Django密码存储策略分析

一、源码分析

Django 发布的 1.4 版本中包含了一些安全方面的重要提升。其中一个是使用 PBKDF2 密码加密算法代替了 SHA1 。另外一个特性是你可以添加自己的密码加密方法。

Django 会使用你提供的第一个密码加密方法（在你的 setting.py 文件里要至少有一个方法）

PASSWORD_HASHERS = [
  'django.contrib.auth.hashers.PBKDF2PasswordHasher','django.contrib.auth.hashers.PBKDF2SHA1PasswordHasher','django.contrib.auth.hashers.Argon2PasswordHasher','django.contrib.auth.hashers.BCryptSHA256PasswordHasher','django.contrib.auth.hashers.BCryptPasswordHasher',]

我们先一睹自带的PBKDF2PasswordHasher加密方式。

class BasePasswordHasher(object):
  """
  Abstract base class for password hashers
  When creating your own hasher,you need to override algorithm,verify(),encode() and safe_summary().
  PasswordHasher objects are immutable.
  """
  algorithm = None
  library = None

  def _load_library(self):
    if self.library is not None:
      if isinstance(self.library,(tuple,list)):
        name,mod_path = self.library
      else:
        name = mod_path = self.library
      try:
        module = importlib.import_module(mod_path)
      except ImportError:
        raise ValueError("Couldn't load %s password algorithm "
                 "library" % name)
      return module
    raise ValueError("Hasher '%s' doesn't specify a library attribute" %
             self.__class__)

  def salt(self):
    """
    Generates a cryptographically secure nonce salt in ascii
    """
    return get_random_string()

  def verify(self,password,encoded):
    """
    Checks if the given password is correct
    """
    raise NotImplementedError()

  def encode(self,salt):
    """
    Creates an encoded database value
    The result is normally formatted as "algorithm$salt$hash" and
    must be fewer than 128 characters.
    """
    raise NotImplementedError()

  def safe_summary(self,encoded):
    """
    Returns a summary of safe values
    The result is a dictionary and will be used where the password field
    must be displayed to construct a safe representation of the password.
    """
    raise NotImplementedError()

class PBKDF2PasswordHasher(BasePasswordHasher):
  """
  Secure password hashing using the PBKDF2 algorithm (recommended)
  Configured to use PBKDF2 + HMAC + SHA256.
  The result is a 64 byte binary string. Iterations may be changed
  safely but you must rename the algorithm if you change SHA256.
  """
  algorithm = "pbkdf2_sha256"
  iterations = 36000
  digest = hashlib.sha256

  def encode(self,salt,iterations=None):
    assert password is not None
    assert salt and '$' not in salt
    if not iterations:
      iterations = self.iterations
    hash = pbkdf2(password,iterations,digest=self.digest)
    hash = base64.b64encode(hash).decode('ascii').strip()
    return "%s$%d$%s$%s" % (self.algorithm,hash)

  def verify(self,encoded):
    algorithm,hash = encoded.split('$',3)
    assert algorithm == self.algorithm
    encoded_2 = self.encode(password,int(iterations))
    return constant_time_compare(encoded,encoded_2)

  def safe_summary(self,3)
    assert algorithm == self.algorithm
    return OrderedDict([
      (_('algorithm'),algorithm),(_('iterations'),iterations),(_('salt'),mask_hash(salt)),(_('hash'),mask_hash(hash)),])

  def must_update(self,3)
    return int(iterations) != self.iterations

  def harden_runtime(self,3)
    extra_iterations = self.iterations - int(iterations)
    if extra_iterations > 0:
      self.encode(password,extra_iterations)

正如你看到那样，你必须继承自BasePasswordHasher，并且重写 verify() , encode() 以及 safe_summary() 方法。

Django 是使用 PBKDF 2算法与36,000次的迭代使得它不那么容易被暴力破解法轻易攻破。密码用下面的格式储存：

algorithm$number of iterations$salt$password hash”

例：pbkdf2_sha256$36000$Lx7auRCc8FUI$eG9lX66cKFTos9sEcihhiSCjI6uqbr9ZrO+Iq3H9xDU=

二、自定义密码加密方法

1、在settings.py中加入自定义的加密算法：

PASSWORD_HASHERS = [
  'myproject.hashers.MyMD5PasswordHasher','django.contrib.auth.hashers.PBKDF2PasswordHasher',]

2、再来看MyMD5PasswordHasher，这个是我自定义的加密方式，就是基本的md5，而django的MD5PasswordHasher是加盐的：

 from django.contrib.auth.hashers import BasePasswordHasher,MD5PasswordHasher
 from django.contrib.auth.hashers import mask_hash
 import hashlib

 class MyMD5PasswordHasher(MD5PasswordHasher):
   algorithm = "mymd5"

   def encode(self,salt):
     assert password is not None
     hash = hashlib.md5(password).hexdigest().upper()
     return hash

   def verify(self,encoded):
     encoded_2 = self.encode(password,'')
     return encoded.upper() == encoded_2.upper()

   def safe_summary(self,encoded):
     return OrderedDict([
         (_('algorithm'),''),])

之后可以在数据库中看到，密码确实使用了自定义的加密方式。

3、修改认证方式

AUTHENTICATION_BACKENDS = (
  'framework.mybackend.MyBackend',#新加
  'django.contrib.auth.backends.ModelBackend','guardian.backends.ObjectPermissionBackend',)

4、再来看自定义的认证方式

framework.mybackend.py:

 import hashlib
 from pro import models
 from django.contrib.auth.backends import ModelBackend

 class MyBackend(ModelBackend):
   def authenticate(self,username=None,password=None):
     try:
       user = models.M_User.objects.get(username=username)
       print user
     except Exception:
       print 'no user'
       return None
     if hashlib.md5(password).hexdigest().upper() == user.password:
       return user
     return None

   def get_user(self,user_id):
     try:
       return models.M_User.objects.get(id=user_id)
     except Exception:
       return None

当然经过这些修改后最终的安全性比起django自带的降低很多，但是需求就是这样的，必须满足。

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（编辑：李大同）

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