All, Python has never exposed the ability to use TLS backends other than OpenSSL in the standard library. As Christian and I discussed back at the Developer Summit at PyCon US 2016, the ssl module would more properly be called the openssl module, due to exposing many OpenSSL-specific concepts and behaviours in its API. This has meant that the Python ecosystem is overwhelmingly also an OpenSSL ecosystem, which is problematic on Windows and macOS (and Unices that for whatever reason aren’t interested in shipping an OpenSSL), as it has meant that Python needs to bring its own OpenSSL, and that it is troublesome to interact with the system trust database. The first step to resolving this would be to provide a new module that exposes TLS concepts in a much more generic manner. There are two possible approaches to this. The first is to have this module be a generic concrete implementation that can be compiled against multiple TLS backends (like curl). This would require that all the relevant bindings be built into the standard library from the get-go, which provides a substantial maintenance burden on a team of people that are already understaffed maintaining the ssl module alone. The second approach is to define a generic high-level TLS interface that provides a minimal usable interface that can be implemented by both first- and third-party modules. This would allow the standard library to continue to ship with only exactly what it needs (for example, OpenSSL, SecureTransport and SChannel), but would give those who want to use more esoteric TLS choices (NSS, GnuTLS, mbedTLS, and BoringSSL are some examples) an API that they can implement that will guarantee that complying modules can use the appropriate TLS backend. To that end, I’ve composed a draft PEP that would define this API: PEP 543. This draft can be found online here[1], and I have reproduced it below for those who want to reply inline. I should apologise in advance that this PEP is quite large: it is this large because it lays out a substantial proportion of the new code that would be added directly in the PEP. This is to help de-risk the work by showing as much of it up-front during the PEP stage, rather than having it emerge over the implementation process. Please note that this proposal is not intended to resolve the current problem pip has with TLSv1.2, and so is not subject to the tight time constraints associated with that problem. That problem will be solved by building a temporary shim into urllib3 (and therefore pip) that can use SecureTransport bindings on the Mac. This proposal is intended as a solution to the more-general problem of supporting different TLS backends in Python, and so is larger in scope and less urgent. I should also mention that there will be a tendency to want to make this API all things to all people from the get-go. I’m going to strongly resist attempts to extend this API too much, because each additional bit of API surface makes it harder for us to encourage module authors to conform to this API. I will encourage people to extend this API over time as needed, but to begin with I think it is most important that basic TLS clients and servers can be constructed with this API. More specialised features should be considered future enhancements, rather than being tacked on to this initial PEP. Please let me know what you think. I’d like significant feedback from the community before I ask the BDFL to pronounce on this PEP, as this is the kind of change that will affect a large amount of the Python ecosystem. It’s important to me that we minimise the risk to ourselves and the community by getting this as right as possible, and while we can help with that by limiting scope, it’s also true that we need as many eyes as possible. Please let me know what you think. Cory [1]: https://www.python.org/dev/peps/pep-0543/ — PEP: 543 Title: A Unified TLS API for Python Version: $Revision$ Last-Modified: $Date$ Author: Cory Benfield , Christian Heimes 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 Abstract ======== 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. Rationale ========= 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. Problems -------- 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 ``ssl`` module 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. Proposal ======== 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: 1. 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. 2. To provide an API that has few or no OpenSSL-specific concepts leak through. The ``ssl`` module today has a number of warts caused by leaking OpenSSL concepts through to the API: the new ABCs would remove those specific concepts. 3. 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. Interfaces ---------- There are several interfaces that require standardisation. Those interfaces are: 1. Configuring TLS, currently implemented by the `SSLContext`_ class in the ``ssl`` module. 2. 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 ``ssl`` module. 3. Wrapping a socket object, currently implemented by the `SSLSocket`_ class in the ``ssl`` module. 4. Applying TLS configuration to the wrapping objects in (2) and (3). Currently this is also implemented by the `SSLContext`_ class in the ``ssl`` module. 5. 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. 6. Specifying application-layer protocols that can be negotiated during the TLS handshake. 7. Specifying TLS versions. 8. Reporting errors to the caller, currently implemented by the `SSLError`_ class in the ``ssl`` module. 9. Specifying certificates to load, either as client or server certificates. 10. Specifying which trust database should be used to validate certificates presented by a remote peer. 11. 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. Configuration ~~~~~~~~~~~~~ The ``TLSConfiguration`` concrete class defines an object that can hold and manage TLS configuration. The goals of this class are as follows: 1. To provide a method of specifying TLS configuration that avoids the risk of errors in typing (this excludes the use of a simple dictionary). 2. 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] _configuration_fields = [ 'validate_certificates', 'certificate_chain', 'ciphers', 'inner_protocols', 'lowest_supported_version', 'highest_supported_version', 'trust_store', 'sni_callback', ] _DEFAULT_VALUE = object() class TLSConfiguration(namedtuple('TLSConfiguration', _configuration_fields)): """ 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 certificate, 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__ = () def __new__(cls, 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 return super().__new__( cls, validate_certificates, certificate_chain, ciphers, inner_protocols, lowest_supported_version, highest_supported_version, trust_store, 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 ) Context ~~~~~~~ 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. """ Buffer ~~~~~~ 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): """ Receives some TLS data from the network and stores it in an internal buffer. """ @abstractmethod def peek_outgoing(self, amt): """ 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): """ 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. """ Socket ~~~~~~ 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. """ Cipher Suites ~~~~~~~~~~~~~ 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 ^^^^^^^ 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 ^^^^^^^^^^^^^^^ 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 ^^^^^^^^ 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. Network Security Services (NSS) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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```. Proposed Interface ^^^^^^^^^^^^^^^^^^ 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. Protocol Negotiation ~~~~~~~~~~~~~~~~~~~~ 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' TLS Versions ~~~~~~~~~~~~ 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() Errors ~~~~~~ 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. """ Certificates ~~~~~~~~~~~~ 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. """ Private Keys ~~~~~~~~~~~~ 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``. """ Trust Store ~~~~~~~~~~~ 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. """ Runtime Access ~~~~~~~~~~~~~~ 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:: Backend = namedtuple( 'Backend', ['client_context', 'server_context', 'certificate', 'private_key', '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() Changes to the Standard Library =============================== 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 Migration of the ssl module --------------------------- 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`_. Future ====== 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. Credits ======= 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. Copyright ========= This document has been placed in the public domain. .. _ssl module: https://docs.python.org/3/library/ssl.html .. _OpenSSL Library: https://www.openssl.org/ .. _PyOpenSSL: https://pypi.org/project/pyOpenSSL/ .. _certifi: https://pypi.org/project/certifi/ .. _SSLContext: https://docs.python.org/3/library/ssl.html#ssl.SSLContext .. _SSLSocket: https://docs.python.org/3/library/ssl.html#ssl.SSLSocket .. _SSLObject: https://docs.python.org/3/library/ssl.html#ssl.SSLObject .. _SSLError: https://docs.python.org/3/library/ssl.html#ssl.SSLError .. _MSDN articles: https://msdn.microsoft.com/en-us/library/windows/desktop/mt490158(v=vs.85).a... .. _TLS enum script: https://github.com/tiran/tlsdb/blob/master/tlspep_ciphersuite.py .. _tlsdb JSON file: https://github.com/tiran/tlsdb/blob/master/tlsdb.json .. _IANA cipher suite registry: https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-par... .. _an email sent to the Security-SIG by Christian Heimes: https://mail.python.org/pipermail/security-sig/2017-January/000213.html .. _s2n: https://github.com/awslabs/s2n .. _working to add it: https://github.com/awslabs/s2n/issues/358 .. 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