bpo-34271: Add ssl debugging helpers (GH-10031)

The ssl module now can dump key material to a keylog file and trace TLS
protocol messages with a tracing callback. The default and stdlib
contexts also support SSLKEYLOGFILE env var.

The msg_callback and related enums are private members. The feature
is designed for internal debugging and not for end users.

Signed-off-by: Christian Heimes <christian@python.org>

Lib/ssl.py

@@ -165,6 +165,90 @@ class TLSVersion(_IntEnum):
     MAXIMUM_SUPPORTED = _ssl.PROTO_MAXIMUM_SUPPORTED
 
 
+class _TLSContentType(_IntEnum):
+    """Content types (record layer)
+
+    See RFC 8446, section B.1
+    """
+    CHANGE_CIPHER_SPEC = 20
+    ALERT = 21
+    HANDSHAKE = 22
+    APPLICATION_DATA = 23
+    # pseudo content types
+    HEADER = 0x100
+    INNER_CONTENT_TYPE = 0x101
+
+
+class _TLSAlertType(_IntEnum):
+    """Alert types for TLSContentType.ALERT messages
+
+    See RFC 8466, section B.2
+    """
+    CLOSE_NOTIFY = 0
+    UNEXPECTED_MESSAGE = 10
+    BAD_RECORD_MAC = 20
+    DECRYPTION_FAILED = 21
+    RECORD_OVERFLOW = 22
+    DECOMPRESSION_FAILURE = 30
+    HANDSHAKE_FAILURE = 40
+    NO_CERTIFICATE = 41
+    BAD_CERTIFICATE = 42
+    UNSUPPORTED_CERTIFICATE = 43
+    CERTIFICATE_REVOKED = 44
+    CERTIFICATE_EXPIRED = 45
+    CERTIFICATE_UNKNOWN = 46
+    ILLEGAL_PARAMETER = 47
+    UNKNOWN_CA = 48
+    ACCESS_DENIED = 49
+    DECODE_ERROR = 50
+    DECRYPT_ERROR = 51
+    EXPORT_RESTRICTION = 60
+    PROTOCOL_VERSION = 70
+    INSUFFICIENT_SECURITY = 71
+    INTERNAL_ERROR = 80
+    INAPPROPRIATE_FALLBACK = 86
+    USER_CANCELED = 90
+    NO_RENEGOTIATION = 100
+    MISSING_EXTENSION = 109
+    UNSUPPORTED_EXTENSION = 110
+    CERTIFICATE_UNOBTAINABLE = 111
+    UNRECOGNIZED_NAME = 112
+    BAD_CERTIFICATE_STATUS_RESPONSE = 113
+    BAD_CERTIFICATE_HASH_VALUE = 114
+    UNKNOWN_PSK_IDENTITY = 115
+    CERTIFICATE_REQUIRED = 116
+    NO_APPLICATION_PROTOCOL = 120
+
+
+class _TLSMessageType(_IntEnum):
+    """Message types (handshake protocol)
+
+    See RFC 8446, section B.3
+    """
+    HELLO_REQUEST = 0
+    CLIENT_HELLO = 1
+    SERVER_HELLO = 2
+    HELLO_VERIFY_REQUEST = 3
+    NEWSESSION_TICKET = 4
+    END_OF_EARLY_DATA = 5
+    HELLO_RETRY_REQUEST = 6
+    ENCRYPTED_EXTENSIONS = 8
+    CERTIFICATE = 11
+    SERVER_KEY_EXCHANGE = 12
+    CERTIFICATE_REQUEST = 13
+    SERVER_DONE = 14
+    CERTIFICATE_VERIFY = 15
+    CLIENT_KEY_EXCHANGE = 16
+    FINISHED = 20
+    CERTIFICATE_URL = 21
+    CERTIFICATE_STATUS = 22
+    SUPPLEMENTAL_DATA = 23
+    KEY_UPDATE = 24
+    NEXT_PROTO = 67
+    MESSAGE_HASH = 254
+    CHANGE_CIPHER_SPEC = 0x0101
+
+
 if sys.platform == "win32":
     from _ssl import enum_certificates, enum_crls
 
