ipa_generate_password algorithm change

A change to the algorithm that generates random passwords
for multiple purposes throught IPA. This spells out the need
to assess password strength by the entropy it contains rather
than its length.

This new password generation should also be compatible with the
NSS implementation of password requirements in FIPS environment
so that newly created databases won't fail with wrong authentication.

https://fedorahosted.org/freeipa/ticket/5695

Reviewed-By: Martin Basti <mbasti@redhat.com>
Reviewed-By: Petr Spacek <pspacek@redhat.com>

ipapython/ipautil.py

@@ -23,6 +23,7 @@ import string
 import tempfile
 import subprocess
 import random
+import math
 import os
 import sys
 import copy
@@ -51,8 +52,8 @@ from six.moves import urllib
 from ipapython.ipa_log_manager import root_logger
 from ipapython.dn import DN
 
-GEN_PWD_LEN = 22
-GEN_TMP_PWD_LEN = 12  # only for OTP password that is manually retyped by user
+# only for OTP password that is manually retyped by user
+TMP_PWD_ENTROPY_BITS = 128
 
 
 PROTOCOL_NAMES = {
@@ -789,34 +790,89 @@ def parse_generalized_time(timestr):
     except ValueError:
         return None
 
-def ipa_generate_password(characters=None,pwd_len=None):
-    ''' Generates password. Password cannot start or end with a whitespace
-    character. It also cannot be formed by whitespace characters only.
-    Length of password as well as string of characters to be used by
-    generator could be optionaly specified by characters and pwd_len
-    parameters, otherwise default values will be used: characters string
-    will be formed by all printable non-whitespace characters and space,
-    pwd_len will be equal to value of GEN_PWD_LEN.
-    '''
-    if not characters:
-        characters=string.digits + string.ascii_letters + string.punctuation + ' '
-    else:
-        if characters.isspace():
-            raise ValueError("password cannot be formed by whitespaces only")
-    if not pwd_len:
-        pwd_len = GEN_PWD_LEN
 
-    upper_bound = len(characters) - 1
-    rndpwd = ''
-    r = random.SystemRandom()
+def ipa_generate_password(entropy_bits=256, uppercase=1, lowercase=1, digits=1,
+                          special=1, min_len=0):
+    """
+    Generate token containing at least `entropy_bits` bits and with the given
+    character restraints.
+
+    :param entropy_bits:
+        The minimal number of entropy bits attacker has to guess:
+           128 bits entropy: secure
+           256 bits of entropy: secure enough if you care about quantum
+                                computers
+
+    Integer values specify minimal number of characters from given
+    character class and length.
+    Value None prevents given character from appearing in the token.
+
+    Example:
+    TokenGenerator(uppercase=3, lowercase=3, digits=0, special=None)
+
+    At least 3 upper and 3 lower case ASCII chars, may contain digits,
+    no special chars.
+    """
+    special_chars = '!$%&()*+,-./:;<>?@[]^_{|}~'
+    pwd_charsets = {
+        'uppercase': {
+            'chars': string.ascii_uppercase,
+            'entropy': math.log(len(string.ascii_uppercase), 2)
+        },
+        'lowercase': {
+            'chars': string.ascii_lowercase,
+            'entropy': math.log(len(string.ascii_lowercase), 2)
+        },
+        'digits': {
+            'chars': string.digits,
+            'entropy': math.log(len(string.digits), 2)
+        },
+        'special': {
+            'chars': special_chars,
+            'entropy': math.log(len(special_chars), 2)
+        },
+    }
+    req_classes = dict(
+        uppercase=uppercase,
+        lowercase=lowercase,
+        digits=digits,
+        special=special
+    )
+    # 'all' class is used when adding entropy to too-short tokens
+    # it contains characters from all allowed classes
+    pwd_charsets['all'] = {
+        'chars': ''.join([
+            charclass['chars'] for charclass_name, charclass
+            in pwd_charsets.items()
+            if req_classes[charclass_name] is not None
+        ])
+    }
+    pwd_charsets['all']['entropy'] = math.log(
+            len(pwd_charsets['all']['chars']), 2)
+    rnd = random.SystemRandom()
+
+    todo_entropy = entropy_bits
+    password = ''
+    # Generate required character classes:
+    # The order of generated characters is fixed to comply with check in
+    # NSS function sftk_newPinCheck() in nss/lib/softoken/fipstokn.c.
+    for charclass_name in ['digits', 'uppercase', 'lowercase', 'special']:
+        charclass = pwd_charsets[charclass_name]
+        todo_characters = req_classes[charclass_name]
+        while todo_characters > 0:
+            password += rnd.choice(charclass['chars'])
+            todo_entropy -= charclass['entropy']
+            todo_characters -= 1
+
+    # required character classes do not provide sufficient entropy
+    # or does not fulfill minimal length constraint
+    allchars = pwd_charsets['all']
+    while todo_entropy > 0 or len(password) < min_len:
+        password += rnd.choice(allchars['chars'])
+        todo_entropy -= allchars['entropy']
+
+    return password
 
-    for x in range(pwd_len):
-        rndchar = characters[r.randint(0,upper_bound)]
-        if (x == 0) or (x == pwd_len-1):
-            while rndchar.isspace():
-                rndchar = characters[r.randint(0,upper_bound)]
-        rndpwd += rndchar
-    return rndpwd
 
 def user_input(prompt, default = None, allow_empty = True):
     if default == None: