18.2. ssl
— TLS/SSL wrapper for socket objects¶
Source code: Lib/ssl.py
This module provides access to Transport Layer Security (often known as «Secure Sockets Layer») encryption and peer authentication facilities for network sockets, both client-side and server-side. This module uses the OpenSSL library. It is available on all modern Unix systems, Windows, Mac OS X, and probably additional platforms, as long as OpenSSL is installed on that platform.
Nota
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.
Advertencia
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.
18.2.1. Functions, Constants, and Exceptions¶
-
exception
ssl.
SSLError
¶ Raised to signal an error from the underlying SSL implementation (currently provided by the OpenSSL library). This signifies some problem in the higher-level encryption and authentication layer that’s superimposed on the underlying network connection. This error is a subtype of
OSError
. The error code and message ofSSLError
instances are provided by the OpenSSL library.Distinto en la versión 3.3:
SSLError
used to be a subtype ofsocket.error
.-
library
¶ A string mnemonic designating the OpenSSL submodule in which the error occurred, such as
SSL
,PEM
orX509
. The range of possible values depends on the OpenSSL version.Nuevo en la versión 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.Nuevo en la versión 3.3.
-
-
exception
ssl.
SSLZeroReturnError
¶ A subclass of
SSLError
raised when trying to read or write and the SSL connection has been closed cleanly. Note that this doesn’t mean that the underlying transport (read TCP) has been closed.Nuevo en la versión 3.3.
-
exception
ssl.
SSLWantReadError
¶ A subclass of
SSLError
raised by a non-blocking SSL socket when trying to read or write data, but more data needs to be received on the underlying TCP transport before the request can be fulfilled.Nuevo en la versión 3.3.
-
exception
ssl.
SSLWantWriteError
¶ A subclass of
SSLError
raised by a non-blocking SSL socket when trying to read or write data, but more data needs to be sent on the underlying TCP transport before the request can be fulfilled.Nuevo en la versión 3.3.
-
exception
ssl.
SSLSyscallError
¶ A subclass of
SSLError
raised when a system error was encountered while trying to fulfill an operation on a SSL socket. Unfortunately, there is no easy way to inspect the original errno number.Nuevo en la versión 3.3.
-
exception
ssl.
SSLEOFError
¶ A subclass of
SSLError
raised when the SSL connection has been terminated abruptly. Generally, you shouldn’t try to reuse the underlying transport when this error is encountered.Nuevo en la versión 3.3.
-
exception
ssl.
CertificateError
¶ Raised to signal an error with a certificate (such as mismatching hostname). Certificate errors detected by OpenSSL, though, raise an
SSLError
.
18.2.1.1. Socket creation¶
The following function allows for standalone socket creation. Starting from
Python 3.2, it can be more flexible to use SSLContext.wrap_socket()
instead.
-
ssl.
wrap_socket
(sock, keyfile=None, certfile=None, server_side=False, cert_reqs=CERT_NONE, ssl_version={see docs}, ca_certs=None, do_handshake_on_connect=True, suppress_ragged_eofs=True, ciphers=None)¶ Takes an instance
sock
ofsocket.socket
, and returns an instance ofssl.SSLSocket
, a subtype ofsocket.socket
, which wraps the underlying socket in an SSL context.sock
must be aSOCK_STREAM
socket; other socket types are unsupported.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.wrap_socket()
may raiseSSLError
.The
keyfile
andcertfile
parameters specify optional files which contain a certificate to be used to identify the local side of the connection. See the discussion of Certificates for more information on how the certificate is stored in thecertfile
.The parameter
server_side
is a boolean which identifies whether server-side or client-side behavior is desired from this socket.The parameter
cert_reqs
specifies whether a certificate is required from the other side of the connection, and whether it will be validated if provided. It must be one of the three valuesCERT_NONE
(certificates ignored),CERT_OPTIONAL
(not required, but validated if provided), orCERT_REQUIRED
(required and validated). If the value of this parameter is notCERT_NONE
, then theca_certs
parameter must point to a file of CA certificates.The
ca_certs
file contains a set of concatenated «certification authority» certificates, which are used to validate certificates passed from the other end of the connection. See the discussion of Certificates for more information about how to arrange the certificates in this file.The parameter
ssl_version
specifies which version of the SSL protocol to use. Typically, the server chooses a particular protocol version, and the client must adapt to the server’s choice. Most of the versions are not interoperable with the other versions. If not specified, the default 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):
client / server SSLv2 SSLv3 TLS TLSv1 TLSv1.1 TLSv1.2 SSLv2 yes no yes no no no SSLv3 no yes yes no no no TLS (SSLv23) no yes 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 Nota
Which connections succeed will vary depending on the version of OpenSSL. For example, before OpenSSL 1.0.0, an SSLv23 client would always attempt SSLv2 connections.
