Mailman 3 January 2017 - Security-SIG

Re: [Security-sig] Unified TLS API for Python: Draft 3
by Cory Benfield 10 Feb '17

10 Feb '17

> On 26 Jan 2017, at 01:33, Steve Dower <steve.dower(a)python.org> wrote: > > Looks good to me, but I wonder why we need to define all the algorithms in the PEP (by reference)? Could they also use a similar model to certificates, where the implementation provides a constructor that takes a string (in a format defined here, e.g. "OpenSSL style") and does the best it can to return something it will know how to use later? It involves some trust, but I honestly don't see a world where we end up with implementations deliberately trying to be incompatible with each other (which would seem to be the only reason to define the full enum ahead of time). There’s a thread running through this PEP which I’d call a rejection of “stringly-typed” APIs. I have never liked OpenSSL’s cipher string format: it’s a constant source of footguns where a given cipher string provides an extremely non-deterministic result. Because OpenSSL needs to tolerate the possibility that you are referring to ciphers that are only present in older or newer versions of OpenSSL, there is little error checking done on a cipher string. You can see this by running the openssl ciphers command on two very similar cipher strings: openssl ciphers "ECDHE+AESGCM:RSA+AESGCM" ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-GCM-SHA256 vs openssl ciphers "ECDHE+AESGCM:RSA+AESGMC" ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256 Note that the typo in the second string wipes out a whole bunch of cipher suites, but OpenSSL doesn’t flag this as a problem because it doesn’t know whether AESGMC is a thing that might appear in the future or have appeared in the past. The only way to actually error check this is to compare to the hard-coded list of what cipher suites you expect, except of course at that point you may as well have just written the full list of cipher suites out or, more aptly, used a typed API. OpenSSL only errors out on a cipher string if it leads to no ciphers being supported, which is an extremely unlikely outcome for any project taking rigorous control of their cipher string. The other problem here is that we need to define a grammar of how to translate the cipher string into a list of actual ciphers. This grammar needs to be forward-looking, which is a problem that OpenSSL is running directly into as we speak[0]. For this reason I’m inclined to lean towards the more verbose approach of just writing down what all of the cipher suites are in an enum. That way, it gets much easier to validate what’s going on. There’s still no requirement to actually support them all: an implementation is allowed to quietly ignore any cipher suites it doesn’t support. But that can no longer happen due to typos, because typos now cause AttributeErrors at runtime in a way that is very obvious and clear. Does that make sense? Cory [0]: This is a digression, but worth harping on about. OpenSSL’s cipher suite names have traditionally been of the form “key exchange-signing algo-stream cipher-mode-MAC”. However, OpenSSL has allowed some of those to be unspecified and to mean their defaults (e.g. AES128-GCM-SHA256 actually means RSA-RSA-AES128-GCM-SHA256). This has caused them problems with TLSv1.3, which has defined all new cipher suites that no longer include their key exchange mechanism (e.g. TLS_AES_128_GCM_SHA256 is now a complete cipher string). Because of OpenSSL’s weird defaulty grammar, right now their cipher string assumes that this new cipher uses RSA key exchange, even though it absolutely does not. All of this is a very long form way of saying that defining a stable string-to-cipher-suite parser is very hard unless we define it as saying “just use the actual IANA names for the cipher suites”, and the second we’ve done that we may as well have just defined an enum and called it good. ;)

