PEP: 543
Title: A Unified TLS API for Python
Version:
Christian Heimes <[email protected]>
Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 17-Oct-2016 Python-Version: 3.7 Post-History: 11-Jan-2017, 19-Jan-2017, 02-Feb-2017, 09-Feb-2017
This PEP would define a standard TLS interface in the form of a collection of abstract base classes. This interface would allow Python implementations and third-party libraries to provide bindings to TLS libraries other than OpenSSL that can be used by tools that expect the interface provided by the Python standard library, with the goal of reducing the dependence of the Python ecosystem on OpenSSL.
In the 21st century it has become increasingly clear that robust and user-friendly TLS support is an extremely important part of the ecosystem of any popular programming language. For most of its lifetime, this role in the Python ecosystem has primarily been served by the ssl module, which provides a Python API to the OpenSSL library.
Because the ssl module is distributed with the Python standard library, it
has become the overwhelmingly most-popular method for handling TLS in Python.
An extraordinary majority of Python libraries, both in the standard library and
on the Python Package Index, rely on the ssl module for their TLS
connectivity.
Unfortunately, the preeminence of the ssl module has had a number of
unforeseen side-effects that have had the effect of tying the entire Python
ecosystem tightly to OpenSSL. This has forced Python users to use OpenSSL even
in situations where it may provide a worse user experience than alternative TLS
implementations, which imposes a cognitive burden and makes it hard to provide
"platform-native" experiences.
The fact that the ssl module is built into the standard library has meant
that all standard-library Python networking libraries are entirely reliant on
the OpenSSL that the Python implementation has been linked against. This
leads to the following issues:
It is difficult to take advantage of new, higher-security TLS without recompiling Python to get a new OpenSSL. While there are third-party bindings to OpenSSL (e.g. pyOpenSSL), these need to be shimmed into a format that the standard library understands, forcing projects that want to use them to maintain substantial compatibility layers.
For Windows distributions of Python, they need to be shipped with a copy of OpenSSL. This puts the CPython development team in the position of being OpenSSL redistributors, potentially needing to ship security updates to the Windows Python distributions when OpenSSL vulnerabilities are released.
For macOS distributions of Python, they need either to be shipped with a copy of OpenSSL or linked against the system OpenSSL library. Apple has formally deprecated linking against the system OpenSSL library, and even if they had not, that library version has been unsupported by upstream for nearly one year as of the time of writing. The CPython development team has started shipping newer OpenSSLs with the Python available from python.org, but this has the same problem as with Windows.
Many systems, including but not limited to Windows and macOS, do not make their system certificate stores available to OpenSSL. This forces users to either obtain their trust roots from elsewhere (e.g. certifi) or to attempt to export their system trust stores in some form.
Relying on certifi is less than ideal, as most system administrators do not expect to receive security-critical software updates from PyPI. Additionally, it is not easy to extend the certifi trust bundle to include custom roots, or to centrally manage trust using the certifi model.
Even in situations where the system certificate stores are made available to OpenSSL in some form, the experience is still sub-standard, as OpenSSL will perform different validation checks than the platform-native TLS implementation. This can lead to users experiencing different behaviour on their browsers or other platform-native tools than they experience in Python, with little or no recourse to resolve the problem.
Users may wish to integrate with TLS libraries other than OpenSSL for many other reasons, such as OpenSSL missing features (e.g. TLS 1.3 support), or because OpenSSL is simply too large and unwieldy for the platform (e.g. for embedded Python). Those users are left with the requirement to use third-party networking libraries that can interact with their preferred TLS library or to shim their preferred library into the OpenSSL-specific
sslmodule API.
Additionally, the ssl module as implemented today limits the ability of
CPython itself to add support for alternative TLS backends, or remove OpenSSL
support entirely, should either of these become necessary or useful. The
ssl module exposes too many OpenSSL-specific function calls and features to
easily map to an alternative TLS backend.
This PEP proposes to introduce a few new Abstract Base Classes in Python 3.7 to provide TLS functionality that is not so strongly tied to OpenSSL. It also proposes to update standard library modules to use only the interface exposed by these abstract base classes wherever possible. There are three goals here:
- To provide a common API surface for both core and third-party developers to target their TLS implementations to. This allows TLS developers to provide interfaces that can be used by most Python code, and allows network developers to have an interface that they can target that will work with a wide range of TLS implementations.
- To provide an API that has few or no OpenSSL-specific concepts leak through.
The
sslmodule today has a number of warts caused by leaking OpenSSL concepts through to the API: the new ABCs would remove those specific concepts. - To provide a path for the core development team to make OpenSSL one of many possible TLS backends, rather than requiring that it be present on a system in order for Python to have TLS support.
The proposed interface is laid out below.
There are several interfaces that require standardisation. Those interfaces are:
- Configuring TLS, currently implemented by the SSLContext class in the
sslmodule. - Providing an in-memory buffer for doing in-memory encryption or decryption
with no actual I/O (necessary for asynchronous I/O models), currently
implemented by the SSLObject class in the
sslmodule. - Wrapping a socket object, currently implemented by the SSLSocket class
in the
sslmodule. - Applying TLS configuration to the wrapping objects in (2) and (3). Currently
this is also implemented by the SSLContext class in the
sslmodule. - Specifying TLS cipher suites. There is currently no code for doing this in the standard library: instead, the standard library uses OpenSSL cipher suite strings.
- Specifying application-layer protocols that can be negotiated during the TLS handshake.
- Specifying TLS versions.
- Reporting errors to the caller, currently implemented by the SSLError
class in the
sslmodule. - Specifying certificates to load, either as client or server certificates.
- Specifying which trust database should be used to validate certificates presented by a remote peer.
- Finding a way to get hold of these interfaces at run time.
For the sake of simplicitly, this PEP proposes to take a unified approach to (2) and (3) (that is, buffers and sockets). The Python socket API is a sizeable one, and implementing a wrapped socket that has the same behaviour as a regular Python socket is a subtle and tricky thing to do. However, it is entirely possible to implement a generic wrapped socket in terms of wrapped buffers: that is, it is possible to write a wrapped socket (3) that will work for any implementation that provides (2). For this reason, this PEP proposes to provide an ABC for wrapped buffers (2) but a concrete class for wrapped sockets (3).
This decision has the effect of making it impossible to bind a small number of TLS libraries to this ABC, because those TLS libraries cannot provide a wrapped buffer implementation. The most notable of these at this time appears to be Amazon's s2n, which currently does not provide an I/O abstraction layer. However, even this library consider this a missing feature and are working to add it. For this reason, it is safe to assume that a concrete implementation of (3) in terms of (2) will be a substantial effort-saving device and a great tool for correctness. Therefore, this PEP proposes doing just that.
Obviously, (5) doesn't require an abstract base class: instead, it requires a richer API for configuring supported cipher suites that can be easily updated with supported cipher suites for different implementations.
(9) is a thorny problem, because in an ideal world the private keys associated with these certificates would never end up in-memory in the Python process (that is, the TLS library would collaborate with a Hardware Security Module (HSM) to provide the private key in such a way that it cannot be extracted from process memory). Thus, we need to provide an extensible model of providing certificates that allows concrete implementations the ability to provide this higher level of security, while also allowing a lower bar for those implementations that cannot. This lower bar would be the same as the status quo: that is, the certificate may be loaded from an in-memory buffer or from a file on disk.