The ciphers parameter sets the available ciphers for this SSL object. It should be a string in the OpenSSL cipher list format.
The parameter
do_handshake_on_connect
specifies whether to do the SSL handshake automatically after doing 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_eofs
specifies 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.Distinto en la versión 3.2: New optional argument ciphers.
18.2.1.2. 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
SSLContext
object with default settings for the given purpose. The settings are chosen by thessl
module, and usually represent a higher security level than when calling theSSLContext
constructor 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_SSLv3
with high encryption cipher suites without RC4 and without unauthenticated cipher suites. PassingSERVER_AUTH
as purpose setsverify_mode
toCERT_REQUIRED
and 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.Nota
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
SSLContext
and apply the settings yourself.Nota
If you find that when certain older clients or servers attempt to connect with a
SSLContext
created 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
Nuevo en la versión 3.4.
Distinto en la versión 3.4.4: RC4 was dropped from the default cipher string.
Distinto en la versión 3.5.3: ChaCha20/Poly1305 was added to the default cipher string.
3DES was dropped from the default cipher string.
18.2.1.3. Random generation¶
-
ssl.
RAND_bytes
(num)¶ Return num cryptographically strong pseudo-random bytes. Raises an
SSLError
if the PRNG has not been seeded with enough data or if the operation is not supported by the current RAND method.RAND_status()
can be used to check the status of the PRNG 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 generator.
Nuevo en la versión 3.3.
-
ssl.
RAND_pseudo_bytes
(num)¶ Return (bytes, is_cryptographic): bytes are num pseudo-random bytes, is_cryptographic is
True
if the bytes generated are cryptographically strong. Raises anSSLError
if the operation is not supported by the current RAND method.Generated pseudo-random byte sequences will be unique if they are of sufficient length, but are not necessarily unpredictable. They can be used for non-cryptographic purposes and for certain purposes in cryptographic protocols, but usually not for key generation etc.
For almost all applications
os.urandom()
is preferable.Nuevo en la versión 3.3.
Obsoleto desde la versión 3.5.3: OpenSSL has deprecated
ssl.RAND_pseudo_bytes()
, usessl.RAND_bytes()
instead.
-
ssl.
RAND_status
()¶ Return
True
if the SSL pseudo-random number generator has been seeded with “enough” randomness, andFalse
otherwise. 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.Distinto en la versión 3.5: Writable bytes-like object is now accepted.
18.2.1.4. Certificate handling¶
-
ssl.
match_hostname
(cert, hostname)¶ Verify that cert (in decoded format as returned by
SSLSocket.getpeercert()
) matches the given hostname. The rules applied are those for checking the identity of HTTPS servers as outlined in RFC 2818 and RFC 6125. In addition to HTTPS, this function should be suitable for checking the identity of servers in various SSL-based protocols such as FTPS, IMAPS, POPS and others.CertificateError
is raised on failure. On success, the function returns nothing:>>> cert = {'subject': ((('commonName', 'example.com'),),)} >>> ssl.match_hostname(cert, "example.com") >>> ssl.match_hostname(cert, "example.org") Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/home/py3k/Lib/ssl.py", line 130, in match_hostname ssl.CertificateError: hostname 'example.org' doesn't match 'example.com'
Nuevo en la versión 3.2.
Distinto en la versión 3.3.3: The function now follows RFC 6125, section 6.4.3 and does neither match multiple wildcards (e.g.
