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. For example, TLSv1.1 and TLSv1.2 come with openssl version 1.0.1.
Warning
Don’t use this module without reading the Security considerations. Doing so may lead to a false sense of security, as the default settings of the ssl module are not necessarily appropriate for your application.
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.
Changed in version 3.5.3: Updated to support linking with OpenSSL 1.1.0
Changed in version 3.6: OpenSSL 0.9.8, 1.0.0 and 1.0.1 are deprecated and no longer supported. In the future the ssl module will require at least OpenSSL 1.0.2 or 1.1.0.
Functions, Constants, and Exceptions
Socket creation
Since Python 3.2 and 2.7.9, it is recommended to use the
SSLContext.wrap_socket() of an SSLContext instance to wrap
sockets as SSLSocket objects. The helper functions
create_default_context() returns a new context with secure default
settings. The old wrap_socket() function is deprecated since it is
both inefficient and has no support for server name indication (SNI) and
hostname matching.
Client socket example with default context and IPv4/IPv6 dual stack:
import socket
import ssl
hostname = 'www.python.org'
context = ssl.create_default_context()
with socket.create_connection((hostname, 443)) as sock:
with context.wrap_socket(sock, server_hostname=hostname) as ssock:
print(ssock.version())
Client socket example with custom context and IPv4:
hostname = 'www.python.org'
# PROTOCOL_TLS_CLIENT requires valid cert chain and hostname
context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
context.load_verify_locations('path/to/cabundle.pem')
with socket.socket(socket.AF_INET, socket.SOCK_STREAM, 0) as sock:
with context.wrap_socket(sock, server_hostname=hostname) as ssock:
print(ssock.version())
Server socket example listening on localhost IPv4:
context = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
context.load_cert_chain('/path/to/certchain.pem', '/path/to/private.key')
with socket.socket(socket.AF_INET, socket.SOCK_STREAM, 0) as sock:
sock.bind(('127.0.0.1', 8443))
sock.listen(5)
with context.wrap_socket(sock, server_side=True) as ssock:
conn, addr = ssock.accept()
...
Context creation
A convenience function helps create SSLContext objects for common
purposes.
-
ssl.create_default_context(purpose=Purpose.SERVER_AUTH, cafile=None, capath=None, cadata=None) Return a new
SSLContextobject with default settings for the given purpose. The settings are chosen by thesslmodule, and usually represent a higher security level than when calling theSSLContextconstructor directly.cafile, capath, cadata represent optional CA certificates to trust for certificate verification, as in
SSLContext.load_verify_locations(). If all three areNone, this function can choose to trust the system’s default CA certificates instead.The settings are:
PROTOCOL_TLS,OP_NO_SSLv2, andOP_NO_SSLv3with high encryption cipher suites without RC4 and without unauthenticated cipher suites. PassingSERVER_AUTHas purpose setsverify_modetoCERT_REQUIREDand either loads CA certificates (when at least one of cafile, capath or cadata is given) or usesSSLContext.load_default_certs()to load default CA certificates.When
keylog_filenameis supported and the environment variableSSLKEYLOGFILEis set,create_default_context()enables key logging.Note
The protocol, options, cipher and other settings may change to more restrictive values anytime without prior deprecation. The values represent a fair balance between compatibility and security.
If your application needs specific settings, you should create a
SSLContextand apply the settings yourself.Note
If you find that when certain older clients or servers attempt to connect with a
SSLContextcreated by this function that they get an error stating “Protocol or cipher suite mismatch”, it may be that they only support SSL3.0 which this function excludes using theOP_NO_SSLv3. SSL3.0 is widely considered to be completely broken. If you still wish to continue to use this function but still allow SSL 3.0 connections you can re-enable them using:ctx = ssl.create_default_context(Purpose.CLIENT_AUTH) ctx.options &= ~ssl.OP_NO_SSLv3
New in version 3.4.
Changed in version 3.4.4: RC4 was dropped from the default cipher string.
Changed in version 3.6: ChaCha20/Poly1305 was added to the default cipher string.
3DES was dropped from the default cipher string.
Changed in version 3.8: Support for key logging to
SSLKEYLOGFILEwas added.
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 ofSSLErrorinstances are provided by the OpenSSL library.Changed in version 3.3:
SSLErrorused to be a subtype ofsocket.error.-
library A string mnemonic designating the OpenSSL submodule in which the error occurred, such as
SSL,PEMorX509. 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
SSLErrorraised 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
SSLErrorraised 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
SSLErrorraised 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
SSLErrorraised 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
SSLErrorraised 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.SSLCertVerificationError A subclass of
SSLErrorraised when certificate validation has failed.New in version 3.7.
-
verify_code A numeric error number that denotes the verification error.
-
verify_message A human readable string of the verification error.
-
-
exception
ssl.CertificateError An alias for
SSLCertVerificationError.Changed in version 3.7: The exception is now an alias for
SSLCertVerificationError.
Random generation
-
ssl.RAND_bytes(num) Return num cryptographically strong pseudo-random bytes. Raises an
SSLErrorif 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 andRAND_add()can be used to seed the PRNG.For almost all applications
os.urandom()is preferable.Read the Wikipedia article, Cryptographically secure pseudorandom number generator (CSPRNG), to get the requirements of a cryptographically strong generator.
New in version 3.3.
-
ssl.RAND_pseudo_bytes(num) Return (bytes, is_cryptographic): bytes are num pseudo-random bytes, is_cryptographic is
Trueif the bytes generated are cryptographically strong. Raises anSSLErrorif 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.
For almost all applications
os.urandom()is preferable.New in version 3.3.
Deprecated since version 3.6: OpenSSL has deprecated
ssl.RAND_pseudo_bytes(), usessl.RAND_bytes()instead.
-
ssl.RAND_status() Return
Trueif the SSL pseudo-random number generator has been seeded with ‘enough’ randomness, andFalseotherwise. You can usessl.RAND_egd()andssl.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.
Availability: not available with LibreSSL and OpenSSL > 1.1.0.
-
ssl.RAND_add(bytes, entropy) Mix 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.Changed in version 3.5: Writable bytes-like object is now accepted.
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, RFC 5280 and RFC 6125. 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.CertificateErroris 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.
*.*.comor*a*.example.org) nor a wildcard inside an internationalized domain names (IDN) fragment. IDN A-labels such aswww*.xn--pthon-kva.orgare still supported, butx*.python.orgno longer matchesxn--tda.python.org.Changed in version 3.5: Matching of IP addresses, when present in the subjectAltName field of the certificate, is now supported.
Changed in version 3.7: The function is no longer used to TLS connections. Hostname matching is now performed by OpenSSL.
Allow wildcard when it is the leftmost and the only character in that segment. Partial wildcards like
www*.example.comare no longer supported.Deprecated since version 3.7.
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ssl.cert_time_to_seconds(cert_time) Return the time in seconds since the Epoch, given the
cert_timestring representing the “notBefore” or “notAfter” date from a certificate in"%b %d %H:%M:%S %Y %Z"strptime format (C locale).Here’s an example:
>>> import ssl >>> timestamp = ssl.cert_time_to_seconds("Jan 5 09:34:43 2018 GMT") >>> timestamp 1515144883 >>> from datetime import datetime >>> print(datetime.utcfromtimestamp(timestamp)) 2018-01-05 09:34:43
“notBefore” or “notAfter” dates must use GMT (RFC 5280).
Changed in version 3.5: Interpret the input time as a time in UTC as specified by ‘GMT’ timezone in the input string. Local timezone was used previously. Return an integer (no fractions of a second in the input format)
-
ssl.get_server_certificate(addr, ssl_version=PROTOCOL_TLS, ca_certs=None) Given the address
addrof an SSL-protected server, as a (hostname, port-number) pair, fetches the server’s certificate, and returns it as a PEM-encoded string. Ifssl_versionis specified, uses that version of the SSL protocol to attempt to connect to the server. Ifca_certsis specified, it should be a file containing a list of root certificates, the same format as used for the same parameter inSSLContext.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.