(10) also represents an issue because different TLS implementations vary wildly
in how they allow users to select trust stores. Some implementations have
specific trust store formats that only they can use (such as the OpenSSL CA
directory format that is created by c_rehash), and others may not allow you
to specify a trust store that does not include their default trust store.
For this reason, we need to provide a model that assumes very little about the form that trust stores take. The "Trust Store" section below goes into more detail about how this is achieved.
Finally, this API will split the responsibilities currently assumed by the SSLContext object: specifically, the responsibility for holding and managing configuration and the responsibility for using that configuration to build wrapper objects.
This is necessarily primarily for supporting functionality like Server Name
Indication (SNI). In OpenSSL (and thus in the ssl module), the server has
the ability to modify the TLS configuration in response to the client telling
the server what hostname it is trying to reach. This is mostly used to change
certificate chain so as to present the correct TLS certificate chain for the
given hostname. The specific mechanism by which this is done is by returning
a new SSLContext object with the appropriate configuration.
This is not a model that maps well to other TLS implementations. Instead, we need to make it possible to provide a return value from the SNI callback that can be used to indicate what configuration changes should be made. This means providing an object that can hold TLS configuration. This object needs to be applied to specific TLSWrappedBuffer, and TLSWrappedSocket objects.
For this reason, we split the responsibility of SSLContext into two separate
objects. The TLSConfiguration object is an object that acts as container
for TLS configuration: the ClientContext and ServerContext objects are
objects that are instantiated with a TLSConfiguration object. All three
objects would be immutable.
Note
The following API declarations uniformly use type hints to aid reading. Some of these type hints cannot actually be used in practice because they are circularly referential. Consider them more a guideline than a reflection of the final code in the module.
The TLSConfiguration concrete class defines an object that can hold and
manage TLS configuration. The goals of this class are as follows:
- To provide a method of specifying TLS configuration that avoids the risk of errors in typing (this excludes the use of a simple dictionary).
- To provide an object that can be safely compared to other configuration objects to detect changes in TLS configuration, for use with the SNI callback.
This class is not an ABC, primarily because it is not expected to have
implementation-specific behaviour. The responsibility for transforming a
TLSConfiguration object into a useful set of configuration for a given TLS
implementation belongs to the Context objects discussed below.
This class has one other notable property: it is immutable. This is a desirable trait for a few reasons. The most important one is that it allows these objects to be used as dictionary keys, which is potentially extremely valuable for certain TLS backends and their SNI configuration. On top of this, it frees implementations from needing to worry about their configuration objects being changed under their feet, which allows them to avoid needing to carefully synchronize changes between their concrete data structures and the configuration object.
This object is extendable: that is, future releases of Python may add configuration fields to this object as they become useful. For backwards-compatibility purposes, new fields are only appended to this object. Existing fields will never be removed, renamed, or reordered.
The TLSConfiguration object would be defined by the following code:
ServerNameCallback = Callable[[TLSBufferObject, Optional[str], TLSConfiguration], Any]
_DEFAULT_VALUE = object()
class TLSConfiguration:
"""
An immutable TLS Configuration object. This object has the following
properties:
:param validate_certificates bool: Whether to validate the TLS
certificates. This switch operates at a very broad scope: either
validation is enabled, in which case all forms of validation are
performed including hostname validation if possible, or validation
is disabled, in which case no validation is performed.
Not all backends support having their certificate validation
disabled. If a backend does not support having their certificate
validation disabled, attempting to set this property to ``False``
will throw a ``TLSError`` when this object is passed into a
context object.
:param certificate_chain Tuple[Tuple[Certificate],PrivateKey]: The
certificate, intermediate certificates, and the corresponding
private key for the leaf certificate. These certificates will be
offered to the remote peer during the handshake if required.
The first Certificate in the list must be the leaf certificate. All
subsequent certificates will be offered as intermediate additional
certificates.
:param ciphers Tuple[Union[CipherSuite, int]]:
The available ciphers for TLS connections created with this
configuration, in priority order.
:param inner_protocols Tuple[Union[NextProtocol, bytes]]:
Protocols that connections created with this configuration should
advertise as supported during the TLS handshake. These may be
advertised using either or both of ALPN or NPN. This list of
protocols should be ordered by preference.
:param lowest_supported_version TLSVersion:
The minimum version of TLS that should be allowed on TLS
connections using this configuration.
:param highest_supported_version TLSVersion:
The maximum version of TLS that should be allowed on TLS
connections using this configuration.
:param trust_store TrustStore:
The trust store that connections using this configuration will use
to validate certificates.
:param sni_callback Optional[ServerNameCallback]:
A callback function that will be called after the TLS Client Hello
handshake message has been received by the TLS server when the TLS
client specifies a server name indication.
Only one callback can be set per ``TLSConfiguration``. If the
``sni_callback`` is ``None`` then the callback is disabled. If the
``TLSConfiguration`` is used for a ``ClientContext`` then this
setting will be ignored.
The ``callback`` function will be called with three arguments: the
first will be the ``TLSBufferObject`` for the connection; the
second will be 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 will
be the original ``TLSConfiguration`` that configured the
connection. The server name argument will be the IDNA *decoded*
server name.
The ``callback`` must return a ``TLSConfiguration`` to allow
negotiation to continue. Other return values signal errors.
Attempting to control what error is signaled by the underlying TLS
implementation is not specified in this API, but is up to the
concrete implementation to handle.
The Context will do its best to apply the ``TLSConfiguration``
changes from its original configuration to the incoming connection.
This will usually include changing the certificate chain, but may
also include changes to allowable ciphers or any other
configuration settings.
"""
__slots__ = (
'_validate_certificates', '_certificate_chain', '_ciphers',
'_inner_protocols', '_lowest_supported_version',
'_highest_supported_version', '_trust_store', '_sni_callback'
)
def __init__(self, validate_certificates: Optional[bool] = None,
certificate_chain: Optional[Tuple[Tuple[Certificate], PrivateKey]] = None,
ciphers: Optional[Tuple[Union[CipherSuite, int]]] = None,
inner_protocols: Optional[Tuple[Union[NextProtocol, bytes]]] = None,
lowest_supported_version: Optional[TLSVersion] = None,
highest_supported_version: Optional[TLSVersion] = None,
trust_store: Optional[TrustStore] = None,
sni_callback: Optional[ServerNameCallback] = None):
if validate_certificates is None:
validate_certificates = True
if ciphers is None:
ciphers = DEFAULT_CIPHER_LIST
if inner_protocols is None:
inner_protocols = ()
if lowest_supported_version is None:
lowest_supported_version = TLSVersion.TLSv1
if highest_supported_version is None:
highest_supported_version = TLSVersion.MAXIMUM_SUPPORTED
self._validate_certificates = validate_certificates
self._certificate_chain = certificate_chain
self._ciphers = ciphers
self._inner_protocols = inner_protocols
self._lowest_supported_version = lowest_supported_version
self._highest_supported_version = highest_supported_version
self._trust_store = trust_store
self._sni_callback = sni_callback
def update(self, validate_certificates=_DEFAULT_VALUE,
certificate_chain=_DEFAULT_VALUE,
ciphers=_DEFAULT_VALUE,
inner_protocols=_DEFAULT_VALUE,
lowest_supported_version=_DEFAULT_VALUE,
highest_supported_version=_DEFAULT_VALUE,
trust_store=_DEFAULT_VALUE,
sni_callback=_DEFAULT_VALUE):
"""
Create a new ``TLSConfiguration``, overriding some of the settings
on the original configuration with the new settings.