*.*.com
or*a*.example.org
) nor a wildcard inside an internationalized domain names (IDN) fragment. IDN A-labels such aswww*.xn--pthon-kva.org
are still supported, butx*.python.org
no longer matchesxn--tda.python.org
.Distinto en la versión 3.5: Matching of IP addresses, when present in the subjectAltName field of the certificate, is now supported.
-
ssl.
cert_time_to_seconds
(cert_time)¶ Return the time in seconds since the Epoch, given the
cert_time
string 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).
Distinto en la versión 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
addr
of an SSL-protected server, as a (hostname, port-number) pair, fetches the server’s certificate, and returns it as a PEM-encoded string. Ifssl_version
is specified, uses that version of the SSL protocol to attempt to connect to the server. Ifca_certs
is specified, it should be a file containing a list of root certificates, the same format as used for the same parameter inwrap_socket()
. The call will attempt to validate the server certificate against that set of root certificates, and will fail if the validation attempt fails.Distinto en la versión 3.3: This function is now IPv6-compatible.
Distinto en la versión 3.5: The default ssl_version is changed from
PROTOCOL_SSLv3
toPROTOCOL_TLS
for maximum compatibility with modern servers.
-
ssl.
DER_cert_to_PEM_cert
(DER_cert_bytes)¶ Given a certificate as a DER-encoded blob of bytes, returns a PEM-encoded string version of the same certificate.
-
ssl.
PEM_cert_to_DER_cert
(PEM_cert_string)¶ Given a certificate as an ASCII PEM string, returns a DER-encoded sequence of bytes for that same certificate.
-
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 orNone
if the file doesn’t exist,capath
- resolved path to capath orNone
if 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_env
andopenssl_capath_env
Nuevo en la versión 3.4.
-
ssl.
enum_certificates
(store_name)¶ Retrieve certificates from Windows” system cert store. store_name may be one of
CA
,ROOT
orMY
. 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_asn
for X.509 ASN.1 data orpkcs_7_asn
for PKCS#7 ASN.1 data. Trust specifies the purpose of the certificate as a set of OIDS or exactlyTrue
if 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.
Nuevo en la versión 3.4.
-
ssl.
enum_crls
(store_name)¶ Retrieve CRLs from Windows” system cert store. store_name may be one of
CA
,ROOT
orMY
. 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_asn
for X.509 ASN.1 data orpkcs_7_asn
for PKCS#7 ASN.1 data.Availability: Windows.
Nuevo en la versión 3.4.
18.2.1.5. Constants¶
-
ssl.
CERT_NONE
¶ Possible value for
SSLContext.verify_mode
, or thecert_reqs
parameter towrap_socket()
. In this mode (the default), no certificates will be required from the other side of the socket connection. If a certificate is received from the other end, no attempt to validate it is made.See the discussion of Security considerations below.
-
ssl.
CERT_OPTIONAL
¶ Possible value for
SSLContext.verify_mode
, or thecert_reqs
parameter towrap_socket()
. In this mode no certificates will be required from the other side of the socket connection; but if they are provided, validation will be attempted and anSSLError
will be raised on failure.Use of this setting requires a valid set of CA certificates to be passed, either to
SSLContext.load_verify_locations()
or as a value of theca_certs
parameter towrap_socket()
.
-
ssl.
CERT_REQUIRED
¶ Possible value for
SSLContext.verify_mode
, or thecert_reqs
parameter towrap_socket()
. In this mode, certificates are required from the other side of the socket connection; anSSLError
will be raised if no certificate is provided, or if its validation fails.Use of this setting requires a valid set of CA certificates to be passed, either to
SSLContext.load_verify_locations()
or as a value of theca_certs
parameter towrap_socket()
.
-
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.Nuevo en la versión 3.4.
-
ssl.
VERIFY_CRL_CHECK_LEAF
¶ Possible value for
SSLContext.verify_flags
. In this mode, only the peer cert is check but non 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 has been loadedSSLContext.load_verify_locations
, validation will fail.Nuevo en la versión 3.4.
-
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.Nuevo en la versión 3.4.
-
ssl.
VERIFY_X509_STRICT
¶ Possible value for
SSLContext.verify_flags
to disable workarounds for broken X.509 certificates.Nuevo en la versión 3.4.
