18.2. ssl — TLS/SSL wrapper for socket objects

Python 3.3

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18.2. `ssl` — TLS/SSL wrapper for socket objects

Source code: Lib/ssl.py

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. The installed version of OpenSSL may also cause variations in behavior.

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 “See Also” section at the bottom.

This module provides a class, ssl.SSLSocket, which is derived from the socket.socket type, and provides a socket-like wrapper that also encrypts and decrypts the data going over the socket with SSL. It supports additional methods such as getpeercert(), which retrieves the certificate of the other side of the connection, and cipher(),which retrieves the cipher being used for the secure connection.

For more sophisticated applications, the ssl.SSLContext class helps manage settings and certificates, which can then be inherited by SSL sockets created through the SSLContext.wrap_socket() method.

18.2.1. Functions, Constants, and Exceptions

exception ssl.SSLError

Raised to signal an error from the underlying SSL implementation (currently provided by the OpenSSL library). This signifies some problem in the higher-level encryption and authentication layer that’s superimposed on the underlying network connection. This error is a subtype of OSError. The error code and message of SSLError instances are provided by the OpenSSL library.

Changed in version 3.3: SSLError used to be a subtype of socket.error.

library

A string mnemonic designating the OpenSSL submodule in which the error occurred, such as SSL, PEM or X509. The range of possible values depends on the OpenSSL version.

New in version 3.3.

reason

A string mnemonic designating the reason this error occurred, for example CERTIFICATE_VERIFY_FAILED. The range of possible values depends on the OpenSSL version.

New in version 3.3.

exception ssl.SSLZeroReturnError

A subclass of SSLError raised when trying to read or write and the SSL connection has been closed cleanly. Note that this doesn’t mean that the underlying transport (read TCP) has been closed.

New in version 3.3.

exception ssl.SSLWantReadError

A subclass of SSLError raised by a non-blocking SSL socket when trying to read or write data, but more data needs to be received on the underlying TCP transport before the request can be fulfilled.

New in version 3.3.

exception ssl.SSLWantWriteError

A subclass of SSLError raised by a non-blocking SSL socket when trying to read or write data, but more data needs to be sent on the underlying TCP transport before the request can be fulfilled.

New in version 3.3.

exception ssl.SSLSyscallError

A subclass of SSLError raised when a system error was encountered while trying to fulfill an operation on a SSL socket. Unfortunately, there is no easy way to inspect the original errno number.

New in version 3.3.

exception ssl.SSLEOFError

A subclass of SSLError raised when the SSL connection has been terminated abruptly. Generally, you shouldn’t try to reuse the underlying transport when this error is encountered.

New in version 3.3.

exception ssl.CertificateError: Raised to signal an error with a certificate (such as mismatching hostname). Certificate errors detected by OpenSSL, though, raise an SSLError.

18.2.1.1. Socket creation

The following function allows for standalone socket creation. Starting from Python 3.2, it can be more flexible to use SSLContext.wrap_socket() instead.

ssl.wrap_socket(sock, keyfile=None, certfile=None, server_side=False, cert_reqs=CERT_NONE, ssl_version={see docs}, ca_certs=None, do_handshake_on_connect=True, suppress_ragged_eofs=True, ciphers=None)

Takes an instance sock of socket.socket, and returns an instance of ssl.SSLSocket, a subtype of 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 connect() is called on the socket. For server-side sockets, if the socket has no remote peer, it is assumed to be a listening socket, and the server-side SSL wrapping is automatically performed on client connections accepted via the accept() method. wrap_socket() may raise SSLError.

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 discussion of Certificates for more information on how the certificate is stored in the certfile.

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 CERT_NONE (certificates ignored), CERT_OPTIONAL (not required, but validated if provided), or CERT_REQUIRED (required and validated). If the value of this parameter is not CERT_NONE, then the ca_certs parameter must point to a file of CA certificates.

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 discussion of Certificates for more information about how to arrange the certificates in this file.

The parameter ssl_version specifies which version of the SSL protocol to use. Typically, the server chooses a particular protocol version, and the client must adapt to the server’s choice. Most of the versions are not interoperable with the other versions. If not specified, the default is PROTOCOL_SSLv23; it provides the most compatibility with other versions.

