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Editing: low_level.py
""" Low-level helpers for the SecureTransport bindings. These are Python functions that are not directly related to the high-level APIs but are necessary to get them to work. They include a whole bunch of low-level CoreFoundation messing about and memory management. The concerns in this module are almost entirely about trying to avoid memory leaks and providing appropriate and useful assistance to the higher-level code. """ import base64 import ctypes import itertools import re import os import ssl import tempfile from .bindings import Security, CoreFoundation, CFConst # This regular expression is used to grab PEM data out of a PEM bundle. _PEM_CERTS_RE = re.compile( b"-----BEGIN CERTIFICATE-----\n(.*?)\n-----END CERTIFICATE-----", re.DOTALL ) def _cf_data_from_bytes(bytestring): """ Given a bytestring, create a CFData object from it. This CFData object must be CFReleased by the caller. """ return CoreFoundation.CFDataCreate( CoreFoundation.kCFAllocatorDefault, bytestring, len(bytestring) ) def _cf_dictionary_from_tuples(tuples): """ Given a list of Python tuples, create an associated CFDictionary. """ dictionary_size = len(tuples) # We need to get the dictionary keys and values out in the same order. keys = (t[0] for t in tuples) values = (t[1] for t in tuples) cf_keys = (CoreFoundation.CFTypeRef * dictionary_size)(*keys) cf_values = (CoreFoundation.CFTypeRef * dictionary_size)(*values) return CoreFoundation.CFDictionaryCreate( CoreFoundation.kCFAllocatorDefault, cf_keys, cf_values, dictionary_size, CoreFoundation.kCFTypeDictionaryKeyCallBacks, CoreFoundation.kCFTypeDictionaryValueCallBacks, ) def _cf_string_to_unicode(value): """ Creates a Unicode string from a CFString object. Used entirely for error reporting. Yes, it annoys me quite a lot that this function is this complex. """ value_as_void_p = ctypes.cast(value, ctypes.POINTER(ctypes.c_void_p)) string = CoreFoundation.CFStringGetCStringPtr( value_as_void_p, CFConst.kCFStringEncodingUTF8 ) if string is None: buffer = ctypes.create_string_buffer(1024) result = CoreFoundation.CFStringGetCString( value_as_void_p, buffer, 1024, CFConst.kCFStringEncodingUTF8 ) if not result: raise OSError('Error copying C string from CFStringRef') string = buffer.value if string is not None: string = string.decode('utf-8') return string def _assert_no_error(error, exception_class=None): """ Checks the return code and throws an exception if there is an error to report """ if error == 0: return cf_error_string = Security.SecCopyErrorMessageString(error, None) output = _cf_string_to_unicode(cf_error_string) CoreFoundation.CFRelease(cf_error_string) if output is None or output == u'': output = u'OSStatus %s' % error if exception_class is None: exception_class = ssl.SSLError raise exception_class(output) def _cert_array_from_pem(pem_bundle): """ Given a bundle of certs in PEM format, turns them into a CFArray of certs that can be used to validate a cert chain. """ der_certs = [ base64.b64decode(match.group(1)) for match in _PEM_CERTS_RE.finditer(pem_bundle) ] if not der_certs: raise ssl.SSLError("No root certificates specified") cert_array = CoreFoundation.CFArrayCreateMutable( CoreFoundation.kCFAllocatorDefault, 0, ctypes.byref(CoreFoundation.kCFTypeArrayCallBacks) ) if not cert_array: raise ssl.SSLError("Unable to allocate memory!") try: for der_bytes in der_certs: certdata = _cf_data_from_bytes(der_bytes) if not certdata: raise ssl.SSLError("Unable to allocate memory!") cert = Security.SecCertificateCreateWithData( CoreFoundation.kCFAllocatorDefault, certdata ) CoreFoundation.CFRelease(certdata) if not cert: raise ssl.SSLError("Unable to build cert object!") CoreFoundation.CFArrayAppendValue(cert_array, cert) CoreFoundation.CFRelease(cert) except Exception: # We need to free the array before the exception bubbles further. # We only want to do that if an error occurs: otherwise, the caller # should free. CoreFoundation.CFRelease(cert_array) return cert_array def _is_cert(item): """ Returns True if a given CFTypeRef is a certificate. """ expected = Security.SecCertificateGetTypeID() return CoreFoundation.CFGetTypeID(item) == expected def _is_identity(item): """ Returns True if a given CFTypeRef is an identity. """ expected = Security.SecIdentityGetTypeID() return CoreFoundation.CFGetTypeID(item) == expected def _temporary_keychain(): """ This function creates a temporary Mac keychain that we can use to work with credentials. This keychain uses a one-time password and a temporary file to store the data. We expect to have one keychain per socket. The returned SecKeychainRef must be freed by the caller, including calling SecKeychainDelete. Returns a tuple of the SecKeychainRef and the path to the temporary directory that contains it. """ # Unfortunately, SecKeychainCreate requires a path to a keychain. This # means we cannot use mkstemp to use a generic temporary file. Instead, # we're going to create a temporary directory and a filename to use there. # This filename will be 8 random bytes expanded into base64. We also need # some random bytes to