-
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.Nuevo en la versión 3.4.4.
-
ssl.
PROTOCOL_TLS
¶ Selects the highest protocol version that both the client and server support. Despite the name, this option can select «TLS» protocols as well as «SSL».
Nuevo en la versión 3.5.3.
-
ssl.
PROTOCOL_SSLv23
¶ Alias for data:PROTOCOL_TLS.
Obsoleto desde la versión 3.5.3: Use data:PROTOCOL_TLS instead.
-
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_SSL2
flag.Advertencia
SSL version 2 is insecure. Its use is highly discouraged.
Obsoleto desde la versión 3.5.3: OpenSSL has removed support for SSLv2.
-
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_SSLv3
flag.Advertencia
SSL version 3 is insecure. Its use is highly discouraged.
Obsoleto desde la versión 3.5.3: OpenSSL has deprecated all version specific protocols. Use the default protocol data:PROTOCOL_TLS with flags like data:OP_NO_SSLv3 instead.
-
ssl.
PROTOCOL_TLSv1
¶ Selects TLS version 1.0 as the channel encryption protocol.
Obsoleto desde la versión 3.5.3: OpenSSL has deprecated all version specific protocols. Use the default protocol data:PROTOCOL_TLS with flags like data:OP_NO_SSLv3 instead.
-
ssl.
PROTOCOL_TLSv1_1
¶ Selects TLS version 1.1 as the channel encryption protocol. Available only with openssl version 1.0.1+.
Nuevo en la versión 3.4.
Obsoleto desde la versión 3.5.3: OpenSSL has deprecated all version specific protocols. Use the default protocol data:PROTOCOL_TLS with flags like data:OP_NO_SSLv3 instead.
-
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+.
Nuevo en la versión 3.4.
Obsoleto desde la versión 3.5.3: OpenSSL has deprecated all version specific protocols. Use the default protocol data:PROTOCOL_TLS with flags like data:OP_NO_SSLv3 instead.
-
ssl.
OP_ALL
¶ Enables workarounds for various bugs present in other SSL implementations. This option is set by default. It does not necessarily set the same flags as OpenSSL’s
SSL_OP_ALL
constant.Nuevo en la versión 3.2.
-
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.Nuevo en la versión 3.2.
Obsoleto desde la versión 3.5.3: 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.Nuevo en la versión 3.2.
Obsoleto desde la versión 3.5.3: 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.Nuevo en la versión 3.2.
-
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+.Nuevo en la versión 3.4.
-
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+.Nuevo en la versión 3.4.
-
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.
Nuevo en la versión 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.
Nuevo en la versión 3.3.
-
ssl.
OP_SINGLE_ECDH_USE
¶ Prevents re-use of the same ECDH key for distinct SSL sessions. This improves forward secrecy but requires more computational resources. This option only applies to server sockets.
Nuevo en la versión 3.3.
-
ssl.
OP_NO_COMPRESSION
¶ Disable compression on the SSL channel. This is useful if the application protocol supports its own compression scheme.
This option is only available with OpenSSL 1.0.0 and later.
Nuevo en la versión 3.3.
-
ssl.
HAS_ALPN
¶ Whether the OpenSSL library has built-in support for the Application-Layer Protocol Negotiation TLS extension as described in RFC 7301.
Nuevo en la versión 3.5.
-
ssl.
HAS_ECDH
¶ Whether the OpenSSL library has built-in support for Elliptic Curve-based Diffie-Hellman key exchange. This should be true unless the feature was explicitly disabled by the distributor.
Nuevo en la versión 3.3.
-
ssl.
HAS_SNI
¶ Whether the OpenSSL library has built-in support for the Server Name Indication extension (as defined in RFC 4366).
Nuevo en la versión 3.2.
-
ssl.
HAS_NPN
¶ Whether the OpenSSL library has built-in support for Next Protocol Negotiation as described in the NPN draft specification. When true, you can use the
SSLContext.set_npn_protocols()
method to advertise which protocols you want to support.Nuevo en la versión 3.3.
-
ssl.