Here’s a table showing which versions in a client (down the side) can connect to which versions in a server (along the top):

client / server SSLv2 SSLv3 SSLv23 TLSv1

SSLv2 yes no yes no

SSLv3 no yes yes no

SSLv23 yes no yes no

TLSv1 no no yes yes

Note

Which connections succeed will vary depending on the version of OpenSSL. For instance, in some older versions of OpenSSL (such as 0.9.7l on OS X 10.4), an SSLv2 client could not connect to an SSLv23 server. Another example: beginning with OpenSSL 1.0.0, an SSLv23 client will not actually attempt SSLv2 connections unless you explicitly enable SSLv2 ciphers; for example, you might specify "ALL" or "SSLv2" as the ciphers parameter to enable them.

The ciphers parameter sets the available ciphers for this SSL object. It should be a string in the OpenSSL cipher list format.

The parameter do_handshake_on_connect specifies whether to do the SSL handshake automatically after doing a socket.connect(), or whether the application program will call it explicitly, by invoking the SSLSocket.do_handshake() method. Calling SSLSocket.do_handshake() explicitly gives the program control over the blocking behavior of the socket I/O involved in the handshake.

The parameter suppress_ragged_eofs specifies how the SSLSocket.recv() method should signal unexpected EOF from the other end of the connection. If specified as True (the default), it returns a normal EOF (an empty bytes object) in response to unexpected EOF errors raised from the underlying socket; if False, it will raise the exceptions back to the caller.

Changed in version 3.2: New optional argument ciphers.

18.2.1.2. Random generation

ssl.RAND_bytes(num)

Returns num cryptographically strong pseudo-random bytes. Raises an SSLError if the PRNG has not been seeded with enough data or if the operation is not supported by the current RAND method. RAND_status() can be used to check the status of the PRNG and RAND_add() can be used to seed the PRNG.

Read the Wikipedia article, Cryptographically secure pseudorandom number generator (CSPRNG), to get the requirements of a cryptographically generator.

New in version 3.3.

ssl.RAND_pseudo_bytes(num)

Returns (bytes, is_cryptographic): bytes are num pseudo-random bytes, is_cryptographic is True if the bytes generated are cryptographically strong. Raises an SSLError if the operation is not supported by the current RAND method.

Generated pseudo-random byte sequences will be unique if they are of sufficient length, but are not necessarily unpredictable. They can be used for non-cryptographic purposes and for certain purposes in cryptographic protocols, but usually not for key generation etc.

New in version 3.3.

ssl.RAND_status(): Returns True if the SSL pseudo-random number generator has been seeded with ‘enough’ randomness, and False otherwise. You can use ssl.RAND_egd() and ssl.RAND_add() to increase the randomness of the pseudo-random number generator.

ssl.RAND_egd(path)

If you are running an entropy-gathering daemon (EGD) somewhere, and path is the pathname of a socket connection open to it, this will read 256 bytes of randomness from the socket, and add it to the SSL pseudo-random number generator to increase the security of generated secret keys. This is typically only necessary on systems without better sources of randomness.

See http://egd.sourceforge.net/ or http://prngd.sourceforge.net/ for sources of entropy-gathering daemons.

ssl.RAND_add(bytes, entropy): Mixes the given bytes into the SSL pseudo-random number generator. The parameter entropy (a float) is a lower bound on the entropy contained in string (so you can always use 0.0). See RFC 1750 for more information on sources of entropy.

18.2.1.3. Certificate handling

ssl.match_hostname(cert, hostname)

Verify that cert (in decoded format as returned by SSLSocket.getpeercert()) matches the given hostname. The rules applied are those for checking the identity of HTTPS servers as outlined in RFC 2818 and RFC 6125, except that IP addresses are not currently supported. In addition to HTTPS, this function should be suitable for checking the identity of servers in various SSL-based protocols such as FTPS, IMAPS, POPS and others.

CertificateError is raised on failure. On success, the function returns nothing:

>>> cert = {'subject': ((('commonName', 'example.com'),),)}
>>> ssl.match_hostname(cert, "example.com")
>>> ssl.match_hostname(cert, "example.org")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/py3k/Lib/ssl.py", line 130, in match_hostname
ssl.CertificateError: hostname 'example.org' doesn't match 'example.com'

New in version 3.2.