"""
if validate_certificates is _DEFAULT_VALUE:
validate_certificates = self.validate_certificates
if certificate_chain is _DEFAULT_VALUE:
certificate_chain = self.certificate_chain
if ciphers is _DEFAULT_VALUE:
ciphers = self.ciphers
if inner_protocols is _DEFAULT_VALUE:
inner_protocols = self.inner_protocols
if lowest_supported_version is _DEFAULT_VALUE:
lowest_supported_version = self.lowest_supported_version
if highest_supported_version is _DEFAULT_VALUE:
highest_supported_version = self.highest_supported_version
if trust_store is _DEFAULT_VALUE:
trust_store = self.trust_store
if sni_callback is _DEFAULT_VALUE:
sni_callback = self.sni_callback
return self.__class__(
validate_certificates, certificate_chain, ciphers,
inner_protocols, lowest_supported_version,
highest_supported_version, trust_store, sni_callback
)
@property
def validate_certificates(self) -> bool:
"""
Whether to validate the TLS certificates.
"""
return self._validate_certificates
@property
def certificate_chain(self) -> Tuple[Tuple[Certificate],PrivateKey]:
"""
The certificate, intermediate certificates, and the corresponding
private key for the leaf certificate. These certificates will be
offered to the remote peer during the handshake if required.
The first Certificate in the list is the leaf certificate. All
subsequent certificates will be offered as intermediate additional
certificates.
"""
return self._certificate_chain
@property
def ciphers(self) -> Tuple[Union[CipherSuite, int]]:
"""
The available ciphers for TLS connections created with this
configuration, in priority order.
"""
return self._ciphers
@property
def inner_protocols(self) -> Tuple[Union[NextProtocol, bytes]]:
"""
Protocols that connections created with this configuration should
advertise as supported during the TLS handshake. These may be
advertised using either or both of ALPN or NPN. This list of
protocols is ordered by preference.
"""
return self._inner_protocols
@property
def lowest_supported_version(self) -> TLSVersion:
"""
The minimum version of TLS that is allowed on TLS connections using
this configuration.
"""
return self._lowest_supported_version
@property
def highest_supported_version(self) -> TLSVersion:
"""
The maximum version of TLS that will be allowed on TLS connections
using this configuration.
"""
return self._highest_supported_version
@property
def trust_store(self) -> TrustStore:
"""
The trust store that connections using this configuration will use
to validate certificates.
"""
return self._trust_store
@property
def sni_callback(self) -> Optional[ServerNameCallback]:
"""
The callback function that will be called after the TLS Client
Hello handshake message has been received by the TLS server when
the TLS client specifies a server name indication, if any.
"""
return self._sni_callback
We define two Context abstract base classes. These ABCs define objects that
allow configuration of TLS to be applied to specific connections. They can be
thought of as factories for TLSWrappedSocket and TLSWrappedBuffer
objects.
Unlike the current ssl module, we provide two context classes instead of
one. Specifically, we provide the ClientContext and ServerContext
classes. This simplifies the APIs (for example, there is no sense in the server
providing the server_hostname parameter to ssl.SSLContext.wrap_socket,
but because there is only one context class that parameter is still available),
and ensures that implementations know as early as possible which side of a TLS
connection they will serve. Additionally, it allows implementations to opt-out
of one or either side of the connection. For example, SecureTransport on macOS
is not really intended for server use and has an enormous amount of
functionality missing for server-side use. This would allow SecureTransport
implementations to simply not define a concrete subclass of ServerContext
to signal their lack of support.
One of the other major differences to the current ssl module is that a
number of flags and options have been removed. Most of these are self-evident,
but it is worth noting that auto_handshake has been removed from
wrap_socket. This was removed because it fundamentally represents an odd
design wart that saves very minimal effort at the cost of a complexity increase
both for users and implementers. This PEP requires that all users call
do_handshake explicitly after connecting.
As much as possible implementers should aim to make these classes immutable: that is, they should prefer not to allow users to mutate their internal state directly, instead preferring to create new contexts from new TLSConfiguration objects. Obviously, the ABCs cannot enforce this constraint, and so they do not attempt to.
The Context abstract base class has the following class definition:
TLSBufferObject = Union[TLSWrappedSocket, TLSWrappedBuffer]
class _BaseContext(metaclass=ABCMeta):
@abstractmethod
def __init__(self, configuration: TLSConfiguration):
"""
Create a new context object from a given TLS configuration.
"""
@property
@abstractmethod
def configuration(self) -> TLSConfiguration:
"""
Returns the TLS configuration that was used to create the context.
"""
class ClientContext(_BaseContext):
def wrap_socket(self,
socket: socket.socket,
server_hostname: Optional[str]) -> TLSWrappedSocket:
"""
Wrap an existing Python socket object ``socket`` and return a
``TLSWrappedSocket`` object. ``socket`` must be a ``SOCK_STREAM``
socket: all other socket types are unsupported.
The returned SSL socket is tied to the context, its settings and
certificates. The socket object originally passed to this method
should not be used again: attempting to use it in any way will lead
to undefined behaviour, especially across different TLS
implementations. To get the original socket object back once it has
been wrapped in TLS, see the ``unwrap`` method of the
TLSWrappedSocket.
The 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. This is also used to validate the
TLS certificate for the given hostname. If hostname validation is
not desired, then pass ``None`` for this parameter. This parameter
has no default value because opting-out of hostname validation is
dangerous, and should not be the default behaviour.
"""
buffer = self.wrap_buffers(server_hostname)
return TLSWrappedSocket(socket, buffer)
@abstractmethod
def wrap_buffers(self, server_hostname: Optional[str]) -> TLSWrappedBuffer:
"""
Create an in-memory stream for TLS, using memory buffers to store
incoming and outgoing ciphertext. The TLS routines will read
received TLS data from one buffer, and write TLS data that needs to
be emitted to another buffer.
The implementation details of how this buffering works are up to
the individual TLS implementation. This allows TLS libraries that
have their own specialised support to continue to do so, while
allowing those without to use whatever Python objects they see fit.
The ``server_hostname`` parameter has the same meaning as in
``wrap_socket``.
"""
class ServerContext(_BaseContext):
def wrap_socket(self, socket: socket.socket) -> TLSWrappedSocket:
"""
Wrap an existing Python socket object ``socket`` and return a
``TLSWrappedSocket`` object. ``socket`` must be a ``SOCK_STREAM``
socket: all other socket types are unsupported.
The returned SSL socket is tied to the context, its settings and
certificates. The socket object originally passed to this method
should not be used again: attempting to use it in any way will lead
to undefined behaviour, especially across different TLS
implementations. To get the original socket object back once it has
been wrapped in TLS, see the ``unwrap`` method of the
TLSWrappedSocket.
"""
buffer = self.wrap_buffers()
return TLSWrappedSocket(socket, buffer)
@abstractmethod
def wrap_buffers(self) -> TLSWrappedBuffer:
"""
Create an in-memory stream for TLS, using memory buffers to store
incoming and outgoing ciphertext. The TLS routines will read
received TLS data from one buffer, and write TLS data that needs to
be emitted to another buffer.