CHANNEL_BINDING_TYPES
¶ List of supported TLS channel binding types. Strings in this list can be used as arguments to
SSLSocket.get_channel_binding()
.Nuevo en la versión 3.3.
-
ssl.
OPENSSL_VERSION
¶ The version string of the OpenSSL library loaded by the interpreter:
>>> ssl.OPENSSL_VERSION 'OpenSSL 0.9.8k 25 Mar 2009'
Nuevo en la versión 3.2.
-
ssl.
OPENSSL_VERSION_INFO
¶ A tuple of five integers representing version information about the OpenSSL library:
>>> ssl.OPENSSL_VERSION_INFO (0, 9, 8, 11, 15)
Nuevo en la versión 3.2.
-
ssl.
OPENSSL_VERSION_NUMBER
¶ The raw version number of the OpenSSL library, as a single integer:
>>> ssl.OPENSSL_VERSION_NUMBER 9470143 >>> hex(ssl.OPENSSL_VERSION_NUMBER) '0x9080bf'
Nuevo en la versión 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()
.Nuevo en la versión 3.4.
-
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).Nuevo en la versión 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).Nuevo en la versión 3.4.
18.2.2. 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-zeroflags
argument is not allowed)send()
,sendall()
(with the same limitation)sendfile()
(butos.sendfile
will 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.
Usually,
SSLSocket
are not created directly, but using thewrap_socket()
function or theSSLContext.wrap_socket()
method.Distinto en la versión 3.5: The
sendfile()
method was added.Distinto en la versión 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.
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
orSSLWantWriteError
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.Distinto en la versión 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.
-
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
SSLWantReadError
orSSLWantWriteError
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.Distinto en la versión 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.
Nota
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.
Distinto en la versión 3.4: The handshake method also performs
match_hostname()
when thecheck_hostname
attribute of the socket’scontext
is true.Distinto en la versión 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.
-
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_form
parameter isFalse
, and a certificate was received from the peer, this method returns adict
instance. 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 asubjectAltName
key in the dictionary.The
subject
andissuer
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}
Nota
To validate a certificate for a particular service, you can use the
match_hostname()
function.If the
binary_form
parameter isTrue
, and a certificate was provided, this method returns the DER-encoded form of the entire certificate as a sequence of bytes, orNone
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 returnNone
if you usedCERT_NONE
(rather thanCERT_OPTIONAL
orCERT_REQUIRED
).
Distinto en la versión 3.2: The returned dictionary includes additional items such as
issuer
andnotBefore
.Distinto en la versión 3.4:
ValueError
is raised when the handshake isn’t done. The returned dictionary includes additional X509v3 extension items such ascrlDistributionPoints
,caIssuers
andOCSP
URIs.
-
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()
returnsNone
if no connection has been established or the socket is a client socket.Nuevo en la versión 3.5.
-
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.Nuevo en la versión 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.Nuevo en la versión 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.Nuevo en la versión 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
.Nuevo en la versión 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.
version
()¶ Return the actual SSL protocol version negotiated by the connection as a string, or
None
is 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.Nuevo en la versión 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. If the SSL socket was created using the top-levelwrap_socket()
function (rather thanSSLContext.wrap_socket()
), this is a custom context object created for this SSL socket.Nuevo en la versión 3.2.
-
SSLSocket.
server_side
¶ A boolean which is
True
for server-side sockets andFalse
for client-side sockets.Nuevo en la versión 3.2.
18.2.3. SSL Contexts¶
Nuevo en la versión 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.PROTOCOL_TLS
is currently recommended for maximum interoperability and default value.Ver también
create_default_context()
lets thessl
module choose security settings for a given purpose.Distinto en la versión 3.5.3:
PROTOCOL_TLS
is the default value.
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}
Nuevo en la versión 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
SSLError
is raised if the private key doesn’t match with the certificate.Distinto en la versión 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
andROOT
system 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_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.Nuevo en la versión 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 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_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.
Distinto en la versión 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 isFalse
each 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.Nota
Certificates in a capath directory aren’t loaded unless they have been used at least once.
Nuevo en la versión 3.4.