Changed in version 3.3.3: The function now follows RFC 6125, section 6.4.3 and does neither match multiple wildcards (e.g. *.*.com or *a*.example.org) nor a wildcard inside an internationalized domain names (IDN) fragment. IDN A-labels such as www*.xn--pthon-kva.org are still supported, but x*.python.org no longer matches xn--tda.python.org.

ssl.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'

ssl.get_server_certificate(addr, ssl_version=PROTOCOL_SSLv3, ca_certs=None)

Given the address addr of an SSL-protected server, as a (hostname, port-number) pair, fetches the server’s certificate, and returns it as a PEM-encoded string. If ssl_version is specified, uses that version of the SSL protocol to attempt to connect to the server. If ca_certs is specified, it should be a file containing a list of root certificates, the same format as used for the same parameter in wrap_socket(). The call will attempt to validate the server certificate against that set of root certificates, and will fail if the validation attempt fails.

Changed in version 3.3: This function is now IPv6-compatible.

ssl.DER_cert_to_PEM_cert(DER_cert_bytes): Given a certificate as a DER-encoded blob of bytes, returns a PEM-encoded string version of the same certificate.

ssl.PEM_cert_to_DER_cert(PEM_cert_string): Given a certificate as an ASCII PEM string, returns a DER-encoded sequence of bytes for that same certificate.

18.2.1.4. Constants

ssl.CERT_NONE

Possible value for SSLContext.verify_mode, or the cert_reqs parameter to wrap_socket(). In this mode (the default), no certificates will be required from the other side of the socket connection. If a certificate is received from the other end, no attempt to validate it is made.

See the discussion of Security considerations below.

ssl.CERT_OPTIONAL

Possible value for SSLContext.verify_mode, or the cert_reqs parameter to wrap_socket(). In this mode no certificates will be required from the other side of the socket connection; but if they are provided, validation will be attempted and an SSLError will be raised on failure.

Use of this setting requires a valid set of CA certificates to be passed, either to SSLContext.load_verify_locations() or as a value of the ca_certs parameter to wrap_socket().

ssl.CERT_REQUIRED

Possible value for SSLContext.verify_mode, or the cert_reqs parameter to wrap_socket(). In this mode, certificates are required from the other side of the socket connection; an SSLError will be raised if no certificate is provided, or if its validation fails.

Use of this setting requires a valid set of CA certificates to be passed, either to SSLContext.load_verify_locations() or as a value of the ca_certs parameter to wrap_socket().

ssl.PROTOCOL_SSLv2

Selects SSL version 2 as the channel encryption protocol.

This protocol is not available if OpenSSL is compiled with OPENSSL_NO_SSL2 flag.

Warning

SSL version 2 is insecure. Its use is highly discouraged.

ssl.PROTOCOL_SSLv23: Selects SSL version 2 or 3 as the channel encryption protocol. This is a setting to use with servers 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.

ssl.PROTOCOL_SSLv3: Selects SSL version 3 as the channel encryption protocol. For clients, this is the maximally compatible SSL variant.

ssl.PROTOCOL_TLSv1: Selects TLS version 1 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.OP_ALL

Enables workarounds for various bugs present in other SSL implementations. This option is set by default. It does not necessarily set the same flags as OpenSSL’s SSL_OP_ALL constant.

New in version 3.2.

ssl.OP_NO_SSLv2

Prevents an SSLv2 connection. This option is only applicable in conjunction with PROTOCOL_SSLv23. It prevents the peers from choosing SSLv2 as the protocol version.

New in version 3.2.

ssl.OP_NO_SSLv3

Prevents an SSLv3 connection. This option is only applicable in conjunction with PROTOCOL_SSLv23. It prevents the peers from choosing SSLv3 as the protocol version.

New in version 3.2.

ssl.OP_NO_TLSv1

Prevents a TLSv1 connection. This option is only applicable in conjunction with PROTOCOL_SSLv23. It prevents the peers from choosing TLSv1 as the protocol version.

New in version 3.2.

ssl.OP_CIPHER_SERVER_PREFERENCE

Use the server’s cipher ordering preference, rather than the client’s. This option has no effect on client sockets and SSLv2 server sockets.

New in version 3.3.

ssl.OP_SINGLE_DH_USE

Prevents re-use of the same DH key for distinct SSL sessions. This improves forward secrecy but requires more computational resources. This option only applies to server sockets.

New in version 3.3.

ssl.OP_SINGLE_ECDH_USE

Prevents re-use of the same ECDH key for distinct SSL sessions. This improves forward secrecy but requires more computational resources. This option only applies to server sockets.

New in version 3.3.

ssl.OP_NO_COMPRESSION

Disable compression on the SSL channel. This is useful if the application protocol supports its own compression scheme.

This option is only available with OpenSSL 1.0.0 and later.