The implementation details of how this buffering works are up to
the individual TLS implementation. This allows TLS libraries that
have their own specialised support to continue to do so, while
allowing those without to use whatever Python objects they see fit.
"""
The buffer-wrapper ABC will be defined by the TLSWrappedBuffer ABC, which
has the following definition:
class TLSWrappedBuffer(metaclass=ABCMeta):
@abstractmethod
def read(self, amt: int) -> bytes:
"""
Read up to ``amt`` bytes of data from the input buffer and return
the result as a ``bytes`` instance.
Once EOF is reached, all further calls to this method return the
empty byte string ``b''``.
May read "short": that is, fewer bytes may be returned than were
requested.
Raise ``WantReadError`` or ``WantWriteError`` if there is
insufficient data in either the input or output buffer and the
operation would have caused data to be written or read.
May raise ``RaggedEOF`` if the connection has been closed without a
graceful TLS shutdown. Whether this is an exception that should be
ignored or not is up to the specific application.
As at any time a re-negotiation is possible, a call to ``read()``
can also cause write operations.
"""
@abstractmethod
def readinto(self, buffer: Any, amt: int) -> int:
"""
Read up to ``amt`` bytes of data from the input buffer into
``buffer``, which must be an object that implements the buffer
protocol. Returns the number of bytes read.
Once EOF is reached, all further calls to this method return the
empty byte string ``b''``.
Raises ``WantReadError`` or ``WantWriteError`` if there is
insufficient data in either the input or output buffer and the
operation would have caused data to be written or read.
May read "short": that is, fewer bytes may be read than were
requested.
May raise ``RaggedEOF`` if the connection has been closed without a
graceful TLS shutdown. Whether this is an exception that should be
ignored or not is up to the specific application.
As at any time a re-negotiation is possible, a call to
``readinto()`` can also cause write operations.
"""
@abstractmethod
def write(self, buf: Any) -> int:
"""
Write ``buf`` in encrypted form to the output buffer and return the
number of bytes written. The ``buf`` argument must be an object
supporting the buffer interface.
Raise ``WantReadError`` or ``WantWriteError`` if there is
insufficient data in either the input or output buffer and the
operation would have caused data to be written or read. In either
case, users should endeavour to resolve that situation and then
re-call this method. When re-calling this method users *should*
re-use the exact same ``buf`` object, as some backends require that
the exact same buffer be used.
This operation may write "short": that is, fewer bytes may be
written than were in the buffer.
As at any time a re-negotiation is possible, a call to ``write()``
can also cause read operations.
"""
@abstractmethod
def do_handshake(self) -> None:
"""
Performs the TLS handshake. Also performs certificate validation
and hostname verification.
"""
@abstractmethod
def cipher(self) -> Optional[Union[CipherSuite, int]]:
"""
Returns the CipherSuite entry for the cipher that has been
negotiated on the connection. If no connection has been negotiated,
returns ``None``. If the cipher negotiated is not defined in
CipherSuite, returns the 16-bit integer representing that cipher
directly.
"""
@abstractmethod
def negotiated_protocol(self) -> Optional[Union[NextProtocol, bytes]]:
"""
Returns the protocol that was selected during the TLS handshake.
This selection may have been made using ALPN, NPN, or some future
negotiation mechanism.
If the negotiated protocol is one of the protocols defined in the
``NextProtocol`` enum, the value from that enum will be returned.
Otherwise, the raw bytestring of the negotiated protocol will be
returned.
If ``Context.set_inner_protocols()`` was not called, if the other
party does not support protocol negotiation, if this socket does
not support any of the peer's proposed protocols, or if the
handshake has not happened yet, ``None`` is returned.
"""
@property
@abstractmethod
def context(self) -> Context:
"""
The ``Context`` object this buffer is tied to.
"""
@abstractproperty
def negotiated_tls_version(self) -> Optional[TLSVersion]:
"""
The version of TLS that has been negotiated on this connection.
"""
@abstractmethod
def shutdown(self) -> None:
"""
Performs a clean TLS shut down. This should generally be used
whenever possible to signal to the remote peer that the content is
finished.
"""
@abstractmethod
def receive_from_network(self, data: bytes) -> None:
"""
Receives some TLS data from the network and stores it in an
internal buffer.
If the internal buffer is overfull, this method will raise
``WantReadError`` and store no data. At this point, the user must
call ``read`` or ``readinto`` to remove some data from the internal
buffer before repeating this call.
"""
@abstractmethod
def peek_outgoing(self, amt: int) -> bytes:
"""
Returns the next ``amt`` bytes of data that should be written to
the network from the outgoing data buffer, without removing it from
the internal buffer.
"""
@abstractmethod
def consume_outgoing(self, amt: int) -> None:
"""
Discard the next ``amt`` bytes from the outgoing data buffer. This
should be used when ``amt`` bytes have been sent on the network, to
signal that the data no longer needs to be buffered.
"""
@abstractmethod
def bytes_buffered(self) -> int:
"""
Returns how many bytes are in the send buffer waiting to be sent.
"""
The socket-wrapper class will be a concrete class that accepts two items in its
constructor: a regular socket object, and a TLSWrappedBuffer object. This
object will be too large to recreate in this PEP, but will be submitted as part
of the work to build the module.
The wrapped socket will implement all of the socket API, though it will have
stub implementations of methods that only work for sockets with types other
than SOCK_STREAM (e.g. sendto/recvfrom). That limitation can be
lifted as-and-when support for DTLS is added to this module.
In addition, the socket class will include the following extra methods on top of the regular socket methods:
class TLSWrappedSocket:
def do_handshake(self) -> None:
"""
Performs the TLS handshake. Also performs certificate validation
and hostname verification. This must be called after the socket has
connected (either via ``connect`` or ``accept``), before any other
operation is performed on the socket.
"""
def cipher(self) -> Optional[Union[CipherSuite, int]]:
"""
Returns the CipherSuite entry for the cipher that has been
negotiated on the connection. If no connection has been negotiated,
returns ``None``. If the cipher negotiated is not defined in
CipherSuite, returns the 16-bit integer representing that cipher
directly.
"""
def negotiated_protocol(self) -> Optional[Union[NextProtocol, bytes]]:
"""
Returns the protocol that was selected during the TLS handshake.
This selection may have been made using ALPN, NPN, or some future
negotiation mechanism.
If the negotiated protocol is one of the protocols defined in the
``NextProtocol`` enum, the value from that enum will be returned.
Otherwise, the raw bytestring of the negotiated protocol will be
returned.
If ``Context.set_inner_protocols()`` was not called, if the other
party does not support protocol negotiation, if this socket does
not support any of the peer's proposed protocols, or if the
handshake has not happened yet, ``None`` is returned.
"""
@property
def context(self) -> Context:
"""
The ``Context`` object this socket is tied to.
"""
def negotiated_tls_version(self) -> Optional[TLSVersion]:
"""
The version of TLS that has been negotiated on this connection.
"""
def unwrap(self) -> socket.socket:
"""
Cleanly terminate the TLS connection on this wrapped socket. Once
called, this ``TLSWrappedSocket`` can no longer be used to transmit
data. Returns the socket that was wrapped with TLS.