-
SSLContext.
set_default_verify_paths
()¶ Load a set of default «certification authority» (CA) certificates from a filesystem path defined when building the OpenSSL library. Unfortunately, there’s no easy way to know whether this method succeeds: no error is returned if no certificates are to be found. When the OpenSSL library is provided as part of the operating system, though, it is likely to be configured properly.
-
SSLContext.
set_ciphers
(ciphers)¶ Set the available ciphers for sockets created with this context. It should be a string in the OpenSSL cipher list format. If no cipher can be selected (because compile-time options or other configuration forbids use of all the specified ciphers), an
SSLError
will be raised.Nota
when connected, the
SSLSocket.cipher()
method of SSL sockets will give the currently selected cipher.
-
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
NotImplementedError
ifHAS_ALPN
is False.OpenSSL 1.1.0+ will abort the handshake and raise
SSLError
when both sides support ALPN but cannot agree on a protocol.Nuevo en la versión 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 NPN draft specification. After a successful handshake, theSSLSocket.selected_npn_protocol()
method will return the agreed-upon protocol.This method will raise
NotImplementedError
ifHAS_NPN
is False.Nuevo en la versión 3.3.
-
SSLContext.
set_servername_callback
(server_name_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 server_name_callback isNone
then the callback is disabled. Calling this function a subsequent time will disable the previously registered callback.The callback function, server_name_callback, 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 (orNone
if the TLS Client Hello does not contain a server name) and the third argument is the originalSSLContext
. The server name argument is the IDNA decoded server name.A typical use of this callback is to change the
ssl.SSLSocket
’sSSLSocket.context
attribute to a new object of typeSSLContext
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()
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 server_name_callback function must return
None
to 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 there is an IDNA decoding error on the server name, the TLS connection will terminate with an
ALERT_DESCRIPTION_INTERNAL_ERROR
fatal TLS alert message to the client.If an exception is raised from the server_name_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.Nuevo en la versión 3.4.
-
SSLContext.
load_dh_params
(dhfile)¶ Load the key generation parameters for Diffie-Helman (DH) key exchange. Using DH key exchange improves forward secrecy at the expense of computational resources (both on the server and on the client). The dhfile parameter should be the path to a file containing DH parameters in PEM format.
This setting doesn’t apply to client sockets. You can also use the
OP_SINGLE_DH_USE
option to further improve security.Nuevo en la versión 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
isFalse
.Nuevo en la versión 3.3.
Ver también
- 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)¶ Wrap an existing Python socket sock and return an
SSLSocket
object. sock must be aSOCK_STREAM
socket; other socket types are unsupported.The returned SSL socket is tied to the context, its settings and certificates. The parameters server_side, do_handshake_on_connect and suppress_ragged_eofs have the same meaning as in the top-level
wrap_socket()
function.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.Distinto en la versión 3.5: Always allow a server_hostname to be passed, even if OpenSSL does not have SNI.
-
SSLContext.
wrap_bio
(incoming, outgoing, server_side=False, server_hostname=None)¶ Create a new
SSLObject
instance by wrapping the BIO objects incoming and outgoing. The SSL routines will read input data from the incoming BIO and write data to the outgoing BIO.The server_side and server_hostname parameters have the same meaning as in
SSLContext.wrap_socket()
.
-
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_mode
must be set toCERT_OPTIONAL
orCERT_REQUIRED
, and you must pass server_hostname towrap_socket()
in order to match the hostname.Example:
import socket, ssl context = ssl.SSLContext(ssl.PROTOCOL_TLSv1) 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))
Nuevo en la versión 3.4.
Nota
This features requires OpenSSL 0.9.8f or newer.
-
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_SSLv2
by ORing them together.Nota
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
.
-
SSLContext.
protocol
¶ The protocol version chosen when constructing the context. This attribute is read-only.
-
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). Available only with openssl version 0.9.8+.Nuevo en la versión 3.4.
-
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
orCERT_REQUIRED
.
18.2.4. Certificates¶
Certificates in general are part of a public-key / private-key system. In this system, each principal, (which may be a machine, or a person, or an organization) is assigned a unique two-part encryption key. One part of the key is public, and is called the public key; the other part is kept secret, and is called the private key. The two parts are related, in that if you encrypt a message with one of the parts, you can decrypt it with the other part, and only with the other part.
