ssl — TLS/SSL wrapper for socket objects¶

Functions, constants, and exceptions¶

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())

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())

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()
        ...

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-zero flags argument is not allowed)

• send(), sendall() (with the same limitation)

• sendfile() (it may be high-performant only when the kernel TLS is enabled by setting OP_ENABLE_KTLS or when a socket is plain-text, else send() 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 SSLSocket must be created using the SSLContext.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 the maximum total duration of the shutdown.

Deprecated since version 3.6: It is deprecated to create a SSLSocket instance directly, use SSLContext.wrap_socket() to wrap a socket.

Changed in version 3.7: SSLSocket instances must be created with wrap_socket(). In earlier versions, it was possible to create instances directly. This was never documented or officially supported.

Changed in version 3.10: Python now uses SSL_read_ex and SSL_write_ex internally. The functions support reading and writing of data larger than 2 GB. Writing zero-length data no longer fails with a protocol violation error.

Changed in version 3.15: Python now uses SSL_sendfile internally when possible. The function sends a file more efficiently because it performs TLS encryption in the kernel to avoid additional context switches.

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 bytes instance. If buffer is specified, then read into the buffer instead, and return the number of bytes read.

Raise SSLWantReadError or SSLWantWriteError if 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 longer reset each time bytes are received or sent. The socket timeout is now the maximum total duration to read up to len bytes.

Deprecated since version 3.6: Use recv() instead of read().

SSLSocket.write(data)¶

Write data to the SSL socket and return the number of bytes written. The data argument must be an object supporting the buffer interface.

Raise SSLWantReadError or SSLWantWriteError if 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 longer reset each time bytes are received or sent. The socket timeout is now the maximum total duration to write data.

Deprecated since version 3.6: Use send() instead of write().

SSLSocket.do_handshake(block=False)¶

Perform the SSL setup handshake.

If block is true and the timeout obtained by gettimeout() is zero, the socket is set in blocking mode until the handshake is performed.

Changed in version 3.4: The handshake method also performs match_hostname() when the check_hostname attribute of the socket’s context is true.

Changed in version 3.5: The socket timeout is no longer reset each time bytes are received or sent. The socket timeout is now the 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 sent 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, raise ValueError.

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', 'hostmaster@eff.org'),)),
 'subjectAltName': (('DNS', '*.eff.org'), ('DNS', 'eff.org')),
 'version': 3}

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).

See also SSLContext.check_hostname.

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

Changed in version 3.4: ValueError is raised when the handshake isn’t done. The returned dictionary includes additional X509v3 extension items such as crlDistributionPoints, caIssuers and OCSP URIs.

Changed in version 3.9: IPv6 address strings no longer have a trailing new line.

SSLSocket.get_verified_chain()¶

Returns verified certificate chain provided by the other end of the SSL channel as a list of DER-encoded bytes. If certificate verification was disabled method acts the same as get_unverified_chain().

Added in version 3.13.

SSLSocket.get_unverified_chain()¶

Returns raw certificate chain provided by the other end of the SSL channel as a list of DER-encoded bytes.

Added in version 3.13.

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.shared_ciphers()¶

Return the list of ciphers available in both the client and server. 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() returns None if no connection has been established or the socket is a client socket.

Added in version 3.5.

SSLSocket.group()¶

Return the group used for doing key agreement on this connection. If no connection has been established, returns None.

Added in version 3.15.

SSLSocket.client_sigalg()¶

Return the signature algorithm used for performing certificate-based client authentication on this connection, or None if no connection has been established or client authentication didn’t occur.

Added in version 3.15.

SSLSocket.server_sigalg()¶

Return the signature algorithm used by the server to complete the TLS handshake on this connection, or None if no connection has been established or the cipher suite has no signature.

Added in version 3.15.

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.

Added 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.

Added 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, None is returned.

Added 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 return None.

Added in version 3.3.

Deprecated since version 3.10: NPN has been superseded by ALPN

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 SSLError is raised.

Note

Only available with OpenSSL 1.1.1 and TLS 1.3 enabled. Without TLS 1.3 support, the method raises NotImplementedError.

Added in version 3.8.

SSLSocket.version()¶

Return the actual SSL protocol version negotiated by the connection as a string, or None if 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.

Added in version 3.5.

