Viewing file:  low_level.py (13.58 KB) -rw-r--r--
Select action/file-type:
 (+) |  (+) |  (+) | Code (+) | Session (+) |  (+) | SDB (+) |  (+) |  (+) |  (+) |  (+) |  (+) |
"""
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 os
import re
import ssl
import struct
import tempfile
from .bindings import CFConst, CoreFoundation, Security
# 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 _cfstr(py_bstr):
"""
Given a Python binary data, create a CFString.
The string must be CFReleased by the caller.
"""
c_str = ctypes.c_char_p(py_bstr)
cf_str = CoreFoundation.CFStringCreateWithCString(
CoreFoundation.kCFAllocatorDefault,
c_str,
CFConst.kCFStringEncodingUTF8,
)
return cf_str
def _create_cfstring_array(lst):
"""
Given a list of Python binary data, create an associated CFMutableArray.
The array must be CFReleased by the caller.
Raises an ssl.SSLError on failure.
"""
cf_arr = None
try:
cf_arr = CoreFoundation.CFArrayCreateMutable(
CoreFoundation.kCFAllocatorDefault,
0,
ctypes.byref(CoreFoundation.kCFTypeArrayCallBacks),
)
if not cf_arr:
raise MemoryError("Unable to allocate memory!")
for item in lst:
cf_str = _cfstr(item)
if not cf_str:
raise MemoryError("Unable to allocate memory!")
try:
CoreFoundation.CFArrayAppendValue(cf_arr, cf_str)
finally:
CoreFoundation.CFRelease(cf_str)
except BaseException as e:
if cf_arr:
CoreFoundation.CFRelease(cf_arr)
raise ssl.SSLError("Unable to allocate array: %s" % (e,))
return cf_arr
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.
"""
# Normalize the PEM bundle's line endings.
pem_bundle = pem_bundle.replace(b"\r\n", b"\n")
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.b16encode(random_bytes[:8]).decode("utf-8")
password = base64.b16encode(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)
TLS_PROTOCOL_VERSIONS = {
"SSLv2": (0, 2),
"SSLv3": (3, 0),
"TLSv1": (3, 1),
"TLSv1.1": (3, 2),
"TLSv1.2": (3, 3),
}
def _build_tls_unknown_ca_alert(version):
"""
Builds a TLS alert record for an unknown CA.
"""
ver_maj, ver_min = TLS_PROTOCOL_VERSIONS[version]
severity_fatal = 0x02
description_unknown_ca = 0x30
msg = struct.pack(">BB", severity_fatal, description_unknown_ca)
msg_len = len(msg)
record_type_alert = 0x15
record = struct.pack(">BBBH", record_type_alert, ver_maj, ver_min, msg_len) + msg
return record
|