"""
Supporting cipher suites in a truly library-agnostic fashion is a remarkably difficult undertaking. Different TLS implementations often have radically different APIs for specifying cipher suites, but more problematically these APIs frequently differ in capability as well as in style. Some examples are shown below:
OpenSSL uses a well-known cipher string format. This format has been adopted as a configuration language by most products that use OpenSSL, including Python. This format is relatively easy to read, but has a number of downsides: it is a string, which makes it remarkably easy to provide bad inputs; it lacks much detailed validation, meaning that it is possible to configure OpenSSL in a way that doesn't allow it to negotiate any cipher at all; and it allows specifying cipher suites in a number of different ways that make it tricky to parse. The biggest problem with this format is that there is no formal specification for it, meaning that the only way to parse a given string the way OpenSSL would is to get OpenSSL to parse it.
OpenSSL's cipher strings can look like this:
'ECDH+AESGCM:ECDH+CHACHA20:DH+AESGCM:DH+CHACHA20:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:RSA+AESGCM:RSA+AES:!aNULL:!eNULL:!MD5'
This string demonstrates some of the complexity of the OpenSSL format. For
example, it is possible for one entry to specify multiple cipher suites: the
entry ECDH+AESGCM means "all ciphers suites that include both
elliptic-curve Diffie-Hellman key exchange and AES in Galois Counter Mode".
More explicitly, that will expand to four cipher suites:
"ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256"
That makes parsing a complete OpenSSL cipher string extremely tricky. Add to the fact that there are other meta-characters, such as "!" (exclude all cipher suites that match this criterion, even if they would otherwise be included: "!MD5" means that no cipher suites using the MD5 hash algorithm should be included), "-" (exclude matching ciphers if they were already included, but allow them to be re-added later if they get included again), and "+" (include the matching ciphers, but place them at the end of the list), and you get an extremely complex format to parse. On top of this complexity it should be noted that the actual result depends on the OpenSSL version, as an OpenSSL cipher string is valid so long as it contains at least one cipher that OpenSSL recognises.
OpenSSL also uses different names for its ciphers than the names used in the
relevant specifications. See the manual page for ciphers(1) for more
details.
The actual API inside OpenSSL for the cipher string is simple:
char *cipher_list = <some cipher list>; int rc = SSL_CTX_set_cipher_list(context, cipher_list);
This means that any format that is used by this module must be able to be converted to an OpenSSL cipher string for use with OpenSSL.
SecureTransport is the macOS system TLS library. This library is substantially more restricted than OpenSSL in many ways, as it has a much more restricted class of users. One of these substantial restrictions is in controlling supported cipher suites.
Ciphers in SecureTransport are represented by a C enum. This enum has one
entry per cipher suite, with no aggregate entries, meaning that it is not
possible to reproduce the meaning of an OpenSSL cipher string like
"ECDH+AESGCM" without hand-coding which categories each enum member falls into.
However, the names of most of the enum members are in line with the formal names of the cipher suites: that is, the cipher suite that OpenSSL calls "ECDHE-ECDSA-AES256-GCM-SHA384" is called "TLS_ECDHE_ECDHSA_WITH_AES_256_GCM_SHA384" in SecureTransport.
The API for configuring cipher suites inside SecureTransport is simple:
SSLCipherSuite ciphers[] = {TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, ...};
OSStatus status = SSLSetEnabledCiphers(context, ciphers, sizeof(ciphers));
SChannel is the Windows system TLS library.
SChannel has extremely restrictive support for controlling available TLS cipher suites, and additionally adopts a third method of expressing what TLS cipher suites are supported.
Specifically, SChannel defines a set of ALG_ID constants (C unsigned ints).
Each of these constants does not refer to an entire cipher suite, but instead
an individual algorithm. Some examples are CALG_3DES and CALG_AES_256,
which refer to the bulk encryption algorithm used in a cipher suite,
CALG_DH_EPHEM and CALG_RSA_KEYX which refer to part of the key exchange
algorithm used in a cipher suite, CALG_SHA1 and CALG_MD5 which refer to
the message authentication code used in a cipher suite, and CALG_ECDSA and
CALG_RSA_SIGN which refer to the signing portions of the key exchange
algorithm.
This can be thought of as the half of OpenSSL's functionality that SecureTransport doesn't have: SecureTransport only allows specifying exact cipher suites, while SChannel only allows specifying parts of the cipher suite, while OpenSSL allows both.
Determining which cipher suites are allowed on a given connection is done by
providing a pointer to an array of these ALG_ID constants. This means that
any suitable API must allow the Python code to determine which ALG_ID
constants must be provided.
NSS is Mozilla's crypto and TLS library. It's used in Firefox, Thunderbird, and as alternative to OpenSSL in multiple libraries, e.g. curl.
By default, NSS comes with secure configuration of allowed ciphers. On some platforms such as Fedora, the list of enabled ciphers is globally configured in a system policy. Generally, applications should not modify cipher suites unless they have specific reasons to do so.
NSS has both process global and per-connection settings for cipher suites. It
does not have a concept of SSLContext like OpenSSL. A SSLContext-like behavior
can be easily emulated. Specifically, ciphers can be enabled or disabled
globally with `SSL_CipherPrefSetDefault(PRInt32 cipher, PRBool enabled)`,
and `SSL_CipherPrefSet(PRFileDesc *fd, PRInt32 cipher, PRBool enabled)`
for a connection. The cipher `PRInt32` number is a signed 32bit integer
that directly corresponds to an registered IANA id, e.g. `0x1301`
is `TLS_AES_128_GCM_SHA256`. Contrary to OpenSSL, the preference order
of ciphers is fixed and cannot be modified at runtime.
Like SecureTransport, NSS has no API for aggregated entries. Some consumers
of NSS have implemented custom mappings from OpenSSL cipher names and rules
to NSS ciphers, e.g. `mod_nss`.
The proposed interface for the new module is influenced by the combined set of limitations of the above implementations. Specifically, as every implementation except OpenSSL requires that each individual cipher be provided, there is no option but to provide that lowest-common denominator approach.
The simplest approach is to provide an enumerated type that includes a large subset of the cipher suites defined for TLS. The values of the enum members will be their two-octet cipher identifier as used in the TLS handshake, stored as a 16 bit integer. The names of the enum members will be their IANA-registered cipher suite names.
As of now, the IANA cipher suite registry contains over 320 cipher suites. A large portion of the cipher suites are irrelevant for TLS connections to network services. Other suites specify deprecated and insecure algorithms that are no longer provided by recent versions of implementations. The enum does not contain ciphers with:
- key exchange: NULL, Kerberos (KRB5), pre-shared key (PSK), secure remote transport (TLS-SRP)
- authentication: NULL, anonymous, export grade, Kerberos (KRB5), pre-shared key (PSK), secure remote transport (TLS-SRP), DSA cert (DSS)
- encryption: NULL, ARIA, DES, RC2, export grade 40bit
- PRF: MD5
- SCSV cipher suites
3DES, RC4, SEED, and IDEA are included for legacy applications. Further more five additional cipher suites from the TLS 1.3 draft (draft-ietf-tls-tls13-18) are included, too. TLS 1.3 does not share any cipher suites with TLS 1.2 and earlier. The resulting enum will contain roughly 110 suites.
Because of these limitations, and because the enum doesn't contain every
defined cipher, and also to allow for forward-looking applications, all parts
of this API that accept CipherSuite objects will also accept raw 16-bit
integers directly.
Rather than populate this enum by hand, we have a TLS enum script that builds it from Christian Heimes' tlsdb JSON file (warning: large file) and IANA cipher suite registry. The TLSDB also opens up the possibility of extending the API with additional querying function, such as determining which TLS versions support which ciphers, if that functionality is found to be useful or necessary.