Changed in version 3.5: The default ssl_version is changed from
PROTOCOL_SSLv3toPROTOCOL_TLSfor maximum compatibility with modern servers.
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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.
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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.
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ssl.get_default_verify_paths() Returns a named tuple with paths to OpenSSL’s default cafile and capath. The paths are the same as used by
SSLContext.set_default_verify_paths(). The return value is a named tupleDefaultVerifyPaths:cafile- resolved path to cafile orNoneif the file doesn’t exist,capath- resolved path to capath orNoneif the directory doesn’t exist,openssl_cafile_env- OpenSSL’s environment key that points to a cafile,openssl_cafile- hard coded path to a cafile,openssl_capath_env- OpenSSL’s environment key that points to a capath,openssl_capath- hard coded path to a capath directory
Availability: LibreSSL ignores the environment vars
openssl_cafile_envandopenssl_capath_env.New in version 3.4.
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ssl.enum_certificates(store_name) Retrieve certificates from Windows’ system cert store. store_name may be one of
CA,ROOTorMY. Windows may provide additional cert stores, too.The function returns a list of (cert_bytes, encoding_type, trust) tuples. The encoding_type specifies the encoding of cert_bytes. It is either
x509_asnfor X.509 ASN.1 data orpkcs_7_asnfor PKCS#7 ASN.1 data. Trust specifies the purpose of the certificate as a set of OIDS or exactlyTrueif the certificate is trustworthy for all purposes.Example:
>>> ssl.enum_certificates("CA") [(b'data...', 'x509_asn', {'1.3.6.1.5.5.7.3.1', '1.3.6.1.5.5.7.3.2'}), (b'data...', 'x509_asn', True)]
Availability: Windows.
New in version 3.4.
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ssl.enum_crls(store_name) Retrieve CRLs from Windows’ system cert store. store_name may be one of
CA,ROOTorMY. Windows may provide additional cert stores, too.The function returns a list of (cert_bytes, encoding_type, trust) tuples. The encoding_type specifies the encoding of cert_bytes. It is either
x509_asnfor X.509 ASN.1 data orpkcs_7_asnfor PKCS#7 ASN.1 data.Availability: Windows.
New in version 3.4.
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ssl.wrap_socket(sock, keyfile=None, certfile=None, server_side=False, cert_reqs=CERT_NONE, ssl_version=PROTOCOL_TLS, ca_certs=None, do_handshake_on_connect=True, suppress_ragged_eofs=True, ciphers=None) Takes an instance
sockofsocket.socket, and returns an instance ofssl.SSLSocket, a subtype ofsocket.socket, which wraps the underlying socket in an SSL context.sockmust be aSOCK_STREAMsocket; other socket types are unsupported.Internally, function creates a
SSLContextwith protocol ssl_version andSSLContext.optionsset to cert_reqs. If parameters keyfile, certfile, ca_certs or ciphers are set, then the values are passed toSSLContext.load_cert_chain(),SSLContext.load_verify_locations(), andSSLContext.set_ciphers().The arguments server_side, do_handshake_on_connect, and suppress_ragged_eofs have the same meaning as
SSLContext.wrap_socket().Deprecated since version 3.7: Since Python 3.2 and 2.7.9, it is recommended to use the
SSLContext.wrap_socket()instead ofwrap_socket(). The top-level function is limited and creates an insecure client socket without server name indication or hostname matching.
Constants
All constants are now
enum.IntEnumorenum.IntFlagcollections.New in version 3.6.
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ssl.CERT_NONE Possible value for
SSLContext.verify_mode, or thecert_reqsparameter towrap_socket(). Except forPROTOCOL_TLS_CLIENT, it is the default mode. With client-side sockets, just about any cert is accepted. Validation errors, such as untrusted or expired cert, are ignored and do not abort the TLS/SSL handshake.In server mode, no certificate is requested from the client, so the client does not send any for client cert authentication.
See the discussion of Security considerations below.
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ssl.CERT_OPTIONAL Possible value for
SSLContext.verify_mode, or thecert_reqsparameter towrap_socket(). In client mode,CERT_OPTIONALhas the same meaning asCERT_REQUIRED. It is recommended to useCERT_REQUIREDfor client-side sockets instead.In server mode, a client certificate request is sent to the client. The client may either ignore the request or send a certificate in order perform TLS client cert authentication. If the client chooses to send a certificate, it is verified. Any verification error immediately aborts the TLS handshake.
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 theca_certsparameter towrap_socket().
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ssl.CERT_REQUIRED Possible value for
SSLContext.verify_mode, or thecert_reqsparameter towrap_socket(). In this mode, certificates are required from the other side of the socket connection; anSSLErrorwill be raised if no certificate is provided, or if its validation fails. This mode is not sufficient to verify a certificate in client mode as it does not match hostnames.check_hostnamemust be enabled as well to verify the authenticity of a cert.PROTOCOL_TLS_CLIENTusesCERT_REQUIREDand enablescheck_hostnameby default.With server socket, this mode provides mandatory TLS client cert authentication. A client certificate request is sent to the client and the client must provide a valid and trusted certificate.
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 theca_certsparameter towrap_socket().
-
class
ssl.VerifyMode enum.IntEnumcollection of CERT_* constants.New in version 3.6.
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ssl.VERIFY_DEFAULT Possible value for
SSLContext.verify_flags. In this mode, certificate revocation lists (CRLs) are not checked. By default OpenSSL does neither require nor verify CRLs.New in version 3.4.
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ssl.VERIFY_CRL_CHECK_LEAF Possible value for
SSLContext.verify_flags. In this mode, only the peer cert is checked but none of the intermediate CA certificates. The mode requires a valid CRL that is signed by the peer cert’s issuer (its direct ancestor CA). If no proper CRL has been loaded withSSLContext.load_verify_locations, validation will fail.New in version 3.4.
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ssl.VERIFY_CRL_CHECK_CHAIN Possible value for
SSLContext.verify_flags. In this mode, CRLs of all certificates in the peer cert chain are checked.New in version 3.4.
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ssl.VERIFY_X509_STRICT Possible value for
SSLContext.verify_flagsto disable workarounds for broken X.509 certificates.New in version 3.4.
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ssl.VERIFY_X509_TRUSTED_FIRST Possible value for
SSLContext.verify_flags. It instructs OpenSSL to prefer trusted certificates when building the trust chain to validate a certificate. This flag is enabled by default.New in version 3.4.4.
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class
ssl.VerifyFlags enum.IntFlagcollection of VERIFY_* constants.New in version 3.6.
-
ssl.PROTOCOL_TLS Selects the highest protocol version that both the client and server support. Despite the name, this option can select both “SSL” and “TLS” protocols.
New in version 3.6.
-
ssl.PROTOCOL_TLS_CLIENT Auto-negotiate the highest protocol version like
PROTOCOL_TLS, but only support client-sideSSLSocketconnections. The protocol enablesCERT_REQUIREDandcheck_hostnameby default.New in version 3.6.
-
ssl.PROTOCOL_TLS_SERVER Auto-negotiate the highest protocol version like
PROTOCOL_TLS, but only support server-sideSSLSocketconnections.New in version 3.6.
-
ssl.PROTOCOL_SSLv23 Alias for
PROTOCOL_TLS.Deprecated since version 3.6: Use
PROTOCOL_TLSinstead.
-
ssl.PROTOCOL_SSLv2 Selects SSL version 2 as the channel encryption protocol.
This protocol is not available if OpenSSL is compiled with the
OPENSSL_NO_SSL2flag.Warning
SSL version 2 is insecure. Its use is highly discouraged.
Deprecated since version 3.6: OpenSSL has removed support for SSLv2.
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ssl.PROTOCOL_SSLv3 Selects SSL version 3 as the channel encryption protocol.