3 6

Unified TLS API for Python: Round 2
by Cory Benfield 26 Jan '17

26 Jan '17

All, Thanks for your feedback on the draft PEP I proposed last week! There was a lot of really enthusiastic and valuable feedback both on this mailing list and on GitHub. I believe I’ve addressed a lot of the concerns that were brought up with the PEP now, so I’d like to ask that interested parties take another look. Some quick hits on the changes: - I’ve introduced a TLSConfiguration object and extracted all configuration from the Context onto it. This object is an immutable one (it’s strictly a slightly-fancy namedtuple), which provides some really huge advantages in terms of managing them and allowing concrete implementations to use Configuration objects as dictionary keys. - I’ve fleshed out the cipher suite section. - I’ve split out read_into. - I’ve stopped allowing passwords to be strings: they must now always be bytes. - I’ve dramatically changed the SNI callback to take advantage of the TLSConfiguration. - I’ve added support for trust stores. - Several other smaller changes. Please let me know what you think. The version of the draft PEP, from commit ce74bc60, is reproduced below. Thanks! Cory — PEP: XXX Title: TLS Abstract Base Classes Version: $Revision$ Last-Modified: $Date$ Author: Cory Benfield <cory(a)lukasa.co.uk> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 17-Oct-2016 Python-Version: 3.7 Post-History: 30-Aug-2002 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 unforseen 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 build 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 unweildy 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. Abstract Base Classes --------------------- There are several interfaces that require standardisation. Those interfaces are: 1. Configuring TLS, currently implemented by the `SSLContext`_ class in the ``ssl`` module. 2. Wrapping a socket object, currently implemented by the `SSLSocket`_ class in the ``ssl`` module. 3. 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. 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. While it is technically possible to define (2) in terms of (3), for the sake of simplicity it is easier to define these as two separate ABCs. Implementations are of course free to implement the concrete subclasses however they see fit. 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, becuase 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. Both objects would be immutable. 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. 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 imutable 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[CipherSuite]: 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 ``Context``. 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=None: Optional[bool], certificate_chain=None: Optional[Tuple[Tuple[Certificate], PrivateKey]], ciphers=None: Optional[Tuple[CipherSuite]], inner_protocols=None: Optional[Tuple[Union[NextProtocol, bytes]]], lowest_supported_version=None: Optional[TLSVersion], highest_supported_version=None: Optional[TLSVersion], trust_store=None: Optional[TrustStore], sni_callback=None: Optional[ServerNameCallback]): 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 ~~~~~~~ The ``Context`` abstract base class defines an object that allows configuration of TLS. It can be thought of as a factory for ``TLSWrappedSocket`` and ``TLSWrappedBuffer`` objects. 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): @abstractmethod def wrap_socket(self, socket: socket.socket, auto_handshake=True: bool, server_hostname=None: 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 parameter ``auto_handshake`` specifies whether to do the SSL handshake automatically after doing a ``socket.connect()``, or whether the application program will call it explicitly, by invoking the ``TLSWrappedSocket.do_handshake()`` method. Calling ``TLSWrappedSocket.do_handshake()`` explicitly gives the program control over the blocking behavior of the socket I/O involved in the handshake. The optional parameter ``server_hostname`` specifies the hostname of the service which we are connecting to. This allows a single server to host multiple SSL-based services with distinct certificates, quite similarly to HTTP virtual hosts. """ @abstractmethod def wrap_buffers(self, incoming: Any, outgoing: Any, server_hostname=None: Optional[str]) -> TLSWrappedBuffer: """ Wrap a pair of buffer objects (``incoming`` and ``outgoing``) to create an in-memory stream for TLS. The SSL routines will read data from ``incoming`` and decrypt it, and write encrypted data to ``outgoing``. The ``server_hostname`` parameter has the same meaning as in ``wrap_socket``. """ class ServerContext(_BaseContext): @abstractmethod def wrap_socket(self, socket: socket.socket, auto_handshake=True: bool) -> 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 parameter ``auto_handshake`` specifies whether to do the SSL handshake automatically after doing a ``socket.connect()``, or whether the application program will call it explicitly, by invoking the ``TLSWrappedSocket.do_handshake()`` method. Calling ``TLSWrappedSocket.do_handshake()`` explicitly gives the program control over the blocking behavior of the socket I/O involved in the handshake. """ @abstractmethod def wrap_buffers(self, incoming: Any, outgoing: Any) -> TLSWrappedBuffer: """ Wrap a pair of buffer objects (``incoming`` and ``outgoing``) to create an in-memory stream