New in version 3.3.

ssl.HAS_ECDH

Whether the OpenSSL library has built-in support for Elliptic Curve-based Diffie-Hellman key exchange. This should be true unless the feature was explicitly disabled by the distributor.

New in version 3.3.

ssl.HAS_SNI

Whether the OpenSSL library has built-in support for the Server Name Indication extension to the SSLv3 and TLSv1 protocols (as defined in RFC 4366). When true, you can use the server_hostname argument to SSLContext.wrap_socket().

New in version 3.2.

ssl.HAS_NPN

Whether the OpenSSL library has built-in support for Next Protocol Negotiation as described in the NPN draft specification. When true, you can use the SSLContext.set_npn_protocols() method to advertise which protocols you want to support.

New in version 3.3.

ssl.CHANNEL_BINDING_TYPES

List of supported TLS channel binding types. Strings in this list can be used as arguments to SSLSocket.get_channel_binding().

New in version 3.3.

ssl.OPENSSL_VERSION

The version string of the OpenSSL library loaded by the interpreter:

>>> ssl.OPENSSL_VERSION
'OpenSSL 0.9.8k 25 Mar 2009'

New in version 3.2.

ssl.OPENSSL_VERSION_INFO

A tuple of five integers representing version information about the OpenSSL library:

>>> ssl.OPENSSL_VERSION_INFO
(0, 9, 8, 11, 15)

New in version 3.2.

ssl.OPENSSL_VERSION_NUMBER

The raw version number of the OpenSSL library, as a single integer:

>>> ssl.OPENSSL_VERSION_NUMBER
9470143
>>> hex(ssl.OPENSSL_VERSION_NUMBER)
'0x9080bf'

New in version 3.2.

18.2.2. SSL Sockets

SSL sockets provide the following methods of Socket Objects:

accept()
bind()
close()
connect()
detach()
fileno()
getpeername(), getsockname()
getsockopt(), setsockopt()
gettimeout(), settimeout(), setblocking()
listen()
makefile()
recv(), recv_into() (but passing a non-zero flags argument is not allowed)
send(), sendall() (with the same limitation)
shutdown()

However, since the SSL (and TLS) protocol has its own framing atop of TCP, the SSL sockets abstraction can, in certain respects, diverge from the specification of normal, OS-level sockets. See especially the notes on non-blocking sockets.

SSL sockets also have the following additional methods and attributes:

SSLSocket.do_handshake(): Perform the SSL setup handshake.

SSLSocket.getpeercert(binary_form=False)

If there is no certificate for the peer on the other end of the connection, returns None.

If the binary_form parameter is False, and a certificate was received from the peer, this method returns a dict instance. If the certificate was not validated, the dict is empty. If the certificate was validated, it returns a dict with several keys, amongst them subject (the principal for which the certificate was issued) and issuer (the principal issuing the certificate). If a certificate contains an instance of the Subject Alternative Name extension (see RFC 3280), there will also be a subjectAltName key in the dictionary.

The subject and issuer fields are tuples containing the sequence of relative distinguished names (RDNs) given in the certificate’s data structure for the respective fields, and each RDN is a sequence of name-value pairs. Here is a real-world example:

{'issuer': ((('countryName', 'IL'),),
            (('organizationName', 'StartCom Ltd.'),),
            (('organizationalUnitName',
              'Secure Digital Certificate Signing'),),
            (('commonName',
              'StartCom Class 2 Primary Intermediate Server CA'),)),
 'notAfter': 'Nov 22 08:15:19 2013 GMT',
 'notBefore': 'Nov 21 03:09:52 2011 GMT',
 'serialNumber': '95F0',
 'subject': ((('description', '571208-SLe257oHY9fVQ07Z'),),
             (('countryName', 'US'),),
             (('stateOrProvinceName', 'California'),),
             (('localityName', 'San Francisco'),),
             (('organizationName', 'Electronic Frontier Foundation, Inc.'),),
             (('commonName', '*.eff.org'),),
             (('emailAddress', '[email protected]'),)),
 'subjectAltName': (('DNS', '*.eff.org'), ('DNS', 'eff.org')),
 'version': 3}

Note

To validate a certificate for a particular service, you can use the match_hostname() function.

If the binary_form parameter is True, and a certificate was provided, this method returns the DER-encoded form of the entire certificate as a sequence of bytes, or None if the peer did not provide a certificate. Whether the peer provides a certificate depends on the SSL socket’s role:

for a client SSL socket, the server will always provide a certificate, regardless of whether validation was required;
for a server SSL socket, the client will only provide a certificate when requested by the server; therefore getpeercert() will return None if you used CERT_NONE (rather than CERT_OPTIONAL or CERT_REQUIRED).