This protocol is not be available if OpenSSL is compiled with the
OPENSSL_NO_SSLv3flag.Warning
SSL version 3 is insecure. Its use is highly discouraged.
Deprecated since version 3.6: OpenSSL has deprecated all version specific protocols. Use the default protocol
PROTOCOL_TLSwith flags likeOP_NO_SSLv3instead.
-
ssl.PROTOCOL_TLSv1 Selects TLS version 1.0 as the channel encryption protocol.
Deprecated since version 3.6: OpenSSL has deprecated all version specific protocols. Use the default protocol
PROTOCOL_TLSwith flags likeOP_NO_SSLv3instead.
-
ssl.PROTOCOL_TLSv1_1 Selects TLS version 1.1 as the channel encryption protocol. Available only with openssl version 1.0.1+.
New in version 3.4.
Deprecated since version 3.6: OpenSSL has deprecated all version specific protocols. Use the default protocol
PROTOCOL_TLSwith flags likeOP_NO_SSLv3instead.
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ssl.PROTOCOL_TLSv1_2 Selects TLS version 1.2 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. Available only with openssl version 1.0.1+.
New in version 3.4.
Deprecated since version 3.6: OpenSSL has deprecated all version specific protocols. Use the default protocol
PROTOCOL_TLSwith flags likeOP_NO_SSLv3instead.
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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_ALLconstant.New in version 3.2.
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ssl.OP_NO_SSLv2 Prevents an SSLv2 connection. This option is only applicable in conjunction with
PROTOCOL_TLS. It prevents the peers from choosing SSLv2 as the protocol version.New in version 3.2.
Deprecated since version 3.6: SSLv2 is deprecated
-
ssl.OP_NO_SSLv3 Prevents an SSLv3 connection. This option is only applicable in conjunction with
PROTOCOL_TLS. It prevents the peers from choosing SSLv3 as the protocol version.New in version 3.2.
Deprecated since version 3.6: SSLv3 is deprecated
-
ssl.OP_NO_TLSv1 Prevents a TLSv1 connection. This option is only applicable in conjunction with
PROTOCOL_TLS. It prevents the peers from choosing TLSv1 as the protocol version.New in version 3.2.
Deprecated since version 3.7: The option is deprecated since OpenSSL 1.1.0, use the new
SSLContext.minimum_versionandSSLContext.maximum_versioninstead.
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ssl.OP_NO_TLSv1_1 Prevents a TLSv1.1 connection. This option is only applicable in conjunction with
PROTOCOL_TLS. It prevents the peers from choosing TLSv1.1 as the protocol version. Available only with openssl version 1.0.1+.New in version 3.4.
Deprecated since version 3.7: The option is deprecated since OpenSSL 1.1.0.
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ssl.OP_NO_TLSv1_2 Prevents a TLSv1.2 connection. This option is only applicable in conjunction with
PROTOCOL_TLS. It prevents the peers from choosing TLSv1.2 as the protocol version. Available only with openssl version 1.0.1+.New in version 3.4.
Deprecated since version 3.7: The option is deprecated since OpenSSL 1.1.0.
-
ssl.OP_NO_TLSv1_3 Prevents a TLSv1.3 connection. This option is only applicable in conjunction with
PROTOCOL_TLS. It prevents the peers from choosing TLSv1.3 as the protocol version. TLS 1.3 is available with OpenSSL 1.1.1 or later. When Python has been compiled against an older version of OpenSSL, the flag defaults to 0.New in version 3.7.
Deprecated since version 3.7: The option is deprecated since OpenSSL 1.1.0. It was added to 2.7.15, 3.6.3 and 3.7.0 for backwards compatibility with OpenSSL 1.0.2.
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ssl.OP_NO_RENEGOTIATION Disable all renegotiation in TLSv1.2 and earlier. Do not send HelloRequest messages, and ignore renegotiation requests via ClientHello.
This option is only available with OpenSSL 1.1.0h and later.
New in version 3.7.
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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.
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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.
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ssl.OP_ENABLE_MIDDLEBOX_COMPAT Send dummy Change Cipher Spec (CCS) messages in TLS 1.3 handshake to make a TLS 1.3 connection look more like a TLS 1.2 connection.
This option is only available with OpenSSL 1.1.1 and later.
New in version 3.8.
-
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.
-
class
ssl.Options enum.IntFlagcollection of OP_* constants.
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ssl.OP_NO_TICKET Prevent client side from requesting a session ticket.
New in version 3.6.
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ssl.HAS_ALPN Whether the OpenSSL library has built-in support for the Application-Layer Protocol Negotiation TLS extension as described in RFC 7301.
New in version 3.5.
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ssl.HAS_NEVER_CHECK_COMMON_NAME Whether the OpenSSL library has built-in support not checking subject common name and
SSLContext.hostname_checks_common_nameis writeable.New in version 3.7.
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ssl.HAS_ECDH Whether the OpenSSL library has built-in support for the 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.
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ssl.HAS_SNI Whether the OpenSSL library has built-in support for the Server Name Indication extension (as defined in RFC 6066).
New in version 3.2.
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ssl.HAS_NPN Whether the OpenSSL library has built-in support for the Next Protocol Negotiation as described in the Application Layer Protocol Negotiation. When true, you can use the
SSLContext.set_npn_protocols()method to advertise which protocols you want to support.New in version 3.3.
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ssl.HAS_SSLv2 Whether the OpenSSL library has built-in support for the SSL 2.0 protocol.
New in version 3.7.
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ssl.HAS_SSLv3 Whether the OpenSSL library has built-in support for the SSL 3.0 protocol.
New in version 3.7.
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ssl.HAS_TLSv1 Whether the OpenSSL library has built-in support for the TLS 1.0 protocol.
New in version 3.7.
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ssl.HAS_TLSv1_1 Whether the OpenSSL library has built-in support for the TLS 1.1 protocol.
New in version 3.7.
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ssl.HAS_TLSv1_2 Whether the OpenSSL library has built-in support for the TLS 1.2 protocol.
New in version 3.7.
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ssl.HAS_TLSv1_3 Whether the OpenSSL library has built-in support for the TLS 1.3 protocol.
New in version 3.7.
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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 1.0.2k 26 Jan 2017'
New in version 3.2.
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ssl.OPENSSL_VERSION_INFO A tuple of five integers representing version information about the OpenSSL library:
>>> ssl.OPENSSL_VERSION_INFO (1, 0, 2, 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 268443839 >>> hex(ssl.OPENSSL_VERSION_NUMBER) '0x100020bf'
New in version 3.2.
-
ssl.ALERT_DESCRIPTION_HANDSHAKE_FAILURE -
ssl.ALERT_DESCRIPTION_INTERNAL_ERROR -
ALERT_DESCRIPTION_* Alert Descriptions from RFC 5246 and others. The IANA TLS Alert Registry contains this list and references to the RFCs where their meaning is defined.
Used as the return value of the callback function in
SSLContext.set_servername_callback().New in version 3.4.
-
class
ssl.AlertDescription enum.IntEnumcollection of ALERT_DESCRIPTION_* constants.New in version 3.6.
-
Purpose.SERVER_AUTH Option for
create_default_context()andSSLContext.load_default_certs(). This value indicates that the context may be used to authenticate Web servers (therefore, it will be used to create client-side sockets).New in version 3.4.
-
Purpose.CLIENT_AUTH Option for
create_default_context()andSSLContext.load_default_certs(). This value indicates that the context may be used to authenticate Web clients (therefore, it will be used to create server-side sockets).New in version 3.4.
-
class
ssl.SSLErrorNumber enum.IntEnumcollection of SSL_ERROR_* constants.New in version 3.6.
-
class
ssl.TLSVersion enum.IntEnumcollection of SSL and TLS versions forSSLContext.maximum_versionandSSLContext.minimum_version.New in version 3.7.
-
TLSVersion.MINIMUM_SUPPORTED
-
TLSVersion.MAXIMUM_SUPPORTED The minimum or maximum supported SSL or TLS version. These are magic constants. Their values don’t reflect the lowest and highest available TLS/SSL versions.