SSLSocket.pending()¶

Returns the number of already decrypted bytes available for read, pending on the connection.

SSLSocket.context¶

The SSLContext object this SSL socket is tied to.

Added in version 3.2.

SSLSocket.server_side¶

A boolean which is True for server-side sockets and False for client-side sockets.

Added in version 3.2.

SSLSocket.server_hostname¶

Hostname of the server: str type, or None for server-side socket or if the hostname was not specified in the constructor.

Added in version 3.2.

Changed in version 3.7: The attribute is now always ASCII text. When server_hostname is 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 SSLSession for 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 before do_handshake() has been called to reuse a session.

Added in version 3.6.

SSLSocket.session_reused¶

Added in version 3.6.

SSL contexts¶

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=None)¶

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 is PROTOCOL_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):

client / server	SSLv2	SSLv3	TLS [3]	TLSv1	TLSv1.1	TLSv1.2
SSLv2	yes	no	no [1]	no	no	no
SSLv3	no	yes	no [2]	no	no	no
TLS (SSLv23) [3]	no [1]	no [2]	yes	yes	yes	yes
TLSv1	no	no	yes	yes	no	no
TLSv1.1	no	no	yes	no	yes	no
TLSv1.2	no	no	yes	no	no	yes

Footnotes

See also

create_default_context() lets the ssl module 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, and OP_NO_SSLv3 (except for PROTOCOL_SSLv3) are set by default. The initial cipher suite list contains only HIGH ciphers, no NULL ciphers and no MD5 ciphers.

Deprecated since version 3.10: SSLContext without protocol argument is deprecated. The context class will either require PROTOCOL_TLS_CLIENT or PROTOCOL_TLS_SERVER protocol in the future.

Changed in version 3.10: The default cipher suites now include only secure AES and ChaCha20 ciphers with forward secrecy and security level 2. RSA and DH keys with less than 2048 bits and ECC keys with less than 224 bits are prohibited. PROTOCOL_TLS, PROTOCOL_TLS_CLIENT, and PROTOCOL_TLS_SERVER use TLS 1.2 as minimum TLS version.

Note

SSLContext only supports limited mutation once it has been used by a connection. Adding new certificates to the internal trust store is allowed, but changing ciphers, verification settings, or mTLS certificates may result in surprising behavior.

Note

SSLContext is designed to be shared and used by multiple connections. Thus, it is thread-safe as long as it is not reconfigured after being used by a connection.

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}

Added 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. 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_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 CA and ROOT system stores. On all systems it calls SSLContext.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_AUTH loads certificates, that are flagged and trusted for TLS web server authentication (client side sockets). Purpose.CLIENT_AUTH loads CA certificates for client certificate verification on the server side.

Added 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_mode is other than CERT_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_flags must 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_form parameter is False each list entry is a dict like the output of SSLSocket.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.

Added 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()
[{'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'}]

Added in version 3.6.

SSLContext.get_groups(*, include_aliases=False)¶

Get a list of groups implemented for key agreement, taking into account the current TLS minimum_version and maximum_version values. For example:

>>> ctx = ssl.create_default_context()
>>> ctx.minimum_version = ssl.TLSVersion.TLSv1_3
>>> ctx.maximum_version = ssl.TLSVersion.TLSv1_3
>>> ctx.get_groups()
['secp256r1', 'secp384r1', 'secp521r1', 'x25519', 'x448', ...]

By default, this method returns only the preferred IANA names for the available groups. However, if the include_aliases parameter is set to True this method will also return any associated aliases such as the ECDH curve names supported in older versions of OpenSSL.

Added in version 3.15.

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 allowed ciphers for sockets created with this context when connecting using TLS 1.2 and earlier. The ciphers argument should be a string in the OpenSSL cipher list format. To set allowed TLS 1.3 ciphers, use SSLContext.set_ciphersuites().

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 return details about the negotiated cipher.

SSLContext.set_ciphersuites(ciphersuites, /)¶

Set the allowed ciphers for sockets created with this context when connecting using TLS 1.3. The ciphersuites argument should be a colon-separate string of TLS 1.3 cipher names. 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 return details about the negotiated cipher.

Added in version 3.15.

SSLContext.set_groups(groups, /)¶

Set the groups allowed for key agreement for sockets created with this context. It should be a string in the OpenSSL group list format.