If users find this approach to be onerous, a future extension to this API can provide helpers that can reintroduce OpenSSL's aggregation functionality.
class CipherSuite(IntEnum):
TLS_RSA_WITH_RC4_128_SHA = 0x0005
TLS_RSA_WITH_IDEA_CBC_SHA = 0x0007
TLS_RSA_WITH_3DES_EDE_CBC_SHA = 0x000a
TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA = 0x0010
TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA = 0x0016
TLS_RSA_WITH_AES_128_CBC_SHA = 0x002f
TLS_DH_RSA_WITH_AES_128_CBC_SHA = 0x0031
TLS_DHE_RSA_WITH_AES_128_CBC_SHA = 0x0033
TLS_RSA_WITH_AES_256_CBC_SHA = 0x0035
TLS_DH_RSA_WITH_AES_256_CBC_SHA = 0x0037
TLS_DHE_RSA_WITH_AES_256_CBC_SHA = 0x0039
TLS_RSA_WITH_AES_128_CBC_SHA256 = 0x003c
TLS_RSA_WITH_AES_256_CBC_SHA256 = 0x003d
TLS_DH_RSA_WITH_AES_128_CBC_SHA256 = 0x003f
TLS_RSA_WITH_CAMELLIA_128_CBC_SHA = 0x0041
TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA = 0x0043
TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA = 0x0045
TLS_DHE_RSA_WITH_AES_128_CBC_SHA256 = 0x0067
TLS_DH_RSA_WITH_AES_256_CBC_SHA256 = 0x0069
TLS_DHE_RSA_WITH_AES_256_CBC_SHA256 = 0x006b
TLS_RSA_WITH_CAMELLIA_256_CBC_SHA = 0x0084
TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA = 0x0086
TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA = 0x0088
TLS_RSA_WITH_SEED_CBC_SHA = 0x0096
TLS_DH_RSA_WITH_SEED_CBC_SHA = 0x0098
TLS_DHE_RSA_WITH_SEED_CBC_SHA = 0x009a
TLS_RSA_WITH_AES_128_GCM_SHA256 = 0x009c
TLS_RSA_WITH_AES_256_GCM_SHA384 = 0x009d
TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = 0x009e
TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = 0x009f
TLS_DH_RSA_WITH_AES_128_GCM_SHA256 = 0x00a0
TLS_DH_RSA_WITH_AES_256_GCM_SHA384 = 0x00a1
TLS_RSA_WITH_CAMELLIA_128_CBC_SHA256 = 0x00ba
TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA256 = 0x00bc
TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA256 = 0x00be
TLS_RSA_WITH_CAMELLIA_256_CBC_SHA256 = 0x00c0
TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA256 = 0x00c2
TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA256 = 0x00c4
TLS_AES_128_GCM_SHA256 = 0x1301
TLS_AES_256_GCM_SHA384 = 0x1302
TLS_CHACHA20_POLY1305_SHA256 = 0x1303
TLS_AES_128_CCM_SHA256 = 0x1304
TLS_AES_128_CCM_8_SHA256 = 0x1305
TLS_ECDH_ECDSA_WITH_RC4_128_SHA = 0xc002
TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA = 0xc003
TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA = 0xc004
TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA = 0xc005
TLS_ECDHE_ECDSA_WITH_RC4_128_SHA = 0xc007
TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA = 0xc008
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA = 0xc009
TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA = 0xc00a
TLS_ECDH_RSA_WITH_RC4_128_SHA = 0xc00c
TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA = 0xc00d
TLS_ECDH_RSA_WITH_AES_128_CBC_SHA = 0xc00e
TLS_ECDH_RSA_WITH_AES_256_CBC_SHA = 0xc00f
TLS_ECDHE_RSA_WITH_RC4_128_SHA = 0xc011
TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA = 0xc012
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA = 0xc013
TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA = 0xc014
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 = 0xc023
TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 = 0xc024
TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256 = 0xc025
TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384 = 0xc026
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256 = 0xc027
TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384 = 0xc028
TLS_ECDH_RSA_WITH_AES_128_CBC_SHA256 = 0xc029
TLS_ECDH_RSA_WITH_AES_256_CBC_SHA384 = 0xc02a
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 = 0xc02b
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 = 0xc02c
TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 = 0xc02d
TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384 = 0xc02e
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 = 0xc02f
TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 = 0xc030
TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256 = 0xc031
TLS_ECDH_RSA_WITH_AES_256_GCM_SHA384 = 0xc032
TLS_ECDHE_ECDSA_WITH_CAMELLIA_128_CBC_SHA256 = 0xc072
TLS_ECDHE_ECDSA_WITH_CAMELLIA_256_CBC_SHA384 = 0xc073
TLS_ECDH_ECDSA_WITH_CAMELLIA_128_CBC_SHA256 = 0xc074
TLS_ECDH_ECDSA_WITH_CAMELLIA_256_CBC_SHA384 = 0xc075
TLS_ECDHE_RSA_WITH_CAMELLIA_128_CBC_SHA256 = 0xc076
TLS_ECDHE_RSA_WITH_CAMELLIA_256_CBC_SHA384 = 0xc077
TLS_ECDH_RSA_WITH_CAMELLIA_128_CBC_SHA256 = 0xc078
TLS_ECDH_RSA_WITH_CAMELLIA_256_CBC_SHA384 = 0xc079
TLS_RSA_WITH_CAMELLIA_128_GCM_SHA256 = 0xc07a
TLS_RSA_WITH_CAMELLIA_256_GCM_SHA384 = 0xc07b
TLS_DHE_RSA_WITH_CAMELLIA_128_GCM_SHA256 = 0xc07c
TLS_DHE_RSA_WITH_CAMELLIA_256_GCM_SHA384 = 0xc07d
TLS_DH_RSA_WITH_CAMELLIA_128_GCM_SHA256 = 0xc07e
TLS_DH_RSA_WITH_CAMELLIA_256_GCM_SHA384 = 0xc07f
TLS_ECDHE_ECDSA_WITH_CAMELLIA_128_GCM_SHA256 = 0xc086
TLS_ECDHE_ECDSA_WITH_CAMELLIA_256_GCM_SHA384 = 0xc087
TLS_ECDH_ECDSA_WITH_CAMELLIA_128_GCM_SHA256 = 0xc088
TLS_ECDH_ECDSA_WITH_CAMELLIA_256_GCM_SHA384 = 0xc089
TLS_ECDHE_RSA_WITH_CAMELLIA_128_GCM_SHA256 = 0xc08a
TLS_ECDHE_RSA_WITH_CAMELLIA_256_GCM_SHA384 = 0xc08b
TLS_ECDH_RSA_WITH_CAMELLIA_128_GCM_SHA256 = 0xc08c
TLS_ECDH_RSA_WITH_CAMELLIA_256_GCM_SHA384 = 0xc08d
TLS_RSA_WITH_AES_128_CCM = 0xc09c
TLS_RSA_WITH_AES_256_CCM = 0xc09d
TLS_DHE_RSA_WITH_AES_128_CCM = 0xc09e
TLS_DHE_RSA_WITH_AES_256_CCM = 0xc09f
TLS_RSA_WITH_AES_128_CCM_8 = 0xc0a0
TLS_RSA_WITH_AES_256_CCM_8 = 0xc0a1
TLS_DHE_RSA_WITH_AES_128_CCM_8 = 0xc0a2
TLS_DHE_RSA_WITH_AES_256_CCM_8 = 0xc0a3
TLS_ECDHE_ECDSA_WITH_AES_128_CCM = 0xc0ac
TLS_ECDHE_ECDSA_WITH_AES_256_CCM = 0xc0ad
TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 = 0xc0ae
TLS_ECDHE_ECDSA_WITH_AES_256_CCM_8 = 0xc0af
TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256 = 0xcca8
TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256 = 0xcca9
TLS_DHE_RSA_WITH_CHACHA20_POLY1305_SHA256 = 0xccaa
Enum members can be mapped to OpenSSL cipher names:
>>> import ssl
>>> ctx = ssl.SSLContext(ssl.PROTOCOL_TLS)
>>> ctx.set_ciphers('ALL:COMPLEMENTOFALL')
>>> ciphers = {c['id'] & 0xffff: c['name'] for c in ctx.get_ciphers()}
>>> ciphers[CipherSuite.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256]
'ECDHE-RSA-AES128-GCM-SHA256'
For SecureTransport, these enum members directly refer to the values of the
cipher suite constants. For example, SecureTransport defines the cipher suite
enum member TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 as having the value
0xC02C. Not coincidentally, that is identical to its value in the above
enum. This makes mapping between SecureTransport and the above enum very easy
indeed.
