      for security purposes.
      """
      request.META.update({
          "CSRF_COOKIE_USED": True,
          "CSRF_COOKIE": _get_new_csrf_key(),
      })
  
  
  def _sanitize_token(token):
      # Allow only alphanum
 
</s> remove     # Allow only alphanum
    if len(token) > CSRF_KEY_LENGTH:
        return _get_new_csrf_key()
    token = re.sub('[^a-zA-Z0-9]+', '', force_text(token))
    if token == "":
        # In case the cookie has been truncated to nothing at some point.
        return _get_new_csrf_key()
    return token
 </s> add     # Allow only ASCII alphanumerics
    if re.search('[^a-zA-Z0-9]', force_text(token)):
        return _get_new_csrf_token()
    elif len(token) == CSRF_TOKEN_LENGTH:
        return token
    elif len(token) == CSRF_SECRET_LENGTH:
        # Older Django versions set cookies to values of CSRF_SECRET_LENGTH
        # alphanumeric characters. For backwards compatibility, accept
        # such values as unsalted secrets.
        # It's easier to salt here and be consistent later, rather than add
        # different code paths in the checks, although that might be a tad more
        # efficient.
        return _salt_cipher_secret(token)
    return _get_new_csrf_token()


def _compare_salted_tokens(request_csrf_token, csrf_token):
    # Assume both arguments are sanitized -- that is, strings of
    # length CSRF_TOKEN_LENGTH, all CSRF_ALLOWED_CHARS.
    return constant_time_compare(
        _unsalt_cipher_token(request_csrf_token),
        _unsalt_cipher_token(csrf_token),
    ) </s> remove def _get_new_csrf_key():
    return get_random_string(CSRF_KEY_LENGTH)
 </s> add def _get_new_csrf_string():
    return get_random_string(CSRF_SECRET_LENGTH, allowed_chars=CSRF_ALLOWED_CHARS)


def _salt_cipher_secret(secret):
    """
    Given a secret (assumed to be a string of CSRF_ALLOWED_CHARS), generate a
    token by adding a salt and using it to encrypt the secret.
    """
    salt = _get_new_csrf_string()
    chars = CSRF_ALLOWED_CHARS
    pairs = zip((chars.index(x) for x in secret), (chars.index(x) for x in salt))
    cipher = ''.join(chars[(x + y) % len(chars)] for x, y in pairs)
    return salt + cipher


def _unsalt_cipher_token(token):
    """
    Given a token (assumed to be a string of CSRF_ALLOWED_CHARS, of length
    CSRF_TOKEN_LENGTH, and that its first half is a salt), use it to decrypt
    the second half to produce the original secret.
    """
    salt = token[:CSRF_SECRET_LENGTH]
    token = token[CSRF_SECRET_LENGTH:]
    chars = CSRF_ALLOWED_CHARS
    pairs = zip((chars.index(x) for x in token), (chars.index(x) for x in salt))
    secret = ''.join(chars[x - y] for x, y in pairs)  # Note negative values are ok
    return secret


def _get_new_csrf_token():
    return _salt_cipher_secret(_get_new_csrf_string()) </s> remove     return request.META["CSRF_COOKIE"]
 </s> add     return _salt_cipher_secret(csrf_secret) </s> remove             if not constant_time_compare(request_csrf_token, csrf_token):
 </s> add             request_csrf_token = _sanitize_token(request_csrf_token)
            if not _compare_salted_tokens(request_csrf_token, csrf_token):
        "CSRF_COOKIE": _get_new_csrf_token(),
      for security purposes.
      """
      request.META.update({
          "CSRF_COOKIE_USED": True,
        "CSRF_COOKIE": _get_new_csrf_token(),
      })
  
  
  def _sanitize_token(token):
      # Allow only alphanum
keep keep keep keep replace keep keep keep keep keep
 Fixed #20869 -- made CSRF tokens change every request by salt-encrypting them

Note that the cookie is not changed every request, just the token retrieved
by the `get_token()` method (used also by the `{% csrf_token %}` tag).

While at it, made token validation strict: Where, before, any length was
accepted and non-ASCII chars were ignored, we now treat anything other than
`[A-Za-z0-9]{64}` as invalid (except for 32-char tokens, which, for
backwards-compatibility, are accepted and replaced by 64-char ones).

Thanks Trac user patrys for reporting, github user adambrenecki
for initial patch, Tim Graham for help, and Curtis Maloney,
Collin Anderson, Florian Apolloner, Markus Holtermann & Jon Dufresne
for reviews. 
django/middleware/csrf.py