Note

When connected, the SSLSocket.group() method of SSL sockets will return the group used for key agreement on that connection.

Added in version 3.15.

SSLContext.set_client_sigalgs(sigalgs, /)¶

Set the signature algorithms allowed for certificate-based client authentication. It should be a string in the OpenSSL client sigalgs list format.

Note

When connected, the SSLSocket.client_sigalg() method of SSL sockets will return the signature algorithm used for performing certificate-based client authentication on that connection.

Added in version 3.15.

SSLContext.set_server_sigalgs(sigalgs, /)¶

Set the signature algorithms allowed for the server to complete the TLS handshake. It should be a string in the OpenSSL sigalgs list format.

Note

When connected, the SSLSocket.server_sigalg() method of SSL sockets will return the signature algorithm used by the server to complete the TLS handshake on that connection.

Added in version 3.15.

SSLContext.set_alpn_protocols(alpn_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, the SSLSocket.selected_alpn_protocol() method will return the agreed-upon protocol.

This method will raise NotImplementedError if HAS_ALPN is False.

Added in version 3.5.

SSLContext.set_npn_protocols(npn_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, the SSLSocket.selected_npn_protocol() method will return the agreed-upon protocol.

This method will raise NotImplementedError if HAS_NPN is False.

Added in version 3.3.

Deprecated since version 3.10: NPN has been superseded by ALPN

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 to None then 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 (or None if the TLS Client Hello does not contain a server name) and the third argument is the original SSLContext. 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’s SSLSocket.context attribute to a new object of type SSLContext representing 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() and SSLSocket.context. The SSLSocket.getpeercert(), SSLSocket.get_verified_chain(), SSLSocket.get_unverified_chain() SSLSocket.cipher() and SSLSocket.compression() methods require that the TLS connection has progressed beyond the TLS Client Hello and therefore will not return meaningful values nor can they be called safely.

The sni_callback function must return None to allow the TLS negotiation to continue. If a TLS failure is required, a constant ALERT_DESCRIPTION_* can be returned. Other return values will result in a TLS fatal error with ALERT_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 NotImplementedError if the OpenSSL library had OPENSSL_NO_TLSEXT defined when it was built.

Added 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_callback instead. 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 a decoding error on the server name, the TLS connection will terminate with an ALERT_DESCRIPTION_INTERNAL_ERROR fatal TLS alert message to the client.

Added 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_USE option to further improve security.

Added 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.

Added 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 (default SSLSocket). The returned SSL socket is tied to the context, its settings and certificates. sock must be a SOCK_STREAM socket; other socket types are unsupported.

The parameter server_side is 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 the accept() method. The method may raise SSLError.

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 ValueError if server_side is true.

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.

session, see session.

To wrap an SSLSocket in another SSLSocket, use SSLContext.wrap_bio().

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 an instance of SSLContext.sslsocket_class instead of hard-coded SSLSocket.

SSLContext.sslsocket_class¶

The return type of SSLContext.wrap_socket(), defaults to SSLSocket. The attribute can be assigned to on instances of SSLContext in order to return a custom subclass of SSLSocket.

Added 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 (default SSLObject). 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 an instance of SSLContext.sslobject_class instead of hard-coded SSLObject.

SSLContext.sslobject_class¶

The return type of SSLContext.wrap_bio(), defaults to SSLObject. The attribute can be overridden on instance of class in order to return a custom subclass of SSLObject.

Added 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 in SSLSocket.do_handshake(). The context’s verify_mode must be set to CERT_OPTIONAL or CERT_REQUIRED, and you must pass server_hostname to wrap_socket() in order to match the hostname. Enabling hostname checking automatically sets verify_mode from CERT_NONE to CERT_REQUIRED. It cannot be set back to CERT_NONE as long as hostname checking is enabled. The PROTOCOL_TLS_CLIENT protocol 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))

Added in version 3.4.

Changed in version 3.7: verify_mode is now automatically changed to CERT_REQUIRED when hostname checking is enabled and verify_mode is CERT_NONE. Previously the same operation would have failed with a ValueError.

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.

Added in version 3.8.

