I need to use existing field(i.e password) of registration form(it is not new field) to use on the seperate table while posting

my code is as below

from django.contrib.auth.models import User
from django.db.models.signals import post_save

class Token(models.Model):
user = models.OneToOneField(User)
api_key=models.CharField(max_length=50)


def create_user_token(sender, instance, created, **kwargs):
if created:
profile, created = Token.objects.get_or_create(user=instance)

#this will definitely not work because i have no field name api_key on form
post_save.connect(create_user_token, sender=User)
and my logic to create apikey is below

#password=posted from django-registration form as i have used django-registration
digest = hmac.new('Secret', password, hashlib.sha256).hexdigest()
I am using django registration and i need to set api_key=digest whenever user register or change the password