openstack/heat
Code Issues Proposed changes

Fix entropy problems with OS::Random::String

Browse Source 
When generating a random string, once we had selected from the various
required pools, we continued by selecting a pool at random and then
selecting a character from that pool at random. This did not take into
account the differing sizes of the available pools, nor the fact that the
same character could appear in multiple pools, which resulted in a
non-uniform probability distribution of characters. Since users mostly make
use of this feature to generate default passwords for services they are
deploying, this would result in the generated passwords having slightly
less entropy than expected (and pathological cases were possible).

Rectify this by always selecting non-constrained characters from a single
combined pool, and by ensuring that each character appears only once in any
pool we're selecting from.

Since we also want to use this method to generate passwords for OpenStack
Users, the new implementation is in a separate module in heat.common rather
than mixed in with the resource's logic. Also, use a StringIO object to
collect the characters rather than repeatedly appending to a string.

Change-Id: Ia7b63e72c1e3c0649290caf4fea8a32f7f89560b
Closes-Bug: #1757300
Related-Bug: #1666129
Related-Bug: #1444429
This commit is contained in:
Zane Bitter 2018-03-20 20:48:38 -04:00
parent b7c03a7c46
commit 6e16c051ba
4 changed files with 256 additions and 80 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

109
heat/common/password_gen.py Normal file
View File

@ -0,0 +1,109 @@
#
# Licensed under the Apache License, Version 2.0 (the "License"); you may
# not use this file except in compliance with the License. You may obtain
# a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
# License for the specific language governing permissions and limitations
# under the License.
import collections
import random as random_module
import string
import six
# NOTE(pas-ha) Heat officially supports only POSIX::Linux platform
# where os.urandom() and random.SystemRandom() are available
random = random_module.SystemRandom()
CHARACTER_CLASSES = (
LETTERS_DIGITS, LETTERS, LOWERCASE, UPPERCASE,
DIGITS, HEXDIGITS, OCTDIGITS,
) = (
'lettersdigits', 'letters', 'lowercase', 'uppercase',
'digits', 'hexdigits', 'octdigits',
)
_char_class_members = {
LETTERS_DIGITS: string.ascii_letters + string.digits,
LETTERS: string.ascii_letters,
LOWERCASE: string.ascii_lowercase,
UPPERCASE: string.ascii_uppercase,
DIGITS: string.digits,
HEXDIGITS: string.digits + 'ABCDEF',
OCTDIGITS: string.octdigits,
}
CharClass = collections.namedtuple('CharClass',
('allowed_chars', 'min_count'))
def named_char_class(char_class, min_count=0):
"""Return a predefined character class.
The result of this function can be passed to :func:`generate_password` as
one of the character classes to use in generating a password.
:param char_class: Any of the character classes named in
:const:`CHARACTER_CLASSES`
:param min_count: The minimum number of members of this class to appear in
a generated password
"""
assert char_class in CHARACTER_CLASSES
return CharClass(frozenset(_char_class_members[char_class]), min_count)
def special_char_class(allowed_chars, min_count=0):
"""Return a character class containing custom characters.
The result of this function can be passed to :func:`generate_password` as
one of the character classes to use in generating a password.
:param allowed_chars: Iterable of the characters in the character class
:param min_count: The minimum number of members of this class to appear in
a generated password
"""
return CharClass(frozenset(allowed_chars), min_count)
def generate_password(length, char_classes):
"""Generate a random password.
The password will be of the specified length, and comprised of characters
from the specified character classes, which can be generated using the
:func:`named_char_class` and :func:`special_char_class` functions. Where
a minimum count is specified in the character class, at least that number
of characters in the resulting password are guaranteed to be from that
character class.
:param length: The length of the password to generate, in characters
:param char_classes: Iterable over classes of characters from which to
generate a password
"""
char_buffer = six.StringIO()
all_allowed_chars = set()
# Add the minimum number of chars from each char class
for char_class in char_classes:
all_allowed_chars |= char_class.allowed_chars
allowed_chars = tuple(char_class.allowed_chars)
for i in six.moves.xrange(char_class.min_count):
char_buffer.write(random.choice(allowed_chars))
# Fill up rest with random chars from provided classes
combined_chars = tuple(all_allowed_chars)
for i in six.moves.xrange(max(0, length - char_buffer.tell())):
char_buffer.write(random.choice(combined_chars))
# Shuffle string
selected_chars = char_buffer.getvalue()
char_buffer.close()
return ''.join(random.sample(selected_chars, length))