-
TLSVersion.SSLv3
-
TLSVersion.TLSv1
-
TLSVersion.TLSv1_1
-
TLSVersion.TLSv1_2
-
TLSVersion.TLSv1_3 SSL 3.0 to TLS 1.3.
SSL Sockets
-
class
ssl.SSLSocket(socket.socket) 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-zeroflagsargument is not allowed)send(),sendall()(with the same limitation)sendfile()(butos.sendfilewill be used for plain-text sockets only, elsesend()will be used)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.
Instances of
SSLSocketmust be created using theSSLContext.wrap_socket()method.Changed in version 3.5: The
sendfile()method was added.Changed in version 3.5: The
shutdown()does not reset the socket timeout each time bytes are received or sent. The socket timeout is now to maximum total duration of the shutdown.Deprecated since version 3.6: It is deprecated to create a
SSLSocketinstance directly, useSSLContext.wrap_socket()to wrap a socket.Changed in version 3.7:
SSLSocketinstances must to created withwrap_socket(). In earlier versions, it was possible to create instances directly. This was never documented or officially supported.
SSL sockets also have the following additional methods and attributes:
-
SSLSocket.read(len=1024, buffer=None) Read up to len bytes of data from the SSL socket and return the result as a
bytesinstance. If buffer is specified, then read into the buffer instead, and return the number of bytes read.Raise
SSLWantReadErrororSSLWantWriteErrorif the socket is non-blocking and the read would block.As at any time a re-negotiation is possible, a call to
read()can also cause write operations.Changed in version 3.5: The socket timeout is no more reset each time bytes are received or sent. The socket timeout is now to maximum total duration to read up to len bytes.
Deprecated since version 3.6: Use
recv()instead ofread().
-
SSLSocket.write(buf) Write buf to the SSL socket and return the number of bytes written. The buf argument must be an object supporting the buffer interface.
Raise
SSLWantReadErrororSSLWantWriteErrorif the socket is non-blocking and the write would block.As at any time a re-negotiation is possible, a call to
write()can also cause read operations.Changed in version 3.5: The socket timeout is no more reset each time bytes are received or sent. The socket timeout is now to maximum total duration to write buf.
Deprecated since version 3.6: Use
send()instead ofwrite().
Note
The read() and write() methods are the
low-level methods that read and write unencrypted, application-level data
and decrypt/encrypt it to encrypted, wire-level data. These methods
require an active SSL connection, i.e. the handshake was completed and
SSLSocket.unwrap() was not called.
Normally you should use the socket API methods like
recv() and send() instead of these
methods.
-
SSLSocket.do_handshake() Perform the SSL setup handshake.
Changed in version 3.4: The handshake method also performs
match_hostname()when thecheck_hostnameattribute of the socket’scontextis true.Changed in version 3.5: The socket timeout is no more reset each time bytes are received or sent. The socket timeout is now to maximum total duration of the handshake.
Changed in version 3.7: Hostname or IP address is matched by OpenSSL during handshake. The function
match_hostname()is no longer used. In case OpenSSL refuses a hostname or IP address, the handshake is aborted early and a TLS alert message is send to the peer.
-
SSLSocket.getpeercert(binary_form=False) If there is no certificate for the peer on the other end of the connection, return
None. If the SSL handshake hasn’t been done yet, raiseValueError.If the
binary_formparameter isFalse, and a certificate was received from the peer, this method returns adictinstance. If the certificate was not validated, the dict is empty. If the certificate was validated, it returns a dict with several keys, amongst themsubject(the principal for which the certificate was issued) andissuer(the principal issuing the certificate). If a certificate contains an instance of the Subject Alternative Name extension (see RFC 3280), there will also be asubjectAltNamekey in the dictionary.The
subjectandissuerfields 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', 'hostmaster@eff.org'),)), '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_formparameter isTrue, and a certificate was provided, this method returns the DER-encoded form of the entire certificate as a sequence of bytes, orNoneif 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 returnNoneif you usedCERT_NONE(rather thanCERT_OPTIONALorCERT_REQUIRED).
Changed in version 3.2: The returned dictionary includes additional items such as
issuerandnotBefore.Changed in version 3.4:
ValueErroris raised when the handshake isn’t done. The returned dictionary includes additional X509v3 extension items such ascrlDistributionPoints,caIssuersandOCSPURIs.Changed in version 3.9: IPv6 address strings no longer have a trailing new line.
-
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.
Return the list of ciphers shared by the client during the handshake. Each entry of the returned list is a three-value tuple containing the name of the cipher, the version of the SSL protocol that defines its use, and the number of secret bits the cipher uses.
shared_ciphers()returnsNoneif no connection has been established or the socket is a client socket.New in version 3.5.
-
SSLSocket.compression() Return the compression algorithm being used as a string, or
Noneif the connection isn’t compressed.If the higher-level protocol supports its own compression mechanism, you can use
OP_NO_COMPRESSIONto 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
Noneif 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_TYPESlist. Currently only the ‘tls-unique’ channel binding, defined by RFC 5929, is supported.ValueErrorwill be raised if an unsupported channel binding type is requested.New in version 3.3.
-
SSLSocket.selected_alpn_protocol() Return the protocol that was selected during the TLS handshake. If
SSLContext.set_alpn_protocols()was not called, if the other party does not support ALPN, if this socket does not support any of the client’s proposed protocols, or if the handshake has not happened yet,Noneis returned.New in version 3.5.
-
SSLSocket.selected_npn_protocol() Return the higher-level 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 returnNone.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.verify_client_post_handshake() Requests post-handshake authentication (PHA) from a TLS 1.3 client. PHA can only be initiated for a TLS 1.3 connection from a server-side socket, after the initial TLS handshake and with PHA enabled on both sides, see
SSLContext.post_handshake_auth.The method does not perform a cert exchange immediately. The server-side sends a CertificateRequest during the next write event and expects the client to respond with a certificate on the next read event.
If any precondition isn’t met (e.g. not TLS 1.3, PHA not enabled), an
SSLErroris raised.Note
Only available with OpenSSL 1.1.1 and TLS 1.3 enabled. Without TLS 1.3 support, the method raises
NotImplementedError.New in version 3.8.
-
SSLSocket.version() Return the actual SSL protocol version negotiated by the connection as a string, or
Noneis no secure connection is established. As of this writing, possible return values include"SSLv2","SSLv3","TLSv1","TLSv1.1"and"TLSv1.2". Recent OpenSSL versions may define more return values.New in version 3.5.
-
SSLSocket.pending() Returns the number of already decrypted bytes available for read, pending on the connection.
-
SSLSocket.context The
SSLContextobject this SSL socket is tied to. If the SSL socket was created using the deprecatedwrap_socket()function (rather thanSSLContext.wrap_socket()), this is a custom context object created for this SSL socket.New in version 3.2.
-
SSLSocket.server_side A boolean which is
Truefor server-side sockets andFalsefor client-side sockets.New in version 3.2.
-
SSLSocket.server_hostname Hostname of the server:
strtype, orNonefor server-side socket or if the hostname was not specified in the constructor.New in version 3.2.
Changed in version 3.7: The attribute is now always ASCII text. When
server_hostnameis an internationalized domain name (IDN), this attribute now stores the A-label form ("xn--pythn-mua.org"), rather than the U-label form ("pythön.org").
-
SSLSocket.session The
SSLSessionfor this SSL connection. The session is available for client and server side sockets after the TLS handshake has been performed. For client sockets the session can be set beforedo_handshake()has been called to reuse a session.New in version 3.6.
-
SSLSocket.session_reused New in version 3.6.
