Persistent Crypto Dictionary

This class implements a persistent dictionary using sqlite3 and encrypts the keys and the values of the dictionary in a way, that makes it very hard to bruteforce either the key or the values in the db.

example usage::

>>> from pcd import PersistentCryptoDict
>>> d=PersistentCryptoDict()
>>> print d
<pcd.PersistentCryptoDict instance at 0x8dcb54c>
>>> print d['my key']
None
>>> d['my key']='secret value'
>>> print d['my key']
secret value
>>> d['my key']='top secret value'
>>> print d['my key']
top secret value

Crypto

The key and the value in the dict is transformed according to the following algorithm (credit: dnet):

Setting values

1. we calculate they keyhash - a hmac-sha512(salt,key)
2. we split the key in half, the first half as a hexdigest (ascii), the second we keep as a binary
3. we use the second binary half from step 2 of the keyhash to encrypt the value
4. we use the ascii keyhash from step 2 as a key to the database, and the value is the encrypted result from step 3.

Getting values

1. we calculate they keyhash - a hmac-sha512(salt,key)
2. we split the key in half, the first half as a hexdigest (ascii), the second we keep as a binary
3. we query the database using the ascii keyhash from step 2 as a key
4. we use the second binary half from step 2 of the keyhash to decrypt the value

The database contains only the following pairs of data:

(hmac-sha512(key, salt).hexdigest()[:64], # key aes256-ofb(hmac-sha512(key, salt).digest()[32:], value)) # value

we diligently obey Schneier's law: https://www.schneier.com/blog/archives/2011/04/schneiers_law.html, and thus we would consider the task to retrieve any meaningful data without huge rainbow tables from such a database a futile task. :)