Changed in version 3.2: The returned dictionary includes additional items such as issuer and notBefore.

SSLSocket.cipher(): Returns a three-value tuple containing the name of the cipher being used, the version of the SSL protocol that defines its use, and the number of secret bits being used. If no connection has been established, returns None.

SSLSocket.compression()

Return the compression algorithm being used as a string, or None if the connection isn’t compressed.

If the higher-level protocol supports its own compression mechanism, you can use OP_NO_COMPRESSION to disable SSL-level compression.

New in version 3.3.

SSLSocket.get_channel_binding(cb_type="tls-unique")

Get channel binding data for current connection, as a bytes object. Returns None if not connected or the handshake has not been completed.

The cb_type parameter allow selection of the desired channel binding type. Valid channel binding types are listed in the CHANNEL_BINDING_TYPES list. Currently only the ‘tls-unique’ channel binding, defined by RFC 5929, is supported. ValueError will be raised if an unsupported channel binding type is requested.

New in version 3.3.

SSLSocket.selected_npn_protocol()

Returns the protocol that was selected during the TLS/SSL handshake. If SSLContext.set_npn_protocols() was not called, or if the other party does not support NPN, or if the handshake has not yet happened, this will return None.

New in version 3.3.

SSLSocket.unwrap(): Performs the SSL shutdown handshake, which removes the TLS layer from the underlying socket, and returns the underlying socket object. This can be used to go from encrypted operation over a connection to unencrypted. The returned socket should always be used for further communication with the other side of the connection, rather than the original socket.

SSLSocket.context

The SSLContext object this SSL socket is tied to. If the SSL socket was created using the top-level wrap_socket() function (rather than SSLContext.wrap_socket()), this is a custom context object created for this SSL socket.

New in version 3.2.

18.2.3. SSL Contexts

New in version 3.2.

An SSL context holds various data longer-lived than single SSL connections, such as SSL configuration options, certificate(s) and private key(s). It also manages a cache of SSL sessions for server-side sockets, in order to speed up repeated connections from the same clients.

class ssl.SSLContext(protocol): Create a new SSL context. You must pass protocol which must be one of the PROTOCOL_* constants defined in this module. PROTOCOL_SSLv23 is recommended for maximum interoperability.

SSLContext objects have the following methods and attributes:

SSLContext.load_cert_chain(certfile, keyfile=None, password=None)

Load a private key and the corresponding certificate. The certfile string must be the path to a single file in PEM format containing the certificate as well as any number of CA certificates needed to establish the certificate’s authenticity. The keyfile string, if present, must point to a file containing the private key in. Otherwise the private key will be taken from certfile as well. See the discussion of Certificates for more information on how the certificate is stored in the certfile.

The password argument may be a function to call to get the password for decrypting the private key. It will only be called if the private key is encrypted and a password is necessary. It will be called with no arguments, and it should return a string, bytes, or bytearray. If the return value is a string it will be encoded as UTF-8 before using it to decrypt the key. Alternatively a string, bytes, or bytearray value may be supplied directly as the password argument. It will be ignored if the private key is not encrypted and no password is needed.

If the password argument is not specified and a password is required, OpenSSL’s built-in password prompting mechanism will be used to interactively prompt the user for a password.

An SSLError is raised if the private key doesn’t match with the certificate.

Changed in version 3.3: New optional argument password.

SSLContext.load_verify_locations(cafile=None, capath=None)

Load a set of “certification authority” (CA) certificates used to validate other peers’ certificates when verify_mode is other than CERT_NONE. At least one of cafile or capath must be specified.

The cafile string, if present, is the path to a file of concatenated CA certificates in PEM format. See the discussion of Certificates for more information about how to arrange the certificates in this file.

The capath string, if present, is the path to a directory containing several CA certificates in PEM format, following an OpenSSL specific layout.

SSLContext.set_default_verify_paths(): Load a set of default “certification authority” (CA) certificates from a filesystem path defined when building the OpenSSL library. Unfortunately, there’s no easy way to know whether this method succeeds: no error is returned if no certificates are to be found. When the OpenSSL library is provided as part of the operating system, though, it is likely to be configured properly.