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=PROTOCOL_TLS) Create a new SSL context. You may pass protocol which must be one of the
PROTOCOL_*constants defined in this module. The parameter 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 isPROTOCOL_TLS; 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):
Footnotes
[1] (1, 2) SSLContextdisables SSLv2 withOP_NO_SSLv2by default.[2] (1, 2) SSLContextdisables SSLv3 withOP_NO_SSLv3by default.[3] (1, 2) TLS 1.3 protocol will be available with PROTOCOL_TLSin OpenSSL >= 1.1.1. There is no dedicated PROTOCOL constant for just TLS 1.3.See also
create_default_context()lets thesslmodule choose security settings for a given purpose.Changed in version 3.6: The context is created with secure default values. The options
OP_NO_COMPRESSION,OP_CIPHER_SERVER_PREFERENCE,OP_SINGLE_DH_USE,OP_SINGLE_ECDH_USE,OP_NO_SSLv2(except forPROTOCOL_SSLv2), andOP_NO_SSLv3(except forPROTOCOL_SSLv3) are set by default. The initial cipher suite list contains onlyHIGHciphers, noNULLciphers and noMD5ciphers (except forPROTOCOL_SSLv2).
SSLContext objects have the following methods and attributes:
-
SSLContext.cert_store_stats() Get statistics about quantities of loaded X.509 certificates, count of X.509 certificates flagged as CA certificates and certificate revocation lists as dictionary.
Example for a context with one CA cert and one other cert:
>>> context.cert_store_stats() {'crl': 0, 'x509_ca': 1, 'x509': 2}
New in version 3.4.
-
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
SSLErroris raised if the private key doesn’t match with the certificate.Changed in version 3.3: New optional argument password.
-
SSLContext.load_default_certs(purpose=Purpose.SERVER_AUTH) Load a set of default “certification authority” (CA) certificates from default locations. On Windows it loads CA certs from the
CAandROOTsystem stores. On other systems it callsSSLContext.set_default_verify_paths(). In the future the method may load CA certificates from other locations, too.The purpose flag specifies what kind of CA certificates are loaded. The default settings
Purpose.SERVER_AUTHloads certificates, that are flagged and trusted for TLS web server authentication (client side sockets).Purpose.CLIENT_AUTHloads CA certificates for client certificate verification on the server side.New in version 3.4.
-
SSLContext.load_verify_locations(cafile=None, capath=None, cadata=None) Load a set of “certification authority” (CA) certificates used to validate other peers’ certificates when
verify_modeis other thanCERT_NONE. At least one of cafile or capath must be specified.This method can also load certification revocation lists (CRLs) in PEM or DER format. In order to make use of CRLs,
SSLContext.verify_flagsmust be configured properly.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.
The cadata object, if present, is either an ASCII string of one or more PEM-encoded certificates or a bytes-like object of DER-encoded certificates. Like with capath extra lines around PEM-encoded certificates are ignored but at least one certificate must be present.
Changed in version 3.4: New optional argument cadata
-
SSLContext.get_ca_certs(binary_form=False) Get a list of loaded “certification authority” (CA) certificates. If the
binary_formparameter isFalseeach list entry is a dict like the output ofSSLSocket.getpeercert(). Otherwise the method returns a list of DER-encoded certificates. The returned list does not contain certificates from capath unless a certificate was requested and loaded by a SSL connection.Note
Certificates in a capath directory aren’t loaded unless they have been used at least once.
New in version 3.4.
-
SSLContext.get_ciphers() Get a list of enabled ciphers. The list is in order of cipher priority. See
SSLContext.set_ciphers().Example:
>>> ctx = ssl.SSLContext(ssl.PROTOCOL_SSLv23) >>> ctx.set_ciphers('ECDHE+AESGCM:!ECDSA') >>> ctx.get_ciphers() # OpenSSL 1.0.x [{'alg_bits': 256, 'description': 'ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=RSA ' 'Enc=AESGCM(256) Mac=AEAD', 'id': 50380848, 'name': 'ECDHE-RSA-AES256-GCM-SHA384', 'protocol': 'TLSv1/SSLv3', 'strength_bits': 256}, {'alg_bits': 128, 'description': 'ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH Au=RSA ' 'Enc=AESGCM(128) Mac=AEAD', 'id': 50380847, 'name': 'ECDHE-RSA-AES128-GCM-SHA256', 'protocol': 'TLSv1/SSLv3', 'strength_bits': 128}]
On OpenSSL 1.1 and newer the cipher dict contains additional fields:
>>> ctx.get_ciphers() # OpenSSL 1.1+ [{'aead': True, 'alg_bits': 256, 'auth': 'auth-rsa', 'description': 'ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=RSA ' 'Enc=AESGCM(256) Mac=AEAD', 'digest': None, 'id': 50380848, 'kea': 'kx-ecdhe', 'name': 'ECDHE-RSA-AES256-GCM-SHA384', 'protocol': 'TLSv1.2', 'strength_bits': 256, 'symmetric': 'aes-256-gcm'}, {'aead': True, 'alg_bits': 128, 'auth': 'auth-rsa', 'description': 'ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH Au=RSA ' 'Enc=AESGCM(128) Mac=AEAD', 'digest': None, 'id': 50380847, 'kea': 'kx-ecdhe', 'name': 'ECDHE-RSA-AES128-GCM-SHA256', 'protocol': 'TLSv1.2', 'strength_bits': 128, 'symmetric': 'aes-128-gcm'}]
Availability: OpenSSL 1.0.2+.
New in version 3.6.
-
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
SSLErrorwill be raised.Note
when connected, the
SSLSocket.cipher()method of SSL sockets will give the currently selected cipher.OpenSSL 1.1.1 has TLS 1.3 cipher suites enabled by default. The suites cannot be disabled with
set_ciphers().
-
SSLContext.set_alpn_protocols(protocols) Specify which protocols the socket should advertise during the SSL/TLS handshake. It should be a list of ASCII 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 RFC 7301. After a successful handshake, theSSLSocket.selected_alpn_protocol()method will return the agreed-upon protocol.This method will raise
NotImplementedErrorifHAS_ALPNisFalse.OpenSSL 1.1.0 to 1.1.0e will abort the handshake and raise
SSLErrorwhen both sides support ALPN but cannot agree on a protocol. 1.1.0f+ behaves like 1.0.2,SSLSocket.selected_alpn_protocol()returns None.New in version 3.5.
-
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 Application Layer Protocol Negotiation. After a successful handshake, theSSLSocket.selected_npn_protocol()method will return the agreed-upon protocol.This method will raise
NotImplementedErrorifHAS_NPNisFalse.New in version 3.3.
-
SSLContext.sni_callback Register a callback function that will be called after the TLS Client Hello handshake message has been received by the SSL/TLS server when the TLS client specifies a server name indication. The server name indication mechanism is specified in RFC 6066 section 3 - Server Name Indication.
Only one callback can be set per
SSLContext. If sni_callback is set toNonethen the callback is disabled. Calling this function a subsequent time will disable the previously registered callback.The callback function will be called with three arguments; the first being the
ssl.SSLSocket, the second is a string that represents the server name that the client is intending to communicate (orNoneif the TLS Client Hello does not contain a server name) and the third argument is the originalSSLContext. The server name argument is text. For internationalized domain name, the server name is an IDN A-label ("xn--pythn-mua.org").A typical use of this callback is to change the
ssl.SSLSocket’sSSLSocket.contextattribute to a new object of typeSSLContextrepresenting a certificate chain that matches the server name.Due to the early negotiation phase of the TLS connection, only limited methods and attributes are usable like
SSLSocket.selected_alpn_protocol()andSSLSocket.context.SSLSocket.getpeercert(),SSLSocket.getpeercert(),SSLSocket.cipher()andSSLSocket.compress()methods require that the TLS connection has progressed beyond the TLS Client Hello and therefore will not contain return meaningful values nor can they be called safely.The sni_callback function must return
Noneto allow the TLS negotiation to continue. If a TLS failure is required, a constantALERT_DESCRIPTION_*can be returned. Other return values will result in a TLS fatal error withALERT_DESCRIPTION_INTERNAL_ERROR.If an exception is raised from the sni_callback function the TLS connection will terminate with a fatal TLS alert message
ALERT_DESCRIPTION_HANDSHAKE_FAILURE.This method will raise
NotImplementedErrorif the OpenSSL library had OPENSSL_NO_TLSEXT defined when it was built.New in version 3.7.