A certificate contains information about two principals. It contains the name of a subject, and the subject’s public key. It also contains a statement by a second principal, the issuer, that the subject is who he claims to be, and that this is indeed the subject’s public key. The issuer’s statement is signed with the issuer’s private key, which only the issuer knows. However, anyone can verify the issuer’s statement by finding the issuer’s public key, decrypting the statement with it, and comparing it to the other information in the certificate. The certificate also contains information about the time period over which it is valid. This is expressed as two fields, called «notBefore» and «notAfter».
In the Python use of certificates, a client or server can use a certificate to prove who they are. The other side of a network connection can also be required to produce a certificate, and that certificate can be validated to the satisfaction of the client or server that requires such validation. The connection attempt can be set to raise an exception if the validation fails. Validation is done automatically, by the underlying OpenSSL framework; the application need not concern itself with its mechanics. But the application does usually need to provide sets of certificates to allow this process to take place.
Python uses files to contain certificates. They should be formatted as «PEM» (see RFC 1422), which is a base-64 encoded form wrapped with a header line and a footer line:
-----BEGIN CERTIFICATE-----
... (certificate in base64 PEM encoding) ...
-----END CERTIFICATE-----
18.2.4.1. Certificate chains¶
The Python files which contain certificates can contain a sequence of certificates, sometimes called a certificate chain. This chain should start with the specific certificate for the principal who «is» the client or server, and then the certificate for the issuer of that certificate, and then the certificate for the issuer of that certificate, and so on up the chain till you get to a certificate which is self-signed, that is, a certificate which has the same subject and issuer, sometimes called a root certificate. The certificates should just be concatenated together in the certificate file. For example, suppose we had a three certificate chain, from our server certificate to the certificate of the certification authority that signed our server certificate, to the root certificate of the agency which issued the certification authority’s certificate:
-----BEGIN CERTIFICATE-----
... (certificate for your server)...
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
... (the certificate for the CA)...
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
... (the root certificate for the CA's issuer)...
-----END CERTIFICATE-----
18.2.4.2. CA certificates¶
If you are going to require validation of the other side of the connection’s
certificate, you need to provide a «CA certs» file, filled with the certificate
chains for each issuer you are willing to trust. Again, this file just contains
these chains concatenated together. For validation, Python will use the first
chain it finds in the file which matches. The platform’s certificates file can
be used by calling SSLContext.load_default_certs()
, this is done
automatically with create_default_context()
.
18.2.4.3. Combined key and certificate¶
Often the private key is stored in the same file as the certificate; in this
case, only the certfile
parameter to SSLContext.load_cert_chain()
and wrap_socket()
needs to be passed. If the private key is stored
with the certificate, it should come before the first certificate in
the certificate chain:
-----BEGIN RSA PRIVATE KEY-----
... (private key in base64 encoding) ...
-----END RSA PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
... (certificate in base64 PEM encoding) ...
-----END CERTIFICATE-----
18.2.4.4. Self-signed certificates¶
If you are going to create a server that provides SSL-encrypted connection services, you will need to acquire a certificate for that service. There are many ways of acquiring appropriate certificates, such as buying one from a certification authority. Another common practice is to generate a self-signed certificate. The simplest way to do this is with the OpenSSL package, using something like the following:
% openssl req -new -x509 -days 365 -nodes -out cert.pem -keyout cert.pem
Generating a 1024 bit RSA private key
.......++++++
.............................++++++
writing new private key to 'cert.pem'
-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:US
State or Province Name (full name) [Some-State]:MyState
Locality Name (eg, city) []:Some City
Organization Name (eg, company) [Internet Widgits Pty Ltd]:My Organization, Inc.
Organizational Unit Name (eg, section) []:My Group
Common Name (eg, YOUR name) []:myserver.mygroup.myorganization.com
Email Address []:ops@myserver.mygroup.myorganization.com
%
The disadvantage of a self-signed certificate is that it is its own root certificate, and no one else will have it in their cache of known (and trusted) root certificates.
