when no certificates will be required from the other side of the socket connection, but if they are provided, will be validated. Note that use of this setting requires a valid certificate validation file also be passed as a value of the ``ca_certs`` parameter. .. data:: CERT_REQUIRED Value to pass to the ``cert_reqs`` parameter to :func:`sslobject` when certificates will be required from the other side of the socket connection. Note that use of this setting requires a valid certificate validation file also be passed as a value of the ``ca_certs`` parameter. .. data:: PROTOCOL_SSLv2 Selects SSL version 2 as the channel encryption protocol. .. data:: PROTOCOL_SSLv23 Selects SSL version 2 or 3 as the channel encryption protocol. This is a setting to use for maximum compatibility with the other end of an SSL connection, but it may cause the specific ciphers chosen for the encryption to be of fairly low quality. .. data:: PROTOCOL_SSLv3 Selects SSL version 3 as the channel encryption protocol. .. data:: PROTOCOL_TLSv1 Selects SSL version 2 as the channel encryption protocol. This is the most modern version, and probably the best choice for maximum protection, if both sides can speak it. .. _ssl-certificates: Certificates ------------ Certificates in general are part of a public-key / private-key system. In this system, each *principal*, (which may be a machine, or a person, or an organization) is assigned a unique two-part encryption key. One part of the key is public, and is called the *public key*; the other part is kept secret, and is called the *private key*. The two parts are related, in that if you encrypt a message with one of the parts, you can decrypt it with the other part, and **only** with the other part. A certificate contains information about two principals. It contains the name of a *subject*, and the subject's public key. It also contains a statement by a second principal, the *issuer*, that the subject is who he claims to be, and that this is indeed the subject's public key. The issuer's statement is signed with the issuer's private key, which only the issuer knows. However, anyone can verify the issuer's statement by finding the issuer's public key, decrypting the statement with it, and comparing it to the other information in the certificate. The certificate also contains information about the time period over which it is valid. This is expressed as two fields, called "notBefore" and "notAfter". In the Python use of certificates, a client or server can use a certificate to prove who they are. The other side of a network connection can also be required to produce a certificate, and that certificate can be validated to the satisfaction of the client or server that requires such validation. The connection can be set to fail automatically if such validation is not achieved. Python uses files to contain certificates. They should be formatted as "PEM" (see :rfc:`1422`), which is a base-64 encoded form wrapped with a header line and a footer line:: -----BEGIN CERTIFICATE----- ... (certificate in base64 PEM encoding) ... -----END CERTIFICATE----- The Python files which contain certificates can contain a sequence of certificates, sometimes called a *certificate chain*. This chain should start with the specific certificate for the principal who "is" the client or server, and then the certificate for the issuer of that certificate, and then the certificate for the issuer of *that* certificate, and so on up the chain till you get to a certificate which is *self-signed*, that is, a certificate which has the same subject and issuer, sometimes called a *root certificate*. The certificates should just be concatenated together in the certificate file. For example, suppose we had a three certificate chain, from our server certificate to the certificate of the certification authority that signed our server certificate, to the root certificate of the agency which issued the certification authority's certificate:: -----BEGIN CERTIFICATE----- ... (certificate for your server)... -----END CERTIFICATE----- -----BEGIN CERTIFICATE----- ... (the certificate for the CA)... -----END CERTIFICATE----- -----BEGIN CERTIFICATE----- ... (the root certificate for the CA's issuer)... -----END CERTIFICATE----- If you are going to require validation of the other side of the connection's certificate, you need to provide a "CA certs" file, filled with the certificate chains for each issuer you are willing to trust. Again, this file just contains these chains concatenated together. For validation, Python will use the first chain it finds in the file which matches. Some "standard" root certificates are available at http://www.thawte.com/roots/ (for Thawte roots) and http://www.verisign.com/support/roots.html (for Verisign roots). sslsocket Objects ----------------- .. class:: sslsocket(sock [, keyfile=None, certfile=None, server_side=False, cert_reqs=CERT_NONE, ssl_version=PROTOCOL_SSLv23, ca_certs=None]) Takes an instance ``sock`` of :class:`socket.socket`, and returns an instance of a subtype of :class:`socket.socket` which wraps the underlying socket in an SSL context. For client-side sockets, the context construction is lazy; if the underlying socket isn't connected yet, the context construction will be performed after :meth:`connect` is called on the socket. The ``keyfile`` and ``certfile`` parameters specify optional files which contain a certificate to be used to identify the local side of the connection. See the above discussion of :ref:`ssl-certificates` for more information on how the certificate is stored in the ``certfile``. Often the private key is stored in the same file as the certificate; in this case, only the ``certfile`` parameter need be passed. If the private key is stored in a separate file, both parameters must be used. If the private key is stored in the ``certfile``, it should come before the first certificate in the certificate chain:: -----BEGIN RSA PRIVATE KEY----- ... (private key in base64 encoding) ... -----END RSA PRIVATE KEY----- -----BEGIN CERTIFICATE----- ... (certificate in base64 PEM encoding) ... -----END CERTIFICATE----- The parameter ``server_side`` is a boolean which identifies whether server-side or client-side behavior is desired from this socket. The parameter ``cert_reqs`` specifies whether a certificate is required from the other side of the connection, and whether it will be validated if provided. It must be one of the three values :const:`CERT_NONE` (certificates ignored), :const:`CERT_OPTIONAL` (not required, but validated if provided), or :const:`CERT_REQUIRED` (required and validated). If the value of this parameter is not :const:`CERT_NONE`, then the ``ca_certs`` parameter must point to a file of CA certificates. The parameter ``ssl_version`` specifies which version of the SSL protocol to use. Typically, the server specifies this, and a client connecting to it must use the same protocol. An SSL server using :const:`PROTOCOL_SSLv23` can understand a client connecting via SSL2, SSL3, or TLS1, but a client using :const:`PROTOCOL_SSLv23` can only connect to an SSL2 server. The ``ca_certs`` file contains a set of concatenated "certification authority" certificates, which are used to validate certificates passed from the other end of the connection. See the above discussion of :ref:`ssl-certificates` for more information about how to arrange the certificates in this file. .. method:: sslsocket.read([nbytes]) Reads up to ``nbytes`` bytes from the SSL-encrypted channel and returns them. .. method:: sslsocket.write(data) Writes the ``data`` to the other side of the connection, using the SSL channel to encrypt. Returns the number of bytes written. .. method:: sslsocket.getpeercert() If there is no certificate for the peer on the other end of the connection, returns ``None``. If a certificate was received from the peer, but not validated, returns an empty ``dict`` instance. If a certificate was received and validated, returns a ``dict`` instance with the fields ``subject`` (the principal for which the certificate was issued), ``issuer`` (the signer of the certificate), ``notBefore`` (the time before which the certificate should not be trusted), and ``notAfter`` (the time after which the certificate should not be trusted) filled in. The "subject" and "issuer" fields are themselves dictionaries containing the fields given in the certificate's data structure for each principal:: {'issuer': {'commonName': u'somemachine.python.org', 'countryName': u'US', 'localityName': u'Wilmington', 'organizationName': u'Python