97
heat/engine/resources/openstack/heat/random_string.py
View File

@ -11,13 +11,11 @@
# License for the specific language governing permissions and limitations
# under the License.
import random as random_module
import string
import six
from heat.common import exception
from heat.common.i18n import _
from heat.common import password_gen
from heat.engine import attributes
from heat.engine import constraints
from heat.engine import properties
@ -25,10 +23,6 @@ from heat.engine import resource
from heat.engine import support
from heat.engine import translation
# NOTE(pas-ha) Heat officially supports only POSIX::Linux platform
# where os.urandom() and random.SystemRandom() are available
random = random_module.SystemRandom()
class RandomString(resource.Resource):
"""A resource which generates a random string.
@ -83,10 +77,7 @@ class RandomString(resource.Resource):
properties.Schema.STRING,
_('Sequence of characters to build the random string from.'),
constraints=[
constraints.AllowedValues(['lettersdigits', 'letters',
'lowercase', 'uppercase',
'digits', 'hexdigits',
'octdigits']),
constraints.AllowedValues(password_gen.CHARACTER_CLASSES),
],
support_status=support.SupportStatus(
status=support.HIDDEN,
@ -112,11 +103,9 @@ class RandomString(resource.Resource):
% {'min': CHARACTER_CLASSES_MIN}),
constraints=[
constraints.AllowedValues(
['lettersdigits', 'letters', 'lowercase',
'uppercase', 'digits', 'hexdigits',
'octdigits']),
password_gen.CHARACTER_CLASSES),
],
default='lettersdigits'),
default=password_gen.LETTERS_DIGITS),
CHARACTER_CLASSES_MIN: properties.Schema(
properties.Schema.INTEGER,
_('The minimum number of characters from this '
@ -130,7 +119,7 @@ class RandomString(resource.Resource):
}
),
# add defaults for backward compatibility
default=[{CHARACTER_CLASSES_CLASS: 'lettersdigits',
default=[{CHARACTER_CLASSES_CLASS: password_gen.LETTERS_DIGITS,
CHARACTER_CLASSES_MIN: 1}]
),
@ -177,16 +166,6 @@ class RandomString(resource.Resource):
),
}
_sequences = {
'lettersdigits': string.ascii_letters + string.digits,
'letters': string.ascii_letters,
'lowercase': string.ascii_lowercase,
'uppercase': string.ascii_uppercase,
'digits': string.digits,
'hexdigits': string.digits + 'ABCDEF',
'octdigits': string.octdigits
}
def translation_rules(self, props):
if props.get(self.SEQUENCE):
return [
@ -205,57 +184,19 @@ class RandomString(resource.Resource):
]
def _generate_random_string(self, char_sequences, char_classes, length):
random_string = ""
seq_mins = [
password_gen.special_char_class(
char_seq[self.CHARACTER_SEQUENCES_SEQUENCE],
char_seq[self.CHARACTER_SEQUENCES_MIN])
for char_seq in char_sequences]
char_class_mins = [
password_gen.named_char_class(
char_class[self.CHARACTER_CLASSES_CLASS],
char_class[self.CHARACTER_CLASSES_MIN])
for char_class in char_classes]
# Add the minimum number of chars from each char sequence & char class
if char_sequences:
for char_seq in char_sequences:
seq = char_seq[self.CHARACTER_SEQUENCES_SEQUENCE]
seq_min = char_seq[self.CHARACTER_SEQUENCES_MIN]
for i in six.moves.xrange(seq_min):
random_string += random.choice(seq)
if char_classes:
for char_class in char_classes:
cclass_class = char_class[self.CHARACTER_CLASSES_CLASS]
cclass_seq = self._sequences[cclass_class]
cclass_min = char_class[self.CHARACTER_CLASSES_MIN]
for i in six.moves.xrange(cclass_min):
random_string += random.choice(cclass_seq)
def random_class_char():
cclass_dict = random.choice(char_classes)
cclass_class = cclass_dict[self.CHARACTER_CLASSES_CLASS]
cclass_seq = self._sequences[cclass_class]
return random.choice(cclass_seq)
def random_seq_char():
seq_dict = random.choice(char_sequences)
seq = seq_dict[self.CHARACTER_SEQUENCES_SEQUENCE]
return random.choice(seq)
# Fill up rest with random chars from provided sequences & classes
if char_sequences and char_classes:
weighted_choices = ([True] * len(char_classes) +
[False] * len(char_sequences))
while len(random_string) < length:
if random.choice(weighted_choices):
random_string += random_class_char()
else:
random_string += random_seq_char()
elif char_sequences:
while len(random_string) < length:
random_string += random_seq_char()
else:
while len(random_string) < length:
random_string += random_class_char()
# Randomize string
random_string = ''.join(random.sample(random_string,
len(random_string)))
return random_string
return password_gen.generate_password(length,
seq_mins + char_class_mins)
def validate(self):
super(RandomString, self).validate()
@ -276,8 +217,8 @@ class RandomString(resource.Resource):
raise exception.StackValidationFailed(message=msg)
def handle_create(self):
char_sequences = self.properties[self.CHARACTER_SEQUENCES]
char_classes = self.properties[self.CHARACTER_CLASSES]
char_sequences = self.properties[self.CHARACTER_SEQUENCES] or []
char_classes = self.properties[self.CHARACTER_CLASSES] or []
length = self.properties[self.LENGTH]
random_string = self._generate_random_string(char_sequences,