For SChannel there is no easy direct mapping, due to the fact that SChannel configures ciphers, instead of cipher suites. This represents an ongoing concern with SChannel, which is that it is very difficult to configure in a specific manner compared to other TLS implementations.
For the purposes of this PEP, any SChannel implementation will need to determine which ciphers to choose based on the enum members. This may be more open than the actual cipher suite list actually wants to allow, or it may be more restrictive, depending on the choices of the implementation. This PEP recommends that it be more restrictive, but of course this cannot be enforced.
Both NPN and ALPN allow for protocol negotiation as part of the HTTP/2 handshake. While NPN and ALPN are, at their fundamental level, built on top of bytestrings, string-based APIs are frequently problematic as they allow for errors in typing that can be hard to detect.
For this reason, this module would define a type that protocol negotiation implementations can pass and be passed. This type would wrap a bytestring to allow for aliases for well-known protocols. This allows us to avoid the problems inherent in typos for well-known protocols, while allowing the full extensibility of the protocol negotiation layer if needed by letting users pass byte strings directly.
class NextProtocol(Enum):
H2 = b'h2'
H2C = b'h2c'
HTTP1 = b'http/1.1'
WEBRTC = b'webrtc'
C_WEBRTC = b'c-webrtc'
FTP = b'ftp'
STUN = b'stun.nat-discovery'
TURN = b'stun.turn'
It is often useful to be able to restrict the versions of TLS you're willing to support. There are many security advantages in refusing to use old versions of TLS, and some misbehaving servers will mishandle TLS clients advertising support for newer versions.
The following enumerated type can be used to gate TLS versions. Forward-looking applications should almost never set a maximum TLS version unless they absolutely must, as a TLS backend that is newer than the Python that uses it may support TLS versions that are not in this enumerated type.
Additionally, this enumerated type defines two additional flags that can always be used to request either the lowest or highest TLS version supported by an implementation.
class TLSVersion(Enum):
MINIMUM_SUPPORTED = auto()
SSLv2 = auto()
SSLv3 = auto()
TLSv1 = auto()
TLSv1_1 = auto()
TLSv1_2 = auto()
TLSv1_3 = auto()
MAXIMUM_SUPPORTED = auto()
This module would define four base classes for use with error handling. Unlike many of the the other classes defined here, these classes are not abstract, as they have no behaviour. They exist simply to signal certain common behaviours. Backends should subclass these exceptions in their own packages, but needn't define any behaviour for them.
In general, concrete implementations should subclass these exceptions rather than throw them directly. This makes it moderately easier to determine which concrete TLS implementation is in use during debugging of unexpected errors. However, this is not mandatory.
The definitions of the errors are below:
class TLSError(Exception):
"""
The base exception for all TLS related errors from any backend.
Catching this error should be sufficient to catch *all* TLS errors,
regardless of what backend is used.
"""
class WantWriteError(TLSError):
"""
A special signaling exception used only when non-blocking or
buffer-only I/O is used. This error signals that the requested
operation cannot complete until more data is written to the network,
or until the output buffer is drained.
This error is should only be raised when it is completely impossible
to write any data. If a partial write is achievable then this should
not be raised.
"""
class WantReadError(TLSError):
"""
A special signaling exception used only when non-blocking or
buffer-only I/O is used. This error signals that the requested
operation cannot complete until more data is read from the network, or
until more data is available in the input buffer.
This error should only be raised when it is completely impossible to
write any data. If a partial write is achievable then this should not
be raised.
"""
class RaggedEOF(TLSError):
"""
A special signaling exception used when a TLS connection has been
closed gracelessly: that is, when a TLS CloseNotify was not received
from the peer before the underlying TCP socket reached EOF. This is a
so-called "ragged EOF".
This exception is not guaranteed to be raised in the face of a ragged
EOF: some implementations may not be able to detect or report the
ragged EOF.
This exception is not always a problem. Ragged EOFs are a concern only
when protocols are vulnerable to length truncation attacks. Any
protocol that can detect length truncation attacks at the application
layer (e.g. HTTP/1.1 and HTTP/2) is not vulnerable to this kind of
attack and so can ignore this exception.
"""
This module would define an abstract X509 certificate class. This class would have almost no behaviour, as the goal of this module is not to provide all possible relevant cryptographic functionality that could be provided by X509 certificates. Instead, all we need is the ability to signal the source of a certificate to a concrete implementation.
For that reason, this certificate implementation defines only constructors. In essence, the certificate object in this module could be as abstract as a handle that can be used to locate a specific certificate.
Concrete implementations may choose to provide alternative constructors, e.g. to load certificates from HSMs. If a common interface emerges for doing this, this module may be updated to provide a standard constructor for this use-case as well.
Concrete implementations should aim to have Certificate objects be hashable if at all possible. This will help ensure that TLSConfiguration objects used with an individual concrete implementation are also hashable.
class Certificate(metaclass=ABCMeta):
@abstractclassmethod
def from_buffer(cls, buffer: bytes):
"""
Creates a Certificate object from a byte buffer. This byte buffer
may be either PEM-encoded or DER-encoded. If the buffer is PEM
encoded it *must* begin with the standard PEM preamble (a series of
dashes followed by the ASCII bytes "BEGIN CERTIFICATE" and another
series of dashes). In the absence of that preamble, the
implementation may assume that the certificate is DER-encoded
instead.
"""
@abstractclassmethod
def from_file(cls, path: Union[pathlib.Path, AnyStr]):
"""
Creates a Certificate object from a file on disk. This method may
be a convenience method that wraps ``open`` and ``from_buffer``,
but some TLS implementations may be able to provide more-secure or
faster methods of loading certificates that do not involve Python
code.
"""
This module would define an abstract private key class. Much like the Certificate class, this class has almost no behaviour in order to give as much freedom as possible to the concrete implementations to treat keys carefully.