18.2.5. Examples¶
18.2.5.1. Testing for SSL support¶
To test for the presence of SSL support in a Python installation, user code should use the following idiom:
try:
import ssl
except ImportError:
pass
else:
... # do something that requires SSL support
18.2.5.2. Client-side operation¶
This example 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)
>>> context.verify_mode = ssl.CERT_REQUIRED
>>> context.check_hostname = True
>>> 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)
When you use the context to connect to a server, CERT_REQUIRED
validates the server certificate: it ensures that the server certificate
was signed with one of the CA certificates, and checks the signature for
correctness:
>>> conn = context.wrap_socket(socket.socket(socket.AF_INET),
... 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.python.org'),
('DNS', 'docs.python.org'),
('DNS', 'testpypi.python.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.
18.2.5.3. Server-side operation¶
For server operation, typically you’ll need to have a server certificate, and
private key, each in a file. You’ll first create a context holding the key
and the certificate, so that clients can check your authenticity. Then
you’ll open a socket, bind it to a port, call listen()
on it, and start
waiting for clients to connect:
import socket, ssl
context = ssl.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).
18.2.6. 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 eitherSSLWantWriteError
orSSLWantReadError
instead ofBlockingIOError
if an I/O operation would block.SSLWantReadError
will be raised if a read operation on the underlying socket is necessary, andSSLWantWriteError
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.Distinto en la versión 3.5: In earlier Python versions, the
SSLSocket.send()
method returned zero instead of raisingSSLWantWriteError
orSSLWantReadError
.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 theselectors
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 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], [])
Ver también
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.
18.2.7. Memory BIO Support¶
Nuevo en la versión 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
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.
An
SSLObject
instance can be created using thewrap_bio()
method. This method will create theSSLObject
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
read()
write()
getpeercert()
selected_npn_protocol()
cipher()
shared_ciphers()
compression()
pending()
do_handshake()
unwrap()
get_channel_binding()
When compared to
SSLSocket
, this object lacks the following features:- Any form of network IO incluging methods such as
recv()
andsend()
. - 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 anSSLObject
instance instead of aSSLSocket
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 exampleread()
will raise anSSLWantReadError
if it needs more data than the incoming BIO has available. - There is no module-level
wrap_bio()
call like there is forwrap_socket()
. AnSSLObject
is always created via anSSLContext
.
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.
-
18.2.8. Security considerations¶
18.2.8.1. 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.
18.2.8.2. Manual settings¶
18.2.8.2.1. 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.
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.
Nota
In client mode,
CERT_OPTIONAL
andCERT_REQUIRED
are equivalent unless anonymous ciphers are enabled (they are disabled by default).
18.2.8.2.2. 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
as the protocol version and then
disable SSLv2 and SSLv3 explicitly using the SSLContext.options
attribute:
context = ssl.SSLContext(ssl.PROTOCOL_TLS)
context.options |= ssl.OP_NO_SSLv2
context.options |= ssl.OP_NO_SSLv3
context.options |= ssl.OP_NO_TLSv1
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.
18.2.8.2.3. Cipher selection¶
If you have advanced security requirements, fine-tuning of the ciphers
enabled when negotiating a SSL session is possible through the
SSLContext.set_ciphers()
method. Starting from Python 3.2.3, the
ssl module disables certain weak ciphers by default, but you may want
to further restrict the cipher choice. Be sure to read OpenSSL’s documentation
about the cipher list format.
If you want to check which ciphers are enabled by a given cipher list, use the
openssl ciphers
command on your system.
18.2.8.3. 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.
Ver también
- Class
socket.socket
- Documentation of underlying
socket
class - SSL/TLS Strong Encryption: An Introduction
- Intro from the Apache webserver documentation
- RFC 1422: Privacy Enhancement for Internet Electronic Mail: Part II: Certificate-Based Key Management
- Steve Kent
- RFC 1750: Randomness Recommendations for Security
- D. Eastlake et. al.
- RFC 3280: Internet X.509 Public Key Infrastructure Certificate and CRL Profile
- Housley et. al.
- RFC 4366: Transport Layer Security (TLS) Extensions
- Blake-Wilson et. al.
- 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