-
SSLContext.set_servername_callback(server_name_callback) This is a legacy API retained for backwards compatibility. When possible, you should use
sni_callbackinstead. The given server_name_callback is similar to sni_callback, except that when the server hostname is an IDN-encoded internationalized domain name, the server_name_callback receives a decoded U-label ("pythön.org").If there is an decoding error on the server name, the TLS connection will terminate with an
ALERT_DESCRIPTION_INTERNAL_ERRORfatal TLS alert message to the client.New in version 3.4.
-
SSLContext.load_dh_params(dhfile) Load the key generation parameters for Diffie-Hellman (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_USEoption 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
prime256v1for a widely supported curve.This setting doesn’t apply to client sockets. You can also use the
OP_SINGLE_ECDH_USEoption to further improve security.This method is not available if
HAS_ECDHisFalse.New in version 3.3.
See also
- SSL/TLS & Perfect Forward Secrecy
- Vincent Bernat.
-
SSLContext.wrap_socket(sock, server_side=False, do_handshake_on_connect=True, suppress_ragged_eofs=True, server_hostname=None, session=None) Wrap an existing Python socket sock and return an instance of
SSLContext.sslsocket_class(defaultSSLSocket). The returned SSL socket is tied to the context, its settings and certificates. sock must be aSOCK_STREAMsocket; other socket types are unsupported.The parameter
server_sideis a boolean which identifies whether server-side or client-side behavior is desired from this socket.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 theaccept()method. The method may raiseSSLError.On client connections, the optional parameter server_hostname specifies the hostname of the service which we are connecting to. This allows a single server to host multiple SSL-based services with distinct certificates, quite similarly to HTTP virtual hosts. Specifying server_hostname will raise a
ValueErrorif server_side is true.The parameter
do_handshake_on_connectspecifies whether to do the SSL handshake automatically after doing asocket.connect(), or whether the application program will call it explicitly, by invoking theSSLSocket.do_handshake()method. CallingSSLSocket.do_handshake()explicitly gives the program control over the blocking behavior of the socket I/O involved in the handshake.The parameter
suppress_ragged_eofsspecifies how theSSLSocket.recv()method should signal unexpected EOF from the other end of the connection. If specified asTrue(the default), it returns a normal EOF (an empty bytes object) in response to unexpected EOF errors raised from the underlying socket; ifFalse, it will raise the exceptions back to the caller.session, see
session.Changed in version 3.5: Always allow a server_hostname to be passed, even if OpenSSL does not have SNI.
Changed in version 3.6: session argument was added.
Changed in version 3.7: The method returns on instance of
SSLContext.sslsocket_classinstead of hard-codedSSLSocket.
-
SSLContext.sslsocket_class The return type of
SSLContext.wrap_socket(), defaults toSSLSocket. The attribute can be overridden on instance of class in order to return a custom subclass ofSSLSocket.New in version 3.7.
-
SSLContext.wrap_bio(incoming, outgoing, server_side=False, server_hostname=None, session=None) Wrap the BIO objects incoming and outgoing and return an instance of
SSLContext.sslobject_class(defaultSSLObject). The SSL routines will read input data from the incoming BIO and write data to the outgoing BIO.The server_side, server_hostname and session parameters have the same meaning as in
SSLContext.wrap_socket().Changed in version 3.6: session argument was added.
Changed in version 3.7: The method returns on instance of
SSLContext.sslobject_classinstead of hard-codedSSLObject.
-
SSLContext.sslobject_class The return type of
SSLContext.wrap_bio(), defaults toSSLObject. The attribute can be overridden on instance of class in order to return a custom subclass ofSSLObject.New in version 3.7.
-
SSLContext.session_stats() Get statistics about the SSL sessions created or managed by this context. A dictionary is returned which maps the names of each piece of information to their numeric values. For example, here is the total number of hits and misses in the session cache since the context was created:
>>> stats = context.session_stats() >>> stats['hits'], stats['misses'] (0, 0)
-
SSLContext.check_hostname Whether to match the peer cert’s hostname with
match_hostname()inSSLSocket.do_handshake(). The context’sverify_modemust be set toCERT_OPTIONALorCERT_REQUIRED, and you must pass server_hostname towrap_socket()in order to match the hostname. Enabling hostname checking automatically setsverify_modefromCERT_NONEtoCERT_REQUIRED. It cannot be set back toCERT_NONEas long as hostname checking is enabled. ThePROTOCOL_TLS_CLIENTprotocol enables hostname checking by default. With other protocols, hostname checking must be enabled explicitly.Example:
import socket, ssl context = ssl.SSLContext(ssl.PROTOCOL_TLSv1_2) context.verify_mode = ssl.CERT_REQUIRED context.check_hostname = True context.load_default_certs() s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) ssl_sock = context.wrap_socket(s, server_hostname='www.verisign.com') ssl_sock.connect(('www.verisign.com', 443))
New in version 3.4.
Changed in version 3.7:
verify_modeis now automatically changed toCERT_REQUIREDwhen hostname checking is enabled andverify_modeisCERT_NONE. Previously the same operation would have failed with aValueError.Note
This features requires OpenSSL 0.9.8f or newer.
-
SSLContext.keylog_filename Write TLS keys to a keylog file, whenever key material is generated or received. The keylog file is designed for debugging purposes only. The file format is specified by NSS and used by many traffic analyzers such as Wireshark. The log file is opened in append-only mode. Writes are synchronized between threads, but not between processes.
New in version 3.8.
Note
This features requires OpenSSL 1.1.1 or newer.
-
SSLContext.maximum_version A
TLSVersionenum member representing the highest supported TLS version. The value defaults toTLSVersion.MAXIMUM_SUPPORTED. The attribute is read-only for protocols other thanPROTOCOL_TLS,PROTOCOL_TLS_CLIENT, andPROTOCOL_TLS_SERVER.The attributes
maximum_version,minimum_versionandSSLContext.optionsall affect the supported SSL and TLS versions of the context. The implementation does not prevent invalid combination. For example a context withOP_NO_TLSv1_2inoptionsandmaximum_versionset toTLSVersion.TLSv1_2will not be able to establish a TLS 1.2 connection.Note
This attribute is not available unless the ssl module is compiled with OpenSSL 1.1.0g or newer.
New in version 3.7.
-
SSLContext.minimum_version Like
SSLContext.maximum_versionexcept it is the lowest supported version orTLSVersion.MINIMUM_SUPPORTED.Note
This attribute is not available unless the ssl module is compiled with OpenSSL 1.1.0g or newer.
New in version 3.7.
-
SSLContext.num_tickets Control the number of TLS 1.3 session tickets of a
TLS_PROTOCOL_SERVERcontext. The setting has no impact on TLS 1.0 to 1.2 connections.Note
This attribute is not available unless the ssl module is compiled with OpenSSL 1.1.1 or newer.
New in version 3.8.
-
SSLContext.options An integer representing the set of SSL options enabled on this context. The default value is
OP_ALL, but you can specify other options such asOP_NO_SSLv2by ORing them together.Note
With versions of OpenSSL older than 0.9.8m, it is only possible to set options, not to clear them. Attempting to clear an option (by resetting the corresponding bits) will raise a
ValueError.Changed in version 3.6:
SSLContext.optionsreturnsOptionsflags:>>> ssl.create_default_context().options <Options.OP_ALL|OP_NO_SSLv3|OP_NO_SSLv2|OP_NO_COMPRESSION: 2197947391>
-
SSLContext.post_handshake_auth Enable TLS 1.3 post-handshake client authentication. Post-handshake auth is disabled by default and a server can only request a TLS client certificate during the initial handshake. When enabled, a server may request a TLS client certificate at any time after the handshake.