SSLContext.maximum_version¶

A TLSVersion enum member representing the highest supported TLS version. The value defaults to TLSVersion.MAXIMUM_SUPPORTED. The attribute is read-only for protocols other than PROTOCOL_TLS, PROTOCOL_TLS_CLIENT, and PROTOCOL_TLS_SERVER.

The attributes maximum_version, minimum_version and SSLContext.options all affect the supported SSL and TLS versions of the context. The implementation does not prevent invalid combinations. For example a context with OP_NO_TLSv1_2 in options and maximum_version set to TLSVersion.TLSv1_2 will not be able to establish a TLS 1.2 connection.

Added in version 3.7.

SSLContext.minimum_version¶

Like SSLContext.maximum_version except it is the lowest supported version or TLSVersion.MINIMUM_SUPPORTED.

Added in version 3.7.

SSLContext.num_tickets¶

Control the number of TLS 1.3 session tickets of a PROTOCOL_TLS_SERVER context. The setting has no impact on TLS 1.0 to 1.2 connections.

Added 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 as OP_NO_SSLv2 by ORing them together.

Changed in version 3.6: SSLContext.options returns Options flags:

>>> ssl.create_default_context().options
<Options.OP_ALL|OP_NO_SSLv3|OP_NO_SSLv2|OP_NO_COMPRESSION: 2197947391>

Deprecated since version 3.7: All OP_NO_SSL* and OP_NO_TLS* options have been deprecated since Python 3.7. Use SSLContext.minimum_version and SSLContext.maximum_version instead.

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_mode must be set to CERT_OPTIONAL or CERT_REQUIRED, too. The actual client cert exchange is delayed until SSLSocket.verify_client_post_handshake() is called and some I/O is performed.

Added 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_hostname falls back to verify the cert’s subject common name in the absence of a subject alternative name extension (default: true).

Added in version 3.7.

Changed in version 3.10: The flag had no effect with OpenSSL before version 1.1.1l. Python 3.8.9, 3.9.3, and 3.10 include workarounds for previous versions.

SSLContext.security_level¶

An integer representing the security level for the context. This attribute is read-only.

Added in version 3.10.

SSLContext.verify_flags¶

The flags for certificate verification operations. You can set flags like VERIFY_CRL_CHECK_LEAF by ORing them together. By default OpenSSL does neither require nor verify certificate revocation lists (CRLs).

Added in version 3.4.

Changed in version 3.6: SSLContext.verify_flags returns VerifyFlags flags:

>>> 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_OPTIONAL or CERT_REQUIRED.

Changed in version 3.6: SSLContext.verify_mode returns VerifyMode enum:

>>> ssl.create_default_context().verify_mode
<VerifyMode.CERT_REQUIRED: 2>

SSLContext.set_psk_client_callback(callback)¶

Enables TLS-PSK (pre-shared key) authentication on a client-side connection.

In general, certificate based authentication should be preferred over this method.

The parameter callback is a callable object with the signature: def callback(hint: str | None) -> tuple[str | None, bytes]. The hint parameter is an optional identity hint sent by the server. The return value is a tuple in the form (client-identity, psk). Client-identity is an optional string which may be used by the server to select a corresponding PSK for the client. The string must be less than or equal to 256 octets when UTF-8 encoded. PSK is a bytes-like object representing the pre-shared key. Return a zero length PSK to reject the connection.

Setting callback to None removes any existing callback.

Note

When using TLS 1.3:

• the hint parameter is always None.

• client-identity must be a non-empty string.

Example usage:

context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
context.check_hostname = False
context.verify_mode = ssl.CERT_NONE
context.maximum_version = ssl.TLSVersion.TLSv1_2
context.set_ciphers('PSK')

# A simple lambda:
psk = bytes.fromhex('c0ffee')
context.set_psk_client_callback(lambda hint: (None, psk))

# A table using the hint from the server:
psk_table = { 'ServerId_1': bytes.fromhex('c0ffee'),
              'ServerId_2': bytes.fromhex('facade')
}
def callback(hint):
    return 'ClientId_1', psk_table.get(hint, b'')
context.set_psk_client_callback(callback)

This method will raise NotImplementedError if HAS_PSK is False.

Added in version 3.13.

SSLContext.set_psk_server_callback(callback, identity_hint=None)¶

Enables TLS-PSK (pre-shared key) authentication on a server-side connection.

In general, certificate based authentication should be preferred over this method.