@@ -523,6 +607,83 @@ class SSLContext(_SSLContext):
         def hostname_checks_common_name(self):
             return True
 
+    @property
+    def _msg_callback(self):
+        """TLS message callback
+
+        The message callback provides a debugging hook to analyze TLS
+        connections. The callback is called for any TLS protocol message
+        (header, handshake, alert, and more), but not for application data.
+        Due to technical  limitations, the callback can't be used to filter
+        traffic or to abort a connection. Any exception raised in the
+        callback is delayed until the handshake, read, or write operation
+        has been performed.
+
+        def msg_cb(conn, direction, version, content_type, msg_type, data):
+            pass
+
+        conn
+            :class:`SSLSocket` or :class:`SSLObject` instance
+        direction
+            ``read`` or ``write``
+        version
+            :class:`TLSVersion` enum member or int for unknown version. For a
+            frame header, it's the header version.
+        content_type
+            :class:`_TLSContentType` enum member or int for unsupported
+            content type.
+        msg_type
+            Either a :class:`_TLSContentType` enum number for a header
+            message, a :class:`_TLSAlertType` enum member for an alert
+            message, a :class:`_TLSMessageType` enum member for other
+            messages, or int for unsupported message types.
+        data
+            Raw, decrypted message content as bytes
+        """
+        inner = super()._msg_callback
+        if inner is not None:
+            return inner.user_function
+        else:
+            return None
+
+    @_msg_callback.setter
+    def _msg_callback(self, callback):
+        if callback is None:
+            super(SSLContext, SSLContext)._msg_callback.__set__(self, None)
+            return
+
+        if not hasattr(callback, '__call__'):
+            raise TypeError(f"{callback} is not callable.")
+
+        def inner(conn, direction, version, content_type, msg_type, data):
+            try:
+                version = TLSVersion(version)
+            except TypeError:
+                pass
+
+            try:
+                content_type = _TLSContentType(content_type)
+            except TypeError:
+                pass
+
+            if content_type == _TLSContentType.HEADER:
+                msg_enum = _TLSContentType
+            elif content_type == _TLSContentType.ALERT:
+                msg_enum = _TLSAlertType
+            else:
+                msg_enum = _TLSMessageType
+            try:
+                msg_type = msg_enum(msg_type)
+            except TypeError:
+                pass
+
+            return callback(conn, direction, version,
+                            content_type, msg_type, data)
+
+        inner.user_function = callback
+
+        super(SSLContext, SSLContext)._msg_callback.__set__(self, inner)
+
     @property
     def protocol(self):
         return _SSLMethod(super().protocol)
@@ -576,6 +737,11 @@ def create_default_context(purpose=Purpose.SERVER_AUTH, *, cafile=None,
         # CERT_OPTIONAL or CERT_REQUIRED. Let's try to load default system
         # root CA certificates for the given purpose. This may fail silently.
         context.load_default_certs(purpose)
+    # OpenSSL 1.1.1 keylog file
+    if hasattr(context, 'keylog_filename'):
+        keylogfile = os.environ.get('SSLKEYLOGFILE')
+        if keylogfile and not sys.flags.ignore_environment:
+            context.keylog_filename = keylogfile
     return context
 
 def _create_unverified_context(protocol=PROTOCOL_TLS, *, cert_reqs=CERT_NONE,
@@ -617,7 +783,11 @@ def _create_unverified_context(protocol=PROTOCOL_TLS, *, cert_reqs=CERT_NONE,
         # CERT_OPTIONAL or CERT_REQUIRED. Let's try to load default system
         # root CA certificates for the given purpose. This may fail silently.
         context.load_default_certs(purpose)
-
+    # OpenSSL 1.1.1 keylog file
+    if hasattr(context, 'keylog_filename'):
+        keylogfile = os.environ.get('SSLKEYLOGFILE')
+        if keylogfile and not sys.flags.ignore_environment:
+            context.keylog_filename = keylogfile
     return context
 
 # Used by http.client if no context is explicitly passed.