When enabled on client-side sockets, the client signals the server that it supports post-handshake authentication.
When enabled on server-side sockets,
SSLContext.verify_modemust be set toCERT_OPTIONALorCERT_REQUIRED, too. The actual client cert exchange is delayed untilSSLSocket.verify_client_post_handshake()is called and some I/O is performed.Note
Only available with OpenSSL 1.1.1 and TLS 1.3 enabled. Without TLS 1.3 support, the property value is None and can’t be modified
New in version 3.8.
-
SSLContext.protocol The protocol version chosen when constructing the context. This attribute is read-only.
-
SSLContext.hostname_checks_common_name Whether
check_hostnamefalls back to verify the cert’s subject common name in the absence of a subject alternative name extension (default: true).Note
Only writeable with OpenSSL 1.1.0 or higher.
New in version 3.7.
-
SSLContext.verify_flags The flags for certificate verification operations. You can set flags like
VERIFY_CRL_CHECK_LEAFby ORing them together. By default OpenSSL does neither require nor verify certificate revocation lists (CRLs). Available only with openssl version 0.9.8+.New in version 3.4.
Changed in version 3.6:
SSLContext.verify_flagsreturnsVerifyFlagsflags:>>> ssl.create_default_context().verify_flags <VerifyFlags.VERIFY_X509_TRUSTED_FIRST: 32768>
-
SSLContext.verify_mode Whether to try to verify other peers’ certificates and how to behave if verification fails. This attribute must be one of
CERT_NONE,CERT_OPTIONALorCERT_REQUIRED.Changed in version 3.6:
SSLContext.verify_modereturnsVerifyModeenum:>>> ssl.create_default_context().verify_mode <VerifyMode.CERT_REQUIRED: 2>
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 they claim 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-----
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-----
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. The platform’s certificates file can
be used by calling SSLContext.load_default_certs(), this is done
automatically with create_default_context().
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-----
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.
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 creates a SSL context with the recommended security settings for client sockets, including automatic certificate verification:
>>> context = ssl.create_default_context()
If you prefer to tune security settings yourself, you might create a context from scratch (but beware that you might not get the settings right):
>>> context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
>>> context.load_verify_locations("/etc/ssl/certs/ca-bundle.crt")
(this snippet assumes 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)
The PROTOCOL_TLS_CLIENT protocol configures the context for cert
validation and hostname verification. verify_mode is
set to CERT_REQUIRED and check_hostname is set
to True. All other protocols create SSL contexts with insecure defaults.
When you use the context to connect to a server, CERT_REQUIRED
and check_hostname validate the server certificate: it
ensures that the server certificate was signed with one of the CA
certificates, checks the signature for correctness, and verifies other
properties like validity and identity of the hostname:
>>> conn = context.wrap_socket(socket.socket(socket.AF_INET),
... server_hostname="www.python.org")
>>> conn.connect(("www.python.org", 443))
You may then fetch the certificate:
>>> cert = conn.getpeercert()
Visual inspection shows that the certificate does identify the desired service
(that is, the HTTPS host www.python.org):
>>> pprint.pprint(cert)
{'OCSP': ('http://ocsp.digicert.com',),
'caIssuers': ('http://cacerts.digicert.com/DigiCertSHA2ExtendedValidationServerCA.crt',),
'crlDistributionPoints': ('http://crl3.digicert.com/sha2-ev-server-g1.crl',
'http://crl4.digicert.com/sha2-ev-server-g1.crl'),
'issuer': ((('countryName', 'US'),),
(('organizationName', 'DigiCert Inc'),),
(('organizationalUnitName', 'www.digicert.com'),),
(('commonName', 'DigiCert SHA2 Extended Validation Server CA'),)),
'notAfter': 'Sep 9 12:00:00 2016 GMT',
'notBefore': 'Sep 5 00:00:00 2014 GMT',
'serialNumber': '01BB6F00122B177F36CAB49CEA8B6B26',
'subject': ((('businessCategory', 'Private Organization'),),
(('1.3.6.1.4.1.311.60.2.1.3', 'US'),),
(('1.3.6.1.4.1.311.60.2.1.2', 'Delaware'),),
(('serialNumber', '3359300'),),
(('streetAddress', '16 Allen Rd'),),
(('postalCode', '03894-4801'),),
(('countryName', 'US'),),
(('stateOrProvinceName', 'NH'),),
(('localityName', 'Wolfeboro'),),
(('organizationName', 'Python Software Foundation'),),
(('commonName', 'www.python.org'),)),
'subjectAltName': (('DNS', 'www.python.org'),
('DNS', 'python.org'),
('DNS', 'pypi.org'),
('DNS', 'docs.python.org'),
('DNS', 'testpypi.org'),
('DNS', 'bugs.python.org'),
('DNS', 'wiki.python.org'),
('DNS', 'hg.python.org'),
('DNS', 'mail.python.org'),
('DNS', 'packaging.python.org'),
('DNS', 'pythonhosted.org'),
('DNS', 'www.pythonhosted.org'),
('DNS', 'test.pythonhosted.org'),
('DNS', 'us.pycon.org'),
('DNS', 'id.python.org')),
'version': 3}
Now the SSL channel is established and the certificate verified, 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/1.1 200 OK',
b'Date: Sat, 18 Oct 2014 18:27:20 GMT',
b'Server: nginx',
b'Content-Type: text/html; charset=utf-8',
b'X-Frame-Options: SAMEORIGIN',
b'Content-Length: 45679',
b'Accept-Ranges: bytes',
b'Via: 1.1 varnish',
b'Age: 2188',
b'X-Served-By: cache-lcy1134-LCY',
b'X-Cache: HIT',
b'X-Cache-Hits: 11',
b'Vary: Cookie',
b'Strict-Transport-Security: max-age=63072000; includeSubDomains',
b'Connection: close',
b'',
b'']
See the discussion of Security considerations below.
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.create_default_context(ssl.Purpose.CLIENT_AUTH)
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).
Notes on non-blocking sockets
SSL sockets behave slightly different than regular sockets in non-blocking mode. When working with non-blocking sockets, there are thus several things you need to be aware of:
Most
SSLSocketmethods will raise eitherSSLWantWriteErrororSSLWantReadErrorinstead ofBlockingIOErrorif an I/O operation would block.SSLWantReadErrorwill be raised if a read operation on the underlying socket is necessary, andSSLWantWriteErrorfor a write operation on the underlying socket. Note that attempts to write to an SSL socket may require reading from the underlying socket first, and attempts to read from the SSL socket may require a prior write to the underlying socket.Changed in version 3.5: In earlier Python versions, the
SSLSocket.send()method returned zero instead of raisingSSLWantWriteErrororSSLWantReadError.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 handleSSLSocket.recv()andSSLSocket.send()failures, and retry after another call toselect().Conversely, since the SSL layer has its own framing, a SSL socket may still have data available for reading without
select()being aware of it. Therefore, you should first callSSLSocket.recv()to drain any potentially available data, and then only block on aselect()call if still necessary.(of course, similar provisions apply when using other primitives such as
poll(), or those in theselectorsmodule)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 usingselect()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], [])
See also
The asyncio module supports non-blocking SSL sockets and provides a
higher level API. It polls for events using the selectors module and
handles SSLWantWriteError, SSLWantReadError and
BlockingIOError exceptions. It runs the SSL handshake asynchronously
as well.
Memory BIO Support
New in version 3.5.
Ever since the SSL module was introduced in Python 2.6, the SSLSocket
class has provided two related but distinct areas of functionality:
- SSL protocol handling
- Network IO
The network IO API is identical to that provided by socket.socket,
from which SSLSocket also inherits. This allows an SSL socket to be
used as a drop-in replacement for a regular socket, making it very easy to add
SSL support to an existing application.