The parameter callback is a callable object with the signature: def callback(identity: str | None) -> bytes. The identity parameter is an optional identity sent by the client which can be used to select a corresponding PSK. The return value is a bytes-like object representing the pre-shared key. Return a zero length PSK to reject the connection.

Setting callback to None removes any existing callback.

The parameter identity_hint is an optional identity hint string sent to the client. The string must be less than or equal to 256 octets when UTF-8 encoded.

Note

When using TLS 1.3 the identity_hint parameter is not sent to the client.

Example usage:

context = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
context.maximum_version = ssl.TLSVersion.TLSv1_2
context.set_ciphers('PSK')

# A simple lambda:
psk = bytes.fromhex('c0ffee')
context.set_psk_server_callback(lambda identity: psk)

# A table using the identity of the client:
psk_table = { 'ClientId_1': bytes.fromhex('c0ffee'),
              'ClientId_2': bytes.fromhex('facade')
}
def callback(identity):
    return psk_table.get(identity, b'')
context.set_psk_server_callback(callback, 'ServerId_1')

This method will raise NotImplementedError if HAS_PSK is False.

Added in version 3.13.

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-----

-----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-----

-----BEGIN RSA PRIVATE KEY-----
... (private key in base64 encoding) ...
-----END RSA PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
... (certificate in base64 PEM encoding) ...
-----END CERTIFICATE-----

% 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
%

Examples¶

try:
    import ssl
except ImportError:
    pass
else:
    ...  # do something that requires SSL support

>>> context = ssl.create_default_context()

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

>>> conn = context.wrap_socket(socket.socket(socket.AF_INET),
...                            server_hostname="www.python.org")
>>> conn.connect(("www.python.org", 443))

>>> cert = conn.getpeercert()

>>> 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}

>>> 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'']

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.example.com', 10023))
bindsocket.listen(5)

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()

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

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 SSLSocket methods will raise either SSLWantWriteError or SSLWantReadError instead of BlockingIOError if an I/O operation would block. SSLWantReadError will be raised if a read operation on the underlying socket is necessary, and SSLWantWriteError for 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 raising SSLWantWriteError or SSLWantReadError.

• 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().

• 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 call SSLSocket.recv() to drain any potentially available data, and then only block on a select() call if still necessary.

  (of course, similar provisions apply when using other primitives such as poll(), or those in the selectors module)

• 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], [])
  

Memory BIO support¶

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 SSLSocket representing 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 SSLObject instance must be created using the wrap_bio() method. This method will create the SSLObject instance 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:

• context

• server_side

• server_hostname

• session

• session_reused

• read()

• write()

• getpeercert()

• get_verified_chain()

• get_unverified_chain()

• 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() and send() read and write only to the underlying MemoryBIO buffers.

• 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 SSLEOFError exception.

• 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 an SSLObject instance instead of a SSLSocket instance as its first parameter.

Some notes related to the use of SSLObject:

• All IO on an SSLObject is non-blocking. This means that for example read() will raise an SSLWantReadError if it needs more data than the incoming BIO has available.

Changed in version 3.7: SSLObject instances must be created with wrap_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.

write_eof()¶

Write an EOF marker to the memory BIO. After this method has been called, it is illegal to call write(). The attribute eof will become true after all data currently in the buffer has been read.

SSL session¶

class ssl.SSLSession¶

Session object used by session.

id¶

time¶

timeout¶

ticket_lifetime_hint¶

has_ticket¶

Security considerations¶

>>> 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')

>>> client_context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
>>> client_context.minimum_version = ssl.TLSVersion.TLSv1_2
>>> client_context.maximum_version = ssl.TLSVersion.TLSv1_3

TLS 1.3¶

The TLS 1.3 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. To restrict which TLS 1.3 ciphers are allowed, the SSLContext.set_ciphersuites() method should be called instead of SSLContext.set_ciphers(), which only affects ciphers in older TLS versions. The SSLContext.get_ciphers() method returns information about ciphers for both TLS 1.3 and earlier versions and the method SSLSocket.cipher() returns information about the negotiated cipher for both TLS 1.3 and earlier versions once a connection is established.

• Session tickets are no longer sent as part of the initial handshake and are handled differently. SSLSocket.session and SSLSession are 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, and rekeying are not supported yet.