Alternative to Captchas (Without Human Interaction)

# -*- coding: UTF-8 -*-

import time
import datetime
import base64

from django import forms
from django.utils.translation import ugettext
from django.conf import settings

MIN_TIME = getattr(settings, "MIN_TIME", 5) # 5 seconds
MAX_TIME = getattr(settings, "MAX_TIME", 3600) # 1 hour

def cryptString(plain):
    # Your implementation for encrypting a string.
    # For example:
    return base64.encodestring(plain)    

def decryptString(cipher):
    # Your implementation for decrypting a string
    # For example:
    return base64.decodestring(cipher)

class SecurityField(forms.CharField):
    """
    A field which checks whether the form was filled in within the 
    given range of time
    
    The concept works only for Unbounded forms.
    """
    time_elapsed = 0
    
    def generate_value(self):
        started = cryptString(str(int(time.mktime(datetime.datetime.now().timetuple()))))
        return started
    def _pass_test(self, value):
        started = int(decryptString(value))
        current = int(time.mktime(datetime.datetime.now().timetuple()))
        self.time_elapsed = current - started
        return self.MIN_TIME < current - started < self.MAX_TIME
    def __init__(self, *args, **kwargs):
        super(type(self), self).__init__(*args, **kwargs)
        self.widget = forms.HiddenInput()
        self.initial = self.generate_value()
        self.required = True
        self.MIN_TIME = MIN_TIME
        self.MAX_TIME = MAX_TIME
    def clean(self, value):
        value = super(type(self), self).clean(value)
        if not self._pass_test(value):
            raise forms.ValidationError(ugettext(u"The data transfer didn't pass the security test. You are considered as a spambot."))
        return value