Combining SSL protocol handling and network IO usually works well, but there
are some cases where it doesn’t. An example is async IO frameworks that want to
use a different IO multiplexing model than the “select/poll on a file
descriptor” (readiness based) model that is assumed by socket.socket
and by the internal OpenSSL socket IO routines. This is mostly relevant for
platforms like Windows where this model is not efficient. For this purpose, a
reduced scope variant of SSLSocket called SSLObject is
provided.
-
class
ssl.SSLObject A reduced-scope variant of
SSLSocketrepresenting an SSL protocol instance that does not contain any network IO methods. This class is typically used by framework authors that want to implement asynchronous IO for SSL through memory buffers.This class implements an interface on top of a low-level SSL object as implemented by OpenSSL. This object captures the state of an SSL connection but does not provide any network IO itself. IO needs to be performed through separate “BIO” objects which are OpenSSL’s IO abstraction layer.
This class has no public constructor. An
SSLObjectinstance must be created using thewrap_bio()method. This method will create theSSLObjectinstance and bind it to a pair of BIOs. The incoming BIO is used to pass data from Python to the SSL protocol instance, while the outgoing BIO is used to pass data the other way around.The following methods are available:
contextserver_sideserver_hostnamesessionsession_reusedread()write()getpeercert()selected_alpn_protocol()selected_npn_protocol()cipher()shared_ciphers()compression()pending()do_handshake()verify_client_post_handshake()unwrap()get_channel_binding()version()
When compared to
SSLSocket, this object lacks the following features:- Any form of network IO;
recv()andsend()read and write only to the underlyingMemoryBIObuffers. - There is no do_handshake_on_connect machinery. You must always manually
call
do_handshake()to start the handshake. - There is no handling of suppress_ragged_eofs. All end-of-file conditions
that are in violation of the protocol are reported via the
SSLEOFErrorexception. - The method
unwrap()call does not return anything, unlike for an SSL socket where it returns the underlying socket. - The server_name_callback callback passed to
SSLContext.set_servername_callback()will get anSSLObjectinstance instead of aSSLSocketinstance as its first parameter.
Some notes related to the use of
SSLObject:- All IO on an
SSLObjectis non-blocking. This means that for exampleread()will raise anSSLWantReadErrorif it needs more data than the incoming BIO has available. - There is no module-level
wrap_bio()call like there is forwrap_socket(). AnSSLObjectis always created via anSSLContext.
Changed in version 3.7:
SSLObjectinstances must to created withwrap_bio(). In earlier versions, it was possible to create instances directly. This was never documented or officially supported.
An SSLObject communicates with the outside world using memory buffers. The
class MemoryBIO provides a memory buffer that can be used for this
purpose. It wraps an OpenSSL memory BIO (Basic IO) object:
-
class
ssl.MemoryBIO A memory buffer that can be used to pass data between Python and an SSL protocol instance.
-
pending Return the number of bytes currently in the memory buffer.
-
eof A boolean indicating whether the memory BIO is current at the end-of-file position.
-
read(n=-1) Read up to n bytes from the memory buffer. If n is not specified or negative, all bytes are returned.
-
write(buf) Write the bytes from buf to the memory BIO. The buf argument must be an object supporting the buffer protocol.
The return value is the number of bytes written, which is always equal to the length of buf.
-
SSL session
New in version 3.6.
-
class
ssl.SSLSession Session object used by
session.-
id
-
time
-
timeout
-
ticket_lifetime_hint
-
has_ticket
-
Security considerations
Best defaults
For client use, if you don’t have any special requirements for your
security policy, it is highly recommended that you use the
create_default_context() function to create your SSL context.
It will load the system’s trusted CA certificates, enable certificate
validation and hostname checking, and try to choose reasonably secure
protocol and cipher settings.
For example, here is how you would use the smtplib.SMTP class to
create a trusted, secure connection to a SMTP server:
>>> import ssl, smtplib
>>> smtp = smtplib.SMTP("mail.python.org", port=587)
>>> context = ssl.create_default_context()
>>> smtp.starttls(context=context)
(220, b'2.0.0 Ready to start TLS')
If a client certificate is needed for the connection, it can be added with
SSLContext.load_cert_chain().
By contrast, if you create the SSL context by calling the SSLContext
constructor yourself, it will not have certificate validation nor hostname
checking enabled by default. If you do so, please read the paragraphs below
to achieve a good security level.
Manual settings
Verifying certificates
When calling the SSLContext constructor directly,
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. This common
check is automatically performed when SSLContext.check_hostname is
enabled.
Changed in version 3.7: Hostname matchings is now performed by OpenSSL. Python no longer uses
match_hostname().
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.
Protocol versions
SSL versions 2 and 3 are considered insecure and are therefore dangerous to
use. If you want maximum compatibility between clients and servers, it is
recommended to use PROTOCOL_TLS_CLIENT or
PROTOCOL_TLS_SERVER as the protocol version. SSLv2 and SSLv3 are
disabled by default.
>>> client_context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
>>> client_context.options |= ssl.OP_NO_TLSv1
>>> client_context.options |= ssl.OP_NO_TLSv1_1
The SSL context created above will only allow TLSv1.2 and later (if
supported by your system) connections to a server. PROTOCOL_TLS_CLIENT
implies certificate validation and hostname checks by default. You have to
load certificates into the context.
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. 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
SSLContext.get_ciphers() or the openssl ciphers command on your
system.
Multi-processing
If using this module as part of a multi-processed application (using,
for example the multiprocessing or concurrent.futures modules),
be aware that OpenSSL’s internal random number generator does not properly
handle forked processes. Applications must change the PRNG state of the
parent process if they use any SSL feature with os.fork(). Any
successful call of RAND_add(), RAND_bytes() or
RAND_pseudo_bytes() is sufficient.
TLS 1.3
New in version 3.7.
Python has provisional and experimental support for TLS 1.3 with OpenSSL 1.1.1. The new protocol behaves slightly differently than previous version of TLS/SSL. Some new TLS 1.3 features are not yet available.
- TLS 1.3 uses a disjunct set of cipher suites. All AES-GCM and
ChaCha20 cipher suites are enabled by default. The method
SSLContext.set_ciphers()cannot enable or disable any TLS 1.3 ciphers yet, butSSLContext.get_ciphers()returns them. - Session tickets are no longer sent as part of the initial handshake and
are handled differently.
SSLSocket.sessionandSSLSessionare not compatible with TLS 1.3. - Client-side certificates are also no longer verified during the initial handshake. A server can request a certificate at any time. Clients process certificate requests while they send or receive application data from the server.
- TLS 1.3 features like early data, deferred TLS client cert request, signature algorithm configuration, and rekeying are not supported yet.
LibreSSL support
LibreSSL is a fork of OpenSSL 1.0.1. The ssl module has limited support for LibreSSL. Some features are not available when the ssl module is compiled with LibreSSL.
- LibreSSL >= 2.6.1 no longer supports NPN. The methods
SSLContext.set_npn_protocols()andSSLSocket.selected_npn_protocol()are not available. SSLContext.set_default_verify_paths()ignores the env varsSSL_CERT_FILEandSSL_CERT_PATHalthoughget_default_verify_paths()still reports them.
See also
- Class
socket.socket - Documentation of underlying
socketclass - SSL/TLS Strong Encryption: An Introduction
- Intro from the Apache HTTP Server documentation
- RFC 1422: Privacy Enhancement for Internet Electronic Mail: Part II: Certificate-Based Key Management
- Steve Kent
- RFC 4086: Randomness Requirements for Security
- Donald E., Jeffrey I. Schiller
- RFC 5280: Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile
- D. Cooper
- RFC 5246: The Transport Layer Security (TLS) Protocol Version 1.2
- T. Dierks et. al.
- RFC 6066: Transport Layer Security (TLS) Extensions
- D. Eastlake
- IANA TLS: Transport Layer Security (TLS) Parameters
- IANA
- RFC 7525: Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)
- IETF
- Mozilla’s Server Side TLS recommendations
- Mozilla