Model Encryption

from django.db import models
from django.core.exceptions import ImproperlyConfigured
from django import forms
from django.conf import settings

try:
    from keyczar import keyczar
except ImportError:
    raise ImportError('Using an encrypted field requires the Keyczar module.  You can obtain Keyczar from http://www.keyczar.org/.')

class BaseEncryptedField(models.Field):
    prefix = 'enc_str:::'
    def __init__(self, *args, **kwargs):
        if not hasattr(settings, 'ENCRYPTED_FIELD_KEYS_DIR'):
            raise ImproperlyConfigured('You must set settings.ENCRYPTED_FIELD_KEYS_DIR to your Keyczar keys directory.')
        self.crypt = keyczar.Crypter.Read(settings.ENCRYPTED_FIELD_KEYS_DIR)
        super(BaseEncryptedField, self).__init__(*args, **kwargs)
    
    def to_python(self, value):
        if (value.startswith(self.prefix)):
            retval = self.crypt.Decrypt(value[len(self.prefix):])
        else:
            retval = value
            
        return retval
    
    def get_db_prep_value(self, value):
        if not value.startswith(self.prefix):
            value = self.prefix + self.crypt.Encrypt(value)
        return value

class EncryptedTextField(BaseEncryptedField):
    __metaclass__ = models.SubfieldBase

    def get_internal_type(self): 
        return 'TextField'
    
    def formfield(self, **kwargs):
        defaults = {'widget': forms.Textarea}
        defaults.update(kwargs)
        return super(EncryptedTextField, self).formfield(**defaults)

class EncryptedCharField(BaseEncryptedField):
    __metaclass__ = models.SubfieldBase
    
    def __init__(self, max_length=None, *args, **kwargs):
        if max_length:
            max_length += len(self.prefix)
        
        super(EncryptedCharField, self).__init__(max_length=max_length, *args, **kwargs)
        
    def get_internal_type(self):
        return "CharField"
    
    def formfield(self, **kwargs):
        defaults = {'max_length': self.max_length}
        defaults.update(kwargs)
        return super(EncryptedCharField, self).formfield(**defaults)