This class has all the caveats of the Certificate class.
class PrivateKey(metaclass=ABCMeta):
@abstractclassmethod
def from_buffer(cls,
buffer: bytes,
password: Optional[Union[Callable[[], Union[bytes, bytearray]], bytes, bytearray]] = None):
"""
Creates a PrivateKey object from a byte buffer. This byte buffer
may be either PEM-encoded or DER-encoded. If the buffer is PEM
encoded it *must* begin with the standard PEM preamble (a series of
dashes followed by the ASCII bytes "BEGIN", the key type, and
another series of dashes). In the absence of that preamble, the
implementation may assume that the certificate is DER-encoded
instead.
The key may additionally be encrypted. If it is, the ``password``
argument can be used to decrypt the key. 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 either bytes or bytearray containing the
password. Alternatively a 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.
"""
@abstractclassmethod
def from_file(cls,
path: Union[pathlib.Path, bytes, str],
password: Optional[Union[Callable[[], Union[bytes, bytearray]], bytes, bytearray]] = None):
"""
Creates a PrivateKey object from a file on disk. This method may
be a convenience method that wraps ``open`` and ``from_buffer``,
but some TLS implementations may be able to provide more-secure or
faster methods of loading certificates that do not involve Python
code.
The ``password`` parameter behaves exactly as the equivalent
parameter on ``from_buffer``.
"""
As discussed above, loading a trust store represents an issue because different TLS implementations vary wildly in how they allow users to select trust stores. For this reason, we need to provide a model that assumes very little about the form that trust stores take.
This problem is the same as the one that the Certificate and PrivateKey types need to solve. For this reason, we use the exact same model, by creating an opaque type that can encapsulate the various means that TLS backends may open a trust store.
A given TLS implementation is not required to implement all of the
constructors. However, it is strongly recommended that a given TLS
implementation provide the system constructor if at all possible, as this
is the most common validation trust store that is used. Concrete
implementations may also add their own constructors.
Concrete implementations should aim to have TrustStore objects be hashable if at all possible. This will help ensure that TLSConfiguration objects used with an individual concrete implementation are also hashable.
class TrustStore(metaclass=ABCMeta):
@abstractclassmethod
def system(cls) -> TrustStore:
"""
Returns a TrustStore object that represents the system trust
database.
"""
@abstractclassmethod
def from_pem_file(cls, path: Union[pathlib.Path, bytes, str]) -> TrustStore:
"""
Initializes a trust store from a single file full of PEMs.
"""
A not-uncommon use case for library users is to want to allow the library to control the TLS configuration, but to want to select what backend is in use. For example, users of Requests may want to be able to select between OpenSSL or a platform-native solution on Windows and macOS, or between OpenSSL and NSS on some Linux platforms. These users, however, may not care about exactly how their TLS configuration is done.
This poses a problem: given an arbitrary concrete implementation, how can a library work out how to load certificates into the trust store? There are two options: either all concrete implementations can be required to fit into a specific naming scheme, or we can provide an API that makes it possible to grab these objects.
This PEP proposes that we use the second approach. This grants the greatest freedom to concrete implementations to structure their code as they see fit, requiring only that they provide a single object that has the appropriate properties in place. Users can then pass this "backend" object to libraries that support it, and those libraries can take care of configuring and using the concrete implementation.
All concrete implementations must provide a method of obtaining a Backend
object. The Backend object can be a global singleton or can be created by a
callable if there is an advantage in doing that.
The Backend object has the following definition:
class Backend:
"""
An object representing the collection of classes that implement the
PEP 543 abstract TLS API for a specific TLS implementation.
"""
__slots__ = (
'_client_context', '_server_context', '_certificate',
'_private_key', '_trust_store'
)
def __init__(self, client_context,
server_context,
certificate,
private_key,
trust_store):
self._client_context = client_context
self._server_context = server_context
self._certificate = certificate
self._private_key = private_key
self._trust_store = trust_store
@property
def client_context(self) -> Optional[ClientContext]:
"""
The concrete implementation of the PEP 543 Client Context object,
if this TLS backend supports being the client on a TLS connection.
"""
return self._client_context
@property
def server_context(self) -> Optional[ServerContext]:
"""
The concrete implementation of the PEP 543 Server Context object,
if this TLS backend supports being a server on a TLS connection.
"""
return self._server_context
@property
def certificate(self) -> Certificate:
"""
The concrete implementation of the PEP 543 Certificate object used
by this TLS backend.
"""
return self._certificate
@property
def private_key(self) -> PrivateKey:
"""
The concrete implementation of the PEP 543 Private Key object used
by this TLS backend.
"""
return self._private_key
@property
def trust_store(self) -> TrustStore:
"""
The concrete implementation of the PEP 543 TrustStore object used
by this TLS backend.
"""
return self._trust_store
Each of the properties must provide the concrete implementation of the relevant ABC. This ensures that code like this will work for any backend:
trust_store = backend.trust_store.system()
The portions of the standard library that interact with TLS should be revised to use these ABCs. This will allow them to function with other TLS backends. This includes the following modules:
- asyncio
- ftplib
- http
- imaplib
- nntplib
- poplib
- smtplib
- urllib
Naturally, we will need to extend the ssl module itself to conform to these
ABCs. This extension will take the form of new classes, potentially in an
entirely new module. This will allow applications that take advantage of the
current ssl module to continue to do so, while enabling the new APIs for
applications and libraries that want to use them.
In general, migrating from the ssl module to the new ABCs is not expected
to be one-to-one. This is normally acceptable: most tools that use the ssl
module hide it from the user, and so refactoring to use the new module should
be invisible.
However, a specific problem comes from libraries or applications that leak
exceptions from the ssl module, either as part of their defined API or by
accident (which is easily done). Users of those tools may have written code
that tolerates and handles exceptions from the ssl module being raised:
migrating to the ABCs presented here would potentially cause the exceptions
defined above to be thrown instead, and existing except blocks will not
catch them.
For this reason, part of the migration of the ssl module would require that
the exceptions in the ssl module alias those defined above. That is, they
would require the following statements to all succeed:
assert ssl.SSLError is tls.TLSError assert ssl.SSLWantReadError is tls.WantReadError assert ssl.SSLWantWriteError is tls.WantWriteError
The exact mechanics of how this will be done are beyond the scope of this PEP,
as they are made more complex due to the fact that the current ssl
exceptions are defined in C code, but more details can be found in
an email sent to the Security-SIG by Christian Heimes.
Major future TLS features may require revisions of these ABCs. These revisions should be made cautiously: many backends may not be able to move forward swiftly, and will be invalidated by changes in these ABCs. This is acceptable, but wherever possible features that are specific to individual implementations should not be added to the ABCs. The ABCs should restrict themselves to high-level descriptions of IETF-specified features.
However, well-justified extensions to this API absolutely should be made. The focus of this API is to provide a unifying lowest-common-denominator configuration option for the Python community. TLS is not a static target, and as TLS evolves so must this API.
This document has received extensive review from a number of individuals in the community who have substantially helped shape it. Detailed review was provided by:
- Alex Chan
- Alex Gaynor
- Antoine Pitrou
- Ashwini Oruganti
- Donald Stufft
- Ethan Furman
- Glyph
- Hynek Schlawack
- Jim J Jewett
- Nathaniel J. Smith
- Nick Coghlan
- Paul Kehrer
- Steve Dower
- Steven Fackler
- Wes Turner
- Will Bond
Further review was provided by the Security-SIG and python-ideas mailing lists.
This document has been placed in the public domain.