password-protect the keychain we're creating, so we # ask for 40 random bytes. random_bytes = os.urandom(40) filename = base64.b64encode(random_bytes[:8]).decode('utf-8') password = base64.b64encode(random_bytes[8:]) # Must be valid UTF-8 tempdirectory = tempfile.mkdtemp() keychain_path = os.path.join(tempdirectory, filename).encode('utf-8') # We now want to create the keychain itself. keychain = Security.SecKeychainRef() status = Security.SecKeychainCreate( keychain_path, len(password), password, False, None, ctypes.byref(keychain) ) _assert_no_error(status) # Having created the keychain, we want to pass it off to the caller. return keychain, tempdirectory def _load_items_from_file(keychain, path): """ Given a single file, loads all the trust objects from it into arrays and the keychain. Returns a tuple of lists: the first list is a list of identities, the second a list of certs. """ certificates = [] identities = [] result_array = None with open(path, 'rb') as f: raw_filedata = f.read() try: filedata = CoreFoundation.CFDataCreate( CoreFoundation.kCFAllocatorDefault, raw_filedata, len(raw_filedata) ) result_array = CoreFoundation.CFArrayRef() result = Security.SecItemImport( filedata, # cert data None, # Filename, leaving it out for now None, # What the type of the file is, we don't care None, # what's in the file, we don't care 0, # import flags None, # key params, can include passphrase in the future keychain, # The keychain to insert into ctypes.byref(result_array) # Results ) _assert_no_error(result) # A CFArray is not very useful to us as an intermediary # representation, so we are going to extract the objects we want # and then free the array. We don't need to keep hold of keys: the # keychain already has them! result_count = CoreFoundation.CFArrayGetCount(result_array) for index in range(result_count): item = CoreFoundation.CFArrayGetValueAtIndex( result_array, index ) item = ctypes.cast(item, CoreFoundation.CFTypeRef) if _is_cert(item): CoreFoundation.CFRetain(item) certificates.append(item) elif _is_identity(item): CoreFoundation.CFRetain(item) identities.append(item) finally: if result_array: CoreFoundation.CFRelease(result_array) CoreFoundation.CFRelease(filedata) return (identities, certificates) def _load_client_cert_chain(keychain, *paths): """ Load certificates and maybe keys from a number of files. Has the end goal of returning a CFArray containing one SecIdentityRef, and then zero or more SecCertificateRef objects, suitable for use as a client certificate trust chain. """ # Ok, the strategy. # # This relies on knowing that macOS will not give you a SecIdentityRef # unless you have imported a key into a keychain. This is a somewhat # artificial limitation of macOS (for example, it doesn't necessarily # affect iOS), but there is nothing inside Security.framework that lets you # get a SecIdentityRef without having a key in a keychain. # # So the policy here is we take all the files and iterate them in order. # Each one will use SecItemImport to have one or more objects loaded from # it. We will also point at a keychain that macOS can use to work with the # private key. # # Once we have all the objects, we'll check what we actually have. If we # already have a SecIdentityRef in hand, fab: we'll use that. Otherwise, # we'll take the first certificate (which we assume to be our leaf) and # ask the keychain to give us a SecIdentityRef with that cert's associated # key. # # We'll then return a CFArray containing the trust chain: one # SecIdentityRef and then zero-or-more SecCertificateRef objects. The # responsibility for freeing this CFArray will be with the caller. This # CFArray must remain alive for the entire connection, so in practice it # will be stored with a single SSLSocket, along with the reference to the # keychain. certificates = [] identities = [] # Filter out bad paths. paths = (path for path in paths if path) try: for file_path in paths: new_identities, new_certs = _load_items_from_file( keychain, file_path ) identities.extend(new_identities) certificates.extend(new_certs) # Ok, we have everything. The question is: do we have an identity? If # not, we want to grab one from the first cert we have. if not identities: new_identity = Security.SecIdentityRef() status = Security.SecIdentityCreateWithCertificate( keychain, certificates[0], ctypes.byref(new_identity) ) _assert_no_error(status) identities.append(new_identity) # We now want to release the original certificate, as we no longer # need it. CoreFoundation.CFRelease(certificates.pop(0)) # We now need to build a new CFArray that holds the trust chain. trust_chain = CoreFoundation.CFArrayCreateMutable( CoreFoundation.kCFAllocatorDefault, 0, ctypes.byref(CoreFoundation.kCFTypeArrayCallBacks), ) for item in itertools.chain(identities, certificates): # ArrayAppendValue does a CFRetain on the item. That's fine, # because the finally block will release our other refs to them. CoreFoundation.CFArrayAppendValue(trust_chain, item) return trust_chain finally: for obj in itertools.chain(identities, certificates): CoreFoundation.CFRelease(obj)
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