for TLS. The SSL routines will read data from ``incoming`` and decrypt it, and write encrypted data to ``outgoing``. """ Socket ~~~~~~ The socket-wrapper ABC will be defined by the ``TLSWrappedSocket`` ABC, which has the following definition:: class TLSWrappedSocket(metaclass=ABCMeta): # The various socket methods all must be implemented. Their definitions # have been elided from this class defintion in the PEP because they # aren't instructive. @abstractmethod def do_handshake(self) -> None: """ Performs the TLS handshake. Also performs certificate validation and hostname verification. """ @abstractmethod def cipher(self) -> Optional[CipherSuite]: """ Returns the CipherSuite entry for the cipher that has been negotiated on the connection. If no connection has been negotiated, returns ``None``. """ @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 socket is tied to. """ @abstractproperty def negotiated_tls_version(self) -> Optional[TLSVersion]: """ The version of TLS that has been negotiated on this connection. """ @abstractmethod 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. """ 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=None: int) -> bytes: """ Read up to ``amt`` bytes of data from the input buffer and return the result as a ``bytes`` instance. If ``amt`` is ``None``, will attempt to read until either EOF is reached or until further attempts to read would raise either ``WantReadError`` or ``WantWriteError``. 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. As at any time a re-negotiation is possible, a call to ``read()`` can also cause write operations. """ @abstractmethod def read_into(self, buffer: Any, amt=None: 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. If ``amt`` is ``None``, will attempt to read until either EOF is reached or until further attempts to read would raise either ``WantReadError`` or ``WantWriteError``, or until the buffer is full. 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. As at any time a re-negotiation is possible, a call to ``read_into()`` 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. 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[CipherSuite]: """ Returns the CipherSuite entry for the cipher that has been negotiated on the connection. If no connection has been negotiated, returns ``None``. """ @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 socket 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. """ 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(cphers)); 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. 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 all 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 tuple of integers. The names of the enum members will be their IANA-registered cipher suite names. Rather than populate this enum by hand, it is likely that we'll define a script that can build it from Christian Heimes' `tlsdb JSON file`_ (warning: large file). This 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. Because this enum would be enormous, the entire enum is not provided here. Instead, a small sample of entries is provided to give a flavor of how it will appear. class CipherSuite(Enum): ... TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA = (0xC0, 0x12) ... TLS_ECDHE_ECDSA_WITH_AES_128_CCM = (0xC0, 0xAC) ... TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 = (0xC0, 0x2B) ... 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 SSLv2 SSLv3 TLSv1 TLSv1_1 TLSv1_2 TLSv1_3 MAXIMUM_SUPPORTED Errors ~~~~~~ This module would define three base classes for use with error handling. Unlike 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. """ 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. """ 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) -> Certificate: """ 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]) -> Certificate: """ 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=None: Optional[Union[Callable[[], Union[bytes, bytearray]], bytes, bytearray]) -> PrivateKey: """ 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=None: Optional[Union[Callable[[], Union[bytes, bytearray]], bytes, bytearray]) -> PrivateKey: """ 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. """ 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.client - imaplib - nntplib - poplib - smtplib 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. ToDo ==== * Consider adding a new parameter (``valid_subjects``?) to ``wrap_socket`` and ``wrap_buffers`` that specifies in a *typed* manner what kind of entries in the SAN field are acceptable. This would break the union between SNI and cert validation, which may be a good thing (you can't SNI an IP address, but you can validate a cert with one if you want). * It's annoying that there's no type signature for fileobj. Do I really have to define one as part of this PEP? Otherwise, how do I define the types of the arguments to ``wrap_buffers``? * Do we need ways to control hostname validation? * Do we need to support getpeercert? Should we always return DER instead of the weird semi-structured thing? * How do we load certs from locations on disk? What about HSMs? * How do we signal to load certs from the OS? What happens if an implementation doesn't let you *not* load those certs? References ========== .. _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).… .. _tlsdb JSON file: https://github.com/tiran/tlsdb/blob/master/tlsdb.json Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End:

6 25

email & phones
by Ethan Furman 26 Jan '17

26 Jan '17

Question for everyone -- particularly those that reply to messages using their phones (an honest question, I'm not trying to be a jerk): Is it very difficult to trim quoted text that you are not replying to? On Python List I'm a stickler for kicking back messages where large parts of a previous email (usually the whole thing) are quoted for apparently no good reason as I feel such replies seriously drop the signal-to-noise ratio. However, this list is very low traffic, so if trimming quoted text is a huge pain when using phone email I could stop worrying about that. Thoughts? -- ~Ethan~

4 4

Unified TLS API for Python: Draft 3
by Cory Benfield 26 Jan '17

26 Jan '17

All, Once again, thanks for your feedback: it’s extremely helpful to be able to have a fairly short cycle time with dedicated review. I think I’ve addressed several of the concerns raised before. The third draft of this document is below. Cory — PEP: XXX Title: TLS Abstract Base Classes Version: $Revision$ Last-Modified: $Date$ Author: Cory Benfield <cory(a)lukasa.co.uk> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 17-Oct-2016 Python-Version: 3.7 Post-History: 30-Aug-2002 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 build 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 unweildy 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. Abstract Base Classes --------------------- There are several interfaces that require standardisation. Those interfaces are: 1. Configuring TLS, currently implemented by the `SSLContext`_ class in the ``ssl`` module. 2. Wrapping a socket object, currently implemented by the `SSLSocket`_ class in the ``ssl`` module. 3. 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. 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. While it is technically possible to define (2) in terms of (3), for the sake of simplicity it is easier to define these as two separate ABCs. Implementations are of course free to implement the concrete subclasses however they see fit. 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, becuase 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. Both objects would be immutable. 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[CipherSuite]: 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 ``Context``. 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=None: Optional[bool], certificate_chain=None: Optional[Tuple[Tuple[Certificate], PrivateKey]], ciphers=None: Optional[Tuple[CipherSuite]], inner_protocols=None: Optional[Tuple[Union[NextProtocol, bytes]]], lowest_supported_version=None: Optional[TLSVersion], highest_supported_version=None: Optional[TLSVersion], trust_store=None: Optional[TrustStore], sni_callback=None: Optional[ServerNameCallback]): 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, SChannel on macOS is not really intended for server use and has an enormous amount of functionality missing for server-side use. This would allow SChannel implementations to simply not define a concrete subclass of ``ServerContext`` to signal their lack of support. 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): @abstractmethod def wrap_socket(self, socket: socket.socket, server_hostname: Optional[str], auto_handshake=True: bool) -> 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 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. The parameter ``auto_handshake`` specifies whether to do the SSL handshake automatically after doing a ``socket.connect()``, or whether the application program will call it explicitly, by invoking the ``TLSWrappedSocket.do_handshake()`` method. Calling ``TLSWrappedSocket.do_handshake()`` explicitly gives the program control over the blocking behavior of the socket I/O involved in the handshake. """ @abstractmethod def wrap_buffers(self, incoming: Any, outgoing: Any, server_hostname: Optional[str]) -> TLSWrappedBuffer: """ Wrap a pair of buffer objects (``incoming`` and ``outgoing``) to create an in-memory stream for TLS. The SSL routines will read data from ``incoming`` and decrypt it, and write encrypted data to ``outgoing``. The buffer objects must be either file objects or objects that implement the buffer protocol. The ``server_hostname`` parameter has the same meaning as in ``wrap_socket``. """ class ServerContext(_BaseContext): @abstractmethod def wrap_socket(self, socket: socket.socket, auto_handshake=True: bool) -> 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 parameter ``auto_handshake`` specifies whether to do the SSL handshake automatically after doing a ``socket.accept()``, or whether the application program will call it explicitly, by invoking the ``TLSWrappedSocket.do_handshake()`` method. Calling ``TLSWrappedSocket.do_handshake()`` explicitly gives the program control over the blocking behavior of the socket I/O involved in the handshake. """ @abstractmethod def wrap_buffers(self, incoming: Any, outgoing: Any) -> TLSWrappedBuffer: """ Wrap a pair of buffer objects (``incoming`` and ``outgoing``) to create an in-memory stream for TLS. The SSL routines will read data from ``incoming`` and decrypt it, and write encrypted data to ``outgoing``. The buffer objects must be either file objects or objects that implement the buffer protocol. """ Socket ~~~~~~ The socket-wrapper ABC will be defined by the ``TLSWrappedSocket`` ABC, which has the following definition:: class TLSWrappedSocket(metaclass=ABCMeta): # The various socket methods all must be implemented. Their definitions # have been elided from this class defintion in the PEP because they # aren't instructive. @abstractmethod def do_handshake(self) -> None: """ Performs the TLS handshake. Also performs certificate validation and hostname verification. """ @abstractmethod def cipher(self) -> Optional[CipherSuite]: """ Returns the CipherSuite entry for the cipher that has been negotiated on the connection. If no connection has been negotiated, returns ``None``. """ @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 socket is tied to. """ @abstractproperty def negotiated_tls_version(self) -> Optional[TLSVersion]: """ The version of TLS that has been negotiated on this connection. """ @abstractmethod 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. """ 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=None: int) -> bytes: """ Read up to ``amt`` bytes of data from the input buffer and return the result as a ``bytes`` instance. If ``amt`` is ``None``, will attempt to read until either EOF is reached or until further attempts to read would raise either ``WantReadError`` or ``WantWriteError``. 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. 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=None: 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. If ``amt`` is ``None``, will attempt to read until either EOF is reached or until further attempts to read would raise either ``WantReadError`` or ``WantWriteError``, or until the buffer is full. 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. 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. 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[CipherSuite]: """ Returns the CipherSuite entry for the cipher that has been negotiated on the connection. If no connection has been negotiated, returns ``None``. """ @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 socket 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. """ 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(cphers)); 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. 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 all 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 tuple of integers. The names of the enum members will be their IANA-registered cipher suite names. Rather than populate this enum by hand, it is likely that we'll define a script that can build it from Christian Heimes' `tlsdb JSON file`_ (warning: large file). This 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. Because this enum would be enormous, the entire enum is not provided here. Instead, a small sample of entries is provided to give a flavor of how it will appear. :: class CipherSuite(Enum): ... TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA = (0xC0, 0x12) ... TLS_ECDHE_ECDSA_WITH_AES_128_CCM = (0xC0, 0xAC) ... TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 = (0xC0, 0x2B) ... 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 SSLv2 SSLv3 TLSv1 TLSv1_1 TLSv1_2 TLSv1_3 MAXIMUM_SUPPORTED Errors ~~~~~~ This module would define three 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. """ 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. """ 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) -> Certificate: """ 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]) -> Certificate: """ 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=None: Optional[Union[Callable[[], Union[bytes, bytearray]], bytes, bytearray]) -> PrivateKey: """ 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=None: Optional[Union[Callable[[], Union[bytes, bytearray]], bytes, bytearray]) -> PrivateKey: """ 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.client - imaplib - nntplib - poplib - smtplib 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. References ========== .. _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).… .. _tlsdb JSON file: https://github.com/tiran/tlsdb/blob/master/tlsdb.json Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End:

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Re: [Security-sig] Unified TLS API for Python: Round 2
by Cory Benfield 20 Jan '17

20 Jan '17

> On 20 Jan 2017, at 04:38, Wes Turner <wes.turner(a)gmail.com> wrote: > > Thanks! TLSConfiguration looks much easier to review; and should make other implementations easier. > > I read a great (illustrated) intro to TLS1.3 the other day: > > https://temen.io/resources/tls-gets-an-upgrade:-welcome-1.3/ <https://temen.io/resources/tls-gets-an-upgrade:-welcome-1.3/> > > - 1-RTT and 0-RTT look useful. > - There's a reduced set of cipher suites > > https://tlswg.github.io/tls13-spec/ <https://tlswg.github.io/tls13-spec/> #rfc.section.4.3 > > - Are there additional parameters relevant to TLS1.3 for the TLSConfiguration object? > - If necessary, how should TLSConfiguration parameter fields be added? So both 0-RTT and 1-RTT have little to no library support at this time. Certainly the three main implementations that this proposal considers (OpenSSL, SChannel, SecureTransport) have no public APIs to control this kind of functionality. Indeed, none of those implementations is shipping TLSv1.3 yet as far as I’m aware. OpenSSL has a date by which they plan to have TLSv1.3 support complete, and based on Apple’s adoption of TLS tech I’d expect to see TLSv1.3 in SecureTransport within 18 months and possibly as soon as September. The TL;DR here is that I think we should not attempt to spec APIs for 1-RTT and 0-RTT yet, until we understand how implementations plan to support it. As to the reduced set of cipher suites, all either do or will have IANA assigned names, and so can be managed as per the cipher enum. As to adding TLSConfiguration parameter fields, I think they are subject to the regular standard library update process. Generally speaking we should have a policy that adding fields should be a difficult thing to do unless there is substantial need for their addition, to avoid making the object almost impossible to manage. However, otherwise the regular standard library concerns should all apply. Cory

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Re: [Security-sig] Unified TLS API for Python
by Nathaniel Smith 13 Jan '17

13 Jan '17

On Jan 13, 2017 3:17 AM, "Nick Coghlan" <ncoghlan(a)gmail.com> wrote: On 13 January 2017 at 19:33, Nathaniel Smith <njs(a)pobox.com> wrote: > On Fri, Jan 13, 2017 at 1:09 AM, Cory Benfield <cory(a)lukasa.co.uk> wrote: > I get what you're saying about a typed API -- basically in the current > setup, the way _BaseContext is written as a bunch of Python methods > means that the interpreter will automatically catch if someone tries > to call set_cihpers, whereas in the dict version each implementation > would have to catch this itself. But in practice this difference seems > really minimal to me -- either way it's a runtime check The difference isn't minimal at all from the perspective of: - static type analysis - runtime introspection - IDEs (which use a mixture of both) - documentation Config dictionary based APIs have *major* problems in all those respects, as they're ultimately just a bunch of opaque-to-the-compiler keys and values. Network security is a sufficiently hard problem that we want to make it as easy as possible for developers to bring additional tools for ensuring code correctness to bear :) > and it's > really difficult to write a loop like > > for key, value in config.items(): > do_some_quirky_cffi_thing_based_on_key() > > that doesn't implicitly validate the keys anyway. There's also the > option of using e.g. JSON-Schema to write down a formal description of > what goes in the dict -- this could actually be substantially stricter > than what Python gets you, because you can actually statically enforce > that validate_certificates is a bool. For whatever that's worth -- > probably not much. I've generally found that it's easier to build a declarative API atop a programmatic API than it is to tackle a problem the other way around: 1. Even if the only public API is declarative, there's going to be a programmatic API that implements the actual processing of the declarative requests 2. Working with the programmatic API provides insight into which features the declarative API should cover, and which it can skip 3. Programmatic APIs are often easier to test, since you can more readily isolate the effects of the individual operations 4. A public programmatic API serves as a natural constraint and test case for any subsequent declarative API I totally buy these as advantages of a generic programmatic API over a generic declarative API, but I'm not sure how much it applies given that in this case the "programmatic" API we're talking about is literally *nothing* but getters and setters. Spelling set_ciphers correctly is certainly important and it's nice if your IDE can help, no doubt. But let's keep this in perspective - I don't think this is the *hard* part of network security, really :-). And I can't resist pointing out that in your other email sent at the same time, you're musing about out how nice it would be if only there were some way the stdlib could export its idea of a "default tls configuration" as some sort of concrete object that arbitrary tls implementations could ingest... which is *exactly* what the config-dict approach provides :-). I guess another possible point in the design space would be to split the difference: instead of an abstract context class or a dict, define a concrete context class that acts as a simple transparent container for these settings, effectively layering the type safety etc over its __dict__. -n