Software Foundation', 'organizationalUnitName': u'SSL', 'stateOrProvinceName': u'Delaware'}, 'subject': {'commonName': u'somemachine.python.org', 'countryName': u'US', 'localityName': u'Wilmington', 'organizationName': u'Python Software Foundation', 'organizationalUnitName': u'SSL', 'stateOrProvinceName': u'Delaware'}, 'notAfter': 'Sep 4 21:54:26 2007 GMT', 'notBefore': 'Aug 25 21:54:26 2007 GMT', 'version': 2} This certificate is said to be *self-signed*, because the subject and issuer are the same entity. The *version* field refers to the X509 version that's used for the certificate. .. method:: sslsocket.ssl_shutdown() Closes the SSL context (if any) over the socket, but leaves the socket connection open for further use, if both sides are willing. This is different from :meth:`socket.socket.shutdown`, which will close the connection, but leave the local socket available for further use. Examples -------- Testing for SSL support ^^^^^^^^^^^^^^^^^^^^^^^ To test for the presence of SSL support in a Python installation, user code should use the following idiom:: try: import ssl except ImportError: pass else: [ do something that requires SSL support ] Client-side operation ^^^^^^^^^^^^^^^^^^^^^ This example connects to an SSL server, prints the server's address and certificate, sends some bytes, and reads part of the response:: import socket, ssl, pprint s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) ssl_sock = ssl.sslsocket(s, ca_certs="/etc/ca_certs_file", cert_reqs=ssl.CERT_REQUIRED) ssl_sock.connect(('www.verisign.com', 443)) print(repr(ssl_sock.getpeername())) pprint.pprint(ssl_sock.getpeercert()) # Set a simple HTTP request -- use httplib in actual code. ssl_sock.write("""GET / HTTP/1.0\r Host: www.verisign.com\r\n\r\n""") # Read a chunk of data. Will not necessarily # read all the data returned by the server. data = ssl_sock.read() # note that closing the sslsocket will also close the underlying socket ssl_sock.close() As of August 25, 2007, the certificate printed by this program looked like this:: {'issuer': {'commonName': u'VeriSign Class 3 Extended Validation SSL SGC CA', 'countryName': u'US', 'organizationName': u'VeriSign, Inc.', 'organizationalUnitName': u'Terms of use at https://www.verisign.com/rpa (c)06'}, 'subject': {'1.3.6.1.4.1.311.60.2.1.2': u'Delaware', '1.3.6.1.4.1.311.60.2.1.3': u'US', 'commonName': u'www.verisign.com', 'countryName': u'US', 'localityName': u'Mountain View', 'organizationName': u'VeriSign, Inc.', 'organizationalUnitName': u'Terms of use at www.verisign.com/rpa (c)06', 'postalCode': u'94043', 'serialNumber': u'2497886', 'stateOrProvinceName': u'California', 'streetAddress': u'487 East Middlefield Road'}, 'notAfter': 'May 8 23:59:59 2009 GMT', 'notBefore': 'May 9 00:00:00 2007 GMT', 'version': 2} Server-side operation ^^^^^^^^^^^^^^^^^^^^^ For server operation, typically you'd need to have a server certificate, and private key, each in a file. You'd open a socket, bind it to a port, call :meth:`listen` on it, then start waiting for clients to connect:: import socket, ssl bindsocket = socket.socket() bindsocket.bind(('myaddr.mydomain.com', 10023)) bindsocket.listen(5) When one did, you'd call :meth:`accept` on the socket to get the new socket from the other end, and use :func:`sslsocket` to create a server-side SSL context for it:: while True: newsocket, fromaddr = bindsocket.accept() connstream = ssl.sslsocket(newsocket, server_side=True, certfile="mycertfile", keyfile="mykeyfile", ssl_protocol=ssl.PROTOCOL_TLSv1) deal_with_client(connstream) 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):: def deal_with_client(connstream): data = connstream.read() # null data means the client is finished with us while data: if not do_something(connstream, data): # we'll assume do_something returns False when we're finished with client break data = connstream.read() # finished with client connstream.close() And go back to listening for new client connections. .. _ssl-references: References ---------- Class :class:`socket.socket` Documentation of underlying :mod:`socket` class `Introducing SSL and Certificates using OpenSSL `_, by Frederick J. Hirsch `Privacy Enhancement for Internet Electronic Mail: Part II: Certificate-Based Key Management`, :rfc:`1422`, by Steve Kent