SSLContext.set_ciphers(ciphers): Set the available ciphers for sockets created with this context. It should be a string in the OpenSSL cipher list format. If no cipher can be selected (because compile-time options or other configuration forbids use of all the specified ciphers), an SSLError will be raised.

Note

when connected, the SSLSocket.cipher() method of SSL sockets will give the currently selected cipher.

SSLContext.set_npn_protocols(protocols)

Specify which protocols the socket should advertise during the SSL/TLS handshake. It should be a list of strings, like ['http/1.1', 'spdy/2'], ordered by preference. The selection of a protocol will happen during the handshake, and will play out according to the NPN draft specification. After a successful handshake, the SSLSocket.selected_npn_protocol() method will return the agreed-upon protocol.

This method will raise NotImplementedError if HAS_NPN is False.

New in version 3.3.

SSLContext.load_dh_params(dhfile)

Load the key generation parameters for Diffie-Helman (DH) key exchange. Using DH key exchange improves forward secrecy at the expense of computational resources (both on the server and on the client). The dhfile parameter should be the path to a file containing DH parameters in PEM format.

This setting doesn’t apply to client sockets. You can also use the OP_SINGLE_DH_USE option to further improve security.

New in version 3.3.

SSLContext.set_ecdh_curve(curve_name)

Set the curve name for Elliptic Curve-based Diffie-Hellman (ECDH) key exchange. ECDH is significantly faster than regular DH while arguably as secure. The curve_name parameter should be a string describing a well-known elliptic curve, for example prime256v1 for a widely supported curve.

This setting doesn’t apply to client sockets. You can also use the OP_SINGLE_ECDH_USE option to further improve security.

This method is not available if HAS_ECDH is False.

New in version 3.3.

18.2.4. 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 attempt can be set to raise an exception if the validation fails. Validation is done automatically, by the underlying OpenSSL framework; the application need not concern itself with its mechanics. But the application does usually need to provide sets of certificates to allow this process to take place.

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-----

18.2.4.1. Certificate chains

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-----

18.2.4.2. CA certificates

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 from various certification authorities: CACert.org, Thawte, Verisign, Positive SSL (used by python.org), Equifax and GeoTrust.

In general, if you are using SSL3 or TLS1, you don’t need to put the full chain in your “CA certs” file; you only need the root certificates, and the remote peer is supposed to furnish the other certificates necessary to chain from its certificate to a root certificate. See RFC 4158 for more discussion of the way in which certification chains can be built.

18.2.4.3. Combined key and certificate

Often the private key is stored in the same file as the certificate; in this case, only the certfile parameter to SSLContext.load_cert_chain() and wrap_socket() needs to be passed. If the private key is stored with the certificate, 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-----

18.2.4.4. Self-signed certificates

If you are going to create a server that provides SSL-encrypted connection services, you will need to acquire a certificate for that service. There are many ways of acquiring appropriate certificates, such as buying one from a certification authority. Another common practice is to generate a self-signed certificate. The simplest way to do this is with the OpenSSL package, using something like the following:

% openssl req -new -x509 -days 365 -nodes -out cert.pem -keyout cert.pem
Generating a 1024 bit RSA private key
.......++++++
.............................++++++
writing new private key to 'cert.pem'
-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:US
State or Province Name (full name) [Some-State]:MyState
Locality Name (eg, city) []:Some City
Organization Name (eg, company) [Internet Widgits Pty Ltd]:My Organization, Inc.
Organizational Unit Name (eg, section) []:My Group
Common Name (eg, YOUR name) []:myserver.mygroup.myorganization.com
Email Address []:ops@myserver.mygroup.myorganization.com
%

The disadvantage of a self-signed certificate is that it is its own root certificate, and no one else will have it in their cache of known (and trusted) root certificates.

18.2.5. Examples

18.2.5.1. 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

18.2.5.2. Client-side operation

This example connects to an SSL server and prints the server’s certificate:

import socket, ssl, pprint

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# require a certificate from the server
ssl_sock = ssl.wrap_socket(s,
                           ca_certs="/etc/ca_certs_file",
                           cert_reqs=ssl.CERT_REQUIRED)
ssl_sock.connect(('www.verisign.com', 443))

pprint.pprint(ssl_sock.getpeercert())
# note that closing the SSLSocket will also close the underlying socket
ssl_sock.close()

As of January 6, 2012, the certificate printed by this program looks like this:

{'issuer': ((('countryName', 'US'),),
            (('organizationName', 'VeriSign, Inc.'),),
            (('organizationalUnitName', 'VeriSign Trust Network'),),
            (('organizationalUnitName',
              'Terms of use at https://www.verisign.com/rpa (c)06'),),
            (('commonName',
              'VeriSign Class 3 Extended Validation SSL SGC CA'),)),
 'notAfter': 'May 25 23:59:59 2012 GMT',
 'notBefore': 'May 26 00:00:00 2010 GMT',
 'serialNumber': '53D2BEF924A7245E83CA01E46CAA2477',
 'subject': ((('1.3.6.1.4.1.311.60.2.1.3', 'US'),),
             (('1.3.6.1.4.1.311.60.2.1.2', 'Delaware'),),
             (('businessCategory', 'V1.0, Clause 5.(b)'),),
             (('serialNumber', '2497886'),),
             (('countryName', 'US'),),
             (('postalCode', '94043'),),
             (('stateOrProvinceName', 'California'),),
             (('localityName', 'Mountain View'),),
             (('streetAddress', '487 East Middlefield Road'),),
             (('organizationName', 'VeriSign, Inc.'),),
             (('organizationalUnitName', ' Production Security Services'),),
             (('commonName', 'www.verisign.com'),)),
 'subjectAltName': (('DNS', 'www.verisign.com'),
                    ('DNS', 'verisign.com'),
                    ('DNS', 'www.verisign.net'),
                    ('DNS', 'verisign.net'),
                    ('DNS', 'www.verisign.mobi'),
                    ('DNS', 'verisign.mobi'),
                    ('DNS', 'www.verisign.eu'),
                    ('DNS', 'verisign.eu')),
 'version': 3}

This other example first creates an SSL context, instructs it to verify certificates sent by peers, and feeds it a set of recognized certificate authorities (CA):

>>> context = ssl.SSLContext(ssl.PROTOCOL_SSLv23)
>>> context.verify_mode = ssl.CERT_REQUIRED
>>> context.load_verify_locations("/etc/ssl/certs/ca-bundle.crt")

(it is assumed your operating system places a bundle of all CA certificates in /etc/ssl/certs/ca-bundle.crt; if not, you’ll get an error and have to adjust the location)

When you use the context to connect to a server, CERT_REQUIRED validates the server certificate: it ensures that the server certificate was signed with one of the CA certificates, and checks the signature for correctness:

>>> conn = context.wrap_socket(socket.socket(socket.AF_INET))
>>> conn.connect(("linuxfr.org", 443))

You should then fetch the certificate and check its fields for conformity:

>>> cert = conn.getpeercert()
>>> ssl.match_hostname(cert, "linuxfr.org")

Visual inspection shows that the certificate does identify the desired service (that is, the HTTPS host linuxfr.org):

>>> pprint.pprint(cert)
{'issuer': ((('organizationName', 'CAcert Inc.'),),
            (('organizationalUnitName', 'http://www.CAcert.org'),),
            (('commonName', 'CAcert Class 3 Root'),)),
 'notAfter': 'Jun  7 21:02:24 2013 GMT',
 'notBefore': 'Jun  8 21:02:24 2011 GMT',
 'serialNumber': 'D3E9',
 'subject': ((('commonName', 'linuxfr.org'),),),
 'subjectAltName': (('DNS', 'linuxfr.org'),
                    ('othername', '<unsupported>'),
                    ('DNS', 'linuxfr.org'),
                    ('othername', '<unsupported>'),
                    ('DNS', 'dev.linuxfr.org'),
                    ('othername', '<unsupported>'),
                    ('DNS', 'prod.linuxfr.org'),
                    ('othername', '<unsupported>'),
                    ('DNS', 'alpha.linuxfr.org'),
                    ('othername', '<unsupported>'),
                    ('DNS', '*.linuxfr.org'),
                    ('othername', '<unsupported>')),
 'version': 3}

Now that you are assured of its authenticity, you can proceed to talk with the server:

>>> conn.sendall(b"HEAD / HTTP/1.0\r\nHost: linuxfr.org\r\n\r\n")
>>> pprint.pprint(conn.recv(1024).split(b"\r\n"))
[b'HTTP/1.1 302 Found',
 b'Date: Sun, 16 May 2010 13:43:28 GMT',
 b'Server: Apache/2.2',
 b'Location: https://linuxfr.org/pub/',
 b'Vary: Accept-Encoding',
 b'Connection: close',
 b'Content-Type: text/html; charset=iso-8859-1',
 b'',
 b'']

See the discussion of Security considerations below.