4 9

Re: [Security-sig] Unified TLS API for Python
by Cory Benfield 13 Jan '17

13 Jan '17

> On 11 Jan 2017, at 21:44, Wes Turner <wes.turner(a)gmail.com> wrote: > > This may be a bit of a different use case (and possibly worth having in the first version of a new tls module): > > "Hitless TLS Certificate Rotation in Go" > https://diogomonica.com/2017/01/11/hitless-tls-certificate-rotation-in-go/ <https://diogomonica.com/2017/01/11/hitless-tls-certificate-rotation-in-go/> > > - Can/could this be done with only set_sni_callback ? Yes, it can be. Twisted has an extension module, txsni, that uses the SNI callback to choose which certificate to provide. This is basically identical to the Go GetCertificate callback function. Cory

7 20

Unified TLS API for Python
by Cory Benfield 12 Jan '17

12 Jan '17

All, A recent change in the Fastly policy has led the distutils-sig to discover that there is a very real risk of many macOS installations of Python being entirely unable to access PyPI using pip, due to the fact that they are linked against the ancient system OpenSSL (see [1]). This problem ultimately stems from the fact that Python has never exposed the ability to use TLS backends other than OpenSSL in the standard library. As Christian and I discussed back at PyCon US 2015, 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 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. The current copy can be found on GitHub[2]. I’d like to see if there is interest from the Python security community in pursuing this approach to generic TLS, and what changes would be necessary to this API before we submitted it to python-dev. Please let me know what you think. Cory [1]: https://mail.python.org/pipermail/distutils-sig/2017-January/029970.html [2]: https://github.com/Lukasa/peps/pull/1 - - - 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 unforseen 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 build 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 unweildy 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. Abstract Base Classes --------------------- There are several interfaces that require standardisation. Those interfaces are: 1. Configuring TLS, currently implemented by the `SSLContext`_ class in the ``ssl`` module. 2. Wrapping a socket object, currently implemented by the `SSLSocket`_ class in the ``ssl`` module. 3. 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. 4. 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. 5. Specifying application-layer protocols that can be negotiated during the TLS handshake. 6. Specifying TLS versions. 7. Reporting errors to the caller, currently implemented by the `SSLError`_ class in the ``ssl`` module. While it is technically possible to define (2) in terms of (3), for the sake of simplicity it is easier to define these as two separate ABCs. Implementations are of course free to implement the concrete subclasses however they see fit. Obviously, (4) 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. Context ~~~~~~~ The ``Context`` abstract base class defines an object that allows configuration of TLS. It can be thought of as a factory for ``TLSWrappedSocket`` and ``TLSWrappedBuffer`` objects. The ``Context`` abstract base class has the following class definition:: TLSBufferObject = Union[TLSWrappedSocket, TLSWrappedBuffer] ServerNameCallback = Callable[[TLSBufferObject, Optional[str], Context], Any] class _BaseContext(metaclass=ABCMeta): @property @abstractmethod def validate_certificates(self) -> 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``. """ @validate_certificates.setter @abstractmethod def validate_certificates(self, val: bool) -> None: pass @abstractmethod def register_certificates(self, certificates: str, key=None: Optional[str], password=None: Optional[Callable[[], Union[AnyStr, bytearray]]]) -> None: """ Loads a certificate, a number of intermediate certificates, and the corresponding private key. These certificates will be offered to the remote peer during the handshake if required. The ``certificates`` argument must be a bytestring containing the PEM-encoded certificates. The first PEM-encoded certificate must be the leaf certificate. All subsequence certificates will be offered as intermediate additional certificates. The ``key`` argument, if present, must contain the PEM-encoded private key associated with the leaf certificate. If not present, the private key will be extracted from ``certificates``. The ``password`` argument may be a function to call to get the password for decrypting the private key. It will only be called if the private key is encrypted and a password is necessary. It will be called with no arguments, and it should return a string, bytes, or bytearray. If the return value is a string it will be encoded as UTF-8 before using it to decrypt the key. Alternatively a string, bytes, or bytearray value may be supplied directly as the password argument. It will be ignored if the private key is not encrypted and no password is needed. """ @abstractmethod def set_ciphers(self, ciphers: List[Ciphers]) -> None: """ Set the available ciphers for TLS connections created with this context. ``ciphers`` should be a list of ciphers from the ``Cipher`` registry. If none of the ``ciphers`` provided to this object are supported or available, a ``TLSError`` will be raised. """ @abstractmethod def set_inner_protocols(self, protocols: List[NextProtocol]) -> None: """ Specify which protocols the socket should advertise as supported during the TLS handshake. This may be advertised using either or both of ALPN or NPN. ``protocols`` should be a list of acceptable protocols in the form of ``NextProtocol`` objects, such as ``[H2, HTTP1]``, ordered by preference. The selection of the protocol will happen during the handshake, and will use whatever protocol negotiation mechanisms are available and supported by both peers. If the TLS implementation doesn't support protocol negotiation, this method will raise ``NotImplementedError``. """ @abstractmethod def set_sni_callback(self, callback: Optional[ServerNameCallback]) -> None: """ Register 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 ``Context``. If ``callback`` is ``None`` then the callback is disabled. Calling this function a subsequent time will disable the previously registered callback. The ``callback`` function will be called with three arguments: the first 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 ``Context``. The server name argument will be the IDNA *decoded* server name. The ``callback`` must return ``None`` 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. """ @abstractmethod def set_version_range(self, lower_bound=None: Optional[TLSVersion], upper_bound=None: Optional[TLSVersion]) -> None: """ Set the minumum and maximum versions of TLS that should be allowed on TLS connections made by this context. If present, ``lower_bound`` will set the lowest acceptable TLS version. If present, ``upper_bound`` will set the highest acceptable TLS version. If either argument is ``None``, this will leave that bound unchanged. """ @abstractmethod def wrap_socket(self, socket: socket.socket, server_side=False: bool, auto_handshake=True: bool, server_hostname=None: 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 parameter ``server_side`` is a boolean which identifies whether server-side or client-side behavior is desired from this socket. The parameter ``auto_handshake`` specifies whether to do the SSL handshake automatically after doing a ``socket.connect()``, or whether the application program will call it explicitly, by invoking the ``TLSWrappedSocket.do_handshake()`` method. Calling ``TLSWrappedSocket.do_handshake()`` explicitly gives the program control over the blocking behavior of the socket I/O involved in the handshake. On client connections, the optional parameter ``server_hostname`` specifies the hostname of the service which we are connecting to. This allows a single server to host multiple SSL-based services with distinct certificates, quite similarly to HTTP virtual hosts. Specifying ``server_hostname`` will raise a ValueError if ``server_side`` is ``True``. """ @abstractmethod def wrap_buffers(self, incoming: Any, outgoing: Any, server_side=False: bool, server_hostname=None: Optional[str]) -> TLSWrappedBuffer: """ Wrap a pair of buffer objects (``incoming`` and ``outgoing``) to create an in-memory stream for TLS. The SSL routines will read data from ``incoming`` and decrypt it, and write encrypted data to ``outgoing``. The ``server_side`` and ``server_hostname`` parameters have the same meaning as in ``wrap_socket``. """ class ClientContext(metaclass=ABCMeta): @abstractmethod def wrap_socket(self, socket: socket.socket, auto_handshake=True: bool, server_hostname=None: 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 parameter ``auto_handshake`` specifies whether to do the SSL handshake automatically after doing a ``socket.connect()``, or whether the application program will call it explicitly, by invoking the ``TLSWrappedSocket.do_handshake()`` method. Calling ``TLSWrappedSocket.do_handshake()`` explicitly gives the program control over the blocking behavior of the socket I/O involved in the handshake. The optional parameter ``server_hostname`` specifies the hostname of the service which we are connecting to. This allows a single server to host multiple SSL-based services with distinct certificates, quite similarly to HTTP virtual hosts. """ @abstractmethod def wrap_buffers(self, incoming: Any, outgoing: Any, server_hostname=None: Optional[str]) -> TLSWrappedBuffer: """ Wrap a pair of buffer objects (``incoming`` and ``outgoing``) to create an in-memory stream for TLS. The SSL routines will read data from ``incoming`` and decrypt it, and write encrypted data to ``outgoing``. The ``server_hostname`` parameter has the same meaning as in ``wrap_socket``. """ class ServerContext(metaclass=ABCMeta): @abstractmethod def wrap_socket(self, socket: socket.socket, auto_handshake=True: bool) -> 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 parameter ``auto_handshake`` specifies whether to do the SSL handshake automatically after doing a ``socket.connect()``, or whether the application program will call it explicitly, by invoking the ``TLSWrappedSocket.do_handshake()`` method. Calling ``TLSWrappedSocket.do_handshake()`` explicitly gives the program control over the blocking behavior of the socket I/O involved in the handshake. """ @abstractmethod def