121
heat/tests/openstack/heat/test_random_string.py
View File

@ -259,7 +259,7 @@ Resources:
return stack
# test was saved to test backward compatibility with old behavior
def test_generate_random_string_backward_compatiable(self):
def test_generate_random_string_backward_compatible(self):
stack = self.parse_stack(template_format.parse(self.template_rs))
secret = stack['secret']
char_classes = secret.properties['character_classes']
@ -268,8 +268,125 @@ Resources:
# run each test multiple times to confirm random generator
# doesn't generate a matching pattern by chance
for i in range(1, 32):
r = secret._generate_random_string(None, char_classes, self.length)
r = secret._generate_random_string([], char_classes, self.length)
self.assertThat(r, matchers.HasLength(self.length))
regex = '%s{%s}' % (self.pattern, self.length)
self.assertThat(r, matchers.MatchesRegex(regex))
class TestGenerateRandomStringDistribution(common.HeatTestCase):
def run_test(self, tmpl, iterations=5):
stack = utils.parse_stack(template_format.parse(tmpl))
secret = stack['secret']
secret.data_set = mock.Mock()
for i in range(iterations):
secret.handle_create()
return [call[1][1] for call in secret.data_set.mock_calls]
def char_counts(self, random_strings, char):
return [rs.count(char) for rs in random_strings]
def check_stats(self, char_counts, expected_mean, allowed_variance,
expected_minimum=0):
mean = float(sum(char_counts)) / len(char_counts)
self.assertLess(mean, expected_mean + allowed_variance)
self.assertGreater(mean, max(0, expected_mean - allowed_variance))
if expected_minimum:
self.assertGreaterEqual(min(char_counts), expected_minimum)
def test_class_uniformity(self):
template_rs = '''
HeatTemplateFormatVersion: '2012-12-12'
Resources:
secret:
Type: OS::Heat::RandomString
Properties:
length: 66
character_classes:
- class: lettersdigits
character_sequences:
- sequence: "*$"
'''
results = self.run_test(template_rs, 10)
for char in '$*':
self.check_stats(self.char_counts(results, char), 1.5, 2)
def test_repeated_sequence(self):
template_rs = '''
HeatTemplateFormatVersion: '2012-12-12'
Resources:
secret:
Type: OS::Heat::RandomString
Properties:
length: 40
character_classes: []
character_sequences:
- sequence: "**********$*****************************"
'''
results = self.run_test(template_rs)
for char in '$*':
self.check_stats(self.char_counts(results, char), 20, 6)
def test_overlapping_classes(self):
template_rs = '''
HeatTemplateFormatVersion: '2012-12-12'
Resources:
secret:
Type: OS::Heat::RandomString
Properties:
length: 624
character_classes:
- class: lettersdigits
- class: digits
- class: octdigits
- class: hexdigits
'''
results = self.run_test(template_rs, 20)
self.check_stats(self.char_counts(results, '0'), 10.3, 2.5)
def test_overlapping_sequences(self):
template_rs = '''
HeatTemplateFormatVersion: '2012-12-12'
Resources:
secret:
Type: OS::Heat::RandomString
Properties:
length: 60
character_classes: []
character_sequences:
- sequence: "01"
- sequence: "02"
- sequence: "03"
- sequence: "04"
- sequence: "05"
- sequence: "06"
- sequence: "07"
- sequence: "08"
- sequence: "09"
'''
results = self.run_test(template_rs)
self.check_stats(self.char_counts(results, '0'), 10, 5)
def test_overlapping_class_sequence(self):
template_rs = '''
HeatTemplateFormatVersion: '2012-12-12'
Resources:
secret:
Type: OS::Heat::RandomString
Properties:
length: 402
character_classes:
- class: octdigits
character_sequences:
- sequence: "0"
'''
results = self.run_test(template_rs, 10)
self.check_stats(self.char_counts(results, '0'), 51.125, 8, 1)

9
releasenotes/notes/random-string-entropy-9b8e23874cd79b8f.yaml Normal file
View File

@ -0,0 +1,9 @@
---
security:
- |
Passwords generated by the OS::Heat::RandomString resource may have had
less entropy than expected, depending on what is specified in the
``character_class`` and ``character_sequence`` properties. This has been
corrected so that each character present in any of the specified classes
or sequences now has an equal probability of appearing at each point in
the generated random string.