18.2.5.3. Server-side operation

For server operation, typically you’ll need to have a server certificate, and private key, each in a file. You’ll first create a context holding the key and the certificate, so that clients can check your authenticity. Then you’ll open a socket, bind it to a port, call listen() on it, and start waiting for clients to connect:

import socket, ssl

context = ssl.SSLContext(ssl.PROTOCOL_TLSv1)
context.load_cert_chain(certfile="mycertfile", keyfile="mykeyfile")

bindsocket = socket.socket()
bindsocket.bind(('myaddr.mydomain.com', 10023))
bindsocket.listen(5)

When a client connects, you’ll call accept() on the socket to get the new socket from the other end, and use the context’s SSLContext.wrap_socket() method to create a server-side SSL socket for the connection:

while True:
    newsocket, fromaddr = bindsocket.accept()
    connstream = context.wrap_socket(newsocket, server_side=True)
    try:
        deal_with_client(connstream)
    finally:
        connstream.shutdown(socket.SHUT_RDWR)
        connstream.close()

Then you’ll 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.recv(1024)
    # empty 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.recv(1024)
    # finished with client

And go back to listening for new client connections (of course, a real server would probably handle each client connection in a separate thread, or put the sockets in non-blocking mode and use an event loop).

18.2.6. Notes on non-blocking sockets

When working with non-blocking sockets, there are several things you need to be aware of:

Calling select() tells you that the OS-level socket can be read from (or written to), but it does not imply that there is sufficient data at the upper SSL layer. For example, only part of an SSL frame might have arrived. Therefore, you must be ready to handle SSLSocket.recv() and SSLSocket.send() failures, and retry after another call to select().

(of course, similar provisions apply when using other primitives such as poll())

The SSL handshake itself will be non-blocking: the SSLSocket.do_handshake() method has to be retried until it returns successfully. Here is a synopsis using select() to wait for the socket’s readiness:

while True:
    try:
        sock.do_handshake()
        break
    except ssl.SSLWantReadError:
        select.select([sock], [], [])
    except ssl.SSLWantWriteError:
        select.select([], [sock], [])

18.2.7. Security considerations

18.2.7.1. Verifying certificates

CERT_NONE is the default. Since it does not authenticate the other peer, it can be insecure, especially in client mode where most of time you would like to ensure the authenticity of the server you’re talking to. Therefore, when in client mode, it is highly recommended to use CERT_REQUIRED. However, it is in itself not sufficient; you also have to check that the server certificate, which can be obtained by calling SSLSocket.getpeercert(), matches the desired service. For many protocols and applications, the service can be identified by the hostname; in this case, the match_hostname() function can be used.

In server mode, if you want to authenticate your clients using the SSL layer (rather than using a higher-level authentication mechanism), you’ll also have to specify CERT_REQUIRED and similarly check the client certificate.

Note

In client mode, CERT_OPTIONAL and CERT_REQUIRED are equivalent unless anonymous ciphers are enabled (they are disabled by default).

18.2.7.2. Protocol versions

SSL version 2 is considered insecure and is therefore dangerous to use. If you want maximum compatibility between clients and servers, it is recommended to use PROTOCOL_SSLv23 as the protocol version and then disable SSLv2 explicitly using the SSLContext.options attribute:

context = ssl.SSLContext(ssl.PROTOCOL_SSLv23)
context.options |= ssl.OP_NO_SSLv2

The SSL context created above will allow SSLv3 and TLSv1 connections, but not SSLv2.

18.2.7.3. Cipher selection

If you have advanced security requirements, fine-tuning of the ciphers enabled when negotiating a SSL session is possible through the SSLContext.set_ciphers() method. Starting from Python 3.2.3, the ssl module disables certain weak ciphers by default, but you may want to further restrict the cipher choice. For example:

context = ssl.SSLContext(ssl.PROTOCOL_TLSv1)
context.set_ciphers('HIGH:!aNULL:!eNULL')

The !aNULL:!eNULL part of the cipher spec is necessary to disable ciphers which don’t provide both encryption and authentication. Be sure to read OpenSSL’s documentation about the cipher list format. If you want to check which ciphers are enabled by a given cipher list, use the openssl ciphers command on your system.

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client / server	SSLv2	SSLv3	SSLv23	TLSv1
SSLv2	yes	no	yes	no
SSLv3	no	yes	yes	no
SSLv23	yes	no	yes	no
TLSv1	no	no	yes	yes