wrap_buffers(self, incoming: Any, outgoing: Any) -> TLSWrappedBuffer: """ Wrap a pair of buffer objects (``incoming`` and ``outgoing``) to create an in-memory stream for TLS. The SSL routines will read data from ``incoming`` and decrypt it, and write encrypted data to ``outgoing``. """ Socket ~~~~~~ The socket-wrapper ABC will be defined by the ``TLSWrappedSocket`` ABC, which has the following definition:: class TLSWrappedSocket(metaclass=ABCMeta): # The various socket methods all must be implemented. Their definitions # have been elided from this class defintion in the PEP because they # aren't instructive. @abstractmethod def do_handshake(self) -> None: """ Performs the TLS handshake. Also performs certificate validation and hostname verification. """ @abstractmethod def cipher(self) -> Optional[Cipher]: """ Returns the Cipher entry for the cipher that has been negotiated on the connection. If no connection has been negotiated, returns ``None``. """ @abstractmethod def negotiated_protocol(self) -> Optional[NextProtocol]: """ 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 ``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 socket is tied to. """ @context.setter @abstractmethod def context(self, value: Context) -> None: """ Set the value of the ``Context`` object this socket is tied to. This operation (changing the context) may not always be supported. """ @abstractproperty def negotiated_tls_version(self) -> Optional[TLSVersion]: """ The version of TLS that has been negotiated on this connection. """ 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, len=1024: int, buffer=None: Any) -> Union[bytes, int]: """ Read up to ``len`` bytes of data from the input buffer and return the result as a ``bytes`` instance. If ``buffer`` is specified, then read into the buffer instead, and return the number of bytes read. Raise ``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. As at any time a re-negotiation is possible, a call to ``read()`` 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. 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[Cipher]: """ Returns the Cipher entry for the cipher that has been negotiated on the connection. If no connection has been negotiated, returns ``None``. """ @abstractmethod def negotiated_protocol(self) -> Optional[NextProtocol]: """ 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 ``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 socket is tied to. """ @context.setter @abstractmethod def context(self, value: Context) -> None: """ Set the value of the ``Context`` object this socket is tied to. This operation (changing the context) may not always be supported. """ @abstractproperty def negotiated_tls_version(self) -> Optional[TLSVersion]: """ The version of TLS that has been negotiated on this connection. """ Cipher Suites ~~~~~~~~~~~~~ Todo 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, but 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. :: NextProtocol = namedtuple('NextProtocol', ['name']) H2 = NextProtocol(b'h2') H2C = NextProtocol(b'h2c') HTTP1 = NextProtocol(b'http/1.1') WEBRTC = NextProtocol(b'webrtc') C_WEBRTC = NextProtocol(b'c-webrtc') FTP = NextProtocol(b'ftp') STUN = NextProtocol(b'stun.nat-discovery') TURN = NextProtocol(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 SSLv2 SSLv3 TLSv1 TLSv1_1 TLSv1_2 TLSv1_3 MAXIMUM_SUPPORTED Errors ~~~~~~ This module would define three base classes for use with error handling. Unlike 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. These exceptions should *never* be thrown directly, they should always be subclassed. 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. """ 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. """ 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.client - imaplib - nntplib - poplib - smtplib 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. ToDo ==== * Consider adding a new parameter (``valid_subjects``?) to ``wrap_socket`` and ``wrap_buffers`` that specifies in a *typed* manner what kind of entries in the SAN field are acceptable. This would break the union between SNI and cert validation, which may be a good thing (you can't SNI an IP address, but you can validate a cert with one if you want). * It's annoying that there's no type signature for fileobj. Do I really have to define one as part of this PEP? Otherwise, how do I define the types of the arguments to ``wrap_buffers``? * Do we need ways to control hostname validation? * Do we need to support getpeercert? Should we always return DER instead of the weird semi-structured thing? * How do we load certs from locations on disk? What about HSMs? * How do we signal to load certs from the OS? What happens if an implementation doesn't let you *not* load those certs? References ========== .. _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 Copyright ========= This document has been placed in the public domain.

4 17

new TLS module
by Ethan Furman 12 Jan '17

12 Jan '17

With TLSv1.2 becoming mandatory in the near future and the various issues with the ssl module there is more interest in a tls module for Python. What should such a module include? What should it not include? Any reason to not also have a backport of this module? -- ~Ethan~

2 1